WorldWideScience

Sample records for wake velocity deficits

  1. Velocity Measurements of Turbulent Wake Flow Over a Circular Cylinder

    Science.gov (United States)

    Shih, Chang-Lung; Chen, Wei-Cheng; Chang, Keh-Chin; Wang, Muh-Rong

    2016-06-01

    There are two general concerns in the velocity measurements of turbulence. One is the temporal characteristics which governs the turbulent mixing process. Turbulence is rotational and is characterized by high levels of fluctuating vorticity. In order to obtain the information of vorticity dynamics, the spatial characteristics is the other concern. These varying needs can be satisfied by using a variety of diagnostic techniques such as invasive physical probes and non-invasive optical instruments. Probe techniques for the turbulent measurements are inherently simple and less expensive than optical methods. However, the presence of a physical probe may alter the flow field, and velocity measurements usually become questionable when probing recirculation zones. The non-invasive optical methods are mostly made of the foreign particles (or seeding) instead of the fluid flow and are, thus, of indirect method. The difference between the velocities of fluid and foreign particles is always an issue to be discussed particularly in the measurements of complicated turbulent flows. Velocity measurements of the turbulent wake flow over a circular cylinder will be made by using two invasive instruments, namely, a cross-type hot-wire anemometry (HWA) and a split-fiber hot-film anemometry (HFA), and a non-invasive optical instrument, namely, particle image velocimetry (PIV) in this study. Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, particularly for the PIV measurements. It is demonstrated that HFA possesses more capability than HWA in the flow measurements of wake flow. Wake width is determined in terms of either the flatness factor or shear-induced vorticity. It is demonstrated that flow data obtained with the three employed diagnostic techniques are capable of yielding accurate determination of wake width.

  2. Wing Kinematics and Wake Velocity Characteristics of Bat Flight

    Science.gov (United States)

    Swartz, Sharon

    2005-11-01

    Bats demonstrate unequalled flight characteristics and are capable of highly efficient flight as well as extreme maneuverability at high speeds. They have morphological properties that are unique in the animal world including jointed wings skeletons, elastic wing membranes and very complex wing motions. We report on a series of experiments on bats flying in a flight cage along both a straight path and through a 90-degree turn. Measurements of their kinematic wing motion (using high speed photography) and wake velocity structures (using stereo PIV) are reported. The live animal measurements are also interpreted with the help of a series of companion wind tunnel experiments using model structures that mimic some key features of bat flight mechanics. The results reveal a complex vortex wake structure which is compared and contrasted to that found in bird and insect flight.

  3. Velocity field measurements in the wake of a propeller model

    Science.gov (United States)

    Mukund, R.; Kumar, A. Chandan

    2016-10-01

    Turboprop configurations are being revisited for the modern-day regional transport aircrafts for their fuel efficiency. The use of laminar flow wings is an effort in this direction. One way to further improve their efficiency is by optimizing the flow over the wing in the propeller wake. Previous studies have focused on improving the gross aerodynamic characteristics of the wing. It is known that the propeller slipstream causes early transition of the boundary layer on the wing. However, an optimized design of the propeller and wing combination could delay this transition and decrease the skin friction drag. Such a wing design would require the detailed knowledge of the development of the slipstream in isolated conditions. There are very few studies in the literature addressing the requirements of transport aircraft having six-bladed propeller and cruising at a high propeller advance ratio. Low-speed wind tunnel experiments have been conducted on a powered propeller model in isolated conditions, measuring the velocity field in the vertical plane behind the propeller using two-component hot-wire anemometry. The data obtained clearly resolved the mean velocity, the turbulence, the ensemble phase averages and the structure and development of the tip vortex. The turbulence in the slipstream showed that transition could be close to the leading edge of the wing, making it a fine case for optimization. The development of the wake with distance shows some interesting flow features, and the data are valuable for flow computation and optimization.

  4. Numerical performance analysis of acoustic Doppler velocity profilers in the wake of an axial-flow marine hydrokinetic turbine

    Energy Technology Data Exchange (ETDEWEB)

    Richmond, Marshall C.; Harding, Samuel F.; Romero Gomez, Pedro DJ

    2015-09-01

    The use of acoustic Doppler current profilers (ADCPs) for the characterization of flow conditions in the vicinity of both experimental and full scale marine hydrokinetic (MHK) turbines is becoming increasingly prevalent. The computation of a three dimensional velocity measurement from divergent acoustic beams requires the assumption that the flow conditions are homogeneous between all beams at a particular axial distance from the instrument. In the near wake of MHK devices, the mean fluid motion is observed to be highly spatially dependent as a result of torque generation and energy extraction. This paper examines the performance of ADCP measurements in such scenarios through the modelling of a virtual ADCP (VADCP) instrument in the velocity field in the wake of an MHK turbine resolved using unsteady computational fluid dynamics (CFD). This is achieved by sampling the CFD velocity field at equivalent locations to the sample bins of an ADCP and performing the coordinate transformation from beam coordinates to instrument coordinates and finally to global coordinates. The error in the mean velocity calculated by the VADCP relative to the reference velocity along the instrument axis is calculated for a range of instrument locations and orientations. The stream-wise velocity deficit and tangential swirl velocity caused by the rotor rotation lead to significant misrepresentation of the true flow velocity profiles by the VADCP, with the most significant errors in the transverse (cross-flow) velocity direction.

  5. Flat Plate Wake Velocity Statistics Obtained With Circular And Elliptic Trailing Edges

    Science.gov (United States)

    Rai, Man Mohan

    2016-01-01

    The near wake of a flat plate with circular and elliptic trailing edges is investigated with data from direct numerical simulations. The plate length and thickness are the same in both cases. The separating boundary layers are turbulent and statistically identical. Therefore the wake is symmetric in the two cases. The emphasis in this study is on a comparison of the wake-distributions of velocity components, normal intensity and fluctuating shear stress obtained in the two cases.

  6. Wind turbine wakes; power deficit in clusters and wind farms

    DEFF Research Database (Denmark)

    Hansen, Kurt Schaldemose; Barthelmie, Rebecca

    2012-01-01

    The purpose of this presentation is to present recent power deficit analysis based on wind farm measurements. The power deficit is used to validate wind farm prediction models for different inflow conditions......The purpose of this presentation is to present recent power deficit analysis based on wind farm measurements. The power deficit is used to validate wind farm prediction models for different inflow conditions...

  7. Mean velocity and moments of turbulent velocity fluctuations in the wake of a model ship propulsor

    Energy Technology Data Exchange (ETDEWEB)

    Pego, J.P. [Universitaet Erlangen-Nuernberg, LSTM, Erlangen, Lehrstuhl fuer Stroemungsmechanik, Erlangen (Germany); Faculdade de Engenharia da Universidade do Porto, Porto (Portugal); Lienhart, H.; Durst, F. [Universitaet Erlangen-Nuernberg, LSTM, Erlangen, Lehrstuhl fuer Stroemungsmechanik, Erlangen (Germany)

    2007-08-15

    Pod drives are modern outboard ship propulsion systems with a motor encapsulated in a watertight pod, whose shaft is connected directly to one or two propellers. The whole unit hangs from the stern of the ship and rotates azimuthally, thus providing thrust and steering without the need of a rudder. Force/momentum and phase-resolved laser Doppler anemometry (LDA) measurements were performed for in line co-rotating and contra-rotating propellers pod drive models. The measurements permitted to characterize these ship propulsion systems in terms of their hydrodynamic characteristics. The torque delivered to the propellers and the thrust of the system were measured for different operation conditions of the propellers. These measurements lead to the hydrodynamic optimization of the ship propulsion system. The parameters under focus revealed the influence of distance between propeller planes, propeller frequency of rotation ratio and type of propellers (co- or contra-rotating) on the overall efficiency of the system. Two of the ship propulsion systems under consideration were chosen, based on their hydrodynamic characteristics, for a detailed study of the swirling wake flow by means of laser Doppler anemometry. A two-component laser Doppler system was employed for the velocity measurements. A light barrier mounted on the axle of the rear propeller motor supplied a TTL signal to mark the beginning of each period, thus providing angle information for the LDA measurements. Measurements were conducted for four axial positions in the slipstream of the pod drive models. The results show that the wake of contra-rotating propeller is more homogeneous than when they co-rotate. In agreement with the results of the force/momentum measurements and with hypotheses put forward in the literature (see e.g. Poehls in Entwurfsgrundlagen fuer Schraubenpropeller, 1984; Schneekluth in Hydromechanik zum Schiffsentwurf, 1988; Breslin and Andersen in Hydrodynamics of ship propellers, 1996

  8. Mean velocity and moments of turbulent velocity fluctuations in the wake of a model ship propulsor

    Science.gov (United States)

    Pêgo, J. P.; Lienhart, H.; Durst, F.

    2007-08-01

    Pod drives are modern outboard ship propulsion systems with a motor encapsulated in a watertight pod, whose shaft is connected directly to one or two propellers. The whole unit hangs from the stern of the ship and rotates azimuthally, thus providing thrust and steering without the need of a rudder. Force/momentum and phase-resolved laser Doppler anemometry (LDA) measurements were performed for in line co-rotating and contra-rotating propellers pod drive models. The measurements permitted to characterize these ship propulsion systems in terms of their hydrodynamic characteristics. The torque delivered to the propellers and the thrust of the system were measured for different operation conditions of the propellers. These measurements lead to the hydrodynamic optimization of the ship propulsion system. The parameters under focus revealed the influence of distance between propeller planes, propeller frequency of rotation ratio and type of propellers (co- or contra-rotating) on the overall efficiency of the system. Two of the ship propulsion systems under consideration were chosen, based on their hydrodynamic characteristics, for a detailed study of the swirling wake flow by means of laser Doppler anemometry. A two-component laser Doppler system was employed for the velocity measurements. A light barrier mounted on the axle of the rear propeller motor supplied a TTL signal to mark the beginning of each period, thus providing angle information for the LDA measurements. Measurements were conducted for four axial positions in the slipstream of the pod drive models. The results show that the wake of contra-rotating propeller is more homogeneous than when they co-rotate. In agreement with the results of the force/momentum measurements and with hypotheses put forward in the literature (see e.g. Poehls in Entwurfsgrundlagen für Schraubenpropeller, 1984; Schneekluth in Hydromechanik zum Schiffsentwurf, 1988; Breslin and Andersen in Hydrodynamics of ship propellers, 1996

  9. Ship heading and velocity analysis by wake detection in SAR images

    Science.gov (United States)

    Graziano, Maria Daniela; D'Errico, Marco; Rufino, Giancarlo

    2016-11-01

    With the aim of ship-route estimation, a wake detection method is developed and applied to COSMO/SkyMed and TerraSAR-X Stripmap SAR images over the Gulf of Naples, Italy. In order to mitigate the intrinsic limitations of the threshold logic, the algorithm identifies the wake features according to the hydrodynamic theory. A post-detection validation phase is performed to classify the features as real wake structures by means of merit indexes defined in the intensity domain. After wake reconstruction, ship heading is evaluated on the basis of turbulent wake direction and ship velocity is estimated by both techniques of azimuth shift and Kelvin pattern wavelength. The method is tested over 34 ship wakes identified by visual inspection in both HH and VV images at different incidence angles. For all wakes, no missed detections are reported and at least the turbulent and one narrow-V wakes are correctly identified, with ship heading successfully estimated. Also, the azimuth shift method is applied to estimate velocity for the 10 ships having route with sufficient angular separation from the satellite ground track. In one case ship velocity is successfully estimated with both methods, showing agreement within 14%.

  10. Spanwise loading distribution and wake velocity surveys of a semi-span wing

    Science.gov (United States)

    Felker, F. F., III; Piziali, R. A.; Gall, J. K.

    1982-01-01

    The spanwise distribution of bound circulation on a semi-span wing and the flow velocities in its wake were measured in a wind tunnel. Particular attention was given to documenting the flow velocities in and around the development tip vortex. A two-component laser velocimeter was used to make the velocity measurements. The spanwise distribution of bound circulation, three components of the time-averaged velocities throughout the near wake their standard deviations, and the integrated forces and moments on a metric tip as measured by an internal strain gage balance are presented without discussion.

  11. Wake Component Detection in X-Band SAR Images for Ship Heading and Velocity Estimation

    Directory of Open Access Journals (Sweden)

    Maria Daniela Graziano

    2016-06-01

    Full Text Available A new algorithm for ship wake detection is developed with the aim of ship heading and velocity estimation. It exploits the Radon transform and utilizes merit indexes in the intensity domain to validate the detected linear features as real components of the ship wake. Finally, ship velocity is estimated by state-of-the-art techniques of azimuth shift and Kelvin arm wavelength. The algorithm is applied to 13 X-band SAR images from the TerraSAR-X and COSMO/SkyMed missions with different polarization and incidence angles. Results show that the vast majority of wake features are correctly detected and validated also in critical situations, i.e., when multiple wake appearances or dark areas not related to wake features are imaged. The ship route estimations are validated with truth-at-sea in seven cases. Finally, it is also verified that the algorithm does not detect wakes in the surroundings of 10 ships without wake appearances.

  12. Characterization of wind velocities in the wake of a full scale wind turbine using three ground-based synchronized WindScanners

    Science.gov (United States)

    Yazicioglu, Hasan; Angelou, Nikolas; Mikkelsen, Torben; José Trujillo, Juan

    2016-09-01

    The wind energy community is in need of detailed full-field measurements in the wake of wind turbines. Here, three dimensional(3D) wind vector field measurements obtained in the near-wake region behind a full-scale test turbine are presented. Specifically, the wake of a NEG Nordtank turbine, installed at Risoe test field, has been measured from 0 to 2 diameters downstream. For this, three ground-based synchronised short-range WindScanners and a spinner lidar have been used. The 3D wind velocity field has been reconstructed in horizontal and vertical planes crossing the hub. The 10-min mean values of the three wind components reveal detailed information regarding the wake properties while propagating downwind over flat terrain. Furthermore, the wake centre is tracked from the measurements and its meander is investigated as function of yaw misalignment of the turbine. The centre-line wake deficit is calculated both in a Nacelle and Moving Frame of Reference. The results can be used in quantitative validation of numerical wake models.

  13. Characterization of wind velocities in the wake of a full scale wind turbine using three ground-based synchronized WindScanners

    DEFF Research Database (Denmark)

    Yazicioglu, Hasan; Angelou, Nikolas; Mikkelsen, Torben Krogh;

    2016-01-01

    , installed at Risoe test field, has been measured from 0 to 2 diameters downstream. For this, three ground-based synchronised short-range WindScanners and a spinner lidar have been used. The 3D wind velocity field has been reconstructed in horizontal and vertical planes crossing the hub. The 10-min mean...... deficit is calculated both in a Nacelle and Moving Frame of Reference. The results can be used in quantitative validation of numerical wake models....

  14. Coalescing Wind Turbine Wakes

    Science.gov (United States)

    Lee, S.; Churchfield, M.; Sirnivas, S.; Moriarty, P.; Nielsen, F. G.; Skaare, B.; Byklum, E.

    2015-06-01

    A team of researchers from the National Renewable Energy Laboratory and Statoil used large-eddy simulations to numerically investigate the merging wakes from upstream offshore wind turbines. Merging wakes are typical phenomena in wind farm flows in which neighboring turbine wakes consolidate to form complex flow patterns that are as yet not well understood. In the present study, three 6-MW turbines in a row were subjected to a neutrally stable atmospheric boundary layer flow. As a result, the wake from the farthest upstream turbine conjoined the downstream wake, which significantly altered the subsequent velocity deficit structures, turbulence intensity, and the global meandering behavior. The complexity increased even more when the combined wakes from the two upstream turbines mixed with the wake generated by the last turbine, thereby forming a “triplet” structure. Although the influence of the wake generated by the first turbine decayed with downstream distance, the mutated wakes from the second turbine continued to influence the downstream wake. Two mirror-image angles of wind directions that yielded partial wakes impinging on the downstream turbines yielded asymmetric wake profiles that could be attributed to the changing flow directions in the rotor plane induced by the Coriolis force. The turbine wakes persisted for extended distances in the present study, which is a result of low aerodynamic surface roughness typically found in offshore conditions.

  15. Wind farm array wake losses

    Energy Technology Data Exchange (ETDEWEB)

    Baker, R.W. [Impact Weather, Washougal, WA (United States); McCarthy, E.F. [Wind Economics & Technology, Inc., Martinez, CA (United States)

    1997-12-31

    A wind turbine wake study was conducted in the summer of 1987 at an Altamont Pass wind electric generating facility. The wind speed deficits, turbulence, and power deficits from an array consisting of several rows of wind turbines is discussed. A total of nine different test configurations were evaluated for a downwind spacing ranging from 7 rotor diameters (RD) to 34 RD and a cross wind spacing of 1.3 RD and 2.7 RD. Wake power deficits of 15% were measured at 16 RD and power losses of a few percent were even measurable at 27 RD for the closer cross wind spacing. For several rows of turbines separated by 7-9 RD the wake zones overlapped and formed compound wakes with higher velocity deficits. The wind speed and direction turbulence in the wake was much higher than the ambient turbulence. The results from this study are compared to the findings from other similar field measurements.

  16. Validation of the Eddy Viscosity and Lange Wake Models using Measured Wake Flow Characteristics Behind a Large Wind Turbine Rotor

    Energy Technology Data Exchange (ETDEWEB)

    Jeon, Sang Hyeon; Kim, Bum Suk; Huh, Jong Chul [Jeju National Univ., Jeju (Korea, Republic of); Go, Young Jun [Hanjin Ind, Co., Ltd., Yangsan (Korea, Republic of)

    2016-01-15

    The wake effects behind wind turbines were investigated by using data from a Met Mast tower and the SCADA (Supervisory Control and Data Acquisition) system for a wind turbine. The results of the wake investigations and predicted values for the velocity deficit based on the eddy viscosity model were compared with the turbulence intensity from the Lange model. As a result, the velocity deficit and turbulence intensity of the wake increased as the free stream wind speed decreased. In addition, the magnitude of the velocity deficit for the center of the wake using the eddy viscosity model was overestimated while the turbulence intensity from the Lange model showed similarities with measured values.

  17. Method of radial velocities for the estimation of aircraft wake vortex parameters from data measured by coherent Doppler lidar.

    Science.gov (United States)

    Smalikho, I N; Banakh, V A; Holzäpfel, F; Rahm, S

    2015-09-21

    The method of radial velocities (RV) is applied to estimate aircraft wake vortex parameters from measurements conducted with pulsed coherent Doppler lidar (PCDL). Operations of the Stream Line lidar and the 2-µm PCDL are simulated numerically to analyze the accuracy of the estimated wake vortex parameters with the RV method. The RV method is also used to estimate wake vortex trajectories and circulation from lidar measurements at Tomsk and Munich airports. The method of velocity envelopes and the RV method are compared employing data gathered with the 2-µm PCDL. The domain of applicability of the RV method is determined.

  18. Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations

    Science.gov (United States)

    Dhanya, M. B.; Bhardwaj, Anil; Futaana, Yoshifumi; Barabash, Stas; Alok, Abhinaw; Wieser, Martin; Holmström, Mats; Wurz, Peter

    2016-06-01

    Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The solar wind, either directly or after interaction with the lunar surface (including magnetic anomalies), is the source of these protons in the near-wake region. Using the entire data from the SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analyzed the velocity distribution of the protons observed in the near-lunar wake. The average velocity distribution functions, computed in the solar wind rest frame, were further separated based on the angle between the upstream solar wind velocity and the IMF. Although the protons enter the wake parallel as well as perpendicular to the IMF, the velocity distribution were not identical for the different IMF orientations, indicating the control of IMF in the proton entry processes. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar wind protons into the wake along IMF, (ii) the solar wind protons with finite gyro-radii that are aided by the wake boundary electric field, (iii) solar wind protons with gyro-radii larger than lunar radii from the tail of the solar wind velocity distribution, and (iv) scattering of solar wind protons from the dayside lunar surface or from magnetic anomalies. In order to gain more insight into the entry mechanisms associated with different populations, backtracing is carried out for each of these populations. For most of the populations, the source of the protons obtained from backtracing is found to be in agreement with that inferred from the velocity distribution. There are few populations that could not be explained by the known mechanisms

  19. Doppler lidar investigation of wind turbine wake characteristics and atmospheric turbulence under different surface roughness.

    Science.gov (United States)

    Zhai, Xiaochun; Wu, Songhua; Liu, Bingyi

    2017-06-12

    Four field experiments based on Pulsed Coherent Doppler Lidar with different surface roughness have been carried out in 2013-2015 to study the turbulent wind field in the vicinity of operating wind turbine in the onshore and offshore wind parks. The turbulence characteristics in ambient atmosphere and wake area was analyzed using transverse structure function based on Plane Position Indicator scanning mode. An automatic wake processing procedure was developed to determine the wake velocity deficit by considering the effect of ambient velocity disturbance and wake meandering with the mean wind direction. It is found that the turbine wake obviously enhances the atmospheric turbulence mixing, and the difference in the correlation of turbulence parameters under different surface roughness is significant. The dependence of wake parameters including the wake velocity deficit and wake length on wind velocity and turbulence intensity are analyzed and compared with other studies, which validates the empirical model and simulation of a turbine wake for various atmosphere conditions.

  20. Analysis of turbulent wake behind a wind turbine

    DEFF Research Database (Denmark)

    Kermani, Nasrin Arjomand; Andersen, Søren Juhl; Sørensen, Jens Nørkær;

    2013-01-01

    The aim of this study is to improve the classical analytical model for estimation of the rate of wake expansion and the decay of wake velocity deficit in the far wake region behind a wind turbine. The relations for a fully turbulent axisymmetric far wake were derived by applying the mass and mome......The aim of this study is to improve the classical analytical model for estimation of the rate of wake expansion and the decay of wake velocity deficit in the far wake region behind a wind turbine. The relations for a fully turbulent axisymmetric far wake were derived by applying the mass...... and momentum conservations, the selfsimilarity of mean velocity profile and the eddy viscosity closure. The theoretical approach is validated using the numerical results obtained from large eddy simulations with an actuator line technique at 0.1% and 3% ambient turbulence level and ambient wind velocity of 10....... Therefore the theoretical determination of the power law for the wake expansion and the decay of the wake velocity deficit may not be valid in the case of the wake generated behind a wind turbine with low ambient turbulence and high thrust coefficient. Although at higher ambient turbulence levels or lower...

  1. Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations

    CERN Document Server

    Dhanya, M B; Futaana, Yoshifumi; Barabash, Stas; Alok, Abhinaw; Wieser, Martin; Holmström, Mats; Wurz, Peter

    2016-01-01

    Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The solar wind, either directly or after interaction with the lunar surface (including magnetic anomalies), is the source of these protons in the near-wake region. Using the entire data from the SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed the velocity distribution of the protons observed in the near-lunar wake. The average velocity distribution functions, computed in the solar wind rest frame, were further separated based on the angle between the upstream solar wind velocity and the IMF. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar w...

  2. On the phase velocity of plasma waves in a self-modulated laser wake-field accelerator

    NARCIS (Netherlands)

    Andreev, N. E.; Kirsanov, V. I.; Sakharov, A. S.; van Amersfoort, P. W.; Goloviznin, V. V.

    1996-01-01

    The properties of the wake field excited by a flattop laser pulse with a sharp leading edge and a power below the critical one for relativistic self-focusing are studied analytically and numerically with emphasis on the phase velocity of the plasma wave. The paraxial model describing modulation of t

  3. On the phase velocity of plasma waves in a self-modulated laser wake-field accelerator

    NARCIS (Netherlands)

    Andreev, N. E.; Kirsanov, V. I.; Sakharov, A. S.; van Amersfoort, P. W.; Goloviznin, V. V.

    1996-01-01

    The properties of the wake field excited by a flattop laser pulse with a sharp leading edge and a power below the critical one for relativistic self-focusing are studied analytically and numerically with emphasis on the phase velocity of the plasma wave. The paraxial model describing modulation of t

  4. Efficient Turbulence Modeling for CFD Wake Simulations

    DEFF Research Database (Denmark)

    van der Laan, Paul

    , that can accurately and efficiently simulate wind turbine wakes. The linear k-ε eddy viscosity model (EVM) is a popular turbulence model in RANS; however, it underpredicts the velocity wake deficit and cannot predict the anisotropic Reynolds-stresses in the wake. In the current work, nonlinear eddy...... viscosity models (NLEVM) are applied to wind turbine wakes. NLEVMs can model anisotropic turbulence through a nonlinear stress-strain relation, and they can improve the velocity deficit by the use of a variable eddy viscosity coefficient, that delays the wake recovery. Unfortunately, all tested NLEVMs show...... numerically unstable behavior for fine grids, which inhibits a grid dependency study for numerical verification. Therefore, a simpler EVM is proposed, labeled as the k-ε - fp EVM, that has a linear stress-strain relation, but still has a variable eddy viscosity coefficient. The k-ε - fp EVM is numerically...

  5. Effects of Movement Velocity and Maximal Concentric and Eccentric Actions on the Bilateral Deficit

    Science.gov (United States)

    Dickin, D. Clark; Too, Danny

    2006-01-01

    This study was performed to examine the effects of movement velocity and maximal concentric and eccentric actions on the bilateral deficit. Eighteen female participants performed maximal unilateral and bilateral knee extensions concentrically and eccentrically across six movement velocities (30, 60, 90, 120, 150, and 180[degrees]/s). Repeated…

  6. From Research Assistant to Researcher: Being Wakeful in a Mentorship Journey About Methodology, Poverty, and Deficit Thinking

    Directory of Open Access Journals (Sweden)

    Heather Grenville

    2013-01-01

    Full Text Available This article explores how insights and new knowledge were incorporated about narrative inquiry methodology, poverty, and deficit ways of thinking through a journey of mentorship. The experiences of a graduate student, as she journeys through the roles of a research assistant and graduate researcher, all the while being part of a positive mentorship experience, are relayed. The article describes the journey of an evolving researcher who becomes wakeful through the narrative inquiry methodology while engaged as a research assistant as well as a graduate student alongside her supervisor.

  7. Proper orthogonal decomposition of velocity gradient fields in a simulated stratified turbulent wake: analysis of vorticity and internal waves

    Science.gov (United States)

    Gurka, R.; Diamessis, P.; Liberzon, A.

    2009-04-01

    The characterization of three-dimensional space and time-dependent coherent structures and internal waves in stratified environment is one of the most challenging tasks in geophysical fluid dynamics. Proper orthogonal decomposition (POD) is applied to 2-D slices of vorticity and horizontal divergence obtained from 3-D DNS of a stratified turbulent wake of a towed sphere at Re=5x103 and Fr=4. The numerical method employed solves the incompressible Navier-Stokes equations under the Boussinesq approximation. The temporal discretization consists of three fractional steps: an explicit advancement of the nonlinear terms, an implicit solution of the Poisson equation for the pseudo-pressure (which enforces incompressibility), and an implicit solution of the Helmholtz equation for the viscous terms (where boundary conditions are imposed). The computational domain is assumed to be periodic in the horizontal direction and non-periodic in the vertical direction. The 2-D slices are sampled along the stream-depth (Oxz), span-depth (Oyz) and stream-span planes (Oxy) for 231 times during the interval, Nt ∈ [12,35] (N is the stratification frequency). During this interval, internal wave radiation from the wake is most pronounced and the vorticity field in the wake undergoes distinct structural transitions. POD was chosen amongst the available statistical tools due to its advantage in characterization of simulated and experimentally measured velocity gradient fields. The computational procedure, applied to any random vector field, finds the most coherent feature from the given ensemble of field realizations. The decomposed empirical eigenfunctions could be referred to as "coherent structures", since they are highly correlated in an average sense with the flow field. In our analysis, we follow the computationally efficient method of 'snapshots' to find the POD eigenfunctions of the ensemble of vorticity field realizations. The results contains of the separate POD modes, along with

  8. The effects of the canopy created velocity inflection in the wake development

    Science.gov (United States)

    Agafonova, O.; Avramenko, A.; Chaudhari, A.; Hellsten, A.

    2016-06-01

    The aim of this paper is to study the effects of forest on the turbine wakes. Initially, the ACL (actuator line) model as well as a Canopy model are validated with the experiments separately. The models are further applied to simulate the flow over two wind turbines in a row located within the forest.

  9. A stochastic wind turbine wake model based on new metrics for wake characterization: A stochastic wind turbine wake model based on new metrics for wake characterization

    Energy Technology Data Exchange (ETDEWEB)

    Doubrawa, Paula [Sibley School of Mechanical and Aerospace Engineering, Cornell University, Upson Hall Ithaca 14850 New York USA; Barthelmie, Rebecca J. [Sibley School of Mechanical and Aerospace Engineering, Cornell University, Upson Hall Ithaca 14850 New York USA; Wang, Hui [Sibley School of Mechanical and Aerospace Engineering, Cornell University, Upson Hall Ithaca 14850 New York USA; Churchfield, Matthew J. [National Renewable Energy Laboratory, Golden 80401 Colorado USA

    2016-08-04

    Understanding the detailed dynamics of wind turbine wakes is critical to predicting the performance and maximizing the efficiency of wind farms. This knowledge requires atmospheric data at a high spatial and temporal resolution, which are not easily obtained from direct measurements. Therefore, research is often based on numerical models, which vary in fidelity and computational cost. The simplest models produce axisymmetric wakes and are only valid beyond the near wake. Higher-fidelity results can be obtained by solving the filtered Navier-Stokes equations at a resolution that is sufficient to resolve the relevant turbulence scales. This work addresses the gap between these two extremes by proposing a stochastic model that produces an unsteady asymmetric wake. The model is developed based on a large-eddy simulation (LES) of an offshore wind farm. Because there are several ways of characterizing wakes, the first part of this work explores different approaches to defining global wake characteristics. From these, a model is developed that captures essential features of a LES-generated wake at a small fraction of the cost. The synthetic wake successfully reproduces the mean characteristics of the original LES wake, including its area and stretching patterns, and statistics of the mean azimuthal radius. The mean and standard deviation of the wake width and height are also reproduced. This preliminary study focuses on reproducing the wake shape, while future work will incorporate velocity deficit and meandering, as well as different stability scenarios.

  10. Wall-wake velocity profile for compressible non-adiabatic flows

    Science.gov (United States)

    Sun, C. C.; Childs, M. E.

    1975-01-01

    A form of the wall-wake profile, which is applicable to flows with heat transfer, and for which a variation in y = O at y = delta, was suggested. The modified profile, which takes into account the effect of turbulent Prandtl number, was found to provide a good representation of experimental data for a wide range numbers and heat transfer. The Cf values which are determined by a least squares fit of the profile to the data agree well with values which were measured by the floating element technique. In addition, the values of delta determined by the fit correspond more closely to the outer edge of the viscous flow region than those obtained with earlier versions of the wall-wake profile.

  11. Flow visualization study of the MOD-2 wind turbine wake

    Energy Technology Data Exchange (ETDEWEB)

    Liu H.T.; Waite, J.W.; Hiester, T.R.; Tacheron, P.H.; Srnsky, R.A.

    1983-06-01

    The specific objectives of the study reported were: to determine the geometry of the MOD-2 wind turbine wake in terms of wake height and width as a function of downstream distance under two conditions of atmospheric stability; to estimate the mean velocity deficit at several downstream stations in the turbine wake; and to investigate the behavior of the rotor-generated vortices, particularly their configuration and persistence. The background of the wake problem is briefly examined, including a discussion of the critical issues that the flow visualization study addresses. Experimental techniques and data analysis methods are described in detail. (LEW)

  12. Shed Vortex Structure and Phase-Averaged Velocity Statistics in Symmetric/Asymmetric Turbulent Flat Plate Wakes

    Science.gov (United States)

    Rai, Man Mohan

    2017-01-01

    The near wake of a flat plate is investigated via direct numerical simulations (DNS). Many earlier experimental investigations have used thin plates with sharp trailing edges and turbulent boundary layers to create the wake. This results in large theta divided by D (sub TE) values (theta is the boundary layer momentum thickness towards the end of the plate and D (sub TE) is the trailing edge thickness). In the present study the emphasis is on relatively thick plates with circular trailing edges (CTE) resulting in theta divided by D values less than one (D is the plate thickness and the diameter of the CTE), and vigorous vortex shedding. The Reynolds numbers based on the plate length and D are 1.255 x 10 (sup 6) and 10,000, respectively. Two cases are computed; one with turbulent boundary layers on both the upper and lower surfaces of the plate (statistically the same, symmetric wake, Case TT) and, a second with turbulent and laminar boundary layers on the upper and lower surfaces, respectively (asymmetric case, Case TL). The data and understanding obtained is of considerable engineering interest, particularly in turbomachinery where the pressure side of an airfoil can remain laminar or transitional because of a favorable pressure gradient and the suction side is turbulent. Shed-vortex structure and phase-averaged velocity statistics obtained in the two cases are compared here. The upper negative shed vortices in Case TL (turbulent separating boundary layer) are weaker than the lower positive ones (laminar separating boundary layer) at inception (a factor 1.27 weaker in terms of peak phase-averaged spanwise vorticity at first appearance of a peak). The upper vortices weaken rapidly as they travel downstream. A second feature of interest in Case TL is a considerable increase in the peak phase-averaged, streamwise normal intensity (random component) with increasing streamwise distance (x divided by D) that occurs nears the positive vortex cores. This behavior is

  13. Correlations Between Supermassive Black Holes, Velocity Dispersions, and Mass Deficits in Elliptical Galaxies with Cores

    CERN Document Server

    Kormendy, John; 10.1088/0004-637X/691/2/L142

    2009-01-01

    High-dynamic-range surface photometry in a companion paper makes possible accurate measurement of the stellar light deficits L_def and mass deficits M_def associated with the cores of elliptical galaxies. We show that L_def correlates with the velocity dispersion sigma of the host galaxy bulge averaged outside the central region that may be affected by a supermassive black hole (BH). We confirm that L_def correlates with BH mass MBH. Also, the fractional light deficit L_def/L correlates with MBH/M, the ratio of BH mass to the galaxy stellar mass. All three correlations have scatter similar to or smaller than the scatter in the well known correlation between MBH and sigma. The new correlations are remarkable in view of the dichotomy between ellipticals with cores and those with central extra light. Core light deficit correlates closely with MBH and sigma, but extra light does not. This supports the suggestion that extra light Es are made in wet mergers with starbursts whereas core Es are made in dry mergers. A...

  14. Wake Behavior behind Turbine Cascades in Compressible Two-Dimensional Flows

    OpenAIRE

    2005-01-01

    The goal of the paper is to describe wake parameters of wakes from turbine cascades in compressible flows especially in planes where the leading edge of the following blade row would be located. Data from experiments with turbine cascades in compressible flow will be used to derive a theoretical approach which describes the wake growth and the recovery of the velocity deficit. The theory is based on similarity assumptions. The derived equations depend on simple and readily available parameter...

  15. Observations and Analysis of Turbulent Wake of Wind Turbine by Coherent Doppler Lidar

    Directory of Open Access Journals (Sweden)

    Wu Songhua

    2016-01-01

    Full Text Available Turbulent wake of wind turbine will reduce the power output of wind farm. The access to real turbulent wake of wind turbine blades with different spatial and temporal scales is provided by the pulsed Coherent Doppler Lidar (CDL which operates by transmitting a laser beam and detecting the radiation backscattered by atmospheric aerosol particles. In this paper, the authors discuss the possibility of using lidar measurements to characterize the complicated wind field, specifically wind velocity deficit by the turbine wake.

  16. Flowfield characterisation in the wake of a low-velocity heated sphere anemometer

    Energy Technology Data Exchange (ETDEWEB)

    Olim, A.M. [Associacao para o Desenvolvimento da Aerodinamica Industrial (ADAI), Coimbra (Portugal); Riethmuller, M.L. [Von Karman Institute for Fluid Dynamics (VKI), St. Genese (Belgium); Gameiro da Silva, M.C. [Departamento de Engenharia Mecanica, Faculdade de Ciencias e Tecnologia, Universidade de Coimbra Polo II, Coimbra (Portugal)

    2002-06-01

    Heated sphere anemometers (HSA) are the most widely used instruments for low-velocity measurements in the heating, ventilation and air-conditioning industry. Experiments were conducted to characterise the flowfield around the spherically shaped sensor and upper probe assembly of a HSA. Particle image velocimetry was the main quantitative experimental technique. Measurements of the flowfield around a HSA probe and a 2:1 scaled-up model were performed in a uniform isothermal axisymmetrical jet air flow at Re around 350, based on sensor diameter, for different pitch angle incident flows. Additionally, extensive flow visualisation studies around scaled-up models of the HSA probe were performed. (orig.)

  17. On the spread and decay of wind turbine wakes in ambient turbulence

    Science.gov (United States)

    Johnson, P. B.; Jonsson, C.; Achilleos, S.; Eames, I.

    2014-12-01

    The decay of the downstream wake of a wind turbine plays an important role in the performance of wind farms. The spread and decay of a wake depend both on wake meandering (advection of the wake as a whole) and wake diffusion (widening of the wake within its meandering frame of reference). Both of these effects depend strongly on the intensity of the ambient turbulence relative to the velocity deficit in the wake, and on the integral length scale of the turbulence relative to the wake width. Recent theory, which we review here, shows how intense large-scale turbulence can lead to a rapid x-2 decay in the time-averaged centreline velocity deficit, as compared to a x-1 decay for smaller scale turbulence, where x is distance downstream. We emphasise in this paper that common wind farm models do not predict this rapid decay. We present new experimental measurements of the velocity deficit downstream of a porous disc in relatively large-scale ambient turbulence which corroborate predictions of a x-2 decay, and we show theoretically that the commonly used k-epsilon model does not capture this effect. We further show that a commercial CFD package, configured to match our experiments and employing the k-epsilon model, fails to predict such rapid decay. We conclude that steady simulations of wind turbine wake dynamics are insufficient for informing wind farm layout optimisation.

  18. Characteristics of velocity distribution functions and entry mechanisms of protons in the near-lunar wake from SWIM/SARA on Chandrayaan-1

    Science.gov (United States)

    Dhanya, M. B.; Barabash, Stas; Wieser, Martin; Holmström, Mats; Bhardwaj, Anil; Wurz, Peter; Alok, Abhinaw; Futaana, Yoshifumi

    2016-07-01

    Moon is an airless body with no global magnetic field, although regions of crustal magnetic fields known as magnetic anomalies exist on Moon. Solar wind, the magnetized plasma flow from the Sun, continuously impinges on Moon. Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The source of these protons have been found to be the solar wind that enter the wake either directly or after interaction with the lunar surface or with the magnetic anomalies. Using the entire data from the SWIM sensor, which was an ion-mass analyzer, of the SARA experiment onboard Chandrayaan-1, the characteristics of velocity distribution of these protons were investigated to understand the entry mechanisms to near lunar wake. The velocity distribution functions were computed in the two dimensional velocity space, namely in the directions parallel and perpendicular to the IMF (v_allel and v_perp) in the solar wind rest frame. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar wind protons into the wake along IMF, (ii) the solar wind protons with finite gyro-radii that are aided by the wake boundary electric field, (iii) solar wind protons with gyro-radii larger than lunar radii from the tail of the solar wind velocity distribution, and (iv) scattering of solar wind protons from the dayside lunar surface or from magnetic anomalies. In order to gain more insight into the entry mechanisms associated with different populations, the trajectories of the protons were computed backward in time (backtracing) for each of these populations. For most of the populations, the source mechanism obtained from

  19. A CFD code comparison of wind turbine wakes

    DEFF Research Database (Denmark)

    Laan, van der, Paul Maarten; Storey, R. C.; Sørensen, Niels N.;

    2014-01-01

    A comparison is made between the EllipSys3D and SnS CFD codes. Both codes are used to perform Large-Eddy Simulations (LES) of single wind turbine wakes, using the actuator disk method. The comparison shows that both LES models predict similar velocity deficits and stream-wise Reynolds...

  20. Wake Identification Based Wake Impact Alleviation Control

    OpenAIRE

    Ehlers, Jana; Fischenberg, Dietrich; Niedermeier, Dominik

    2014-01-01

    During a wake vortex encounter an aircraft is exposed to strong unsteady disturbance velocities which can lead to significant changes in the aircraft attitude and flight path. This can represent a severe safety risk and can result in injuries to the passengers and crew as well as damages to the aircraft. The application of a wake impact alleviation control system can considerably decrease the aircraft’s response during the wake encounter, and hence diminish the pilot workload while reducing t...

  1. Effects of wind turbine wake on atmospheric sound propagation

    DEFF Research Database (Denmark)

    Barlas, Emre; Zhu, Wei Jun; Shen, Wen Zhong

    2017-01-01

    In this paper, we investigate the sound propagation from a wind turbine considering the effects of wake-induced velocity deficit and turbulence. In order to address this issue, an advanced approach was developed in which both scalar and vector parabolic equations in two dimensions are solved. Flow...... field input was obtained using the actuator line (AL) technique with Large Eddy Simulation (LES) to model the wind turbine and its wake and from an analytical wake model. The effect of incoming wind speed and atmospheric stability was investigated with the analytical wake input using a single point...... source. Unsteady acoustic simulations were carried out with the AL/LES input for three cases with different incoming turbulence intensity, and a moving source approach to mimic the rotating turbine blades. The results show a non-negligible effect of the wake on far-field noise prediction. Particularly...

  2. Three-Dimensional Structure of Wind Turbine Wakes as Measured by Scanning Lidar

    Energy Technology Data Exchange (ETDEWEB)

    Lundquist, Julie [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Bodini, Nicola [University of Colorado; University of Trento; Zardi, Dino [University of Trento

    2017-08-14

    The lower wind speeds and increased turbulence that are characteristic of turbine wakes have considerable consequences on large wind farms: turbines located downwind generate less power and experience increased turbulent loads. The structures of wakes and their downwind impacts are sensitive to wind speed and atmospheric variability. Wake characterization can provide important insights for turbine layout optimization in view of decreasing the cost of wind energy. The CWEX-13 field campaign, which took place between June and September 2013 in a wind farm in Iowa, was designed to explore the interaction of multiple wakes in a range of atmospheric stability conditions. Based on lidar wind measurements, we extend, present, and apply a quantitative algorithm to assess wake parameters such as the velocity deficits, the size of the wake boundaries, and the location of the wake centerlines. We focus on wakes from a row of four turbines at the leading edge of the wind farm to explore variations between wakes from the edge of the row (outer wakes) and those from turbines in the center of the row (inner wakes). Using multiple horizontal scans at different elevations, a three-dimensional structure of wakes from the row of turbines can be created. Wakes erode very quickly during unstable conditions and can in fact be detected primarily in stable conditions in the conditions measured here. During stable conditions, important differences emerge between the wakes of inner turbines and the wakes of outer turbines. Further, the strong wind veer associated with stable conditions results in a stretching of the wake structures, and this stretching manifests differently for inner and outer wakes. These insights can be incorporated into low-order wake models for wind farm layout optimization or for wind power forecasting.

  3. Dynamic wake meandering modeling

    DEFF Research Database (Denmark)

    Larsen, Gunner Chr.; Madsen Aagaard, Helge; Bingöl, Ferhat;

    , are an integrated part the model complex. For design applications, the computational efficiency of wake deficit prediction is a key issue. Two computationally low cost models are developed for this purpose. The character of the added wake turbulence, generated by the up-stream turbine in the form of shed......We present a consistent, physically based theory for the wake meandering phenomenon, which we consider of crucial importance for the overall description of wind turbine loadings in wind farms. In its present version the model is confined to single wake situations. The model philosophy does, however......, have the potential to include also mutual wake interaction phenomenons. The basic conjecture behind the dynamic wake meandering model is that wake transportation in the atmospheric boundary layer is driven by the large scale lateral- and vertical turbulence components. Based on this conjecture...

  4. Wake effect on a uniform flow behind wind-turbine model

    DEFF Research Database (Denmark)

    Okulov, Valery; Naumov, I. V.; Mikkelsen, Robert Flemming;

    2015-01-01

    speed ratios from 3 to 9, and at different cross-sections from 10 to 100 rotor radii downstream from the rotor. By using regression techniques to fit the velocity profiles it was possible to obtain accurate velocity deficits and estimate length scales of the wake attenuation. The data are compared...... the influence of external disturbances on the development of the inherent wake instability. The rotor is three-bladed and designed using Glauert’s optimum theory at a tip speed ratio λ =5 with a constant of the lift coefficient along the span, CL= 0.8. The wake development has been studied in the range of tip...

  5. Performance and wake conditions of a rotor located in the wake of an obstacle

    Science.gov (United States)

    Naumov, I. V.; Kabardin, I. K.; Mikkelsen, R. F.; Okulov, V. L.; Sørensen, J. N.

    2016-09-01

    Obstacles like forests, ridges and hills can strongly affect the velocity profile in front of a wind turbine rotor. The present work aims at quantifying the influence of nearby located obstacles on the performance and wake characteristics of a downstream located wind turbine. Here the influence of an obstacle in the form of a cylindrical disk was investigated experimentally in a water flume. A model of a three-bladed rotor, designed using Glauert's optimum theory at a tip speed ratio λ = 5, was placed in the wake of a disk with a diameter close to the one of the rotor. The distance from the disk to the rotor was changed from 4 to 8 rotor diameters, with the vertical distance from the rotor axis varied 0.5 and 1 rotor diameters. The associated turbulent intensity of the incoming flow to the rotor changed 3 to '6% due to the influence of the disk wake. In the experiment, thrust characteristics and associated pulsations as a function of the incoming flow structures were measured by strain gauges. The flow condition in front of the rotor was measured with high temporal accuracy using LDA and power coefficients were determine as function of tip speed ratio for different obstacle positions. Furthermore, PIV measurements were carried out to study the development of the mean velocity deficit profiles of the wake behind the wind turbine model under the influence of the wake generated by the obstacle. By use of regression techniques to fit the velocity profiles it was possible to determine velocity deficits and estimate length scales of the wake attenuation.

  6. Influence of Coriolis forces on the structure and evolution of wind-turbine wakes

    Science.gov (United States)

    Abkar, Mahdi; Porté-Agel, Fernando

    2015-11-01

    In this study, large-eddy simulation (LES) is combined with a turbine model to investigate the effect of Coriolis forces on the structure and evolution of wind-turbine wakes. In order to isolate the Coriolis effect on the turbulent wake flow, two set of simulations are performed. In the first set of simulations, atmospheric boundary layer (ABL) flow is driven by the geostrophic forces including the effect of Earth's rotation, while in the second case, the ABL flow is driven by a unidirectional pressure gradient forcing. Both cases have the same mean horizontal velocity and turbulence intensity at the hub height. The simulation results show that the Coriolis forces significantly affect the spatial distribution of the mean velocity deficit and turbulence statistics in the wake region. In particular, it is found that the Coriolis effect, responsible for vertical wind veer, has important lateral wake stretching effects, which in turn significantly impacts the wake recovery and wake meandering characteristics downwind of the turbines. We also apply the proper orthogonal decomposition (POD) to LES data of the wake. The results indicate a very high correlation between the most energetic modes and both maximum velocity deficit and wake meandering characteristics.

  7. A consistent turbulence formulation for the dynamic wake meandering model in the atmospheric boundary layer

    Energy Technology Data Exchange (ETDEWEB)

    Keck, R.-E.

    2013-07-15

    This thesis describes the further development and validation of the dynamic meandering wake model for simulating the flow field and power production of wind farms operating in the atmospheric boundary layer (ABL). The overall objective of the conducted research is to improve the modelling capability of the dynamics wake meandering model to a level where it is sufficiently mature to be applied in industrial applications and for an augmentation of the IEC-standard for wind turbine wake modelling. Based on a comparison of capabilities of the dynamic wake meandering model to the requirement of the wind industry, four areas were identified as high prioritizations for further research: 1. the turbulence distribution in a single wake. 2. multiple wake deficits and build-up of turbulence over a row of turbines. 3. the effect of the atmospheric boundary layer on wake turbulence and wake deficit evolution. 4. atmospheric stability effects on wake deficit evolution and meandering. The conducted research is to a large extent based on detailed wake investigations and reference data generated through computational fluid dynamics simulations, where the wind turbine rotor has been represented by an actuator line model. As a consequence, part of the research also targets the performance of the actuator line model when generating wind turbine wakes in the atmospheric boundary layer. Highlights of the conducted research: 1. A description is given for using the dynamic wake meandering model as a standalone flow-solver for the velocity and turbulence distribution, and power production in a wind farm. The performance of the standalone implementation is validated against field data, higher-order computational fluid dynamics models, as well as the most common engineering wake models in the wind industry. 2. The EllipSys3D actuator line model, including the synthetic methods used to model atmospheric boundary layer shear and turbulence, is verified for modelling the evolution of wind

  8. Modelling wind turbine wakes using the turbulent entrainment hypothesis

    Science.gov (United States)

    Luzzatto-Fegiz, Paolo

    2015-11-01

    Simple models for turbine wakes have been used extensively in the wind energy community, both as independent tools, as well as to complement more refined and computationally-intensive techniques. Jensen (1983; see also Katić et al. 1986) developed a model assuming that the wake radius grows linearly with distance x, approximating the velocity deficit with a top-hat profile. While this model has been widely implemented in the wind energy community, recently Bastankhah & Porté-Agel (2014) showed that it does not conserve momentum. They proposed a momentum-conserving theory, which assumed a Gaussian velocity deficit and retained the linear-spreading assumption, significantly improving agreement with experiments and LES. While the linear spreading assumption facilitates conceptual modeling, it requires empirical estimates of the spreading rate, and does not readily enable generalizations to other turbine designs. Furthermore, field measurements show sub-linear wake growth with x in the far-wake, consistently with results from fundamental turbulence studies. We develop a model by relying on a simple and general turbulence parameterization, namely the entrainment hypothesis, which has been used extensively in other areas of geophysical fluid dynamics. Without assuming similarity, we derive an analytical solution for a circular turbine wake, which predicts a far-wake radius increasing with x 1 / 3, and is consistent with field measurements and fundamental turbulence studies. Finally, we discuss developments accounting for effects of stratification, as well as generalizations to other turbine designs.

  9. Drag Prediction in the Near Wake of a Circular Cylinder based on DPIV Data

    Directory of Open Access Journals (Sweden)

    Onur Son

    2016-01-01

    Full Text Available This study focuses on drag prediction in the near-wake of a circular cylinder by use of mean velocity profiles and discusses the closest location where a wake survey would yield an accurate result. Although the investigation considers both the mean and fluctuating velocities, the main focus is on the mean momentum deficit which should be handled properly beyond a critical distance. Digital Particle Image Velocimetry (DPIV experiments are performed in a Reynolds number range of 100 to 1250. Wake characteristics such as vortex formation length (L and wake width (t are determined and their relations to drag prediction are presented. Drag coefficients determined by momentum deficit formula are found to be in good agreement with experimental and numerical literature data in present Reynolds number regime.

  10. Verification and validation studies of the time-averaged velocity field in the very near-wake of a finite elliptical cylinder

    Science.gov (United States)

    Flynn, Michael R.; Eisner, Alfred D.

    2004-04-01

    This paper presents verification and validation results for the time-averaged, three-dimensional velocity field immediately downstream of a finite elliptic cylinder at a Reynolds number of 1.35 × 10 4. Numerical simulations were performed with the finite element package, Fidap, using the steady state, standard k-epsilon model. The ratio of the cylinder height to the major axis of the elliptical cross section is 5.0; the aspect ratio of the cross section is 0.5625. This particular geometry is selected as a crude surrogate for the human form in consideration of further applied occupational and environmental health studies. Predictions of the velocity and turbulence kinetic energy fields in the very near-wake are compared to measurements taken in a wind tunnel using laser Doppler anemometry. Results show that at all locations where a reliable grid convergence index can be calculated there is not a demonstrable difference between simulated and measured values. The overall topology of the time-averaged flow field is reasonably well predicted, although the simulated near-wake is narrower than the measured one.

  11. Deficit

    CERN Multimedia

    2002-01-01

    UCL's former provost, Sir Derek Roberts, has been drafted in for a year to run the college. UCL is expected to have a 6 million pounds deficit this year and up to a 10 million pounds deficit next year. Sir Christopher Llewellyn-Smith took over at UCL nearly 4 years ago and decided then that the finanical situation was serious enough to warrant a reduction in the vast expansion policy undertaken by his predecessor (1 page).

  12. Turbulent axisymmetric swirling wake: equilibrium similarity solution and experiments with a wind turbine as wake generator

    Science.gov (United States)

    Wosnik, Martin; Dufresne, Nathaniel

    2013-11-01

    An analytical and experimental investigation of the turbulent axisymmetric swirling wake was carried out. An equilibrium similarity theory was derived that obtained scaling functions from conditions for similarity from the equations of motion, leading to a new scaling function for the decay of the swirling velocity component. Axial and azimuthal (swirl) velocity fields were measured in the wake of a single 3-bladed model wind turbine with rotor diameter of 0.91 m, up to 20 diameters downstream, using X-wire constant temperature hot-wire anemometry. The turbine was positioned in the free stream, near the entrance of the UNH Flow Physics Facility, which has a test section of 6m × 2.7m cross section and 72m length. Measurements were conducted at different rotor loading conditions with blade tip-speed ratios up to 2.8. At U∞ = 7 m/s, the Reynolds number based on turbine diameter was approximately 5 ×105 . Both mean velocity deficit and mean swirl were found to persist beyond 20 diameters downstream. First evidence for a new scaling function for the mean swirl, Wmax ~Uo3 / 2 ~x-1 was found. The similarity solution thus predicts that in the axisymmetric swirling wake mean swirl decays faster with x-1 than mean velocity deficit with x - 2 / 3.

  13. Wake effects of large offshore wind farms on the mesoscale atmosphere

    DEFF Research Database (Denmark)

    Volker, Patrick; Badger, Jake; Hahmann, Andrea N.

    to the fact that its typical horizontal grid spacing is on the order of 2km, the energy extracted by the turbine, as well as the wake development inside the turbine-containing grid-cells, are not described explicitly, but are parametrized as another sub-grid scale process. In order to appropriately capture...... the wind farm wake recovery and its direction, two properties are important, the total energy extracted by the wind farm and its velocity deficit distribution. In the considered parametrization the individual turbines apply a thrust dependent on a local sub grid scale velocity, which is influenced...... by the up-stream turbines. For the sub-grid scale velocity deficit, the entrainment from the free atmospheric flow into the wake region, is taken into account. Furthermore, since the model horizontal distance is several times larger then the turbine diameter, it has been assumed that the generated...

  14. The Explicit Wake Parametrisation V1.0: a wind farm parametrisation in the mesoscale model WRF

    DEFF Research Database (Denmark)

    Volker, P. H. J.; Badger, Jake; Hahmann, Andrea N.;

    2015-01-01

    Parametrisation (EWP), uses classical wake theory to describe the unresolved wake expansion. The EWP scheme is validated against filtered in situ measurements from two meteorological masts situated a few kilometres away from the Danish oshore wind farm Horns Rev I. The simulated velocity deficit in the wake...... of the wind farm compares well to that observed in the measurements and the velocity profile is qualitatively similar to that simulated with large eddy simulation models and from wind tunnel studies. At the same time, the validation process highlights the challenges in verifying such models with real...... observations....

  15. A new approach for evaluating measured wake data

    Energy Technology Data Exchange (ETDEWEB)

    Magnusson, Mikael [Uppsala Univ. (Sweden). Dept. of Meteorology

    1996-12-01

    Wind turbine wakes have been studied by analysing a large set of atmospheric data, from a wind farm with four turbines sited on a flat coastal area. The results obtained have ben generalized to allow tests against data from other full scale wind turbines as well as wind tunnel simulations. These comparisons are found to give very satisfactory results. The thrust coefficient is found to be a better parameter for description than wind speed, of wake characteristics because it implicitly includes the effect of regulation. It is also found that down-wind travel time is more convenient to use than down-wind distance in this context. The travel time to the end of the near wake region, i.e. to the point where a single velocity deficit peak first appears, is found to be inversely proportional to the rotational frequency of the turbine and to the turbulence intensity of the ambient air flow and proportional to the ratio of the wake radius and the hub height. For larger travel times, i.e. for the far wake region, it is found that the centre line relative velocity deficit decreases with the logarithm of the time traveled and is parametrically dependent on the time constant and the thrust coefficient. 3 refs, 5 figs

  16. Wake effects in Alsvik wind park: Comparison between measurements and predictions

    Energy Technology Data Exchange (ETDEWEB)

    Magnusson, Mikael [Uppsala Univ. (Sweden). Dept. of Meteorology; Rados, K.G.; Pothou, K.P. [National Technical Univ., Athen (Greece). Fluid section

    1996-12-01

    In this paper characteristic parameters in a wind turbine wake are studied. The data used are full-scale measurements from a wind farm in Sweden, Alsvik, and results from a numerical model calculated for the same site. The results are valid for neutral stratification. The model employs a particle-vortex approach at the rotor plane, a Navier-Stokes solver in the near wake and applies self preservation in the far wake. The parameters investigated are the relative velocity deficit at centre line and hub height, and the radial distribution of the turbulent kinetic energy. 6 refs, 9 figs

  17. Wind turbine wake characterization using long-range Doppler lidar

    Science.gov (United States)

    Aitken, M.; Lundquist, J. K.; Hestmark, K.; Banta, R. M.; Pichugina, Y.; Brewer, A.

    2012-12-01

    Wind turbines extract energy from the freestream flow, resulting in a waked region behind the rotor which is characterized by reduced wind speed and increased turbulence. The velocity deficit in the wake diminishes with distance, as faster-moving air outside is gradually entrained. In a concentrated group of turbines, then, downwind machines experience very different inflow conditions compared to those in the front row. As utility-scale turbines rarely exist in isolation, detailed knowledge of the mean flow and turbulence structure inside wakes is needed to correctly model both power production and turbine loading at modern wind farms. To this end, the Turbine Wake and Inflow Characterization Study (TWICS) was conducted in the spring of 2011 to determine the reduction in wind speeds downstream from a multi-MW turbine located at the National Renewable Energy Laboratory's National Wind Technology Center (NWTC) near Boulder, Colorado. Full-scale measurements of wake dynamics are hardly practical or even possible with conventional sensors, such as cup anemometers mounted on meteorological (met) masts. Accordingly, the High Resolution Doppler Lidar (HRDL) developed by the National Oceanic and Atmospheric Administration's Earth System Research Laboratory was employed to investigate the formation and propagation of wakes under varying levels of ambient wind speed, shear, atmospheric stability, and turbulence. HRDL remotely senses line-of-sight wind velocities and has been used in several previous studies of boundary layer aerodynamics. With a fully steerable beam and a maximum range up to about 5 km, depending on atmospheric conditions, HRDL performed a comprehensive survey of the wind flow in front of and behind the turbine to study the shape, meandering, and attenuation of wakes. Due in large part to limited experimental data availability, wind farm wake modeling is still subject to an unacceptable amount of uncertainty, particularly in complex terrain. Here, analytical

  18. A Scanning laser-velocimeter technique for measuring two-dimensional wake-vortex velocity distributions. [Langley Vortex Research Facility

    Science.gov (United States)

    Gartrell, L. R.; Rhodes, D. B.

    1980-01-01

    A rapid scanning two dimensional laser velocimeter (LV) has been used to measure simultaneously the vortex vertical and axial velocity distributions in the Langley Vortex Research Facility. This system utilized a two dimensional Bragg cell for removing flow direction ambiguity by translating the optical frequency for each velocity component, which was separated by band-pass filters. A rotational scan mechanism provided an incremental rapid scan to compensate for the large displacement of the vortex with time. The data were processed with a digital counter and an on-line minicomputer. Vaporized kerosene (0.5 micron to 5 micron particle sizes) was used for flow visualization and LV scattering centers. The overall measured mean-velocity uncertainity is less than 2 percent. These measurements were obtained from ensemble averaging of individual realizations.

  19. Dynamic wake meandering modeling

    Energy Technology Data Exchange (ETDEWEB)

    Larsen, Gunner C.; Aagaard Madsen, H.; Bingoel, F. (and others)

    2007-06-15

    We present a consistent, physically based theory for the wake meandering phenomenon, which we consider of crucial importance for the overall description of wind turbine loadings in wind farms. In its present version the model is confined to single wake situations. The model philosophy does, however, have the potential to include also mutual wake interaction phenomenons. The basic conjecture behind the dynamic wake meandering model is that wake transportation in the atmospheric boundary layer is driven by the large scale lateral- and vertical turbulence components. Based on this conjecture a stochastic model of the downstream wake meandering is formulated. In addition to the kinematic formulation of the dynamics of the 'meandering frame of reference', models characterizing the mean wake deficit as well as the added wake turbulence, described in the meandering frame of reference, are an integrated part the model complex. For design applications, the computational efficiency of wake deficit prediction is a key issue. Two computationally low cost models are developed for this purpose. The character of the added wake turbulence, generated by the up-stream turbine in the form of shed and trailed vorticity, has been approached by analytical as well as by numerical studies. The dynamic wake meandering philosophy has been verified by comparing model predictions with extensive full-scale measurements. These comparisons have demonstrated good agreement, both qualitatively and quantitatively, concerning both flow characteristics and turbine load characteristics. Contrary to previous attempts to model wake loading, the dynamic wake meandering approach opens for a unifying description in the sense that turbine power and load aspects can be treated simultaneously. This capability is a direct and attractive consequence of the model being based on the underlying physical process, and it potentially opens for optimization of wind farm topology, of wind farm operation as

  20. An LES study of vertical-axis wind turbine wakes aerodynamics

    Science.gov (United States)

    Abkar, Mahdi; Dabiri, John O.

    2016-11-01

    In this study, large-eddy simulation (LES) combined with a turbine model is used to investigate the structure of the wake behind a vertical-axis wind turbine (VAWT). In the simulations, a recently developed minimum dissipation model is used to parameterize the subgrid-scale stress tensor, while the turbine-induced forces are modeled with an actuator-line technique. The LES framework is first tested in the simulation of the wake behind a model straight-bladed VAWT placed in the water channel, and then used to study the wake structure downwind of a full-scale VAWT sited in the atmospheric boundary layer. In particular, the self-similarity of the wake is examined, and it is found that the wake velocity deficit is well characterized by a two-dimensional elliptical Gaussian distribution. By assuming a self-similar Gaussian distribution of the velocity deficit, and applying mass and momentum conservation, an analytical model is developed and tested to predict the maximum velocity deficit downwind of the turbine.

  1. A comparison between 2-and 3-bladed wind turbine rotors with focus on wake characteristics

    Science.gov (United States)

    Mühle, Franz; Adaramola, Muyiwa Samuel; Sætran, Lars

    2016-11-01

    Due to cost benefit and weight reduction, 2-bladed wind turbines have the potential to become more important for offshore wind applications. In order to optimize the arrangement of wind turbines in wind farms and for accurate forecasts of the power production, a detailed knowledge of the wake flow is needed. In the presented study, three different rotors with varying number of blades and similar performance behavior have been designed and manufactured using the 3-dimensional (3D) printing technology. The performance characteristics of these rotors as well as their wake features are measured experimentally in wind tunnel tests and compared. The velocity deficit is seen to vary only insignificantly for the wakes in distances of 3D (where D is the rotor diameter), 5D and 7D behind the turbine. However, higher turbulence intensity levels are recorded in the wake of the 2-bladed rotors. This could have potential for a faster wake recovery and thus a narrower turbine spacing.

  2. HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud

    CERN Document Server

    Matthews, L D; Gerard, E; Bertre, T Le; Johnson, M C; Dame, T M

    2010-01-01

    We report HI 21-cm line observations of the AGB star X Her obtained with the Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope of the star. The HI distribution exhibits a head-tail morphology, similar to those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24 pc) in the plane of the sky, along the direction of the star's space motion. We also detect a velocity gradient of ~6.5 km/s across the envelope, consistent with the HI tracing a turbulent wake that arises from the motion of a mass-losing star through the ISM. GBT mapping of a 2x2deg region around X Her reveals that the star lies (in projection) near the periphery of a much larger HI cloud that also exhibits signatures of ISM interaction. The properties of the cloud are consistent with those of compact high-velocity clouds. Using CO observations, we have placed an upper limit on its molecular gas content of N_H22.4~M_sun) an...

  3. Comparison of far wakes behind a solid disk and a three-blade rotor

    DEFF Research Database (Denmark)

    Litvinov, I. V.; Naumov, I.V.; Okulov, Valery;

    2015-01-01

    A comparison of the wakes developed behind an immobile bluff body (solid disk) and a three-blade rotor at different rotational speeds is studied to find a correlation between them. LDA and PIV were applied to study the wakes behind both models in a water flume (Re ≈ 2.3·105). Everywhere in both...... deficit and rms of the velocity in both far wakes for the disk or the rotor shows a rational decay with the same power –2/3. It has good agreement with the analytical formula for the decay of the velocity deficit behind bluff bodies. A limit for using this model restricted by the turbulence level...

  4. Volumetric LiDAR scanning of a wind turbine wake and comparison with a 3D analytical wake model

    Science.gov (United States)

    Carbajo Fuertes, Fernando; Porté-Agel, Fernando

    2016-04-01

    A correct estimation of the future power production is of capital importance whenever the feasibility of a future wind farm is being studied. This power estimation relies mostly on three aspects: (1) a reliable measurement of the wind resource in the area, (2) a well-established power curve of the future wind turbines and, (3) an accurate characterization of the wake effects; the latter being arguably the most challenging one due to the complexity of the phenomenon and the lack of extensive full-scale data sets that could be used to validate analytical or numerical models. The current project addresses the problem of obtaining a volumetric description of a full-scale wake of a 2MW wind turbine in terms of velocity deficit and turbulence intensity using three scanning wind LiDARs and two sonic anemometers. The characterization of the upstream flow conditions is done by one scanning LiDAR and two sonic anemometers, which have been used to calculate incoming vertical profiles of horizontal wind speed, wind direction and an approximation to turbulence intensity, as well as the thermal stability of the atmospheric boundary layer. The characterization of the wake is done by two scanning LiDARs working simultaneously and pointing downstream from the base of the wind turbine. The direct LiDAR measurements in terms of radial wind speed can be corrected using the upstream conditions in order to provide good estimations of the horizontal wind speed at any point downstream of the wind turbine. All this data combined allow for the volumetric reconstruction of the wake in terms of velocity deficit as well as turbulence intensity. Finally, the predictions of a 3D analytical model [1] are compared to the 3D LiDAR measurements of the wind turbine. The model is derived by applying the laws of conservation of mass and momentum and assuming a Gaussian distribution for the velocity deficit in the wake. This model has already been validated using high resolution wind-tunnel measurements

  5. Wind-Turbine Wakes in a Convective Boundary Layer: A Wind-Tunnel Study

    Science.gov (United States)

    Zhang, Wei; Markfort, Corey D.; Porté-Agel, Fernando

    2013-02-01

    Thermal stability changes the properties of the turbulent atmospheric boundary layer, and in turn affects the behaviour of wind-turbine wakes. To better understand the effects of thermal stability on the wind-turbine wake structure, wind-tunnel experiments were carried out with a simulated convective boundary layer (CBL) and a neutral boundary layer. The CBL was generated by cooling the airflow to 12-15 °C and heating up the test section floor to 73-75 °C. The freestream wind speed was set at about 2.5 m s-1, resulting in a bulk Richardson number of -0.13. The wake of a horizontal-axis 3-blade wind-turbine model, whose height was within the lowest one third of the boundary layer, was studied using stereoscopic particle image velocimetry (S-PIV) and triple-wire (x-wire/cold-wire) anemometry. Data acquired with the S-PIV were analyzed to characterize the highly three-dimensional turbulent flow in the near wake (0.2-3.2 rotor diameters) as well as to visualize the shedding of tip vortices. Profiles of the mean flow, turbulence intensity, and turbulent momentum and heat fluxes were measured with the triple-wire anemometer at downwind locations from 2-20 rotor diameters in the centre plane of the wake. In comparison with the wake of the same wind turbine in a neutral boundary layer, a smaller velocity deficit (about 15 % at the wake centre) is observed in the CBL, where an enhanced radial momentum transport leads to a more rapid momentum recovery, particularly in the lower part of the wake. The velocity deficit at the wake centre decays following a power law regardless of the thermal stability. While the peak turbulence intensity (and the maximum added turbulence) occurs at the top-tip height at a downwind distance of about three rotor diameters in both cases, the magnitude is about 20 % higher in the CBL than in the neutral boundary layer. Correspondingly, the turbulent heat flux is also enhanced by approximately 25 % in the lower part of the wake, compared to that

  6. Spectral coherence in windturbine wakes

    Energy Technology Data Exchange (ETDEWEB)

    Hojstrup, J. [Riso National Lab., Roskilde (Denmark)

    1996-12-31

    This paper describes an experiment at a Danish wind farm to investigate the lateral and vertical coherences in the nonequilibrium turbulence of a wind turbine wake. Two meteorological masts were instrumented for measuring profiles of mean speed, turbulence, and temperature. Results are provided graphically for turbulence intensities, velocity spectra, lateral coherence, and vertical coherence. The turbulence was somewhat influenced by the wake, or possibly from aggregated wakes further upstream, even at 14.5 diameters. Lateral coherence (separation 5m) seemed to be unaffected by the wake at 7.5 diameters, but the flow was less coherent in the near wake. The wake appeared to have little influence on vertical coherence (separation 13m). Simple, conventional models for coherence appeared to be adequate descriptions for wake turbulence except for the near wake situation. 3 refs., 7 figs., 1 tab.

  7. Comparison of the far wake behind dual rotor and dual disk configurations

    Science.gov (United States)

    Okulov, V. L.; Mikkelsen, R. F.; Naumov, I. V.; Litvinov, I. V.; Gesheva, E.; Sørensen, J. N.

    2016-09-01

    There is an increasing interest in studying the development of far wakes behind two or more interacting wind turbines in order to determine the influence of wake interaction in relation to the design of wind farms. The focus of this experimental study is to understand and describe the resulting wake features for two rotors subjected to different operating and spatial conditions. As a part of this, a comparison with the wake development behind two disks replacing the rotor models was performed to determine the difference between the two wake systems. LDA and Stereo PIV experiments were carried out to study the development of far wakes behind configurations of dual HAWT wind turbine rotors and dual circular disks. The setups were placed in the middle of a water flume. The initial flow in the flume is subjected to a very low turbulence level, limiting the influence of all external disturbances in order to focus the study to the inherent wake instability. As a result of the investigation, we obtained decays of profiles for the velocity deficit and turbulent pulsations in the far wakes behind both dual rotor and dual disk configurations. By using regression techniques to fit the obtained velocity profiles the experimental data were approximated by identical analytical models and compared to each other. An identical rational dependence with the same powers, but with different coefficients, was found for the two configurations.

  8. Spanwise lift distributions and wake velocity surveys of a semi-span wing with a discontinuous twist

    Science.gov (United States)

    Kumagai, Hiroyuki

    1989-01-01

    A wind tunnel test was conducted in the NASA-Ames 7 x 10 ft wind tunnel to investigate the lift distribution on a semispan wing with a discontinuous change in spanwise twist. The semispan wing had a tip with an adjustable pitch angle independent on the inboard section pitch angle simulating the free tip rotor blade when its free tip is at a deflected position. The spanwise lift distribution over the wing and the tip were measured and three component velocity surveys behind the wing were obtained with a 3-D laser Doppler velocimeter (LV) with the wing at one angle of attack and the tip deflected at different pitch angles. A six-component internal strain gage balance was also used to measure total forces and moments on the tip. The 3-D lift was computed from the 2-D lift distributions obtained from the LV and from the strain gage balance. The results from both experimental methods are shown to be in agreement with predictions made by a steady, 3-D panel code, VSAERO.

  9. Comparison of two LES codes for wind turbine wake studies

    DEFF Research Database (Denmark)

    Chivaee, Hamid Sarlak; Pierella, F.; Mikkelsen, Robert Flemming

    2014-01-01

    are compared in terms of velocity deficit, turbulence kinetic energy and eddy viscosity. It is seen that both codes predict similar near-wake flow structures with the exception of OpenFoam's simulations without the subgrid-scale model. The differences begin to increase with increasing the distance from......For the third time a blind test comparison in Norway 2013, was conducted comparing numerical simulations for the rotor Cp and Ct and wake profiles with the experimental results. As the only large eddy simulation study among participants, results of the Technical University of Denmark (DTU) using...... their in-house CFD solver, EllipSys3D, proved to be more reliable among the other models for capturing the wake profiles and the turbulence intensities downstream the turbine. It was therefore remarked in the workshop to investigate other LES codes to compare their performance with EllipSys3D. The aim...

  10. Influence of the Coriolis force on the structure and evolution of wind turbine wakes

    Science.gov (United States)

    Abkar, Mahdi; Porté-Agel, Fernando

    2016-10-01

    In this study, large-eddy simulation combined with a turbine model is used to investigate the effect of vertical wind veer associated with the Coriolis force on the structure and evolution of wind turbine wakes. In order to isolate the Coriolis effect on the wake, two cases are considered. In the first case, atmospheric boundary-layer flow is driven by a geostrophic wind, including the effect of Earth's rotation and the Coriolis force. In the second case, the boundary-layer flow is unidirectional and is forced by an imposed pressure gradient. Both cases have the same mean horizontal velocity and turbulence intensity at the hub height. The simulation results show that the Coriolis force significantly affects the aerodynamics of the wake including the mean velocity deficit, turbulence statistics, and wake-meandering characteristics downwind of the turbine. In particular, when the flow is forced by a geostrophic wind, vertical wind veer causes a skewed spatial structure in the wake. Moreover, the presence of lateral wind shear, in addition to the vertical one, enhances the shear production of turbulent kinetic energy and the turbulent momentum flux. This leads to a larger flow entrainment and, thus, a faster wake recovery compared to the case forced by unidirectional pressure gradient. Consistent with this result, wake meandering is also stronger in both lateral and vertical directions in the case of geostrophic forcing compared to the case with pressure-gradient forcing.

  11. The effect of the number of blades on wind turbine wake - a comparison between 2-and 3-bladed rotors

    Science.gov (United States)

    Mühle, Franz; Adaramola, Muyiwa S.; Sretran, Lars

    2016-09-01

    Due to cost benefit and weight reduction, 2-bladed wind turbines have the potential to become more important for offshore wind applications. In order to optimize the arrangement of wind turbines in wind farms and for accurate forecasts of the power production, a detailed knowledge of the wake flow is needed. In this study, three different rotors with varying number of blades and similar performance behaviour have been designed and manufactured using the 3dimensional (3D) printing technology. The performance characteristics of these rotors as well as their wake features are measured experimentally in wind tunnel tests and compared. The velocity deficit is seen to vary only insignificantly for the wakes in distances of 3D (where D is the rotor diameter), 5D and 7D behind the turbine. However, higher turbulence intensity levels are recorded in the wake of the 2-bladed rotors. This could have potential for a faster wake recovery and thus a narrower turbine spacing.

  12. Development and validation of a new two-dimensional wake model for wind turbine wakes

    DEFF Research Database (Denmark)

    Tian, Linlin; Zhu, Wei Jun; Shen, Wen Zhong;

    2015-01-01

    of the wake deficit in the crosswind direction. Moreover, a variable wake decay rate is proposed to take into account both the ambient turbulence and the rotor generated turbulence, different from a constant wake decay rate used in the Jensen model. The obtained results are compared to field measurements...

  13. Trailing Vortex Measurements in the Wake of a Hovering Rotor Blade with Various Tip Shapes

    Science.gov (United States)

    Martin, Preston B.; Leishman, J. Gordon

    2003-01-01

    This work examined the wake aerodynamics of a single helicopter rotor blade with several tip shapes operating on a hover test stand. Velocity field measurements were conducted using three-component laser Doppler velocimetry (LDV). The objective of these measurements was to document the vortex velocity profiles and then extract the core properties, such as the core radius, peak swirl velocity, and axial velocity. The measured test cases covered a wide range of wake-ages and several tip shapes, including rectangular, tapered, swept, and a subwing tip. One of the primary differences shown by the change in tip shape was the wake geometry. The effect of blade taper reduced the initial peak swirl velocity by a significant fraction. It appears that this is accomplished by decreasing the vortex strength for a given blade loading. The subwing measurements showed that the interaction and merging of the subwing and primary vortices created a less coherent vortical structure. A source of vortex core instability is shown to be the ratio of the peak swirl velocity to the axial velocity deficit. The results show that if there is a turbulence producing region of the vortex structure, it will be outside of the core boundary. The LDV measurements were supported by laser light-sheet flow visualization. The results provide several benchmark test cases for future validation of theoretical vortex models, numerical free-wake models, and computational fluid dynamics results.

  14. Effect of nacelle on the wake meandering in Horns Rev wind farm

    Science.gov (United States)

    Yang, Xiaolei; Foti, Daniel; Sotiropoulos, Fotis

    2016-11-01

    Turbine wake meandering has considerable effects on the velocity deficit and turbulence intensity in the wake. However, the mechanism for wake meandering is still not well understood and low-order models cannot take into account the wake meandering effects accurately. A recent work by Kang, Yang and Sotiropoulos showed that the nacelle has a significant effect on the wake meandering of a hydrokinetic turbine. To examine the nacelle contributions to wake meandering and wake interactions in utility-scale wind farms, we simulate the atmospheric turbulent flow over the Horns Rev wind farm using large-eddy simulation with actuator type models. In a preliminary simulation on a coarse grid using actuator line model for turbine blades without a nacelle model, the computed power shows overall good agreement with field measurements. Fine grid simulations using an actuator surface model for turbine blades with and without a nacelle model are being carried out. The corresponding results will be presented with analysis on wake meandering dynamics using the technique proposed by Horward et al. and Foti et al.. This work was supported by DOE (DE-AC04-94AL85000), Xcel Energy (Grant RD4-13) and Sandia National Laboratories. Computational resources were provided by National Renewable Energy Laboratory and University of Minnesota Supercomputing Institute.

  15. Wake Behavior behind Turbine Cascades in Compressible Two-Dimensional Flows

    Directory of Open Access Journals (Sweden)

    Kurz Rainer

    2005-01-01

    Full Text Available The goal of the paper is to describe wake parameters of wakes from turbine cascades in compressible flows especially in planes where the leading edge of the following blade row would be located. Data from experiments with turbine cascades in compressible flow will be used to derive a theoretical approach which describes the wake growth and the recovery of the velocity deficit. The theory is based on similarity assumptions. The derived equations depend on simple and readily available parameters such as overall losses, exit angle, and Mach or Laval number. In compressible turbine flows, the influence of the inviscid flow field is of great importance. In this paper, an approach to take this influence into account when determining the behavior of the wake is presented. Correlations for basic characteristics of wakes in compressible flows are not readily available. Such correlations are necessary as input to unsteady flow and heat transfer calculation procedures for turbomachine blades. Based on available data on wake behavior in the compressible flow behind turbine blades, the correlations presented describe the wake behavior from the trailing edge to the confluence of the wakes of adjacent blades.

  16. The Explicit Wake Parametrisation V1.0: a wind farm parametrisation in the mesoscale model WRF

    Directory of Open Access Journals (Sweden)

    P. J. H. Volker

    2015-11-01

    Full Text Available We describe the theoretical basis, implementation, and validation of a new parametrisation that accounts for the effect of large offshore wind farms on the atmosphere and can be used in mesoscale and large-scale atmospheric models. This new parametrisation, referred to as the Explicit Wake Parametrisation (EWP, uses classical wake theory to describe the unresolved wake expansion. The EWP scheme is validated for a neutral atmospheric boundary layer against filtered in situ measurements from two meteorological masts situated a few kilometres away from the Danish offshore wind farm Horns Rev I. The simulated velocity deficit in the wake of the wind farm compares well to that observed in the measurements, and the velocity profile is qualitatively similar to that simulated with large eddy simulation models and from wind tunnel studies. At the same time, the validation process highlights the challenges in verifying such models with real observations.

  17. Experimental investigation of the wake behind a model of wind turbine in a water flume

    Science.gov (United States)

    Okulov, V. L.; Naumov, I. N.; Kabardin, I.; Mikkelsen, R.; Sørensen, J. N.

    2014-12-01

    The flow behind the model of wind turbine rotor is investigated experimentally in a water flume using Particle Image Velocimetry. The study carried out involves rotors of three bladed wind turbine designed using Glauert's optimization. The transitional regime, generally characterized as in between the regime governed by stable organized vortical structures and the turbulent wake, develops from disturbances of the tip and root vorticies through vortex paring and further complex behaviour towards the fully turbulent wake. Our PIV measurements pay special attention to the onset of the instabilities. The near wake characteristics (development of expansion, tip vortex position, deficit velocity and rotation in the wake) have been measured for different tip speed ratio to compare with main assumptions and conclusions of various rotor theories.

  18. Wind Turbine Wake Characterization from Temporally Disjunct 3-D Measurements

    Energy Technology Data Exchange (ETDEWEB)

    Doubrawa, Paula; Barthelmie, Rebecca J.; Wang, Hui; Pryor, S. C.; Churchfield, Matthew

    2016-11-01

    Scanning LiDARs can be used to obtain three-dimensional wind measurements in and beyond the atmospheric surface layer. In this work, metrics characterizing wind turbine wakes are derived from LiDAR observations and from large-eddy simulation (LES) data, which are used to recreate the LiDAR scanning geometry. The metrics are calculated for two-dimensional planes in the vertical and cross-stream directions at discrete distances downstream of a turbine under single-wake conditions. The simulation data are used to estimate the uncertainty when mean wake characteristics are quantified from scanning LiDAR measurements, which are temporally disjunct due to the time that the instrument takes to probe a large volume of air. Based on LES output, we determine that wind speeds sampled with the synthetic LiDAR are within 10% of the actual mean values and that the disjunct nature of the scan does not compromise the spatial variation of wind speeds within the planes. We propose scanning geometry density and coverage indices, which quantify the spatial distribution of the sampled points in the area of interest and are valuable to design LiDAR measurement campaigns for wake characterization. We find that scanning geometry coverage is important for estimates of the wake center, orientation and length scales, while density is more important when seeking to characterize the velocity deficit distribution.

  19. Wind Turbine Wake Characterization from Temporally Disjunct 3-D Measurements

    Directory of Open Access Journals (Sweden)

    Paula Doubrawa

    2016-11-01

    Full Text Available Scanning LiDARs can be used to obtain three-dimensional wind measurements in and beyond the atmospheric surface layer. In this work, metrics characterizing wind turbine wakes are derived from LiDAR observations and from large-eddy simulation (LES data, which are used to recreate the LiDAR scanning geometry. The metrics are calculated for two-dimensional planes in the vertical and cross-stream directions at discrete distances downstream of a turbine under single-wake conditions. The simulation data are used to estimate the uncertainty when mean wake characteristics are quantified from scanning LiDAR measurements, which are temporally disjunct due to the time that the instrument takes to probe a large volume of air. Based on LES output, we determine that wind speeds sampled with the synthetic LiDAR are within 10% of the actual mean values and that the disjunct nature of the scan does not compromise the spatial variation of wind speeds within the planes. We propose scanning geometry density and coverage indices, which quantify the spatial distribution of the sampled points in the area of interest and are valuable to design LiDAR measurement campaigns for wake characterization. We find that scanning geometry coverage is important for estimates of the wake center, orientation and length scales, while density is more important when seeking to characterize the velocity deficit distribution.

  20. An improved k-ε model applied to a wind turbine wake in atmospheric turbulence

    DEFF Research Database (Denmark)

    Laan, van der, Paul Maarten; Sørensen, Niels N.; Réthoré, Pierre-Elouan

    2015-01-01

    turbine wake. The modified k-ε model is compared with the original k-ε eddy viscosity model, Large-Eddy Simulations and field measurements using eight test cases. The comparison shows that the velocity wake deficits, predicted by the proposed model are much closer to the ones calculated by the Large......An improved k-ε turbulence model is developed and applied to a single wind turbine wake in a neutral atmospheric boundary layer using a Reynolds averaged Navier–Stokes solver. The proposed model includes a flow-dependent Cμ that is sensitive to high velocity gradients, e.g., at the edge of a wind......-Eddy Simulation and those observed in the measurements, than predicted by the original k-ε model. Copyright © 2014 John Wiley & Sons, Ltd....

  1. Study of large-scale vertical axis wind turbine wake through numerical modelling and fullscale experiments

    DEFF Research Database (Denmark)

    Immas, Alexandre; Kluczewska-Bordier, Joanna; Beneditti, Pascal

    Offshore wind capacity is increasing exponentially over the years in Europe, taking advantage of the strong winds available over the ocean and of the political incentives to reduce greenhouse gases. The technology is however not yet competitive when compared to fossil fuels or onshore wind. One key...... improvement that could make offshore wind more attractive is the reduction of the wake effect [1]. The latter corresponds to the velocity deficit generated by each wind turbine wake which affects the production of the others. This effect accounts for approximately 10% of the energy losses for a typical...

  2. Influence of turbulence on the wake of a marine current turbine simulator.

    Science.gov (United States)

    Blackmore, T; Batten, W M J; Bahaj, A S

    2014-10-08

    Marine current turbine commercial prototypes have now been deployed and arrays of multiple turbines under design. The tidal flows in which they operate are highly turbulent, but the characteristics of the inflow turbulence have not being considered in present design methods. This work considers the effects of inflow turbulence on the wake behind an actuator disc representation of a marine current turbine. Different turbulence intensities and integral length scales were generated in a large eddy simulation using a gridInlet, which produces turbulence from a grid pattern on the inlet boundary. The results highlight the significance of turbulence on the wake profile, with a different flow regime occurring for the zero turbulence case. Increasing the turbulence intensity reduced the velocity deficit and shifted the maximum deficit closer to the turbine. Increasing the integral length scale increased the velocity deficit close to the turbine due to an increased production of turbulent energy. However, the wake recovery was increased due to the higher rate of turbulent mixing causing the wake to expand. The implication of this work is that marine current turbine arrays could be further optimized, increasing the energy yield of the array when the site-specific turbulence characteristics are considered.

  3. Wake flow characteristics at high wind speed

    DEFF Research Database (Denmark)

    Aagaard Madsen, Helge; Larsen, Torben J.; Larsen, Gunner Chr.

    2016-01-01

    power. In the present study we simulate the wake flow for a row of turbines with the wind aligned with the row using a simplified approach. The velocity deficit, being a function of the thrust coefficient, is simulated based on the BEM solution for wake expansion. An axis-symmetric boundary layer...... equation model (the same as implemented in the DWM model) is subsequently used to develop the deficit down to the next turbine, and then the approach is successively repeated. Simulation results for four different spacing’s in a row with eight turbines show that there are two major flow regimes...... in the intersection region between the two flow regimes a strong variation in power and thrust occur, e.g. going from almost zero power to rated power for a wind speed change of 4m/s. Another result is that the inflow profile to the last turbine in the row at a wind speed of 16m/s for a spacing of 3D shows...

  4. Wind turbine wake visualization and characteristics analysis by Doppler lidar.

    Science.gov (United States)

    Wu, Songhua; Liu, Bingyi; Liu, Jintao; Zhai, Xiaochun; Feng, Changzhong; Wang, Guining; Zhang, Hongwei; Yin, Jiaping; Wang, Xitao; Li, Rongzhong; Gallacher, Daniel

    2016-05-16

    Wind power generation is growing fast as one of the most promising renewable energy sources that can serve as an alternative to fossil fuel-generated electricity. When the wind turbine generator (WTG) extracts power from the wind, the wake evolves and leads to a considerable reduction in the efficiency of the actual power generation. Furthermore, the wake effect can lead to the increase of turbulence induced fatigue loads that reduce the life time of WTGs. In this work, a pulsed coherent Doppler lidar (PCDL) has been developed and deployed to visualize wind turbine wakes and to characterize the geometry and dynamics of wakes. As compared with the commercial off-the-shelf coherent lidars, the PCDL in this work has higher updating rate of 4 Hz and variable physical spatial resolution from 15 to 60 m, which improves its capability to observation the instantaneous turbulent wind field. The wind speed estimation method from the arc scan technique was evaluated in comparison with wind mast measurements. Field experiments were performed to study the turbulent wind field in the vicinity of operating WTGs in the onshore and offshore wind parks from 2013 to 2015. Techniques based on a single and a dual Doppler lidar were employed for elucidating main features of turbine wakes, including wind velocity deficit, wake dimension, velocity profile, 2D wind vector with resolution of 10 m, turbulence dissipation rate and turbulence intensity under different conditions of surface roughness. The paper shows that the PCDL is a practical tool for wind energy research and will provide a significant basis for wind farm site selection, design and optimization.

  5. Wind turbine wakes in forest and neutral plane wall boundary layer large-eddy simulations

    Science.gov (United States)

    Schröttle, Josef; Piotrowski, Zbigniew; Gerz, Thomas; Englberger, Antonia; Dörnbrack, Andreas

    2016-09-01

    Wind turbine wake flow characteristics are studied in a strongly sheared and turbulent forest boundary layer and a neutral plane wall boundary layer flow. The reference simulations without wind turbine yield similar results as earlier large-eddy simulations by Shaw and Schumann (1992) and Porte-Agel et al. (2000). To use the fields from the homogeneous turbulent boundary layers on the fly as inflow fields for the wind turbine wake simulations, a new and efficient methodology was developed for the multiscale geophysical flow solver EULAG. With this method fully developed turbulent flow fields can be achieved upstream of the wind turbine which are independent of the wake flow. The large-eddy simulations reproduce known boundary-layer statistics as mean wind profile, momentum flux profile, and eddy dissipation rate of the plane wall and the forest boundary layer. The wake velocity deficit is more asymmetric above the forest and recovers faster downstream compared to the velocity deficit in the plane wall boundary layer. This is due to the inflection point in the mean streamwise velocity profile with corresponding turbulent coherent structures of high turbulence intensity in the strong shear flow above the forest.

  6. Investigation of wake characteristics in wind farm varying turbulent inflow condition

    Science.gov (United States)

    Na, Jisung; Koo, Eunmo; Domingo, Munoz-Esparza; Jin, Emilia Kyung; Linn, Rodman; Lee, Joon Sang

    2016-11-01

    In this study, we investigate the wake characteristics in wind farm varying turbulent property at inlet condition. To solve the flow with wind turbines and its wake, we use large eddy simulation (LES) technique with actuator line method (ALM). The wake characteristics in wind farm is important mainly in performance of wind farm because non-fully recovered wake induced by upstream wind turbines interferes power generation at downstream wind turbines. Turbulent inflow which contains the information of turbulence in atmospheric boundary layer is one of the key factors for describing the wake in wind farm accurately. We perform the quantitative analysis of velocity deficit and turbulent intensity in whole cases. In the comparison between cases with and without turbulent inflow, we observe that wake in case with turbulent inflow is more diffused to span-wise direction. And we analyze the coherent structures behind wind turbines at each row. Through above-analysis, we reveal how the wake is interacted with performance of wind farm. This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No.2015R1A5A1037668).

  7. Hub vortex instability within wind turbine wakes: Effects of wind turbulence, loading conditions, and blade aerodynamics

    Science.gov (United States)

    Ashton, Ryan; Viola, Francesco; Camarri, Simone; Gallaire, Francois; Iungo, Giacomo Valerio

    2016-11-01

    The near wake of wind turbines is characterized by the presence of the hub vortex, which is a coherent vorticity structure generated from the interaction between the root vortices and the boundary layer evolving over the turbine nacelle. By moving downstream, the hub vortex undergoes an instability with growth rate, azimuthal and axial wavenumbers determined by the characteristics of the incoming wind and turbine aerodynamics. Thus, a large variability of the hub vortex instability is expected for wind energy applications with consequent effects on wake downstream evolution, wake interactions within a wind farm, power production, and fatigue loads on turbines invested by wakes generated upstream. In order to predict characteristics of the hub vortex instability for different operating conditions, linear stability analysis is carried out by considering different statistics of the incoming wind turbulence, thrust coefficient, tip speed ratio, and blade lift distribution of a wind turbine. Axial and azimuthal wake velocity fields are modeled through Carton-McWilliams velocity profiles by mimicking the presence of the hub vortex, helicoidal tip vortices, and matching the wind turbine thrust coefficient predicted through the actuator disk model. The linear stability analysis shows that hub vortex instability is strongly affected by the wind turbine loading conditions, and specifically it is promoted by a larger thrust coefficient. A higher load of the wind turbines produces an enhanced axial velocity deficit and, in turn, higher shear in the radial direction of the streamwise velocity. The axial velocity shear within the turbine wake is also the main physical mechanism promoting the hub vortex instability when varying the lift distribution over the blade span for a specific loading condition. Cases with a larger velocity deficit in proximity of the wake center and less aerodynamic load towards the blade tip result to be more unstable. Moreover, wake swirl promotes hub

  8. Stratified wake of an accelerating hydrofoil

    CERN Document Server

    Ben-Gida, Hadar; Gurka, Roi

    2015-01-01

    Wakes of towed and self-propelled bodies in stratified fluids are significantly different from non-stratified wakes. Long time effects of stratification on the development of the wakes of bluff bodies moving at constant speed are well known. In this experimental study we demonstrate how buoyancy affects the initial growth of vortices developing in the wake of a hydrofoil accelerating from rest. Particle image velocimetry measurements were applied to characterize the wake evolution behind a NACA 0015 hydrofoil accelerating in water and for low Reynolds number and relatively strong and stably stratified fluid (Re=5,000, Fr~O(1)). The analysis of velocity and vorticity fields, following vortex identification and an estimate of the circulation, reveal that the vortices in the stratified fluid case are stretched along the streamwise direction in the near wake. The momentum thickness profiles show lower momentum thickness values for the stratified late wake compared to the non-stratified wake, implying that the dra...

  9. Synergistic Effects of Turbine Wakes and Atmospheric Stability on Power Production at an Onshore Wind Farm

    Energy Technology Data Exchange (ETDEWEB)

    Wharton, S; Lundquist, J K; Marjanovic, N

    2012-01-25

    recovers to its inflow velocity is dependent on the amount ambient turbulence, the amount of wind shear, and topographical and structural effects. The maximum velocity deficit is estimated to occur at 1-2 D but can be longer under low levels of ambient turbulence. Our understanding of turbine wakes comes from wind tunnel experiments, field experiments, numerical simulations, and from studies utilizing both experimental and modeling methods. It is well documented that downwind turbines in multi-Megawatt wind farms often produce less power than upwind turbine rows. These wake-induced power losses have been estimated from 5% to up to 40% depending on the turbine operating settings (e.g., thrust coefficient), number of turbine rows, turbine size (e.g., rotor diameter and hub-height), wind farm terrain, and atmospheric flow conditions (e.g., ambient wind speed, turbulence, and atmospheric stability). Early work by Elliott and Cadogan suggested that power data for different turbulent conditions be segregated to distinguish the effects of turbulence on wind farm power production. This may be especially important for downwind turbines within wind farms, as chaotic and turbulent wake flows increase stress on downstream turbines. Impacts of stability on turbine wakes and power production have been examined for a flat terrain, moderate size (43 turbines) wind farm in Minnesota and for an offshore, 80 turbine wind farm off the coast of Denmark. Conzemius found it difficult to distinguish wakes (i.e., downwind velocity deficits) when the atmosphere was convective as large amounts of scatter were present in the turbine nacelle wind speed data. This suggested that high levels of turbulence broke-up the wake via large buoyancy effects, which are generally on the order of 1 km in size. On the other hand, they found pronounced wake effects when the atmosphere was very stable and turbulence was either suppressed or the length scale was reduced as turbulence in this case was mechanically

  10. Measurements of wakes originated from 2-bladed and 3-bladed rotors

    Science.gov (United States)

    Wu, Yu-Ting; Lyu, Shao-Dong; Chen, Bo-Wei

    2016-04-01

    Measurements of wakes originated from 2-bladed and 3-bladed rotors were carried out using a hot-wire probe system in an open jet wind tunnel. Hot-wire anemometry was adopted to characterize the spanwise profiles of mean wind speed, turbulence intensity and momentum flux for downwind locations at 0.5, 1, 2, 3, and 4 rotor diameters. The results showed that the 2-bladed rotor spun faster than the 3-bladed one, where the ratio of the two blade angular velocities was 1.065:1 under the same inflow condition with a uniform distribution of 5.4 m/s flow velocity. The turbulence flow statistics of the rotor wakes showed that the wake originated from the 3-bladed rotor has larger velocity deficit, streamwise turbulence intensity, momentum flux magnitude, but smaller spanwise turbulence intensity. The velocity spectrum showed peaks associated with the presence of the blade-induced tip vortices in the near wake region (approximately within 3 rotor diameters).

  11. Nonlinear growth of electron holes in cross-field wakes

    Science.gov (United States)

    Hutchinson, Ian; Haakonsen, C. B.; Zhou, C.

    2015-11-01

    Cross-field plasma flow past an obstacle is key to the physics underlying Mach-probes, space-craft charging, and the wakes of non-magnetic bodies: the solar-wind wake of the moon is a typical example. We report associated new nonlinear instability mechanisms. Ions are accelerated along the B-field into the wake, forming two beams, but they are not initially unstable to ion two-stream instabilities. Electron Langmuir waves become unstable much earlier because of an electron velocity-distribution distortion called the ``dimple''. The magnetic field, perpendicular to the flow, defines the 1-D direction of particle dynamics. In high-fidelity PIC simulations at realistic mass ratio, small electron holes--non-linearly self-binding electron density deficits--are spawned by the dimple in fe (v) near the phase-space separatrix. Most holes accelerate rapidly out of the wake, along B. However, some remain at very low speed, and grow until they are large enough to disrupt the two ion-streams, well before the ions are themselves linearly unstable. This non-linear hole growth is caused by the same mechanism that causes the dimple: cross-field drift from a lower to a higher density. Related mechanisms cause plasma near magnetized Langmuir probes to be unsteady. Partially supported by the NSF/DOE Basic Plasma Science Partnership grant DE-SC0010491.

  12. Similarity considerations for a turbulent axisymmetric wake with rotation subjected to a boundary layer flow

    Science.gov (United States)

    Wosnik, Martin

    2015-11-01

    Recently an analytical and experimental investigation of the turbulent axisymmetric wake with rotation found a new asymptotic scaling function for the mean swirl, Wmax ~Uo3/ 2 ~x-1 (Dufresne and Wosnik, Mar Technol Soc J, 47, no.4, 193-205, 2013). An equilibrium similarity theory derived scaling functions from the conditions for the existence of similarity directly from the equations of motion. Axial and azimuthal (swirl) velocities were measured in the wake of a single 3-bladed wind turbine in a free stream up to 20 diameters downstream, and the data were found to support the theoretical results. The scaling implies that the mean swirl decays faster, with x-1, than the mean velocity deficit, with x - 2 / 3. Real wind turbines, however, operate in the atmospheric boundary layer. They are subjected to mean shear and turbulence, both have been observed to improve wake recovery. Similarity considerations are extended to place a turbulent axisymmetric wake with rotation in a boundary layer flow, and the scaling implications are examined. Corresponding experiments were carried out in the UNH Flow Physics Facility, using model wind turbines of various sizes as swirling wake generators. Supported by NSF CBET grant 1150797.

  13. A neural network based wake model for small wind turbine siting near obstacles

    Science.gov (United States)

    Brunskill, Andrew William

    Many potential small wind turbine locations are near obstacles such as buildings and shelterbelts, which can have a significant, detrimental effect on the local wind climate. This thesis describes the creation of a new model which can predict the wind speed, turbulence intensity, and wind power density at any point in an obstacle's region of influence, relative to unsheltered conditions. Artificial neural networks were used to learn the relationship between an obstacle's characteristics and its effects on the local wind. The neural network was trained using measurements collected in the wakes of scale models exposed to a simulated atmospheric boundary layer in a wind tunnel. A field experiment was conducted to validate the wind tunnel measurements. Model predictions are most accurate in the far wake region. The estimated mean uncertainties associated with model predictions of velocity deficit, power density deficit, and turbulence intensity excess are 5.0%, 15%, and 12.8%, respectively.

  14. An experimental study on the aeromechanics and wake characteristics of a novel twin-rotor wind turbine in a turbulent boundary layer flow

    Science.gov (United States)

    Wang, Zhenyu; Tian, Wei; Ozbay, Ahmet; Sharma, Anupam; Hu, Hui

    2016-09-01

    The aeromechanic performance and wake characteristics of a novel twin-rotor wind turbine (TRWT) design, which has an extra set of smaller, auxiliary rotor blades appended in front of the main rotor, was evaluated experimentally, in comparison with those of a conventional single-rotor wind turbine (SRWT) design. The comparative study was performed in a large-scale wind tunnel with scaled TRWT and SRWT models mounted in the same incoming turbulent boundary layer flow. In addition to quantifying power outputs and the dynamic wind loadings acting on the model turbines, the wake characteristics behind the model turbines were also measured by using a particle image velocimetry system and a Cobra anemometry probe. The measurement results reveal that, while the TRWT design is capable of harnessing more wind energy from the same incoming airflow by reducing the roots losses incurred in the region near the roots of the main rotor blades, it also cause much greater dynamic wind loadings acting on the TRWT model and higher velocity deficits in the near wake behind the TRWT model, in comparison with those of the SRWT case. Due to the existence of the auxiliary rotor, more complex vortex structures were found to be generated in the wake behind the TRWT model, which greatly enhanced the turbulent mixing in the turbine wake, and caused a much faster recovery of the velocity deficits in the turbine far wake. As a result, the TRWT design was also found to enable the same downstream turbine to generate more power when sited in the wake behind the TRWT model than that in the SRWT wake, i.e., by mitigating wake losses in typical wind farm settings.

  15. Wake characteristics of a model ornithopter

    Science.gov (United States)

    Juarez, Alfredo; Harlow, Jacob; Allen, James; Ferreira de Sousa, Paulo

    2006-03-01

    This paper details unsteady wake measurements from a model Ornithopther flying in a wind tunnel at representative flight conditions. Testing over a range of Strouhal number, 0.1-0.3, shows that the unsteady wake is composed of coherent vortical structures that resemble vortex rings. A single ring is formed in the wake of each wing during one wing beat. Momentum balance from velocity field measurements are reconciled with unsteady lift and drag measurements from a drag balance.

  16. Experimental investigation of the turbulent axisymmetric wake with rotation generated by a wind turbine

    Science.gov (United States)

    Dufresne, Nathaniel P.

    An experimental investigation of the axial and azimuthal (swirl) velocity field in the wake of a single 3-bladed wind turbine with rotor diameter of 0.91m was conducted, up to 20 diameters downstream. The turbine was positioned in the free stream, near the entrance of the 6m x 2.7m cross section of the University of New Hampshire (UNH) Flow Physics Facility. Velocity measurements were conducted at different rotor loading conditions with blade tip-speed ratios from 2.0 to 2.8. A Pitot-static tube and constant temperature hot-wire anemometer with a multi-wire sensor were used to measure velocity fields. An equilibrium similarity theory for the turbulent axisymmetric wake with rotation was outlined, and first evidence for a new scaling function for the mean swirling velocity component, Wmax ∝ x-1 ∝ U3/2o a was found; where W represents swirl, x represents downstream distance, and Uo, represents the centerline velocity deficit in the wake.

  17. When do Acacia mellifera trees use water? Responses of sap velocity to soil water availability, vapour pressure deficit and global radiation.

    Science.gov (United States)

    de Blécourt, Marleen; Thomsen, Simon; Gröngröft, Alexander; Eschenbach, Annette

    2017-04-01

    Acacia mellifera (multi-stem deciduous tree) is one of the dominant woody species responsible for bush encroachment in southern African savannahs. However, very little is known on water use, transpiration or xylem sap flow of A. mellifera. We analyzed the responses of sap velocity in A. mellifera to soil moisture, vapour pressure deficit and global radiation. This knowledge is necessary to improve hydrological modelling and will as such contribute to our understanding of the impacts of bush encroachment in (semi) arid savannahs on the soil water balance. We monitored sap velocities at two sites that differed in tree density in a semi-arid thornbush savannah in central Namibia (mean annual precipitation = 346 mm). Sap velocities were derived using the Heat Ratio Method. Measurements were done in four periods of 3-4 months between November 2014 and September 2016. The measurement periods covered the transitions between the dry and rainy season and vice versa, and the dry season. In two of these periods we did measurements at all stems of three trees per site (a total of 17-19 stems), while in the other two periods sap velocities were measured on one stem per tree for six to eight trees per site. The study was done in the framework of SASSCAL (Southern African Science Service Centre for Climate Change and Adaptive Land Management) granted by the German Federal Ministry of Education and Research (BMBF). Preliminary results indicate that the day-to-day fluctuations in cumulative daily sap velocity showed a three-phase interaction with soil water tension (minimum soil water tension of four sensors to 1-m depth). Phase I: At soil water tension radiation. Phase II: At soil water tensions between pF 2.5 and pF 3.2, sap velocities were negatively related to soil water tension. Phase III: At soil water tensions > pF 3.2 no sap flow could be detected.

  18. Large Eddy Simulation of Wind Turbine Wake Dynamics in the Stable Boundary Layer Using the Weather Research and Forecasting Model

    Science.gov (United States)

    Aitken, M.; Kosovic, B.; Mirocha, J. D.; Lundquist, J. K.

    2014-12-01

    To thoroughly verify the actuator disk model recently implemented in WRF for large eddy simulation (LES) of wind turbine wakes, simulations of various types of turbines and atmospheric conditions must be compared to full-scale field measurements of the real atmosphere. Here, numerical simulations are compared to nacelle-based scanning lidar measurements taken in stable atmospheric conditions during a field campaign conducted at a wind farm in the western United States. Using several wake characteristics—such as the velocity deficit, centerline location, and wake width—as metrics for model verification, the simulations show good agreement with the observations. Notably, the average velocity deficit was seen to be quite high in both the experiment and simulation, resulting from a low average wind speed and therefore high average turbine thrust coefficient. Moreover, new features—namely rotor tilt and drag from the nacelle and tower—were added to the existing actuator disk model in WRF-LES. Compared to the rotor, the effect of the tower and nacelle on the flow is relatively small but nevertheless important for an accurate representation of the entire turbine. Adding rotor tilt to the model causes the vertical location of the wake center to shift upward. Continued advancement of the actuator disk model in WRF-LES will help lead to optimized turbine siting and controls at wind farms.

  19. SIMULATION OF AIRCRAFT CONDENSATION TRAILS AND WAKE VORTICES INTERACTION

    Directory of Open Access Journals (Sweden)

    T. O. Aubakirov

    2015-01-01

    Full Text Available A technique of calculation of aircraft condensation trails (contrails and wake vortices interaction is described. The technique is based on a suitable for real-time applications mathematical model of far wake utilizes the method of discrete vortices. The technique is supplemented by account of the influence of axial velocities in the vortex nucleus on contrail and wake vortex location. Results of calculations of contrails and wake vortices interaction for Il-76 and B-747 aircraft are presented.

  20. Wind Speed Estimation and Wake model Re-calibration for Downregulated Offshore Wind Farms

    Science.gov (United States)

    Göçmen Bozkurt, Tuhfe; Giebel, Gregor; Kjølstad Poulsen, Niels; Réthoré, Pierre-Elouan; Mirzaei, Mahmood

    2014-05-01

    In recent years, the wind farm sizes have increased tremendously and with increasing installed capacity, the wind farms are requested to downregulate from their maximum possible power more frequently, especially in the offshore environment. Determination of the possible (or available) power is crucial not only because the reserve power has considerable market value but also for wind farm developers to be properly compensated for the loss during downregulation. While the available power calculation is straightforward for a single turbine, it gets rather complicated for the whole wind farm due to the change in the wake characteristics. In fact, the wake losses generated by the upstream turbine(s) decrease during downregulation and the downstream turbines therefore see more wind compared to the normal operation case. Currently, the Transmission System Operators (TSOs) have no real way to determine exactly the available power of a whole wind farm which is downregulated. Therefore, the PossPOW project aims to develop a verified and internationally accepted way to determine the possible power of a down-regulated offshore wind farm. The first phase of the project is to estimate the rotor effective wind speed. Since the nacelle anemometers are not readily available and are known to have reliability issues, the proposed method is to use power, pitch angle and rotational speed as inputs and combine it with a generic Cp model to estimate the wind speed. The performance of the model has been evaluated for both normal operation and downregulation periods using two different case studies: Horns Rev-I wind farm and NREL 5MW single turbine. During downregulation, the wake losses are not as severe and the velocity deficits at the downstream turbines are smaller as if also the wake is "downregulated". On the other hand, in order to calculate the available power, the wakes that would have been produced normally (if the turbines were not curtailed) are of importance, not the

  1. Numerical study of the wake acceleration effect using CFD approach

    OpenAIRE

    Chai, Xiang

    2014-01-01

    A new drag force model was proposed to consider the wake acceleration effect. The proposed model is dependent on wake velocity. Based on a systematic CFD simulation, correlations were proposed to describe the velocity profile of turbulent wake. The proposed model was validated against the experimental data of bubble pairs and employed in the simulation of bubbly flow. It revealed better prediction accuracy than the existing models. It was also used to assess the bubble induced turbulence.

  2. Wind Turbine Wake Characterization with Remote Sensing and Computational Fluid Dynamics

    Science.gov (United States)

    Aitken, Matthew Lawrence

    Because of the dense arrays at most wind farms, the region of disturbed flow downstream of an individual turbine leads to reduced power production and increased structural loading for its leeward counterparts. Currently, wind farm wake modeling, and hence turbine layout optimization, suffer from an unacceptable degree of uncertainty, largely because of a lack of adequate experimental data for model verification. Accordingly, wake measurements were taken in two separate experiments, (1) using the ground-based High Resolution Doppler Lidar (HRDL) developed by the National Oceanic and Atmospheric Administration (NOAA) in the Turbine Wake and Inflow Characterization Study (TWICS) at the National Renewable Energy Laboratory (NREL), and (2) using nacelle-based long-range lidar at a wind farm in the western United States. The vantage point from the nacelle is favorable in that scans can more consistently transect the central part of the wake. The work presented here outlines a set of quantitative procedures for determining critical parameters from these extensive datasets---such as the velocity deficit, the size of the wake boundary, and the location of the wake centerline---and the results are categorized by ambient wind speed, turbulence, and atmospheric stability. Despite specific reference to lidar, the methodology is general and can be applied to extract wake characteristics from other remote sensor datasets, as well as output from numerical simulations. In an effort to help advance computational fluid dynamics (CFD) models of wind turbine wake dynamics, experimental results are compared to a large eddy simulation (LES) of a turbine operating in the stable boundary layer using the actuator disk parameterization in the Weather Research and Forecasting (WRF) Model. With the wake characteristics described above as metrics for model verification, the simulations show good agreement with the observations. Moreover, new features---namely rotor tilt and drag from the

  3. A simple stationary semi-analytical wake model

    DEFF Research Database (Denmark)

    Larsen, Gunner Chr.

    -uniform mean wind field, although the modelling of the individual stationary wake flow fields includes non-linear terms. The simulation of the individual wake contributions are based on an analytical solution of the thin shear layer approximation of the NS equations. The wake flow fields are assumed......We present an idealized simple, but fast, semi-analytical algorithm for computation of stationary wind farm wind fields with a possible potential within a multi-fidelity strategy for wind farm topology optimization. Basically, the model considers wakes as linear perturbations on the ambient non...... rotationally symmetric, and the rotor inflow fields are consistently assumed uniform. Expansion of stationary wake fields is believed to be significantly affected by meandering of wake deficits as e.g. described by the Dynamic Wake Meandering model. In the present context, this effect is approximately...

  4. CFD modelling approaches against single wind turbine wake measurements using RANS

    Science.gov (United States)

    Stergiannis, N.; Lacor, C.; Beeck, J. V.; Donnelly, R.

    2016-09-01

    Numerical simulations of two wind turbine generators including the exact geometry of their blades and hub are compared against a simplified actuator disk model (ADM). The wake expansion of the upstream rotor is investigated and compared with measurements. Computational Fluid Dynamics (CFD) simulations have been performed using the open-source platform OpenFOAM [1]. The multiple reference frame (MRF) approach was used to model the inner rotating reference frames in a stationary computational mesh and outer reference frame for the full wind turbine rotor simulations. The standard k — ε and k — ω turbulence closure schemes have been used to solve the steady state, three dimensional Reynolds Averaged Navier- Stokes (RANS) equations. Results of near and far wake regions are compared with wind tunnel measurements along three horizontal lines downstream. The ADM under-predicted the velocity deficit at the wake for both turbulence models. Full wind turbine rotor simulations showed good agreement against the experimental data at the near wake, amplifying the differences between the simplified models.

  5. 轴流泵叶轮出口尾迹区非定常压力和速度场特性%Property of unsteady pressure and meridional velocity in wake region of axial-flow pump impeller

    Institute of Scientific and Technical Information of China (English)

    张德胜; 施卫东; 李通通; 张华; 关醒凡

    2012-01-01

    为了分析轴流泵叶轮出口尾迹和势流交替干扰特性,基于RNGκ-ε湍流模型和SIMPLE算法,对南水北调工程用轴流泵模型进行了数值计算.通过定常预测的外特性结果与试验值进行比较,验证了计算网格和湍流模型的适用性,并在此基础上计算了轴流泵叶轮出口尾迹区非定常流场特性.研究结果表明,通过轴流泵全流场数值计算结果与试验值对比,在最优工况下计算扬程相对误差为4.56%,效率相对误差为2.78%,较好反映了轴流泵内部流动特性;在小流量工况下,轴流泵叶轮出口圆周方向轴面速度存在与叶片数相同的3个主波峰和3个次波峰;随着流量增大,叶轮出口圆周方向速度分布图中的波峰与导叶叶片数相同.在小流量工况和设计工况下,叶轮出口尾迹区压力脉动时域图出现3个主波峰,随着流量增大,额外产生了3个次波峰.基于FFT变换发现不同流量工况下的压力脉动主频均以叶频为主,其他谐频以叶频为基频,呈倍数出现,且主频的幅值随着流量减小而迅速上升.%In order to study the rotor-stator interaction in the wake region of the axial-flow impeller, the axial-flow pump model for South-to-North Water Diversion Project model was simulated based RNG k-e turbulence model and the SIMPLE algorithm. The calculated values by steady simulation were compared with the experimental results to verify the computational grid and the applicability of turbulence model, and the unsteady field was simulated based on the steady results. The numerical results show that the relative error of Head is 4.56% and efficiency is 2.78% in optimal condition compared with the experimental data. In small flow rata condition, three peaks and three second peaks occur on the meridional velocity at axial flow impeller outlet in the circumferential direction. As the flow rate increases, the number of the velocity peak at impeller outlet is same to the guide

  6. Identification of tower-wake distortions using sonic anemometer and lidar measurements

    Science.gov (United States)

    McCaffrey, Katherine; Quelet, Paul T.; Choukulkar, Aditya; Wilczak, James M.; Wolfe, Daniel E.; Oncley, Steven P.; Brewer, W. Alan; Debnath, Mithu; Ashton, Ryan; Valerio Iungo, G.; Lundquist, Julie K.

    2017-02-01

    The eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) field campaign took place in March through May 2015 at the Boulder Atmospheric Observatory, utilizing its 300 m meteorological tower, instrumented with two sonic anemometers mounted on opposite sides of the tower at six heights. This allowed for at least one sonic anemometer at each level to be upstream of the tower at all times and for identification of the times when a sonic anemometer is in the wake of the tower frame. Other instrumentation, including profiling and scanning lidars aided in the identification of the tower wake. Here we compare pairs of sonic anemometers at the same heights to identify the range of directions that are affected by the tower for each of the opposing booms. The mean velocity and turbulent kinetic energy are used to quantify the wake impact on these first- and second-order wind measurements, showing up to a 50 % reduction in wind speed and an order of magnitude increase in turbulent kinetic energy. Comparisons of wind speeds from profiling and scanning lidars confirmed the extent of the tower wake, with the same reduction in wind speed observed in the tower wake, and a speed-up effect around the wake boundaries. Wind direction differences between pairs of sonic anemometers and between sonic anemometers and lidars can also be significant, as the flow is deflected by the tower structure. Comparisons of lengths of averaging intervals showed a decrease in wind speed deficit with longer averages, but the flow deflection remains constant over longer averages. Furthermore, asymmetry exists in the tower effects due to the geometry and placement of the booms on the triangular tower. An analysis of the percentage of observations in the wake that must be removed from 2 min mean wind speed and 20 min turbulent values showed that removing even small portions of the time interval due to wakes impacts these two quantities. However, a vast majority of intervals have no

  7. Identification of tower-wake distortions using sonic anemometer and lidar measurements

    Energy Technology Data Exchange (ETDEWEB)

    McCaffrey, Katherine; Quelet, Paul T.; Choukulkar, Aditya; Wilczak, James M.; Wolfe, Daniel E.; Oncley, Steven P.; Brewer, W. Alan; Debnath, Mithu; Ashton, Ryan; Iungo, G. Valerio; Lundquist, Julie K.

    2017-02-02

    The eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) field campaign took place in March through May 2015 at the Boulder Atmospheric Observatory, utilizing its 300 m meteorological tower, instrumented with two sonic anemometers mounted on opposite sides of the tower at six heights. This allowed for at least one sonic anemometer at each level to be upstream of the tower at all times and for identification of the times when a sonic anemometer is in the wake of the tower frame. Other instrumentation, including profiling and scanning lidars aided in the identification of the tower wake. Here we compare pairs of sonic anemometers at the same heights to identify the range of directions that are affected by the tower for each of the opposing booms. The mean velocity and turbulent kinetic energy are used to quantify the wake impact on these first- and second-order wind measurements, showing up to a 50 % reduction in wind speed and an order of magnitude increase in turbulent kinetic energy. Comparisons of wind speeds from profiling and scanning lidars confirmed the extent of the tower wake, with the same reduction in wind speed observed in the tower wake, and a speed-up effect around the wake boundaries. Wind direction differences between pairs of sonic anemometers and between sonic anemometers and lidars can also be significant, as the flow is deflected by the tower structure. Comparisons of lengths of averaging intervals showed a decrease in wind speed deficit with longer averages, but the flow deflection remains constant over longer averages. Furthermore, asymmetry exists in the tower effects due to the geometry and placement of the booms on the triangular tower. An analysis of the percentage of observations in the wake that must be removed from 2 min mean wind speed and 20 min turbulent values showed that removing even small portions of the time interval due to wakes impacts these two quantities. However, a vast majority of intervals have no

  8. Wake up and Die

    DEFF Research Database (Denmark)

    Schubart, Rikke

    2013-01-01

    Anmeldelse af den colombianske gyserfilm Volver a morir (eng. titel Wake Up and Die) af Miguel Urrutia......Anmeldelse af den colombianske gyserfilm Volver a morir (eng. titel Wake Up and Die) af Miguel Urrutia...

  9. Empirical modeling of single-wake advection and expansion using full-scale pulsed lidar-based measurements

    DEFF Research Database (Denmark)

    Machefaux, Ewan; Larsen, Gunner Chr.; Troldborg, Niels;

    2015-01-01

    fairly well in the far wake but lacks accuracy in the outer region of the near wake. An empirical relationship, relating maximum wake induction and wake advection velocity, is derived and linked to the characteristics of a spherical vortex structure. Furthermore, a new empirical model for single-wake......In the present paper, single-wake dynamics have been studied both experimentally and numerically. The use of pulsed lidar measurements allows for validation of basic dynamic wake meandering modeling assumptions. Wake center tracking is used to estimate the wake advection velocity experimentally...... and to obtain an estimate of the wake expansion in a fixed frame of reference. A comparison shows good agreement between the measured average expansion and the Computational Fluid Dynamics (CFD) large eddy simulation–actuator line computations. Frandsen’s expansion model seems to predict the wake expansion...

  10. Performance and wake predictions of HAWTs in wind farms

    Energy Technology Data Exchange (ETDEWEB)

    Leclerc, C.; Masson, C.; Paraschivoiu, I. [Ecole Polytechnique, Montreal (Canada)

    1997-12-31

    The present contribution proposes and describes a promising way towards performance prediction of an arbitrary array of turbines. It is based on the solution of the time-averaged, steady-state, incompressible Navier-Stokes equations with an appropriate turbulence closure model. The turbines are represented by distributions of momentum sources in the Navier-Stokes equations. In this paper, the applicability and viability of the proposed methodology is demonstrated using an axisymmetric implementation. The k-{epsilon} model has been chosen for the closure of the time-averaged, turbulent flow equations and the properties of the incident flow correspond to those of a neutral atmospheric boundary layer. The proposed mathematical model is solved using a Control-Volume Finite Element Method (CVFEM). Detailed results have been obtained using the proposed method for an isolated wind turbine and for two turbines one behind another. In the case of an isolated turbine, accurate wake velocity deficit predictions are obtained and an increase in power due to atmospheric turbulence is found in agreement with measurements. In the case of two turbines, the proposed methodology provides an appropriate modelling of the wind-turbine wake and a realistic prediction of the performance degradation of the downstream turbine.

  11. Histamine in the regulation of wakefulness.

    Science.gov (United States)

    Thakkar, Mahesh M

    2011-02-01

    The histaminergic system is exclusively localized within the posterior hypothalamus with projection to almost all the major regions of the central nervous system. Strong and consistent evidence exist to suggest that histamine, acting via H₁ and/or H₃ receptor has a pivotal role in the regulation of sleep-wakefulness. Administration of histamine or H₁ receptor agonists induces wakefulness, whereas administration of H₁ receptor antagonists promotes sleep. The H₃ receptor functions as an auto-receptor and regulates the synthesis and release of histamine. Activation of H₃ receptor reduces histamine release and promotes sleep. Conversely, blockade of H₃ receptor promotes wakefulness. Histamine release in the hypothalamus and other target regions is highest during wakefulness. The histaminergic neurons display maximal activity during the state of high vigilance, and cease their activity during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The cerebrospinal levels of histamine are reduced in diseased states where hypersomnolence is a major symptom. The histamine deficient L-histidine decarboxylase knockout (HDC KO) mice display sleep fragmentation and increased REM sleep during the light period along with profound wakefulness deficit at dark onset, and in novel environment. Similar results have been obtained when histamine neurons are lesioned. These studies strongly implicate the histaminergic neurons of the TMN to play a critical role in the maintenance of high vigilance state during wakefulness.

  12. Light detection and ranging measurements of wake dynamics Part I: One-dimensional Scanning

    DEFF Research Database (Denmark)

    Bingöl, Ferhat; Mann, Jakob; Larsen, Gunner Chr.

    2010-01-01

    at the same time widens gradually, primarily because of mixing caused by small-scale atmospheric eddies. In this first paper, we focus on our new measurement technique, and test if the wake meandering follows the wind direction fluctuations, i.e. if it is advected passively in the lateral direction......The vast majority of wind turbines are today erected in wind farms. As a consequence, wake-generated loads are becoming more and more important. In this first of two parts, we present a new experimental technique to measure the instantaneous wake deficit directly, thus allowing for quantification...... of the wake meandering, as well as the instantaneous wake expansion expressed in a meandering frame of reference. The experiment was conducted primarily to test the simple hypothesis that the wake deficit is advected passively by the larger-than-rotor-size eddies in the atmospheric flow, and that the wake...

  13. Comparison of Engineering Wake Models with CFD Simulations

    DEFF Research Database (Denmark)

    Andersen, Søren Juhl; Sørensen, Jens Nørkær; Ivanell, S.;

    2014-01-01

    The engineering wake models by Jensen [1] and Frandsen et al. [2] are assessed for different scenarios simulated using Large Eddy Simulation and the Actuator Line method implemented in the Navier-Stokes equations. The scenarios include the far wake behind a single wind turbine, a long row...... of turbines in an atmospheric boundary layer, idealised cases of an infinitely long row of wind turbines and infinite wind farms with three different spacings. Both models include a wake expansion factor, which is calibrated to fit the simulated wake velocities. The analysis highlights physical deficiencies...

  14. Wakes in Inertial Fusion Plasmas

    Science.gov (United States)

    Ellis, Ian Norman

    Plasma wave wakes, which are the collective oscillatory response near the plasma frequency to the propagation of particles or electromagnetic waves through a plasma, play a critical role in many plasma processes. New results from backwards stimulated Raman scattering (BSRS), in which wakes with phase velocities much less than the speed of light are induced by the beating of counter-propagating light waves, and from electron beam stopping, in which the wakes are produced by the motion of relativistically propagating electrons through the dense plasma, are discussed. Both processes play important roles in Inertial Confinement Fusion (ICF). In BSRS, laser light is scattered backwards out of the plasma, decreasing the energy available to compress the ICF capsule and affecting the symmetry of where the laser energy hits the hohlraum wall in indirect drive ICF. The plasma wave wake can also generate superthermal electrons that can preheat the core and/or the ablator. Electron beam stopping plays a critical role in the Fast Ignition (FI) ICF concept, in which a beam of relativistic electrons is used to heat the target core to ignition temperatures after the compression stage. The beam stopping power determines the effectiveness of the heating process. This dissertation covers new discoveries on the importance of plasma wave wakes in both BSRS and electron beam stopping. In the SRS studies, 1D particle-in-cell (PIC) simulations using OSIRIS are performed, which model a short-duration (˜500/ω0 --1FWHM) counter-propagating scattered light seed pulse in the presence of a constant pump laser with an intensity far below the absolute instability threshold for plasma waves undergoing Landau damping. The seed undergoes linear convective Raman amplification and dominates over the amplification of fluctuations due to particle discreteness. The simulation results are in good agreement with results from a coupled-mode solver when special relativity and the effects of finite size PIC

  15. Wind Farm Layout Optimization through a Crossover-Elitist Evolutionary Algorithm performed over a High Performing Analytical Wake Model

    Science.gov (United States)

    Kirchner-Bossi, Nicolas; Porté-Agel, Fernando

    2017-04-01

    Wind turbine wakes can significantly disrupt the performance of further downstream turbines in a wind farm, thus seriously limiting the overall wind farm power output. Such effect makes the layout design of a wind farm to play a crucial role on the whole performance of the project. An accurate definition of the wake interactions added to a computationally compromised layout optimization strategy can result in an efficient resource when addressing the problem. This work presents a novel soft-computing approach to optimize the wind farm layout by minimizing the overall wake effects that the installed turbines exert on one another. An evolutionary algorithm with an elitist sub-optimization crossover routine and an unconstrained (continuous) turbine positioning set up is developed and tested over an 80-turbine offshore wind farm over the North Sea off Denmark (Horns Rev I). Within every generation of the evolution, the wind power output (cost function) is computed through a recently developed and validated analytical wake model with a Gaussian profile velocity deficit [1], which has shown to outperform the traditionally employed wake models through different LES simulations and wind tunnel experiments. Two schemes with slightly different perimeter constraint conditions (full or partial) are tested. Results show, compared to the baseline, gridded layout, a wind power output increase between 5.5% and 7.7%. In addition, it is observed that the electric cable length at the facilities is reduced by up to 21%. [1] Bastankhah, Majid, and Fernando Porté-Agel. "A new analytical model for wind-turbine wakes." Renewable Energy 70 (2014): 116-123.

  16. Wind Turbines Wake Aerodynamics

    DEFF Research Database (Denmark)

    Vermeer, L.; Sørensen, Jens Nørkær; Crespo, A.

    2003-01-01

    The aerodynamics of horizontal axis wind turbine wakes is studied. The contents is directed towards the physics of power extraction by wind turbines and reviews both the near and the far wake region. For the near wake, the survey is restricted to uniform, steady and parallel flow conditions......, thereby excluding wind shear, wind speed and rotor setting changes and yawed conditions. The emphasis is put on measurements in controlled conditions.For the far wake, the survey focusses on both single turbines and wind farm effects, and the experimental and numerical work are reviewed; the main interest...... is to study how the far wake decays downstream, in order to estimate the effect produced in downstream turbines.The article is further restricted to horizontal axis wind turbines and excludes all other types of turbines....

  17. Wind Turbines Wake Aerodynamics

    DEFF Research Database (Denmark)

    Vermeer, L.; Sørensen, Jens Nørkær; Crespo, A.

    2003-01-01

    The aerodynamics of horizontal axis wind turbine wakes is studied. The contents is directed towards the physics of power extraction by wind turbines and reviews both the near and the far wake region. For the near wake, the survey is restricted to uniform, steady and parallel flow conditions......, thereby excluding wind shear, wind speed and rotor setting changes and yawed conditions. The emphasis is put on measurements in controlled conditions.For the far wake, the survey focusses on both single turbines and wind farm effects, and the experimental and numerical work are reviewed; the main interest...... is to study how the far wake decays downstream, in order to estimate the effect produced in downstream turbines.The article is further restricted to horizontal axis wind turbines and excludes all other types of turbines....

  18. Wake modeling and simulation

    DEFF Research Database (Denmark)

    Larsen, Gunner Chr.; Madsen Aagaard, Helge; Larsen, Torben J.;

    , have the potential to include also mutual wake interaction phenomenons. The basic conjecture behind the dynamic wake meandering (DWM) model is that wake transportation in the atmospheric boundary layer is driven by the large scale lateral- and vertical turbulence components. Based on this conjecture...... and trailed vorticity, has been approached by a simple semi-empirical model essentially based on an eddy viscosity philosophy. Contrary to previous attempts to model wake loading, the DWM approach opens for a unifying description in the sense that turbine power- and load aspects can be treated simultaneously...... methodology has been implemented in the aeroelastic code HAWC2, and example simulations of wake situations, from the small Tjæreborg wind farm, have been performed showing satisfactory agreement between predictions and measurements...

  19. 3D Lagrangian VPM: simulations of the near-wake of an actuator disc and horizontal axis wind turbine

    Science.gov (United States)

    Berdowski, T.; Ferreira, C.; Walther, J.

    2016-09-01

    The application of a 3-dimensional Lagrangian vortex particle method has been assessed for modelling the near-wake of an axisymmetrical actuator disc and 3-bladed horizontal axis wind turbine with prescribed circulation from the MEXICO (Model EXperiments In COntrolled conditions) experiment. The method was developed in the framework of the open- source Parallel Particle-Mesh library for handling the efficient data-parallelism on a CPU (Central Processing Unit) cluster, and utilized a O(N log N)-type fast multipole method for computational acceleration. Simulations with the actuator disc resulted in a wake expansion, velocity deficit profile, and induction factor that showed a close agreement with theoretical, numerical, and experimental results from literature. Also the shear layer expansion was present; the Kelvin-Helmholtz instability in the shear layer was triggered due to the round-off limitations of a numerical method, but this instability was delayed to beyond 1 diameter downstream due to the particle smoothing. Simulations with the 3-bladed turbine demonstrated that a purely 3-dimensional flow representation is challenging to model with particles. The manifestation of local complex flow structures of highly stretched vortices made the simulation unstable, but this was successfully counteracted by the application of a particle strength exchange scheme. The axial and radial velocity profile over the near wake have been compared to that of the original MEXICO experiment, which showed close agreement between results.

  20. A prescribed wake rotor inflow and flow field prediction analysis, user's manual and technical approach

    Science.gov (United States)

    Egolf, T. A.; Landgrebe, A. J.

    1982-01-01

    A user's manual is provided which includes the technical approach for the Prescribed Wake Rotor Inflow and Flow Field Prediction Analysis. The analysis is used to provide the rotor wake induced velocities at the rotor blades for use in blade airloads and response analyses and to provide induced velocities at arbitrary field points such as at a tail surface. This analysis calculates the distribution of rotor wake induced velocities based on a prescribed wake model. Section operating conditions are prescribed from blade motion and controls determined by a separate blade response analysis. The analysis represents each blade by a segmented lifting line, and the rotor wake by discrete segmented trailing vortex filaments. Blade loading and circulation distributions are calculated based on blade element strip theory including the local induced velocity predicted by the numerical integration of the Biot-Savart Law applied to the vortex wake model.

  1. Inlet Guide Vane Wakes Including Rotor Effects

    Science.gov (United States)

    Johnston, R. T.; Fleeter, S.

    2001-02-01

    Fundamental experiments are described directed at the investigation of forcing functions generated by an inlet guide vane (IGV) row, including interactions with the downstream rotor, for application to turbomachine forced response design systems. The experiments are performed in a high-speed research fan facility comprised of an IGV row upstream of a rotor. IGV-rotor axial spacing is variable, with the IGV row able to be indexed circumferentially, thereby allowing measurements to be made across several IGV wakes. With an IGV relative Mach number of 0.29, measurements include the IGV wake pressure and velocity fields for three IGV-rotor axial spacings. The decay characteristics of the IGV wakes are compared to the Majjigi and Gliebe empirical correlations. After Fourier decomposition, a vortical-potential gust splitting analysis is implemented to determine the vortical and potential harmonic wake gust forcing functions both upstream and downstream of the rotor. Higher harmonics of the vortical gust component of the IGV wakes are found to decay at a uniform rate due to viscous diffusion.

  2. The wake of hovering flight in bats

    Science.gov (United States)

    Håkansson, Jonas; Hedenström, Anders; Winter, York; Johansson, L. Christoffer

    2015-01-01

    Hovering means stationary flight at zero net forward speed, which can be achieved by animals through muscle powered flapping flight. Small bats capable of hovering typically do so with a downstroke in an inclined stroke plane, and with an aerodynamically active outer wing during the upstroke. The magnitude and time history of aerodynamic forces should be reflected by vorticity shed into the wake. We thus expect hovering bats to generate a characteristic wake, but this has until now never been studied. Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover at a feeder and using time-resolved stereoscopic particle image velocimetry in conjunction with high-speed kinematic analysis we show that hovering nectar-feeding bats produce a series of bilateral stacked vortex loops. Vortex visualizations suggest that the downstroke produces the majority of the weight support, but that the upstroke contributes positively to the lift production. However, the relative contributions from downstroke and upstroke could not be determined on the basis of the wake, because wake elements from down- and upstroke mix and interact. We also use a modified actuator disc model to estimate lift force, power and flap efficiency. Based on our quantitative wake-induced velocities, the model accounts for weight support well (108%). Estimates of aerodynamic efficiency suggest hovering flight is less efficient than forward flapping flight, while the overall energy conversion efficiency (mechanical power output/metabolic power) was estimated at 13%. PMID:26179990

  3. HARP PRIA- Wake

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — This HARP was first deployed off of Wake Atoll in 2010. It has been recovered and redeployed multiple times (see time frames for information).

  4. Detailed analysis of offshore wakes based on two years data of alpha ventus and comparison with CFD simulations

    Energy Technology Data Exchange (ETDEWEB)

    Westerhellweg, A.; Canadillas, B.; Kinder, F.; Neumann, T. [DEWI, Wilhelmshaven (Germany)

    2013-02-15

    Wind conditions and power output were evaluated for wake effects in the offshore wind farm 'alpha ventus' and compared to CFD simulations. The evaluation of the wind conditions comprises wind speed reduction and turbulence increase in the wake. Power data were assessed for the power deficit in the wake of a single wind turbine and along a row of wind turbines and for the farm efficiency. The wake of a single wind turbine is described by the maximum power deficit and the expansion width of the wake. The wake effects were evaluated with special emphasis on the influence of thermal stability on the wake effects. The atmospheric stability was assessed from temperature difference of air and water and its impact on the power output was quantified. (orig.)

  5. Experimental study of wind turbine wakes in a convective boundary layer

    Science.gov (United States)

    Zhang, W.; Markfort, C. D.; Porte-Agel, F.

    2010-12-01

    Understanding the interaction of atmospheric boundary layer (ABL) flows with wind turbines is important for optimizing the design of wind farms (maximizing energy output and mitigating fatigue loads) and improving the parameterization of wind farms in weather and climate models. Field observations have suggested that atmospheric stability affects ABL flow and its interaction with wind turbines, which in turn affects wind farm performance. However, the fluid mechanics involved has not been fully understood, highlighting the need of acquiring high quality data in clearly defined conditions. Well-controlled wind tunnel experiments of the wake of wind turbines immersed in thermally stratified or convective boundary layers are very limited. In this study, we investigate the wake structure of a miniature three-blade wind turbine model placed in a convective boundary layer (CBL) in the Saint Anthony Falls Laboratory wind tunnel. The objectives of this study are: 1) to understand how the CBL flow affects the wake behind a wind turbine in terms of tip vortices distribution, mean velocity deficit, turbulence intensities, Reynolds shear stress and heat flux modification; 2) to provide reliable data sets for validating and developing new parameterizations of turbulent fluxes and turbine-induced forces in numerical models, such as large-eddy simulation (LES). The CBL was generated by cooling the free stream air flow to 13 οC and heating up the test section floor to 80 οC. The free stream speed was set at 2.5 m/s, resulting in the Obukhov stability of δ/L=-3.15 and the bulk Richardson number about -0.16. The wake of a wind turbine model, whose height is about 1/3 the boundary layer thickness, was systematically studied using Stereo Particle Image Velocimetry (SPIV) and a hot-wire/cold-wire anemometer. Results revealed the top tip vortices (in Fig.1), noticeably degraded velocity deficit and significantly enhanced turbulence. Turbulent momentum and heat fluxes were also

  6. PREFACE: Wake Conference 2015

    Science.gov (United States)

    Barney, Andrew; Nørkær Sørensen, Jens; Ivanell, Stefan

    2015-06-01

    The 44 papers in this volume constitute the proceedings of the 2015 Wake Conference, held in Visby on the island of Gotland in Sweden. It is the fourth time this conference has been held. The Wake Conference series started in Visby, where it was held in 2009 and 2011. In 2013 it took place in Copenhagen where it was combined with the International Conference on Offshore Wind Energy and Ocean Energy. In 2015 it is back where it started in Visby, where it takes place at Uppsala University Campus Gotland, June 9th-11th. The global yearly production of electrical energy by wind turbines has grown tremendously in the past decade and it now comprises more than 3% of the global electrical power consumption. Today the wind power industry has a global annual turnover of more than 50 billion USD and an annual average growth rate of more than 20%. State-of-the-art wind turbines have rotor diameters of up to 150 m and 8 MW installed capacity. These turbines are often placed in large wind farms that have a total production capacity corresponding to that of a nuclear power plant. In order to make a substantial impact on one of the most significant challenges of our time, global warming, the industry's growth has to continue for a decade or two yet. This in turn requires research into the physics of wind turbine wakes and wind farms. Modern wind turbines are today clustered in wind farms in which the turbines are fully or partially influenced by the wake of upstream turbines. As a consequence, the wake behind the wind turbines has a lower mean wind speed and an increased turbulence level, as compared to the undisturbed flow outside the farm. Hence, wake interaction results in decreased total production of power, caused by lower kinetic energy in the wind, and an increase in the turbulence intensity. Therefore, understanding the physical nature of the vortices and their dynamics in the wake of a turbine is important for the optimal design of a wind farm. This conference is aimed

  7. Mach-like capillary-gravity wakes.

    Science.gov (United States)

    Moisy, Frédéric; Rabaud, Marc

    2014-08-01

    We determine experimentally the angle α of maximum wave amplitude in the far-field wake behind a vertical surface-piercing cylinder translated at constant velocity U for Bond numbers Bo(D)=D/λ(c) ranging between 0.1 and 4.2, where D is the cylinder diameter and λ(c) the capillary length. In all cases the wake angle is found to follow a Mach-like law at large velocity, α∼U(-1), but with different prefactors depending on the value of Bo(D). For small Bo(D) (large capillary effects), the wake angle approximately follows the law α≃c(g,min)/U, where c(g,min) is the minimum group velocity of capillary-gravity waves. For larger Bo(D) (weak capillary effects), we recover a law α∼√[gD]/U similar to that found for ship wakes at large velocity [Rabaud and Moisy, Phys. Rev. Lett. 110, 214503 (2013)]. Using the general property of dispersive waves that the characteristic wavelength of the wave packet emitted by a disturbance is of order of the disturbance size, we propose a simple model that describes the transition between these two Mach-like regimes as the Bond number is varied. We show that the new capillary law α≃c(g,min)/U originates from the presence of a capillary cusp angle (distinct from the usual gravity cusp angle), along which the energy radiated by the disturbance accumulates for Bond numbers of order of unity. This model, complemented by numerical simulations of the surface elevation induced by a moving Gaussian pressure disturbance, is in qualitative agreement with experimental measurements.

  8. NEAR WAKE OF A MODEL HORIZONTAL-AXIS WIND TURBINE

    Institute of Scientific and Technical Information of China (English)

    HU Dan-mei; DU Zhao-hui

    2009-01-01

    An experimental investigation on the properties of the near wake behind the rotor of a Horizontal-Axis Wind Turbine (HAWT) was carried out at model scale. Measurements were made with a stationary slanted hot-wire anemometer using the technique of phase-locked averaging. The primary aim is to study the formation and development of the three-dimensional wake. Five axial locations were chosen within four chord lengths of the blades over a range of tip speed ratios. The results show that during the downstream development of the wake, the wake centre traces a helical curve with its rotation direction opposite to that of the rotor. The distribution of mean velocity behind the HAWT rotor reveals an expansion and a decay of the three-dimensional wake. The shapes of the mean velocity distribution are similar along the blades span at the same downstream axial location. It is shown that the turbulence levels in the wake are higher than those in the non-wake region. The circumferential component and the radial component of the turbulence intensity are higher than the axial component. Our study offers some food of thought for better understanding of the physical features of the flow field as well as the performance of HAWT.

  9. Stable Propagating Waves and Wake Fields in Relativistic Electromagnetic Plasma

    Institute of Scientific and Technical Information of China (English)

    DUAN Yi-Shi; XIE Bai-Song; TIAN Miao; YIN Xin-Tao; ZHANG Xin-Hui

    2008-01-01

    Stable propagating waves and wake fields in relativistic electromagnetic plasma are investigated. The incident electromagnetic field has a finite initial constant amplitude meanwhile the longitudinal momentum of electrons is taken into account in the problem. It is found that in the moving frame with transverse wave group velocity the stable propagating transverse electromagnetic waves and longitudinal plasma wake fields can exist in the appropriate regime of plasma.

  10. A SIMILARITY METHOD FOR LAMINAR WAKE OF POWER-LAW FLUID FLOW AROUND A FLAT PLATE

    Institute of Scientific and Technical Information of China (English)

    Liu Cun-fang; Wang Mei-xia

    2003-01-01

    Based on the characteristic equation for power-law fluid and the Prandtl boundary layer equation, using the similarity method similar to that of Newtonian fluids, two similarity variables were given and a normal differential equation was derived for the laminar wake of power-law fluid flow produced by a flat plate. And numerical results were obtained. The results show that the power-law index n has evident influence on the velocity distribution in the wake. In the wake, velocity gradient is larger, and the wake is narrower for larger n.

  11. Coupled wake boundary layer model of wind-farms

    CERN Document Server

    Stevens, Richard J A M; Meneveau, Charles

    2014-01-01

    We present and test a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall wind-farm boundary layer structure. The wake expansion/superposition model captures the effect of turbine positioning, while the top-down portion adds the interaction between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the model requires specification of a parameter that is not known a-priori. For the wake model the wake expansion coefficient is required, while the top-down model requires an effective span-wise turbine spacing within which the model's momentum balance is relevant. The wake expansion coefficient is obtained by matching the predicted mean velocity at the turbine from both approaches, while the effective span-wise turbine spacing depends on turbine positioning and thus can be determined from the wake expansion...

  12. Data-driven RANS for prediction of wind turbine wakes

    Science.gov (United States)

    Iungo, Giacomo Valerio; Viola, Francesco; Ciri, Umberto; Camarri, Simone; Rotea, Mario A.; Leonardi, Stefano

    2015-11-01

    Wind turbine wakes are highly turbulent flows resulting from the interaction between the atmospheric boundary layer and wake vorticity structures. Measurement technologies, such as wind LiDARs, are currently available to perform velocity measurements in a set of locations of wakes past utility-scale wind turbines; however, computational methods are still needed to predict wake downstream evolution. In this work, a low-computational cost and accurate algorithm is proposed for prediction of the spatial evolution of wind turbine wakes. Reynolds-averaged Navier Stokes equations (RANS) are formulated in cylindrical coordinates and simplified by using a boundary layer type approximation. Turbulence effects are taken into account with a mixing length model calibrated on the available observations. In this study, observations of wind turbine wakes consist in LES data of wakes produced by a wind turbine operating with different incoming wind and loading conditions. The mixing length calibrated on the LES data is constant in the near wake and only affected by the incoming turbulence, whereas further downstream it increases roughly linearly with the downstream position and with increased slope for increasing rotational speed of the turbine.

  13. Wind turbine wake meandering at the laboratory and field scales

    Science.gov (United States)

    Heisel, Michael; Musa, Mirko; Hong, Jiarong; Guala, Michele

    2016-11-01

    Flow measurements were collected in the wake of the utility-scale (2.5MW) Eolos wind turbine using a ground-based light detection and ranging (LiDAR) wind profiler to identify the characteristics of wake meandering at the field scale. The investigation seeks to establish the influence of scale and atmospheric turbulence on wake meandering, which has been observed to leave a strong spectral signature on laboratory measurements in wind tunnel and channel flows. The experimental data include multiple test periods at various downstream distances within the turbine wake. Inflow conditions were assessed using a meteorological tower equipped with sonic anemometers. Additionally, an experiment was conducted in the Saint Anthony Falls Laboratory atmospheric boundary layer wind tunnel to provide a direct comparison for the utility-scale results and to reaffirm the findings of previous laboratory-scale investigations. Estimates of the wake and inflow one-dimensional velocity spectra were compared to determine whether wake meandering characteristics are present at both scales. An empirical correction to the velocity spectra of the LiDAR and a few options to extract a more local velocity signal are discussed to compensate for the inherent limitations of LiDAR in capturing turbulent fluctuations.

  14. Wind turbine wake in atmospheric turbulence

    Energy Technology Data Exchange (ETDEWEB)

    Rethore, P.-E.

    2009-10-15

    This thesis describes the different steps needed to design a steady-state computational fluid dynamics (CFD) wind farm wake model. The ultimate goal of the project was to design a tool that could analyze and extrapolate systematically wind farm measurements to generate wind maps in order to calibrate faster and simpler engineering wind farm wake models. The most attractive solution was the actuator disc method with the steady state k-epsilon turbulence model. The first step to design such a tool is the treatment of the forces. This thesis presents a computationally inexpensive method to apply discrete body forces into the finite-volume flow solver with collocated variable treatment (EllipSys), which avoids the pressure-velocity decoupling issue. The second step is to distribute the body forces in the computational domain accordingly to rotor loading. This thesis presents a generic flexible method that associates any kind of shapes with the computational domain discretization. The special case of the actuator disc performs remarkably well in comparison with Conway's heavily loaded actuator disc analytical solution and a CFD full rotor computation, even with a coarse discretization. The third step is to model the atmospheric turbulence. The standard k-epsilon model is found to be unable to model at the same time the atmospheric turbulence and the actuator disc wake and performs badly in comparison with single wind turbine wake measurements. A comparison with a Large Eddy Simulation (LES) shows that the problem mainly comes from the assumptions of the eddy-viscosity concept, which are deeply invalidated in the wind turbine wake region. Different models that intent to correct the k-epsilon model's issues are investigated, of which none of them is found to be adequate. The mixing of the wake in the atmosphere is a deeply non-local phenomenon that is not handled correctly by an eddy-viscosity model such as k-epsilon. (author)

  15. Multiple Turbine Wakes

    OpenAIRE

    Machefaux, Ewan; Larsen, Gunner Chr.; Mann, Jakob

    2015-01-01

    Formålet med nærværende forskningsprojekt har dels været at studere wakes efter enkeltstående vindmøller, dels at studere flere interagerende wakes - baseret på så vel fuldskala LiDAR målinger som på detaljerede CFD simuleringer. Indledningsvist er dynamikken af enkelt-wakes studeret gennem analyser af fuldskala (nacelle-baserede) pulsede LiDAR målinger udført på en 500kW vindmølle placeret på testområdet ved DTU’s Risø campus. Som en del af den eksperimentelle analyse er der foretaget en val...

  16. Multiple Turbine Wakes

    DEFF Research Database (Denmark)

    Machefaux, Ewan; Mann, Jakob

    as part of the experimental analysis. An inflow wind sector of 30° is selected based on both a wind resource and a lidar data assessment. Wake measurements are averaged within a mean wind speed bin of 1 m/s and classified according to atmospheric stability using 3 different approaches: the Obukhov length...... wind are adapted to the thermal stratification using a newly developed spectral tensor, which includes buoyancy effects. Discrepancies are discussed as basis for future model development and improvement. Moreover, the impact of atmospheric stability and terrain on large/small scale wake flow...

  17. Wake Studies of Ornithopters

    Science.gov (United States)

    Juarez, Alfredo; Harlow, Jacob; Allen, James; Ferreira de Sousa, Paulo

    2006-11-01

    This paper details experiments using a mechanical ornithopter flying in a low speed wind tunnel. Experiments were conducted for a Strouhal number of 0.3 and Reynolds number of 2300, Particle Image Velocimetry (PIV) and flow visualization was used to develop quantitative and qualitative information about the nature of the wake. The data shows that the wake is made of a series of discrete vortex rings. The impulse of these rings has been estimated with PIV data and the results correlate well with the lift required to sustain the ornithopter in flight.

  18. Field investigation of a wake structure downwind of a VAWT in a windfarm array

    Energy Technology Data Exchange (ETDEWEB)

    Liu, H.T.; Buck, J.W.; Germain, A.C.; Hinchee, M.E.; Solt, T.S.; LeRoy, G.M.; Srnsky, R.A.

    1987-10-01

    The effects of upwind turbine wakes on the performance of a FloWind 17-m VAWT were investigated through a series of field experiments conducted at the FloWind windfarm on Cameron Ridge, Tehachapi, California. The field experiment was conducted within a VAWT array consisting of more than nine VAWTs with separations 3D crosswised by 8D downwind (where D is the turbine diameter) in a staggered configuration. The array is the upwind three rows of VAWTS in a total of six rows that are on top of the Cameron Ridge plateau. The terrain features in the vicinity are reasonably regular, with an upslope of 7 deg on the average; however, several local irregularities are present. The annual hourly averaged wind speed exceeds 8 m/s at the site. The wind field and the power-outputs of nine turbines within the array were measured with wind sensors and power transducers. Nine Gill propeller and 18 Maximum cup anemometers and one direction sensor were mounted on portable and stack-up towers installed upwind and within the turbine array. From the field measurements, the velocity and power/energy deficits were derived under various turbine on/off configurations. Much information was provided to characterize the structure of VAWT wakes and to assess their effects on the performance of downwind turbines. Recommendations are made for optimizing windfarm design and operations as well as for wind energy management.

  19. Interaction of two high Reynolds number axisymmetric turbulent wakes

    Science.gov (United States)

    Obligado, M.; Klein, S.; Vassilicos, J. C.

    2015-11-01

    With the recent discovery of non-equilibrium high Reynolds number scalings in the wake of axisymmetric plates (Nedic et al., PRL, 2013), it has become of importance to develop an experimental technique that permits to easily discriminate between different wake scalings. We propose an experimental setup that tests the presence of non-equilibrium turbulence using the streamwise variation of velocity fluctuations between two bluff bodies facing a flow. We have studied two different sets of plates (one with regular and another with irregular peripheries) with Hot-Wire Anemometry in a wind tunnel. By acquiring streamwise profiles for different plate separations and identifying the wake interaction length for each separation it is possible to estimate the streamwise evolution of the single wake width. From this evolution it is also possible to deduce the turbulence dissipation scalings. This work generalizes previous studies on the interaction of plane wakes (see Gomes-Fernandes et al., JFM, 2012) to include axisymmetric wakes. We find that the wake interaction length proposed in this cited work and a constant anisotropy assumption can be used to collapse the streamwise developments of the first three moments.

  20. Experiments on the global instability of confined axisymmetric dense wakes.

    Science.gov (United States)

    Li, Larry; Juniper, Matthew

    2007-11-01

    Recent theoretical studies [M. Juniper, J. Fluid Mech. 565, 171-195 (2006); M. Juniper and S. Candel, J. Fluid Mech. 482, 257-269 (2003)] predict that confinement increases the hydrodynamic instability of wakes by causing the transition from convective to absolute instability to occur at lower values of shear. Experimental evidence supporting this prediction is presented here for a confined, axisymmetric wake at density ratios, S ≡ ρ1 /ρ2> 1 (i.e. dense wake). The wake was produced by a pair of convergent nozzles mounted concentrically, one within the other, in a low-turbulence wind tunnel facility. Variations in S were achieved by employing two high density gases (S = 1.53 and 5.11) in the inner flow with air in the outer flow. For a fixed S, there existed a critical value of shear above which dominant peaks appeared abruptly in the near-wake velocity spectra, as quantified by hot-wire anemometry. Corresponding high-speed video sequences revealed large-scale, sinuous wake motions. Results on the confined wake's response to externally-applied, acoustic forcing are also presented. The presence of discrete spectral peaks and coordinated instability oscillations suggests the emergence of a self-sustained, global mode.

  1. Numerical Simulation on Ship Bubbly Wake

    Institute of Scientific and Technical Information of China (English)

    Huiping Fu; Pengcheng Wan

    2011-01-01

    Based on a volume of fluid two-phase model imbedded in the general computational fluid dynamics code FLUENT6.3.26,the viscous flow with free surface around a model-scaled KRISO container ship(KCS)was first numerically simulated.Then with a rigid-lid-free-surface method,the underwater flow field was computed based on the mixture multiphase model to simulate the bubbly wake around the KCS hull.The realizable k-ε two-equation turbulence model and Reynolds stress model were used to analyze the effects of turbulence model on the ship bubbly wake.The air entrainment model,which is relative to the normal velocity gradient of the free surface,and the solving method were verified by the qualitatively reasonable computed results.

  2. Properties of wind turbine wakes under various atmospheric stability conditions

    Science.gov (United States)

    Xie, Shengbai; Archer, Cristina

    2015-11-01

    Large-eddy simulations (LES) are performed to study the properties of wind turbine wakes under various atmospheric stability conditions. The Wind Turbine and Turbulence Simulator (WiTTS), a 4th-order finite-difference LES code is used for stable, neutral, and unstable conditions. The Coriolis forcing is also considered. Three cases are studied: isolated turbine, finite-size turbine array, and infinite wind farm. The results show strong correlations with stability. For the stable condition, the power extraction by an isolated turbine is highest, but the wake is also longest, thus the relative performance inside the array is lowest. In contrast, although the single-turbine power extraction is low for the unstable condition, the performance of downstream turbines is improved due to faster wake recovery. The wake shape is distorted by the stability-related wind veering. Therefore, the self-similar Gaussian wake deficit is not accurate. Here, a new wake model is proposed for correction. The infinite wind-farm case shows that the temperature near the ground is warmed by about 1 K for the stable condition, but the influence is almost negligible for the unstable and neutral conditions. For all conditions, the near-ground shear stress is reduced.

  3. Spatio-temporal spectral analysis of a forced cylinder wake

    CERN Document Server

    D'Adamo, Juan; Wesfreid, José Eduardo

    2011-01-01

    The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at $Re=100$. Two-dimensional particle image velocimetry on the mid-plane perpendicular to the axis of cylinder is used to characterize the spatial development of the flow and its stability properties. The lock-in phenomenon that determines the boundaries between regions of the forcing parameter space were the wake is globally unstable or convectively unstable is scrutinized using the experimental data. A novel method based on the analysis of power density spectra of the flow allows us to give a detailed description of the forced wake, shedding light on the energy distribution in the different frequency components and in particular on a cascade-like mechanism evidenced for a high amplitude of the forcing oscillation. In addition, a calculation of the drag from the velocity field is performed, allowing us to relate the resulting force on the body to the wake properties.

  4. NASA AVOSS Fast-Time Wake Prediction Models: User's Guide

    Science.gov (United States)

    Ahmad, Nash'at N.; VanValkenburg, Randal L.; Pruis, Matthew

    2014-01-01

    The National Aeronautics and Space Administration (NASA) is developing and testing fast-time wake transport and decay models to safely enhance the capacity of the National Airspace System (NAS). The fast-time wake models are empirical algorithms used for real-time predictions of wake transport and decay based on aircraft parameters and ambient weather conditions. The aircraft dependent parameters include the initial vortex descent velocity and the vortex pair separation distance. The atmospheric initial conditions include vertical profiles of temperature or potential temperature, eddy dissipation rate, and crosswind. The current distribution includes the latest versions of the APA (3.4) and the TDP (2.1) models. This User's Guide provides detailed information on the model inputs, file formats, and the model output. An example of a model run and a brief description of the Memphis 1995 Wake Vortex Dataset is also provided.

  5. Passive Wake Vortex Control

    Energy Technology Data Exchange (ETDEWEB)

    Ortega, J M

    2001-10-18

    The collapse of the Soviet Union and ending of the Cold War brought about many significant changes in military submarine operations. The enemies that the US Navy faces today and in the future will not likely be superpowers armed with nuclear submarines, but rather smaller, rogue nations employing cheaper diesel/electric submarines with advanced air-independent propulsion systems. Unlike Cold War submarine operations, which occurred in deep-water environments, future submarine conflicts are anticipated to occur in shallow, littoral regions that are complex and noisy. Consequently, non-acoustic signatures will become increasingly important and the submarine stealth technology designed for deep-water operations may not be effective in these environments. One such non-acoustic signature is the surface detection of a submarine's trailing vortex wake. If a submarine runs in a slightly buoyant condition, its diving planes must be inclined at a negative angle of attack to generate sufficient downforce, which keeps the submarine from rising to the surface. As a result, the diving planes produce a pair of counter-rotating trailing vortices that propagate to the water surface. In previous deep-water operations, this was not an issue since the submarines could dive deep enough so that the vortex pair became incoherent before it reached the water surface. However, in shallow, littoral environments, submarines do not have the option of diving deep and, hence, the vortex pair can rise to the surface and leave a distinct signature that might be detectable by synthetic aperture radar. Such detection would jeopardize not only the mission of the submarine, but also the lives of military personnel on board. There has been another attempt to solve this problem and reduce the intensity of trailing vortices in the wakes of military submarines. The research of Quackenbush et al. over the past few years has been directed towards an idea called ''vortex leveraging

  6. First Lunar Wake Passage of ARTEMIS: Discrimination of Wake Effects and Solar Wind Fluctuations by 3D Hybrid Simulations

    Science.gov (United States)

    Wiehle, S.; Plaschke, F.; Motschmann, U.; Glassmeier, K. H.; Auster, H. U.; Angelopoulos, V.; Mueller, J.; Kriegel, H.; Georgescu, E.; Halekas, J.; Sibeck, D. G.; McFadden, J. P.

    2011-01-01

    The spacecraft P1 of the new ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) mission passed the lunar wake for the first time on February 13, 2010. We present magnetic field and plasma data of this event and results of 3D hybrid simulations. As the solar wind magnetic field was highly dynamic during the passage, a simulation with stationary solar wind input cannot distinguish whether distortions were caused by these solar wind variations or by the lunar wake; therefore, a dynamic real-time simulation of the flyby has been performed. The input values of this simulation are taken from NASA OMNI data and adapted to the P1 data, resulting in a good agreement between simulation and measurements. Combined with the stationary simulation showing non-transient lunar wake structures, a separation of solar wind and wake effects is achieved. An anisotropy in the magnitude of the plasma bulk flow velocity caused by a non-vanishing magnetic field component parallel to the solar wind flow and perturbations created by counterstreaming ions in the lunar wake are observed in data and simulations. The simulations help to interpret the data granting us the opportunity to examine the entire lunar plasma environment and, thus, extending the possibilities of measurements alone: A comparison of a simulation cross section to theoretical predictions of MHD wave propagation shows that all three basic MHD modes are present in the lunar wake and that their expansion governs the lunar wake refilling process.

  7. Cylindrical vortex wake model: right cylinder

    DEFF Research Database (Denmark)

    Branlard, Emmanuel; Gaunaa, Mac

    2015-01-01

    to recall results from 1D momentum theory. It is shown that a superposition of concentric cylindrical systems predicts the independence of annuli, which is assumed in blade element theory and stream-tube analyses. A simple example of application for the estimation of the velocity deficit upstream of a wind...

  8. A Control-Oriented Dynamic Model for Wakes in Wind Plants

    NARCIS (Netherlands)

    Gebraad, P.M.O.; Van Wingerden, J.W.

    2014-01-01

    In this paper, we present a novel control-oriented model for predicting wake effects in wind plants, called the FLOw Redirection and Induction Dynamics (FLORIDyn) model. The model predicts the wake locations and the effective flow velocities at each turbine, and the resulting turbine electrical

  9. An Experimental Study of Three-Dimensional Characteristics of Turbulent Wakes of Axial Compressor Rotors

    Institute of Scientific and Technical Information of China (English)

    Hongwei MA; Haokang JIANG

    2005-01-01

    This paper presents an experimental investigation of the characteristics of three-dimensional turbulent wakes of an isolated axial compressor rotor and a single-stage axial compressor rotor. The wakes were measured from hub to tip using a single-slant hot-wire and a four-hole conical high frequency pressure probe. The experiments were made at both design and near stall conditions. Variations of mean velocities, total pressure, static pressure and turbulence stresses in the wakes are shown and interpreted. The experimental data from the isolated compressor rotor wake are compared with that from the single-stage compressor rotor.

  10. Effect of trailing edge shape on the wake and propulsive performance of pitching panels

    Science.gov (United States)

    van Buren, Tyler; Floryan, Daniel; Brunner, Daniel; Senturk, Utku; Smits, Alexander

    2016-11-01

    We present the effects of the trailing edge shape on the wake and propulsive performance of a pitching panel with an aspect ratio of 1. The trailing edges are symmetric chevron shapes with convex and concave orientations of varying degree. Concave trailing edges delay the natural vortex bending and compression of the wake, and the streamwise velocity field contains a single jet-like structure. Conversely, convex trailing edges promote wake compression and produce a wake split into four jets. Deviation from the square trailing edge mostly reduces the thrust and efficiency. Supported by the Office of Naval Research under MURI Grant Number N00014-14-1-0533.

  11. Impact of trailing edge shape on the wake and propulsive performance of pitching panels

    Science.gov (United States)

    Van Buren, T.; Floryan, D.; Brunner, D.; Senturk, U.; Smits, A. J.

    2017-01-01

    The effects of changing the trailing edge shape on the wake and propulsive performance of a pitching rigid panel are examined experimentally. The panel aspect ratio is AR=1 , and the trailing edges are symmetric chevron shapes with convex and concave orientations of varying degree. Concave trailing edges delay the natural vortex bending and compression of the wake, and the mean streamwise velocity field contains a single jet. Conversely, convex trailing edges promote wake compression and produce a quadfurcated wake with four jets. As the trailing edge shape changes from the most concave to the most convex, the thrust and efficiency increase significantly.

  12. An Analytical Model of Wake Deflection Due to Shear Flow

    NARCIS (Netherlands)

    Micallef, D.; Simao Ferreira, C.J.; Sant, T.; Van Bussel, G.J.W.

    2010-01-01

    The main motivation behind this work is to create a purely analytical engineering model for wind turbine wake upward deflection due to shear flow, by developing a closed form solution of the velocity field due to an oblique vortex ring. The effectiveness of the model is evaluated by comparing the re

  13. The k-ε-fP model applied to double wind turbine wakes using different actuator disk force methods

    DEFF Research Database (Denmark)

    Laan, van der, Paul Maarten; Sørensen, Niels N.; Réthoré, Pierre-Elouan

    2015-01-01

    The newly developed k-ε-fP  eddy viscosity model is applied to double wind turbine wake configurations in a neutral atmospheric boundary layer, using a Reynolds-Averaged Navier–Stokes solver. The wind turbines are represented by actuator disks. A proposed variable actuator disk force method...... is employed to estimate the power production of the interacting wind turbines, and the results are compared with two existing methods: a method based on tabulated airfoil data and a method based on the axial induction from 1D momentum theory. The proposed method calculates the correct power, while the other...... two methods overpredict it. The results of the k-ε-fP  eddy viscosity model are also compared with the original k-ε eddy viscosity model and large-eddy simulations. Compared to the large-eddy simulations-predicted velocity and power deficits, the k-ε-fP  is superior to the original k-ε model...

  14. Wake interaction and power production of variable height model wind farms

    DEFF Research Database (Denmark)

    Vested, Malene Hovgaard; Hamilton, N.; Sørensen, Jens Nørkær;

    2014-01-01

    of comparison. It was found that downstream of the exit row wind turbine, the power was increased by 25% in the case of a staggered height configuration. This is partly due to the fact that the taller turbines reach into a flow area with a softened velocity gradient. Another aspect is that the wake downstream......Understanding wake dynamics is an ongoing research topic in wind energy, since wakes have considerable effects on the power production when wind turbines are placed in a wind farm. Wind tunnel experiments have been conducted to study the wake to wake interaction in a model wind farm in tandem...... with measurements of the extracted power. The aim is to investigate how alternating mast height influences the interaction of the wakes and the power production. Via the use of stereo-particle image velocimetry, the flow field was obtained in the first and last rows of the wind turbine array as a basis...

  15. Offshore and onshore wind turbine wake meandering studied in an ABL wind tunnel

    DEFF Research Database (Denmark)

    Barlas, Emre; Buckingham, Sophia; Glabeke, Gertjan;

    2015-01-01

    diameter downstream. The results show an earlier wake recovery for the onshore case. The effect of inflow conditions and the wind turbine’s working conditions on wake meandering was investigated. Wake meandering was detected by hot wire anemometry through a low frequency peak in the turbulent power......Scaled wind turbine models have been installed in the VKI L1-B atmospheric boundary layer wind tunnel at offshore and onshore conditions. Time-resolved measurements were carried out with three component hot wire anemometry and stereo-PIV in the middle vertical plane of the wake up to eleven turbine...... spectrum, present in the entire wake mainly for offshore inflow condition. It was found that the Strouhal number, based on the rotor diameter and the wind velocity at hub height, was in the order of 0.25. Below the meandering frequency, turbulence power spectrum decreased, whereas above it increased. Wake...

  16. Wake potential of swift ion in amorphous carbon target

    Science.gov (United States)

    Al-Bahnam, Nabil janan; Ahmad, Khalid A.; Aboo Al-Numan, Abdullah Ibrahim

    2017-02-01

    The wake potential and wake phenomena for swift proton in an amorphous carbon target were studied by utilising various dielectric function formalisms, including the Drude dielectric function, the Drude-Lorentz dielectric function and quantum dielectric function. The Drude model results exhibited a damped oscillatory behaviour in the longitudinal direction behind the projectile; the pattern of these oscillations decreases exponentially in the transverse direction. In addition, the wake potential extends slightly ahead of the projectile which also depends on the proton coordinate and velocity. The effect of electron binding on the wake potential, characterised by the ratio ωp2 / ω02 = 10 to 0.1, has been studied alongside the Drude-Lorentz dielectric function and quantum dielectric function formalisms; the results evidently show that the wake potential dip depth decreases with more oscillations when the electron density ratio ωp2 / ω02 decreases from 10 to 0.1. One of the primary objectives of the present work is to construct a reasonably realistic procedure for simulating the response of target to swift ions by combining an expression for the induced wake potential along with several important dielectric function models; the aim of this research is to reduce computational complexity without sacrificing accuracy. This is regarded as being an efficient strategy in that it creates suitable computer simulation procedures which are relevant to actual solids. After comparing this method with other models, the main differences and similarities have been noted while the end results have proved encouraging.

  17. Turbulence characteristics in a supersonic cascade wake flow

    Energy Technology Data Exchange (ETDEWEB)

    Andrew, P.L.; Ng, W.F. (Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States))

    1994-10-01

    The turbulent character of the supersonic wake of a linear cascade of fan airfoils has been studied using a two-component laser-doppler anemometer. The cascade was tested in the Virginia Polytechnic Institute and State University intermittent wind tunnel facility, where the Mach and Reynolds numbers were 2.36 and 4.8 [times] 10[sup 6], respectively. In addition to mean flow measurements, Reynolds normal and shear stresses were measured as functions of cascade incidence angle and streamwise locations spanning the near-wake and the far-wake. The extremities of profiles of both the mean and turbulent wake properties were found to be strongly influenced by upstream shock-boundary-layer interactions, the strength of which varied with cascade incidence. In contrast, the peak levels of turbulence properties within the shear layer were found to be largely independent of incidence, and could be characterized in terms of the streamwise position only. The velocity defect turbulence level was found to be 23%, and the generally accepted value of the turbulence structural coefficient of 0.30 was found to be valid for this flow. The degree of similarity of the mean flow wake profiles was established, and those profiles demonstrating the most similarity were found to approach a state of equilibrium between the mean and turbulent properties. In general, this wake flow may be described as a classical free shear flow, upon which the influence of upstream shock-boundary-layer interactions has been superimposed.

  18. Dyspnoea waking from sleep

    Directory of Open Access Journals (Sweden)

    A.O. Yildirim*

    2013-12-01

    Discussion: OSAS, as drivers working people, especially at night, is a disease that leads to serious consequences. Polysomnography is the gold standard for diagnosis and severity of the syndrome, apnoea-hypopnea index (AHI is determined by; AHI = normal  30 is severe. From repetitive episodes of apnoea increased sympathetic nerve activity, oxidative stress, intrathoracic pressure swings, sudden jumps in systemic blood pressure, hypoxia and hypercapnia. Emergency services waking from sleep dyspnoea, lethargy and fatigue in patients admitted with complaints of snoring and OSAS cannot be forgotten.

  19. On the wake flow of asymmetrically beveled trailing edges

    Science.gov (United States)

    Guan, Yaoyi; Pröbsting, Stefan; Stephens, David; Gupta, Abhineet; Morris, Scott C.

    2016-05-01

    Trailing edge and wake flows are of interest for a wide range of applications. Small changes in the design of asymmetrically beveled or semi-rounded trailing edges can result in significant difference in flow features which are relevant for the aerodynamic performance, flow-induced structural vibration and aerodynamically generated sound. The present study describes in detail the flow field characteristics around a family of asymmetrically beveled trailing edges with an enclosed trailing-edge angle of 25° and variable radius of curvature R. The flow fields over the beveled trailing edges are described using data obtained by particle image velocimetry (PIV) experiments. The flow topology for different trailing edges was found to be strongly dependent on the radius of curvature R, with flow separation occurring further downstream as R increases. This variation in the location of flow separation influences the aerodynamic force coefficients, which were evaluated from the PIV data using a control volume approach. Two-point correlations of the in-plane velocity components are considered to assess the structure in the flow field. The analysis shows large-scale coherent motions in the far wake, which are associated with vortex shedding. The wake thickness parameter yf is confirmed as an appropriate length scale to characterize this large-scale roll-up motion in the wake. The development in the very near wake was found to be critically dependent on R. In addition, high-speed PIV measurements provide insight into the spectral characteristics of the turbulent fluctuations. Based on the time-resolved flow field data, the frequency range associated with the shedding of coherent vortex pairs in the wake is identified. By means of time-correlation of the velocity components, turbulent structures are found to convect from the attached or separated shear layers without distinct separation point into the wake.

  20. Improvements in ECN Wake Model

    Energy Technology Data Exchange (ETDEWEB)

    Versteeg, M.C. [University of Twente, Enschede (Netherlands); Ozdemir, H.; Brand, A.J. [ECN Wind Energy, Petten (Netherlands)

    2013-08-15

    Wind turbines extract energy from the flow field so that the flow in the wake of a wind turbine contains less energy and more turbulence than the undisturbed flow, leading to less energy extraction for the downstream turbines. In large wind farms, most turbines are located in the wake of one or more turbines causing the flow characteristics felt by these turbines differ considerably from the free stream flow conditions. The most important wake effect is generally considered to be the lower wind speed behind the turbine(s) since this decreases the energy production and as such the economical performance of a wind farm. The overall loss of a wind farm is very much dependent on the conditions and the lay-out of the farm but it can be in the order of 5-10%. Apart from the loss in energy production an additional wake effect is formed by the increase in turbulence intensity, which leads to higher fatigue loads. In this sense it becomes important to understand the details of wake behavior to improve and/or optimize a wind farm layout. Within this study improvements are presented for the existing ECN wake model which constructs the fundamental basis of ECN's FarmFlow wind farm wake simulation tool. The outline of this paper is as follows: first, the governing equations of the ECN wake farm model are presented. Then the near wake modeling is discussed and the results compared with the original near wake modeling and EWTW (ECN Wind Turbine Test Site Wieringermeer) data as well as the results obtained for various near wake implementation cases are shown. The details of the atmospheric stability model are given and the comparison with the solution obtained for the original surface layer model and with the available data obtained by EWTW measurements are presented. Finally the conclusions are summarized.

  1. Rotating Wheel Wake

    Science.gov (United States)

    Lombard, Jean-Eloi; Xu, Hui; Moxey, Dave; Sherwin, Spencer

    2016-11-01

    For open wheel race-cars, such as Formula One, or IndyCar, the wheels are responsible for 40 % of the total drag. For road cars, drag associated to the wheels and under-carriage can represent 20 - 60 % of total drag at highway cruise speeds. Experimental observations have reported two, three or more pairs of counter rotating vortices, the relative strength of which still remains an open question. The near wake of an unsteady rotating wheel. The numerical investigation by means of direct numerical simulation at ReD =400-1000 is presented here to further the understanding of bifurcations the flow undergoes as the Reynolds number is increased. Direct numerical simulation is performed using Nektar++, the results of which are compared to those of Pirozzoli et al. (2012). Both proper orthogonal decomposition and dynamic mode decomposition, as well as spectral analysis are leveraged to gain unprecedented insight into the bifurcations and subsequent topological differences of the wake as the Reynolds number is increased.

  2. Investigation of wake interaction using full-scale lidar measurements and large eddy simulation

    DEFF Research Database (Denmark)

    Machefaux, Ewan; Larsen, Gunner Chr.; Troldborg, Niels;

    2016-01-01

    dynamics flow solver, using large eddy simulation and fully turbulent inflow. The rotors are modelled using the actuator disc technique. A mutual validation of the computational fluid dynamics model with the measurements is conducted for a selected dataset, where wake interaction occurs. This validation...... is based on a comparison between wake deficit, wake generated turbulence, turbine power production and thrust force. An excellent agreement between measurement and simulation is seen in both the fixed and the meandering frame of reference. Copyright © 2015 John Wiley & Sons, Ltd....

  3. Three-dimensional wake potential in a streaming dusty plasma

    Indian Academy of Sciences (India)

    M Salahuddin; M K Islam; A K Banerjee; M Salimullah; S K Ghosh

    2003-09-01

    The oscillatory wake potential for a slowly moving or static test dust particulate in a finite temperature, collisionless and unmagnetized dusty plasma with a continuous flow of ions and dust particles has been studied. The collective resonant interaction of the moving test particle with the low-frequency and low-phase-velocity dust-acoustic mode is the origin of the periodic attractive force between the like polarity particulates along and perpendicular to the streaming ions and dust grains resulting into dust-Coulomb crystal formation. This wake potential can explain the three-dimensional dust-Coulomb crystal formation in the laboratory conditions.

  4. Study of the wake flow around a circular cylinder

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Jae Sung; Kim, Sang Il; Seung, Sam Sun [Kangwon National University, Chuncheon (Korea, Republic of)

    2015-11-15

    This experimental study investigated the wake flow around an elastically supported circular cylinder. In this study, the Reynolds numbers are varied in the region of 1.4x10{sup 4}≤Re≤3.2x10{sup 4}. Under these conditions, we have captured the process of the wake mechanism and the moving path of the vortex by measuring the velocity at each position in the wake around the cylinder. Further, these facts from the wind tunnel test are proved by a flow visualization test through a water channel. From the result, we have arrived at the following conclusions : i) The process (formationgrowthcollapse) of vortex is observed in the wake around the cylinder, ii) The vortex efflux angle is approximately 16 degrees⁓17 degrees under the experimental conditions. These angles have no relationship with the velocity change and the existence of flow-induced vibrations of the cylinder, and iii) The moving path of the vortex center is obtained by spectrum analysis of the fluctuating velocity behind the cylinder. These are confirmed by conducting visualization tests.

  5. Surface wake in the random-phase approximation

    Energy Technology Data Exchange (ETDEWEB)

    Garcia de Abajo, F.J. (Departamento de Ciencias de la Computacion e Inteligencia Artificial, Facultad de Informatica, Universidad del Pais Vasco, Apartado 649, 20080 San Sebastian (Spain)); Echenique, P.M. (Departamento de Fisica de Materiales, Facultad de Quimica, Universidad del Pais Vasco, Apartado 1072, 20080 San Sebastian (Spain))

    1993-11-01

    The scalar-electric-potential distribution set up by an ion traveling in the vicinity of a plane solid-vacuum interface, that is, the surface-wake potential, is investigated with the specular-reflection model to describe the response of the surface and with the random-phase approximation for the dielectric function of the bulk material. This permits us to address the study of the low-velocity surface wake: the static potential is found to have a dip at the position of the ion; that dip is shifted towards the direction opposite to the velocity vector for velocities smaller than the threshold of creation of plasmons ([approx]1.3[ital v][sub [ital F

  6. Wake-induced vibrations in Tandem Cylinders

    Science.gov (United States)

    Mysa, Ravi Chaithanya; Jaiman, Rajeev Kumar

    2015-11-01

    The upstream cylinder is fixed in the tandem cylinders arrangement. The downstream cylinder is placed at a distance of four diameters from the upstream cylinder in the free stream direction and is mounted on a spring. The dynamic response of the downstream cylinder is studied at Reynolds number of 10,000. The transverse displacement amplitude of the downstream cylinder is larger compared to that of single cylinder in the post-lock-in region. The transverse dynamic response of the downstream cylinder in the post-lock-in region is characterized by a dominant low frequency component compared to shed frequency, which is nearer to the structural natural frequency. The interaction of upstream wake with the downstream cylinder is carefully analyzed to understand the introduction of low frequency component in the transverse load along with the shed frequency. We found that the stagnation point moves in proportional to the velocity of the cylinder and is in-phase with the velocity. The low frequency component in the stagnation point movement on the downstream cylinder is sustained by the interaction of upstream wake. The frequencies in the movement of the stagnation point is reflected in the transverse load resulting in large deformation of the cylinder. The authors wish to acknowledge support from A*STAR- SERC and Singapore Maritime Institute.

  7. Single Wake Meandering, Advection and Expansion - An analysis using an adapted Pulsed Lidar and CFD LES-ACL simulations

    DEFF Research Database (Denmark)

    In this paper, single wake characteristics have been studied both experimentally and numerically. Firstly, the wake is studied experimentally using full-scale measurements from an adapted focused pulsed lidar system, which potentially gives more insight into the wake dynamics as compared to class...... using the EllipSys3D flow solver using Large Eddy Simulation (LES) and Actuator Line Technique (ACL) to model the rotor. Discrepancies due to the uncertainties on the wake advection velocity are observed and discussed....... of the wake, and it is compared to the predictions from the Dynamic Wake Meandering model, for a selected 10 minutes dataset. Secondly, the average wake expansion in the fixed frame of reference is determined from measurements and compared to results from CFD simulations. The CFD simulations were conducted...

  8. Single Wake Meandering, Advection and Expansion - An analysis using an adapted Pulsed Lidar and CFD LES-ACL simulations

    DEFF Research Database (Denmark)

    Machefaux, Ewan; Larsen, Gunner Chr.; Troldborg, Niels;

    2013-01-01

    In this paper, single wake characteristics have been studied both experimentally and numerically. Firstly, the wake is studied experimentally using full-scale measurements from an adapted focused pulsed lidar system, which potentially gives more insight into the wake dynamics as compared to class...... using the EllipSys3D flow solver using Large Eddy Simulation (LES) and Actuator Line Technique (ACL) to model the rotor. Discrepancies due to the uncertainties on the wake advection velocity are observed and discussed....... of the wake, and it is compared to the predictions from the Dynamic Wake Meandering model, for a selected 10 minutes dataset. Secondly, the average wake expansion in the fixed frame of reference is determined from measurements and compared to results from CFD simulations. The CFD simulations were conducted...

  9. Wakes in inhomogeneous plasmas

    CERN Document Server

    Kompaneets, Roman; Nosenko, Vladimir; Morfill, Gregor E

    2014-01-01

    The Debye shielding of a charge immersed in a flowing plasma is an old classic problem in plasma physics. It has been given renewed attention in the last two decades in view of experiments with complex plasmas, where charged dust particles are often levitated in a region with strong ion flow. Efforts to describe the shielding of the dust particles in such conditions have been focused on the homogeneous plasma approximation, which ignores the substantial inhomogeneity of the levitation region. We address the role of the plasma inhomogeneity by rigorously calculating the point charge potential in the collisionless Bohm sheath. We demonstrate that the inhomogeneity can dramatically modify the wake, making it non-oscillatory and weaker.

  10. Gas dynamics, optics and chemistry of an aircraft condensable wake

    Energy Technology Data Exchange (ETDEWEB)

    Grinats, E.S.; Kashevarov, A.V.; Stasenko, A.L. [Central Aerohydrodynamic Inst., Zhukovsky (Russian Federation)

    1997-12-31

    Prediction of the properties of a jet-and-vortex wake from an individual airplane is of great interest as the first step to assessment of the possible global changes in the atmosphere due to the world civil aviation. Several mathematical models of the different regions of an aircraft wake and corresponding numerical results are presented. The axisymmetric exhaust jet was simulated on the base of the well-known k-{epsilon} model of turbulence. Jet chemistry was investigated on the base of kinetic scheme of the gas phase reactions of enriched by including chemisorption by water droplets of several species and by taking into account of the photochemical processes. In the 3D far wake model, the numerical results for distribution of species exhausted by the engines and entrapped by the velocity field of two parallel vortices are shown. (R.P.) 7 refs.

  11. Wind-tunnel modelling of the tip-speed ratio influence on the wake evolution

    Science.gov (United States)

    Stein, Victor P.; Kaltenbach, Hans-Jakob

    2016-09-01

    Wind-tunnel measurements on the near-wake evolution of a three bladed horizontal axis wind turbine model (HAWT) in the scale 1:O(350) operating in uniform flow conditions and within a turbulent boundary layer at different tip speed ratios are presented. Operational conditions are chosen to exclude Reynolds number effects regarding the turbulent boundary layer as well as the rotor performance. Triple-wire anemometry is used to measure all three velocity components in the mid-vertical and mid-horizontal plane, covering the range from the near- to the far-wake region. In order to analyse wake properties systematically, power and thrust coefficients of the turbine were measured additionally. It is confirmed that realistic modelling of the wake evolution is not possible in a low-turbulence uniform approach flow. Profiles of mean velocity and turbulence intensity exhibit large deviations between the low-turbulence uniform flow and the turbulent boundary layer, especially in the far-wake region. For nearly constant thrust coefficients differences in the evolution of the near-wake can be identified for tip speed ratios in the range from 6.5 to 10.5. It is shown that with increasing downstream distances mean velocity profiles become indistinguishable whereas for turbulence statistics a subtle dependency on the tip speed ratio is still noticeable in the far-wake region.

  12. Transition to bluff body dynamics in the wake of vertical axis turbines

    Science.gov (United States)

    Araya, Daniel; Dabiri, John

    2015-11-01

    A unifying characteristic among bluff bodies is a similar wake structure independent of the shape of the body. We present experimental data to demonstrate that the wake of a vertical axis wind/water turbine (VAWT) shares similar features to that of a bluff body, namely a circular cylinder. For a fixed Reynolds number (Re ~ 104) and variable tip-speed ratio, 2D particle image velocimetry (PIV) is used to measure the velocity field in the wake of three different laboratory-scale turbines: a 2-bladed, 3-bladed, and 5-bladed VAWT, each with similar geometry. From the PIV measurements, the time-averaged and dynamic characteristics of the wake are evaluated. In all cases, we observe three distinct regions in the VAWT wake: (1) the near wake, where periodic blade shedding dominates; (2) a transition region, where blade vortices decay and growth of a shear layer instability occurs; (3) the far wake, where bluff body wake oscillations dominate. We further characterize this wake transition with regard to turbine solidity and examine its relation to the mean flow, an important metric for power production within a wind farm.

  13. Wake Structure of Wind Turbines in Yaw under Uniform Inflow Conditions

    CERN Document Server

    Howland, Michael F; Martinez-Tossas, Luis A; Meyers, Johan; Meneveau, Charles

    2016-01-01

    Reducing wake losses in wind farms by deflecting the wakes through turbine yawing has been shown to be a feasible wind farm controls approach. Nonetheless, the effectiveness of yawing depends not only on the degree of wake deflection but also on the resulting shape of the wake. In this work, the deflection and morphology of wakes behind a wind turbine operating in yawed conditions are studied using wind tunnel experiments of a wind turbine modeled as a porous disk in a uniform inflow. First, by measuring velocity distributions at various downstream positions and comparing with prior studies, we confirm that the non-rotating wind turbine model in yaw generates realistic wake deflections. Second, we characterize the wake shape and make first observations of what is termed a curled wake, displaying significant spanwise asymmetry. The wake curling observed in the experiments is also reproduced qualitatively in large eddy simulations using both actuator disk and actuator line models. When a wind turbine is yawed f...

  14. Numerical simulations of wake interaction between two wind turbines at various inflow conditions

    DEFF Research Database (Denmark)

    Troldborg, Niels; Larsen, Gunner Chr.; Aagaard Madsen, Helge;

    2011-01-01

    The phenomenon of wake interaction between two wind turbines was analysed using the actuator line technique and full unsteady Navier–Stokes computations. Results are presented for varying mutual distances between the two turbines and both full wake and half wake situations were considered. Furthe...... as the development of wake generated vortex structures. Moreover, the influence of the wake of the upstream turbine on the external aerodynamic loading on the blades of the downstream turbine was studied. Copyright © 2010 John Wiley & Sons, Ltd.......The phenomenon of wake interaction between two wind turbines was analysed using the actuator line technique and full unsteady Navier–Stokes computations. Results are presented for varying mutual distances between the two turbines and both full wake and half wake situations were considered....... Furthermore, simulations were carried out at different degrees of ambient turbulence intensity representing laminar, offshore and onshore conditions. From the simulations, the main characteristics of the interacting wakes were extracted including the averaged velocity and turbulence fields as well...

  15. A Large-Eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer

    Directory of Open Access Journals (Sweden)

    Sina Shamsoddin

    2016-05-01

    Full Text Available In a future sustainable energy vision, in which diversified conversion of renewable energies is essential, vertical axis wind turbines (VAWTs exhibit some potential as a reliable means of wind energy extraction alongside conventional horizontal axis wind turbines (HAWTs. Nevertheless, there is currently a relative shortage of scientific, academic and technical investigations of VAWTs as compared to HAWTs. Having this in mind, in this work, we aim to, for the first time, study the wake of a single VAWT placed in the atmospheric boundary layer using large-eddy simulation (LES. To do this, we use a previously-validated LES framework in which an actuator line model (ALM is incorporated. First, for a typical three- and straight-bladed 1-MW VAWT design, the variation of the power coefficient with both the chord length of the blades and the tip-speed ratio is analyzed by performing 117 simulations using LES-ALM. The optimum combination of solidity (defined as N c / R , where N is the number of blades, c is the chord length and R is the rotor radius and tip-speed ratio is found to be 0.18 and 4.5, respectively. Subsequently, the wake of a VAWT with these optimum specifications is thoroughly examined by showing different relevant mean and turbulence wake flow statistics. It is found that for this case, the maximum velocity deficit at the equator height of the turbine occurs 2.7 rotor diameters downstream of the center of the turbine, and only after that point, the wake starts to recover. Moreover, it is observed that the maximum turbulence intensity (TI at the equator height of the turbine occurs at a distance of about 3.8 rotor diameters downstream of the turbine. As we move towards the upper and lower edges of the turbine, the maximum TI (at a certain height increases, and its location moves relatively closer to the turbine. Furthermore, whereas both TI and turbulent momentum flux fields show clear vertical asymmetries (with larger magnitudes at the

  16. Vortical structures in the wake of an undulating fin

    Science.gov (United States)

    Dewey, Peter A.; Carriou, Antoine; Smits, Alexander J.

    2010-11-01

    Batoid fish such as the manta ray propel themselves through the water by producing a traveling wave motion along the chord of their pectoral fin. Such a motion produces thrust through the development of an unsteady vortex street that results in a jet-like average flow. Digital particle image velocimetry (DPIV) is used to characterize the vortical patterns and structures developed in the wake of a manta ray-like fin. A DC servo motor powers a gear train to produce the traveling wave motion; whose frequency and wave length can be varied. The amplitude of the traveling wave motion linearly increases along the span of the fin. Wake morphologies for a wide spectrum of oscillation frequencies and traveling wave wavelengths are identified. A bifurcation from a 2S wake structure to a 2P wake structure is observed as the traveling wave wavelength is decreased, which corresponds to a decrease in efficiency as reported by Clark and Smits (2006). Alteration of the oscillation frequency, and thus Strouhal number, affects vortex interaction and is found to significantly modify the resulting velocity profiles in the wake of the fin. Notably, increasing the Strouhal number beyond optimal conditions, reported by Clark and Smits, corresponds to a reduction in the extent that the jet-like average flow is observed downstream of the fin.

  17. Flow visualization in the wake of flapping-wing MAV ‘DelFly II’ in forward flight

    NARCIS (Netherlands)

    Percin, M.; Eisma, H.E.; Van Oudheusden, B.W.; Remes, B.; Ruijsink, R.; De Wagter, C.

    2012-01-01

    Time-resolved velocity field measurements in the wake of the flapping wings of the DelFly II Micro Aerial Vehicle (MAV) in forward flight configuration were obtained by Stereoscopic Particle Image Velocimetry (Stereo-PIV). The PIV measurements were performed at several spanwise planes in the wake of

  18. Wake meandering statistics of a model wind turbine: Insights gained by large eddy simulations

    Science.gov (United States)

    Foti, Daniel; Yang, Xiaolei; Guala, Michele; Sotiropoulos, Fotis

    2016-08-01

    Wind tunnel measurements in the wake of an axial flow miniature wind turbine provide evidence of large-scale motions characteristic of wake meandering [Howard et al., Phys. Fluids 27, 075103 (2015), 10.1063/1.4923334]. A numerical investigation of the wake, using immersed boundary large eddy simulations able to account for all geometrical details of the model wind turbine, is presented here to elucidate the three-dimensional structure of the wake and the mechanisms controlling near and far wake instabilities. Similar to the findings of Kang et al. [Kang et al., J. Fluid Mech. 744, 376 (2014), 10.1017/jfm.2014.82], an energetic coherent helical hub vortex is found to form behind the turbine nacelle, which expands radially outward downstream of the turbine and ultimately interacts with the turbine tip shear layer. Starting from the wake meandering filtering used by Howard et al., a three-dimensional spatiotemporal filtering process is developed to reconstruct a three-dimensional meandering profile in the wake of the turbine. The counterwinding hub vortex undergoes a spiral vortex breakdown and the rotational component of the hub vortex persists downstream, contributing to the rotational direction of the wake meandering. Statistical characteristics of the wake meandering profile, along with triple decomposition of the flow field separating the coherent and incoherent turbulent fluctuations, are used to delineate the near and far wake flow structures and their interactions. In the near wake, the nacelle leads to mostly incoherent turbulence, while in the far wake, turbulent coherent structures, especially the azimuthal velocity component, dominate the flow field.

  19. Droplet depinning in a wake

    Science.gov (United States)

    Hooshanginejad, Alireza; Lee, Sungyon

    2017-03-01

    Pinning and depinning of a windswept droplet on a surface is familiar yet deceptively complex for it depends on the interaction of the contact line with the microscopic features of the solid substrate. This physical picture is further compounded when wind of the Reynolds number greater than 100 blows over pinned drops, leading to the boundary layer separation and wake generation. In this Rapid Communication, we incorporate the well-developed ideas of the classical boundary layer to study partially wetting droplets in a wake created by a leader object. Depending on its distance from the leader, the droplet is observed to exhibit drafting, upstream motion, and splitting, due to the wake-induced hydrodynamic coupling that is analogous to drafting of moving bodies. We successfully rationalize the onset of the upstream motion regime using a reduced model that computes the droplet shape governed by the pressure field inside the wake.

  20. Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation

    Science.gov (United States)

    Bomphrey, Richard J.; Henningsson, Per; Michaelis, Dirk; Hollis, David

    2012-01-01

    Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread. PMID:22977102

  1. Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation.

    Science.gov (United States)

    Bomphrey, Richard J; Henningsson, Per; Michaelis, Dirk; Hollis, David

    2012-12-07

    Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread.

  2. CFD Study on Effective Wake of Conventional and Tip-modified Propellers

    DEFF Research Database (Denmark)

    Shin, K. W.; Andersen, Poul

    2016-01-01

    by integrating velocity fields at a section 40% of the propeller radius upstream from the propeller plane in self-propulsion simulations. The difference of effective wake fraction from integrating velocity fields between tip-modified and conventional propellers is less than 1%. Based on the open-water simulation...

  3. Reconstructing three-dimensional wake topology based on planar PIV measurements and pattern recognition analysis

    Science.gov (United States)

    Morton, C.; Yarusevych, S.

    2016-10-01

    The present study presents a new technique for reconstructing the salient aspects of three-dimensional wake topology based on time-resolved, planar, two-component particle image velocimetry data collected in multiple orthogonal planes. The technique produces conditionally averaged flow field reconstructions based on a pattern recognition analysis of velocity fields. It is validated on the wake of a low-aspect ratio dual step cylinder geometry, consisting of a large diameter cylinder ( D) with small aspect ratio ( L/ D) attached to the mid-span of a small diameter cylinder ( d). For a dual step cylinders with D/ d = 2, and L/ D = 1, numerical and experimental data are considered for ReD = 150 (laminar wake) and for ReD = 2100 (turbulent wake). The results show that the proposed technique successfully reconstructs the dominant periodic wake vortex interactions and can be extended to a wide range of turbulent flows.

  4. Investigating fundamental properties of wind turbine wake structure using particle image velocimetry

    Energy Technology Data Exchange (ETDEWEB)

    Whale, J. [Univ. of Edinburgh, Dept. of Mechanical Engineering, Edinburgh (United Kingdom)

    1997-08-01

    Low Reynolds number flow visualization tests are often used for showing the flow pattern changes associated with changes in lift-coefficients at a higher Reynolds number. In wind turbine studies, analysis of measured wake structures at small scale may reveal fundamental properties of the wake which will offer wake modellers a more complete understanding of rotor flows. Measurements are presented from experiments on a model wind turbine rig conducted in a water channel. The laser-optics technique of Particle Image Velocimetry (PIV) is used to make simultaneous multi-point measurements of the wake flow behind small-scale rotors. Analysis of the PIV data shows trends in velocity and vorticity structure in the wake. Study of the flow close to the rotor plane reveals information on stalled flow and blade performance. (au)

  5. Energy velocity and group velocity

    Institute of Scientific and Technical Information of China (English)

    陈宇

    1995-01-01

    A new Lagrangian method for studying the relationship between the energy velocity and the group velocity is described. It is proved that under the usual quasistatic electric field, the energy velocity is identical to the group velocity for acoustic waves in anisotropic piezoelectric (or non-piezoelectric) media.

  6. A Study of the Development of Steady and Periodic Unsteady Turbulent Wakes Through Curved Channels at Positive, Zero, and Negative Streamwise Pressure Gradients, Part 1

    Science.gov (United States)

    Schobeiri, M. T.; John, J.

    1996-01-01

    The turbomachinery wake flow development is largely influenced by streamline curvature and streamwise pressure gradient. The objective of this investigation is to study the development of the wake under the influence of streamline curvature and streamwise pressure gradient. The experimental investigation is carried out in two phases. The first phase involves the study of the wake behind a stationary circular cylinder (steady wake) in curved channels at positive, zero, and negative streamwise pressure gradients. The mean velocity and Reynolds stress components are measured using a X-hot-film probe. The measured quantities obtained in probe coordinates are transformed to a curvilinear coordinate system along the wake centerline and are presented in similarity coordinates. The results of the steady wakes suggest strong asymmetry in velocity and Reynolds stress components. However, the velocity defect profiles in similarity coordinates are almost symmetrical and follow the same distribution as the zero pressure gradient straight wake. The results of Reynolds stress distributions show higher values on the inner side of the wake than the outer side. Other quantities, including the decay of maximum velocity defect, growth of wake width, and wake integral parameters, are also presented for the three different pressure gradient cases of steady wake. The decay rate of velocity defect is fastest for the negative streamwise pressure gradient case and slowest for the positive pressure gradient case. Conversely, the growth of the wake width is fastest for the positive streamwise pressure gradient case and slowest for the negative streamwise pressure gradient. The second phase studies the development of periodic unsteady wakes generated by the circular cylinders of the rotating wake generator in a curved channel at zero streamwise pressure gradient. Instantaneous velocity components of the periodic unsteady wakes, measured with a stationary X-hot-film probe, are analyzed by the

  7. Reynolds Number Scaling and Parameterization of Stratified Turbulent Wakes

    Science.gov (United States)

    2017-04-17

    the Lagrangian drift, and hence particulate dispersion, induced by a internal wave beam reflecting off a rigid lid surface in an idealized linear...to explore the signature of wake-radiated internal waves on an rigid lid surface within an idealized linear stratification. The datasets span...motivates one to revert to linear theory. The definition of the vertical group velocity, according to linear theory, ultimately leads to a relationship

  8. Nonlinear wake amplification by an active medium in a cylindrical waveguide using a modulated trigger bunch

    Institute of Scientific and Technical Information of China (English)

    Zeev; Toroker; Miron; Voin; Levi; Schchter

    2014-01-01

    Cerenkov wake amplification can be used as an accelerating scheme,in which a trigger bunch of electrons propagating inside a cylindrical waveguide filled with an active medium generates an initial wake field.Due to the multiple reflections inside the waveguide,the wake may be amplified significantly more strongly than when propagating in a boundless medium.Sufficiently far away from the trigger bunch the wake,which travels with the same phase velocity as the bunch,reaches saturation and it can accelerate a second bunch of electrons trailing behind.For a CO2 gas mixture our numerical and analytical calculations indicate that a short saturation length and a high gradient can be achieved with a large waveguide radius filled with a high density of excited atoms and a trigger bunch that travels at a velocity slightly above the Cerenkov velocity.To obtain a stable level of saturated wake that will be suitable for particle acceleration,it is crucial to satisfy the single-mode resonance condition,which requires high accuracy in the waveguide radius and the ratio between the electron phase velocity and the Cerenkov velocity.For single-mode propagation our model indicates that it is feasible to obtain gradients as high as GV m-1 in a waveguide length of cm.

  9. Numerical Study of Wake Characteristics in a Horizontal-Axis Hydrokinetic Turbine

    Directory of Open Access Journals (Sweden)

    PAULO A.S.F. SILVA

    Full Text Available ABSTRACT Over the years most studies on wake characteristics have been devoted to wind turbines, while few works are related to hydrokinetic turbines. Among studies applied to rivers, depth and width are important parameters for a suitable design. In this work, a numerical study of the wake in a horizontal-axis hydrokinetic turbine is performed, where the main objective is an investigation on the wake structure, which can be a constraining factor in rivers. The present paper uses the Reynolds Averaged Navier Stokes (RANS flow simulation technique, in which the Shear-Stress Transport (SST turbulent model is considered, in order to simulate a free hydrokinetic runner in a typical river flow. The NREL-PHASE VI wind turbine was used to validate the numerical approach. Simulations for a 3-bladed axial hydrokinetic turbine with 10 m diameter were carried out, depicting the expanded helical behavior of the wake. The axial velocity, in this case, is fully recovered at 12 diameters downstream in the wake. The results are compared with others available in the literature and also a study of the turbulence kinetic energy and mean axial velocity is presented so as to assess the influence of proximity of river surface from rotor in the wake geometry. Hence, even for a single turbine facility it is still necessary to consider the propagation of the wake over the spatial domain.

  10. Experimental Study on Influence of Pitch Motion on the Wake of a Floating Wind Turbine Model

    Directory of Open Access Journals (Sweden)

    Stanislav Rockel

    2014-03-01

    Full Text Available Wind tunnel experiments were performed, where the development of the wake of a model wind turbine was measured using stereo Particle Image Velocimetry to observe the influence of platform pitch motion. The wakes of a classical bottom fixed turbine and a streamwise oscillating turbine are compared. Results indicate that platform pitch creates an upward shift in all components of the flow and their fluctuations. The vertical flow created by the pitch motion as well as the reduced entrainment of kinetic energy from undisturbed flows above the turbine result in potentially higher loads and less available kinetic energy for a downwind turbine. Experimental results are compared with four wake models. The wake models employed are consistent with experimental results in describing the shapes and magnitudes of the streamwise velocity component of the wake for a fixed turbine. Inconsistencies between the model predictions and experimental results arise in the floating case particularly regarding the vertical displacement of the velocity components of the flow. Furthermore, it is found that the additional degrees of freedom of a floating wind turbine add to the complexity of the wake aerodynamics and improved wake models are needed, considering vertical flows and displacements due to pitch motion.

  11. Basic research in wake vortex alleviation using a variable twist wing

    Science.gov (United States)

    Morris, D. J.; Holbrook, G. T.

    1981-01-01

    The variable twist wing concept was used to investigate the relative effects of lift and turbulence distribution on the rolled up vortex wake. Several methods of reducing the vortex strength behind an aircraft were identified. These involve the redistribution of lift spanwise on the wing and drag distribution along the wing. Initial attempts to use the variable twist wing velocity data to validate the WAKE computer code have shown a strong correlation, although the vorticity levels were not exactly matched.

  12. On the influence of far-wake resolution on wind turbine flow simulations

    DEFF Research Database (Denmark)

    Zahle, Frederik; Sørensen, Niels N.

    2007-01-01

    resolving up to 13 tip vortex revolutions. It was found that the resolution of the tip vortices in the far-wake beyond half a diameter downstream did not influence the induced velocities in the rotor plane significantly, with the low speed shaft torque differing less than approximately 1% for the rotor......  A study was made on the influence of the wake mesh resolution on the integrated forces acting on a wind turbine rotor using an unsteady incompressible Navier-Stokes flow solver. Solutions were obtained for five wake resolutions ranging from 0.5 rotor diameters to 7.5 diameters downstream...

  13. Numerical investigation of the wake interaction between two model wind turbines with span-wise offset

    DEFF Research Database (Denmark)

    Sarmast, Sasan; Chivaee, Hamid Sarlak; Ivanell, Stefan;

    2014-01-01

    Wake interaction between two model scale wind turbines with span-wise offset is investigated numerically using Large Eddy Simulation (LES) and the results are validated against the experimental data. An actuator line technique is used for modeling the rotor. The investigated setup refers...... to a series of experimental measurements of two model scale turbines conducted by NTNU in low speed wind tunnel in which the two wind turbines are aligned with a span-wise offset resulting in half wake interaction. Two levels of free-stream turbulence are tested, the minimum undisturbed level of about Ti ≈ 0....... The interacting wake development is captured in great details in terms of wake deficit and streamwise turbulence kinetic energy. The present work is done in connection with Blind test 3 workshops organized jointly by NOWITECH and NORCOWE....

  14. Gas transfer in a bubbly wake flow

    Science.gov (United States)

    Karn, A.; Gulliver, J. S.; Monson, G. M.; Ellis, C.; Arndt, R. E. A.; Hong, J.

    2016-05-01

    The present work reports simultaneous bubble size and gas transfer measurements in a bubbly wake flow of a hydrofoil, designed to be similar to a hydroturbine blade. Bubble size was measured by a shadow imaging technique and found to have a Sauter mean diameter of 0.9 mm for a reference case. A lower gas flow rate, greater liquid velocities, and a larger angle of attack all resulted in an increased number of small size bubbles and a reduced weighted mean bubble size. Bubble-water gas transfer is measured by the disturbed equilibrium technique. The gas transfer model of Azbel (1981) is utilized to characterize the liquid film coefficient for gas transfer, with one scaling coefficient to reflect the fact that characteristic turbulent velocity is replaced by cross-sectional mean velocity. The coefficient was found to stay constant at a particular hydrofoil configuration while it varied within a narrow range of 0.52-0.60 for different gas/water flow conditions.

  15. Impact of Neutral Boundary-Layer Turbulence on Wind-Turbine Wakes: A Numerical Modelling Study

    Science.gov (United States)

    Englberger, Antonia; Dörnbrack, Andreas

    2017-03-01

    The wake characteristics of a wind turbine in a turbulent boundary layer under neutral stratification are investigated systematically by means of large-eddy simulations. A methodology to maintain the turbulence of the background flow for simulations with open horizontal boundaries, without the necessity of the permanent import of turbulence data from a precursor simulation, was implemented in the geophysical flow solver EULAG. These requirements are fulfilled by applying the spectral energy distribution of a neutral boundary layer in the wind-turbine simulations. A detailed analysis of the wake response towards different turbulence levels of the background flow results in a more rapid recovery of the wake for a higher level of turbulence. A modified version of the Rankine-Froude actuator disc model and the blade element momentum method are tested as wind-turbine parametrizations resulting in a strong dependence of the near-wake wind field on the parametrization, whereas the far-wake flow is fairly insensitive to it. The wake characteristics are influenced by the two considered airfoils in the blade element momentum method up to a streamwise distance of 14 D ( D = rotor diameter). In addition, the swirl induced by the rotation has an impact on the velocity field of the wind turbine even in the far wake. Further, a wake response study reveals a considerable effect of different subgrid-scale closure models on the streamwise turbulent intensity.

  16. Rotor Wake Development During the First Revolution

    Science.gov (United States)

    McAlister, Kenneth W.

    2003-01-01

    The wake behind a two-bladed model rotor in light climb was measured using particle image velocimetry, with particular emphasis on the development of the trailing vortex during the first revolution of the rotor. The distribution of vorticity was distinguished from the slightly elliptical swirl pattern. Peculiar dynamics within the void region may explain why the peak vorticity appeared to shift away from the center as the vortex aged, suggesting the onset of instability. The swirl and axial velocities (which reached 44 and 12 percent of the rotor-tip speed, respectively) were found to be asymmetric relative to the vortex center. In particular, the axial flow was composed of two concentrated zones moving in opposite directions. The radial distribution of the circulation rapidly increased in magnitude until reaching a point just beyond the core radius, after which the rate of growth decreased significantly. The core-radius circulation increased slightly with wake age, but the large-radius circulation appeared to remain relatively constant. The radial distributions of swirl velocity and vorticity exhibit self-similar behaviors, especially within the core. The diameter of the vortex core was initially about 10 percent of the rotor-blade chord, but more than doubled its size after one revolution of the rotor. According to vortex models that approximate the measured data, the core-radius circulation was about 79 percent of the large-radius circulation, and the large-radius circulation was about 67 percent of the maximum bound circulation on the rotor blade. On average, about 53 percent of the maximum bound circulation resides within the vortex core during the first revolution of the rotor.

  17. Maximization of the annual energy production of wind power plants by optimization of layout and yaw-based wake control: Maximization of wind plant AEP by optimization of layout and wake control

    Energy Technology Data Exchange (ETDEWEB)

    Gebraad, Pieter [National Renewable Energy Laboratory, Golden CO USA; Thomas, Jared J. [Brigham Young University, Provo UT USA; Ning, Andrew [Brigham Young University, Provo UT USA; Fleming, Paul [National Renewable Energy Laboratory, Golden CO USA; Dykes, Katherine [National Renewable Energy Laboratory, Golden CO USA

    2016-05-24

    This paper presents a wind plant modeling and optimization tool that enables the maximization of wind plant annual energy production (AEP) using yaw-based wake steering control and layout changes. The tool is an extension of a wake engineering model describing the steady-state effects of yaw on wake velocity profiles and power productions of wind turbines in a wind plant. To make predictions of a wind plant's AEP, necessary extensions of the original wake model include coupling it with a detailed rotor model and a control policy for turbine blade pitch and rotor speed. This enables the prediction of power production with wake effects throughout a range of wind speeds. We use the tool to perform an example optimization study on a wind plant based on the Princess Amalia Wind Park. In this case study, combined optimization of layout and wake steering control increases AEP by 5%. The power gains from wake steering control are highest for region 1.5 inflow wind speeds, and they continue to be present to some extent for the above-rated inflow wind speeds. The results show that layout optimization and wake steering are complementary because significant AEP improvements can be achieved with wake steering in a wind plant layout that is already optimized to reduce wake losses.

  18. Turbulence in wind turbine wakes under different atmospheric conditions from static and scanning Doppler LiDARs

    Science.gov (United States)

    Kumer, Valerie; Reuder, Joachim

    2016-04-01

    Wake characteristics are of great importance for wind park performances and turbine loads. Wind tunnel experiments helped to validate wake model simulations under neutral atmospheric conditions. However, recent studies show strongest wake characteristics and power losses in stable atmospheric conditions. Considering all three occurring atmospheric conditions this study presents a turbulence analysis of wind turbine wake flows measured by static and scanning Doppler LiDARs at the coast of the Netherlands. We use data collected by three Windcubes v1, a scanning Windcube 100S and sonic anemometers during the Wind Turbine Wake Experiment - Wieringermeer (WINTWEX-W). Turbulence parameters such as Turbulence Intensity (TI) and turbulent kinetic energy (TKE) are retrieved from the collected raw data. Results show highest turbulence on the flanks of the wake where strong wind shear dominates. On average the spatial turbulence distribution becomes more homogeneous with conical areas of enhanced TI. Highest turbulence and strongest wind deficits occur during stable weather conditions. Despite the ongoing research on the reliability of turbulence retrievals of Doppler LiDAR data, the results are consistent with sonic anemometer measurements and show promising opportunities for a qualitative study of wake characteristics such as wake strength and wake peak frequencies.

  19. Elastohydrodynamic wake and wave resistance

    CERN Document Server

    Arutkin, Maxence; Salez, Thomas; Raphaël, Elie

    2016-01-01

    The dynamics of a thin elastic sheet lubricated by a narrow layer of liquid is relevant to various situations and length scales. In the continuity of our previous work on viscous wakes, we study theoretically the effects of an external pressure disturbance moving at constant speed along the surface of a thin lubricated elastic sheet. In the comoving frame, the imposed pressure field creates a stationary deformation of the free interface that spatially vanishes in the far-field region. The shape of the wake and the way it decays depend on the speed and size of the external disturbance, as well as the rheological properties of both the elastic and liquid layers. The wave resistance, namely the force that has to be externally furnished in order to maintain the wake, is analyzed in detail.

  20. Linearised CFD Models for Wakes

    DEFF Research Database (Denmark)

    Ott, Søren; Berg, Jacob; Nielsen, Morten

    This report describes the development of a fast and reasonably accurate model for the prediction of energy production in oshore wind farms taking wake eects into account. The model has been implemented as a windows application called Fuga which can run in batch mode or as a graphical user interface....... Fuga is brie y described. The model is based on alinearization technique which is described in some detail, and linearized, governing equations are derived and written in a standard form based on a mixed{spectral formulation. A new solution method is used to solve the equations which involves intensive...... use of look{up tables for storage of intermediate results. Due to the linearity of the model, multiple wakes from many turbines can be constructed from the wake of a single, solitary turbine. These are in turn constructed from Fourier components by a fast Fourier integral transform of results derived...

  1. Linearised CFD models for wakes

    Energy Technology Data Exchange (ETDEWEB)

    Ott, S.; Berg, J.; Nielsen, Morten

    2011-12-15

    This report describes the development of a fast and reasonably accurate model for the prediction of energy production in offshore wind farms taking wake effects into account. The model has been implemented as a windows application called Fuga which can run in batch mode or as a graphical user interface. Fuga is briefly described. The model is based on a linearization technique which is described in some detail, and linearized, governing equations are derived and written in a standard form based on a mixed-spectral formulation. A new solution method is used to solve the equations which involves intensive use of look-up tables for storage of intermediate results. Due to the linearity of the model, multiple wakes from many turbines can be constructed from the wake of a single, solitary turbine. These are in turn constructed from Fourier components by a fast Fourier integral transform of results derived from generic look-up tables. Three different models, based on three different closures, are examined: 1) the 'simple closure' using an unperturbed eddy viscosity kucentre dotz. 2) the mixing length closure. 3) the E-epsilon closure. Model results are evaluated against offshore wind farm production data from Horns Rev I and the Nysted wind farm, and a comparison with direct wake measurements in an onshore turbine (Nibe B) is also made. A very satisfactory agreement with data is found for the simple closure. The exception is the near wake, just behind the rotor, where all three linearized models fail. The mixing length closure underestimates wake effects in all cases. The E-epsilon closure overestimates wake losses in the offshore farms while it predicts a too shallow and too wide the wake in the onshore case. The simple closure performs distinctly better than the other two. Wind speed data from the the Horns rev met masts are used to further validate Fuga results with the 'simple' closure. Finally, Roedsand 1 and 2 are used as an example to illustrate

  2. Evaluation of the wind direction uncertainty and its impact on wake modeling at the Horns Rev offshore wind farm

    DEFF Research Database (Denmark)

    Gaumond, M.; Réthoré, Pierre-Elouan; Ott, Søren;

    2014-01-01

    of the power deficit in a single wake situation is improved. The robustness of the method is verified using the Jensen model, the Larsen model and Fuga, which are three different engineering wake models. The results indicate that the discrepancies between the traditional numerical simulations and power...... production data for narrow wind direction sectors are not caused by an inherent inaccuracy of the current wake models, but rather by the large wind direction uncertainty included in the dataset. The technique can potentially improve wind farm control algorithms and layout optimization because both...

  3. Counter-rotating vortex pairs in the wake of a vertical axis wind turbine

    Science.gov (United States)

    Rolin, Vincent; Porté-Agel, Fernando

    2017-04-01

    Despite the rising popularity of vertical axis wind turbines, or VAWTs, the wakes behind these machines is much less well understood than those behind horizontal axis wind turbines, or HAWTs. A thorough understanding of wakes is important as they can cause turbines in wind farms to produce less power than anticipated and increase the fatigue loading on turbines due to vibrations. In order to gain a deeper understanding of the wake behind a vertical axis wind turbine in atmospheric flow stereo-PIV is implemented in a boundary-layer wind tunnel to produce snapshots of the 3-component velocity field in the wake at various downstream positions. The boundaries of the wake are readily observed due to the high velocity gradients and turbulence present here. Two pairs of counter-rotating vortices similar to those in the wake of yawed HAWTs are also observed. An examination of the momentum fluxes behind the turbine demonstrates that the mean flow induced by these vortices entrains a large quantity of momentum from the unperturbed boundary layer flow above the wake. This effect proves to play an even more significant role than turbulence in reintroducing momentum into the wake. In order to comprehend why the VAWT produces these vortices we modify the double-multiple stream-tube model typically used to predict VAWT performance to incorporate crosswind forces. The similarity between VAWT and yawed HAWT wakes is found not to be coincidental as both cases feature rotors which exert a lateral thrust on the incoming wind which leads to the creation of counter-rotating vortex pairs.

  4. Determination of real-time predictors of the wind turbine wake meandering

    Science.gov (United States)

    Muller, Yann-Aël; Aubrun, Sandrine; Masson, Christian

    2015-03-01

    The present work proposes an experimental methodology to characterize the unsteady properties of a wind turbine wake, called meandering, and particularly its ability to follow the large-scale motions induced by large turbulent eddies contained in the approach flow. The measurements were made in an atmospheric boundary layer wind tunnel. The wind turbine model is based on the actuator disc concept. One part of the work has been dedicated to the development of a methodology for horizontal wake tracking by mean of a transverse hot wire rake, whose dynamic response is adequate for spectral analysis. Spectral coherence analysis shows that the horizontal position of the wake correlates well with the upstream transverse velocity, especially for wavelength larger than three times the diameter of the disc but less so for smaller scales. Therefore, it is concluded that the wake is actually a rather passive tracer of the large surrounding turbulent structures. The influence of the rotor size and downstream distance on the wake meandering is studied. The fluctuations of the lateral force and the yawing torque affecting the wind turbine model are also measured and correlated with the wake meandering. Two approach flow configurations are then tested: an undisturbed incoming flow (modelled atmospheric boundary layer) and a disturbed incoming flow, with a wind turbine model located upstream. Results showed that the meandering process is amplified by the presence of the upstream wake. It is shown that the coherence between the lateral force fluctuations and the horizontal wake position is significant up to length scales larger than twice the wind turbine model diameter. This leads to the conclusion that the lateral force is a better candidate than the upstream transverse velocity to predict in real time the meandering process, for either undisturbed (wake free) or disturbed incoming atmospheric flows.

  5. Transient wake and trajectory of free falling cones with various apex angles

    Science.gov (United States)

    Jin, Yaqing; Hamed, Ali M.; Chamorro, Leonardo P.

    2015-11-01

    The early free-fall stages of cones with a density ratio 1.18 and apex angles of 30°, 45°, 60°, and 90° were studied using a wireless 3-axis gyroscope and accelerometer to describe the cone 3D motions, while the induced flow in the near wake was captured using particle image velocimetry. The Reynolds number based on the cone diameter and the velocity at which the cone reaches the first local velocity maximum is found to set the limit between two distinctive states. Before this Re is reached the departure from the vertical path and cone rotations are insignificant, while relatively rapid growth is observed after this Re. Sequences of vertical velocity, swirling strength, LES-decomposed velocity, and pressure fields shows the formation and growth of a large and initially symmetric recirculation bubble at the cone base and highlights the presence of a symmetric 3D vortex rollup dominating the near-wake in the early stages of the fall. Later, the shear layer at the edge of the wake manifests in the shedding of Kelvin-Helmholtz vortices that, due to the nature of the recirculation bubble, reorganize to constitute a part of the rollup. Later in the fall, the wake loses its symmetry and shows a high population of vortical structures leading to turbulence. The asymmetric wake leads to strong interactions between the flow field and the cone creating complex feedback loops.

  6. Wind Farm Wake: The Horns Rev Photo Case

    Directory of Open Access Journals (Sweden)

    Pierre-Elouan Réthoré

    2013-02-01

    Full Text Available The aim of the paper is to examine the nowadays well-known wind farm wake photographs taken on 12 February 2008 at the offshore Horns Rev 1 wind farm. The meteorological conditions are described from observations from several satellite sensors quantifying clouds, surface wind vectors and sea surface temperature as well as ground-based information at and near the wind farm, including Supervisory Control and Data Acquisition (SCADA data. The SCADA data reveal that the case of fog formation occurred 12 February 2008 on the 10:10 UTC. The fog formation is due to very special atmospheric conditions where a layer of cold humid air above a warmer sea surface re-condensates to fog in the wake of the turbines. The process is fed by warm humid air up-drafted from below in the counter-rotating swirl generated by the clock-wise rotating rotors. The condensation appears to take place primarily in the wake regions with relatively high axial velocities and high turbulent kinetic energy. The wind speed is near cut-in and most turbines produce very little power. The rotational pattern of spiraling bands produces the large-scale structure of the wake fog.

  7. Harbor seal whiskers synchronize with frequency of upstream wake

    Science.gov (United States)

    Beem, Heather; Triantafyllou, Michael

    2013-11-01

    Harbor seals are able to use their whiskers to track minute water movements, such as those left in the wake of a fish. The current study is a simple representation of what the whiskers experience as the seal chases a fish. A scaled whisker model (average cross-flow diameter: dw) is first tested in a towing tank by itself and then towed behind a larger cylinder (dc = 2 . 5dw), which serves as a wake generator. A flexing plate attached to the model base allows the whisker to freely vibrate in response to the flow. Measurements from strain gages on the plate are calibrated to tip deflections. While in the cylinder wake, the whisker vibrates with an amplitude up to ten times higher than it does on its own (A /dw = 0 . 15). Also, the whisker synchronizes with the vortex shedding frequency (fs =0/. 2 U dc) of the upstream cylinder over the range of reduced velocities tested, whereas on its own, the whisker oscillates around its own natural frequency in water. Seals may use the difference in vibration amplitude and frequency between these two cases to help detect the presence of a vortex wake.

  8. Wake Survey of a Marine Current Turbine Under Steady Conditions

    Science.gov (United States)

    Lust, Ethan; Luznik, Luksa; Flack, Karen

    2016-11-01

    A submersible particle image velocimetry (PIV) system was used to study the wake of a horizontal axis marine current turbine. The turbine was tested in a large tow tank facility at the United States Naval Academy. The turbine is a 1/25th scale model of the U.S. National Renewable Energy Laboratory's Reference Model 1 (RM1) tidal turbine. It is a two-bladed turbine measuring 0.8 m in diameter and featuring a NACA 63-618 airfoil cross section. Separate wind tunnel testing has shown the foil section used on the turbine to be Reynolds number independent with respect to lift at the experimental parameters of tow carriage speed (Utow = 1 . 68 m/s) and tip speed ratio (TSR = 7). The wake survey was conducted over an area extending 0.25D forward of the turbine tip path to 2.0D aft, and to a depth of 1.0D beneath the turbine output shaft in the streamwise plane. Each field of view was approximately 30 cm by 30 cm, and each overlapped the adjacent fields of view by 5 cm. The entire flow field was then reconstructed into a single field of investigation. Results include streamwise and vertical ensemble average velocity fields averaged over approximately 1,000 realizations, as well as higher-order statistics. Turbine tip vortex centers were identified and plotted showing increasing aperiodicity with wake age. keywords: horizontal axis marine current turbine, particle image velocimetry, towing tank, wake survey

  9. Towards a Simplified Dynamic Wake Model using POD Analysis

    CERN Document Server

    Bastine, David; Wächter, Matthias; Peinke, Joachim

    2014-01-01

    We apply the proper orthogonal decomposition (POD) to large eddy simulation data of a wind turbine wake in a turbulent atmospheric boundary layer. The turbine is modeled as an actuator disk. Our analyis mainly focuses on the question whether POD could be a useful tool to develop a simplified dynamic wake model. The extracted POD modes are used to obtain approximate descriptions of the velocity field. To assess the quality of these POD reconstructions, we define simple measures which are believed to be relevant for a sequential turbine in the wake such as the energy flux through a disk in the wake. It is shown that only a few modes are necessary to capture basic dynamical aspects of these measures even though only a small part of the turbulent kinetic energy is restored. Furthermore, we show that the importance of the individual modes depends on the measure chosen. Therefore, the optimal choice of modes for a possible model could in principle depend on the application of interest. We additionally present a pos...

  10. How to perform measurements in a hovering animal's wake: physical modelling of the vortex wake of the hawkmoth, Manduca sexta.

    Science.gov (United States)

    Tytell, Eric D; Ellington, Charles P

    2003-09-29

    The vortex wake structure of the hawkmoth, Manduca sexta, was investigated using a vortex ring generator. Based on existing kinematic and morphological data, a piston and tube apparatus was constructed to produce circular vortex rings with the same size and disc loading as a hovering hawkmoth. Results show that the artificial rings were initially laminar, but developed turbulence owing to azimuthal wave instability. The initial impulse and circulation were accurately estimated for laminar rings using particle image velocimetry; after the transition to turbulence, initial circulation was generally underestimated. The underestimate for turbulent rings can be corrected if the transition time and velocity profile are accurately known, but this correction will not be feasible for experiments on real animals. It is therefore crucial that the circulation and impulse be estimated while the wake vortices are still laminar. The scaling of the ring Reynolds number suggests that flying animals of about the size of hawkmoths may be the largest animals whose wakes stay laminar for long enough to perform such measurements during hovering. Thus, at low advance ratios, they may be the largest animals for which wake circulation and impulse can be accurately measured.

  11. Wake measurements for code validations

    DEFF Research Database (Denmark)

    Hansen, Kurt Schaldemose

    2009-01-01

    As part of the EU-TOPFARM project a large number of datasets have been identified for verification of wind farm climate models, aeroelastic load and production models of turbines subjected to three dimensional dynamic wake wind field and the aeroelastic production modeling of a whole wind farm de...

  12. Cooling Signs in Wake Debate

    Science.gov (United States)

    Samuels, Christina A.

    2011-01-01

    More than a year after dismantling a student-assignment policy based on socioeconomic diversity and setting off a wave of reaction that drew national attention, the Wake County, North Carolina, school board took a step that may turn down the temperature of the intense debate. The board, which has been deeply split on an assignment plan for the…

  13. Irregular sleep-wake syndrome

    Science.gov (United States)

    ... normal, but the body clock loses its normal circadian cycle. People with changing work shifts and travelers who ... Abbott SM, Reid KJ, Zee PC. Circadian disorders of the sleep-wake ... Medicine . 6th ed. Philadelphia, PA: Elsevier; 2017:chap 40. ...

  14. Computation of Rotorcraft Wake Geometry using NURBS

    NARCIS (Netherlands)

    Van Hoydonck, W.R.M.

    2013-01-01

    This thesis contains the results of research in the area of rotorcraft aerodynamics with a focus on method development related to the vortical wake generated by rotor blades. It is applied to a vortex tube representation of the wake (using a single NURBS surface) and a simplified filament wake model

  15. Evolution and breakdown of helical vortex wakes behind a wind turbine

    Science.gov (United States)

    Nemes, A.; Sherry, M.; Lo Jacono, D.; Blackburn, H. M.; Sheridan, J.

    2014-12-01

    The wake behind a three-bladed Glauert model rotor in a water channel was investigated. Planar particle image velocimetry was used to measure the velocity fields on the wake centre-line, with snapshots phase-locked to blade position of the rotor. Phase- locked averages of the velocity and vorticity fields are shown, with tip vortex interaction and entanglement of the helical filaments elucidated. Proper orthogonal decomposition and topology-based vortex identification are used to filter the PIV images for coherent structures and locate vortex cores. Application of these methods to the instantaneous data reveals unsteady behaviour of the helical filaments that is statistically quantifiable.

  16. Two Methods for Viscous and Inviscid Free-Wake Analysis of Helicopter Rotors.

    Science.gov (United States)

    1985-08-01

    states that the circulation r = jCv" dx (2.7) over a material contour C (i.e.. a contour which is made up of material particles) remains constant in time (dr...function defined only over a surface. Equation 2.2 may be integrated to yield A , = constant in time (2.32) following a wake point x,. which has velocity v...points on the wake, the velocity is given by (see Eq. 2.29) v (V;( - V;p) (2.37) whereas, (see Eq. 2.32) A;p = constant in time (2.38) following x

  17. Lagrangian transport and chaos in the near wake of the flow around an obstacle: a numerical implementation of lobe dynamics

    Directory of Open Access Journals (Sweden)

    J. Duan

    1997-01-01

    Full Text Available In this paper we study Lagrangian transport in the near wake of the flow around an obstacle, which we take to be a cylinder. In this case, for the range of Reynolds numbers investigated, the flow is two-dimensional and time periodic. We use ideas and methods from transport theory in dynamical systems to describe and quantify transport in the near wake. We numerically solve the Navier-Stokes equations for the velocity field and apply these methods to the resulting numerical representation of the velocity field. We show that the method of lobe dynamics can be used in conjunction with computational fluid dynamics methods to give very detailed and quantitative information about Lagrangian transport. In particular, we show how the stable and unstable manifolds of certain saddle-type stagnation points on the cylinder, and one in the wake, can be used to divide the flow into three distinct regions, an upper wake, a lower wake, and a wake cavity. The significance of the division using stable and unstable manifolds lies in the fact that these invariant manifolds form a template on which the transport occurs. Using this, we compute fluxes from the upper and lower wakes into the wake cavity using the associated turnstile lobes. We also compute escape time distributions as well as compare transport properties for two different Reynolds numbers.

  18. Recent NASA Wake-Vortex Flight Tests, Flow-Physics Database and Wake-Development Analysis

    Science.gov (United States)

    Vicroy, Dan D.; Vijgen, Paul M.; Reimer, Heidi M.; Gallegos, Joey L.; Spalart, Philippe R.

    1998-01-01

    A series of flight tests over the ocean of a four engine turboprop airplane in the cruise configuration have provided a data set for improved understanding of wake vortex physics and atmospheric interaction. An integrated database has been compiled for wake characterization and validation of wake-vortex computational models. This paper describes the wake-vortex flight tests, the data processing, the database development and access, and results obtained from preliminary wake-characterization analysis using the data sets.

  19. Cylinder wake influence on the tonal noise and aerodynamic characteristics of a NACA0018 airfoil

    Science.gov (United States)

    Takagi, Y.; Fujisawa, N.; Nakano, T.; Nashimoto, A.

    2006-11-01

    The influence of cylinder wake on discrete tonal noise and aerodynamic characteristics of a NACA0018 airfoil is studied experimentally in a uniform flow at a moderate Reynolds number. The experiments are carried out by measuring sound pressure levels and spectrum, separation and the reattachment points, pressure distribution, fluid forces, mean-flow and turbulence characteristics around the airfoil with and without the cylinder wake. Present results indicate that the tonal noise from the airfoil is suppressed by the influence of the cylinder wake and the aerodynamic characteristics are improved in comparison with the case without the cylinder wake. These are mainly due to the separation control of boundary layers over the airfoil caused by the wake-induced transition, which is observed by surface flow visualization with liquid- crystal coating. The PIV measurements of the flow field around the airfoil confirm that highly turbulent velocity fluctuation of the cylinder wake induces the transition of the boundary layers and produces an attached boundary layer over the airfoil. Then, the vortex shedding phenomenon near the trailing edge of pressure surface is removed by the influence of the wake and results in the suppression of tonal noise.

  20. Effects of bupropion SR on anterior paralimbic function during waking and REM sleep in depression: preliminary findings using.

    Science.gov (United States)

    Nofzinger, E A; Berman, S; Fasiczka, A; Miewald, J M; Meltzer, C C; Price, J C; Sembrat, R C; Wood, A; Thase, M E

    2001-04-10

    This study sought to clarify the effects of bupropion SR on anterior paralimbic function in depressed patients by studying changes in the activation of these structures from waking to REM sleep both before and after treatment. Twelve depressed patients underwent concurrent EEG sleep studies and [18F]fluoro-2-deoxy-D-glucose ([18F]-FDG) positron emission tomography (PET) scans during waking and during their second REM period of sleep before and after treatment with bupropion SR. Nine subjects completed pre- and post-treatment waking PET studies. Five subjects completed pre- and post-treatment waking and REM sleep PET studies. Bupropion SR treatment did not suppress electrophysiologic measures of REM sleep, nor did it alter an indirect measure of global metabolism during either waking or REM sleep. Bupropion SR treatment reversed the previously observed deficit in anterior cingulate, medial prefrontal cortex and right anterior insula activation from waking to REM sleep. In secondary analyses, this effect was related to a reduction in waking relative metabolism in these structures following treatment in the absence of a significant effect on REM sleep relative metabolism. The implications of these findings for the relative importance of anterior paralimbic function in REM sleep in depression and for the differential effects of anti-depressant treatment on brain function during waking vs. REM sleep are discussed.

  1. Design and analysis of small wind turbine blades with wakes similar to those of industrial scale turbines

    Science.gov (United States)

    Hassanzadeh, Arash; Naughton, Jonathan

    2016-11-01

    A new design approach has been developed for wind turbine blades to be used in wind tunnel experiments that study wind turbine wakes. The approach allows wakes of small scale (2 m diameter) wind turbine rotors to simulate the important physics of wakes generated by a "parent" industrial scale wind turbine rotor despite the difference in size. The design approach forces the normalized normal and tangential force distributions of the small scale wind turbine blades to match those of the "parent" industrial scale wind turbine blades. The wake arises from the interaction between the flow and the blade, which imparts a momentum deficit and rotation to the flow due to the forces created by the blade on the flow. In addition, the wake dynamics and stability are affected by the load distribution across the blade. Thus, it is expected that matching normalized force distributions should result in similar wake structure. To independently assess the blades designed using this approach, the "parent" industrial scale and small scale wind turbine rotors are modeled using a free vortex wake method to study the generation and evolution of the two wakes. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0012671.

  2. Time-resolved vortex wake of a common swift flying over a range of flight speeds.

    Science.gov (United States)

    Henningsson, P; Muijres, F T; Hedenström, A

    2011-06-06

    The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s(-1). The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of 'wing root vortices' are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings.

  3. Self-similarity of far wake behind tandem of two disks

    DEFF Research Database (Denmark)

    Okulov, Valery; Litvinov, I. V.; Naumov, I. V.

    2017-01-01

    In this work we used digital particle image visualization (PIV) to experimentally establish the self-similarity of far wake behind a tandem of two disks of a diameter D (300 mm) with a common axis along the incident flow. The research was performed in a water flume (Re ≈ 2 · 105) with variation...... the tandem exceeded the corresponding value for a single disk, being independent of the distance between the disks (L = 4–8D). The velocity fluctuations behind the tandem did not differ much from the level of fluctuations in the case of a single disk up to a distance of forty calibers downstream, where...... the wake ceased to differ from the background of natural turbulent fluctuations of the incident flow. It has been found that the position of the second disk in the tandem affects the energy loss in the wake due to its expansion but does not influence the decay. The revealed patterns in the wake development...

  4. Propulsive jet simulation with air and helium in launcher wake flows

    Science.gov (United States)

    Stephan, Sören; Radespiel, Rolf

    2016-12-01

    The influence on the turbulent wake of a generic space launcher model due to the presence of an under-expanded jet is investigated experimentally. Wake flow phenomena represent a significant source of uncertainties in the design of a space launcher. Especially critical are dynamic loads on the structure. The wake flow is investigated at supersonic (M=2.9 ) and hypersonic (M=5.9 ) flow regimes. The jet flow is simulated using air and helium as working gas. Due to the lower molar mass of helium, higher jet velocities are realized, and therefore, velocity ratios similar to space launchers can be simulated. The degree of under-expansion of the jet is moderate for the supersonic case (p_e/p_∞ ≈ 5 ) and high for the hypersonic case (p_e/p_∞ ≈ 90 ). The flow topology is described by Schlieren visualization and mean-pressure measurements. Unsteady pressure measurements are performed to describe the dynamic wake flow. The influences of the under-expanded jet and different jet velocities are reported. On the base fluctuations at a Strouhal number, around St_D ≈ 0.25 dominate for supersonic free-stream flows. With air jet, a fluctuation-level increase on the base is observed for Strouhal numbers above St_D ≈ 0.75 in hypersonic flow regime. With helium jet, distinct peaks at higher frequencies are found. This is attributed to the interactions of wake flow and jet.

  5. VELOCITY PROFILES OF TURBULENT OPEN CHANNEL FLOWS

    Institute of Scientific and Technical Information of China (English)

    WANG Dianchang; WANG Xingkui; YU Mingzhong; LI Danxun

    2001-01-01

    The log-law and the wake law of velocity profile for open channel flows are discussed and compared in this paper. Experimental data from eight sources are used to verify the velocity distribution models.The effect of bed level on the velocity profile is analyzed. A formula to calculate the maximum velocity is proposed. In the region of y <δm , the velocity profile approximately follows the log-law. For the region of y >δm , the effect of the aspect ratio is considered. A new velocity profile model on the basis of log-law that can unify all of the hydraulic bed roughness is presented.

  6. Characterization of wake effects and loading status of wind turbine arrays under different inflow conditions

    Science.gov (United States)

    Gao, Xiangyu

    The objective of the present work is to improve the accuracy of Actuator Line Modeling (ALM) in predicting the unsteady aerodynamic loadings on turbine blades and turbine wake by assessing different methods used to determine the relative velocity between the rotating blades and wind. ALM is incorporated into a Large Eddy Simulation (LES) solver in OpenFOAM (Open Field Operations and Manipulations). The aerodynamic loadings are validated by experiment results from National Renewable Energy Laboratory (NREL). Turbine wakes are validated by predictions of large eddy simulation using exact 3D blade geometries from a two-blade NREL Phase VI turbine. Three different relative velocity calculation methods are presented: iterative process in Blade Element Momentum (BEM) theory, local velocity sampling, and Lagrange-Euler Interpolation (LEI). Loadings and wakes obtained from these three methods are compared. It is discovered that LEI functions better than the conventional BEM with iterative process in both loading and wake prediction. Then LES-ALM with LEI is performed on a small wind farm deploying five NREL Phase VI turbines in full wake setting. The power outputs and force coefficients of downstream turbines are evaluated. The LES-ALM with LEI is also performed on a small wind farm deploying 25 NREL Phase VI turbines with different inflow angles (from full wake setting to partial wake setting). The power outputs and force coefficients of each turbine are evaluated under different inflow angles (the angle the rotor has to turn to make the rotor plane face the incoming wind) (0, 5, 15, 30 and 45 degree). The power coefficient distributions and thrust coefficient distributions of the wind farm under each inflow angle are compared. The range of inflow angle which is best for power generation is also discussed. The results demonstrate that the LES-ALM with LEI has the potential to optimize wind farm arrangement and pitch angle of individual turbines.

  7. Turbulent wakes of fractal objects.

    Science.gov (United States)

    Staicu, Adrian; Mazzi, Biagio; Vassilicos, J C; van de Water, Willem

    2003-06-01

    Turbulence of a windtunnel flow is stirred using objects that have a fractal structure. The strong turbulent wakes resulting from three such objects which have different fractal dimensions are probed using multiprobe hot-wire anemometry in various configurations. Statistical turbulent quantities are studied within inertial and dissipative range scales in an attempt to relate changes in their self-similar behavior to the scaling of the fractal objects.

  8. Rotor Wake Vortex Definition Using 3C-PIV Measurements: Corrected for Vortex Orientation

    Science.gov (United States)

    Burley, Casey L.; Brooks, Thomas F.; vanderWall, Berend; Richard, Hughues Richard; Raffel, Markus; Beaumier, Philippe; Delrieux, Yves; Lim, Joon W.; Yu, Yung H.; Tung, Chee

    2003-01-01

    Three-component (3-C) particle image velocimetry (PIV) measurements, within the wake across a rotor disk plane, are used to determine wake vortex definitions important for BVI (Blade Vortex Interaction) and broadband noise prediction. This study is part of the HART II test program conducted using a 40 percent scale BO-105 helicopter main rotor in the German-Dutch Wind Tunnel (DNW). In this paper, measurements are presented of the wake vortex field over the advancing side of the rotor operating at a typical descent landing condition. The orientations of the vortex (tube) axes are found to have non-zero tilt angles with respect to the chosen PIV measurement cut planes, often on the order of 45 degrees. Methods for determining the orientation of the vortex axis and reorienting the measured PIV velocity maps (by rotation/projection) are presented. One method utilizes the vortex core axial velocity component, the other utilizes the swirl velocity components. Key vortex parameters such as vortex core size, strength, and core velocity distribution characteristics are determined from the reoriented PIV velocity maps. The results are compared with those determined from velocity maps that are not corrected for orientation. Knowledge of magnitudes and directions of the vortex axial and swirl velocity components as a function of streamwise location provide a basis for insight into the vortex evolution.

  9. PIV MEASUREMENTS OF THE NEAR-WAKE FLOW OF AN AIRFOIL ABOVE A FREE SURFACE

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    The near-wake flow of a NACA0012 airfoils mounted above a water surface were experimentally studied in a wind/wave tunnel. The main objective of this study is to investigate the influence of the free surface on the structure of the airfoil trailing wake. The flow structure was measured with different ride heights between the airfoil and free surface using a Particle Image Velocimetry (PIV) system. The Reynolds number based on the chord length of the airfoil was about 3.5×103. For each experimental condition, large amount of instantaneous velocity fields were captured and ensemble-averaged to get the spatial distributions of mean velocity and mean vorticity, as well as turbulence statistics. The results show that the flow structures of the airfoil wake varies remarkably with the change in the ride height.

  10. Experimental study of a wake behind a barrier

    Science.gov (United States)

    Tomáš, Dufek; Katarína, Ratkovská

    2017-09-01

    This article describes in detail an experiment which was carried out on a wind tunnel in the Laboratory of the Department of Power Machines, Faculty of Mechanical Engineering, at the University of West Bohemia (UWB), using Particle Image Velocimetry and Stereo Particle Image Velocimetry. PIV is a non-invasive method that allows you to simultaneously measure the flow velocity across the entire field under investigation. In the experiment, the field was located behind the exit of the wind tunnel. The experiment dealt with the measurement of the wake behind a barrier. Measurement with Stereo PIV was carried out in several vertical parallel planes perpendicular to the axis of the tunnel. Conventional PIV method was then used for a horizontal plane passing through the axis of the tunnel at half the height of the barrier. The velocities in the measured plane are expressed by a vector map. In areas not affected by the wake, the speed in the w direction is about 16 m / s. The wake is formed behind the barrier. A shear layer is formed at the boundary between the flowing air and the braked air. A backflow occurs in the area just behind the barrier. The highest speed in the area is achieved in places just behind the exit of the tunnel, where the current is not affected by the barrier. In the direction from the axis and the obstacle, the speed gradually rises from the negative values of the return flow through the zero speed. In addition to the velocity fields, the output from the experimental measurement was also the distribution of the sum of variances, standard deviation and correlation coefficient in the measured planes.

  11. Investigation of Inlet Guide Vane Wakes in a F109 Turbofan Engine with and without Flow Control

    OpenAIRE

    Kozak, Jeffrey D

    2000-01-01

    A series of experiments were conducted in a F109 turbofan engine to investigate the unsteady wake profiles of an Inlet Guide Vane (IGV) at a typical spacing to the downstream fan at subsonic and transonic relative blade velocities. The sharp trailing-edge vanes were designed to produce a wake profile consistent with modern IGV. Time averaged baseline measurements were first performed with the IGV located upstream of the aerodynamic influence of the fan. Unsteady experiments were performed ...

  12. Wind turbine wake structure and yawed effect analysis

    Science.gov (United States)

    Ge, Yuntian

    In this research, the three-dimensional CFD simulation of the complex flow around a two-bladed rotor based on NREL Phase VI Experiment is presented. Its accuracy is demonstrated by comparing torque, thrust, power and power coefficient with experimental data. Then the wake structure behind the rotor is analyzed by showing the vorticity contour and comparing the velocity distribution behind the rotor with previous studies. Furthermore, a triangle relationship among power, upstream wind speed and yaw angle is given by setting control groups simulation. Reasonable explanations to the simulation results are presented. Finally, a linear relationship between power output and cosine value of yaw angle is found and analyzed.

  13. Wake flow variability in a wind farm throughout the diurnal cycle

    Science.gov (United States)

    Abkar, Mahdi; Sharifi, Ahmad; Porté-Agel, Fernando

    2015-04-01

    The atmospheric boundary layer (ABL) undergoes substantial changes in its structure and dynamics in the course of a day due to the transient nature of forcing factors such as the surface fluxes of heat and momentum. The non-stationary nature of the mean wind and turbulence in the ABL, associated with the diurnal cycle, can in turn affect the structure of wind turbine wakes and their effects on power losses within wind farms. In this research, large-eddy simulation (LES) is used to study the evolution of the turbine wakes and their effects on power losses inside an idealized finite-size wind farm in the course of two full diurnal cycles. In the LES, turbulent subgrid-scale stresses are modeled using tuning-free Lagrangian scale-dependent dynamic models, while the turbine-induced forces are parameterized using a dynamic actuator disk model with rotation. To minimize the effects of the initial conditions on the results, our analysis is focused on the second diurnal cycle. The simulation results show a strong effect of atmospheric stability on the wind farm wakes and associated power losses. During the night, the relatively low turbulence intensity of the ambient ABL flow results in a relatively slow rate of entrainment of momentum into the wake and, consequently, a slow wake recovery. In contrast, during the day the positive buoyancy flux and associated turbulence production lead to a relatively high turbulence level in the background ABL flow, which enhances turbulent mixing and wake recovery. As a result, the averaged power deficit in the wind farm is found to increase with increasing thermal stability. In particular for that day, the averaged power deficit increased from 28% under the most convective condition to about 66% under the most stable condition.

  14. Numerical Investigation of Viscous Flow Velocity Field around a Marine Cavitating Propeller

    Directory of Open Access Journals (Sweden)

    Zhifeng Zhu

    2014-11-01

    Full Text Available Velocity field around a ship cavitating propeller is investigated based on the viscous multiphase flow theory. Using a hybrid grid, the unsteady Navier-stokes (N-S and the bubble dynamics equations are solved in this paper to predict the velocity in a propeller wake and the vapor volume fraction on the back side of propeller blade for a uniform inflow. Compared with experimental results, the numerical predictions of cavitation and axial velocity coincide with the measured data. The evolution of tip vortex is shown, and the interaction between the tip vortex of the current blade and the wake of the next one occurs in the far propeller wake. The frequency of velocity signals changes from shaft rate to blade rate. The phenomena reflect the instability of propeller wake.

  15. Evaluation of Fast-Time Wake Vortex Models using Wake Encounter Flight Test Data

    Science.gov (United States)

    Ahmad, Nashat N.; VanValkenburg, Randal L.; Bowles, Roland L.; Limon Duparcmeur, Fanny M.; Gloudesman, Thijs; van Lochem, Sander; Ras, Eelco

    2014-01-01

    This paper describes a methodology for the integration and evaluation of fast-time wake models with flight data. The National Aeronautics and Space Administration conducted detailed flight tests in 1995 and 1997 under the Aircraft Vortex Spacing System Program to characterize wake vortex decay and wake encounter dynamics. In this study, data collected during Flight 705 were used to evaluate NASA's fast-time wake transport and decay models. Deterministic and Monte-Carlo simulations were conducted to define wake hazard bounds behind the wake generator. The methodology described in this paper can be used for further validation of fast-time wake models using en-route flight data, and for determining wake turbulence constraints in the design of air traffic management concepts.

  16. Observing the Evolution of Typhoon Wakes

    Science.gov (United States)

    2010-09-30

    the formation of a sufficiently intense typhoon to begin the cold-wake survey. 1 RESULTS The planning for the ...2010 2. REPORT TYPE 3. DATES COVERED 00-00-2010 to 00-00-2010 4. TITLE AND SUBTITLE Observing the Evolution of Typhoon Wakes 5a. CONTRACT NUMBER...Approved for public release; distribution is unlimited. Observing the Evolution of Typhoon Wakes Steven R. Jayne Woods Hole Oceanographic

  17. The chaotic dynamics of a turbulent wake

    CERN Document Server

    Varon, Eliott; Edwige, Stephie; Gilotte, Philippe; Aider, Jean-Luc

    2016-01-01

    The dynamics of a 3D bi-stable turbulent wake downstream a square-back Ahmed body are experimentally studied in a wind-tunnel through high-frequency wall pressure probes mapping the rear of the model and a horizontal 2D velocity field. The barycenters of the pressure distribution over the rear part of the model and the intensity recirculation are found highly correlated. Focusing on the pressure, its barycenter trajectory exhibits the same characteristics as a Lorenz dynamical system, with two well defined attractors and a 2D Poincar\\'e section displaying a well-defined "butterfly-like" shape. The signal is firstly transformed and analyzed as a telegraph signal showing that its dynamics corresponds to a quasi-random telegraph signal. Then the largest Lyapunov exponent is estimated, leading to a positive value characteristic of strange attractors and chaotic systems. Finally, analyzing the autocorrelation function of the time-series, we compute the correlation dimension, slightly larger than two, very similar ...

  18. Use of Individual Flight Corridors to Avoid Vortex Wakes

    Science.gov (United States)

    Rossow, Vernon J.

    2001-01-01

    Vortex wakes of aircraft pose a hazard to following aircraft until the energetic parts of their flow fields have decayed to a harmless level. It is suggested here that in-trail spacings between aircraft can be significantly and safely reduced by designing an individual, vortex-free flight corridor for each aircraft. Because each aircraft will then have its own flight corridor, which is free of vortex wakes while in use by the assigned aircraft, the time intervals between aircraft operations can be safely reduced to the order of seconds. The productivity of airports can then be substantially increased. How large the offset distances between operational corridors need to be to have them vortex free, and how airports need to be changed to accommodate an individual flight-corridor process for landing and takeoff operations, are explored. Estimates are then made of the productivity of an individual flight-corridor system as a function of the in-trail time interval between operations for various values of wake decay time, runway width, and the velocity of a sidewind. The results confirm the need for short time intervals between aircraft operations if smaller offset distances and increased productivity are to be achieved.

  19. Simulation of spray dispersion in a simplified heavy vehicle wake

    Energy Technology Data Exchange (ETDEWEB)

    Paschkewitz, J S

    2006-01-13

    Simulations of spray dispersion in a simplified tractor-trailer wake have been completed with the goal of obtaining a better understanding of how to mitigate this safety hazard. The Generic Conventional Model (GCM) for the tractor-trailer was used. The impact of aerodynamic drag reduction devices, specifically trailer-mounted base flaps, on the transport of spray in the vehicle wake was considered using the GCM. This analysis demonstrated that base flaps including a bottom plate may actually worsen motorist visibility because of the interaction of fine spray with large vortex flows in the wake. This work suggests that to use computational fluid dynamics (CFD) to design and evaluate spray mitigation strategies the jet or sheet breakup processes can be modeled using an array of injectors of small (< 0.1 mm) water droplets; however the choice of size distribution, injection locations, directions and velocities is largely unknown and requires further study. Possible containment strategies would include using flow structures to 'focus' particles into regions away from passing cars or surface treatments to capture small drops.

  20. Revolutionary Wake Hazard Assessment Tool Project

    Data.gov (United States)

    National Aeronautics and Space Administration — Continuum Dynamics, Inc. (CDI) has developed a Multiple Aircraft Simulation Tool (MAST) that revolutionizes the ability to predict and assess wake interactions...

  1. Wind turbine wake measurement in complex terrain

    Science.gov (United States)

    Hansen, KS; Larsen, GC; Menke, R.; Vasiljevic, N.; Angelou, N.; Feng, J.; Zhu, WJ; Vignaroli, A.; W, W. Liu; Xu, C.; Shen, WZ

    2016-09-01

    SCADA data from a wind farm and high frequency time series measurements obtained with remote scanning systems have been analysed with focus on identification of wind turbine wake properties in complex terrain. The analysis indicates that within the flow regime characterized by medium to large downstream distances (more than 5 diameters) from the wake generating turbine, the wake changes according to local atmospheric conditions e.g. vertical wind speed. In very complex terrain the wake effects are often “overruled” by distortion effects due to the terrain complexity or topology.

  2. Actuator Line Modeling of Wind Turbine Wakes

    DEFF Research Database (Denmark)

    Troldborg, Niels

    2009-01-01

    inflow shows that besides an expected vertical skewed wake the wake also becomes increasingly asymmetric in the horizontal direction as it is convected downstream. The latter phenomena, which is also often observed in measurements, is argued to be caused by the rotation of the wake. A detailed study....... Furthermore, it is shown that the main effect governing the large scale meandering of wakes is the large scale structures of the ambient turbulence field. Finally studies are conducted on rows of respectively two and three turbines. The investigation includes evaluation of the loading on the rotors...

  3. Panel/full-span free-wake coupled method for unsteady aerodynamics of helicopter rotor blade

    Institute of Scientific and Technical Information of China (English)

    Tan Jianfeng; Wang Haowen

    2013-01-01

    A full-span free-wake method is coupled with an unsteady panel method to accurately predict the unsteady aerodynamics of helicopter rotor blades in hover and forward flight.The unsteady potential-based panel method is used to consider aerodynamics of finite thickness multi-bladed rotors,and the full-span free-wake method is applied to simulating dynamics of rotor wake.These methods are tightly coupled through trailing-edge Kutta condition and by converting doublet-wake panels to full-span vortex filaments.A velocity-field integration technique is also adopted to overcome singularity problem during the interaction between the rotor wake and blades.Helicopter rotors including Caradonna-Tung,UH-60A,and AH-1G rotors,are simulated in hover and forward flight to validate the accuracy of this approach.The predicted aerodynamic loads of rotor blades agree well with available measured data and computational fluid dynamics (CFD) results,and the unsteady dynamics of rotor wake is also well simulated.Compared to CFD,the present method obtains accurate results more efficiently and is suitable to rotorcraft aeroelastic analysis.

  4. A Critical Review of the Transport and Decay of Wake Vortices in Ground Effect

    Science.gov (United States)

    Sarpkaya, T.

    2004-01-01

    This slide presentation reviews the transport and decay of wake vortices in ground effect and cites a need for a physics-based parametric model. The encounter of a vortex with a solid body is always a complex event involving turbulence enhancement, unsteadiness, and very large gradients of velocity and pressure. Wake counter in ground effect is the most dangerous of them all. The interaction of diverging, area-varying, and decaying aircraft wake vortices with the ground is very complex because both the vortices and the flow field generated by them are altered to accommodate the presence of the ground (where there is very little room to maneuver) and the background turbulent flow. Previous research regarding vortex models, wake vortex decay mechanisms, time evolution within in ground effect of a wake vortex pair, laminar flow in ground effect, and the interaction of the existing boundary layer with a convected vortex are reviewed. Additionally, numerical simulations, 3-dimensional large-eddy simulations, a probabilistic 2-phase wake vortex decay and transport model and a vortex element method are discussed. The devising of physics-based, parametric models for the prediction of (operational) real-time response, mindful of the highly three-dimensional and unsteady structure of vortices, boundary layers, atmospheric thermodynamics, and weather convective phenomena is required. In creating a model, LES and field data will be the most powerful tools.

  5. Measurements of Wake-Riding Electrons in Antiproton-Carbon-Foil Collisions

    CERN Multimedia

    2002-01-01

    When a charged particle passes through dielectric media, e.g. a thin carbon foil, a ``wake'' is induced. The characteristic wake-potential shows an oscillatory behaviour, with a wavelength of about $ 2 \\pi v _{p} / \\omega _{p} _{l} $ where $ v _{p} $ is the projectile velocity and $ \\omega _{p} _{l} $ the plasmon energy of the target. This induced wake potential is superimposed on the Coulomb potential of the projectile, the latter leading to a pronounced ``cusp'' of electrons leaving the solid at $ v _{e} app v _{p} $ for positively charged projectiles in the MeV region. Correspondingly, an ``anti-cusp'' is expected for antiprotons. \\\\ \\\\ In the solid, the wake-potential leads to an attractive force on electrons, and a dynamic electronic state is predicted both for proton and antiproton projectiles. In the solid, the wake-riding electrons are travelling with the projectile speed $ v _{p} $ Upon exit of the foil, the electron released from the wake-riding state of an antiproton will suddenly find itself in th...

  6. Proton entry into the near-lunar plasma wake for magnetic field aligned flow

    CERN Document Server

    Dhanya, M B; Futaana, Y; Fatemi, S; Holmström, M; Barabash, S; Wieser, M; Wurz, P; Alok, A; Thampi, R S

    2016-01-01

    We report the first observation of protons in the near (100--200 km from surface) and deeper (near anti-subsolar point) lunar plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity ($v_{sw}$) are parallel (aligned flow, angle between IMF and ($v_{sw} \\le 10^\\circ$). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the SWIM sensor of the SARA experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back-tracing of the observed protons suggests that their source is the solar wind. The larger gyro-radii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyro-radii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons in to...

  7. Vortex Wakes of Conventional Aircraft

    Science.gov (United States)

    1975-05-01

    Lewellen and Milton Teske , have developed, under the sponsorship of the United States Navy, a digital computer code which allows one to obtain such...178, June 1972. 67. Lewellen, W.S., M.E. Teske , and C.duP. Donaldson: Turbulent Wakes in a Stratified Fluid. Aeronautical Research Associates of...Princeton, Inc. Report No. 226, September 1974. 68. Lewellen, W.S., M.E. Teske , R.M. Contiliano, G.R. Hilst, and C.duP. Donaldson: Invariant Modeling

  8. Characterization of the supersonic wake of a generic space launcher

    Science.gov (United States)

    Schreyer, A.-M.; Stephan, S.; Radespiel, R.

    2017-03-01

    The wake flow of a generic axisymmetric space-launcher model is investigated experimentally for flow cases with and without propulsive jet to gain insight into the wake-flow phenomena at a supersonic stage of the flight trajectory which is especially critical with respect to dynamic loads on the structure. Measurements are performed at Mach 2.9 and a Reynolds number Re D = 1.3 × 106 based on model diameter D. The nozzle exit velocity of the jet is at Mach 2.5, and the flow is moderately underexpanded ( p e/ p ∞ = 5.7). The flow topology is described based on velocity measurements in the wake by means of particle image velocimetry and schlieren visualizations. Mean and fluctuating mass-flux profiles are obtained from hot-wire measurements, and unsteady wall-pressure measurements on the main-body base are performed simultaneously. This way, the evolution of the wake flow and its spectral content can be observed along with the footprint of this highly dynamic flow on the launcher main-body base. For the case without propulsive jet, a large separated zone is forming downstream of the main body shoulder, and the flow is reattaching further downstream on the afterbody. The afterexpanding propulsive jet (air) causes a displacement of the shear layer away from the wall, preventing the reattachment of the flow. In the spectral analysis of the baseline case, a dominant frequency around St D = 0.25 is found in the pressure-fluctuation signal at the main-body base of the launcher. This frequency is related to the shedding of the separation bubble and is less pronounced in the presence of the propulsive jet. In the shear layer itself, the spectra obtained from the hot-wire signal have a more broadband low-frequency content, which also reflects the characteristic frequency of turbulent structures convected in the shear layer, a swinging motion ( St D = 0.6), as well as the radial flapping motion of the shear layer ( St D = 0.85), respectively. Moving downstream along the

  9. Calibration and Validation of the Dynamic Wake Meandering Model for Implementation in an Aeroelastic Code

    DEFF Research Database (Denmark)

    Aagaard Madsen, Helge; Larsen, Gunner Chr.; Larsen, Torben J.;

    2010-01-01

    on this background, and the basic physical mechanisms in the wake—i.e., the velocity deficit, the meandering of the deficit, and the added turbulence—are modeled as simply as possible in order to make fast computations. In the present paper, the DWM model is presented in a version suitable for full integration...

  10. PIV and LDA measurements of the wake behind a wind turbine model

    Science.gov (United States)

    Naumov, I. V.; Mikkelsen, R. F.; Okulov, V. L.; Sørensen, J. N.

    2014-06-01

    In the present work we review the results of a series of measurements of the flow behind a model scale of a horizontal axis wind turbine rotor carried out at the water flume at Technical University of Denmark (DTU). The rotor is three-bladed and designed using Glauert theory for tip speed ratio λ =5 with a constant design lift coefficient along the span, CLdesign= 0.8. The measurements include dye visualization, Particle Image Velocimetry and Laser Doppler Anemometry. The wake instability has been studied in the range λ =3 - 9 at different cross-sections from the very near wake up to 10 rotor diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different processes and instabilities in the rotor wake. In the far wake a constant frequency corresponding to the Strouhal number was found for the long-scale instabilities. This Strouhal number is in good agreement with the well-known constant that usually characterizes the oscillation in wakes behind bluff bodies. From associated visualizations and reconstructions of the flow field, it was found that the dynamics of the far wake is associated with the precession (rotation) of a helical vortex core. The data indicate that Strouhal number of this precession is independent of the rotor angular speed.

  11. Modeling the Lunar plasma wake

    CERN Document Server

    Holmstrom, M

    2013-01-01

    Bodies that lack a significant atmosphere and internal magnetic fields, such as the Moon and asteroids, can to a first approximation be considered passive absorbers of the solar wind. The solar wind ions and electrons directly impact the surface of these bodies due to the lack of atmosphere, and the interplanetary magnetic field passes through the obstacle relatively undisturbed because the bodies are assumed to be non-conductive. Since the solar wind is absorbed by the body, a wake is created behind the object. This wake is gradually filled by solar wind plasma downstream of the body, through thermal expansion and the resulting ambipolar electric field, along the magnetic field lines. Here we study this plasma expansion into a vacuum using a hybrid plasma solver. In the hybrid approximation, ions are treated as particles, and electrons as a massless fluid. We also derive corresponding one- and two-dimensional model problems that account for the absorbing obstacle. It is found that the absorbing obstacle crea...

  12. Separation control in low pressure turbines using plasma actuators with passing wakes

    Science.gov (United States)

    Burman, Debashish

    A Dielectric Barrier Discharge (DBD) plasma actuator is operated in flow over the suction surface of a Pack-B Low Pressure Turbine (LPT) airfoil at a Reynolds number of 50,000 (based on exit velocity and suction surface length) and inlet free-stream turbulence intensity of 2.5%. Preliminary characterization studies were made of the effect of varying actuator pulsing frequency and duty cycle, actuator edge effects, and orientation of the actuator with the flow. Flow control was demonstrated with the actuator imparting momentum opposite to the stream-wise flow direction, showing that it is possible to use disturbances alone to destabilize the flow and effect transition. No frequencies of strong influence were found over the range tested, indicating that a broad band of effective frequencies exists. Edge effects were found to considerably enhance separation control. Total pressure measurements of the flow without passing wakes were taken using a glass total-pressure tube. Corrections for streamline displacement due to shear and wall effects were made, and comparisons with previous hot-wire measurements were used to validate data. Performance features of conventional two-electrode and a novel three-electrode actuator configuration were compared. Hot-wire anemometry was used to take time-varying ensemble-averaged near-wall velocity measurements of the flow with periodic passing wakes. Corrections were made for near-wall effects, temperature effects, and interference of the electric field. The wakes were generated by a wake generator mechanism located upstream of the airfoil passage. The near-suction-surface total pressure field (flow without wakes) and velocity field (flow with wakes) in the trailing part of the airfoil passage, and the wall-normal gradient of these quantities, were used to demonstrate effective prevention of flow separation using the plasma actuator. Both flows (with and without passing wakes) showed fully attached flow (or very thin separation zones

  13. Direct numerical simulations and modeling of a spatially-evolving turbulent wake

    Science.gov (United States)

    Cimbala, John M.

    1994-01-01

    appropriate noise. After some adjustment period, the flow downstream of the inlet develops into a fully three-dimensional turbulent wake. Of particular interest in the present study is the far wake spreading rate and the self-similar mean and turbulence profiles. At the time of this writing, grid resolution studies are underway, and a code is being written to calculate turbulence statistics from these wake calculations; the statistics will be compared to those from the ongoing PIV wake measurements, those of previous experiments, and those predicted by the various turbulence models. These calculations will lead to significant long-term benefits for the turbulence modeling effort. In particular, quantities such as the pressure-strain correlation and the dissipation rate tensor can be easily calculated from the DNS results, whereas these quantities are nearly impossible to measure experimentally. Improvements to existing turbulence models (and development of new models) require knowledge about flow quantities such as these. Present turbulence models do a very good job at prediction of the shape of the mean velocity and Reynolds stress profiles in a turbulent wake, but significantly underpredict the magnitude of the stresses and the spreading rate of the wake. Thus, the turbulent wake is an ideal flow for turbulence modeling research. By careful comparison and analysis of each term in the modeled Reynolds stress equations, the DNS data can show where deficiencies in the models exist; improvements to the models can then be attempted.

  14. Do trout swim better than eels? Challenges for estimating performance based on the wake of self-propelled bodies

    Science.gov (United States)

    Tytell, Eric D.

    Engineers and biologists have long desired to compare propulsive performance for fishes and underwater vehicles of different sizes, shapes, and modes of propulsion. Ideally, such a comparison would be made on the basis of either propulsive efficiency, total power output or both. However, estimating the efficiency and power output of self-propelled bodies, and particularly fishes, is methodologically challenging because it requires an estimate of thrust. For such systems traveling at a constant velocity, thrust and drag are equal, and can rarely be separated on the basis of flow measured in the wake. This problem is demonstrated using flow fields from swimming American eels, Anguilla rostrata, measured using particle image velocimetry (PIV) and high-speed video. Eels balance thrust and drag quite evenly, resulting in virtually no wake momentum in the swimming (axial) direction. On average, their wakes resemble those of self-propelled jet propulsors, which have been studied extensively. Theoretical studies of such wakes may provide methods for the estimation of thrust separately from drag. These flow fields are compared with those measured in the wakes of rainbow trout, Oncorhynchus mykiss, and bluegill sunfish, Lepomis macrochirus. In contrast to eels, these fishes produce wakes with axial momentum. Although the net momentum flux must be zero on average, it is neither spatially nor temporally homogeneous; the heterogeneity may provide an alternative route for estimating thrust. This review shows examples of wakes and velocity profiles from the three fishes, indicating challenges in estimating efficiency and power output and suggesting several routes for further experiments. Because these estimates will be complicated, a much simpler method for comparing performance is outlined, using as a point of comparison the power lost producing the wake. This wake power, a component of the efficiency and total power, can be estimated in a straightforward way from the flow

  15. Wake-vortex decay in external turbulence

    NARCIS (Netherlands)

    Kuczaj, Arkadiusz K.; Armenio, V.; Fröhlich, J.; Geurts, Bernardus J.

    2010-01-01

    Wake vortices that form behind a moving aircraft represent 11 safety COil cern for other aircraft.s that follow. These tornado-like wake structures may persist for several minutes, extending for many kilometers across the sky. This safety issue is particularly important close to major airports where

  16. Wake-vortex decay in external turbulence

    NARCIS (Netherlands)

    Geurts, Bernard J.; Kuczaj, Arkadiusz K.; Armenio, V.; Fröhlich, J.; Geurts, B.J.

    2010-01-01

    Wake vortices that form behind a moving aircraft represent 11 safety COil cern for other aircraft.s that follow. These tornado-like wake structures may persist for several minutes, extending for many kilometers across the sky. This safety issue is particularly important close to major airports where

  17. Statistical analyses of a screen cylinder wake

    Science.gov (United States)

    Mohd Azmi, Azlin; Zhou, Tongming; Zhou, Yu; Cheng, Liang

    2017-02-01

    The evolution of a screen cylinder wake was studied by analysing its statistical properties over a streamwise range of x/d={10-60}. The screen cylinder was made of a stainless steel screen mesh of 67% porosity. The experiments were conducted in a wind tunnel at a Reynolds number of 7000 using an X-probe. The results were compared with those obtained in the wake generated by a solid cylinder. It was observed that the evolution of the statistics in the wake of the screen cylinder was different from that of a solid cylinder, reflecting the differences in the formation of the organized large-scale vortices in both wakes. The streamwise evolution of the Reynolds stresses, energy spectra and cross-correlation coefficients indicated that there exists a critical location that differentiates the screen cylinder wake into two regions over the measured streamwise range. The formation of the fully formed large-scale vortices was delayed until this critical location. Comparison with existing results for screen strips showed that although the near-wake characteristics and the vortex formation mechanism were similar between the two wake generators, variation in the Strouhal frequencies was observed and the self-preservation states were non-universal, reconfirming the dependence of a wake on its initial condition.

  18. Detecting wind turbine wakes with nacelle lidars

    DEFF Research Database (Denmark)

    Held, D. P.; Larvol, A.; Mann, Jakob

    2017-01-01

    variance is used as a detection parameter for wakes. A one month long measurement campaign, where a continuous-wave lidar on a turbine has been exposed to multiple wake situations, is used to test the detection capabilities. The results show that it is possible to identify situation where a downstream...

  19. PIV and LDA measurements of the wake behind a wind turbine model

    DEFF Research Database (Denmark)

    Naumov, I. V.; Mikkelsen, Robert Flemming; Okulov, Valery

    2014-01-01

    diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid...... to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different...

  20. Strongly Stratified Turbulence Wakes and Mixing Produced by Fractal Wakes

    Science.gov (United States)

    Dimitrieva, Natalia; Redondo, Jose Manuel; Chashechkin, Yuli; Fraunie, Philippe; Velascos, David

    2017-04-01

    This paper describes Shliering and Shadowgraph experiments of the wake induced mixing produced by tranversing a vertical or horizontal fractal grid through the interfase between two miscible fluids at low Atwood and Reynolds numbers. This is a configuration design to models the mixing across isopycnals in stably-stratified flows in many environmental relevant situations (either in the atmosphere or in the ocean. The initial unstable stratification is characterized by a reduced gravity: g' = gΔρ ρ where g is gravity, Δρ being the initial density step and ρ the reference density. Here the Atwood number is A = g' _ 2 g . The topology of the fractal wake within the strong stratification, and the internal wave field produces both a turbulent cascade and a wave cascade, with frecuen parametric resonances, the envelope of the mixing front is found to follow a complex non steady 3rd order polinomial function with a maximum at about 4-5 Brunt-Vaisalla non-dimensional time scales: t/N δ = c1(t/N) + c2g Δρ ρ (t/N)2 -c3(t/N)3. Conductivity probes and Shliering and Shadowgraph visual techniques, including CIV with (Laser induced fluorescence and digitization of the light attenuation across the tank) are used in order to investigate the density gradients and the three-dimensionality of the expanding and contracting wake. Fractal analysis is also used in order to estimate the fastest and slowest growing wavelengths. The large scale structures are observed to increase in wave-length as the mixing progresses, and the processes involved in this increase in scale are also examined.Measurements of the pointwise and horizontally averaged concentrations confirm the picture obtained from past flow visualization studies. They show that the fluid passes through the mixing region with relatively small amounts of molecular mixing,and the molecular effects only dominate on longer time scales when the small scales have penetrated through the large scale structures. The Non

  1. Wake Measurements in ECN's Scaled Wind Farm

    Energy Technology Data Exchange (ETDEWEB)

    Wagenaar, J.W.; Schepers, J.G. [ECN Wind Energy, Petten (Netherlands)

    2013-02-15

    In ECN's scaled wind farm the wake evolution is studied in two different situations. A single wake is studied at two different locations downstream of a turbine and a single wake is studied in conjunction with a triple wake. Here, the wake is characterized by the wind speed ratio, the turbulence intensity, the vertical wind speed and the turbulence (an)isotropy. Per situation all wake measurements are taken simultaneously together with the inflow conditions.

  2. Comparison of a Coupled Near and Far Wake Model With a Free Wake Vortex Code

    DEFF Research Database (Denmark)

    Pirrung, Georg; Riziotis, Vasilis; Aagaard Madsen, Helge

    2016-01-01

    This paper presents the integration of a near wake model for trailing vorticity, which is based on a prescribed wake lifting line model proposed by Beddoes, with a BEM-based far wake model and a 2D shed vorticity model. The resulting coupled aerodynamics model is validated against lifting surface...... computations performed using a free wake panel code. The focus of the description of the aerodynamics model is on the numerical stability, the computation speed and the accuracy of 5 unsteady simulations. To stabilize the near wake model, it has to be iterated to convergence, using a relaxation factor that has...... induction modeling at slow time scales. Finally, the unsteady airfoil aerodynamics model is extended to provide the unsteady bound circulation for the near wake model and to improve 10 the modeling of the unsteady behavior of cambered airfoils. The model comparison with results from a free wake panel code...

  3. Bifurcation and instability problems in vortex wakes

    Energy Technology Data Exchange (ETDEWEB)

    Aref, H [Center for Fluid Dynamics and Department of Physics, Technical University of Denmark, Kgs. Lyngby, DK-2800 (Denmark); Broens, M [Center for Fluid Dynamics and Department of Mathematics, Technical University of Denmark, Kgs. Lyngby, DK-2800 (Denmark); Stremler, M A [Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (United States)

    2007-04-15

    A number of instability and bifurcation problems related to the dynamics of vortex wake flows are addressed using various analytical tools and approaches. We discuss the bifurcations of the streamline pattern behind a bluff body as a vortex wake is produced, a theory of the universal Strouhal-Reynolds number relation for vortex wakes, the bifurcation diagram for 'exotic' wake patterns behind an oscillating cylinder first determined experimentally by Williamson and Roshko, and the bifurcations in topology of the streamlines pattern in point vortex streets. The Hamiltonian dynamics of point vortices in a periodic strip is considered. The classical results of von Karman concerning the structure of the vortex street follow from the two-vortices-in-a-strip problem, while the stability results follow largely from a four-vortices-in-a-strip analysis. The three-vortices-in-a-strip problem is argued to be relevant to the wake behind an oscillating body.

  4. On the evolution of waking and sleeping.

    Science.gov (United States)

    Rial, R; Nicolau, M C; Lopez-Garcia, J A; Almirall, H

    1993-02-01

    1. The aim of this paper is to present a new hypothesis to explain the evolution of the sleeping and waking states. 2. We propose that the reptilian waking state and the mammalian slow wave sleep are homologous states. 3. We also propose that instead of looking at the polygraphic sleep as a new evolutive acquisition of mammals and birds, it seems more convenient to look at the full waking state; the "advanced wakefulness" as the true new evolutionary acquisition of these animals. 4. These conclusions are reached after examining some available reports of slow wave electroencephalogram in waking reptiles, some other reports showing signs of rapid eye movement sleep in this same group and the coevolution between sleep states and thermoregulation. Finally, a clear parallelism between sleep ontogeny and phylogeny is shown under the light of the proposed hypothesis.

  5. Experimental study on the near wake behind two side-by-side cylinders of unequal diameters

    Science.gov (United States)

    Gao, Yangyang; Yu, Dingyong; Tan, Soonkeat; Wang, Xikun; Hao, Zhiyong

    2010-10-01

    The wake structure behind two side-by-side circular cylinders with unequal diameter was investigated using the particle image velocimetry (PIV) technique. This investigation focused on the asymmetrical flow within the subcritical Reynolds number regime. A comparison between the time-averaged mean flow field of one cylinder and that of two side-by-side cylinders of unequal diameter was made for elucidating the mean flow characteristics attributable to the latter. The distribution of mean velocity behind two side-by-side unequal cylinders was distinctly different from that behind one cylinder, and an asymmetric combination was observed behind the two cylinders. The present paper also presents the authors' interpretation of the generation mechanism of gap flow deflection based on the instantaneous vorticity contours and velocity distribution. The effect of Reynolds number on flow structure was studied by analyzing the mean velocity distribution and time-averaged wake structures. The results showed that vortex formation length decreases with Reynolds number.

  6. Effects of Freestream Turbulence in a Model Wind Turbine Wake

    Directory of Open Access Journals (Sweden)

    Yaqing Jin

    2016-10-01

    Full Text Available The flow structure in the wake of a model wind turbine is explored under negligible and high turbulence in the freestream region of a wind tunnel at R e ∼ 7 × 10 4 . Attention is placed on the evolution of the integral scale and the contribution of the large-scale motions from the background flow. Hotwire anemometry was used to obtain the streamwise velocity at various streamwise and spanwise locations. The pre-multiplied spectral difference of the velocity fluctuations between the two cases shows a significant energy contribution from the background turbulence on scales larger than the rotor diameter. The integral scale along the rotor axis is found to grow linearly with distance, independent of the incoming turbulence levels. This scale appears to reach that of the incoming flow in the high turbulence case at x / d ∼ 35–40. The energy contribution from the turbine to the large-scale flow structures in the low turbulence case increases monotonically with distance. Its growth rate is reduced past x / d ∼ 6–7. There, motions larger than the rotor contribute ∼ 50 % of the total energy, suggesting that the population of large-scale motions is more intense in the intermediate field. In contrast, the wake in the high incoming turbulence is quickly populated with large-scale motions and plateau at x / d ∼ 3 .

  7. Anisotropy of the Reynolds stress tensor in the wakes of wind turbine arrays in Cartesian arrangements with counter-rotating rotors

    Science.gov (United States)

    Hamilton, Nicholas; Cal, Raúl Bayoán

    2015-01-01

    A 4 × 3 wind turbine array in a Cartesian arrangement was constructed in a wind tunnel setting with four configurations based on the rotational sense of the rotor blades. The fourth row of devices is considered to be in the fully developed turbine canopy for a Cartesian arrangement. Measurements of the flow field were made with stereo particle-image velocimetry immediately upstream and downstream of the selected model turbines. Rotational sense of the turbine blades is evident in the mean spanwise velocity W and the Reynolds shear stress - v w ¯ . The flux of kinetic energy is shown to be of greater magnitude following turbines in arrays where direction of rotation of the blades varies. Invariants of the normalized Reynolds stress anisotropy tensor (η and ξ) are plotted in the Lumley triangle and indicate that distinct characters of turbulence exist in regions of the wake following the nacelle and the rotor blade tips. Eigendecomposition of the tensor yields principle components and corresponding coordinate system transformations. Characteristic spheroids representing the balance of components in the normalized anisotropy tensor are composed with the eigenvalues yielding shapes predicted by the Lumley triangle. Rotation of the coordinate system defined by the eigenvectors demonstrates trends in the streamwise coordinate following the rotors, especially trailing the top-tip of the rotor and below the hub. Direction of rotation of rotor blades is shown by the orientation of characteristic spheroids according to principle axes. In the inflows of exit row turbines, the normalized Reynolds stress anisotropy tensor shows cumulative effects of the upstream turbines, tending toward prolate shapes for uniform rotational sense, oblate spheroids for streamwise organization of rotational senses, and a mixture of characteristic shapes when the rotation varies by row. Comparison between the invariants of the Reynolds stress anisotropy tensor and terms from the mean

  8. Effects of beam velocity and density on an ion-beam pulse moving in magnetized plasmas

    CERN Document Server

    Zhao, Xiao-ying; Zhao, Yong-tao; Qi, Xin; Yang, Lei

    2016-01-01

    The wakefield and stopping power of an ion-beam pulse moving in magnetized plasmas are investigated by particle-in-cell (PIC) simulations. The effects of beam velocity and density on the wake and stopping power are discussed. In the presence of magnetic field, it is found that beside the longitudinal conversed V-shaped wakes, the strong whistler wave are observed when low-density and low-velocity pulses moving in plasmas. The corresponding stopping powers are enhanced due to the drag of these whistler waves. As beam velocities increase, the whistler waves disappear, and only are conversed V-shape wakes observed. The corresponding stopping powers are reduced compared with these in isotropic plasmas. When high-density pulses transport in the magnetized plasmas, the whistler waves are greatly inhibited for low-velocity pulses and disappear for high-velocity pulses. Additionally, the magnetic field reduces the stopping powers for all high-density cases.

  9. Experimental Study of Turbulent Wake Behind a Sine Shaped Island in a Shallow-Water Layer

    Institute of Scientific and Technical Information of China (English)

    李玲; 李玉梁; 陈嘉范

    2002-01-01

    A series of experiments is conducted to study shallow-water flow in the wake of a sine shaped island. Digital particle imaging velocimetry (DPIV) is used to measure velocities in the turbulent wake behind a sine shaped island for different characteristic coefficients S. Flow streamlines are given for the wake flows. The measured results show that the characteristic coefficient S is uniquely related to the flow pattern around a sine shaped island in a shallow water layer. An S value of approximately 0.20 is the critical value for transition from a vortex street to unsteady flow and a value of approximately 0.40 is the critical value for transition from unsteady flow to steady flow.

  10. Non-linear Ion-wake Excitation by Ultra-relativistic Electron Wakefields

    CERN Document Server

    Sahai, Aakash A

    2015-01-01

    The excitation of a non-linear ion-wake by a train of ultra-relativistic plasmons is modeled and its use for a novel regime of positron acceleration is explored. Its channel-like structure is independent of the energy-source driving the bubble-shaped slowly-propagating high phase-velocity electron density waves. The back of the bubble electron compression sucks-in the ions and the space-charge within the bubble expels them, forming a near-void channel with on-axis and bubble-edge density-spikes. The channel-edge density-spike is driven radially outwards as a non-linear ion acoustic-wave by the wake electron thermal pressure. OSIRIS PIC simulations are used to study the ion-wake structure, its evolution and its use for positron acceleration.

  11. Investigation on the wake evolution of contra-rotating propeller using RANS computation and SPIV measurement

    Directory of Open Access Journals (Sweden)

    Kwang-Jun Paik

    2015-05-01

    Full Text Available The wake characteristics of Contra-Rotating Propeller (CRP were investigated using numerical simulation and flow measurement. The numerical simulation was carried out with a commercial CFD code based on a Reynolds Averaged Navier-Stokes (RANS equations solver, and the flow measurement was performed with Stereoscopic Particle Image Velocimetry (SPIV system. The simulation results were validated through the comparison with the experiment results measured around the leading edge of rudder to investigate the effect of propeller operation under the conditions without propeller, with forward propeller alone, and with both forward and aft propellers. The evolution of CRP wake was analyzed through velocity and vorticity contours on three transverse planes and one longitudinal plane based on CFD results. The trajectories of propeller tip vortex core in the cases with and without aft propeller were also compared, and larger wake contraction with CRP was confirmed.

  12. Wake-Mediated Propulsion of an Upstream Particle in Two-Dimensional Plasma Crystals

    Science.gov (United States)

    Laut, I.; Räth, C.; Zhdanov, S. K.; Nosenko, V.; Morfill, G. E.; Thomas, H. M.

    2017-02-01

    The wake-mediated propulsion of an "extra" particle in a channel of two neighboring rows of a two-dimensional plasma crystal, observed experimentally by Du et al. [Phys. Rev. E 89, 021101(R) (2014), 10.1103/PhysRevE.89.021101], is explained in simulations and theory. We use the simple model of a pointlike ion wake charge to reproduce this intriguing effect in simulations, allowing for a detailed investigation and a deeper understanding of the underlying dynamics. We show that the nonreciprocity of the particle interaction, owing to the wake charges, is responsible for a broken symmetry of the channel that enables a persistent self-propelled motion of the extra particle. We find good agreement of the terminal extra-particle velocity with our theoretical considerations and with experiments.

  13. Investigation on the wake evolution of contra-rotating propeller using RANS computation and SPIV measurement

    Directory of Open Access Journals (Sweden)

    Paik Kwang-Jun

    2015-05-01

    Full Text Available The wake characteristics of Contra-Rotating Propeller (CRP were investigated using numerical simulation and flow measurement. The numerical simulation was carried out with a commercial CFD code based on a Reynolds Averaged Navier-Stokes (RANS equations solver, and the flow measurement was performed with Stereoscopic Particle Image Velocimetry (SPIV system. The simulation results were validated through the comparison with the experiment results measured around the leading edge of rudder to investigate the effect of propeller operation under the conditions without propeller, with forward propeller alone, and with both forward and aft propellers. The evolution of CRP wake was analyzed through velocity and vorticity contours on three transverse planes and one longitudinal plane based on CFD results. The trajectories of propeller tip vortex core in the cases with and without aft propeller were also compared, and larger wake contraction with CRP was confirmed.

  14. Wake patterns of the wings and tail of hovering hummingbirds

    Science.gov (United States)

    Altshuler, Douglas L.; Princevac, Marko; Pan, Hansheng; Lozano, Jesse

    The flow fields of slowly flying bats and fasterflying birds differ in that bats produce two vortex loops during each stroke, one per wing, and birds produce a single vortex loop per stroke. In addition, the circulation at stroke transition approaches zero in bats but remains strong in birds. It is unknown if these difference derive from fundamental differences in wing morphology or are a consequence of flight speed. Here, we present an analysis of the horizontal flow field underneath hovering Anna's hummingbirds (Calypte anna) to describe the wake of a bird flying at zero forward velocity. We also consider how the hummingbird tail interacts with the wake generated by the wings. High-speed image recording and analysis from three orthogonal perspectives revealed that the wing tips reach peak velocities in the middle of each stroke and approach zero velocity at stroke transition. Hummingbirds use complex tail kinematic patterns ranging from in phase to antiphase cycling with respect to the wings, covering several phase shifted patterns. We employed particle image velocimetry to attain detailed horizontal flow measurements at three levels with respect to the tail: in the tail, at the tail tip, and just below the tail. The velocity patterns underneath the wings indicate that flow oscillates along the ventral-dorsal axis in response to the down- and up-strokes and that the sideways flows with respect to the bird are consistently from the lateral to medial. The region around the tail is dominated by axial flows in dorsal to ventral direction. We propose that these flows are generated by interaction between the wakes of the two wings at the end of the upstroke, and that the tail actively defects flows to generate moments that contribute to pitch stability. The flow fields images also revealed distinct vortex loops underneath each wing, which were generated during each stroke. From these data, we propose a model for the primary flow structures of hummingbirds that more

  15. NUMERICAL STUDY ON THE WAKE EVOLUTION OF CONTRA-ROTATING PROPELLER IN PROPELLER OPEN WATER AND SELF-PROPULSION CONDITIONS

    Directory of Open Access Journals (Sweden)

    Kwang-Jun Paik

    2017-01-01

    Full Text Available In this study, the wake characteristics of a contra-rotating propeller (CRP were investigated using a numerical simulation. The numerical simulation was carried out with a Reynolds averaged Navier−Stokes equations solver. The numerical simulations were performed on CRPs in both propeller open water and self-propulsion conditions to investigate their wake evolution characteristics. To study the effect of the rudder on the wake in the self-propulsion condition, the numerical simulations with and without a rudder were compared. The evolution of the CRP wake was analysed through velocity and vorticity contours on one transverse plane between the forward and aft propellers and two transverse planes located downstream of the CRP. The variations of thrust and torque of the forward and aft propellers during one revolution of the CRP were compared to investigate the interaction between forward and aft propellers and the effect of a rudder.

  16. Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel.

    Science.gov (United States)

    Henningsson, P; Spedding, G R; Hedenström, A

    2008-03-01

    In this paper we describe the flight characteristics of a swift (Apus apus) in cruising flight at three different flight speeds (8.0, 8.4 and 9.2 m s(-1)) in a low turbulence wind tunnel. The wingbeat kinematics were recorded by high-speed filming and the wake of the bird was visualized by digital particle image velocimetry (DPIV). Certain flight characteristics of the swift differ from those of previously studied species. As the flight speed increases, the angular velocity of the wingbeat remains constant, and so as the wingbeat amplitude increases, the frequency decreases accordingly, as though the flight muscles were contracting at a fixed rate. The wings are also comparatively inflexible and are flexed or retracted rather little during the upstroke. The upstroke is always aerodynamically active and this is reflected in the wake, where shedding of spanwise vorticity occurs throughout the wingbeat. Although the wake superficially resembles those of other birds in cruising flight, with a pair of trailing wingtip vortices connected by spanwise vortices, the continuous shedding of first positive vorticity during the downstroke and then negative vorticity during the upstroke suggests a wing whose circulation is gradually increasing and then decreasing during the wingbeat cycle. The wake (and implied wing aerodynamics) are not well described by discrete vortex loop models, but a new wake-based model, where incremental spanwise and streamwise variations of the wake impulse are integrated over the wingbeat, shows good agreement of the vertical momentum flux with the required weight support. The total drag was also estimated from the wake alone, and the calculated lift:drag ratio of approximately 13 for flapping flight is the highest measured yet for birds.

  17. Wake-induced unsteady flows: Their impact on rotor performance and wake rectification

    Energy Technology Data Exchange (ETDEWEB)

    Adamczyk, J.J. [National Aeronautics and Space Administration, Brook Park, OH (United States). Lewis Research Center; Celestina, M.L. [Sverdrup Technology, Inc., Brook Park, OH (United States). Dept. of Aeromechanics; Chen, J.P. [Mississippi State Univ., MS (United States). NSF Engineering Research Center

    1996-01-01

    The impact of wake-induced unsteady flows on blade row performance and the wake rectification process is examined by means of numerical simulation. The passage of a stator wake through a downstream rotor is first simulated using a three-dimensional unsteady viscous flow code. The results from this simulation are used to define two steady-state inlet conditions for a three-dimensional viscous flow simulation of a rotor operating in isolation. The results obtained from these numerical simulations are then compared to those obtained form the unsteady simulation both to quantify the impact of the wake-induced unsteady flow field on rotor performance and to identify the flow processes which impact wake rectification. Finally, the results from this comparison study are related to an existing model, which attempts to account for the impact of wake-induced unsteady flows on the performance of multistage turbomachinery.

  18. Ion potential in warm dense matter: wake effects due to streaming degenerate electrons.

    Science.gov (United States)

    Moldabekov, Zhandos; Ludwig, Patrick; Bonitz, Michael; Ramazanov, Tlekkabul

    2015-02-01

    The effective dynamically screened potential of a classical ion in a stationary flowing quantum plasma at finite temperature is investigated. This is a key quantity for thermodynamics and transport of dense plasmas in the warm-dense-matter regime. This potential has been studied before within hydrodynamic approaches or based on the zero temperature Lindhard dielectric function. Here we extend the kinetic analysis by including the effects of finite temperature and of collisions based on the Mermin dielectric function. The resulting ion potential exhibits an oscillatory structure with attractive minima (wakes) and, thus, strongly deviates from the static Yukawa potential of equilibrium plasmas. This potential is analyzed in detail for high-density plasmas with values of the Brueckner parameter in the range 0.1≤r(s)≤1 for a broad range of plasma temperature and electron streaming velocity. It is shown that wake effects become weaker with increasing temperature of the electrons. Finally, we obtain the minimal electron streaming velocity for which attraction between ions occurs. This velocity turns out to be less than the electron Fermi velocity. Our results allow for reliable predictions of the strength of wake effects in nonequilibrium quantum plasmas with fast streaming electrons showing that these effects are crucial for transport under warm-dense-matter conditions, in particular for laser-matter interaction, electron-ion temperature equilibration, and stopping power.

  19. Spatial characterization of vortical structures and internal waves in stratified turbulent wake using POD

    Science.gov (United States)

    Diamessis, P.; Gurka, R.; Liberzon, A.

    2008-11-01

    Proper orthogonal decomposition (POD) is applied to 2-D slices of vorticity and horizontal divergence obtained from the 3-D DNS of the stratified turbulent wake of a towed sphere at Re=5x10^3 and Fr=4. Slices are sampled along the stream-depth (Oxz) and stream-span planes (Oxy) at 231 times during the interval Nt[12,35]. POD was chosen amongst the available statistical tools due to its advantage in characterization of simulated and experimentally measured velocity gradient fields, as previously demonstrated for turbulent boundary layers. In the Oxz planes, at the wake centerline, the higher most energetic modes reveal a structure similar of the structure of late-time stratified wakes. Off-set from centerline, the signature of internal waves in the form of forward-inclined coherent beams extending into the ambient becomes evident. The angle of inclination becomes progressively vertical with increasing POD mode. Lower POD modes on the Oyz planes show a layered structure in the wake core with coherent beams radiating out into the ambient over a broad range of angles. Further insight is provided through the relative energy spectra distribution of the vorticity eigenmodes. POD analysis has provided a statistical description of the geometrical features previously observed in instantaneous flow fields of stratified turbulent wakes.

  20. Computational hydrodynamics of animal swimming: boundary element method and three-dimensional vortex wake structure.

    Science.gov (United States)

    Cheng, J Y; Chahine, G L

    2001-12-01

    The slender body theory, lifting surface theories, and more recently panel methods and Navier-Stokes solvers have been used to study the hydrodynamics of fish swimming. This paper presents progress on swimming hydrodynamics using a boundary integral equation method (or boundary element method) based on potential flow model. The unsteady three-dimensional BEM code 3DynaFS that we developed and used is able to model realistic body geometries, arbitrary movements, and resulting wake evolution. Pressure distribution over the body surface, vorticity in the wake, and the velocity field around the body can be computed. The structure and dynamic behavior of the vortex wakes generated by the swimming body are responsible for the underlying fluid dynamic mechanisms to realize the high-efficiency propulsion and high-agility maneuvering. Three-dimensional vortex wake structures are not well known, although two-dimensional structures termed 'reverse Karman Vortex Street' have been observed and studied. In this paper, simulations about a swimming saithe (Pollachius virens) using our BEM code have demonstrated that undulatory swimming reduces three-dimensional effects due to substantially weakened tail tip vortex, resulting in a reverse Karman Vortex Street as the major flow pattern in the three-dimensional wake of an undulating swimming fish.

  1. CFD simulation on Kappel propeller with a hull wake field

    DEFF Research Database (Denmark)

    Shin, Keun Woo; Andersen, Poul; Møller Bering, Rasmus

    2013-01-01

    -water characteristics. The hull wake field is simulated without the propeller flow to check whether it is preserved at the propeller plane or not. Propeller flow simulations are made with mean axial wake varying only along the radius (i.e. circumferentially uniform), whole axial wake and upstream transverse wake...

  2. Comparing satellite SAR and wind farm wake models

    DEFF Research Database (Denmark)

    Hasager, Charlotte Bay; Vincent, P.; Husson, R.

    2015-01-01

    The aim of the paper is to present offshore wind farm wake observed from satellite Synthetic Aperture Radar (SAR) wind fields from RADARSAT-1/-2 and Envisat and to compare these wakes qualitatively to wind farm wake model results. From some satellite SAR wind maps very long wakes are observed. Th...

  3. PIV measurements of near wake behind a U-grooved cylinder

    Science.gov (United States)

    Lim, H.-C.; Lee, S.-J.

    2003-08-01

    The flow structure around a circular cylinder with U-grooved surfaces has been investigated experimentally. The results were compared with that of a smooth cylinder having the same diameter. Drag force and turbulence statistics of wake behind each cylinder were measured for Reynolds numbers based on the cylinder diameter (/D=60mm) in the range ReD=8×103-1.4×105. At ReD=1.4×105, the U-type grooves reduce the drag coefficient acting on the cylinder by 18.6%, compared with that of smooth cylinder. The flow characteristics of wake behind the U-grooved cylinder have been analyzed using two kinds of particle image velocimetry (PIV) velocity measurement techniques, cinematic PIV and high-resolution PIV. Consecutive instantaneous velocity fields were measured using the cinematic PIV technique at time interval of 5ms, corresponding to about 1% of the vortex shedding frequency of the wake. The instantaneous velocity fields measured with the high-resolution PIV technique were ensemble-averaged to get the spatial distributions of turbulent statistics including turbulent intensities and turbulent kinetic energy. For the case of smooth cylinder, large-scale vortices formed behind the cylinder maintain round shape and do not spread out noticeably in the near wake. However, for the case of U-grooved cylinder, the vortices are largely distorted and spread out significantly as they go downstream. The longitudinal grooves seem to shift the location of spanwise vortices toward the cylinder, reducing the vortex formation region, compared with the smooth cylinder. The sharp peaks of longitudinal U-shaped grooves also suppress the formation of large-scale secondary streamwise vortices. The secondary vortices are broken into smaller eddies, reducing turbulent kinetic energy in the near-wake region.

  4. X-Ray Wakes in Abell 160

    CERN Document Server

    Drake, N; Sakelliou, I; Pinkney, J C; Drake, Nick; Merrifield, Michael R.; Sakelliou, Irini; Pinkney, Jason C.

    2000-01-01

    `Wakes' of X-ray emission have now been detected trailing behind a few (at least seven) elliptical galaxies in clusters. To quantify how widespread this phenomenon is, and what its nature might be, we have obtained a deep (70 ksec) X-ray image of the poor cluster Abell 160 using the ROSAT HRI. Combining the X-ray data with optical positions of confirmed cluster members, and applying a statistic designed to search for wake-like excesses, we confirm that this phenomenon is observed in galaxies in this cluster. The probability that the detections arise from chance is less than 0.0038. Further, the wakes are not randomly distributed in direction, but are preferentially oriented pointing away from the cluster centre. This arrangement can be explained by a simple model in which wakes arise from the stripping of their host galaxies' interstellar media due to ram pressure against the intracluster medium through which they travel.

  5. CRED REA Algal Assessments Wake Atoll, 2005

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Twelve quadrats were sampled along 2 consecutively-placed, 25m transect lines as part of Rapid Ecological Assessments conducted at 14 sites at Wake Atoll in October...

  6. Free wake models for vortex methods

    Energy Technology Data Exchange (ETDEWEB)

    Kaiser, K. [Technical Univ. Berlin, Aerospace Inst. (Germany)

    1997-08-01

    The blade element method works fast and good. For some problems (rotor shapes or flow conditions) it could be better to use vortex methods. Different methods for calculating a wake geometry will be presented. (au)

  7. Cockpit-based Wake Vortex Visualization Project

    Data.gov (United States)

    National Aeronautics and Space Administration — To prevent aircraft accidents due to wake vortex hazards, FAA procedures specify the minimum separation required between different categories of aircraft. However, a...

  8. Bifurcation and instability problems in vortex wakes

    DEFF Research Database (Denmark)

    Aref, Hassan; Brøns, Morten; Stremler, Mark A.

    2007-01-01

    A number of instability and bifurcation problems related to the dynamics of vortex wake flows are addressed using various analytical tools and approaches. We discuss the bifurcations of the streamline pattern behind a bluff body as a vortex wake is produced, a theory of the universal Strouhal......-Reynolds number relation for vortex wakes, the bifurcation diagram for "exotic" wake patterns behind an oscillating cylinder first determined experimentally by Williamson & Roshko, and the bifurcations in topology of the streamlines pattern in point vortex streets. The Hamiltonian dynamics of point vortices...... in a periodic strip is considered. The classical results of von Kármán concerning the structure of the vortex street follow from the two-vortices-in-a-strip problem, while the stability results follow largely from a four-vortices-in-a-strip analysis. The three-vortices-in-a-strip problem is argued...

  9. 2014 CERN Accelerator Schools: Plasma Wake Acceleration

    CERN Multimedia

    2014-01-01

    A specialised school on Plasma Wake Acceleration will be held at CERN, Switzerland from 23-29 November, 2014.   This course will be of interest to staff and students in accelerator laboratories, university departments and companies working in or having an interest in the field of new acceleration techniques. Following introductory lectures on plasma and laser physics, the course will cover the different components of a plasma wake accelerator and plasma beam systems. An overview of the experimental studies, diagnostic tools and state of the art wake acceleration facilities, both present and planned, will complement the theoretical part. Topical seminars and a visit of CERN will complete the programme. Further information can be found at: http://cas.web.cern.ch/cas/PlasmaWake2014/CERN-advert.html http://indico.cern.ch/event/285444/

  10. Investigation of aircraft vortex wake structure

    Science.gov (United States)

    Baranov, N. A.; Turchak, L. I.

    2014-11-01

    In this work we analyze the mechanisms of formation of the vortex wake structure of aircraft with different wing shape in the plan flying close to or away from the underlying surface cleaned or released mechanization wing.

  11. CRED REA Algal Assessments Wake Atoll, 2007

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Twelve quadrats were sampled along 2 consecutively-placed, 25m transect lines as part of Rapid Ecological Assessments conducted at 12 sites at Wake Atoll in April...

  12. Model of the Human Sleep Wake System

    CERN Document Server

    Rogers, Lisa

    2012-01-01

    A model and analysis of the human sleep/wake system is presented. The model is derived using the known neuronal groups, and their various projections, involved with sleep and wake. Inherent in the derivation is the existence of a slow time scale associated with homeostatic regulation, and a faster time scale associated with the dynamics within the sleep phase. A significant feature of the model is that it does not contain a periodic forcing term, common in other models, reflecting the fact that sleep/wake is not dependent upon a diurnal stimulus. Once derived, the model is analyzed using a linearized stability analysis. We then use experimental data from normal sleep-wake systems and orexin knockout systems to verify the physiological validity of the equations.

  13. Numerical Simulations of a VAWT in the Wake of a Moving Car

    Directory of Open Access Journals (Sweden)

    Wenlong Tian

    2017-04-01

    Full Text Available Wind energy generated from the wake of moving cars has a large energy potential that has not yet been utilized. In this study, a vertical axis wind turbine (VAWT was used to recover energy from the wakes of moving cars. The turbine was designed to be planted by the side of the car lane and driven by the wake produced by the car. Transient computational fluid dynamics (CFD simulations were performed to evaluate the performance of the VAWT. The influence of two main factors on the performance of the VAWT, the velocity of the car and the gap between the car and the rotor, were studied. The simulations confirmed the feasibility of this plan, and in the tested cases, the VAWT was able to generate a maximum energy output of 100.49 J from the wake of a car. The results also showed that the performance of the VAWT decreased with the velocity of the car, and the increased gap between the car and the VAWT.

  14. Aircraft Wake Vortex Deformation in Turbulent Atmosphere

    OpenAIRE

    Hennemann, Ingo; Holzaepfel, Frank

    2007-01-01

    Large-scale distortion of aircraft wake vortices appears to play a crucial role for aircraft safety during approach and landing. Vortex distortion is investigated based on large eddy simulations of wake vortex evolution in a turbulent atmosphere. A vortex identification method is developed that can be adapted to the vortex scales of interest. Based on the identified vortex center tracks, a statistics of vortex curvature radii is established. This statistics constitutes the basis for understan...

  15. Wake-Vortex Hazards During Cruise

    Science.gov (United States)

    Rossow, Vernon J.; James, Kevin D.; Nixon, David (Technical Monitor)

    1998-01-01

    Even though the hazard posed by lift-generated wakes of subsonic transport aircraft has been studied extensively for approach and departure at airports, only a small amount of effort has gone into the potential hazard at cruise altitude. This paper reports on a studio of the wake-vortex hazard during cruise because encounters may become more prevalent when free-flight becomes available and each aircraft, is free to choose its own route between destinations. In order to address the problem, the various fluid-dynamic stages that vortex wakes usually go through as they age will be described along with estimates of the potential hazard that each stage poses. It appears that a rolling-moment hazard can be just as severe at cruise as for approach at airports, but it only persists for several minutes. However, the hazard posed by the downwash in the wake due to the lift on the generator aircraft persists for tens of minutes in a long narrow region behind the generating aircraft. The hazard consists of severe vertical loads when an encountering aircraft crosses the wake. A technique for avoiding vortex wakes at cruise altitude will be described. To date the hazard posed by lift-generated vortex wakes and their persistence at cruise altitudes has been identified and subdivided into several tasks. Analyses of the loads to be encounter and are underway and should be completed shortly. A review of published literature on the subject has been nearly completed (see text) and photographs of vortex wakes at cruise altitudes have been taken and the various stages of decay have been identified. It remains to study and sort the photographs for those that best illustrate the various stages of decay after they are shed by subsonic transport aircraft at cruise altitudes. The present status of the analysis and the paper are described.

  16. Distributed chaos in turbulent wakes

    CERN Document Server

    Bershadskii, A

    2016-01-01

    Soft and hard spontaneous breaking of space translational symmetry (homogeneity) have been studied in turbulent wakes by means of distributed chaos. In the case of the soft translational symmetry breaking the vorticity correlation integral $\\int_{V} \\langle {\\boldsymbol \\omega} ({\\bf x},t) \\cdot {\\boldsymbol \\omega} ({\\bf x} + {\\bf r},t) \\rangle_{V} d{\\bf r}$ dominates the distributed chaos and the chaotic spectra $\\exp-(k/k_{\\beta})^{\\beta }$ have $\\beta =1/2$. In the case of the hard translational symmetry breaking, control on the distributed chaos is switched from one type of fundamental symmetry to another (in this case to Lagrangian relabeling symmetry). Due to the Noether's theorem the relabeling symmetry results in the inviscid helicity conservation and helicity correlation integral $I=\\int \\langle h({\\bf x},t)~h({\\bf x}+{\\bf r}, t)\\rangle d{\\bf r}$ (Levich-Tsinober invariant) dominates the distributed chaos with $\\beta =1/3$. Good agreement with the experimatal data has been established for turbulent ...

  17. Vortex wakes of a flapping foil

    DEFF Research Database (Denmark)

    Schnipper, Teis; Andersen, Anders Peter; Bohr, Tomas

    2009-01-01

    and the shedding process at the sharp trailing edge in detail. This allows us to identify the origins of the vortices in the 2P wake, to understand that two distinct 2P regions are present in the phase diagram due to the timing of the vortex shedding at the leading edge and the trailing edge and to propose......We present an experimental study of a symmetric foil performing pitching oscillations in a vertically flowing soap film. By varying the frequency and amplitude of the oscillation we visualize a variety of wakes with up to 46 vortices per oscillation period, including von Karman vortex street......, inverted von Karman vortex street, 2P wake, 2P+2S wake and novel wakes ranging from 4P to 8P. We map out the wake types in a phase diagram spanned by the width-based Strouhal number and the dimensionless amplitude. We follow the time evolution of the vortex formation near the round leading edge...

  18. Utilizing Surface Sensors to Identify Wake Regimes

    Science.gov (United States)

    Wang, Mengying; Hemati, Maziar S.

    2016-11-01

    Marine swimmers often exploit external flow structures to reduce locomotive effort. To achieve this advantage, these swimmers utilize mechanosensory organs on the surface of their bodies to detect hydrodynamic signals from the surrounding fluid, which can then be used to inform the control task. Recently, there has been a growing interest in developing similar flow sensing systems to achieve enhanced propulsive efficiency and maneuverability in human-engineered underwater vehicles. In particular, much attention has been given to the problem of wake sensing; however, these investigations have concentrated on a restricted class of wakes-i.e., Kármán-type vortex streets-whereas more complicated wake structures can arise in practice. In this talk, we will explore the possibility of identifying wake regimes through the use of surface sensors. Potential flow theory is adopted to simulate the interactions of various wakes with a fish-like body. Wakes in different dynamical regimes impart distinct hydrodynamic signatures on the body, which permits these regimes to be distinguished from one another in an automated fashion. Our results can provide guidance for improving flow sensing capabilities in human-engineered systems and hint at how marine swimmers may sense their hydrodynamic surroundings.

  19. Investigation of Wake-Vortex Aircraft Encounters

    Science.gov (United States)

    Smith, Sonya T.

    1999-01-01

    The National Aeronautics and Space Administration is addressing airport capacity enhancements during instrument meteorological conditions though the Terminal Area Productivity (TAP) program. The major goal of the TAP program is to develop the technology that will allow air traffic levels during instrument meteorological condition to approach those achieved during visual operations. The Reduced Spacing Operations (RSO) subelement of TAP at the NASA Langley Research Center (LaRC) will develop the Aircraft Vortex Spacing System (AVOSS). The purpose of the AVOSS is to integrate current and predicted weather conditions, wake vortex transport and decay knowledge, wake vortex sensor data, and operational definitions of acceptable strengths for vortex encounters to produce dynamic wake vortex separation criteria. The proposed research is in support of the wake vortex hazard definition component of the LaRC AVOSS development research. The research program described in the next section provided an analysis of the static test data and uses this data to evaluate the accuracy vortex/wake-encounter models. The accuracy of these models has not before been evaluated using experimental data. The research results also presented the first analysis of the forces and moments imparted on an airplane during a wake vortex encounter using actual flight test data.

  20. [Testing results of telemechanic system controlling train operators wakefulness].

    Science.gov (United States)

    Serikov, V V; Zakrevskaia, A A; Zakharchenko, D V; Alpaev, D V; At'kova, E O

    2015-01-01

    Expert and instrumental assessment covered efficiency of telemechanic system controlling train operators wakefulness in simulation of real night travel, through special simulator complex "Locomotive operator cabin". The telemechanic system controlling train operators wakefulness, if exploited correctly, provides wakefulness of the train operators at the level sufficient for the effective work. That is supported by distribution of falling asleep cases in experiments with activated or deactivated telemechanic system controlling train operators wakefulness. The study proved efficiency of telemechanic system controlling train operators wakefulness.

  1. Experimental and theoretical analysis of vortex breakdown in the wake of the 25∘ Ahmed body

    Science.gov (United States)

    Jermann, Cyril; Meliga, Philippe; Pujals, Gregory; Gallaire, Francois; Serre, Eric

    2014-11-01

    We study experimentally and theoretically the wake of the 25circ; Ahmed body, considered a suitable test-case to reproduce the two counter-rotating longitudinal vortices widely encountered in automotive aerodynamics. The three-dimensional experimental mean flow is reconstructed at high Reynolds number (Re = 2 . 8 ×106) from a series of cross-flow time-averaged planes acquired with a moving automated Stereo-PIV system. We observe a sharp decay of the axial velocity and vorticity in the near-wake, 0 . 5 times the projected length of the slanted surface downstream the square back, where the streamwise vortices is subjected to a strong adverse pressure gradient and the turbulent kinetic energy exhibits a peak in the vortex core. A stability analysis of the experimental velocity shows that the flow undergoes vortex breakdown roughly at the same position, through a transition from supercritical (x 0 . 5) conditions and the accumulation of upstream propagating axisymmetric waves.

  2. The ion potential in warm dense matter: wake effects due to streaming degenerate electrons

    CERN Document Server

    Moldabekov, Zhandos; Bonitz, Michael; Ramazano, Tlekkabul

    2014-01-01

    The effective dynamically screened potential of a classical ion in a stationary flowing quantum plasma at finite temperature is investigated. This is a key quantity for thermodynamics and transport of dense plasmas in the warm dense matter regime. To compute this potential a linear response description of the electrons via the Mermin dielectric function is utilized with electron-electron collisions taken into account via a relaxation time approximation. The ion potential strongly deviates from the static Yukawa potential exhibiting the familiar oscillatory structure with attractive minima (wake potential). This potential is analyzed in detail for high-density plasmas with values of the Brueckner parameter in the range $0.1 \\le r_s \\le 1$, for a broad range of plasma temperature and electron streaming velocity. It is shown that wake effects become weaker with increasing temperature of the electrons. Finally, we obtain the minimal electron streaming velocity for which attraction between ions occurs. This veloci...

  3. Validation of the actuator line method using near wake measurements of the MEXICO rotor

    DEFF Research Database (Denmark)

    Nilsson, Karl; Shen, Wen Zhong; Sørensen, Jens Nørkær

    2015-01-01

    generates significantly larger vortex cores than in the experimental cases, but predicts the expansion, the circulation and the velocity distributions with satisfying results. Additionally, the simulation and experimental data are used to test three different techniques to compute the average axial......The purpose of the present work is to validate the capability of the actuator line method to compute vortex structures in the near wake behind the MEXICO experimental wind turbine rotor. In the MEXICO project/MexNext Annex, particle image velocimetry measurements have made it possible to determine...... of tip speed ratios, have been simulated by large-eddy simulations using a Navier–Stokes code combined with the actuator line method. The flow field is analyzed in terms of wake expansion, vortex core radius, circulation and axial and radial velocity distributions. Generally, the actuator line method...

  4. The computation of thick axisymmetric boundary layers and wakes around bodies of revolution

    Science.gov (United States)

    Markatos, N. C.

    The paper is concerned with the computational investigation of thick, axisymmetric, turbulent boundary layers and wakes around bodies of revolution. The procedures employed take full account of the influence of longitudinal and transverse surface curvatures and normal pressure gradients on the development of the boundary layer and wake, and also the viscous-inviscid interaction in the tail region of the body. The method makes it possible to calculate the static pressure and the velocity profiles along the body as well as the drag components; and it is applicable to both two- and three-dimensional situations, enabling, for example, the prediction of flows around ships' and submarines' hulls to be made. The application of the fully-elliptic calculation procedure to a body of revolution is described, and comparisons made between predictions and experimental measurements. The calculated axial variation of skin friction and pressure coefficient, and the velocity profiles are shown to be in fair agreement with experimental values.

  5. On the transient dynamics of the wake and trajectory of free falling cones with various apex angles

    Science.gov (United States)

    Hamed, Ali M.; Jin, Yaqing; Chamorro, Leonardo P.

    2015-11-01

    The early free fall stages of cones with a density ratio 1.18 and apex angles of 30°, 45°, 60°, and 90° were studied using a wireless 3-axis gyroscope and accelerometer to describe the cone 3D motions, while particle image velocimetry was used to capture the induced flow in the near wake. The Reynolds number based on the cones diameter and the velocity at which the cone reaches the first local velocity maximum is found to consistently set the limit between two distinctive states. Relatively rapid growth in the cone nutation and departure from the vertical axis is observed after this Re is reached. Sequences of vertical velocity, swirling strength, LES-decomposed velocity, and pressure fields show the formation and growth of a large and initially symmetric recirculation bubble at the cone base. Those also highlight the presence of a symmetric 3D vortex rollup dominating the near wake in the early stages of the fall. A shear layer develops at the edge of the wake and manifests in the periodic shedding of Kelvin-Helmholtz vortices that, due to the nature of the recirculation bubble, reorganize to constitute a part of the rollup. Later in the fall, the wake loses symmetry and shows high population of vortical structures leading to turbulence. The asymmetric wake leads to strong interactions between the flow field and the cone evidenced by the shedding of a part of the 3D large-scale vortex rollup. This shedding process along with the cone rotation around its own axis provides a possible explanation of the helical wake structure observed in other studies.

  6. Three-dimensional vortex wake structure of a flapping-wing micro aerial vehicle in forward flight configuration

    Science.gov (United States)

    Percin, M.; van Oudheusden, B. W.; Eisma, H. E.; Remes, B. D. W.

    2014-09-01

    This paper investigates the formation and evolution of the unsteady three-dimensional wake structures generated by the flapping wings of the DelFly II micro aerial vehicle in forward flight configuration. Time-resolved stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at several spanwise-aligned planes in the wake, so as to allow a reconstruction of the temporal development of the wake of the flapping wings throughout the complete flapping cycle. Simultaneous thrust-force measurements were performed to explore the relation between the wake formation and the aerodynamic force generation mechanisms. The three-dimensional wake configuration was subsequently reconstructed from the planar PIV measurements by two different approaches: (1) a spatiotemporal wake reconstruction obtained by convecting the time-resolved, three-component velocity field data of a single measurement plane with the free-stream velocity; (2) for selected phases in the flapping cycle a direct three-dimensional spatial wake reconstruction is interpolated from the data of the different measurement planes, using a Kriging regression technique. Comparing the results derived from both methods in terms of the behavior of the wake formations, their phase and orientation indicate that the spatiotemporal reconstruction method allows to characterize the general three-dimensional structure of the wake, but that the spatial reconstruction method can reveal more details due to higher streamwise resolution. Comparison of the wake reconstructions for different values of the reduced frequency allows assessing the impact of the flapping frequency on the formation and interaction characteristics of the vortical structures. For low values of the reduced frequency, it is observed that the vortex structure formation of instroke and outstroke is relatively independent of each other, but that increasing interaction occurs at higher reduced frequencies. It is further shown that there is a

  7. Vortex and wake-induced vibrations of a tandem arrangement of two flexible circular cylinders with near wake interference

    Science.gov (United States)

    Huera-Huarte, F. J.; Bearman, P. W.

    2011-02-01

    Results showing the dynamic response of a tandem arrangement of two vertical high aspect ratio (length over diameter) and low mass ratio (mass over mass of displaced fluid) flexible cylinders vibrating at low mode number are presented in this paper. Two circular cylinder models were aligned with the flow, so the downstream or trailing cylinder was immersed in the wake of the leading one. Centre-to-centre distances from 2 to 4 diameters were studied. The models were very similar in design, with external diameters of 16 mm and a total length of 1.5 m. Reynolds numbers up to 12 000 were achieved with reduced velocities, based on the fundamental natural frequency of the downstream cylinder in still water, up to 16. The trailing model had a mass ratio of 1.8 with a combined mass-damping parameter of 0.049, whilst the corresponding figures for the leading cylinder were 1.45 and 0.043, respectively. The dynamic response of the trailing model has been analysed by studying cross-flow and in-line amplitudes, dominant frequencies and modal amplitudes. The dynamic response of the leading one is analysed by means of its cross-flow amplitudes and dominant frequencies and it is also related to the motion of the trailing cylinder by studying the synchronisation between their instantaneous cross-flow motions. Planar digital particle image velocimetry (DPIV) was used to visualise the wake. Different response regimes have been identified based on the type of oscillations exhibited by the cylinders: vortex-induced (VIV), wake-induced (WIV) or combinations of both.

  8. Ensemble and phase averaged flow characteristics in the near wake of a square cylinder

    Science.gov (United States)

    Kim, Kyung Chun; Yoon, Sang Youl; Bok Lee, Man

    2001-11-01

    Ensemble and phase averaged velocity fields in the near wake region behind a square cylinder have been successfully obtained using randomly sampled PIV data sets. The Reynolds numbers based on the flow velocity and the vertex height were 1,600 and 3,900. To identify the phase information, we examined the magnitude of circulation and the center of peak vorticity. The center of vorticity was estimated from lowpass filtered vorticity contours (LES decomposition) adopting a sub-pixel searching algorithm. Due to the sinusoidal nature of circulation which is closely related to the instantaneous vorticity, the location of peak voticity fits well with a sine curve of the circulation magnitude. Conditionally averaged velocity and vorticity fields represent the Karman vortex shedding phenomenon quite successfully within ¡ 3/4 5¡Æ phase uncertainty. The oscillating nature of the separated shear layers and the separation bubble at the upper and lower surfaces are clearly observed. With the hot-wire measurement of Strouhal frequency, we found that the convection velocity changes its magnitude very rapidly from 25 to 75 percent of the free stream velocity along the streamwise direction when the flow passes the recirculation region. Comparison with the circular cylinder wake at the same Reynolds number was made to discuss the difference. Both topology and velocity data associated with a given phase could be useful to verify numerical simulation results by an LES method.

  9. Influence of rotating wakes on separation in turbine exhaust diffusers

    Institute of Scientific and Technical Information of China (English)

    Olaf Sieker; Joerg R. Seume

    2008-01-01

    Highly efficient turbine exhaust diffuser cannot be designed without taking into account the unsteady interactions with the last rotating row of the turbine. Former investigations described in the literature show a very high potential compared to that of other parts of turbomachines for improving the diffuser. A scale model of a typical gas turbine exhaust diffuser is investigated experimentally. To investigate the influence of rotating wakes, measuremerits without a spoke wheel as well as measurements with a variable-speed rotating cylindrical spoke wheel with 2 mm- or 10 mm-spekes simulating turbine rotor wakes were made. Miniaturized 3-hole pneumatic probes as well as a 2D-Laser-Doppler-Velocimeter (LDV) were used to investigate velocity profiles. 122 static pressure tapings were used to measure several axial and circumferential static pressure distributions. Without a spoke-wheel the annular diffuser separates at the shroud for all swirl configurations. For the measurements with the 2 mm spoke wheel, the separating diffuser was unstable while keeping the test rig operating parameters constant. For a non-rotating 10mm spoke wheel and at rotational speeds less than 1,000 rpm, the annular diffuser separated at the shroud. Increasing the rotational speed of the 10mm spoke wheel, flow did not separate at the shroud and much higher pressure recovery than without spoke wheel has achieved.

  10. Performance evaluation and wake study of a micro wind turbine

    Energy Technology Data Exchange (ETDEWEB)

    Comyn, G.I.; Nobes, D.S.; Fleck, B.A. [Alberta Univ., Edmonton, AB (Canada). Dept. of Mechanical Engineering

    2010-07-01

    A horizontal axis micro wind turbine was tested in a low speed wind tunnel as part of a study on the icing of wind turbine blades. The turbine was monitored in order to measure the rotational speed of the rotor, axial thrust, and power output. A Kiel probe was used to measure the near wake of the turbine in relation to other wind turbines installed in a wind farm. Performance characteristics were calculated and compared with the turbine manufacturer's published data. A modified 1-D axial momentum theory was used to determine the power extracted from the wind by the rotor. Three-dimensional velocity components in the wake were measured with a hot wire anemometry technique. Results of the study were compared to actual outputs. The study showed that the model over-predicted power by 50 percent, but that the basic trend of the turbine was accurately characterized. Further studies will be conducted to determine the impact of ice growth on rotor imbalances. 27 refs., 13 figs.

  11. Long-term cognitive deficits accompanied by reduced neurogenesis after soman poisoning

    NARCIS (Netherlands)

    Joosen, M.J.A.; Jousma, E.; Boom, T.M. van den; Kuijpers, W.C.; Smit, A.B.; Lucassen, P.J.; Helden, H.P.M. van

    2009-01-01

    To date, treatment of organophosphate (OP) poisoning shows several shortcomings, and OP-victims might suffer from lasting cognitive deficits and sleep–wake disturbances. In the present study, long-term effects of soman poisoning on learning ability, memory and neurogenesis were investigated in rats,

  12. Long-term cognitive deficits accompanied by reduced neurogenesis after soman poisoning

    NARCIS (Netherlands)

    Joosen, M.J.A.; Jousma, E.; Boom, T.M. van den; Kuijpers, W.C.; Smit, A.B.; Lucassen, P.J.; Helden, H.P.M. van

    2009-01-01

    To date, treatment of organophosphate (OP) poisoning shows several shortcomings, and OP-victims might suffer from lasting cognitive deficits and sleep–wake disturbances. In the present study, long-term effects of soman poisoning on learning ability, memory and neurogenesis were investigated in rats,

  13. Long-term cognitive deficits accompanied by reduced neurogenesis after soman poisoning

    NARCIS (Netherlands)

    Joosen, M.J.A.; Jousma, E.; van den Boom, T.M.; Kuijpers, W.C.; Smit, A.B.; Lucassen, P.J.; van Helden, H.P.M.

    2009-01-01

    To date, treatment of organophosphate (OP) poisoning shows several shortcomings, and OP-victims might suffer from lasting cognitive deficits and sleep-wake disturbances. In the present study, long-term effects of soman poisoning on learning ability, memory and neurogenesis were investigated in rats,

  14. TR-PIV measurement of the wake behind a grooved cylinder at low Reynolds number

    Science.gov (United States)

    Liu, Ying Zheng; Shi, Liu Liu; Yu, Jun

    2011-04-01

    A comparative study of the wakes behind cylinders with grooved and smooth surfaces was performed with a view to understand the wake characteristics associated with the adult Saguaro cacti. A low-speed recirculation water channel was established for the experiment; the Reynolds number, based on the free-stream velocity and cylinder diameter (D), was kept at ReD=1500. State-of-the-art time-resolved particle image velocimetry (TR-PIV) was employed to measure a total of 20 480 realizations of the wake field at a frame rate of 250 Hz, enabling a comprehensive view of the time- and phase-averaged wake pattern. In comparison to the wake behind the smooth cylinder, the length of the recirculation zone behind the grooved cylinder was extended by nearly 18.2%, yet the longitudinal velocity fluctuation intensity was considerably weakened. A global view of the peaked spectrum of the longitudinal velocity component revealed that the intermediate region for the grooved cylinder, which approximately corresponds to the transition region where the shear layer vortices interact, merge and shed before the formation of the Karman-like vortex street, was much wider than that for the smooth one. The unsteady events near St=0.3-0.4 were detected in the intermediate region behind the grooved cylinder, but no such events were found in the smooth cylinder system. Although the formation of the Karman-like vortex street was delayed by about 0.6D downstream for the grooved cylinder, no prominent difference in the vortex street region was found in the far wake for both cylinders. The Proper Orthogonal Decomposition (POD) method was used extensively to decompose the vector and swirling strength fields, which gave a close-up view of the vortices in the near wake. The first two POD modes of the swirling strength clarified the spatio-temporal characteristics of the shear layer vortices behind the grooved cylinder. The small-scale vortices superimposed on the shear layers behind the grooved cylinder

  15. Influence of Rigid Body Motions on Rotor Induced Velocities and Aerodynamic Loads of a Floating Horizontal Axis Wind Turbine

    DEFF Research Database (Denmark)

    de Vaal, Jacobus B.; Hansen, Martin Otto Laver; Moan, Torgeir

    2014-01-01

    This paper discusses the influence of rigid body motions on rotor induced velocities and aerodynamic loads of a floating horizontal axis wind turbine. Analyses are performed with a simplified free wake vortex model specifically aimed at capturing the unsteady and non-uniform inflow typically...... experienced by a floating wind turbine. After discussing the simplified model in detail, comparisons are made to a state of the art free wake vortex code, using test cases with prescribed platform motion. It is found that the simplified model compares favourably with a more advanced numerical model......, and captures the essential influences of rigid body motions on the rotor loads, induced velocities and wake influence....

  16. Simulation and Prediction of Wakes and Wake Interaction in Wind Farms

    DEFF Research Database (Denmark)

    Andersen, Søren Juhl

    The highly turbulent wake and the wake interaction of merging wakes between multiple wind turbines are modelled using Large Eddy Simulation (LES) in a general Navier-Stokes solver. The Actuator Line (AL) technique is employed to model the wind turbines, and the aeroelastic computations are fully...... coupled with the flow solver. The numerical simulations include the study of the far wake behind a single turbine, three idealised cases of infinitely long rows of turbines and finally three infinite wind farm scenarios with different spacings. The flow characteristics between the turbines, turbine...... performance, and principal turbulent quantities are examined for the different scenarios. The study focuses on the large coherent structures and movements of the wake behind and between wind turbines. The large coherent structures are analysed using Proper Orthogonal Decoposition (POD). POD constitutes...

  17. Simulation of wind turbine wakes using the actuator line technique.

    Science.gov (United States)

    Sørensen, Jens N; Mikkelsen, Robert F; Henningson, Dan S; Ivanell, Stefan; Sarmast, Sasan; Andersen, Søren J

    2015-02-28

    The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.

  18. Simulation of wind turbine wakes using the actuator line technique

    Science.gov (United States)

    Sørensen, Jens N.; Mikkelsen, Robert F.; Henningson, Dan S.; Ivanell, Stefan; Sarmast, Sasan; Andersen, Søren J.

    2015-01-01

    The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake. PMID:25583862

  19. Wakes behind wind turbines. Studies on tip vortex evolution and stability

    Energy Technology Data Exchange (ETDEWEB)

    Odemark, Ylva

    2012-07-01

    The increased fatigue loads and decreased power output of a wind turbine placed in the wake of another turbine is a well-known problem when building new wind power farms. In order to better estimate the total power output of a wind power farm, knowledge about the development and stability of wind turbine wakes is crucial. In the present thesis, the wake behind a small-scale model turbine was investigated experimentally in a wind tunnel. The velocity in the wake was measured with hot-wire anemometry, for different free stream velocities and tip speed ratios. To characterize the behaviour of the model turbine, the power output, thrust force and rotational frequency of the model were also measured. These results were then compared to calculations using the Blade Element Momentum (BEM) method. New turbine blades for the model was constructed using the same method, in order to get an estimate of the distribution of the lift and drag forces along the blades. This information is needed for comparisons with certain numerical simulations, which however remains to be performed.By placing the turbine at different heights in a turbulent boundary layer, the effects of forest turbulence on wind turbine outputs (power and thrust) could also be investigated.The evolution of the tip vortices shed from the turbine blades was studied by performing velocity measurements around the location of the tip vortex breakdown. The vortices' receptivity to disturbances was then studied by introducing a disturbance in the form of two pulsed jets, located in the rear part of the nacelle. In order to introduce a well-defined disturbance and perform phase-locked measurements, a new experimental setup was constructed and successfully tested for two different disturbance frequencies. The mean stream wise velocity and the stream wise turbulence intensity was found to scale well with the free stream velocity and the spreading of the wake was found to be proportional to the square root of the

  20. Wind tunnel measurements of wake structure and wind farm power for actuator disk model wind turbines in yaw

    Science.gov (United States)

    Howland, Michael; Bossuyt, Juliaan; Kang, Justin; Meyers, Johan; Meneveau, Charles

    2016-11-01

    Reducing wake losses in wind farms by deflecting the wakes through turbine yawing has been shown to be a feasible wind farm control approach. In this work, the deflection and morphology of wakes behind a wind turbine operating in yawed conditions are studied using wind tunnel experiments of a wind turbine modeled as a porous disk in a uniform inflow. First, by measuring velocity distributions at various downstream positions and comparing with prior studies, we confirm that the nonrotating wind turbine model in yaw generates realistic wake deflections. Second, we characterize the wake shape and make observations of what is termed a "curled wake," displaying significant spanwise asymmetry. Through the use of a 100 porous disk micro-wind farm, total wind farm power output is studied for a variety of yaw configurations. Strain gages on the tower of the porous disk models are used to measure the thrust force as a substitute for turbine power. The frequency response of these measurements goes up to the natural frequency of the model and allows studying the spatiotemporal characteristics of the power output under the effects of yawing. This work has been funded by the National Science Foundation (Grants CBET-113380 and IIA-1243482, the WINDINSPIRE project). JB and JM are supported by ERC (ActiveWindFarms, Grant No. 306471).

  1. A comparison of dispersion calculations in bluff body wakes using LES and unsteady RANS

    Energy Technology Data Exchange (ETDEWEB)

    Paschkewitz, J S

    2006-01-19

    Accurate modeling of the dispersion behavior of sprays or particles is critical for a variety of problems including combustion, urban pollution or release events, and splash and spray transport around heavy vehicles. Bluff body wakes are particularly challenging since these flows are both highly separated and strongly unsteady. Attempting to model the dispersion of droplets or particles interacting with bluff body wakes is even more difficult since small differences in the flow field encountered by particles can lead to large differences in the dispersion behavior. Particles with finite inertia can exhibit additional complicating effects such as preferential concentration. In this preliminary study, we consider the dispersion of solid particles in the wake of a rectangular plane at a Reynolds number (Re) of 10000 and that of droplets in the wake of a simplified tractor-trailer geometry at Re = 2 x 10{sup 6} using both the Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) turbulence modeling approaches. The calculations were performed using identical meshes for both the LES and URANS models. Particle stresses are not backcoupled to the carrier fluid velocity solution. In the case of the rectangular plane wake, the LES calculation predicts a finer-scale and more persistent wake structure than the URANS one; the resulting particle dispersion is considerably ({approx} 40%) underpredicted for low inertia particles. For the case of the simplified tractor-trailer geometry, although the LES is underresolved, similar trends are observed with strong differences in the vertical and horizontal dispersion of the smallest particles. These results suggest that it may be necessary to use LES to accurately capture the dispersion behavior of small, low inertia particles or droplets, but that URANS may be sufficient for problems in which only large particles with substantial inertia are of primary concern.

  2. Hub vortex helical instability as the origin of wake meandering in the lee of a model wind-turbine

    Science.gov (United States)

    Viola, Francesco; Iungo, Giacomo Valerio; Camarri, Simone; Porte-Agel, Fernando; Gallaire, Francois

    2012-11-01

    Wind tunnel measurements were performed for the wake produced by a three-bladed wind turbine immersed in uniform flow. These tests show the presence of a vorticity structure in the near wake region mainly oriented along the streamwise direction, which is denoted as hub vortex. The hub vortex is characterized by oscillations with frequencies lower than the one connected to the rotational velocity of the rotor, which are ascribed to wake meandering by previous works. This phenomenon consists in transversal oscillations of the wind turbine wake, which are excited by the shedding of vorticity structures from the rotor disc acting as a bluff body. In this work temporal and spatial linear stability analyses of a wind turbine wake are performed on a base flow obtained through time-averaged wind tunnel velocity measurements. This study shows that the low frequency spectral component detected experimentally is the result of a convective instability of the hub vortex, which is characterized by a counter-winding single-helix structure. Simultaneous hot-wire measurements confirm the presence of a helicoidal unstable mode of the hub vortex with a streamwise wavenumber roughly equal to the one predicted from the linear instability analysis.

  3. DIELECTRIC WAKE FIELD RESONATOR ACCELERATOR MODULE

    Energy Technology Data Exchange (ETDEWEB)

    Hirshfield, Jay L.

    2013-11-06

    Results are presented from experiments, and numerical analysis of wake fields set up by electron bunches passing through a cylindrical or rectangular dielectric-lined structure. These bunches excite many TM-modes, with Ez components of the wake fields sharply localized on the axis of the structure periodically behind the bunches. The experiment with the cylindrical structure, carried out at ATF Brookhaven National Laboratory, used up to three 50 MeV bunches spaced by one wake field period (21 cm) to study the superposition of wake fields by measuring the energy loss of each bunch after it passed through the 53-cm long dielectric element. The millimeter-wave spectrum of radiation excited by the passage of bunches is also studied. Numerical analysis was aimed not only to simulate the behavior of our device, but in general to predict dielectric wake field accelerator performance. It is shown that one needs to match the radius of the cylindrical dielectric channel with the bunch longitudinal rms-length to achieve optimal performance.

  4. The near wake structure and the development of vorticity behind a model horizontal axis wind turbine

    Energy Technology Data Exchange (ETDEWEB)

    Ebert, P.; Wood, D. [The Univ. of Newcastle, Dept. of Mechanical Engineering, Callaghan (Australia)

    1997-08-01

    The wake of a two bladed model HAWT operating at zero yaw angle and in a steady flow in a wind tunnel was measured using hot wire probes. By phase locked averaging and moving the probe axially and radially the full three dimensional mean flow file was determined. All measurements were within two chord lengths of the blades and at tip speed ratios giving high turbine power output, a condition approaching runaway, and a stalled condition. For all tip speed ratios the wakes were significantly three dimensional. Large velocity variations were associated with vortex structures in the wakes, and irrotational fluctuations caused by the blade bound circulation. The vorticity clearly defined the hub and tip vortices that traced helical paths downstream, with the constant tip vortex pitch inversely proportional to tip speed ratio. Close to the blades the flow was complicated, though vortex roll-up was completed within one chord length. Considerable changes in wake structure occurred with tip speed ratio. At high power output the wake showed tip and hub vortices connected by a diffuse vortex sheet of mostly radial vorticity from the blade boundary layers; blade bound circulation was almost constant. The structure approaching runaway was similar though the hub vortex was not well defined and formed a vortex sheet around the hub which lifted away and diffused. The stalled condition was more complicated, with evidence of incomplete tip and hub vortex formation. The stream-wise velocity of the tip vortex core decreased with increasing tip speed ratio, but this was never aligned with local streamlines. The core of the tip vortex was not circular but more elliptical. A phase locked averaged angular momentum analysis was undertaken, the extra terms introduced through phase locked averaging were small. (Abstract Truncated)

  5. Wakes and precursor soliton excitations by a moving charged object in a plasma

    CERN Document Server

    Tiwari, Sanat Kumar

    2015-01-01

    We study the evolution of nonlinear ion acoustic wave excitations due to a moving charged source in a plasma. Our numerical investigations of the full set of cold fluid equations goes beyond the usual weak nonlinearity approximation and shows the existence of a rich variety of solutions including wakes, precursor solitons and pinned solitons that travel with the source velocity. These solutions represent a large amplitude generalization of solutions obtained in the past for the forced Korteweg deVries equation and can find useful applications in a variety of situations in the laboratory and in space wherever there is a large relative velocity between the plasma and a charged object.

  6. Detailed field test of yaw-based wake steering

    DEFF Research Database (Denmark)

    Fleming, P.; Churchfield, M.; Scholbrock, A.

    2016-01-01

    This paper describes a detailed field-test campaign to investigate yaw-based wake steering. In yaw-based wake steering, an upstream turbine intentionally misaligns its yaw with respect to the inflow to deflect its wake away from a downstream turbine, with the goal of increasing total power...... production. In the first phase, a nacelle-mounted scanning lidar was used to verify wake deflection of a misaligned turbine and calibrate wake deflection models. In the second phase, these models were used within a yaw controller to achieve a desired wake deflection. This paper details the experimental...

  7. Multiscale plant wakes, turbulence and non linear scaling flexible effects

    Science.gov (United States)

    Vila, Teresa; Redondo, Jose M.; Velasco, David

    2010-05-01

    We present velocity ADV measurements and flow visualization of the turbulent wakes behind plant arrays, as these are often fractal in nature, we compare the multifractal spectra and the turbulence structure behind the wakes. Both statistical measures allowing to calculate integral lengthscales and their profiles modified by the plant cannopies [1,2] as well as intermittency and spectral behaviour are also measured [3,4]. We distinguish several momentum transfer mechanisms between the cannopy and the flow, an internal one where lateral turbulent tensions are dominant, and another one just above the plant average height dominated by vertical Reynolds stresses. Visualization of flow over individual plant models show the role of coherent vortices triggered by plant elasticity. The deformation rate of the plants and their Youngs modulus may be correlated with overal plant drag and geometry. This is modified strongly in fractal canopies. Large turbulent integral scales are linked to rugosity and the scaling of the waves.[5,6] Pearlescence experiments where local shear is visualized and numerical simulations of Fractal grids are compared following [7]. [1] Nepf,H.M. Drag, turbulence and diffusion in flow through emergent vegetation. Water Resources Res. 35(2)(1999) [2] Ben Mahjoub,O., Redondo J.M. and Babiano A. Jour.Structure functions in complex flows. Flow Turbulence and Combustion 59, 299-313. [3] El-Hakim, O. Salama, M. Velocity distribution inside and above branched flexible roughness. ASCE Journal of Irrigation and Drainage Engineering, Vol. 118, No 6, (November/December 1992) 914-927. [4] Finnigan,J. Turbulence in plant canopies. Annu. Rev. Fluid Mech. 2000 , Vol. 32: 519-571. [5] Ikeda, S., Kanazawa, M. Three- dimensional organized vortices above flexible water plants. ASCE Journal of Hydraulic Engineering, Vol. 122, No 11, (1996) 634-640. [6] Velasco, D.,Bateman A.,Redondo J.M and Medina V. An open channel flow experimental and theorical study of resistance and

  8. Disruptions of Sleep/Wake Patterns in the Stable Tubule Only Polypeptide (STOP) Null Mouse Model of Schizophrenia.

    Science.gov (United States)

    Profitt, Maxine F; Deurveilher, Samuel; Robertson, George S; Rusak, Benjamin; Semba, Kazue

    2016-09-01

    Disruption of sleep/wake cycles is common in patients with schizophrenia and correlates with cognitive and affective abnormalities. Mice deficient in stable tubule only polypeptide (STOP) show cognitive, behavioral, and neurobiological deficits that resemble those seen in patients with schizophrenia, but little is known about their sleep phenotype. We characterized baseline sleep/wake patterns and recovery sleep following sleep deprivation in STOP null mice. Polysomnography was conducted in adult male STOP null and wild-type (WT) mice under a 12:12 hours light:dark cycle before, during, and after 6 hours of sleep deprivation during the light phase. At baseline, STOP null mice spent more time awake and less time in non-rapid eye movement sleep (NREMS) over a 24-hour period, with more frequent transitions between wake and NREMS, compared to WT mice, especially during the dark phase. The distributions of wake, NREMS and REMS across the light and the dark phases differed by genotype, and so did features of the electroencephalogram (EEG). Following sleep deprivation, both genotypes showed homeostatic increases in sleep duration, with no significant genotype differences in the initial compensatory increase in sleep intensity (EEG delta power). These results indicate that STOP null mice sleep less overall, and their sleep and wake periods are more fragmented than those of WT mice. These features in STOP null mice are consistent with the sleep patterns observed in patients with schizophrenia.

  9. Spatial characterization of vortical structures and internal waves in a stratified turbulent wake using proper orthogonal decomposition

    Science.gov (United States)

    Diamessis, Peter J.; Gurka, Roi; Liberzon, Alex

    2010-08-01

    Proper orthogonal decomposition (POD) has been applied to two-dimensional transects of vorticity obtained from numerical simulations of the stratified turbulent wake of a towed sphere at a Reynolds number Re=(UD)/ν =5×103 and Froude number Fr=2U/(ND)=4 (U and D are characteristic velocity and length scales and N is the stratification frequency). At 231 times during the interval 12wake core in terms of the relative influence of buoyancy on flow dynamics. The geometry of the individual eigenmodes shows a vorticity structure that is buoyancy-controlled at the lowest modes and is increasingly more actively turbulent as modal index is increased. In the wake ambient, i.e., the initially quiescent region outside the turbulent wake, the geometry of the POD modes consists of distinct internal wave rays whose angle to the horizontal is strongly dependent on modal index. Reconstruction of vorticity fields from subranges of POD modes indicates that, both inside the wake core but also in the wave-dominated ambient, each modal subrange is not only associated with a particular flow structure but also a characteristic timescale of motion. These preliminary findings suggest that POD may be a highly suitable alternative to globally defined basis functions in analyzing spatially localized internal wave fields emitted from a turbulent source that are also localized in space. In particular, it may serve as a platform toward an improved understanding of two fundamental questions associated with the nonequilibrium regime of stratified wake evolution: the structural transitions of the vorticity field within the wake core and the radiation of internal waves by the wake.

  10. Joyce the Deconstructionist: Finnegans Wake in Context

    Directory of Open Access Journals (Sweden)

    Zangouei J.

    2012-01-01

    Full Text Available Had Finnegans Wake not been written, some seminal post-1950s innovations in the field of modern literary theory and criticism would have been impossible. James Joyce, who seems to have inspiringly influenced the entire sphere of modern literary theory and criticism greatly, is a pioneer of deconstruction too. His last novel, which reflects his deconstructive tendencies, has played a seminal role in the formation of 20th century deconstruction, and comprises an inchoate mass of implicit ideas on the subject. It was perhaps not until Jacques Derrida and his deconstruction techniques that the theory implied by Finnegans Wake really came into focus. This article seeks to delineate Derrida’s theory of deconstruction as well as Joyce's deconstructive aesthetics; and taking a diachronic approach to literary theory and criticism it glances at Finnegans Wake in the light of deconstruction.

  11. Wakes in large offshore wind farms

    DEFF Research Database (Denmark)

    Berthelmie, Rebecca J.; Frandsen, Sten Tronæs; Rathmann, Ole

    2008-01-01

    Power losses due to wind turbine wakes are of the order of 10 and 20% of total power output in large wind farms. The focus of this research carried out within the EC funded UPWIND project is wind speed and turbulence modelling for large wind farms/wind turbines in complex terrain and offshore...... is for five turbines in flat terrain. Finally a complex terrain wind farm will be modelled and compared with observations. For offshore wind farms, the focus is on cases at the Horns Rev wind farm which indicate wind farm models require modification to reduce under-prediction of wake losses while CFD models...... in order to optimise wind farm layouts to reduce wake losses and loads. For complex terrain, a set of three evaluations is underway. The first is a model comparison for a Gaussian Hill where CFD models and wind farm models are being compared for the case of one hilltop wind turbine. The next case...

  12. Actuator Line Modeling of Wind Turbine Wakes

    DEFF Research Database (Denmark)

    Troldborg, Niels

    2009-01-01

    This thesis contains a comprehensive 3D Navier-Stokes computational study of the characteristics of wakes of wind turbines operating in various flow conditions including interacting wakes between a row of turbines. The computations were carried out using the actuator line technique combined...... and it is shown that the turbines are subject to rather severe yaw moments, even in situations where the mean wind is oriented along the row. This observation is indicative of large scale dynamics of the wakes....... with the 3D Navier Stokes solver EllipSys3D and a LES turbulence model. Simple models, based on applying body forces in the computational domain, are developed for imposing sheared and turbulent infow and their validity is discussed. A few computations on stand alone turbines are compared to measurements...

  13. Fast particle tracking with wake fields

    Energy Technology Data Exchange (ETDEWEB)

    Dohlus, M.; Floettmann, K.; Henning, C.

    2012-01-15

    Tracking calculations of charged particles in electromagnetic fields require in principle the simultaneous solution of the equation of motion and of Maxwell's equations. In many tracking codes a simpler and more efficient approach is used: external fields like that of the accelerating structures are provided as field maps, generated in separate computations and for the calculation of self fields the model of a particle bunch in uniform motion is used. We describe how an externally computed wake function can be approximated by a table of Taylor coefficients and how the wake field kick can be calculated for the particle distribution in a tracking calculation. The integrated kick, representing the effect of a distributed structure, is applied at a discrete time. As an example, we use our approach to calculate the emittance growth of a bunch in an undulator beam pipe due to resistive wall wake field effects. (orig.)

  14. Molecular wakes for ultrashort laser pulses

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    The molecular wake-assisted interaction between two collinear femotosecond laser pulses is investigated in air,which leads to the generation of a controllable 1.8 mJ super-continuum pulse with an elongated self-guided channel due to the cross-phase modulation of the impulsively aligned diatomic molecules in air. For two parallel launched femtosecond laser pulses with a certain spatial separation,controllable attraction and repulsion of the pulses are observed due to the counter-balance among molecular wakes,Kerr and plasma effects,where the molecular wakes show a longer interaction distance than the others to control the propagation of the intense ultrashort laser pulses.

  15. Effect of wind turbine wakes on cropland surface fluxes in the US Great Plains during a Nocturnal Low Level Jet

    Science.gov (United States)

    Rhodes, M. E.; Aitken, M.; Lundquist, J. K.; Takle, E. S.; Prueger, J. H.

    2010-12-01

    Installation of large scale wind farms is becoming a common operation in the Midwest, and wind farms frequently are situated among fields of agricultural crops. Each wind turbine is known to alter the behavior of the air mass downwind of the rotor; consequently, the rotor wakes alter the local microclimate. Quantification of the effects of wind turbine wakes on local microclimate is required to understand how large-scale wind deployment affects large-scale agriculture. This study examines the potential effect of wind turbine wakes on a corn crop in central Iowa during summer 2010. The field site consisted of one surface flux tower upwind of a row of five modern wind turbine generators, an identical surface flux station downwind of the turbine row, and a ground based LIDAR system downwind of the wind turbines. Each flux tower was instrumented with an array consisting of radiometers, a three-dimensional sonic anemometer, an open cell CO2 analyzer, a cup anemometer and wind vane, temperature and relative humidity sensors, and a tipping bucket. The LIDAR system reliably obtained readings up to 200 m above ground level (AGL), spanning the entire rotor disk (~40 m to 120 m AGL). This presentation examines wake-surface interaction on one particular night, during which the prevailing winds situated the LIDAR directly behind a wind turbine approximately 2 rotor diameters downwind of the turbine tower. As expected preliminary LIDAR results indicate that in the turbine rotor shadow there is a strong deficit of horizontal momentum. Additionally, a strong nocturnal low-level jet occurred above the turbine rotor disk. Wavelet spectral analysis indicates that oscillatory behavior, with frequencies characteristic of wind turbine wakes, is observed in the LIDAR horizontal and vertical winds and in the downwind flux station datastreams. The characterization of wake effects provided by this unique dataset will allow for better parameterization and modeling of wind turbine wake

  16. Analysis of Numerically Generated Wake Structures

    DEFF Research Database (Denmark)

    Ivanell, S.; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming;

    2009-01-01

    Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology. In the a......Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology...

  17. Anisotropy of turbulence in wind turbine wakes

    Energy Technology Data Exchange (ETDEWEB)

    Gomez-Elvira, Rafael [Comision Nacional de Energia (Spain); Crespo, Antonio; Migoya, Emilio; Manuel, Fernando [Departamento de Ingenieria Energetica y Fluidomecanica, Escuela Tecnica Superior de Ingenieros Industriales, Universidad Politecnica de Madrid, Jose Gutierrez Abascal, 2. 28006 Madrid (Spain); Hernandez, Julio [Departamento de Mecanica, ETSII, Universidad Nacional de Educacion a Distancia, Ciudad Universitaria, 28040 Madrid (Spain)

    2005-10-01

    This work is mainly dedicated to the study of non-isotropic characteristics of turbulence in wind turbine wakes, specifically the shear layer of the near wake. A calculation method based on an explicit algebraic model for the components of the turbulent stress tensor is proposed, and the results are found to be in acceptable agreement with experimental results. Analytical expressions for the estimation of an upper limit of the global turbulence kinetic energy, k, and the individual contributions of each diagonal term in the turbulent stress tensor are proposed. Their predictions are compared with experimental results.

  18. Investigation of the Rotor Wake of Horizontal Axis Wind Turbine under Yawed Condition

    Directory of Open Access Journals (Sweden)

    B. Noura

    2016-01-01

    Full Text Available The wake and the lack of existing velocity behind the wind turbine affect the energy production and the mechanical integrity of wind turbines downstream in the wind farms. This paper presents an investigation of the unsteady flow around a wind turbine under yawed condition. The simulations and experimental measures are made for the yaw angle rotor 30° and 0°. The wind velocity is 9.3 m/s and the rotation velocity rotor of the wind turbine in 1300, 1500 and 1800 rpm. The wind turbine rotor which is modeled is of a commercial wind turbine i.e. Rutland 503. The approach Improved Delayed Detached Eddy Simulation (IDDES based on the SST turbulence model is used in the modeling of the flow. The solutions are obtained by using the solver which uses finite volume method. The particle image velocimetry (PIV method is used in wind tunnel measurements in the experimental laboratory of the ENSAM Paris-Tech. The yawed downstream wake of the rotor is compared with that obtained by the experimental measurements. The results illustrate perfectly the development of the near and far wake of the rotor operation. It is observed that the upstream wind turbine yawed will have a positive impact on the power of the downstream turbine due the distance reduction of the downstream wake of the wind turbine. However the power losses are important for yawed wind turbine when compared with the wind turbine without yaw. The improved understanding of the unsteady environmental of the Horizontal Axis wind Turbine allows optimizing wind turbine structures and the number of wind turbines in wind farms.

  19. Wake effects characterization using wake oscillator model. Comparison on 2D response with experiments

    CERN Document Server

    Gaurier, Benoît; Cébron, David

    2010-01-01

    A model using wake oscillators is developed to predict the 2D motion in a transverse plan of two rigid cylinders in tandem arrangement. This model of the wake dynamics is validated with experimental data from previous trials which took place at the Ifremer flume tank in Boulogne-sur-Mer, France. The agreement between the model and the experimental results allows using this model as a simple computational tool in the prediction of 2D Vortex-Induced Vibrations (VIV) and, after some futher developments, Wake-Induced Oscillations (WIO) effects.

  20. FEATURES OF THE SUPERCRITICAL TRANSITION IN THE WAKE OF A CIRCULAR CYLINDER

    Institute of Scientific and Technical Information of China (English)

    熊俊; 凌国灿; 朱克勤

    2002-01-01

    The features of the wake behind a uniform circular cylinder at Re=200, which is just beyond the critical Reynolds number of 3-D transition, areinvestigated in detail by direct numerical simulations by solving 3-D incompressible Navier-Stokes equations using mixed spectral-spectral-element method. The high-order splitting algorithm based on the mixed stiffly stable scheme is employed inthe time discretization. Due to the nonlinear evolution of the secondary instabilityof the wake, the spanwise modes with different wavelengths emerge. The spanwisecharacteristic length determines the transition features and global properties of thewake. The existence of the spanwise phase difference of the primary vortices sheddingis confirmed by Fourier analysis of the time series of the spanwise vorticity and at-tributed to the dominant spanwise mode. The spatial energy distributions of variousmodes and the velocity profiles in the near wake are obtained. The numerical resultsindicate that the near wake is in 3-D quasi-periodic laminar state with transitionalbehaviors at this supercritical Reynolds number.

  1. Wake field of electron beam accelerated in a RF-gun of free electron laser 'ELSA'

    CERN Document Server

    Salah, W

    1999-01-01

    Wake field effects driven by a coasting relativistic charged particle beam have been studied for various cavity geometries. In the particular case of a cylindrical 'pill-box' cavity, an analytical expression of the (E, B)(x, t) map has been obtained as a development on the complete base cavity normal modes. We extend this method to the case of an accelerated beam, which leaves the downstream face of the cavity with a thermal velocity, and becomes relativistic in a few cm. This situation is very different from the classical wake of an ultrarelativistic beam for two reasons: (a) in the case of an ultrarelativistic beam, the field directly generated by beam particles in their wake can be neglected, and the so-called wake field is the electromagnetic linear response of the cavity to the exciting signal which is the beam. For a transrelativistic beam, the direct field must be taken into account and added to cavity response, which is no longer linear, except for low-intensity beam; (b) causality prevents any beam's...

  2. On point vortex models of exotic bluff body wakes

    Energy Technology Data Exchange (ETDEWEB)

    Stremler, Mark A; Basu, Saikat, E-mail: stremler@vt.edu [Department of Engineering Science and Mechanics, Virginia Tech, Blacksburg, VA 24061 (United States)

    2014-12-01

    Exotic vortex wakes, in which three or more vortices are generated during each shedding cycle, are frequently found in the wake of an oscillating bluff body. Two common examples are P+S wakes (with 3 vortices) and 2P wakes (with 4 vortices). We consider mathematical models of these wakes consisting of N = 3 or 4 point vortices with constant strengths in an inviscid fluid that is otherwise at rest in a singly-periodic domain. By enforcing constraints on the vortex strengths and, in the case of N = 4, on the symmetry of the vortex locations, the mathematical models reduce to integrable Hamiltonian systems. We compare the point vortex trajectories with two exotic wake patterns reported in the literature. Results support the use of point vortex modeling to investigate vortex dynamics in exotic wakes and suggest the need for additional classification of experimental wake patterns. (paper)

  3. TMCI threshold with space charge and different wake fields

    Energy Technology Data Exchange (ETDEWEB)

    Balbekov, V. [Fermilab

    2016-08-22

    Transverse mode coupling instability of a bunch with space charge and wake field is considered within the frameworks of the boxcar model. Eigenfunctions of the bunch without wake are used as a basis for the solution of the equations with the wake field included. A dispersion equation for constant wake is presented in the form of an infinite continued fraction and also as the recursive relation with an arbitrary number of the basis functions. Realistic wake fields are considered as well including resistive wall, square, and oscillating wakes. It is shown that the TMCI threshold of the negative wake grows up in absolute value when the SC tune shift increases. Threshold of positive wake goes down at the increasing SC tune shift. The explanation is developed by an analysis of the bunch spectrum.

  4. TMCI threshold with space charge and different wake fields

    CERN Document Server

    Balbekov, V

    2016-01-01

    Transverse mode coupling instability of a bunch with space charge and wake field is considered within the frameworks of the boxcar model. Eigenfunctions of the bunch without wake are used as a basis for the solution of the equations with the wake field included. A dispersion equation for constant wake is presented in the form of an infinite continued fraction and also as the recursive relation with an arbitrary number of the basis functions. Realistic wake fields are considered as well including resistive wall, square, and oscillating wakes. It is shown that the TMCI threshold of the negative wake grows up in absolute value when the SC tune shift increases. Threshold of positive wake goes down at the increasing SC tune shift. The explanation is developed by an analysis of the bunch spectrum.

  5. Dealing With a Deficit

    Institute of Scientific and Technical Information of China (English)

    LAN XINZHEN

    2010-01-01

    @@ For the first time since April 2004,China experienced a monthly trade deficit as imports surpassed exports.Statistics released by the General Administration of Customs on April 10 showed China's export and import volume reached $112.11billion and $119.35 billion in March,respectively,leading to a trade deficit of $7.24 billion.

  6. Wind turbine wake measurement in complex terrain

    DEFF Research Database (Denmark)

    Hansen, Kurt Schaldemose; Larsen, Gunner Chr.; Menke, Robert;

    2016-01-01

    SCADA data from a wind farm and high frequency time series measurements obtained with remote scanning systems have been analysed with focus on identification of wind turbine wake properties in complex terrain. The analysis indicates that within the flow regime characterized by medium to large...

  7. Meteorological Controls on Wind Turbine Wakes

    DEFF Research Database (Denmark)

    Barthelmie, Rebecca J.; Hansen, Kurt Schaldemose; Pryor, S.C.

    2013-01-01

    The primary control on the magnitude of the power losses induced by wind turbine wakes in large wind farms is the hub-height wind speed via its link to the turbine thrust coefficient. Hence, at low to moderate wind speeds (between cut-in and rated turbine wind speeds) when the thrust coefficient...

  8. Sleep-wake as a biological rhythm.

    Science.gov (United States)

    Lavie, P

    2001-01-01

    Evidence that the sleep-wake rhythm is generated endogenously has been provided by studies employing a variety of experimental paradigms such as sleep deprivation, sleep displacement, isolating subjects in environments free of time cues, or imposing on subjects sleep-wake schedules widely deviating from 24 hours. The initial observations obtained in isolated subjects revealed that the period of the endogenous circadian pacemaker regulating sleep is of approximately 25 hours. More recent studies, however, in which a more rigorous control of subjects' behavior was exerted, particularly over lighting conditions, have shown that the true periodicity of the endogenous pacemaker deviates from 24 hours by a few minutes only. Besides sleep propensity, the circadian pacemaker has been shown to regulate sleep consolidation, sleep stage structure, and electroencephalographic activities. The pattern of light exposure throughout the 24 hours appears to participate in the entrainment of the circadian pacemaker to the geophysical day-night cycle. Melatonin, the pineal hormone produced during the dark hours, participates in communicating both between the environmental light-dark cycle and the circadian pacemaker, and between the circadian pacemaker and the sleep-wake-generating mechanism. In contrast to prevailing views that have placed great emphasis on homeostatic sleep drive, recent data have revealed a potent circadian cycle in the drive for wakefulness, which is generated by the suprachiasmatic nucleus. This drive reaches a peak during the evening hours just before habitual bedtime.

  9. Vortex wakes of a flapping foil

    DEFF Research Database (Denmark)

    Schnipper, Teis; Andersen, Anders Peter; Bohr, Tomas

    2009-01-01

    We present an experimental study of a symmetric foil performing pitching oscillations in a vertically flowing soap film. By varying the frequency and amplitude of the oscillation we visualize a variety of wakes with up to 46 vortices per oscillation period, including von Karman vortex street...

  10. Wakes in large offshore wind farms

    DEFF Research Database (Denmark)

    Berthelmie, Rebecca J.; Frandsen, Sten Tronæs; Rathmann, Ole

    2008-01-01

    Power losses due to wind turbine wakes are of the order of 10 and 20% of total power output in large wind farms. The focus of this research carried out within the EC funded UPWIND project is wind speed and turbulence modelling for large wind farms/wind turbines in complex terrain and offshore in ...

  11. Wake up and Stop Smelling the Coffee

    Institute of Scientific and Technical Information of China (English)

    林芳

    2005-01-01

    The next time someone tells you to wake up and smell the coffee don't take it literally, instead go ahead and chuck2 your coffee making machine as a new study has revealed3 that having more than one cup of coffee a day increases the risk of heart disease and even one cup is ‘risky.'

  12. Simulation of wind turbine wakes using the actuator line technique

    DEFF Research Database (Denmark)

    Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming; Henningson, Dan S.;

    2015-01-01

    The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance...

  13. 32 CFR 707.10 - Wake illumination light.

    Science.gov (United States)

    2010-07-01

    ... 32 National Defense 5 2010-07-01 2010-07-01 false Wake illumination light. 707.10 Section 707.10... RESPECT TO ADDITIONAL STATION AND SIGNAL LIGHTS § 707.10 Wake illumination light. Naval vessels may display a white spot light located near the stern to illuminate the wake....

  14. Wake simulation for wind turbines with a free, prescribed- and hybrid-wake method

    Energy Technology Data Exchange (ETDEWEB)

    Bareiss, R.; Guidati, G.; Wagner, S. [Univ. Stuttgart, Inst. fuer Aerodynamik und Gasdynamik, Stuttgart (Germany)

    1997-08-01

    Calculations of the radial distribution and the time history of the induction factors have been performed with a number of different wake models implemented in a vortex-lattice method for tip-speed ratios in the range 1-13. The new models lead to a significant reduction of the computational effort down to 3-27% compared to a free-wake model with errors less than 5%. (au)

  15. Wake fields in the LHCb vertex detector : alternative designs for the wake field suppressor

    CERN Document Server

    Van Bakel, N; Ferro-Luzzi, M

    2000-01-01

    Two previous LHCb notes showed that wake field suppressors are needed to prevent dissipating excessive power in resonating modes of the vertex detector components. This note concentrates on two different designs of the encapsulations of the silicon detectors which can serve as an alternative to the design which uses long thin strips spanned throughout the vertex detector near the beam axis. The quality of wake field suppression with respect to the depth of a corrugated structure is simulated.

  16. The Near Wake of Bluff Bodies in Stratified Fluids and the Emergence of Late Wake Characteristics

    Science.gov (United States)

    2010-10-29

    Kundu and Ira Cohen. Fluid Mechanics , Third Edition. Elsevier Inc., 2004. [27] S. Lee. A numerical study of the unsteady wake behind a sphere in a...found in the stratified fluid flows is the existence of the standing lee wave, which is a laminar mechanism . Above a Reynolds number of 2000, it is...computational cost to both model the proper fluid mechanics on the sphere and reproduce the far-wake is prohibitively expensive with current

  17. MODIFIED LOG-WAKE LAWS FOR TURBULENT FLOW OF THE OUTER AND INNER REGIONS IN SMOOTH PIPES

    Institute of Scientific and Technical Information of China (English)

    LIU Ya-kun; NI Han-gen

    2007-01-01

    Based on theoretical analysis two modified log-wake laws for turbulent flow in smooth pipes are obtained, one is applicable to the outer region and other one to inner region, the new law for the outer region fits the velocity profile measured in smooth pipes by Zagarola very well and the effect of Reynolds number can be taken into consideration, the velocity profile for inner region satisfies the wall boundary conditions ,equals zero at the pipe wall and smoothly joins up with the velocity profile in outer region, the adopted eddy viscosity model is consistent with Laufer's, Nunner's and Reichardt's experimental data.

  18. WAKE GEOMETRY CALCULATIONS FOR TILT-ROTOR USING VISCOUS VORTEX METHOD

    Institute of Scientific and Technical Information of China (English)

    魏鹏; 史勇杰; 徐国华

    2013-01-01

    A tilt-rotor unsteady flow analytical method has been developed based upon viscous vortex-particle meth-od .In this method ,the vorticity field is divided into small assembled vortex particles .Vortex motion and diffusion are obtained by solving the velocity-vorticity-formed incompressible Navier-Stokes equations using a grid-free La-grangian simulation method .Generation of the newly vortex particles is calculated by using the Weissinger-L lifting surface model .Furthermore ,in order to significantly improve computational efficiency ,a fast multiple method (FMM) is introduced into the calculation of induced velocity and its gradient .Finally ,the joint vertical experimen-tal (JVX) tilt-rotor is taken as numerical examples to analyze .The wake geometry and downwash are investigated for both hover and airplane modes .The proposed method for tilt-rotor flow analysis is verified by comparing its re-sults with those available measured data .Comparison indicates that the current method can accurately capture the complicated tilt-rotor wake variation and be suitable for aerodynamic interaction simulation in complex environ-ments .Additionally ,the aerodynamic interactional characteristics of dual-rotor wake are discussed in different ro-tor distance .Results show that there are significant differences on interactional characteristics between hover mode and airplane mode .

  19. Aerodynamic wake study: oscillating model wind turbine within a turbulent boundary layer

    Science.gov (United States)

    Feist, Christopher J.

    An experimental investigation on the aerodynamic wake behind a pitching and/or heaving model wind turbine was performed. The study was split into two quasi-coupled phases; the first phase characterized the motion of an offshore floating wind turbine subjected to linear wave forcing, the second phase replicated specific motion cases, which were driven by results from phase I, on a model wind turbine within a turbulent boundary layer. Wake measurements were made in an effort to quantify fluctuations in the flow associated with the motion of the turbine. Weak differences were observed in the mean, streamwise velocity and turbulent fluctuations between the static and oscillating turbine cases. These weak differences were a result of opposing trends in the velocity quantities based on turbine motion phases. The wake oscillations created by the turbine motion was characteristic of a 2D wave (with convection in the x plane and amplitude in the z plane) with a relatively small amplitude as compared to urms..

  20. Momentum distribution in the wake of a bio-inspired trapezoidal pitching panel

    Science.gov (United States)

    Kumar, Rajeev; King, Justin; Green, Melissa

    2016-11-01

    A trapezoidal pitching panel that models a fish caudal fin was used to study the distribution of streamwise momentum in its wake. The three-dimensional phase-averaged velocity fields were captured using stereoscopic PIV at Strouhal numbers (St) ranging from 0.17 to 0.56. The pitching trapezoidal panel wake consists of chains of interacting vortex rings that induce significant three-dimensional flows. With increasing Strouhal number, this wake structure induces flow with increasing non-dimensional downstream momentum, which is consistent with greater non-dimensional thrust production at higher St shown previously in the literature. Also at higher St, these chains of vortex rings split and diverge in the transverse direction, giving rise to a pair of downstream jets. At the highest St, a region of downstream momentum lower than the freestream is observed along the centerline between the jet pair. This loss of momentum surplus may be related to a previously described decline in propulsive efficiency at higher St. The momentum distribution is also studied in the time-averaged velocity fields to show how the average momentum is distributed over the same range of St. Research Assistant Professor.

  1. Numerical analysis of the wake of a 10kW HAWT

    Science.gov (United States)

    Gong, S. G.; Deng, Y. B.; Xie, G. L.; Zhang, J. P.

    2017-01-01

    With the rising of wind power industry and the ever-growing scale of wind farm, the research for the wake performance of wind turbine has an important guiding significance for the overall arrangement of wind turbines in the large wind farm. The wake simulation model of 10kW horizontal-axis wind turbine is presented on the basis of Averaged Navier-Stokes (RANS) equations and the RNG k-ε turbulence model for applying to the rotational fluid flow. The sliding mesh technique in ANSYS CFX software is used to solve the coupling equation of velocity and pressure. The characters of the average velocity in the wake zone under rated inlet wind speed and different rotor rotational speeds have been investigated. Based on the analysis results, it is proposed that the horizontal spacing between the wind turbines is less than two times radius of rotor, and its longitudinal spacing is less than five times of radius. And other results have also been obtained, which are of great importance for large wind farms.

  2. A comparative analysis on the shed vortices from the wake of finned, foam-wrapped cylinders

    Science.gov (United States)

    Khashehchi, Morteza; Ashtiani Abdi, Iman; Hooman, Kamel

    2017-08-01

    The wake characteristics behind a finned and a foam-wrapped circular cylinder has been compared in a study (Khashehchi et al 2014 Exp. Therm. Fluid Sci. 52 328-38) done by the Authors. In this paper, the shed vortices from the wake of the same cylinders have been studied. Shedding in a bluff body has an important effect on increasing the pressure drop downstream of the object. Here, we have used particle image velocimetry to investigate the detached vortices from the wake behind a foam-wrapped and a finned cylinder. The standard case of cross-flow over a bare cylinder, i.e. no surface extension, has also been tested as a benchmark. The experiments have been performed for Reynolds numbers 2000 based on the mean air velocity and the cylinder’s outer diameter. To identify the features of each aforementioned case, linear stochastic estimation has been applied to the velocity fields. Results show that unlike the fin, adding foam to the cylinder surface increases the size of detached vortices and amplifies the core strength. Moreover, foam-wrapped cylinder in contrast to the finned one produces strong three-dimensionality. Interestingly, finned cylinder’s results show less three-dimensionality compared to the bare cylinder.

  3. Increasing length of wakefulness and modulation of hypocretin-1 in the wake-consolidated squirrel monkey.

    Science.gov (United States)

    Zeitzer, Jamie M; Buckmaster, Christine L; Lyons, David M; Mignot, Emmanuel

    2007-10-01

    The neuropeptides hypocretins (orexins), the loss of which results in the sleep disorder narcolepsy, are hypothesized to be involved in the consolidation of wakefulness and have been proposed to be part of the circadian-driven alertness signal. To elucidate the role of hypocretins in the consolidation of human wakefulness we examined the effect of wake extension on hypocretin-1 in squirrel monkeys, primates that consolidate wakefulness during the daytime as do humans. Wake was extended up to 7 h with hypocretin-1, cortisol, ghrelin, leptin, locomotion, and feeding, all being assayed. Hypocretin-1 (P sleep deprivation, while ghrelin (P = 0.79) and leptin (P = 1.00) did not change with sleep deprivation. Using cross-correlation and multivariate modeling of these potential covariates along with homeostatic pressure (a measure of time awake/asleep), we found that time of day and homeostatic pressure together explained 44% of the variance in the hypocretin-1 data (P sleep pressure. Concomitants of wakefulness that affect hypocretin-1 in polyphasic species, such as locomotion, food intake, and food deprivation, likely have a more minor role in monophasic species, such as humans.

  4. A theoretical model of a wake of a body towed in a stratified fluid at large Reynolds and Froude numbers

    Directory of Open Access Journals (Sweden)

    Y. I. Troitskaya

    2006-01-01

    Full Text Available The objective of the present paper is to develop a theoretical model describing the evolution of a turbulent wake behind a towed sphere in a stably stratified fluid at large Froude and Reynolds numbers. The wake flow is considered as a quasi two-dimensional (2-D turbulent jet flow whose dynamics is governed by the momentum transfer from the mean flow to a quasi-2-D sinuous mode growing due to hydrodynamic instability. The model employs a quasi-linear approximation to describe this momentum transfer. The model scaling coefficients are defined with the use of available experimental data, and the performance of the model is verified by comparison with the results of a direct numerical simulation of a 2-D turbulent jet flow. The model prediction for the temporal development of the wake axis mean velocity is found to be in good agreement with the experimental data obtained by Spedding (1997.

  5. Pulsed coherent fiber lidar transceiver for aircraft in-flight turbulence and wake-vortex hazard detection

    Science.gov (United States)

    Akbulut, M.; Hwang, J.; Kimpel, F.; Gupta, S.; Verdun, H.

    2011-06-01

    We report on the development of a fiber-optic pulsed coherent lidar transceiver for wind-velocity and aircraft wake-vortex hazard detection. The all-fiber 1.5μm transmitter provides up to 560 μJ energy at 25 kHz with 800 ns pulse width (pump limited). Performance simulations indicate wake-vortex hazard signature detection up to ~2.5km range with a receiver sensitivity of ~2 fW (SNR=6), suited for an aircraft landing scenario. Furthermore, the transceiver is implemented using high-speed FPGA based control and digital-signal-processing, enabling its use as a flexible pulse-format multi-function in-flight lidar sensor. We present the latest laboratory results and preliminary testing of this pulsed coherent lidar transceiver, along with the lidar performance simulation of wake-vortex eddy models.

  6. Vortex Particle-Mesh simulations of Vertical Axis Wind Turbine flows: from the blade aerodynamics to the very far wake

    Science.gov (United States)

    Chatelain, P.; Duponcheel, M.; Caprace, D.-G.; Marichal, Y.; Winckelmans, G.

    2016-09-01

    A Vortex Particle-Mesh (VPM) method with immersed lifting lines has been developed and validated. Based on the vorticity-velocity formulation of the Navier-Stokes equations, it combines the advantages of a particle method and of a mesh-based approach. The immersed lifting lines handle the creation of vorticity from the blade elements and its early development. LES of Vertical Axis Wind Turbine (VAWT) flows are performed. The complex wake development is captured in details and over very long distances: from the blades to the near wake coherent vortices, then through the transitional ones to the fully developed turbulent far wake (beyond 10 rotor diameters). The statistics and topology of the mean flow are studied. The computational sizes also allow insights into the detailed unsteady vortex dynamics, including some unexpected topological flow features.

  7. Interaction of a Boundary Layer with a Turbulent Wake

    Science.gov (United States)

    Piomelli, Ugo

    2004-01-01

    The objective of this grant was to study the transition mechanisms on a flat-plate boundary layer interacting with the wake of a bluff body. This is a simplified configuration presented and designed to exemplify the phenomena that occur in multi-element airfoils, in which the wake of an upstream element impinges on a downstream one. Some experimental data is available for this configuration at various Reynolds numbers. The first task carried out was the implementation and validation of the immersed-boundary method. This was achieved by performing calculations of the flow over a cylinder at low and moderate Reynolds numbers. The low-Reynolds number results are discussed, which is enclosed as Appendix A. The high-Reynolds number results are presented in a paper in preparation for the Journal of Fluid Mechanics. We performed calculations of the wake-boundary-layer interaction at two Reynolds numbers, Re approximately equal to 385 and 1155. The first case is discussed and a comparison of the two calculations is reported. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady Karman vortex street shed by the cylinder. This is shown: long streaky structures appear in the boundary layer in correspondence of the three-dimensionalities in the rollers. The fluctuations, however, cannot be self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer fluctuations persist after the wake has decayed (due, in part, to the higher values of the local Reynolds number Re achieved in this case); some evidence could be observed that a self-sustaining turbulence generation cycle was beginning to be established. A third simulation was subsequently carried out at a higher Reynolds number, Re=3900. This calculation gave results similar to those of the Re=l155 case. Turbulence was established at fairly low

  8. Effect of Large Finite-Size Wind Farms and Their Wakes on Atmospheric Boundary Layer Dynamics

    Science.gov (United States)

    Wu, Ka Ling; Porté-Agel, Fernando

    2016-04-01

    Through the use of large-eddy simulation, the effect of large finite-size wind farms and their wakes on conventionally-neutral atmospheric boundary layer (ABL) dynamics and power extraction is investigated. Specifically, this study focuses on a wind farm that comprises 25 rows of wind turbines, spanning a distance of 10 km. It is shown that large wind farms have a significant effect on internal boundary layer growth both inside and downwind of the wind farms. If the wind farm is large enough, the internal boundary layer interacts with the thermally-stratified free atmosphere above, leading to a modification of the ABL height and power extraction. In addition, it is shown that large wind farms create extensive wakes, which could have an effect on potential downwind wind farms. Specifically, for the case considered here, a power deficit as large as 8% is found at a distance of 10 km downwind from the wind farm. Furthermore, this study compares the wind farm wake dynamics for cases in which the conventionally neutral ABLs are driven by a unidirectional pressure gradient and Coriolis forces.

  9. Velocity anticipation in the optimal velocity model

    Institute of Scientific and Technical Information of China (English)

    DONG Li-yun; WENG Xu-dan; LI Qing-ding

    2009-01-01

    In this paper,the velocity anticipation in the optimal velocity model (OVM) is investigated.The driver adjusts the velocity of his vehicle by the desired headway,which depends on both instantaneous headway and relative velocity.The effect of relative velocity is measured by a sensitivity function.A specific form of the sensitivity function is supposed and the involved parameters are determined by the both numerical simulation and empirical data.It is shown that inclusion of velocity anticipation enhances the stability of traffic flow.Numerical simulations show a good agreement with empirical data.This model provides a better description of real traffic,including the acceleration process from standing states and the deceleration process approaching a stopped car.

  10. PIV measurements of the asymmetric wake of a two dimensional heaving hydrofoil

    Energy Technology Data Exchange (ETDEWEB)

    Ellenrieder, K.D. von [Florida Atlantic University, Department of Ocean Engineering, Dania Beach, FL (United States); Pothos, S. [TSI Inc, Fluid Mechanics Research Instruments, Shoreview, MN (United States)

    2008-05-15

    Particle image velocimetry is used to examine the flow behind a two-dimensional heaving hydrofoil of NACA 0012 cross section, operating with heave amplitude to chord ratio of 0.215 at Strouhal numbers between 0.174 and 0.781 and a Reynolds number of 2,700. The measurements show that for Strouhal numbers larger than 0.434, the wake becomes deflected such that the average velocity profile is asymmetric about the mean heave position of the hydrofoil. The deflection angle of the wake, which is related to the average lift and drag on the hydrofoil, is found to lie between 13 and 18 . An examination of the swirl strength of the vortices generated by the hydrofoil motion reveal that the strongest vortices, which are created at the higher Strouhal numbers, dissipate most rapidly. (orig.)

  11. Relativistic electron beam driven longitudinal wake-wave breaking in a cold plasma

    CERN Document Server

    Bera, Ratan Kumar; Sengupta, Sudip; Das, Amita

    2016-01-01

    Space-time evolution of relativistic electron beam driven wake-field in a cold, homogeneous plasma, is studied using 1D-fluid simulation techniques. It is observed that the wake wave gradu- ally evolves and eventually breaks, exhibiting sharp spikes in the density profile and sawtooth like features in the electric field profile [1]. It is shown here that the excited wakefield is a longitudi- nal Akhiezer-Polovin mode [2] and its steepening (breaking) can be understood in terms of phase mixing of this mode, which arises because of relativistic mass variation effects. Further the phase mixing time (breaking time) is studied as a function of beam density and beam velocity and is found to follow the well known scaling presented in ref.[3].

  12. Cylindrical vortex wake model: skewed cylinder, application to yawed or tilted rotors

    DEFF Research Database (Denmark)

    Branlard, Emmanuel Simon Pierre; Gaunaa, Mac

    2016-01-01

    in Blade Element Momentum method codes for yawed conditions. Here, all the components of the full vortex system are analyzed in view of extending Blade Element Momentum models. The main assumptions of the current study are a constant uniform circulation, an infinite number of blades, an un-expanding wake......A vortex system consisting of a bound vortex disk, a root vortex and a vortex cylinder is presented and applied for skewed wake situations. Both the longitudinal and tangential components of vorticity of the cylinder are considered. A subset of this system leads to a model, which is commonly used...... shape and a finite tip-speed ratio. The investigation remains within the context of inviscid potential flow theory. The model is derived for horizontal-axis rotors in general, but results are presented for wind-turbine applications. For each vortex element, the velocity components in all directions...

  13. Vortex methods with immersed lifting lines applied to LES of wind turbine wakes

    Science.gov (United States)

    Chatelain, Philippe; Bricteux, Laurent; Winckelmans, Gregoire; Koumoutsakos, Petros

    2010-11-01

    We present the coupling of a vortex particle-mesh method with immersed lifting lines. The method relies on the Lagrangian discretization of the Navier-Stokes equations in vorticity-velocity formulation. Advection is handled by the particles while the mesh allows the evaluation of the differential operators and the use of fast Poisson solvers. We use a Fourier-based fast Poisson solver which simultaneously allows unbounded directions and inlet/outlet boundaries. A lifting line approach models the vorticity sources in the flow. Its immersed treatment efficiently captures the development of vorticity from thin sheets into a three-dimensional field. We apply this approach to the simulation of a wind turbine wake at very high Reynolds number. The combined use of particles and multiscale subgrid models allows the capture of wake dynamics with minimal spurious diffusion and dispersion.

  14. Marine propellers performance and flow-field prediction by a free-wake panel method

    Institute of Scientific and Technical Information of China (English)

    GRECO Luca; MUSCARI Roberto; TESTA Claudio; DI MASCI0 Andrea

    2014-01-01

    A Boundary Element Method (BEM) hydrodynamics combined with a flow-alignment technique to evaluate blades shed vorticity is presented and applied to a marine propeller in open water. Potentialities and drawbacks of this approach in capturing propeller performance, slipstream velocities, blade pressure distribution and pressure disturbance in the flow-field are highlighted by comparisons with available experiments and RANSE results. In particular, correlations between the shape of the convected vortex- sheet and the accuracy of BEM results are discussed throughout the paper. To this aim, the analysis of propeller thrust and torque is the starting point towards a detailed discussion on the capability of a 3-D free-wake BEM hydrodynamic approach to describe the local features of the flow-field behind the propeller disk, in view of applications to propulsive configurations where the shed wake plays a dominant role.

  15. High Reynolds Number Studies in the Wake of a Submarine Model

    Science.gov (United States)

    Jimenez, Juan; Reynolds, Ryan; Smits, Alexander

    2005-11-01

    Results are presented from submarine wake studies conducted in Princeton University's High Reynolds Number Test Facility (HRTF). Compressed air is used as a working fluid enabling Reynolds numbers based on length of up to 10^8, about 1/5 of full scale. Measurements at Reynolds numbers up to 3 x10^6 have been completed, and show that, for the model condition without fins, the wake mean velocity was self-similar at locations 6 and 9 diameters downstream. Also, PIV at Reynolds numbers near 10^4 showed that when the yaw angle was varied the sail-tip and sail-hull junction vortices increased in magnitude emphasizing the importance of fully understanding the flow characteristics of a maneuvering submarine.

  16. Wake development behind paired wings with tip and root trailing vortices: consequences for animal flight force estimates.

    Directory of Open Access Journals (Sweden)

    Jan T Horstmann

    Full Text Available Recent experiments on flapping flight in animals have shown that a variety of unrelated species shed a wake behind left and right wings consisting of both tip and root vortices. Here we present an investigation using Particle Image Velocimetry (PIV of the behaviour and interaction of trailing vortices shed by paired, fixed wings that simplify and mimic the wake of a flying animal with a non-lifting body. We measured flow velocities at five positions downstream of two adjacent NACA 0012 aerofoils and systematically varied aspect ratio, the gap between the wings (corresponding to the width of a non-lifting body, angle of attack, and the Reynolds number. The range of aspect ratios and Reynolds number where chosen to be relevant to natural fliers and swimmers, and insect flight in particular. We show that the wake behind the paired wings deformed as a consequence of the induced flow distribution such that the wingtip vortices convected downwards while the root vortices twist around each other. Vortex interaction and wake deformation became more pronounced further downstream of the wing, so the positioning of PIV measurement planes in experiments on flying animals has an important effect on subsequent force estimates due to rotating induced flow vectors. Wake deformation was most severe behind wings with lower aspect ratios and when the distance between the wings was small, suggesting that animals that match this description constitute high-risk groups in terms of measurement error. Our results, therefore, have significant implications for experimental design where wake measurements are used to estimate forces generated in animal flight. In particular, the downstream distance of the measurement plane should be minimised, notwithstanding the animal welfare constraints when measuring the wake behind flying animals.

  17. Proceedings of the NASA First Wake Vortex Dynamic Spacing Workshop

    Science.gov (United States)

    Creduer, Leonard (Editor); Perry, R. Brad (Editor)

    1997-01-01

    A Government and Industry workshop on wake vortex dynamic spacing systems was conducted on May 13-15, 1997, at the NASA Langley Research Center. The purpose of the workshop was to disclose the status of ongoing NASA wake vortex R&D to the international community and to seek feedback on the direction of future work to assure an optimized research approach. Workshop sessions examined wake vortex characterization and physics, wake sensor technologies, aircraft/wake encounters, terminal area weather characterization and prediction, and wake vortex systems integration and implementation. A final workshop session surveyed the Government and Industry perspectives on the NASA research underway and related international wake vortex activities. This document contains the proceedings of the workshop including the presenters' slides, the discussion following each presentation, the wrap-up panel discussion, and the attendees' evaluation feedback.

  18. Scaling of far-field wake angle of nonaxisymmetric pressure disturbance

    Science.gov (United States)

    Moisy, F.; Rabaud, M.

    2014-06-01

    It has been recently emphasized that the angle of maximum wave amplitude α in the wake of a disturbance of finite size can be significantly narrower than the maximum value αK=sin-1(1/3)≃19.47∘ predicted by the classical analysis of Kelvin. For axisymmetric disturbance, a simple argument based on the Cauchy-Poisson initial-value problem suggests that the wake angle decreases following a Mach-like law at large velocity, α ≃FrL-1, where FrL=U/√gL is the Froude number based on the disturbance velocity U, its size L, and gravity g. In this paper we extend this analysis to the case of nonaxisymmetric disturbances, relevant to real ships. We find that, for intermediate Froude numbers, the wake angle follows an intermediate scaling law α ≃FrL-2, in agreement with the recent prediction of Noblesse et al. [Eur. J. Mech. B/Fluids 46, 164 (2014), 10.1016/j.euromechflu.2014.03.012]. We show that beyond a critical Froude number, which scales as A1/2 (where A is the length-to-width aspect ratio of the disturbance), the asymptotic scaling α ≃FrB-1 holds, where now FrB=A1/2FrL is the Froude number based on the disturbance width. We propose a simple model for this transition, and provide a regime diagram of the scaling of the wake angle as a function of parameters (A,FrL).

  19. What I make up when I wake up: anti-experience views and narrative fabrication of dreams.

    Science.gov (United States)

    Rosen, Melanie G

    2013-01-01

    I propose a narrative fabrication thesis of dream reports, according to which dream reports are often not accurate representations of experiences that occur during sleep. I begin with an overview of anti-experience theses of Norman Malcolm and Daniel Dennett who reject the received view of dreams, that dreams are experiences we have during sleep which are reported upon waking. Although rejection of the first claim of the received view, that dreams are experiences that occur during sleep, is implausible, I evaluate in more detail the second assumption of the received view, that dream reports are generally accurate. I then propose a "narrative fabrication" view of dreams as an alternative to the received view. Dream reports are often confabulated or fabricated because of poor memory, bizarre dream content, and cognitive deficits. It is well documented that narratives can be altered between initial rapid eye movement sleep awakenings and subsequent reports. I argue that we have reason to suspect that initial reports are prone to inaccuracy. Experiments demonstrate that subjects rationalize strange elements in narratives, leaving out supernatural or bizarre components when reporting waking memories of stories. Inaccuracies in dream reports are exacerbated by rapid memory loss and bizarre dream content. Waking memory is a process of reconstruction and blending of elements, but unlike waking memory, we cannot reality-test for dream memories. Dream experiences involve imaginative elements, and dream content cannot be verified with external evidence. Some dreams may involve wake-like higher cognitive functions, such as lucid dreams. Such dreams are more likely to elicit accurate reports than cognitively deficient dreams. However, dream reports are generally less accurate than waking reports. I then propose methods which could verify the narrative fabrication view, and argue that although the theory cannot be tested with current methods, new techniques and technologies may

  20. What I make up when I wake up: anti-experience views and narrative fabrication of dreams

    Directory of Open Access Journals (Sweden)

    Melanie Gillespie Rosen

    2013-08-01

    Full Text Available I propose a narrative fabrication thesis of dream reports, according to which dream reports are often not accurate representations of experiences that occur during sleep. I begin with an overview of anti-experience theses of Norman Malcolm and Daniel Dennett who reject the received view of dreams, that dreams are experiences we have during sleep which are reported upon waking. Although rejection of the first claim of the received view, that dreams are experiences that occur during sleep, is implausible, I evaluate in more detail the second assumption of the received view, that dream reports are generally accurate. I then propose a ‘narrative fabrication’ view of dreams as an alternative to the received view. Dream reports are often confabulated or fabricated because of poor memory, bizarre dream content and cognitive deficits. It is well documented that narratives can alter between initial REM sleep awakenings and subsequent reports. I argue that we have reason to suspect that initial reports are prone to inaccuracy. Experiments demonstrate that subjects rationalise strange elements in narratives, leaving out supernatural or bizarre components when reporting waking memories of stories. Inaccuracies in dream reports are exacerbated by rapid memory loss and bizarre dream content. Waking memory is a process of reconstruction and blending of elements, but unlike waking memory, we cannot reality-test for dream memories. Dream experiences involve imaginative elements, and dream content cannot be verified with external evidence. Some dreams may involve wake-like higher cognitive functions, such as lucid dreams. Such dreams more likely to elicit accurate reports than cognitively deficient dreams. However, dream reports are generally less accurate than waking reports. I then propose methods which could verify the narrative fabrication, and argue that although the theory cannot be tested with current methods, new techniques and technologies may be

  1. Influence of Thickness and Angle of Attack on the Dynamics of Rectangular Cylinder Wakes

    Science.gov (United States)

    Mohebi, Meraj

    Stereoscopic Particle Image Velocimetry measurements were taken in the turbulent wake of two-dimensional rectangular cylinders. The influence of post-stall angles of attack and Reynolds number on the flow behind a thin at plate, and for the normal case, the effect of thickness to chord (t=d) ratio over a family of rectangular cylinders were investigated. At all cases, quasi-periodic vortex shedding is observed, the normal direction Reynolds stress becomes very large just downstream of the mean recirculation zone, and the spanwise motions were uncorrelated to the main vortex shedding process. The data were processed to obtain the mean velocities, Reynolds stresses, and forces on the body. All terms in the turbulent kinetic energy equations were measured with the exception of dissipation which was found by difference. The pressure-related terms were estimated from the numerical solution of the Poisson equation for the instantaneous velocity field. Proper Orthogonal Decomposition modes are related via mean-field theory to construct generalized phase-averaging and low-order models capturing coherent cycle-to-cycle variations. The advection, production and pressure diffusion were all significant and mostly coherent. It is shown that high, average, and low amplitude vortex shedding cycles are different in terms of vortex street dimensions, vortex topology, circulation, and decay rate. It is also shown that these flows experience irregular significant decreases in the shedding amplitude associated with shedding of disorganized vortices in a large wake. Reynolds number was found to have imperceptible effects on the wake of a normal thin plate. A reduction in the angle of attack caused the wake to decrease in size and increase in shedding frequency but the global characteristics vary non-linearly. An increase in thickness from thin plate (t=d=0.05), caused the wake to shrink, low cycles to diminish, and local turbulence increase to a peak at t=d=1.0, identified as a

  2. Wake structures of two side by side spheres in a tripped boundary layer flow

    Directory of Open Access Journals (Sweden)

    Canli Eyüb

    2014-03-01

    Full Text Available Two independent spheres were placed in a side by side arrangement and flow structure in the wake region of the spheres was investigated with a Particle Image Velocimetry (PIV system when the spheres were in a boundary layer over a flat plate as a special case. Reynolds number was 5000 based on the sphere diameter which was 42.5 mm. Boundary layer was tripped 8mm away from the leading edge of the flat plate with a 5 mm trip wire. The thickness of the hydrodynamically developed boundary layer was determined as 63mm which was larger than the sphere diameter of D=42.5mm. Wake region of the spheres was examined from point of flow physics for the different sphere locations in the ranges of 0≤G/D ≤1.5 and 0≤S/D ≤1.5 where G and S were the distance between the spheres and the distance between the bottom point of the spheres and the flat plate surface, respectively. Depending on the different sphere locations, instantaneous and time averaged vorticity data, scalar values of time-averaged velocity components and their root mean square (rms values and time averaged vorticity data are presented in the study for the evaluation of wake region of the spheres. It is demonstrated that the gap between the two spheres and the interaction between the gap and the boundary layer greatly affects flow pattern, especially when spheres are located near to the flat plate surface, i.e. S/D=0.1 for 0≤G/D ≤1.5. Different distances between the spheres resulted in various flow patterns as the spheres were approached to the flat plate. The distance S/D=0.1 for all gap values has the strongest effect on the wake structures. Beyond G/D=1.0, the sphere wakes tend to be similar to single sphere case. The instantaneous vorticity fields of the side by side arrangements comprised wavy structures in higher level comparing to an individual sphere case. The gap flow intensifies the occurrence of small scale eddies in the wake region. The submersion rate of the spheres

  3. Wake structures of two side by side spheres in a tripped boundary layer flow

    Science.gov (United States)

    Canli, Eyüb; Özgören, Muammer; Dogan, Sercan; Hilmi Aksoy, Muharrem; Akilli, Huseyin

    2014-03-01

    Two independent spheres were placed in a side by side arrangement and flow structure in the wake region of the spheres was investigated with a Particle Image Velocimetry (PIV) system when the spheres were in a boundary layer over a flat plate as a special case. Reynolds number was 5000 based on the sphere diameter which was 42.5 mm. Boundary layer was tripped 8mm away from the leading edge of the flat plate with a 5 mm trip wire. The thickness of the hydrodynamically developed boundary layer was determined as 63mm which was larger than the sphere diameter of D=42.5mm. Wake region of the spheres was examined from point of flow physics for the different sphere locations in the ranges of 0≤G/D ≤1.5 and 0≤S/D ≤1.5 where G and S were the distance between the spheres and the distance between the bottom point of the spheres and the flat plate surface, respectively. Depending on the different sphere locations, instantaneous and time averaged vorticity data, scalar values of time-averaged velocity components and their root mean square (rms) values and time averaged vorticity data are presented in the study for the evaluation of wake region of the spheres. It is demonstrated that the gap between the two spheres and the interaction between the gap and the boundary layer greatly affects flow pattern, especially when spheres are located near to the flat plate surface, i.e. S/D=0.1 for 0≤G/D ≤1.5. Different distances between the spheres resulted in various flow patterns as the spheres were approached to the flat plate. The distance S/D=0.1 for all gap values has the strongest effect on the wake structures. Beyond G/D=1.0, the sphere wakes tend to be similar to single sphere case. The instantaneous vorticity fields of the side by side arrangements comprised wavy structures in higher level comparing to an individual sphere case. The gap flow intensifies the occurrence of small scale eddies in the wake region. The submersion rate of the spheres actually determines the

  4. [Smith-Magenis syndrome is an association of behavioral and sleep/wake circadian rhythm disorders].

    Science.gov (United States)

    Poisson, A; Nicolas, A; Sanlaville, D; Cochat, P; De Leersnyder, H; Rigard, C; Franco, P; des Portes, V; Edery, P; Demily, C

    2015-06-01

    Smith-Magenis syndrome (SMS) is a genetic disorder characterized by the association of facial dysmorphism, oral speech delay, as well as behavioral and sleep/wake circadian rhythm disorders. Most SMS cases (90%) are due to a 17p11.2 deletion encompassing the RAI1 gene; other cases stem from mutations of the RAI1 gene. Behavioral issues may include frequent outbursts, attention deficit/hyperactivity disorders, self-injuries with onychotillomania and polyembolokoilamania (insertion of objects into bodily orifices), etc. It is noteworthy that the longer the speech delay and the more severe the sleep disorders, the more severe the behavioral issues are. Typical sleep/wake circadian rhythm disorders associate excessive daytime sleepiness with nocturnal agitation. They are related to an inversion of the physiological melatonin secretion cycle. Yet, with an adapted therapeutic strategy, circadian rhythm disorders can radically improve. Usually an association of beta-blockers in the morning (stops daily melatonin secretion) and melatonin in the evening (mimics the evening deficient peak) is used. Once the sleep disorders are controlled, effective treatment of the remaining psychiatric features is needed. Unfortunately, as for many orphan diseases, objective guidelines have not been drawn up. However, efforts should be focused on improving communication skills. In the same vein, attention deficit/hyperactivity disorders, aggressiveness, and anxiety should be identified and specifically treated. This whole appropriate medical management is underpinned by the diagnosis of SMS. Diagnostic strategies include fluorescent in situ hybridization (FISH) or array comparative genomic hybridization (array CGH) when a microdeletion is sought and Sanger sequencing when a point mutation is suspected. Thus, the diagnosis of SMS can be made from a simple blood sample and should be questioned in subjects of any age presenting with an association of facial dysmorphism, speech delay with

  5. Attention Deficit Hyperactivity Disorder

    Science.gov (United States)

    ... If so, your child may have attention deficit hyperactivity disorder (ADHD). Nearly everyone shows some of these ... children. The main features of ADHD are Inattention Hyperactivity Impulsivity No one knows exactly what causes ADHD. ...

  6. Understanding Attention Deficit Disorders.

    Science.gov (United States)

    Villegas, Orlando; And Others

    This booklet provides basic information regarding attention deficit hyperactivity disorders (ADHD), in their separate modalities, with hyperactivity, impulsivity, and inattention. Explanations are offered concerning short attention span, impulsive behavior, hyperactivity, and beginning new activities before completing the previous one. Theories…

  7. Disentangling deficits in adults with attention-deficit/hyperactivity disorder.

    NARCIS (Netherlands)

    Bekker, E.M.; Overtoom, C.C.; Kooij, J.J.; Buitelaar, J.K.; Verbaten, M.N.; Kenemans, J.L.

    2005-01-01

    CONTEXT: A lack of inhibitory control has been suggested to be the core deficit in attention-deficit/hyperactivity disorder (ADHD), especially in adults. This means that a primary deficit in inhibition mediates a cascade of secondary deficits in other executive functions, such as attention. Impaired

  8. CAS course on Plasma Wake Acceleration

    CERN Multimedia

    CERN Accelerator School

    2015-01-01

    The CERN Accelerator School (CAS) recently organised a specialised course on Plasma Wake Acceleration, held at CERN, Geneva, Switzerland, from 23 to 29 November 2014.    Following a number of introductory lectures on laser and plasma physics, as well as an overview of conventional accelerators and their limitations, the course covered a large number of aspects of plasma wake acceleration schemes: the creation of plasma by high power lasers or particle beams, a description of the plasma creation process through simulations and the characteristics of the accelerated particle beams, including results of the latest achievements. Lectures on beam diagnostics, the applications of plasma accelerated beams, and topical seminars completed the programme.  The course was very successful, with 109 students of 26 nationalities attending; most participants coming from European counties, but also from the US, Israel, India, South Korea, Russia and Ukraine. Feedback from the participants was...

  9. Inviscid double wake model for stalled airfoils

    DEFF Research Database (Denmark)

    Marion, Lucas; Ramos García, Néstor; Sørensen, Jens Nørkær

    2014-01-01

    An inviscid double wake model based on a steady two-dimensional panel method has been developed to predict aerodynamic loads of wind turbine airfoils in the deep stall region. The separated flow is modelled using two constant vorticity sheets which are released at the trailing edge and at the sep......An inviscid double wake model based on a steady two-dimensional panel method has been developed to predict aerodynamic loads of wind turbine airfoils in the deep stall region. The separated flow is modelled using two constant vorticity sheets which are released at the trailing edge...... and at the separation point. A calibration of the code through comparison with experiments has been performed using one set of airfoils. A second set of airfoils has been used for the validation of the calibrated model. Predicted aerodynamic forces for a wide range of angles of attack (0 to 90 deg) are in overall good...

  10. Wake Vortex Advisory System (WakeVAS) Evaluation of Impacts on the National Airspace System

    Science.gov (United States)

    Smith, Jeremy C.; Dollyhigh, Samuel M.

    2005-01-01

    This report is one of a series that describes an ongoing effort in high-fidelity modeling/simulation, evaluation and analysis of the benefits and performance metrics of the Wake Vortex Advisory System (WakeVAS) Concept of Operations being developed as part of the Virtual Airspace Modeling and Simulation (VAMS) project. A previous study, determined the overall increases in runway arrival rates that could be achieved at 12 selected airports due to WakeVAS reduced aircraft spacing under Instrument Meteorological Conditions. This study builds on the previous work to evaluate the NAS wide impacts of equipping various numbers of airports with WakeVAS. A queuing network model of the National Airspace System, built by the Logistics Management Institute, Mclean, VA, for NASA (LMINET) was used to estimate the reduction in delay that could be achieved by using WakeVAS under non-visual meteorological conditions for the projected air traffic demand in 2010. The results from LMINET were used to estimate the total annual delay reduction that could be achieved and from this, an estimate of the air carrier variable operating cost saving was made.

  11. A three states sleep-waking model

    Energy Technology Data Exchange (ETDEWEB)

    Comte, J.C. [Laboratoire de Physiopathologie des Reseaux Neuronaux du Cycle Veille-Sommeil, UMR 5167, CNRS/Universite Claude Bernard Lyon1, Faculte de Medecine RTH Laennec 7, Rue Guillaume Paradin 69372 Lyon Cedex 08 (France)]. E-mail: comtejc@gmail.com; Schatzman, M. [MAPLY, Laboratoire de Mathematiques appliquees de Lyon, UMR5585, CNRS/Universite Claude Bernard Lyon1, 21, Avenue Claude Bernard, 69622 Villeurbanne Cedex (France); Ravassard, P. [Laboratoire de Physiopathologie des Reseaux Neuronaux du Cycle Veille-Sommeil, UMR 5167, CNRS/Universite Claude Bernard Lyon1, Faculte de Medecine RTH Laennec 7, Rue Guillaume Paradin 69372 Lyon Cedex 08 (France); Luppi, P.H. [Laboratoire de Physiopathologie des Reseaux Neuronaux du Cycle Veille-Sommeil, UMR 5167, CNRS/Universite Claude Bernard Lyon1, Faculte de Medecine RTH Laennec 7, Rue Guillaume Paradin 69372 Lyon Cedex 08 (France); Salin, P.A. [Laboratoire de Physiopathologie des Reseaux Neuronaux du Cycle Veille-Sommeil, UMR 5167, CNRS/Universite Claude Bernard Lyon1, Faculte de Medecine RTH Laennec 7, Rue Guillaume Paradin 69372 Lyon Cedex 08 (France)

    2006-08-15

    The mechanisms underlying the sleep-states periodicity in animals are a mystery of biology. Recent studies identified a new neuronal population activated during the slow wave sleep (SWS) in the ventral lateral preoptic area of the hypothalamus. Interactions between this neuronal population and the others populations implicated in the vigilance states (paradoxical sleep (PS) and wake (W)) dynamics are not determined. Thus, we propose here a sleep-waking theoretical model that depicts the potential interactions between the neuronal populations responsible for the three vigilance states. First, we pooled data from previous papers regarding the neuronal populations firing rate time course and characterized statistically the experimental hypnograms. Then, we constructed a nonlinear differential equations system describing the neuronal populations activity time course. A simple rule playing the firing threshold role applied to the model allows to construct a theoretical hypnogram. A random modulation of the neuronal activity, shows that theoretical hypnograms present a dynamics close to the experimental observations. Furthermore, we show that the wake promoting neurons activity can predict the next SWS episode duration.

  12. Wake fields in SLAC Linac Collimators

    Energy Technology Data Exchange (ETDEWEB)

    Novokhatski, Alexander [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Decker, F. -J. [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Smith, H. [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States); Sullivan, M. [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)

    2014-12-02

    When a beam travels near collimator jaws, it gets an energy loss and a transverse kick due to the backreaction of the beam field diffracted from the jaws. The effect becomes very important for an intense short bunch when a tight collimation of the background beam halo is required. In the Linac Coherent Light Source at SLAC a collimation system is used to protect the undulators from radiation due to particles in the beam halo. The halo is most likely formed from gun dark current or dark current in some of the accelerating sections. However, collimators are also responsible for the generation of wake fields. The wake field effect from the collimators not only brings an additional energy jitter and change in the trajectory of the beam, but it also rotates the beam on the phase plane, which consequently leads to a degradation of the performance of the Free Electron Laser at the Linac Coherent Light Source. In this paper, we describe a model of the wake field radiation in the SLAC linac collimators. We use the results of a numerical simulation to illustrate the model. Based on the model, we derive simple formulas for the bunch energy loss and the average kick. In addition, we also present results from experimental measurements that confirm our model.

  13. Estimation of particle concentration profiles in a three-phase fluidized bed from experimental data and using the wake model

    Directory of Open Access Journals (Sweden)

    Knesebeck A.

    2004-01-01

    Full Text Available Particles with a size distribution in the range of 34 to 468 µm were fluidized in a three-phase bed using low liquid and gas velocities. Particle size distribution and pressure profile measurements were carried out at different locations in the bed in order to study the influence of fluid velocities on segregation and dispersion of particles in different size classes. The influence of gas velocity on particle mixing was analyzed in terms of internal solid fluxes, calculated by means of the wake model. Based on the experimental results, different particle distribution patterns were identified. Although no significant tendencies were observed for radial profiles, particles of different sizes have significantly different axial profiles, which are mainly affected by the velocity of the liquid phase. Thus, depending on the liquid velocity, smaller particles reach a maximum concentration at different bed heights.

  14. Measurements in the wake of a ventilated hydrofoil: a step towards improved turbine aeration techniques

    Science.gov (United States)

    Ellis, C.; Karn, A.; Hong, J.; Lee, S.-J.; Kawakami, E.; Scott, D.; Gulliver, J.; Arndt, R. E. A.

    2014-03-01

    The purpose of this study is to develop the necessary algorithms to determine the bubble size distribution and velocity in the wake of a ventilated or cavitating hydrofoil utilizing background illumination. A simplified experiment was carried out to validate the automatic bubble detection algorithm at the Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota. The experiment was conducted in the SAFL high-speed water tunnel. First, particle shadow velocimetry (PSV) images of a bubbly flow were collected. Bubbles were identified in the images using an edge detection method based on the Canny algorithm. The utilized algorithm was designed to detect partly overlapping bubbles and reconstruct missing parts. After all images were analyzed, the bubble velocity was determined by applying a tracking algorithm. This study has shown that the algorithm enables reliable analysis of irregularly shaped bubbles even when bubbles are highly overlapped in the wake of the ventilated hydrofoil. It is expected that this technique can be used to determine the bubble velocity field as well as the bubble size distributions.

  15. Experimental study on the near wake behind two side-by-side cylinders of unequal diameters

    Energy Technology Data Exchange (ETDEWEB)

    Gao Yangyang; Yu Dingyong [College of Engineering, Ocean University of China, Qingdao 266100 (China); Tan, Soonkeat [School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798 (Singapore); Wang Xikun [Maritime Research Center, Nanyang Technological University, Singapore 639798 (Singapore); Hao Zhiyong, E-mail: gaoyangyangouc@163.co [Shanghai Maritime University, Shanghai 200135 (China)

    2010-10-15

    The wake structure behind two side-by-side circular cylinders with unequal diameter was investigated using the particle image velocimetry (PIV) technique. This investigation focused on the asymmetrical flow within the subcritical Reynolds number regime. A comparison between the time-averaged mean flow field of one cylinder and that of two side-by-side cylinders of unequal diameter was made for elucidating the mean flow characteristics attributable to the latter. The distribution of mean velocity behind two side-by-side unequal cylinders was distinctly different from that behind one cylinder, and an asymmetric combination was observed behind the two cylinders. The present paper also presents the authors' interpretation of the generation mechanism of gap flow deflection based on the instantaneous vorticity contours and velocity distribution. The effect of Reynolds number on flow structure was studied by analyzing the mean velocity distribution and time-averaged wake structures. The results showed that vortex formation length decreases with Reynolds number.

  16. Wake effect and stopping power for a charged ion moving in magnetized two-component plasmas: two-dimensional particle-in-cell simulation.

    Science.gov (United States)

    Hu, Zhang-Hu; Song, Yuan-Hong; Wang, You-Nian

    2010-08-01

    A two-dimensional particle-in-cell (PIC) model is proposed to study the wake field and stopping power induced by a nonrelativistic charged particle moving perpendicular to the external magnetic field in two-component plasmas. The effects of the magnetic field on the wake potential and the stopping due to the polarization of both the plasma ions and electrons are discussed. The velocity fields of plasma ions and electrons are investigated, respectively, in the weak and strong magnetic field cases. Our simulation results show that in the case of weak magnetic field and high ion velocity, the wakes exhibit typical V-shaped cone structures and the opening cone angles decrease with the increasing ion velocity. As the magnetic field becomes strong, the wakes lose their typical V-shaped structures and become highly asymmetrical. Similar results can be obtained in the case of low ion velocity and strong magnetic field. In addition, stopping power is calculated and compared with previous one-dimensional and full three-dimensional PIC results.

  17. Experimental and Numerical study of Wake to Wake Interaction in Wind Farms

    DEFF Research Database (Denmark)

    Machefaux, Ewan; Troldborg, Niels; Larsen, Gunner Chr.;

    2012-01-01

    In this paper, wake interaction between two wind turbines is analyzed using experimental and numerical approaches. Full-scale wake measurements are conducted at Tjæreborg wind farm and are obtained using a continuous wave lidar mounted on the back of the nacelle of a 2MW NM80 turbine. Numerical...... are compared, and good agreement is seen between the measurements and the computations despite of a lateral offset and other discrepancies due to uncertainties on the measured inflow conditions and lidar mounting alignment....

  18. LES/FDF simulation of particle dispersion in a gas-particle two phase plane wake flow

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    A filtered density function (FDF) transport equation was derived for the fluid velocity seen by the particles in gas-particle two-phase flow. An LES/FDF simulation of a two-phase plane wake flow was carried out. The simulation results were compared with both the experimental photograph and the simulation results without using the FDF model, and proved that the LES/FDF model can clearly improve the spatial dispersion of the particle phase.

  19. Impact of Wake Dispersion on Axial Compressor Performance

    Science.gov (United States)

    Hah, Chunill

    2017-01-01

    Detailed development of wakes and their impact on the performance of a low-speed one and half stage axial compressor are investigated with a large eddy simulation (LES). To investigate effects of wake mixing recovery and wake interaction with the boundary layer of the downstream blade, spacing between the rotor blade and the stator is varied. The calculated LES flow fields based on a fine computational grid are compared with related measurements and analyzed in detail at several radial locations. The current LES calculates the effects of wake recovery very well. The effects of wake recovery vary significantly in the radial direction. Loss generation is higher on the pressure side at the stator exit at both near design and near stall condition. The current investigation indicates that better management of wake development can be achieved for improved compressor performance.

  20. Detailed field test of yaw-based wake steering

    Science.gov (United States)

    Fleming, P.; Churchfield, M.; Scholbrock, A.; Clifton, A.; Schreck, S.; Johnson, K.; Wright, A.; Gebraad, P.; Annoni, J.; Naughton, B.; Berg, J.; Herges, T.; White, J.; Mikkelsen, T.; Sjöholm, M.; Angelou, N.

    2016-09-01

    This paper describes a detailed field-test campaign to investigate yaw-based wake steering. In yaw-based wake steering, an upstream turbine intentionally misaligns its yaw with respect to the inflow to deflect its wake away from a downstream turbine, with the goal of increasing total power production. In the first phase, a nacelle-mounted scanning lidar was used to verify wake deflection of a misaligned turbine and calibrate wake deflection models. In the second phase, these models were used within a yaw controller to achieve a desired wake deflection. This paper details the experimental design and setup. All data collected as part of this field experiment will be archived and made available to the public via the U.S. Department of Energy's Atmosphere to Electrons Data Archive and Portal.

  1. On helical behavior of turbulence in the ship wake

    Institute of Scientific and Technical Information of China (English)

    GOLBRAIKH Ephim; EIDELMAN Alexander; SOLOVIEV Alex

    2013-01-01

    Turbulent ship wake conservation at a long distance is among unsolved problems at present.It is well known that far wakes have a vortical structure and slowly expand with distance.As was obtained by Dubrovin et al.,slow expansion of the wake may be related to the distribution of turbulent viscosity in it.In our work we study the effect of helicity in the wake on the behavior of turbulent viscosity.Taking into account the helical nature of the wake,we can clarify the difference between turbulence inside and outside the wake on the one hand and slowing down of its expansion with time on the other hand.

  2. Stabilization of vortices in the wake of a circular cylinder using harmonic forcing

    DEFF Research Database (Denmark)

    Chamoun, George Chaouki; Schilder, Frank; Brøns, Morten

    2011-01-01

    We explore whether vortex flows in the wake of a fixed circular cylinder can be stabilized using harmonic forcing. We use Fo¨ppl's point vortex model augmented with a harmonic point source-sink mechanism which preserves conservation of mass and leaves the system Hamiltonian. We discover a region...... of Lyapunov-stable vortex motion for an appropriate selection of parameters. We identify four unique parameters that affect the stability of the vortices: the uniform flow velocity, vortex equilibrium positions, forcing amplitude, and forcing frequency. We assess the robustness of the controller using...

  3. High-velocity clouds

    NARCIS (Netherlands)

    Wakker, BP; vanWoerden, H

    1997-01-01

    High-velocity clouds (HVCs) consist of neutral hydrogen (HI) at velocities incompatible with a simple model of differential galactic rotation; in practice one uses \\v(LSR)\\ greater than or equal to 90 km/s to define HVCs. This review describes the main features of the sky and velocity distributions,

  4. Transverse Spectral Velocity Estimation

    DEFF Research Database (Denmark)

    Jensen, Jørgen Arendt

    2014-01-01

    A transverse oscillation (TO)-based method for calculating the velocity spectrum for fully transverse flow is described. Current methods yield the mean velocity at one position, whereas the new method reveals the transverse velocity spectrum as a function of time at one spatial location. A convex...

  5. Evaluation of Fast-Time Wake Vortex Prediction Models

    Science.gov (United States)

    Proctor, Fred H.; Hamilton, David W.

    2009-01-01

    Current fast-time wake models are reviewed and three basic types are defined. Predictions from several of the fast-time models are compared. Previous statistical evaluations of the APA-Sarpkaya and D2P fast-time models are discussed. Root Mean Square errors between fast-time model predictions and Lidar wake measurements are examined for a 24 hr period at Denver International Airport. Shortcomings in current methodology for evaluating wake errors are also discussed.

  6. The decay of wake vortices in the convective boundary layer

    Energy Technology Data Exchange (ETDEWEB)

    Holzaepfel, F.; Gerz, T.; Frech, M.; Doernbrack, A.

    2000-03-01

    The decay of three wake vortex pairs of B-747 aircraft in a convectively driven atmospheric boundary layer is investigated by means of large-eddy simulations (LES). This situation is considered as being hazardous as the updraft velocities of a thermal may compensate the induced descent speed of the vortex pair resulting in vortices stalled in the flight path. The LES results, however, illustrate that (i) the primary rectilinear vortices are rapidly deformed on the scale of the alternating updraft and downdraft regions; (ii) parts of the vortices stay on flight level but are quickly eroded by the enhanced turbulence of an updraft; (iii) longest living sections of the vortices are found in regions of relatively calm downdraft flow which augments their descent. Strip theory calculations are used to illustrate the temporal and spatial development of lift and rolling moments experienced by a following medium weight class B-737 aircraft. Characteristics of the respective distributions are analysed. Initially, the maximum rolling moments slightly exceed the available roll control of the B-737. After 60 seconds the probability of rolling moments exceeding 50% of the roll control, a value which is considered as a threshold for acceptable rolling moments, has decreased to 1% of its initial probability. (orig.)

  7. Observation of supersonic turbulent wakes by laser Fourier densitometry (LFD)

    Science.gov (United States)

    Gresillon, D.; Cabrit, B.; Bonnet, J. P.; Gemaux, G.

    Laser Fourier Densitometry (LFD) is an optical method appropriate for turbulent flow observations. It uses the collective scattering of coherent light, by optical index inhomogeneities. The principle of this method is described. It provides a signal proportional to the space Fourier transform amplitude of index distribution for a wavevector k defined by the optical arrangement. For a fluctuating flow, this amplitude is a function of time, and its frequency spectrum can be observed. The spectrum shape provides elementary parameters of the flow, such as: direction, modulus of mean velocity, and local temperature. It also provides means to distinguish different kinds of density fluctuations, such as convected inhomogeneities, or acoustic waves. The respective level of these different fluctuations types can be measured, as well as their power scale-law and absolute level. A compact optical bench has been set on a nozzle flow. The results of measurements performed in two supersonic wake configurations are presented, for Mach numbers of 1.6 and 4.2. These include density fluctuation spectra in supersonic flows, acoustic waves, variations with position, and comparison with hot wire anemometry.

  8. Investigation on 3Dt wake flow structures of swimming bionic fish

    Institute of Scientific and Technical Information of China (English)

    G.-X.Shen; G.-K.Tan; G.-J.Lai

    2012-01-01

    A bionic experimental platform was designed for the purpose of investigating time accurate three-dimensional flow field,using digital particle image velocimetry (DSPIV).The wake behind the flapping trail of a robotic fish model was studied at high spatial resolution.The study was performed in a water channel.A robot fish model was designed and built.The model was fixed onto a rigid support framework using a cable-supporting method,with twelve stretched wires.The entire tail of the model can perform prescribed motions in two degrees of freedom,mainly in carangiform mode,by driving its afterbody and lunate caudal fin respectively The DSPIV system was set up to operate in a translational manner,measuring velocity field in a series of parallel slices.Phase locked measurements were repeated for a number of runs,allowing reconstruction of phase average flow field.Vortex structures with phase history of the wake were obtained.The study reveals some new and complex three-dimensional flow structures in the wake of the fish,including "reverse hairpin vortex" and "reverse Karman S-H vortex rings",allowing insight into physics of this complex flow.

  9. The Transition from Thick to Thin Plate Wake Physics: Whither Vortex Shedding?

    Science.gov (United States)

    Rai, Man Mohan

    2016-01-01

    The near and very near wake of a flat plate with a circular trailing edge is investigated with data from direct numerical simulations. Computations were performed for six different combinations of the Reynolds numbers based on plate thickness (D) and boundary layer momentum thickness upstream of the trailing edge (theta). Unlike the case of the cylinder, these Reynolds numbers are independent parameters for the flat plate. The separating boundary layers are turbulent in all the cases investigated. One objective of the study is to understand the changes in the wake vortex shedding process as the plate thickness is reduced (increasing theta/D). The value of D varies by a factor of 16 and that of theta by approximately 5 in the computations. Vortex shedding is vigorous in the low theta/D cases with a substantial decrease in shedding intensity in the large theta/D cases. Other shedding characteristics are also significantly altered with increasing theta/D. A visualization of the shedding process in the different cases is provided and discussed. The basic shedding mechanism is explored in depth. The effect of changing theta/D on the time-averaged, near-wake velocity statistics is also discussed. A functional relationship between the shedding frequency and the Reynolds numbers mentioned above is obtained.

  10. Changes in Flat Plate Wake Characteristics Obtained With Decreasing Plate Thickness

    Science.gov (United States)

    Rai, Man Mohan

    2016-01-01

    The near and very near wake of a flat plate with a circular trailing edge is investigated with data from direct numerical simulations. Computations were performed for four different Reynolds numbers based on plate thickness (D) and at constant plate length. The value of ?/D varies by a factor of approximately 20 in the computations (? being the boundary layer momentum thickness at the trailing edge). The separating boundary layers are turbulent in all the cases. One objective of the study is to understand the changes in wake characteristics as the plate thickness is reduced (increasing ?/D). Vortex shedding is vigorous in the low ?/D cases with a substantial decrease in shedding intensity in the largest ?/D case (for all practical purposes shedding becomes almost intermittent). Other characteristics that are significantly altered with increasing ?/D are the roll-up of the detached shear layers and the magnitude of fluctuations in shedding period. These effects are explored in depth. The effects of changing ?/D on the distributions of the time-averaged, near-wake velocity statistics are discussed.

  11. Investigation on 3D t wake flow structures of swimming bionic fish

    Science.gov (United States)

    Shen, G.-X.; Tan, G.-K.; Lai, G.-J.

    2012-10-01

    A bionic experimental platform was designed for the purpose of investigating time accurate three-dimensional flow field, using digital particle image velocimetry (DSPIV). The wake behind the flapping trail of a robotic fish model was studied at high spatial resolution. The study was performed in a water channel. A robot fish model was designed and built. The model was fixed onto a rigid support framework using a cable-supporting method, with twelve stretched wires. The entire tail of the model can perform prescribed motions in two degrees of freedom, mainly in carangiform mode, by driving its afterbody and lunate caudal fin respectively. The DSPIV system was set up to operate in a translational manner, measuring velocity field in a series of parallel slices. Phase locked measurements were repeated for a number of runs, allowing reconstruction of phase average flow field. Vortex structures with phase history of the wake were obtained. The study reveals some new and complex three-dimensional flow structures in the wake of the fish, including "reverse hairpin vortex" and "reverse Karman S-H vortex rings", allowing insight into physics of this complex flow.

  12. Exquisitely sensitive seal whisker-like sensors detect wakes at large distances

    CERN Document Server

    Beem, Heather R

    2015-01-01

    Blindfolded harbor seals are able to use their uniquely shaped whiskers to track vortex wakes left by moving animals and objects that passed by up to 30 seconds earlier; this is an impressive feat as the flow features they detect may have velocity as low as 1 mm/s, and the seals have some capacity to identify the shape of the object as well. They do so while swimming forward at high speed, hence their whiskers are sensitive enough to detect small-scale changes in the external flow field, while rejecting self-generated flow noise. Here we identify and illustrate a novel flow mechanism that allows artificial whiskers with the identical unique geometry as those of the harbor seal to detect the features of minute flow fluctuations in wakes produced by objects far away. This is shown through the study of a model problem, consisting of a harbor seal whisker model interacting with the wake of an upstream circular cylinder. We show that whereas in open water the whisker geometry results in very low vibration, once it...

  13. Kinetic Electron and Ion Instability of the Lunar Wake Simulated at Physical Mass Ratio

    CERN Document Server

    Haakonsen, Christian Bernt; Zhou, Chuteng

    2015-01-01

    The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion streams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly l...

  14. Effects of moving-vehicle wakes on pollutant dispersion inside a highway road tunnel.

    Science.gov (United States)

    Bhautmage, Utkarsh; Gokhale, Sharad

    2016-11-01

    This study investigates the pollutant dispersion in a highway road tunnel in the presence of moving-vehicle wakes by a relative-velocity approach using 3-D CFD (3-Dimensional Computational Fluid Dynamics). The turbulent behavior of airflow around different-shaped vehicles and its impact on the pollutant dispersion have been studied. The different-shaped vehicle geometries were extracted, and simplified and dimensioned basing the typical vehicles on Indian roads. The model has been verified with the literature data of static pressure around a moving vehicle body before applying to simulate concentrations, and validated with on-site data at two locations. The results showed that wakes varied with the size, shape and speed of vehicles. The mixed-traffic flow produced higher near-field wakes and accelerated the piston effect, pushing pollutants toward the tunnel roof and out of exit portal in short-time. The findings have particular significance in the studies related to dispersion inside the tunnels having a mixed traffic of different dimensions and shape.

  15. Turbulence modelling of the aerodynamic interaction ofOGV wakes and diffuser flow

    Institute of Scientific and Technical Information of China (English)

    LI Jing-hua; Page Gary; McGuirk Jim

    2011-01-01

    Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tested based on the classical Reynolds-averaged Navier-Stokes(RANS) approach.Both high-Re and low-Re(Launder-Sharma) versions of the k-ε model were applied to a selected test problem for OGV wake/diffuser flows.The test problem was specifically chosen because experimentally determined inlet conditions and both profile and performance data were available to validate predictions.A preliminary study was also reported of the more advanced large eddy simulation(LES) approach.The LES sub-grid-scale(SGS) model was the basic Smagorinsky eddy viscosity assumption,with a Van-Driest damping function for improved capture of near-wall viscous behaviour.Comparison between the two RANS models showed little difference in terms of velocity contours at OGV trailing edge and diffuser exit.In terms of overall diffuser performance(static pressure recovery and total pressure loss coefficients),the high-Re model was shown to agree well with experimental data.The preliminary LES study indicates the highly unsteady character of the OGV wake flow,but requires improved treatment of inlet conditions.

  16. High-frequency oscillations in human and monkey neocortex during the wake-sleep cycle.

    Science.gov (United States)

    Le Van Quyen, Michel; Muller, Lyle E; Telenczuk, Bartosz; Halgren, Eric; Cash, Sydney; Hatsopoulos, Nicholas G; Dehghani, Nima; Destexhe, Alain

    2016-08-16

    Beta (β)- and gamma (γ)-oscillations are present in different cortical areas and are thought to be inhibition-driven, but it is not known if these properties also apply to γ-oscillations in humans. Here, we analyze such oscillations in high-density microelectrode array recordings in human and monkey during the wake-sleep cycle. In these recordings, units were classified as excitatory and inhibitory cells. We find that γ-oscillations in human and β-oscillations in monkey are characterized by a strong implication of inhibitory neurons, both in terms of their firing rate and their phasic firing with the oscillation cycle. The β- and γ-waves systematically propagate across the array, with similar velocities, during both wake and sleep. However, only in slow-wave sleep (SWS) β- and γ-oscillations are associated with highly coherent and functional interactions across several millimeters of the neocortex. This interaction is specifically pronounced between inhibitory cells. These results suggest that inhibitory cells are dominantly involved in the genesis of β- and γ-oscillations, as well as in the organization of their large-scale coherence in the awake and sleeping brain. The highest oscillation coherence found during SWS suggests that fast oscillations implement a highly coherent reactivation of wake patterns that may support memory consolidation during SWS.

  17. Vortex wakes of a flapping foil in a flowing soap film

    DEFF Research Database (Denmark)

    We present an experimental study of an oscillating, symmetric foil in a vertically flowing soap film. By varying frequency and amplitude of the oscillation we explore and visualize a variety of wake structures, including von K´arm´an wake, reverse von K´arm´an wake, 2P wake, and 2P+2S wake. We...

  18. Wake shape and its effects on aerodynamic characteristics

    Science.gov (United States)

    Emdad, H.; Lan, C. E.

    1986-01-01

    The wake shape under symmetrical flight conditions and its effects on aerodynamic characteristics are examined. In addition, the effect of wake shape in sideslip and discrete vortices such as strake or forebody vortex on lateral characteristics is presented. The present numerical method for airplane configurations, which is based on discretization of the vortex sheet into vortex segments, verified the symmetrical and asymmetrical roll-up process of the trailing vortices. Also, the effect of wing wake on tail planes is calculated. It is concluded that at high lift the assumption of flat wake for longitudinal and lateral-directional characteristics should be reexamined.

  19. Interaction of a swept shock wave and a supersonic wake

    Science.gov (United States)

    He, G.; Zhao, Y. X.; Zhou, J.

    2017-03-01

    The interaction of a swept shock wave and a supersonic wake has been studied. The swept shock wave is generated by a swept compression sidewall, and the supersonic wake is generated by a wake generator. The flow field is visualized with the nanoparticle-based planar laser scattering method, and a supplementary numerical simulation is conducted by solving the Reynolds-averaged Navier-Stokes equations. The results show that the pressure rise induced by the swept shock wave can propagate upstream in the wake, which makes the location where vortices are generated move upstream, thickens the laminar section of the wake, and enlarges the generated vortices. The wake is swept away from the swept compression sidewall by the pressure gradient of the swept shock wave. This pressure gradient is not aligned with the density gradient of the supersonic wake, so the baroclinic torque generates streamwise vorticity and changes the distribution of the spanwise vorticity. The wake shock is curved, so the flow downstream of it is non-uniform, leaving the swept shock wave being distorted. A three-dimensional Mach disk structure is generated when the wake shock interacts with the swept shock wave.

  20. Interaction of a swept shock wave and a supersonic wake

    Science.gov (United States)

    He, G.; Zhao, Y. X.; Zhou, J.

    2017-09-01

    The interaction of a swept shock wave and a supersonic wake has been studied. The swept shock wave is generated by a swept compression sidewall, and the supersonic wake is generated by a wake generator. The flow field is visualized with the nanoparticle-based planar laser scattering method, and a supplementary numerical simulation is conducted by solving the Reynolds-averaged Navier-Stokes equations. The results show that the pressure rise induced by the swept shock wave can propagate upstream in the wake, which makes the location where vortices are generated move upstream, thickens the laminar section of the wake, and enlarges the generated vortices. The wake is swept away from the swept compression sidewall by the pressure gradient of the swept shock wave. This pressure gradient is not aligned with the density gradient of the supersonic wake, so the baroclinic torque generates streamwise vorticity and changes the distribution of the spanwise vorticity. The wake shock is curved, so the flow downstream of it is non-uniform, leaving the swept shock wave being distorted. A three-dimensional Mach disk structure is generated when the wake shock interacts with the swept shock wave.

  1. Contributions of the Stochastic Shape Wake Model to Predictions of Aerodynamic Loads and Power under Single Wake Conditions

    Science.gov (United States)

    Doubrawa, P.; Barthelmie, R. J.; Wang, H.; Churchfield, M. J.

    2016-09-01

    The contribution of wake meandering and shape asymmetry to load and power estimates is quantified by comparing aeroelastic simulations initialized with different inflow conditions: an axisymmetric base wake, an unsteady stochastic shape wake, and a large-eddy simulation with rotating actuator-line turbine representation. Time series of blade-root and tower base bending moments are analyzed. We find that meandering has a large contribution to the fluctuation of the loads. Moreover, considering the wake edge intermittence via the stochastic shape model improves the simulation of load and power fluctuations and of the fatigue damage equivalent loads. These results indicate that the stochastic shape wake simulator is a valuable addition to simplified wake models when seeking to obtain higher-fidelity computationally inexpensive predictions of loads and power.

  2. Power deficits due to wind turbine wakes at Horns Rev wind farm

    DEFF Research Database (Denmark)

    Hansen, Kurt Schaldemose; Barthelmie, Rebecca; Jensen, Leo E.

    2010-01-01

    The wind turbine operational charac-teristics, power measurements and the meteorological measurements as 10 minute statistical data from Horns Rev offshore wind farm have been identified, synchronized, quality screened and stored in a common database. A number of flow cases have been identified...

  3. Attention Demand and Postural Control in Children with Hearing Deficit

    Science.gov (United States)

    Derlich, Malgorzata; Krecisz, Krzysztof; Kuczynski, Michal

    2011-01-01

    To elucidate the mechanisms responsible for deteriorated postural control in children with hearing deficit (CwHD), we measured center-of-pressure (COP) variability, mean velocity and entropy in bipedal quiet stance (feet together) with or without the concurrent cognitive task (reaction to visual stimulus) on hard or foam surface in 29 CwHD and a…

  4. The flow of a power-law fluid in the near-wake of a flat plate

    Science.gov (United States)

    Zhou, Min; Ladeinde, Foluso; Bluestein, Danny

    2006-08-01

    The analysis of the near-wake flow downstream of a flat plate is reported in this paper for the case of a non-Newtonian (power-law) constitutive model. To our knowledge, the present paper is the first to address this problem, as previous work on near-wakes has been limited to the use of a Newtonian model. The motivation for this work comes from the biomedical engineering problem of blood flow around the bileaflet of a mechanical heart valve. In the present paper, the series method has been used to calculate the flow near the centerline of the wake, while an asymptotic method has been used for larger distances from the centerline. The effects of power-law inlet conditions on the wake flow are reported for various values of the power-law index n, within the range 0.7≤n ≤1.3. The present analysis has been successfully validated by comparing the results for n =1 to the near-wake results by Goldstein [Proc. Cambridge Philos. Soc. 26, 1 (1930)]. We generalized the equations for arbitrary values of n, without any special considerations for n =1. Therefore, the accurate results observed for n =1 validate our procedure as a whole. The first major finding is that a fluid with smaller n develops faster downstream, such that decreasing n leads to monotonically increasing velocities compared to fluids with large n values. Another finding is that the non-Newtonian effects become more significant as the downstream distance increases. Finally, these effects tend to be more pronounced in the vicinity of the wake centerline compared to larger y locations.

  5. Wake effects and wind farm modelling

    Energy Technology Data Exchange (ETDEWEB)

    Crespo, A.; Gomez-Elvira, R. [Univ. Politecnica de Madrid, Dept. of Energy and Fluid Mechanic (Spain)

    1999-07-01

    A model has been developed to treat simultaneously the changes in wind characteristics due to the transition from land to sea, and the wake effects in offshore wind farms. There are beneficial effects both in an increase of power production and in a reduction of turbulence loads when the distance from the farm to the land coast is increased, however, particularly for the turbulence loads, this effect reaches a limit beyond a certain distance of the order of 1 to 2 km. A method has been proposed to estimate the performance of offshore wind farms, that has been applied to places of interest to the project. As it was observed in the project East Coast of the UK, wake effects are important and significant increases can be obtained in power production by increasing the distance between machines and a better disposition of them, i.e. alternating positions in contiguous rows normal to the prevailing wind direction. The most efficient wind farms from the point of view of wake interference are Gedser and Omo with an efficiency of 89% and 86% respectively. A large wind farm like Rodsand has lower efficiency because of the cumulative effect of the wakes of many turbines. The calculated value of the capacity factor of the three Danish wind farms is larger than 36%. The value of the added turbulence intensity, averaged over rows normal to the incident wind, increases suddenly in the first rows and then reaches an almost constant value in the downstream rows, that is of the order of 10%. A method to calculate the performance of very large wind farms, such that they can change the planetary boundary layer is presented. This method has been applied to the wind farms of interest in the project, and small reductions of the order of 5% or less in power production may occur. However, for very large wind farms (with a of the order of 100 km), that may perturb the whole planetary boundary layer, much more drastic reductions, of the order of 50%, may appear, particularly in the

  6. Plasma and fields in the wake of Rhea: 3-D hybrid simulation and comparison with Cassini data

    Directory of Open Access Journals (Sweden)

    E. Roussos

    2008-03-01

    Full Text Available Rhea's magnetospheric interaction is simulated using a three-dimensional, hybrid plasma simulation code, where ions are treated as particles and electrons as a massless, charge-neutralizing fluid. In consistency with Cassini observations, Rhea is modeled as a plasma absorbing obstacle. This leads to the formation of a plasma wake (cavity behind the moon. We find that this cavity expands with the ion sound speed along the magnetic field lines, resulting in an extended depletion region north and south of the moon, just a few Rhea radii (RRh downstream. This is a direct consequence of the comparable thermal and bulk plasma velocities at Rhea. Perpendicular to the magnetic field lines the wake's extension is constrained by the magnetic field. A magnetic field compression in the wake and the rarefaction in the wake sides is also observed in our results. This configuration reproduces well the signature in the Cassini magnetometer data, acquired during the close flyby to Rhea on November 2005. Almost all plasma and field parameters show an asymmetric distribution along the plane where the corotational electric field is contained. A diamagnetic current system is found running parallel to the wake boundaries. The presence of this current system shows a direct corelation with the magnetic field configuration downstream of Rhea, while the resulting j×B forces on the ions are responsible for the asymmetric structures seen in the velocity and electric field vector fields in the equatorial plane. As Rhea is one of the many plasma absorbing moons of Saturn, we expect that this case study should be relevant for most lunar-type interactions at Saturn.

  7. Potential load reductions on megawatt turbines exposed to wakes using individual-pitch wake compensator and trailing-edge flaps

    DEFF Research Database (Denmark)

    Markou, Helen; Andersen, Peter Bjørn; Larsen, Gunner Chr.

    2011-01-01

    for the loading conditions of the individual turbines in the farm. The dynamic wake meandering model (DWM) is believed to capture the essential physics of the wake problem, and thus, both load and production aspects can be predicted, which is contrary to the traditional engineering wake prediction methods...... that typically focus on either load or power prediction. As a consequence, the wake affected inflow field generated by the DWM formulation opens for control strategies for the individual turbine. Two different control approaches for load reduction on the individual turbines are implemented in the multi-body aero...

  8. When wings touch wakes: understanding locomotor force control by wake wing interference in insect wings.

    Science.gov (United States)

    Lehmann, Fritz-Olaf

    2008-01-01

    Understanding the fluid dynamics of force control in flying insects requires the exploration of how oscillating wings interact with the surrounding fluid. The production of vorticity and the shedding of vortical structures within the stroke cycle thus depend on two factors: the temporal structure of the flow induced by the wing's own instantaneous motion and the flow components resulting from both the force production in previous wing strokes and the motion of other wings flapping in close proximity. These wake-wing interactions may change on a stroke-by-stroke basis, confronting the neuro-muscular system of the animal with a complex problem for force control. In a single oscillating wing, the flow induced by the preceding half stroke may lower the wing's effective angle of attack but permits the recycling of kinetic energy from the wake via the wake capture mechanism. In two-winged insects, the acceleration fields produced by each wing may strongly interact via the clap-and-fling mechanism during the dorsal stroke reversal. Four-winged insects must cope with the fact that the flow over their hindwings is affected by the presence of the forewings. In these animals, a phase-shift between the stroke cycles of fore- and hindwing modulates aerodynamic performance of the hindwing via leading edge vortex destruction and changes in local flow condition including wake capture. Moreover, robotic wings demonstrate that phase-lag during peak performance and the strength of force modulation depend on the vertical spacing between the two stroke planes and the size ratio between fore- and hindwing. This study broadly summarizes the most prominent mechanisms of wake-wing and wing-wing interactions found in flapping insect wings and evaluates the consequences of these processes for the control of locomotor forces in the behaving animal.

  9. CFD Wake Modelling with a BEM Wind Turbine Sub-Model

    Directory of Open Access Journals (Sweden)

    Anders Hallanger

    2013-01-01

    Full Text Available Modelling of wind farms using computational fluid dynamics (CFD resolving the flow field around each wind turbine's blades on a moving computational grid is still too costly and time consuming in terms of computational capacity and effort. One strategy is to use sub-models for the wind turbines, and sub-grid models for turbulence production and dissipation to model the turbulent viscosity accurately enough to handle interaction of wakes in wind farms. A wind turbine sub-model, based on the Blade Momentum Theory, see Hansen (2008, has been implemented in an in-house CFD code, see Hallanger et al. (2002. The tangential and normal reaction forces from the wind turbine blades are distributed on the control volumes (CVs at the wind turbine rotor location as sources in the conservation equations of momentum. The classical k-epsilon turbulence model of Launder and Spalding (1972 is implemented with sub-grid turbulence (SGT model, see Sha and Launder (1979 and Sand and Salvesen (1994. Steady state CFD simulations were compared with flow and turbulence measurements in the wake of a model scale wind turbine, see Krogstad and Eriksen (2011. The simulated results compared best with experiments when stalling (boundary layer separation on the wind turbine blades did not occur. The SGT model did improve turbulence level in the wake but seems to smear the wake flow structure. It should be noted that the simulations are carried out steady state not including flow oscillations caused by vortex shedding from tower and blades as they were in the experiments. Further improvement of the simulated velocity defect and turbulence level seems to rely on better parameter estimation to the SGT model, improvements to the SGT model, and possibly transient- instead of steady state simulations.

  10. Chaotic dynamics of large-scale structures in a turbulent wake

    Science.gov (United States)

    Varon, Eliott; Eulalie, Yoann; Edwige, Stephie; Gilotte, Philippe; Aider, Jean-Luc

    2017-03-01

    The dynamics of a three-dimensional (3D) bimodal turbulent wake downstream of a square-back Ahmed body are experimentally studied in a wind tunnel through high-frequency wall-pressure probes mapping the rear of the model and a horizontal two-dimensional (2D) velocity field. The barycenters of the pressure distribution over the rear part of the model and the intensity recirculation are found highly correlated. Both described the most energetic large-scale structures dynamics, confirming the relation between the large-scale recirculation bubble and its wall-pressure footprint. Focusing on the pressure, its barycenter trajectory has a stochastic behavior but its low-frequency dynamics exhibit the same characteristics as a weak strange chaotic attractor system, with two well-defined attractors. The low-frequency dynamics associated to the large-scale structures are then analyzed. The largest Lyapunov exponent is first estimated, leading to a low positive value characteristic of strange attractors and weak chaotic systems. Afterwards, analyzing the autocorrelation function of the timeseries, we compute the correlation dimension, larger than two. The signal is finally transformed and analyzed as a telegraph signal, showing that its dynamics correspond to a quasirandom telegraph signal. This is the first demonstration that the low-frequency dynamics of a turbulent 3D wake are not a purely stochastic process but rather a weak chaotic process exhibiting strange attractors. From the flow control point of view, it also opens the path to more simple closed-loop flow-control strategies aiming at the stabilization of the wake and the control of the dynamics of the wake barycenter.

  11. Dissipation of turbulence in the wake of a wind turbine

    Science.gov (United States)

    Lundquist, J. K.; Bariteau, L.

    2013-12-01

    The wake of a wind turbine is characterized by increased turbulence and decreased wind speed. Turbines are generally deployed in large groups in wind farms, and so the behavior of an individual wake as it merges with other wakes and propagates downwind is of great importance in assessing wind farm power production as well as impacts of wind energy deployment on local and regional environments. The rate of turbulence dissipation in the wake quantifies the wake behavior as it propagates. In situ field measurements of turbulence dissipation rate in the wake of wind turbines have not been previously collected although correct modeling of dissipation rate is required for accurate simulations of wake evolution. In Fall 2012, we collected in situ measurements of winds and turbulence dissipation from the wake region of a multi-MW turbine, using the University of Colorado at Boulder's Tethered Lifting System (TLS). The TLS is a unique state-of-the-art tethersonde, proven in numerous boundary-layer field experiments to be able to measure turbulence kinetic energy dissipation rates. Ambient flow measurements were provided from sonic anemometers on a meteorological tower located upwind of the turbine, from a profiling lidar upwind, and from a scanning lidar measuring both inflow to and wake from the turbine. Measurements collected within the wake indicate that dissipation rates are higher in the turbine wake than in the ambient flow. Profiles of dissipation and turbulence throughout the rotor disk suggest that dissipation peaks near the hub height of the turbine. Suggestions for incorporating this information into wind turbine modeling approaches will be provided.

  12. Rainfall-enhanced blooming in typhoon wakes

    Science.gov (United States)

    Lin, Y.-C.; Oey, L.-Y.

    2016-08-01

    Strong phytoplankton blooming in tropical-cyclone (TC) wakes over the oligotrophic oceans potentially contributes to long-term changes in global biogeochemical cycles. Yet blooming has traditionally been discussed using anecdotal events and its biophysical mechanics remain poorly understood. Here we identify dominant blooming patterns using 16 years of ocean-color data in the wakes of 141 typhoons in western North Pacific. We observe right-side asymmetric blooming shortly after the storms, attributed previously to sub-mesoscale re-stratification, but thereafter a left-side asymmetry which coincides with the left-side preference in rainfall due to the large-scale wind shear. Biophysical model experiments and observations demonstrate that heavier rainfall freshens the near-surface water, leading to stronger stratification, decreased turbulence and enhanced blooming. Our results suggest that rainfall plays a previously unrecognized, critical role in TC-induced blooming, with potentially important implications for global biogeochemical cycles especially in view of the recent and projected increases in TC-intensity that harbingers stronger mixing and heavier rain under the storm.

  13. Studies of Particle Wake Potentials in Plasmas

    Science.gov (United States)

    Ellis, Ian; Graziani, Frank; Glosli, James; Strozzi, David; Surh, Michael; Richards, David; Decyk, Viktor; Mori, Warren

    2011-10-01

    Fast Ignition studies require a detailed understanding of electron scattering, stopping, and energy deposition in plasmas with variable values for the number of particles within a Debye sphere. Presently there is disagreement in the literature concerning the proper description of these processes. Developing and validating proper descriptions requires studying the processes using first-principle electrostatic simulations and possibly including magnetic fields. We are using the particle-particle particle-mesh (PPPM) code ddcMD and the particle-in-cell (PIC) code BEPS to perform these simulations. As a starting point in our study, we examine the wake of a particle passing through a plasma in 3D electrostatic simulations performed with ddcMD and with BEPS using various cell sizes. In this poster, we compare the wakes we observe in these simulations with each other and predictions from Vlasov theory. Prepared by LLNL under Contract DE-AC52-07NA27344 and by UCLA under Grant DE-FG52-09NA29552.

  14. Optimization Under Uncertainty for Wake Steering Strategies

    Energy Technology Data Exchange (ETDEWEB)

    Quick, Julian [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Annoni, Jennifer [National Renewable Energy Laboratory (NREL), Golden, CO (United States); King, Ryan N [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Dykes, Katherine L [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Fleming, Paul A [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Ning, Andrew [Brigham Young University

    2017-08-03

    Offsetting turbines' yaw orientations from incoming wind is a powerful tool that may be leveraged to reduce undesirable wake effects on downstream turbines. First, we examine a simple two-turbine case to gain intuition as to how inflow direction uncertainty affects the optimal solution. The turbines are modeled with unidirectional inflow such that one turbine directly wakes the other, using ten rotor diameter spacing. We perform optimization under uncertainty (OUU) via a parameter sweep of the front turbine. The OUU solution generally prefers less steering. We then do this optimization for a 60-turbine wind farm with unidirectional inflow, varying the degree of inflow uncertainty and approaching this OUU problem by nesting a polynomial chaos expansion uncertainty quantification routine within an outer optimization. We examined how different levels of uncertainty in the inflow direction effect the ratio of the expected values of deterministic and OUU solutions for steering strategies in the large wind farm, assuming the directional uncertainty used to reach said OUU solution (this ratio is defined as the value of the stochastic solution or VSS).

  15. [Controlling sleep/wakefulness using optogenetics].

    Science.gov (United States)

    Yamanaka, Akihiro

    2015-08-01

    Optogenetics is a recently developed experimental technique to control the activity of neurons using light. Optogenetics shows its power to reveal the physiological role of specific neural circuits in the brain. In particular, manipulation of a specific type of neurons using optogenetics with high accuracy timing enables us to analyze causality between neural activity and initiation of animal behaviors. However, to manipulate the activity of specific neurons in vivo, there are two critical steps to succeed in manipulation of the neural activity and control of the behavior of individual animals. The first step is an adequate number of molecules of light-activated protein that has to be expressed in the cell membrane of the neurons of interest. The second step is the optical system to illuminate the targeted neurons with enough intensity of light to activate the light-activated protein. We applied optogenetics to hypothalamic peptidergic neurons such as orexin/hypocretin neurons or melanin concentrating hormone (MCH) neurons. These neurons are implicated in sleep/wakefulness regulation. In this mini review, I will show the regulatory mechanism of sleep/wakefulness by these neurons using optogenetics.

  16. Attention deficit hyperactivity disorder (ADHD)

    Science.gov (United States)

    ... this page: //medlineplus.gov/ency/article/001551.htm Attention deficit hyperactivity disorder To use the sharing features on this page, please enable JavaScript. Attention deficit hyperactivity disorder (ADHD) is a problem caused ...

  17. Analytical calculation of the wake potential of a spherical resonator

    Directory of Open Access Journals (Sweden)

    Sebastian Ratschow

    2002-05-01

    Full Text Available An analytical formula for the wake potential of a closed spherical resonator with perfectly conducting walls is presented. Mode analysis is used for the calculation. For every rotationally symmetric TM mode the loss parameter is calculated and the formula for the determination of the corresponding frequency is given. The final wake potential is an infinite sum over all modes mentioned above.

  18. Drone Based Experimental Investigation of Wind Turbine Wake Evolution

    Science.gov (United States)

    Subramanian, Balaji, , Dr.; Chokani, Ndaona, , Dr.; Abhari, Reza, Prof. _., Dr.

    2016-11-01

    The characteristics of the wake downstream of a wind turbine has an important bearing on the optimized micrositing of wind turbines in a given land area, as well as on the loads seen by downstream turbines. We use a novel measurement system to measure the flow field upstream and in the wake of a full-scale wind turbine. The system consists of a fast response aerodynamic probe, mounted on an autonomous drone that is equipped with a suite of sensors. These measurements detail, for the first time at full-scale Reynolds number conditions, the evolution and breakdown of tip vortices that are characteristic of the near wake, as well as the turbulent mixing and entrainment of more energised flow, which are distinctive in the far wake. A short-time Fourier transform (STFT) analysis method is used to derive time-localized TKE along the drone's trajectory. Detailed upstream and wake measurements are needed to understand the flow behavior, as it helps in developing and validating simplified wake models that can approximate the wake qualities. Comparisons of these measurements to recently developed wake prediction models highlights how these measurements can support further model development.

  19. Wind Farm Wake: The 2016 Horns Rev Photo Case

    DEFF Research Database (Denmark)

    Hasager, Charlotte Bay; Nygaard, Nicolai Gayle; Volker, Patrick

    2017-01-01

    Offshore wind farm wakes were observed and photographed in foggy conditions at Horns Rev 2 on 25 January 2016 at 12:45 UTC. These new images show highly contrasting conditions regarding the wind speed, turbulence intensity, atmospheric stability, weather conditions and wind farm wake development...

  20. Bubble wake dynamics in liquids and liquid-solid suspensions

    CERN Document Server

    Fan, Liang-Shih; Brenner, Howard

    1990-01-01

    This book is devoted to a fundamental understanding of the fluid dynamic nature of a bubble wake, more specifically the primary wake, in liquids and liquid-solid suspensions, an dto the role it plays in various important flow phenomena of multiphase systems. Examples of these phenomena are liquid/solids mixing, bubble coalescence and disintergration, particle entrainment to the freeboard, and bed contraction.

  1. Interaction of Aircraft Wakes From Laterally Spaced Aircraft

    Science.gov (United States)

    Proctor, Fred H.

    2009-01-01

    Large Eddy Simulations are used to examine wake interactions from aircraft on closely spaced parallel paths. Two sets of experiments are conducted, with the first set examining wake interactions out of ground effect (OGE) and the second set for in ground effect (IGE). The initial wake field for each aircraft represents a rolled-up wake vortex pair generated by a B-747. Parametric sets include wake interactions from aircraft pairs with lateral separations of 400, 500, 600, and 750 ft. The simulation of a wake from a single aircraft is used as baseline. The study shows that wake vortices from either a pair or a formation of B-747 s that fly with very close lateral spacing, last longer than those from an isolated B-747. For OGE, the inner vortices between the pair of aircraft, ascend, link and quickly dissipate, leaving the outer vortices to decay and descend slowly. For the IGE scenario, the inner vortices ascend and last longer, while the outer vortices decay from ground interaction at a rate similar to that expected from an isolated aircraft. Both OGE and IGE scenarios produce longer-lasting wakes for aircraft with separations less than 600 ft. The results are significant because concepts to increase airport capacity have been proposed that assume either aircraft formations and/or aircraft pairs landing on very closely spaced runways.

  2. On the Turbulent Mixing in Horizontal Axis Wind Turbine Wakes

    NARCIS (Netherlands)

    Lignarolo, L.E.M.

    2016-01-01

    The wake flow of a horizontal axis wind turbine is characterised by lower wind speed and higher turbulence than the free-stream conditions. When clustered in large wind farms, wind turbines regularly operate inside the wake of one or more upstream machines. This is a major cause of energy production

  3. On the Turbulent Mixing in Horizontal Axis Wind Turbine Wakes

    NARCIS (Netherlands)

    Lignarolo, L.E.M.

    2016-01-01

    The wake flow of a horizontal axis wind turbine is characterised by lower wind speed and higher turbulence than the free-stream conditions. When clustered in large wind farms, wind turbines regularly operate inside the wake of one or more upstream machines. This is a major cause of energy production

  4. Dynamics of the vortex wakes of flying and swimming vertebrates.

    Science.gov (United States)

    Rayner, J M

    1995-01-01

    The vortex wakes of flying and swimming animals provide evidence of the history of aero- and hydrodynamic force generation during the locomotor cycle. Vortex-induced momentum flux in the wake is the reaction of forces the animal imposes on its environment, which must be in equilibrium with inertial and external forces. In flying birds and bats, the flapping wings generate lift both to provide thrust and to support the weight. Distinct wingbeat and wake movement patterns can be identified as gaits. In flow visualization experiments, only two wake patterns have been identified: a vortex ring gait with inactive upstroke, and a continuous vortex gait with active upstroke. These gaits may be modelled theoretically by free vortex and lifting line theory to predict mechanical energy consumption, aerodynamic forces and muscle activity. Longer-winged birds undergo a distinct gait change with speed, but shorter-winged species use the vortex ring gait at all speeds. In swimming fish, the situation is more complex: the wake vortices form a reversed von Kármán vortex street, but little is known about the mechanism of generation of the wake, or about how it varies with speed and acceleration or with body form and swimming mode. An unresolved complicating factor is the interaction between the drag wake of the flapping fish body and the thrusting wake from the tail.

  5. Nonlinear Eddy Viscosity Models applied to Wind Turbine Wakes

    DEFF Research Database (Denmark)

    Laan, van der, Paul Maarten; Sørensen, Niels N.; Réthoré, Pierre-Elouan;

    2013-01-01

    The linear k−ε eddy viscosity model and modified versions of two existing nonlinear eddy viscosity models are applied to single wind turbine wake simulations using a Reynolds Averaged Navier-Stokes code. Results are compared with field wake measurements. The nonlinear models give better results...

  6. Velocity selective optical pumping

    OpenAIRE

    Aminoff, C. G.; Pinard, M.

    1982-01-01

    We consider optical pumping with a quasi monochromatic tunable light beam, in the low intensity limit where a rate equation regime is obtained The velocity selective optical pumping (V.S.O.P.) introduces a correlation between atomic velocity and internal variables in the ground (or metastable) state. The aim of this article is to evaluate these atomic observables (orientation, alignment, population) as a function of velocity, using a phenomenological description of the relaxation effect of co...

  7. Bifurcation of space-charge wave in a plasma waveguide including the wake potential effect

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Myoung-Jae [Department of Physics and Research Institute for Natural Sciences, Hanyang University, Seoul 04763 (Korea, Republic of); Jung, Young-Dae, E-mail: ydjung@hanyang.ac.kr [Department of Applied Physics and Department of Bionanotechnology, Hanyang University, Ansan, Kyunggi-Do 15588, South Korea and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180-3590 (United States)

    2016-09-15

    The wake potential effects on the propagation of the space-charge dust ion-acoustic wave are investigated in a cylindrically bounded dusty plasma with the ion flow. The results show that the wake potential would generate the double frequency modes in a cylindrically bounded dusty plasma. It is found that the upper mode of the wave frequency with the root of higher-order is smaller than that with the root of lower-order in intermediate wave number domains. However, the lower mode of the scaled wave frequency with the root of higher-order is found to be greater than that with the root of lower-order. It is found that the influence in the order of the root of the Bessel function on the wave frequency of the space-charge dust-ion-acoustic wave in a cylindrically confined dusty plasma decreases with an increase in the propagation wave number. It is also found that the double frequency modes increase with increasing Mach number due to the ion flow in a cylindrical dusty plasma. In addition, it is found that the upper mode of the group velocity decreases with an increase in the scaled radius of the plasma cylinder. However, it is shown that the lower mode of the scaled group velocity of the space-charge dust ion acoustic wave increases with an increase in the radius of the plasma cylinder. The variation of the space-charge dust-ion-acoustic wave due to the wake potential and geometric effects is also discussed.

  8. Bifurcation of space-charge wave in a plasma waveguide including the wake potential effect

    Science.gov (United States)

    Lee, Myoung-Jae; Jung, Young-Dae

    2016-09-01

    The wake potential effects on the propagation of the space-charge dust ion-acoustic wave are investigated in a cylindrically bounded dusty plasma with the ion flow. The results show that the wake potential would generate the double frequency modes in a cylindrically bounded dusty plasma. It is found that the upper mode of the wave frequency with the root of higher-order is smaller than that with the root of lower-order in intermediate wave number domains. However, the lower mode of the scaled wave frequency with the root of higher-order is found to be greater than that with the root of lower-order. It is found that the influence in the order of the root of the Bessel function on the wave frequency of the space-charge dust-ion-acoustic wave in a cylindrically confined dusty plasma decreases with an increase in the propagation wave number. It is also found that the double frequency modes increase with increasing Mach number due to the ion flow in a cylindrical dusty plasma. In addition, it is found that the upper mode of the group velocity decreases with an increase in the scaled radius of the plasma cylinder. However, it is shown that the lower mode of the scaled group velocity of the space-charge dust ion acoustic wave increases with an increase in the radius of the plasma cylinder. The variation of the space-charge dust-ion-acoustic wave due to the wake potential and geometric effects is also discussed.

  9. Numerical Simulation of Wakes in a Weakly Stratified Fluid

    CERN Document Server

    Rottman, James W; Innis, George E; O'Shea, Thomas T; Novikov, Evgeny

    2014-01-01

    This paper describes some preliminary numerical studies using large eddy simulation of full-scale submarine wakes. Submarine wakes are a combination of the wake generated by a smooth slender body and a number of superimposed vortex pairs generated by various control surfaces and other body appendages. For this preliminary study, we attempt to gain some insight into the behavior of full-scale submarine wakes by computing separately the evolution the self-propelled wake of a slender body and the motion of a single vortex pair in both a non-stratified and a stratified environment. An important aspect of the simulations is the use of an iterative procedure to relax the initial turbulence field so that turbulent production and dissipation are in balance.

  10. Evaluation of wake detection probability of underwater vehicle by IR

    Science.gov (United States)

    Kou, Wei; Chen, Xuan; Yang, Li; Jin, Fang-yuan

    2016-10-01

    The thermal or cold wake of the underwater vehicles will be formed at the sea surface in different region during sailing, then the underwater vehicles will be detected by airborne or space borne infrared detectors easily, which will imperil their security. A model between the detection probability and the Noise Equivalent Temperature Difference (NETD) of the detectors, and the temperature difference between the wake and the sea surface, etc., was established and the evaluation of detection probability in different discrimination levels and other parameters, such as time, location, atmosphere, sea, detector performance, wake temperature, etc., was realized, and a software named Wake Detection of Underwater Vehicle by Infrared (WDPUV-IR) was developed. The results showed that the detection probability to the wake with high detector performance or large temperature difference or short detection distance or low discrimination level was relatively high, but it was difficult to detect targets with small temperature difference and size when the atmospheric transmittance value was low.

  11. Tracking wakes: the nocturnal predatory strategy of piscivorous catfish.

    Science.gov (United States)

    Pohlmann, K; Grasso, F W; Breithaupt, T

    2001-06-19

    Swimming fish leave wakes containing hydrodynamic and chemical traces. These traces mark their swim paths and could guide predators. We now show that nocturnal European catfish (Silurus glanis) locate a piscine prey (guppy, Poecilia reticulata) by accurately tracking its three-dimensional swim path before an attack in the absence of visible light. Wakes that were up to 10 s old were followed over distances up to 55 prey-body lengths in our setup. These results demonstrate that prey wakes remain sufficiently identifiable to guide predators, and to extend considerably the area in which prey is detectable. Moreover, wakes elicit rear attacks, which may be more difficult to detect by prey. Wake tracking may be a common strategy among aquatic predators.

  12. Ram pressure stripping of disc galaxies orbiting in clusters. II. Galactic wakes

    CERN Document Server

    Roediger, E

    2007-01-01

    We present 3D hydrodynamical simulations of ram pressure stripping of a disc galaxy orbiting in a galaxy cluster. In this paper, we focus on the properties of the galaxies' tails of stripped gas. The galactic wakes show a flaring width, where the flaring angle depends on the gas disc's cross-section with respect to the galaxy's direction of motion. The velocity in the wakes shows a significant turbulent component of a few 100 km/s. The stripped gas is deposited in the cluster rather locally, i.e. within ~150 kpc from where it was stripped. We demonstrate that the most important quantity governing the tail density, length and gas mass distribution along the orbit is the galaxy's mass loss per orbital length. This in turn depends on the ram pressure as well as the galaxy's orbital velocity. For a sensitivity limit of ~10^19 cm^-2 in projected gas density, we find typical tail lengths of 40 kpc. Such long tails are seen even at large distances (0.5 to 1 Mpc) from the cluster centre. At this sensitivity limit, th...

  13. Wake Flow Simulation of a Vertical Axis Wind Turbine Under the Influence of Wind Shear

    Science.gov (United States)

    Mendoza, Victor; Goude, Anders

    2017-05-01

    The current trend of the wind energy industry aims for large scale turbines installed in wind farms. This brings a renewed interest in vertical axis wind turbines (VAWTs) since they have several advantages over the traditional Horizontal Axis Wind Tubines (HAWTs) for mitigating the new challenges. However, operating VAWTs are characterized by complex aerodynamics phenomena, presenting considerable challenges for modeling tools. An accurate and reliable simulation tool for predicting the interaction between the obtained wake of an operating VAWT and the flow in atmospheric open sites is fundamental for optimizing the design and location of wind energy facility projects. The present work studies the wake produced by a VAWT and how it is affected by the surface roughness of the terrain, without considering the effects of the ambient turbulence intensity. This study was carried out using an actuator line model (ALM), and it was implemented using the open-source CFD library OpenFOAM to solve the governing equations and to compute the resulting flow fields. An operational H-shaped VAWT model was tested, for which experimental activity has been performed at an open site north of Uppsala-Sweden. Different terrains with similar inflow velocities have been evaluated. Simulated velocity and vorticity of representative sections have been analyzed. Numerical results were validated using normal forces measurements, showing reasonable agreement.

  14. Effects of Flexible Splitter Plate in the Wake of a Cylindrical Body

    Directory of Open Access Journals (Sweden)

    S. Teksin

    2016-01-01

    Full Text Available In the wake of the bluff bodies rigid splitter plates are known to control vortex shedding. In this study, the problem of flexible splitter plate in the wake of circular cylinder was investigated using Particle Image Velocimetry (PIV experimentally. In this case; the splitter plate which has a certain amount of modulus of elasticity freely deforms along its length because of the fluid forces on plate. The diameter of cylinder, D was 60 mm while the Reynolds number based on the cylinder diameter is kept constant as 2500, the characteristics length of the control element, L was tested for four different cases that the values of L/D were 0, 1.25, 2.25, 2.5 in the investigation. As a consequence, turbulent kinetic energy, TKE, velocity vector field , vortex, Reynolds stress , root mean square of streamwise and transverse velocities, /U, /U were analyzed. It is found that the variable parameter of L/D affects the flow structures and also noted that it decreased maximum level of all characteristic values.

  15. Persistent hyperdopaminergia decreases the peak frequency of hippocampal theta oscillations during quiet waking and REM sleep.

    Directory of Open Access Journals (Sweden)

    Kafui Dzirasa

    Full Text Available Long-term changes in dopaminergic signaling are thought to underlie the pathophysiology of a number of psychiatric disorders. Several conditions are associated with cognitive deficits such as disturbances in attention processes and learning and memory, suggesting that persistent changes in dopaminergic signaling may alter neural mechanisms underlying these processes. Dopamine transporter knockout (DAT-KO mice exhibit a persistent five-fold increase in extracellular dopamine levels. Here, we demonstrate that DAT-KO mice display lower hippocampal theta oscillation frequencies during baseline periods of waking and rapid-eye movement sleep. These altered theta oscillations are not reversed via treatment with the antidopaminergic agent haloperidol. Thus, we propose that persistent hyperdopaminergia, together with secondary alterations in other neuromodulatory systems, results in lower frequency activity in neural systems responsible for various cognitive processes.

  16. Research on aircraft/vortex-wake interactions to determine acceptable level of wake intensity

    Science.gov (United States)

    Rossow, Vernon J.; Tinling, Bruce E.

    1988-01-01

    An evaluation of the literature on large aircraft wake-vortex encounters in flight and in flight simulators has furnished an estimate of the level to which the vortex-induced rolling moments must be reduced in order to be perceived as nonhazardous at a 2-n.mi. separation distance. The criteria are based on the ratio of the vortex-induced acceleration in roll to the aileron-induced roll acceleration. A wake is acceptably alleviated if the ratio of vortex-to-aileron rolling moments is less than about 0.5. When a satisfactory alleviation scheme is identified, the alleviated vortex structure should be inserted into a simulator to ascertain whether the maximum bank angles induced are within tolerable limits.

  17. Measurements and modeling of flow structure in the wake of a low profile wishbone vortex generator

    Science.gov (United States)

    Wendt, B. J.; Hingst, W. R.

    1994-01-01

    The results of an experimental examination of the vortex structures shed from a low profile 'wishbone' generator are presented. The vortex generator height relative to the turbulent boundary layer was varied by testing two differently sized models. Measurements of the mean three-dimensional velocity field were conducted in cross-stream planes downstream of the vortex generators. In all cases, a counter-rotating vortex pair was observed. Individual vortices were characterized by three descriptors derived from the velocity data; circulation, peak vorticity, and cross-stream location of peak vorticity. Measurements in the cross plane at two axial locations behind the smaller wishbone characterize the downstream development of the vortex pairs. A single region of stream wise velocity deficit is shared by both vortex cores. This is in contrast to conventional generators, where each core coincides with a region of velocity deficit. The measured cross-stream velocities for each case are compared to an Oseen model with matching descriptors. The best comparison occurs with the data from the larger wishbone.

  18. Estimation of vector velocity

    DEFF Research Database (Denmark)

    2000-01-01

    Using a pulsed ultrasound field, the two-dimensional velocity vector can be determined with the invention. The method uses a transversally modulated ultrasound field for probing the moving medium under investigation. A modified autocorrelation approach is used in the velocity estimation. The new...

  19. Estimation of vector velocity

    DEFF Research Database (Denmark)

    2000-01-01

    Using a pulsed ultrasound field, the two-dimensional velocity vector can be determined with the invention. The method uses a transversally modulated ultrasound field for probing the moving medium under investigation. A modified autocorrelation approach is used in the velocity estimation. The new...

  20. In the wake of the economic crisis

    DEFF Research Database (Denmark)

    Komp, Katrin; Starke, Peter; van Hooren, Franca

    The economic crisis of 2008 is a global phenomenon with far-ranging impact. Its effects reach beyond the economic sphere, impacting the lives of individuals. When confronted with job loss and financial hardship, people look for new income sources, change their world views and attitudes, and remodel......, they take different approaches that reflect the country-specific history and welfare state institutions. This working paper gives an overview of social change and welfare state reform in the wake of the economic crisis. For this purpose, it presents discussions and findings from two classes that were taught...... at universities in Germany. The material includes the outcomes of a grounded theory study that was conducted in the framework of one of these classes, summaries from various essays that students wrote for the classes, and from a book that was used as teaching material. The arguments brought forth in the working...

  1. Altered sleep-wake patterns in blindness

    DEFF Research Database (Denmark)

    Aubin, S.; Gacon, C.; Jennum, P.

    2016-01-01

    discuss variability in the sleep–wake pattern between blind and normal-sighted individuals. Methods Thirty-day actigraphy recordings were collected from 11 blind individuals without residual light perception and 11 age- and sex-matched normal-sighted controls. From these recordings, we extracted...... the Pittsburgh Sleep Quality Index, and chronotype, using the Morningness-Eveningness Questionnaire. Results Although no group differences were found when averaging over the entire recording period, we found a greater variability throughout the 30-days in both sleep efficiency and timing of the night-time sleep...... episode in blind participants as compared to sighted control participants. We also confirm previous reports of reduced sleep quality in blind individuals. Notably, the variability in sleep efficiency and in the timing of sleep correlated with the severity of sleep disturbances. Conclusion The timing...

  2. Wake field effects in APT linac

    Energy Technology Data Exchange (ETDEWEB)

    Kurennoy, S.S.

    1998-12-31

    The 1.7-GeV 100-mA CW proton linac is now under design for the Accelerator Production of Tritium (APT) Project. The high current leads to stringent restrictions on allowable beam losses (<1 nA/m), that requires analyzing carefully all possible loss sources. While wake-field effects are usually considered negligible in proton linacs, the author studies these effects for the APT to exclude potential problems at such a high current. Loss factors and resonance frequency spectra of various discontinuities of the vacuum chamber are investigated, both analytically and using 2-D and 3-D simulation codes with a single bunch as well as with many bunches. Here he concentrates on two features specific to the APT linac: loss factors for the design {beta} < 1 and CW beam structure.

  3. Wake Field Effects in the APT Linac.

    Science.gov (United States)

    Kurennoy, Sergey

    1998-04-01

    The 1.7-GeV 100-mA CW proton linac is now under design for the Accelerator Production of Tritium (APT) Project. While wake-field effects are usually considered negligible in proton linacs, an analysis for the APT accelerator has been performed to exclude potential problems at such a high current leading to beam losses. Loss factors and resonance frequency spectra of various discontinuities of the vacuum chamber are investigated, both analytically and using 2-D and 3-D simulation codes with a single bunch as well as with many bunches. The only noticeable effect is the HOM heating of the 5-cell superconducting cavities. However, it has an acceptable level and will be further reduced by HOM couplers.

  4. Finnegans Wake & embodied cognition. A joycean reading

    Directory of Open Access Journals (Sweden)

    Simone Rebora

    2014-06-01

    Full Text Available This contribution proposes an interpretation of the language theory in Finnegans Wake, identifying its potential matches with the embodied cognition thesis. First, are revised critical texts about the relations between joycean writings, the sciences of the mind and the evolution of technology and informatics. Then, through the close analysis of four excerpt, are outlined the traits of joycean poetics closer to the recent acquisitions in the cognitive sciences. In particular: the bodily roots of the formation of language and the inexhaustible process of interpretation. The first convergence is detected in neurosciences, with reference to the recent proposals of Vittorio Gallese, about “embodiment of language comprehension.” The second convergence is identified in the studies on Artificial Life, referring to the writings of Christopher Langton and the Research Group of Los Alamos.

  5. Dynamic Loads and Wake Prediction for Large Wind Turbines Based on Free Wake Method

    Institute of Scientific and Technical Information of China (English)

    Cao Jiufa; Wang Tongguang; Long Hui; Ke Shitang; Xu Bofeng

    2015-01-01

    With large scale wind turbines ,the issue of aerodynamic elastic response is even more significant on dy-namic behaviour of the system .Unsteady free vortex wake method is proposed to calculate the shape of wake and aerodynamic load .Considering the effect of aerodynamic load ,inertial load and gravity load ,the decoupling dy-namic equations are established by using finite element method in conjunction of the modal method and equations are solved numerically by Newmark approach .Finally ,the numerical simulation of a large scale wind turbine is performed through coupling the free vortex wake modelling with structural modelling .The results show that this coupling model can predict the flexible wind turbine dynamic characteristics effectively and efficiently .Under the influence of the gravitational force ,the dynamic response of flapwise direction contributes to the dynamic behavior of edgewise direction under the operational condition of steady wind speed .The difference in dynamic response be-tween the flexible and rigid wind turbines manifests when the aerodynamics/structure coupling effect is of signifi-cance in both wind turbine design and performance calculation .

  6. Validation of four LES and a vortex model against stereo-PIV measurements in the near wake of an actuator disc and a wind turbine

    DEFF Research Database (Denmark)

    Lignarolo, Lorenzo E.M.; Mehta, Dhruv; Stevens, Richard J.A.M.

    2016-01-01

    stereoscopic Particle Image Velocimetry was employed to obtain the velocity field and turbulence statistics in the near wake of a two-bladed wind turbine model and of a porous disc, which mimics the numerical actuator used in the simulations. Researchers have been invited to simulate the experimental case......In this paper we report the results of a workshop organised by the Delft University of Technology in 2014, aiming at the comparison between different state-of-the-art numerical models for the simulation of wind turbine wakes. The chosen benchmark case is a wind tunnel measurement, where...... based on the disc drag coefficient and the inflow characteristics. Four large eddy simulation (LES) codes from different institutions and a vortex model are part of the comparison. The purpose of this benchmark is to validate the numerical predictions of the flow field statistics in the near wake...

  7. Iodine Tagging Velocimetry and Mechanism in the Hypersonic Near Wake of a MultiPurpose Crew Vehicle

    Science.gov (United States)

    Balla, R. Jeffrey

    2013-01-01

    This study demonstrates a new molecular tagging velocimetry (MTV) method for velocity measurements of high speed flow. It demonstrates offbody Iodine Tagging Velocimetry (ITV) in the hypersonic near wake of a MultiPurpose Crew Vehicle (MPCV) model. Experiments are performed in the NASA-Langley 31-inch Mach 10 air wind tunnel. A 0.5% I2 / N2 mixture is seeded on the leeward backshell of the model using a pressure tap. I2 laser-induced fluorescence is excited along a 5.5 mm line using an ArF excimer laser near 193 nm. Results indicate I2 absorbs at least 2 photons to produce iodine ions and electrons. These recombine as the tagged region is displaced downstream to produce I (2P3/2) whose emission is monitored at 206 nm. Results at P0 = 2.41 MPa (350 psi), T0 = 990K, and 10 micro-sec transit times produce velocities from 630-820 m/sec across the I2 seeded jet at a distance of 38.2 mm (25.5 jet diameters) downstream from the jet orifice. Maximum wake jet velocities near the shear layer are 59% of freestream velocity.

  8. Near and far wake structures behind freely flying bats

    Science.gov (United States)

    Schunk, Cosima; Swartz, Sharon M.; Breuer, Kenneth S.

    2014-11-01

    While pseudo-volumetric reconstructions of the wakes of flying animals, based on transverse (Trefftz) wake measurements, have become a well-established tool in the study of animal aerodynamics in recent years, there are a number of concerns that persist regarding their use in estimating drag and flight efficiency. Here we report on stereo particle image velocimetry (PIV) measurements behind freely flying bats (Eptesicus fuscus) in both the transverse and streamwise planes. The streamwise plane measurements are taken on the wing as well as in the near and far wake region up to eight chord lengths behind the bat. By organizing the data according to the flight speed, wingbeat phase and the spanwise position of the laser sheet on the wing we are able to connect specific features of the wing and body geometry with observed wake structures and thereby construct a detailed time-space map of the wake. Furthermore, we can quantitatively assess wake distortion and assess the validity of lift and drag estimates based on transverse wake measurements. Supported by AFOSR.

  9. Armodafinil in the treatment of sleep/wake disorders.

    Science.gov (United States)

    Schwartz, Jonathan R L; Roth, Thomas; Drake, Chris

    2010-09-07

    Excessive sleepiness (ES) is a major but underestimated public health concern associated with significant impairments in alertness/wakefulness and significant morbidity. The term ES has been used in the sleep medicine literature for years, but due to its nonspecific symptoms (ie tiredness or fatigue), it frequently goes unrecognized or is misdiagnosed in primary care. In some cases ES arises due to poor sleep habits or self-imposed sleep deprivation; however, ES is also a key component of a number of sleep/wake disorders and multiple medical and psychiatric disorders. Identification and treatment of ES is critical to improve the quality of life and well-being of patients and for the safety of the wider community. The inability of patients to recognize the nature, extent, and symptomatic profile of sleep/wake disorders requires vigilance on the part of healthcare professionals. Interventions to address ES and its associated impairments, treatment of the underlying sleep/wake disorder, and follow-up are a priority given the potential for serious consequences if left untreated. Wakefulness-promoting agents are available that treat ES associated with sleep/wake disorders. This review examines current approaches for managing this debilitating and potentially life-threatening condition, focusing on the place of armodafinil as a wakefulness-promoting agent.

  10. Armodafinil-induced wakefulness in animals with ventrolateral preoptic lesions

    Directory of Open Access Journals (Sweden)

    Vetrivelan R

    2014-05-01

    Full Text Available Ramalingam Vetrivelan, Clifford B Saper, Patrick M Fuller Department of Neurology, Harvard Medical School and Beth Israel Deaconess Medical Center, Boston, MA, USA Abstract: Armodafinil is the pharmacologically active R-enantiomer of modafinil, a widely prescribed wake-promoting agent used to treat several sleep-related disorders including excessive daytime sleepiness associated with narcolepsy, shift work sleep disorder, and obstructive sleep apnea/hypopnea syndrome. Remarkably, however, the neuronal circuitry through which modafinil exerts its wake-promoting effects remains unresolved. In the present study, we sought to determine if the wake-promoting effects of armodafinil are mediated, at least in part, by inhibiting the sleep-promoting neurons of the ventrolateral preoptic (VLPO nucleus. To do so, we measured changes in waking following intraperitoneal administration of armodafinil (200 mg/kg or the psychostimulant methamphetamine (1 mg/kg in rats with cell-body specific lesion of the VLPO. Rats with histologically confirmed lesions of the VLPO demonstrated a sustained increase in wakefulness at baseline, but the increase in wakefulness following administration of both armodafinil and methamphetamine was similar to that of intact animals. These data suggest that armodafinil increases wakefulness by mechanisms that extend beyond inhibition of VLPO neurons. Keywords: EEG, sleep, orexin-saporin, methamphetamine

  11. Armodafinil-induced wakefulness in animals with ventrolateral preoptic lesions.

    Science.gov (United States)

    Vetrivelan, Ramalingam; Saper, Clifford B; Fuller, Patrick M

    2014-01-01

    Armodafinil is the pharmacologically active R-enantiomer of modafinil, a widely prescribed wake-promoting agent used to treat several sleep-related disorders including excessive daytime sleepiness associated with narcolepsy, shift work sleep disorder, and obstructive sleep apnea/hypopnea syndrome. Remarkably, however, the neuronal circuitry through which modafinil exerts its wake-promoting effects remains unresolved. In the present study, we sought to determine if the wake-promoting effects of armodafinil are mediated, at least in part, by inhibiting the sleep-promoting neurons of the ventrolateral preoptic (VLPO) nucleus. To do so, we measured changes in waking following intraperitoneal administration of armodafinil (200 mg/kg) or the psychostimulant methamphetamine (1 mg/kg) in rats with cell-body specific lesion of the VLPO. Rats with histologically confirmed lesions of the VLPO demonstrated a sustained increase in wakefulness at baseline, but the increase in wakefulness following administration of both armodafinil and methamphetamine was similar to that of intact animals. These data suggest that armodafinil increases wakefulness by mechanisms that extend beyond inhibition of VLPO neurons.

  12. Activity-based sleep-wake identification in infants.

    Science.gov (United States)

    Sazonov, Edward; Sazonova, Nadezhda; Schuckers, Stephanie; Neuman, Michael

    2004-10-01

    Actigraphy offers one of the best-known alternatives to polysomnography for sleep-wake identification. The advantages of actigraphy include high accuracy, simplicity of use and low intrusiveness. These features allow the use of actigraphy for determining sleep-wake states in such highly sensitive groups as infants. This study utilizes a motion sensor (accelerometer) for a dual purpose: to determine an infant's position in the crib and to identify sleep-wake states. The accelerometer was positioned over the sacral region on the infant's diaper, unlike commonly used attachment to an ankle. Opposed to broadly used discriminant analysis, this study utilized logistic regression and neural networks as predictors. The accuracy of predicted sleep-wake states was established in comparison to the sleep-wake states recorded by technicians in a polysomnograph study. Both statistical and neural predictors of this study provide an accuracy of approximately 77-92% which is comparable to similar studies achieving prediction rates of 85-95%, thus validating the suggested methodology. The results support the use of body motion as a simple and reliable method for determining sleep-wake states in infants. Nonlinear mapping capabilities of the neural network benefit the accuracy of sleep-wake state identification. Utilization of the accelerometer for the dual purpose allows us to minimize intrusiveness of home infant monitors.

  13. Wake Vortex Field Measurement Program at Memphis, Tennessee: Data Guide

    Science.gov (United States)

    Campbell, S. D.; Dasey, T. J.; Freehart, R. E.; Heinrichs, R. M.; Mathews, M. P.; Perras, G. H.; Rowe, G. S.

    1997-01-01

    Eliminating or reducing current restrictions in the air traffic control system due to wake vortex considerations would yield increased capacity, decreased delays, and cost savings. Current wake vortex separation standards are widely viewed as very conservative under most conditions. However, scientific uncertainty about wake vortex behavior under different atmospheric conditions remains a barrier to development of an adaptive vortex spacing system. The objective of the wake vortex field measurement efforts during December, 1994 and August, 1995 at Memphis, TN were to record wake vortex behavior for varying atmospheric conditions and types of aircraft. This effort is part of a larger effort by the NASA Langley Research Center to develop an Aircraft Vortex Spacing System (AVOSS) as an element of the Terminal Area Productivity (TAP) program. The TAP program is being performed in concert with the FAA Terminal Air Traffic Control Automation (TATCA) program and ATC Automation. Wake vortex behavior was observed using a mobile continuous-wave (CW) coherent laser Doppler radar (lidar) developed at Lincoln Laboratory. This lidar features a number of improvements over previous systems, including the first-ever demonstration of an automatic wake vortex detection and tracking algorithm.

  14. Neuroligin-1 links neuronal activity to sleep-wake regulation

    Science.gov (United States)

    El Helou, Janine; Bélanger-Nelson, Erika; Freyburger, Marlène; Dorsaz, Stéphane; Curie, Thomas; La Spada, Francesco; Gaudreault, Pierre-Olivier; Beaumont, Éric; Pouliot, Philippe; Lesage, Frédéric; Frank, Marcos G.; Franken, Paul; Mongrain, Valérie

    2013-01-01

    Maintaining wakefulness is associated with a progressive increase in the need for sleep. This phenomenon has been linked to changes in synaptic function. The synaptic adhesion molecule Neuroligin-1 (NLG1) controls the activity and synaptic localization of N-methyl-d-aspartate receptors, which activity is impaired by prolonged wakefulness. We here highlight that this pathway may underlie both the adverse effects of sleep loss on cognition and the subsequent changes in cortical synchrony. We found that the expression of specific Nlg1 transcript variants is changed by sleep deprivation in three mouse strains. These observations were associated with strain-specific changes in synaptic NLG1 protein content. Importantly, we showed that Nlg1 knockout mice are not able to sustain wakefulness and spend more time in nonrapid eye movement sleep than wild-type mice. These changes occurred with modifications in waking quality as exemplified by low theta/alpha activity during wakefulness and poor preference for social novelty, as well as altered delta synchrony during sleep. Finally, we identified a transcriptional pathway that could underlie the sleep/wake-dependent changes in Nlg1 expression and that involves clock transcription factors. We thus suggest that NLG1 is an element that contributes to the coupling of neuronal activity to sleep/wake regulation. PMID:23716671

  15. A parabolic velocity-decomposition method for wind turbines

    Science.gov (United States)

    Mittal, Anshul; Briley, W. Roger; Sreenivas, Kidambi; Taylor, Lafayette K.

    2017-02-01

    An economical parabolized Navier-Stokes approximation for steady incompressible flow is combined with a compatible wind turbine model to simulate wind turbine flows, both upstream of the turbine and in downstream wake regions. The inviscid parabolizing approximation is based on a Helmholtz decomposition of the secondary velocity vector and physical order-of-magnitude estimates, rather than an axial pressure gradient approximation. The wind turbine is modeled by distributed source-term forces incorporating time-averaged aerodynamic forces generated by a blade-element momentum turbine model. A solution algorithm is given whose dependent variables are streamwise velocity, streamwise vorticity, and pressure, with secondary velocity determined by two-dimensional scalar and vector potentials. In addition to laminar and turbulent boundary-layer test cases, solutions for a streamwise vortex-convection test problem are assessed by mesh refinement and comparison with Navier-Stokes solutions using the same grid. Computed results for a single turbine and a three-turbine array are presented using the NREL offshore 5-MW baseline wind turbine. These are also compared with an unsteady Reynolds-averaged Navier-Stokes solution computed with full rotor resolution. On balance, the agreement in turbine wake predictions for these test cases is very encouraging given the substantial differences in physical modeling fidelity and computer resources required.

  16. Downstream and soaring interfaces and vortices in 2-D stratified wakes and their impact on transport of contaminants

    Directory of Open Access Journals (Sweden)

    Y. D. Chashechkin

    2006-01-01

    Full Text Available The flow of continuously stratified fluids past obstacles was studied analytically, numerically, and experimentally. The obstacles discussed here include a flat strip, aligned with the flow, inclined or transverse to the flow and a horizontal cylinder. In the flow pattern, transient and attached (lee internal waves, downstream wakes with submerged interfaces and vortices, soaring singular interfaces, soaring vortices and vortex systems are distinguished. New components of laminar flow past a horizontally towed strip are presented. Fine transverse streaky structures on the strip in the downstream wake were visualized. Soaring isolated interfaces, which are internal boundary layers forming inside the downstream attached wave field past bluff bodies were observed. With increasing of the body velocity a vortex pair was formed directly at the leading edge of this interface.

  17. Hydrodynamic discrimination of wakes caused by objects of different size or shape in a harbour seal (Phoca vitulina)

    DEFF Research Database (Denmark)

    Wieskotten, S.; Mauck, B.; Miersch, L.

    2011-01-01

    Harbour seals can use their mystacial vibrissae to detect and track hydrodynamic wakes. We investigated the ability of a harbour seal to discriminate objects of different size or shape by their hydrodynamic signature and used particle image velocimetry to identify the hydrodynamic parameters...... to the arrangement of the vortices. We tested whether the seal used highest velocities, the steepness of the gradients and the spatial extension of the wake in a second set of experiments by varying moving speed and paddle size, respectively. The subject was still able to discriminate between the respective object...... that a seal may be using to do so. Hydrodynamic trails were generated by different sized or shaped paddles that were moved in the calm water of an experimental box to produce a characteristic signal. In a two-alternative forced-choice procedure the blindfolded subject was able to discriminate size differences...

  18. The Turbulent/Non-Turbulent Interface Bounding a Far-Wake

    Science.gov (United States)

    Bisset, David K.; Hunt, Julian C. R.; Rogers, Michael M.; Kwak, Dochan (Technical Monitor)

    2000-01-01

    The velocity fields of a turbulent wake behind a flat plate obtained from the direct numerical simulations of Moser et al. are used to study the structure of the flow in the intermittent zone where there are, alternately, regions of fully turbulent flow and non-turbulent velocity fluctuations either side of a thin randomly moving interface. Comparisons are made with a wake that is 'forced' by amplifying initial velocity fluctuations. There is also a random temperature field T in the flow; T varies between constant values of 0.0 and 1.0 on the sides of the wake. The value of the Reynolds number based on the centreplane mean velocity defect and halfwidth b of the wake is Re approx. = 2000. It is found that the thickness of the continuous interface is about equal to 0.07b, whereas the amplitude of fluctuations of the instantaneous interface displacement y(sub I)(t) is an order of magnitude larger, being about 0.5b. This explains why the mean statistics of vorticity in the intermittent zone can be calculated in terms of the probability distribution of y(sub I) and the instantaneous discontinuity in vorticity across the interface. When plotted as functions of y - y(sub I), the conditional mean velocity (U) and temperature (T) profiles show sharp jumps Delta(U) and Delta(T) at the interface adjacent to a thick zone where (U) and (T) vary much more slowly. Statistics for the vorticity and velocity variances, available in such detail only from DNS data, show how streamwise and spanwise components of vorticity are generated by vortex stretching in the bulges of the interface. Flow fields around the interface, analyzed in terms of the local streamline pattern, confirm previous results that the advancement of the vortical interface into the irrotational flow is driven by large-scale eddy motion. It is argued that because this is an inviscid mechanism the entrainment process is not sensitive to the value of Re, and that small-scale nibbling only plays a subsidiary role. While

  19. A family of vortex wakes generated by a thrush nightingale in free flight in a wind tunnel over its entire natural range of flight speeds.

    Science.gov (United States)

    Spedding, G R; Rosén, M; Hedenström, A

    2003-07-01

    In view of the complexity of the wing-beat kinematics and geometry, an important class of theoretical models for analysis and prediction of bird flight performance entirely, or almost entirely, ignores the action of the wing itself and considers only the resulting motions in the air behind the bird. These motions can also be complicated, but some success has previously been recorded in detecting and measuring relatively simple wake structures that can sometimes account for required quantities used to estimate aerodynamic power consumption. To date, all bird wakes, measured or presumed, seem to fall into one of two classes: the closed-loop, discrete vortex model at low flight speeds, and the constant-circulation, continuous vortex model at moderate to high speeds. Here, novel and accurate quantitative measurements of velocity fields in vertical planes aligned with the freestream are used to investigate the wake structure of a thrush nightingale over its entire range of natural flight speeds. At most flight speeds, the wake cannot be categorised as one of the two standard types, but has an intermediate structure, with approximations to the closed-loop and constant-circulation models at the extremes. A careful accounting for all vortical structures revealed with the high-resolution technique permits resolution of the previously unexplained wake momentum paradox. All the measured wake structures have sufficient momentum to provide weight support over the wingbeat. A simple model is formulated and explained that mimics the correct, measured balance of forces in the downstroke- and upstroke-generated wake over the entire range of flight speeds. Pending further work on different bird species, this might form the basis for a generalisable flight model.

  20. Numerical Analysis of the Unsteady Propeller Performance in the Ship Wake Modified By Different Wake Improvement Devices

    Directory of Open Access Journals (Sweden)

    Bugalski Tomasz

    2014-10-01

    Full Text Available The paper presents the summary of results of the numerical analysis of the unsteady propeller performance in the non-uniform ship wake modified by the different wake improvement devices. This analysis is performed using the lifting surface program DUNCAN for unsteady propeller analysis. Te object of the analysis is a 7000 ton chemical tanker, for which four different types of the wake improvement devices have been designed: two vortex generators, a pre-swirl stator, and a boundary layer alignment device. These produced five different cases of the ship wake structure: the original hull and hull equipped alternatively with four wake improvement devices. Two different propellers were analyzed in these five wake fields, one being the original reference propeller P0 and the other - a specially designed, optimized propeller P3. Te analyzed parameters were the pictures of unsteady cavitation on propeller blades, harmonics of pressure pulses generated by the cavitating propellers in the selected points and the fluctuating bearing forces on the propeller shaft. Some of the calculated cavitation phenomena were confronted with the experimental. Te objective of the calculations was to demonstrate the differences in the calculated unsteady propeller performance resulting from the application of different wake improvement devices. Te analysis and discussion of the results, together with the appropriate conclusions, are included in the paper.

  1. Beyond the knowledge deficit

    DEFF Research Database (Denmark)

    Hansen, Janus Staffan; Holm, Lotte; Frewer, Lynn

    2003-01-01

    The paper reviews psychological and social scientific research on lay attitudes to food risks. Many experts (scientists, food producers and public health advisors) regard public unease about food risks as excessive. This expert-lay discrepancy is often attributed to a 'knowledge deficit' among lay...... people. However, much research in psychology and sociology suggests that lay risk assessments are complex, situationally sensitive expressions of personal value systems. The paper is organised around four themes: risk perception, the communication of risk, lay handling of risk, and public trust...... in institutions and experts. It suggests that an interdisciplinary, contextualised and psychologically sound approach to the study of risk is needed....

  2. Neurofibromatozis and Attention Deficit

    Directory of Open Access Journals (Sweden)

    Mehmet ERYILMAZ et al.

    2010-05-01

    Full Text Available Neurofibromatosis type VI, a disease characterized by the presence of café-au-lait spots withoutthe presence of neurofibromas typically present in neurofibromatosis, as well as cognitivefunction and speech problems, often shows neurological involvement. We describe a case of a14-year-old child who has speech problems and isolated cafè-au-lait macules. We performedan IQ test on him and he scored 70 points. His problems started when he was approximately 5years old (school age. He was diagnosed with attention deficit disorder syndrome withouthyperactivity after neuropsychiatric investigation. We reported this case to improve recognitionof NF VI in children who have cognitive function problems.

  3. Superluminal Recession Velocities

    CERN Document Server

    Davis, T M; Davis, Tamara M.; Lineweaver, Charles H.

    2000-01-01

    Hubble's Law, v=HD (recession velocity is proportional to distance), is a theoretical result derived from the Friedmann-Robertson-Walker metric. v=HD applies at least as far as the particle horizon and in principle for all distances. Thus, galaxies with distances greater than D=c/H are receding from us with velocities greater than the speed of light and superluminal recession is a fundamental part of the general relativistic description of the expanding universe. This apparent contradiction of special relativity (SR) is often mistakenly remedied by converting redshift to velocity using SR. Here we show that galaxies with recession velocities faster than the speed of light are observable and that in all viable cosmological models, galaxies above a redshift of three are receding superluminally.

  4. Oculomotor Deficits after Chemotherapy in Childhood.

    Directory of Open Access Journals (Sweden)

    Einar-Jón Einarsson

    Full Text Available Advances in the diagnosis and treatment of pediatric malignancies have substantially increased the number of childhood cancer survivors. However, reports suggest that some of the chemotherapy agents used for treatment can cross the blood brain barrier which may lead to a host of neurological symptoms including oculomotor dysfunction. Whether chemotherapy at young age causes oculomotor dysfunction later in life is unknown. Oculomotor performance was assessed with traditional and novel methods in 23 adults (mean age 25.3 years, treatment age 10.2 years treated with chemotherapy for a solid malignant tumor not affecting the central nervous system. Their results were compared to those from 25 healthy, age-matched controls (mean age 25.1 years. Correlation analysis was performed between the subjective symptoms reported by the chemotherapy treated subjects (CTS and oculomotor performance. In CTS, the temporal control of the smooth pursuit velocity (velocity accuracy was markedly poorer (p<0.001 and the saccades had disproportionally shorter amplitude than normal for the associated saccade peak velocity (main sequence (p = 0.004, whereas smooth pursuit and saccade onset times were shorter (p = 0.004 in CTS compared with controls. The CTS treated before 12 years of age manifested more severe oculomotor deficits. CTS frequently reported subjective symptoms of visual disturbances (70%, unsteadiness, light-headedness and that things around them were spinning or moving (87%. Several subjective symptoms were significantly related to deficits in oculomotor performance. To conclude, chemotherapy in childhood or adolescence can result in severe oculomotor dysfunctions in adulthood. The revealed oculomotor dysfunctions were significantly related to the subjects' self-perception of visual disturbances, dizziness, light-headedness and sensing unsteadiness. Assessments of oculomotor function may, thus, offer an objective method to track and rate the level of

  5. Extreme Velocity Wind Sensor

    Science.gov (United States)

    Perotti, Jose; Voska, Ned (Technical Monitor)

    2002-01-01

    This presentation provides an overview of the development of new hurricane wind sensor (Extreme Velocity Wind Sensor) for the Kennedy Space Center (KSC) which is designed to withstand winds of up to three hundred miles an hour. The proposed Extreme Velocity Wind Sensor contains no moveable components that would be exposed to extreme wind conditions. Topics covered include: need for new hurricane wind sensor, conceptual design, software applications, computational fluid dynamic simulations of design concept, preliminary performance tests, and project status.

  6. Circadian Sleep-Wake Rhythm of Older Adults with Intellectual Disabilities

    Science.gov (United States)

    Maaskant, Marijke; van de Wouw, Ellen; van Wijck, Ruud; Evenhuis, Heleen M.; Echteld, Michael A.

    2013-01-01

    The circadian sleep-wake rhythm changes with aging, resulting in a more fragmented sleep-wake pattern. In individuals with intellectual disabilities (ID), brain structures regulating the sleep-wake rhythm might be affected. The aims of this study were to compare the sleep-wake rhythm of older adults with ID to that of older adults in the general…

  7. Wake Vortex Inverse Model User's Guide

    Science.gov (United States)

    Lai, David; Delisi, Donald

    2008-01-01

    NorthWest Research Associates (NWRA) has developed an inverse model for inverting landing aircraft vortex data. The data used for the inversion are the time evolution of the lateral transport position and vertical position of both the port and starboard vortices. The inverse model performs iterative forward model runs using various estimates of vortex parameters, vertical crosswind profiles, and vortex circulation as a function of wake age. Forward model predictions of lateral transport and altitude are then compared with the observed data. Differences between the data and model predictions guide the choice of vortex parameter values, crosswind profile and circulation evolution in the next iteration. Iterations are performed until a user-defined criterion is satisfied. Currently, the inverse model is set to stop when the improvement in the rms deviation between the data and model predictions is less than 1 percent for two consecutive iterations. The forward model used in this inverse model is a modified version of the Shear-APA model. A detailed description of this forward model, the inverse model, and its validation are presented in a different report (Lai, Mellman, Robins, and Delisi, 2007). This document is a User's Guide for the Wake Vortex Inverse Model. Section 2 presents an overview of the inverse model program. Execution of the inverse model is described in Section 3. When executing the inverse model, a user is requested to provide the name of an input file which contains the inverse model parameters, the various datasets, and directories needed for the inversion. A detailed description of the list of parameters in the inversion input file is presented in Section 4. A user has an option to save the inversion results of each lidar track in a mat-file (a condensed data file in Matlab format). These saved mat-files can be used for post-inversion analysis. A description of the contents of the saved files is given in Section 5. An example of an inversion input

  8. Wind Turbine Wake-Redirection Control at the Fishermen's Atlantic City Windfarm: Preprint

    Energy Technology Data Exchange (ETDEWEB)

    Churchfield, M.; Fleming, P.; Bulder, B.; White, S.

    2015-05-06

    In this paper, we will present our work towards designing a control strategy to mitigate wind turbine wake effects by redirecting the wakes, specifically applied to the Fishermen’s Atlantic City Windfarm (FACW), proposed for deployment off the shore of Atlantic City, New Jersey. As wind turbines extract energy from the air, they create low-speed wakes that extend behind them. Full wake recovery Full wake recovery to the undisturbed wind speed takes a significant distance. In a wind energy plant the wakes of upstream turbines may travel downstream to the next row of turbines, effectively subjecting them to lower wind speeds, meaning these waked turbines will produce less power.

  9. Improvement of a near wake model for trailing vorticity

    DEFF Research Database (Denmark)

    Pirrung, Georg; Hansen, Morten Hartvig; Aagaard Madsen, Helge

    2014-01-01

    to temporal discretization, both regarding numerical stability and quality of the results. The modified near wake model is coupled to an aerodynamics model, which consists of a blade element momentum model with dynamic inflow for the far wake and a 2D shed vorticity model that simulates the unsteady buildup......A near wake model, originally proposed by Beddoes, is further developed. The purpose of the model is to account for the radially dependent time constants of the fast aerodynamic response and to provide a tip loss correction. It is based on lifting line theory and models the downwash due to roughly...... the first 90 degrees of rotation. This restriction of the model to the near wake allows for using a computationally efficient indicial function algorithm. The aim of this study is to improve the accuracy of the downwash close to the root and tip of the blade and to decrease the sensitivity of the model...

  10. Field Test of Wake Steering at an Offshore Wind Farm

    Energy Technology Data Exchange (ETDEWEB)

    Fleming, Paul; Annoni, Jennifer; Shah, Jigar J.; Wang, Linpeng; Ananthan, Shreyas; Zhang, Zhijun; Hutchings, Kyle; Wang, Peng; Chen, Weiguo; Chen, Lin

    2017-02-06

    In this paper, a field test of wake steering control is presented. The field test is the result of a collaboration between the National Renewable Energy Laboratory (NREL) and Envision Energy, a smart energy management company and turbine manufacturer. In the campaign, an array of turbines within an operating commercial offshore wind farm in China have the normal yaw controller modified to implement wake steering according to a yaw control strategy. The strategy was designed using NREL wind farm models, including a computational fluid dynamics model, SOWFA, for understanding wake dynamics and an engineering model, FLORIS, for yaw control optimization. Results indicate that, within the certainty afforded by the data, the wake-steering controller was successful in increasing power capture, by amounts similar to those predicted from the models.

  11. Effects of viscoelasticity in the high Reynolds number cylinder wake

    KAUST Repository

    Richter, David

    2012-01-16

    At Re = 3900, Newtonian flow past a circular cylinder exhibits a wake and detached shear layers which have transitioned to turbulence. It is the goal of the present study to investigate the effects which viscoelasticity has on this state and to identify the mechanisms responsible for wake stabilization. It is found through numerical simulations (employing the FENE-P rheological model) that viscoelasticity greatly reduces the amount of turbulence in the wake, reverting it back to a state which qualitatively appears similar to the Newtonian mode B instability which occurs at lower Re. By focusing on the separated shear layers, it is found that viscoelasticity suppresses the formation of the Kelvin-Helmholtz instability which dominates for Newtonian flows, consistent with previous studies of viscoelastic free shear layers. Through this shear layer stabilization, the viscoelastic far wake is then subject to the same instability mechanisms which dominate for Newtonian flows, but at far lower Reynolds numbers. © Copyright Cambridge University Press 2012.

  12. Noise, vibration and changes in wakefulness during helicopter flight.

    Science.gov (United States)

    Landström, U; Löfstedt, P

    1987-02-01

    The investigation was carried out in cooperation with the helicopter school AF 1 in Boden. Measurements were made in two different types of helicopter, Hkp 3 and Hkp 6. Three different parameters were recorded during the flights: noise, vibrations, and wakefulness. Noise and vibration exposures were mainly correlated to the main rotor energy and frequency. Both types of exposure were dominated by lower frequencies, below 10 Hz. Analyses of wakefulness during long-distance flights, about 4 h, and short-distance flights, less than 2 h, were based on EEG and EKG recordings. As expected the level of wakefulness was influenced by the stress upon the pilots. Take-offs and landings, as well as unexpected events during the flight, were correlated to an increased level of wakefulness. In some cases flying was correlated to a gradual increase of weariness. The correlation between weariness, types of flying, and the external environmental factors of noise and vibration, is also discussed.

  13. Disruption of Cosmic String Wakes by Gaussian Fluctuations

    CERN Document Server

    Brandenberger, Robert H; da Cunha, Disrael C N

    2015-01-01

    We study the stability of cosmic string wakes against the disruption by the dominant Gaussian fluctuations which are present in cosmological models. We find that for a string tension given by $G \\mu = 10^{-7}$ wakes remain locally stable until a redshift of $z = 6$, and for a value of $G \\mu = 10^{-14}$ they are stable beyond a redshift of $z = 20$. We study a global stability criterion which shows that wakes created by strings at times after $t_{eq}$ are identifiable up to the present time, independent of the value of $G \\mu$. Taking into account our criteria it is possible to develop strategies to search for the distinctive position space signals in cosmological maps which are induced by wakes.

  14. Wake Vortex Lidar Monte Carlo Simulation and Visualization Tool Project

    Data.gov (United States)

    National Aeronautics and Space Administration — Light Detection and Ranging (LIDAR) sensors have been successfully demonstrated and implemented capabilities to detect and measure wake vortices in and around the...

  15. SIMULATION OF WAKE VORTEX AIRCRAFT IN GROUND EFFECT

    Directory of Open Access Journals (Sweden)

    Pamfil ŞOMOIAG

    2011-03-01

    Full Text Available The problem developed in this paper is encountered in airplane aerodynamics and concernsthe influence of long life longitudinal wake vortices generated by wing tips or by external obstaclessuch as reactors or landing gears. More generally it concerns 3D bodies of finite extension in crossflow. At the edge of such obstacles, longitudinal vortices are created by pressure differences inside theboundary layers and rotate in opposite senses. It can constitute a danger to another aircraft that fliesin this wake, especially during takeoff and landing. In this case the wake vortex trajectories andstrengths are altered by ground interactions. This study presents the results of a Large EddySimulation of wake vortex in ground effect providing the vorticity flux behavior.

  16. In Situ Wake Vortex Encounter Detection and Reporting System Project

    Data.gov (United States)

    National Aeronautics and Space Administration — Wake vortices are a critical constraint to aircraft separation and therefore airportal throughput, which is already at or near capacity at many major airports in the...

  17. Proceedings of the ENDOW Workshop 'Offshore Wakes: measurements and Modelling'

    DEFF Research Database (Denmark)

    2002-01-01

    The papers presented in these Proceedings are the result of a workshop held at Risø National Laboratory on March 7 and 8 2002 entitled 'Offshore Wakes: Measurements and Modelling'. The Workshop was arranged to showcase results of the European Communitysupported project 'Efficient Development...... of Offshore Windfarms (ENDOW)' and featured a series of eight presentations discussing modelling of wakes and boundary layers, existing and new measurements from offshore wind farms and how these will be linked ina new design tool and applied at planned offshore wind farms. In addition seven invited...... presentations covered a range of relevant topics from analytical approaches to fatigue and extreme loads in wind turbine clusters, current wake modelling in WAsP,optimising power production at Arklow Bank, Particle Image Velocimetry study of a wind turbine wake in a yaw, offshore wind measurements...

  18. Wake redirection: comparison of analytical, numerical and experimental models

    Science.gov (United States)

    Wang, Jiangang; Bottasso, Carlo L.; Campagnolo, Filippo

    2016-09-01

    This paper focuses on wake redirection techniques for wind farm control. Two control strategies are investigated: yaw misalignment and cyclic pitch control. First, analytical formulas are derived for both techniques, with the goal of providing a simple physical interpretation of the behavior of the two methods. Next, more realistic results are obtained by numerical simulations performed with CFD and by experiments conducted with scaled wind turbine models operating in a boundary layer wind tunnel. Comparing the analytical, numerical and experimental models allows for a cross-validation of the results and a better understanding of the two wake redirection techniques. Results indicate that yaw misalignment is more effective than cyclic pitch control in displacing the wake laterally, although the latter may have positive effects on wake recovery.

  19. Wake Island Tsunami Forecast Grids for MOST Model

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The Wake Island Forecast Model Grids provides bathymetric data strictly for tsunami inundation modeling with the Method of Splitting Tsunami (MOST) model. MOST is a...

  20. In Situ Wake Vortex Encounter Detection and Reporting System Project

    Data.gov (United States)

    National Aeronautics and Space Administration — Wake vortices are a critical constraint to aircraft separation and therefore airportal throughput, which is already at or near capacity at many major airports in the...