Analysis of turbulent boundary layers
Cebeci, Tuncer
2012-01-01
Analysis of Turbulent Boundary Layers focuses on turbulent flows meeting the requirements for the boundary-layer or thin-shear-layer approximations. Its approach is devising relatively fundamental, and often subtle, empirical engineering correlations, which are then introduced into various forms of describing equations for final solution. After introducing the topic on turbulence, the book examines the conservation equations for compressible turbulent flows, boundary-layer equations, and general behavior of turbulent boundary layers. The latter chapters describe the CS method for calculati
Cyclone separator having boundary layer turbulence control
International Nuclear Information System (INIS)
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator
Cyclone separator having boundary layer turbulence control
Krishna, Coimbatore R.; Milau, Julius S.
1985-01-01
A cyclone separator including boundary layer turbulence control that is operable to prevent undue build-up of particulate material at selected critical areas on the separator walls, by selectively varying the fluid pressure at those areas to maintain the momentum of the vortex, thereby preventing particulate material from inducing turbulence in the boundary layer of the vortical fluid flow through the separator.
LDV measurements of turbulent baroclinic boundary layers
Energy Technology Data Exchange (ETDEWEB)
Neuwald, P.; Reichenbach, H. [Fraunhofer-Institut fuer Kurzzeitdynamik - Ernst-Mach-Institut (EMI), Freiburg im Breisgau (Germany); Kuhl, A.L. [Lawrence Livermore National Lab., El Segundo, CA (United States)
1993-07-01
Described here are shock tube experiments of nonsteady, turbulent boundary layers with large density variations. A dense-gas layer was created by injecting Freon through the porous floor of the shock tube. As the shock front propagated along the layer, vorticity was created at the air-Freon interface by an inviscid, baroclinic mechanism. Shadow-schlieren photography was used to visualize the turbulent mixing in this baroclinic boundary layer. Laser-Doppler-Velocimetry (LDV) was used to measure the streamwise velocity histories at 14 heights. After transition, the boundary layer profiles may be approximated by a power-law function u {approximately} u{sup {alpha}} where {alpha} {approx_equal} 3/8. This value lies between the clean flat plate value ({alpha} = 1/7) and the dusty boundary layer value ({alpha} {approx_equal} 0.7), and is controlled by the gas density near the wall.
Geometric invariance of compressible turbulent boundary layers
Bi, Wei-Tao; Wu, Bin; She, Zhen-Su; Hussain, Fazle
2015-11-01
A symmetry based approach is applied to analyze the mean velocity and temperature fields of compressible, flat plate turbulent boundary layers (CTBL). A Reynolds stress length scale and a turbulent heat flux length scale are identified to possess the same defect scaling law in the CTBL bulk, which is solely owing to the constraint of the wall to the geometry of the wall-attached eddies, but invariant to compressibility and wall heat transfer. This invariance is called the geometric invariance of CTBL eddies and is likely the origin of the Mach number invariance of Morkovin's hypothesis, as well as the similarity of energy and momentum transports. A closure for the turbulent transport by using the invariant lengths is attainted to predict the mean velocity and temperature profiles in the CTBL bulk- superior to the van Driest transformation and the Reynolds analogy based relations for its sound physics and higher accuracy. Additionally, our approach offers a new understanding of turbulent Prandtl number.
Modelling turbulent spots in swept boundary layers
International Nuclear Information System (INIS)
Highlights: • A linear perturbation method can capture the important flow features within a turbulent spot. • The horseshoe vortex in the perturbed velocity field is the dominant flow feature. • Sweep leads to skewing of the turbulent spot and calmed region. • The effects of pressure gradient are generally reduced by sweep. -- Abstract: A computational technique is presented for determining the fully 3-d viscid unsteady perturbation to a non-developing laminar swept boundary layer. For zero pressure gradient, unswept boundary layers, the perturbation method reveals a strongly three dimensional flow within the turbulent spot and its associated calmed region which is very similar to that observed in experiments and full DNS calculations. The perturbation method cannot predict turbulent motion but nevertheless provides a simple yet accurate means of studying and understanding the development of turbulent spot geometry. The most influential flow feature is the horseshoe vortex observed in the fluctuation velocity field, which is responsible for delivering the fluid found in the calmed region between its trailing legs. The upwards flow around the outer periphery of the vortex is also responsible for delivering low momentum fluid to the spot, but additional high momentum fluid also enters the spot from its rear through the downward sweeping motion of fluid between the vortex legs. The effect of an adverse streamwise pressure gradient is to increase the size of the spot and calmed region whereas a favourable pressure gradient has the opposite effect. When sweep is introduced to the boundary layer the spot is skewed for all non-zero pressure gradients, but the changes in size of the spot and calmed region due to pressure gradient are reduced. For favourable pressure gradients the skew increases monotonically with sweep, but this is not the case for adverse pressure gradients where the effect of sweep is more complex
Analytic prediction for planar turbulent boundary layers
Chen, Xi
2016-01-01
Analytic predictions of mean velocity profile (MVP) and streamwise ($x$) development of related integral quantities are presented for flows in channel and turbulent boundary layer (TBL), based on a symmetry analysis of eddy length and total stress. Specific predictions are the friction velocity $u_\\tau$: ${ U_e/u_\\tau }\\approx 2.22\\ln Re_x+2.86-3.83\\ln(\\ln Re_x)$; the boundary layer thickness $\\delta_e$: $x/\\delta_e \\approx 7.27\\ln Re_x-5.18-12.52\\ln(\\ln Re_x)$; the momentum thickness Reynolds number: $Re_x/Re_\\theta=4.94[{(\\ln {{\\mathop{\\rm Re}\
Local boundary layer scales in turbulent Rayleigh-Benard convection
Scheel, Janet D
2014-01-01
We compute fully local boundary layer scales in three-dimensional turbulent Rayleigh-Benard convection. These scales are directly connected to the highly intermittent fluctuations of the fluxes of momentum and heat at the isothermal top and bottom walls and are statistically distributed around the corresponding mean thickness scales. The local boundary layer scales also reflect the strong spatial inhomogeneities of both boundary layers due to the large-scale, but complex and intermittent, circulation that builds up in closed convection cells. Similar to turbulent boundary layers, we define inner scales based on local shear stress which can be consistently extended to the classical viscous scales in bulk turbulence, e.g. the Kolmogorov scale, and outer scales based on slopes at the wall. We discuss the consequences of our generalization, in particular the scaling of our inner and outer boundary layer thicknesses and the resulting shear Reynolds number with respect to Rayleigh number. The mean outer thickness s...
Numerical simulation of turbulent atmospheric boundary layer flows
Energy Technology Data Exchange (ETDEWEB)
Bennes, L.; Bodnar, T.; Kozel, K.; Sladek, I. [Czech Technical Univ., Prague (Czech Republic). Dept. of Technical Mathematics; Fraunie, P. [Universite Toulon et du Var, La Garde (France). Lab. de Sondages Electromagnetiques de l' Environment Terrestre
2001-07-01
The work deals with the numerical solution of viscous turbulent steady flows in the atmospheric boundary layer including pollution propagation. For its description we use two different mathematical models: - a model based on the Reynolds averaged Navier-Stokes equations for incompressible flows - a model based on a system of boundary layer equations. These systems are completed by two transport equations for the concentration of passive pollutants and the potential temperature in conservative form, respectively, and by an algebraic turbulence model. (orig.)
Theoretical skin-friction law in a turbulent boundary layer
International Nuclear Information System (INIS)
We study transitional and turbulent boundary layers using a turbulent velocity profile equation recently derived from the Navier-Stokes-alpha and Leray-alpha models. From this equation we obtain a theoretical prediction of the skin-friction coefficient in a wide range of Reynolds numbers based on momentum thickness, and deduce the maximal value of cfmax=0.0063 for turbulent velocity profiles. A two-parameter family of solutions to the equation matches experimental data in the transitional boundary layers with different free-stream turbulence intensity, while one-parameter family of solutions, obtained using our skin-friction coefficient law, matches experimental data in the turbulent boundary layer for moderately large Reynolds numbers
Turbulent spots detection during boundary layer by-pass transition
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Elsner, W.; Mazur, Oton; Uruba, Václav; Wysocki, M.
-, č. 80 (2009), s. 16-19. ISSN N R&D Projects: GA AV ČR(CZ) IAA200760614; GA MŠk MEB050810 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulent spot * boundary layer * by-pass transition * turbulent spot detection Subject RIV: BK - Fluid Dynamics
DNS of compressible turbulent boundary layer around a sharp cone
Institute of Scientific and Technical Information of China (English)
2008-01-01
Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh- order up-wind biased finite difference scheme and sixth-order central difference scheme. The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch. The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer. Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow. The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations. The turbulent energy equation in the cylindrical coordinate is given and turbulent en-ergy budget is studied. The computed results show that the effect of circumferen-tial curvature on turbulence characteristics is not obvious.
DNS of compressible turbulent boundary layer around a sharp cone
Institute of Scientific and Technical Information of China (English)
LI XinLiang; FU DeXun; MA YanWen
2008-01-01
Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh-order up-wind biased finite difference scheme and sixth-order central difference scheme.The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch.The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer,Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow.The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations,The turbulent energy equation in the cylindrical coordinate is given and turbulent en-ergy budget is studied.The computed results show that the effect of circumferen-tial curvature on turbulence characteristics is not obvious.
On the growth of turbulent regions in laminar boundary layers
Gad-El-hak, M.; Riley, J. J.; Blackwelder, R. F.
1981-01-01
Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a turbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.
Characteristics of turbulent spots in transitional boundary layers
Marxen, Olaf; Zaki, Tamer
2015-11-01
The laminar-turbulent transition process in a flat-plate boundary layer beneath free-stream turbulence takes place through the inception and spreading of confined patches of turbulence in an otherwise laminar flow. These patches, also referred to as turbulent spots, result from a secondary instability of the Klebanoff streaks in the pre-transitional region. The dynamics of turbulence in the spots are investigated by analyzing data sets obtained from direct numerical simulations. Conditionally-averaged and spot-ensemble-averaged statistics are evaluated and describe the flow in the intermittent transition zone. Both mean-flow and disturbance root mean square levels obtained from conditional averaging agree very well with results for fully turbulent flows, in particular near the wall and at high intermittency levels. At relatively low intermittency, the spatial inhomogeneity of turbulence within the spots is important, and is examined using ensemble averaging of turbulent patches that have comparable volume and a similar streamwise location.
Dynamic Boundary Layer Properties in Turbulent Thermal Convection
Xia, Ke-Qing; Har Cheung, Yin; Sun, Chao
2004-11-01
We report an experimental study on the properties of the velocity and temperature boundary layers in turbulent thermal convection in a rectangular-shaped box over a range of Rayleigh numbers and at a constant Prandtl number. Velocity components both parallel and perpendicular to the conducting plate are measured simultaneously using the PIV technique. Our results show that, for the given geometry of the cell, the velocity boundary layer at the conduction plate is of a Blasius type, i.e. the boundary layer thickness δv scales with the Reynolds number Re as δv ˜ Re-1/2. The measurement further reveals that, at the velocity boundary layer, the turbulent (Reynolds) shear tress becomes larger than the viscous shear stress when Ra reaches 1-2×10^10, indicating that the boundary layer becomes turbulent for Ra >10^10. The viscous dissipation rate calculated based on the measured velocity field shows that it is dominated by contribution from the bulk over that from the boundary layer.
Boundary Layer Turbulence Index: Progress and Recent Developments
Pryor, Kenneth L
2008-01-01
A boundary layer turbulence index (TIBL) product has been developed to assess the potential for turbulence in the lower troposphere, generated using RUC-2 numerical model data. The index algorithm approximates boundary layer turbulent kinetic energy by parameterizing vertical wind shear, responsible for mechanical production of TKE, and kinematic heat flux, parameterized by the vertical temperature lapse rate and responsible for buoyant production of TKE. Validation for the TIBL product has been conducted for selected nonconvective wind events during the 2008 winter season over the Idaho National Laboratory mesonet domain. This paper presents studies of four significant wind events between December 2007 and February 2008 over southeastern Idaho. Based on the favorable results highlighted from validation statistics and in the case studies, the RUC TIBL product has demonstrated operational utility in assessing turbulence hazards to low-flying aircraft and ground transportation, and in the assessment of wildfire...
Turbulence Scales Simulations in Atmospheric Boundary Layer Wind Tunnels
Elena-Carmen Teleman; Radu Silion; Elena Axinte; Radu Pescaru
2008-01-01
The simulation of the air flow over models in atmospheric boundary layer tunnels is a research domain based on advanced scientific technologies imposed by the necessity of studying the turbulent fluid movements in the proximity of the Earth’s surface. The experiment presented herein is developed in the wind tunnel from the Laboratory of Structural Aerodynamics of the Faculty of Civil Engineering and Building Services in Iassy. Measurements necessary for the determination of the turbulence sca...
Two Phases of Coherent Structure Motions in Turbulent Boundary Layer
Institute of Scientific and Technical Information of China (English)
LIU Jian-Hua; JIANG Nan
2007-01-01
Two phases of coherent structure motion are acquired after obtaining conditional phase-averaged waveforms for longitudinal velocity of coherent structures in turbulent boundary layer based on Harr wavelet transfer. The correspondences of the two phases to the two processes (i.e. ejection and sweep) during a burst are determined.
Crosshatch roughness distortions on a hypersonic turbulent boundary layer
Peltier, S. J.; Humble, R. A.; Bowersox, R. D. W.
2016-04-01
The effects of periodic crosshatch roughness (k+ = 160) on a Mach 4.9 turbulent boundary layer (Reθ = 63 000) are examined using particle image velocimetry. The roughness elements generate a series of alternating shock and expansion waves, which span the entire boundary layer, causing significant (up to +50% and -30%) variations in the Reynolds shear stress field. Evidence of the hairpin vortex organization of incompressible flows is found in the comparative smooth-wall boundary layer case (Reθ = 47 000), and can be used to explain several observations regarding the rough-wall vortex organization. In general, the rough-wall boundary layer near-wall vortices no longer appear to be well-organized into streamwise-aligned packets that straddle relatively low-speed regions like their smooth-wall counterpart; instead, they lean farther away from the wall, become more spatially compact, and their populations become altered. In the lower half of the boundary layer, the net vortex swirling strength and outer-scaled Reynolds stresses increase relative to the smooth-wall case, and actually decrease in the outer half of the boundary layer, as ejection and entrainment processes are strengthened and weakened in these two regions, respectively. A spectral analysis of the data suggests a relative homogenizing of the most energetic scales near Λ = ˜ 0.5δ across the rough-wall boundary layer.
Second Law Analysis of the Turbulent Flat Plate Boundary Layer
Directory of Open Access Journals (Sweden)
Dragos Isvoranu
2000-09-01
Full Text Available
Until now the second law analysis of turbulent flow relied only on the irreversibilities performed by the mean velocity and mean temperature gradients. Using the Reynolds decomposition of the volumetric entropy generation rate expression we found that the dissipation rates of both, turbulent kinetic energy and fluctuating temperature variance, also represent the irreversibilities of the flow. Applying the above results, the second law analysis of the turbulent boundary layer shows that the maximum values of the "mean motion irreversibilities" (generated by the mean velocity and mean temperature gradient are located at the wall, while the maximum values of the "turbulent irreversibilities" (performed by the dissipation rate of turbulent kinetic energy and fluctuating temperature variance are located in the buffer sublayer. As a consequence, for a given location on the plate, the integral values of the "mean motion irreversibilities" are approximately constant and the "turbulent irreversibilities" grow up with the boundary layer thickness.
A parametric study of adverse pressure gradient turbulent boundary layers
International Nuclear Information System (INIS)
There are many open questions regarding the behaviour of turbulent boundary layers subjected to pressure gradients and this is confounded by the large parameter space that may affect these flows. While there have been many valuable investigations conducted within this parameter space, there are still insufficient data to attempt to reduce this parameter space. Here, we consider a parametric study of adverse pressure gradient turbulent boundary layers where we restrict our attention to the pressure gradient parameter, β, the Reynolds number and the acceleration parameter, K. The statistics analyzed are limited to the streamwise fluctuating velocity. The data show that the mean velocity profile in strong pressure gradient boundary layers does not conform to the classical logarithmic law. Moreover, there appears to be no measurable logarithmic region in these cases. It is also found that the large-scale motions scaling with outer variables are energised by the pressure gradient. These increasingly strong large-scale motions are found to be the dominant contributor to the increase in turbulence intensity (scaled with friction velocity) with increasing pressure gradient across the boundary layer.
Spatially developing turbulent boundary layer on a flat plate
Lee, J H; Hutchins, N; Monty, J P
2012-01-01
This fluid dynamics video submitted to the Gallery of Fluid motion shows a turbulent boundary layer developing under a 5 metre-long flat plate towed through water. A stationary imaging system provides a unique view of the developing boundary layer as it would form over the hull of a ship or fuselage of an aircraft. The towed plate permits visualisation of the zero-pressure-gradient turbulent boundary layer as it develops from the trip to a high Reynolds number state ($Re_\\tau \\approx 3000$). An evolving large-scale coherent structure will appear almost stationary in this frame of reference. The visualisations provide an unique view of the evolution of fundamental processes in the boundary layer (such as interfacial bulging, entrainment, vortical motions, etc.). In the more traditional laboratory frame of reference, in which fluid passes over a stationary body, it is difficult to observe the full evolution and lifetime of turbulent coherent structures. An equivalent experiment in a wind/water-tunnel would requ...
Direct numerical simulation of supersonic turbulent boundary layers
Guarini, Stephen
The objectives of this research were to develop a method by which the spatially developing compressible turbulent boundary layer could be simulated using a temporally developing numerical simulation and to study the physics of the compressible turbulent boundary layer. We take advantage of the technique developed by Spalart (1987, 1988) for the incompressible case. In this technique, it is recognized that the boundary layer exhibits slow growth in the streamwise direction, so the turbulence can be treated as approximately homogeneous in this direction. The slow growth is accounted for with a coordinate transformation and a multiple scale analysis. The result is a modified system of equations (Navier-Stokes plus some extra terms, which we call "slow growth terms") that are homogeneous in both the streamwise and spanwise directions and represent the state of the boundary layer at a given streamwise location (or, equivalently, a given thickness). The compressible Navier-Stokes equations are solved using a mixed Fourier and B-spline "spectral" method. The dependent variables are expanded in terms of a Fourier representation in the horizontal directions and a B-spline representation in the wall-normal direction. In the wall-normal direction non-reflecting boundary conditions are used at the freestream boundary, and zero-heat-flux no-slip boundary conditions are used at the wall. This combination of splines and Fourier methods produces a very accurate numerical method. Mixed implicit/explicit time discretization is used. Results are presented for a case with a Mach number of 2.5, and a Reynolds number, based on momentum integral thickness and wall viscosity, of Rsb{thetasp'} = 840. The results show that the van Driest transformed velocity satisfies the incompressible scalings and a narrow logarithmic region is obtained. The results for the turbulence intensities compare well with the incompressible simulations of Spalart. Pressure fluctuations are found to be higher than
A Cautionary Note on the Thermal Boundary Layer Similarity Scaling for the Turbulent Boundary Layer
Weyburne, David
2016-01-01
Wang and Castillo have developed empirical parameters for scaling the temperature profile of the turbulent boundary layer flowing over a heated wall in the paper X. Wang and L. Castillo, J. Turbul., 4, 1(2003). They presented experimental data plots that showed similarity type behavior when scaled with their new scaling parameters. However, what was actually plotted, and what actually showed similarity type behavior, was not the temperature profile but the defect profile formed by subtracting the temperature in the boundary layer from the temperature in the bulk flow. We show that if the same data and same scaling is replotted as just the scaled temperature profile, similarity is no longer prevalent. This failure to show both defect profile similarity and temperature profile similarity is indicative of false similarity. The nature of this false similarity problem is discussed in detail.
Numerical analysis of the turbulent natural convection boundary layer
International Nuclear Information System (INIS)
It is considered to be one of options of nuclear fuel cycle policies in Japan to store spent fuel before reprocessing. Then we have to evaluate of the thermal integrity for dry type cask storage system. But the turbulent natural convection boundary layer is a flow with relatively large fluctuations of velocity and temperature at low velocity, and measurements of turbulent quantities near the wall are especially difficult. So, the turbulent structure has not been elucidated. On the other hand, numerical analyses of natural convection using turbulence models have been developed. However, there are not the models which are suitable for prediction of natural convection exactly, so it's effective to analyze of direct numerical simulation (DNS). The propose of this study is to simulate (DNS) for buoyant flow as economical as possible. We calculate two different grid size to investigate to numerical accuracy. (author)
Acoustic Radiation From a Mach 14 Turbulent Boundary Layer
Zhang, Chao; Duan, Lian; Choudhari, Meelan M.
2016-01-01
Direct numerical simulations (DNS) are used to examine the turbulence statistics and the radiation field generated by a high-speed turbulent boundary layer with a nominal freestream Mach number of 14 and wall temperature of 0:18 times the recovery temperature. The flow conditions fall within the range of nozzle exit conditions of the Arnold Engineering Development Center (AEDC) Hypervelocity Tunnel No. 9 facility. The streamwise domain size is approximately 200 times the boundary-layer thickness at the inlet, with a useful range of Reynolds number corresponding to Re 450 ?? 650. Consistent with previous studies of turbulent boundary layer at high Mach numbers, the weak compressibility hypothesis for turbulent boundary layers remains applicable under this flow condition and the computational results confirm the validity of both the van Driest transformation and Morkovin's scaling. The Reynolds analogy is valid at the surface; the RMS of fluctuations in the surface pressure, wall shear stress, and heat flux is 24%, 53%, and 67% of the surface mean, respectively. The magnitude and dominant frequency of pressure fluctuations are found to vary dramatically within the inner layer (z/delta 0.< or approx. 0.08 or z+ < or approx. 50). The peak of the pre-multiplied frequency spectrum of the pressure fluctuation is f(delta)/U(sub infinity) approx. 2.1 at the surface and shifts to a lower frequency of f(delta)/U(sub infinity) approx. 0.7 in the free stream where the pressure signal is predominantly acoustic. The dominant frequency of the pressure spectrum shows a significant dependence on the freestream Mach number both at the wall and in the free stream.
Cebeci, Tuncer
2005-01-01
This second edition of our book extends the modeling and calculation of boundary-layer flows to include compressible flows. The subjects cover laminar, transitional and turbulent boundary layers for two- and three-dimensional incompressible and compressible flows. The viscous-inviscid coupling between the boundary layer and the inviscid flow is also addressed. The book has a large number of homework problems.
Direct numerical simulation of turbulent thermal boundary layers
Kong, Hojin; Choi, Haecheon; Lee, Joon Sik
2000-10-01
In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature fluctuations. It is shown that the behavior of the wall-normal heat flux is similar to that of the Reynolds shear stress, indicating close correlation between the streamwise velocity and temperature. The effect of different thermal boundary conditions at the wall on the near-wall turbulence statistics is also discussed.
Turbulent Boundary Layers in the Vicinity of Separation
Indinger, Thomas; Buschmann, Matthias H.; Gad-El-Hak, Mohamed
2004-11-01
There has been some controversy regarding the behavior of the mean velocity profile of turbulent boundary layers approaching separation. While a number of experiments show that the logarithmic law is sustained even under strong adverse-pressure-gradient and non-equilibrium conditions, other experiments and DNS results reveal that the mean velocity profile breaks down in the vicinity of separation. Measurements at TU Dresden of a decelerated, fully developed turbulent boundary layer over a smooth flat plate in a closed water channel show that the classical log law no longer describes the mean velocity in the overlap region after a certain fraction of the flow travels in the upstream direction. This finding is consistent with the physical explanation advanced by Dengel & Fernholz (J. Fluid Mech. 212, 1990) that the log law failure is caused by the first occurrence of reverse flow. Analyzing adverse-pressure-gradient turbulent boundary layer data from three independent groups, we demonstrate that the log law can be restored by replacing y^+ with a new variable depending both on the wall-normal coordinate and the reverse-flow parameter \\chi_w. This finding is of importance in cases where other complexities such as surface roughness or structured walls (riblets, dimples, etc.) are involved and a universal profile in inner variables is desired.
Turbulent thermal boundary layers subjected to severe acceleration
Araya, Guillermo; Castillo, Luciano
2013-11-01
Favorable turbulent boundary layers are flows of great importance in industry. Particularly, understanding the mechanisms of quasi-laminarization by means of a very strong favorable streamwise pressure gradient is indeed crucial in drag reduction and energy management applications. Furthermore, due to the low Reynolds numbers involved in the quasi-laminarization process, abundant experimental investigation can be found in the literature for the past few decades. However, several grey zones still remain unsolved, principally associated with the difficulties that experiments encounter as the boundary layer becomes smaller. In addition, little attention has been paid to the heat transfer in a quasi-laminarization process. In this investigation, DNS of spatially-developing turbulent thermal boundary layers with prescribed very strong favorable pressure gradients (K = 4 × 10-6) are performed. Realistic inflow conditions are prescribed based on the Dynamic Multi-scale Approach (DMA) [Araya et al. JFM, Vol. 670, pp. 581-605, 2011]. In this sense the flow carries the footprint of turbulence, particularly in the streamwise component of the Reynolds stresses.
Characteristics of turbulent boundary layer flow over algal biofilm
Murphy, Elizabeth; Barros, Julio; Schultz, Michael; Steppe, Cecily; Flack, Karen; Reidenbach, Matthew
2015-11-01
Algal biofilms are an important fouling community on ship hulls, with severe economic consequences due to drag-induced increases in fuel use and cleaning costs. Here, we characterize the boundary layer flow structure in turbulent flow over diatomaceous slime, a type of biofilm. Diatomaceous slime composed of three species of diatoms commonly found on ship hulls was grown on acrylic test plates under shear stress. The slime averages 1.6 mm in thickness and has a high density of streamers, which are flexible elongated growths with a length on the order of 1- 2 mm located at the top of the biofilm that interact with the flow. Fouled acrylic plates were placed in a water tunnel facility specialized for detailed turbulent boundary layer measurements. High resolution Particle Image Velocimetry (PIV) data are analyzed for mean velocity profile as well as local turbulent stresses and turbulent kinetic energy (TKE) production, dissipation and transport. Quadrant analysis is used to characterize the impact of the instantaneous events of Reynolds shear stress (RSS) in the flow. To investigate the coherence of the large-scale motion in the flow two-point correlation analysis is employed. Funding provided by the Office of Naval Research and the National Science Foundation.
Turbulent boundary-layer structure of flows over freshwater biofilms
Walker, J. M.; Sargison, J. E.; Henderson, A. D.
2013-12-01
The structure of the turbulent boundary-layer for flows over freshwater biofilms dominated by the diatom Tabellaria flocculosa was investigated. Biofilms were grown on large test plates under flow conditions in an Australian hydropower canal for periods up to 12 months. Velocity-profile measurements were obtained using LDV in a recirculating water tunnel for biofouled, smooth and artificially sandgrain roughened surfaces over a momentum thickness Reynolds number range of 3,000-8,000. Significant increases in skin friction coefficient of up to 160 % were measured over smooth-wall values. The effective roughnesses of the biofilms, k s, were significantly higher than their physical roughness measured using novel photogrammetry techniques and consisted of the physical roughness and a component due to the vibration of the biofilm mat. The biofilms displayed a k-type roughness function, and a logarithmic relationship was found between the roughness function and roughness Reynolds number based on the maximum peak-to-valley height of the biofilm, R t. The structure of the boundary layer adhered to Townsend's wall-similarity hypothesis even though the scale separation between the effective roughness height and the boundary-layer thickness was small. The biofouled velocity-defect profiles collapsed with smooth and sandgrain profiles in the outer region of the boundary layer. The Reynolds stresses and quadrant analysis also collapsed in the outer region of the boundary layer.
Turbulence Scales Simulations in Atmospheric Boundary Layer Wind Tunnels
Directory of Open Access Journals (Sweden)
Elena-Carmen Teleman
2008-01-01
Full Text Available The simulation of the air flow over models in atmospheric boundary layer tunnels is a research domain based on advanced scientific technologies imposed by the necessity of studying the turbulent fluid movements in the proximity of the Earth’s surface. The experiment presented herein is developed in the wind tunnel from the Laboratory of Structural Aerodynamics of the Faculty of Civil Engineering and Building Services in Iassy. Measurements necessary for the determination of the turbulence scales of the wind action in urban environment were conducted. The data obtained were processed and analyzed and interpreted with specific software. The results are used for a synthesis regarding the scales of turbulence of the model of flow and the actual accuracy of measurements. The paper presents some of the important elements of this synthesis.
Aeroelectric structures and turbulence in the atmospheric boundary layer
Directory of Open Access Journals (Sweden)
S. V. Anisimov
2013-10-01
Full Text Available Complex electrical measurements with the use of sodar data show that electric field pulsation analysis is useful for electrodynamics/turbulence monitoring under different conditions. In particular, the number of aeroelectric structures (AES generated per hour is a convenient measure of the turbulence intensity. During convectively unstable periods, as many as 5–10 AES form per hour. Under stable conditions, AES occasionally form as well, indicating the appearance of occasional mixing events reflected in the electric field perturbations. AES magnitudes under stable conditions are relatively small, except in special cases such as high humidity and fog. The analysis of electric field (EF spectra gives additional useful information on the parameters of the atmospheric boundary layer and its turbulence. A rather sharp change in the spectrum slope takes place in the vicinity of 0.02 Hz under stable conditions. The characteristic slope of the spectrum and its change are reproduced in a simple model of EF formation.
The large Reynolds number - Asymptotic theory of turbulent boundary layers.
Mellor, G. L.
1972-01-01
A self-consistent, asymptotic expansion of the one-point, mean turbulent equations of motion is obtained. Results such as the velocity defect law and the law of the wall evolve in a relatively rigorous manner, and a systematic ordering of the mean velocity boundary layer equations and their interaction with the main stream flow are obtained. The analysis is extended to the turbulent energy equation and to a treatment of the small scale equilibrium range of Kolmogoroff; in velocity correlation space the two-thirds power law is obtained. Thus, the two well-known 'laws' of turbulent flow are imbedded in an analysis which provides a great deal of other information.
Turbulent thermal boundary layers with temperature-dependent viscosity
International Nuclear Information System (INIS)
Highlights: • Turbulent thermal boundary layers with temperature-dependent viscosity are simulated. • Effect of temperature-dependent viscosity on the statistics of the scalar field. • An identity for the Stanton number is derived and analyzed. • Effect of temperature-dependent viscosity on the statistics of scalar transfer rate. • Modification of turbulent flow field leads to an enhanced scalar transfer rate. - Abstract: Direct numerical simulations (DNS) of turbulent boundary layers (TBLs) over isothermally heated walls were performed, and the influence of the wall-heating on the thermal boundary layers was investigated. The DNS adopt an empirical relation for the temperature-dependent viscosity of water. The Prandtl number therefore changes with temperature, while the Péclet number is constant. Two wall temperatures (Tw = 70 °C and 99 °C) were considered relative to T∞ = 30 °C, and a reference simulation of TBL with constant viscosity was also performed for comparison. In the variable viscosity flow, the mean and variance of the scalar, when normalized by the friction temperature deficit, decrease relative to the constant viscosity flow. A relation for the mean scalar which takes into account the variable viscosity is proposed. Appropriate scalings for the scalar fluctuations and the scalar flux are also introduced, and are shown to be applicable for both variable and constant viscosity flows. Due to the modification of the near-wall turbulence, the Stanton number and the Reynolds analogy factor are augmented by 10% and 44%, respectively, in the variable viscosity flow. An identity for the Stanton number is derived and shows that the mean wall-normal velocity and wall-normal scalar flux cause the increase in the heat transfer coefficient. Finally, the augmented near-wall velocity fluctuations lead to an increase of the wall-normal scalar flux, which contributes favorably to the enhanced heat transfer at the wall
Manipulation of Turbulent Boundary Layers Using Synthetic Jets
Berger, Zachary; Gomit, Guillaume; Lavoie, Philippe; Ganapathisubramani, Bharath
2015-11-01
This work focuses on the application of active flow control, in the form of synthetic jet actuators, of turbulent boundary layers. An array of 2 synthetic jets are oriented in the spanwise direction and located approximately 2.7 meters downstream from the leading edge of a flat plate. Actuation is applied perpendicular to the surface of the flat plate with varying blowing ratios and reduced frequencies (open-loop). Two-component large window particle image velocimetry (PIV) was performed at the University of Southampton, in the streamwise-wall-normal plane. Complementary stereo PIV measurements were performed at the University of Toronto Institute for Aerospace Studies (UTIAS), in the spanwise-wall-normal plane. The freestream Reynolds number is 3x104, based on the boundary layer thickness. The skin friction Reynolds number is 1,200 based on the skin friction velocity. The experiments at Southampton allow for the observation of the control effects as the flow propagates downstream. The experiments at UTIAS allow for the observation of the streamwise vorticity induced from the actuation. Overall the two experiments provide a 3D representation of the flow field with respect to actuation effects. The current work focuses on the comparison of the two experiments, as well as the effects of varying blowing ratios and reduced frequencies on the turbulent boundary layer. Funded Supported by Airbus.
The collapse of turbulence in the atmospheric boundary layer
Energy Technology Data Exchange (ETDEWEB)
Van de Wiel, B J H; Clercx, H J H [Department of Physics, Eindhoven University of Technology (Netherlands); Moene, A F [Department of Meteorology and Air Quality, Wageningen University and Research Centre (Netherlands); Jonker, H J J, E-mail: b.j.h.v.d.wiel@tue.nl [Department of Multi-scale Pysics, Delft University of Technology (Netherlands)
2011-12-22
A well-known phenomenon in the atmospheric boundary layer is the fact that winds may become very weak in the evening after a clear sunny day. In these quiet conditions usually hardly any turbulence is present. Consequently this type of boundary layer is referred to as the quasi-laminar boundary layer. In spite of its relevance, the appearance of laminar boundary layers is poorly understood and forms a long standing problem in meteorological research. Here we investigate an analogue problem in the form of a stably stratified channel flow. The flow is studied with a simplified atmospheric model as well as with Direct Numerical Simulations. Both models show remarkably similar behaviour with respect to the mean variables such as temperature and wind speed. The similarity between both models opens new way for understanding and predicting the laminarization process. Mathematical analysis on the simplified model shows that relaminarization can be understood from the existence of a definite limit in the maximum sustainable heat flux under stably stratified conditions. This fascinating aspect will be elaborated in future work.
Numerical studies on laminar-turbulent transition in boundary layers
International Nuclear Information System (INIS)
Laminar-turbulent transition in flat-plate boundary layers is investigated by direct numerical solution of the full Navier-Stokes equations. Both forced transition (in parallel Blasius flow excited by a vibrating ribbon) and natural transition (in a decelerating boundary layer) are studied. In both cases, an initial state containing random noise is employed to eliminate bias in selecting unstable waves. In the simulations of ribbon-induced transition, close agreement with experiments (Saric et al. (1984)) is obtained for low-amplitude two-dimensional Tollmien-Schlichting waves-producing subharmonic breakdown (C- or H-type). For high amplitudes, a mixture of subharmonic and fundamental structures is observed. Clear-cut fundamental breakdown (K-type) is never obtained. In the simulation of the early stages of natural transition in a decelerating boundary layer, two-dimensional and/or slightly oblique waves initially grow due to the inflectional instability. When they become strong enough, they initiate a secondary instability leading to three dimensional distortion and Λ vortices, in good agreement with experiments (Gad-el-Hak et al. (1984)). The tips of the Λ vortices are rarely aligned with the flow direction, and that they appear locally in apace. A simple wave-interference model accounting for these features of natural transition has been developed. It suggests that multiple waves are active in the secondary instability, and that they are determined by unpredictable initial disturbances. The later stages of transition in a decelerating boundary layer were also studied with higher numerical resolution. The naturally-born Λ vortices undergo breakdown processes similar to those of ribbon-induced Λ vortices. Conversely, this justifies the conventional approach to study laminar-turbulent transition-the vibrating-ribbon technique
Laminar Turbulent Transition in a Boundary Layer Subjected to Weak Free Stream Turbulence
Kenchi, Toshiaki; Matsubara, Masaharu; Ikeda, Toshihiko
For revealing the transition process in a flat plate boundary layer subjected to a weak free stream turbulence, flow visualization and hot-wire measurements were performed. A weak free stream turbulence was generated by a turbulence grid mounted upstream of the contraction. The flow visualization clearly displayed a transition scenario in which a local two-dimensional wave packet rapidly forms a Λ shape structure and then breaks down to turbulence, resulting in the generation of a turbulent spot. Quantitative measurements performed by using a hot-wire anemometer also confirmed the existence of local Tollmien-Schlichting waves that agreed with the parallel linear theory in terms of their frequency, phase velocity, and the wall-normal distribution of band-pass-filtered fluctuations. For comparison, a boundary layer subjected to a moderate-intensity free stream turbulence was investigated. This investigation showed that streaky structures play an important role in the boundary layer transition, as shown by Matsubara et al. [J. Fluid Mech., 430, (2001), 149-168.] A drastic change occurred in the transition process and this change could be sensitively determined by employing the intensity and/or spectra of the free stream turbulence.
Investigation of turbulent spot production rate in boundary layer
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Elsner, W.; Mazur, Oton; Uruba, Václav; Wysocki, M.
Žilina : Žilinská univerzita, 2010, s. 1-6. ISBN 978-80-554-0189-8. [Aplikácia experimentálnych a numerických metód v mechanike tekutín a energetike. Bojnice (SK), 28.04.2010-30.04.2010] R&D Projects: GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulent spot * by- pas boundary layer transition * transitional intermittency * wavelet analysis Subject RIV: BK - Fluid Dynamics
Injection-induced turbulence in stagnation-point boundary layers
Park, C.
1984-02-01
A theory is developed for the stagnation point boundary layer with injection under the hypothesis that turbulence is produced at the wall by injection. From the existing experimental heat transfer rate data obtained in wind tunnels, the wall mixing length is deduced to be a product of a time constant and an injection velocity. The theory reproduces the observed increase in heat transfer rates at high injection rates. For graphite and carbon-carbon composite, the time constant is determined to be 0.0002 sec from the existing ablation data taken in an arc-jet tunnel and a balistic range.
Compressible Turbulent Boundary Layers on a Strongly Heated Wall
Institute of Scientific and Technical Information of China (English)
无
1993-01-01
This paper concerns the theoretical and experimental modelling of the flat wall,highly heated,compressible turbulent boundary layer.Its final objective is to develop a numerical Navier-Stokes solver and to conclude on its capability to correctly represent complex aerothermic viscous flows near the wall.The paper presents a constructed numerical method with particular attention given to the turbulence modelling at low Reynolds number and comparisons with supersonic and transonic experimental data.For the transonic experiment,very high wall temperature(Tw=1100K)is realized.The method of this difficult experimental set up is discussed.The comparison between experimental and computational data conducts to the first conclusion and gives some indications for the future work.
Turbulence transition in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M; Veble, Gregor; Duguet, Yohann; Schlatter, Philipp; Henningson, Dan S; Eckhardt, Bruno
2015-01-01
We study the transition to turbulence in the asymptotic suction boundary layer (ASBL) by direct numerical simulation. Tracking the motion of trajectories intermediate between laminar and turbulent states we can identify the invariant object inside the laminar-turbulent boundary, the edge state. In small domains, the flow behaves like a travelling wave over short time intervals. On longer times one notes that the energy shows strong bursts at regular time intervals. During the bursts the streak structure is lost, but it reforms, translated in the spanwise direction by half the domain size. Varying the suction velocity allows to embed the flow into a family of flows that interpolate between plane Couette flow and the ASBL. Near the plane Couette limit, the edge state is a travelling wave. Increasing the suction, the travelling wave and a symmetry-related copy of it undergo a saddle-node infinite-period (SNIPER) bifurcation that leads to bursting and discrete-symmetry shifts. In wider domains, the structures loc...
Coupling between roughness and freestream acceleration in turbulent boundary layers
Yuan, Junlin; Piomelli, Ugo
2015-11-01
To explain various rough-wall flow responses to different types of free-stream conditions previously observed, we carried out a direct numerical simulation of a spatially developing turbulent boundary layer with freestream acceleration. Unlike the equilibrium (self-similar) accelerating scenario, where a strong acceleration leads to complete laminarization and lower friction, in the present non-equilibrium case the friction coefficient increases with acceleration, due to the faster near-wall acceleration than that of the freestream. At the same time, roughness reduces the near-wall time scale of the turbulence, preventing the acceleration from linearly stretching the near-wall eddies and freezing the turbulence intensity as in the smooth case. In addition, acceleration leads to similar decrease of mean-velocity logarithmic slope on rough and smooth walls; this allows a clear definition of the roughness function in a local sense. Interestingly, this roughness function correlates with the roughness Reynolds number in the same way as in self-similar or non-accelerating flows. This study may also help develop benchmark cases for evaluating rough-wall treatments for industrial turbulence models.
Turbulent boundary layer over a convergent and divergent superhydrophobic surface
Nadeem, Muhammad; Hwang, Jinyul; Sung, Hyung Jin
2015-11-01
Direct numerical simulation (DNS) of spatially developing turbulent boundary layer (TBL) over a convergent and divergent superhydrophobic surface (SHS) was performed. The convergent and divergent SHS was aligned in the streamwise direction. The SHS was modeled as a pattern of slip and no-slip surfaces. For comparison, DNS of TBL over a straight SHS was also carried out. The momentum thickness Reynolds number was varied from 800 to 1400. The gas fraction of the convergent and divergent SHS was the same as that of the straight SHS, keeping the slip area constant. The slip velocity in the convergent SHS was higher than that of the straight SHS. An optimal streamwise length of the convergent and divergent SHS was obtained. The convergent and divergent SHS gave more drag reduction than the straight SHS. The convergent and divergent SHS led to the modification of near wall-turbulent structures, resembling the narrowing and widening streaky structures near the wall. The convergent and divergent SHS had a relatively larger damping effect on near-wall turbulence than the straight SHS. These observations will be further analyzed statistically to demonstrate the effect of the convergent and divergent SHS on the interaction of inner and outer regions of TBL.
Subgrid-scale turbulence in shock-boundary layer flows
Jammalamadaka, Avinash; Jaberi, Farhad
2015-04-01
Data generated by direct numerical simulation (DNS) for a Mach 2.75 zero-pressure gradient turbulent boundary layer interacting with shocks of different intensities are used for a priori analysis of subgrid-scale (SGS) turbulence and various terms in the compressible filtered Navier-Stokes equations. The numerical method used for DNS is based on a hybrid scheme that uses a non-dissipative central scheme in the shock-free turbulent regions and a robust monotonicity-preserving scheme in the shock regions. The behavior of SGS stresses and their components, namely Leonard, Cross and Reynolds components, is examined in various regions of the flow for different shock intensities and filter widths. The backscatter in various regions of the flow is found to be significant only instantaneously, while the ensemble-averaged statistics indicate no significant backscatter. The budgets for the SGS kinetic energy equation are examined for a better understanding of shock-tubulence interactions at the subgrid level and also with the aim of providing useful information for one-equation LES models. A term-by-term analysis of SGS terms in the filtered total energy equation indicate that while each term in this equation is significant by itself, the net contribution by all of them is relatively small. This observation is consistent with our a posteriori analysis.
Coherent vorticity extraction in turbulent boundary layers using orthogonal wavelets
Energy Technology Data Exchange (ETDEWEB)
Khujadze, George; Oberlack, Martin [Chair of Fluid Dynamics, Technische Universitaet Darmstadt (Germany); Yen, Romain Nguyen van [Institut fuer Mathematik, Freie Universitaet Berlin (Germany); Schneider, Kai [M2P2-CNRS and CMI, Universite de Provence, Marseille (France); Farge, Marie, E-mail: khujadze@fdy.tu-darmstadt.de [LMD-IPSL-CNRS, Ecole Normale Superieure, Paris (France)
2011-12-22
Turbulent boundary layer data computed by direct numerical simulation are analyzed using orthogonal anisotropic wavelets. The flow fields, originally given on a Chebychev grid, are first interpolated on a locally refined dyadic grid. Then, they are decomposed using a wavelet basis, which accounts for the anisotropy of the flow by using different scales in the wall-normal direction and in the planes parallel to the wall. Thus the vorticity field is decomposed into coherent and incoherent contributions using thresholding of the wavelet coefficients. It is shown that less than 1% of the coefficients retain the coherent structures of the flow, while the majority of the coefficients corresponds to a structureless, i.e., noise-like background flow. Scale-and direction-dependent statistics in wavelet space quantify the flow properties at different wall distances.
Proper orthogonal decomposition of a decelerating turbulent boundary layer
Tutkun, Murat
2010-11-01
Our analysis is based only on streamwise component of velocity fluctuations since the data were simultaneously obtained using a hot-wire rake of 143 single wire probes. The experiment was carried out in the large wind tunnel of Laboratoire de M'ecanique de Lille whose test section is 20 m long, 2 m wide and 1 m high. A 2D bump was used to create converging-diverging flow inside the test section. The thickness of the boundary layer was 25 cm at the measurement location and Reynolds number based on momentum thickness, Reθ, was 17:100 for 10 m s-1 external free stream velocity measured before the bump. Eigenvalue distribution over POD modes shows that approximately 90% of turbulence kinetic energy due to streamwise fluctuations within the domain was captured by the first 5 POD modes. The first POD mode carried more than 45% of turbulence kinetic energy. Resulting eigenspectra are studied for different frequencies and spanwise Fourier indices in order to reduce the number of modes used in reconstructed velocity fields.
A note on turbulent spots over a rough bed in wave boundary layers
DEFF Research Database (Denmark)
Carstensen, Stefan; Sumer, B. Mutlu; Fredsøe, Jørgen
2012-01-01
This study is a continuation of the investigation of turbulent spots in wave boundary layers over a smooth wall reported by Carstensen et al. [J. Fluid Mech. 646, 169–206 (2010)]. The present paper summarises the results of an experimental investigation of turbulent spots in wave boundary layers ...
Tetervin, Neal; Lin, Chia Chiao
1951-01-01
A general integral form of the boundary-layer equation, valid for either laminar or turbulent incompressible boundary-layer flow, is derived. By using the experimental finding that all velocity profiles of the turbulent boundary layer form essentially a single-parameter family, the general equation is changed to an equation for the space rate of change of the velocity-profile shape parameter. The lack of precise knowledge concerning the surface shear and the distribution of the shearing stress across turbulent boundary layers prevented the attainment of a reliable method for calculating the behavior of turbulent boundary layers.
SURFACE ROUGHNESS AND EXTERNAL FLOW TURBULENCE JOINT EFFECT ON TURBULENT BOUNDARY LAYER
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Mazur, Oton; Uruba, Václav
Praha : ÚTAM, 2009 - (Náprstek,, J.; Fischer, C.), s. 122-123 ISBN 978-80-86246-35-2. [Engineering mechanics 2009. Svratka (CZ), 11.05.2009-14.05.2009] R&D Projects: GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * turbulence * surface roughness Subject RIV: BK - Fluid Dynamics
Surface Temperature and Surface-Layer Turbulence in a Convective Boundary Layer
Garai, A.; Pardyjak, E.; Steeneveld, G.J.; Kleissl, J.
2013-01-01
Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared im
A model for turbulent dissipation rate in a constant pressure boundary layer
Indian Academy of Sciences (India)
J DEY; P PHANI KUMAR
2016-04-01
Estimation of the turbulent dissipation rate in a boundary layer is a very involved process.Experimental determination of either the dissipation rate or the Taylor microscale, even in isotropic turbulence,which may occur in a portion of the turbulent boundary layer, is known to be a difficult task. For constant pressure boundary layers, a model for the turbulent dissipation rate is proposed here in terms of the local mean flow quantities. Comparable agreement between the estimated Taylor microscale and Kolmogorov length scale with other data in the logarithmic region suggests usefulness of this model in obtaining these quantitiesexperimentally
Turbulent Suspension Mechanics in Sediment-Laden Boundary Layers
Kiger, K.
2013-05-01
Accurate prediction of benthic sediment transport is a challenging problem due the two-phase nature of the flow near the mobile bed, as well as the large difference in scales between the meso-scale flow and smaller-scale structures interacting with the sediment bed. Of particular importance is the parameterization of the physics at the bottom boundary. This requires estimation of key quantities such as effective bed stress and sediment flux based on the on the outer regional-scale velocity field. An appropriate turbulence/sediment parameterization is needed to specify the correct bottom momentum and sediment flux. Prior work has shown the shortcoming of standard models to properly predict such behavior, which is speculated to result from the dominant role played by large-scale coherent structures in the generation of the bed morphology, suspension of particulates, and important particle-fluid coupling effects. The goal of the current work is to elucidate such relationships through a combination of direct simulation and laboratory-scale experiment, the latter of which will be the primary focus of this paper. Specifically, two-phase PIV is used to provide a novel quantitative description of both phases, allowing for a detailed examination of the flow behavior and particle-turbulence coupling. Experiments were conducted in both a steady, fully-developed turbulent channel flow and an oscillatory boundary layer in order to examine the fundamental behaviour of the suspension and particle coupling mechanisms. The turbulent channel flow measurements indicated an increase in the effective wall stress due to the presence of the sediment on the order of 7%. The sediment suspension was directly correlated with the ejection dynamics of prototypical hairpin structures, but were found to settle back towards the bed in a manner uncorrelated with the fluid structure. In contrast, the measurements of the oscillatory flow reveal it to be dominated by alternating streaming motions and
Volino, Ralph John
1995-01-01
Measurements from transitional, heated boundary layers along a concave-curved test wall are presented and discussed. A boundary layer subject to low free-stream turbulence intensity (FSTI), which contains stationary streamwise (Gortler) vortices, is documented. The low FSTI measurements are followed by measurements in boundary layers subject to high (initially 8%) free-stream turbulence intensity and moderate to strong (K = {nuover U_sp{infty} {2}}{dUinftyover dx} as high as 9times 10^{ -6}) acceleration. The high FSTI experiments are the main focus of the work. Conditions were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. The high FSTI boundary layers undergo transition from a strongly disturbed non-turbulent state to a fully-turbulent state. Due to the stabilizing effect of strong acceleration, the transition zones are of extended length in spite of the high FSTI. Transitional values of skin friction coefficients and Stanton numbers drop below flat-plate, low FSTI, turbulent flow correlations, but remain well above laminar flow values. Mean velocity and temperature profiles exhibit clear changes in shape as the flow passes through transition. Turbulence statistics, including the turbulent shear stress, turbulent heat flux, and turbulent Prandtl number, are documented. Turbulent transport is strongly suppressed below values in unaccelerated turbulent boundary layers. A technique called "octant analysis" is introduced and applied to several cases from the literature as well as to data from the present study. Octant analysis shows a fundamental difference between transitional and fully-turbulent boundary layers. Transitional boundary layers are characterized by incomplete mixing compared to fully-turbulent boundary layers. Similar octant analysis results are observed in both low and high FSTI cases. Spectral analysis suggests that the non-turbulent zone of the high FSTI flow is dominated by large scale
Intermittent turbulence and oscillations in the stable boundary layer over land
Wiel, van de, M.A.
2002-01-01
As the title of this thesis indicates, our main subject of interest is: "Intermittent turbulence and oscillation in the stable boundary layer over land". As such, this theme connects the different chapters. Here, intermittent turbulence is defined as a sequence of events were 'burst' of increased turbulence activity are followed by relatively quiet periods with low turbulence levels. This intermittent turbulence affects the mean structure of the SBL, in a sense that it may cause alternations ...
Properties of the turbulent/non-turbulent interface in boundary layers
Borrell, Guillem
2016-01-01
The turbulent/non-turbulent interface is analysed in a direct numerical simulation of a boundary layer in the range $Re_\\theta=2800-6600$, with emphasis on the behaviour of the relatively large-scale fractal intermittent region. This requires the introduction of a new definition of the distance between a point and a general surface, which is compared with the more usual vertical distance to the top of the layer. Interfaces are obtained by thresholding the enstrophy field and the magnitude of the rate-of-strain tensor, and it is concluded that, while the former are physically relevant features, the latter are not. By varying the threshold, a topological transition is identified as the interface moves from the free stream into the turbulent core. A vorticity scale is defined that collapses that transition for different Reynolds numbers, roughly equivalent to the root-mean-squared vorticity at the edge of the boundary layer. Conditionally averaged flow variables are analysed as functions of the new distance, bot...
Integral method for the calculation of three-dimensional, laminar and turbulent boundary layers
Stock, H. W.
1978-01-01
The method for turbulent flows is a further development of an existing method; profile families with two parameters and a lag entrainment method replace the simple entrainment method and power profiles with one parameter. The method for laminar flows is a new development. Moment of momentum equations were used for the solution of the problem, the profile families were derived from similar solutions of boundary layer equations. Laminar and turbulent flows at the wings were calculated. The influence of wing tapering on the boundary layer development was shown. The turbulent boundary layer for a revolution ellipsoid is calculated for 0 deg and 10 deg incidence angles.
Direct Numerical Simulation of Supersonic Turbulent Boundary Layer with Spanwise Wall Oscillation
Directory of Open Access Journals (Sweden)
Weidan Ni
2016-03-01
Full Text Available Direct numerical simulations (DNS of Mach = 2.9 supersonic turbulent boundary layers with spanwise wall oscillation (SWO are conducted to investigate the turbulent heat transport mechanism and its relation with the turbulent momentum transport. The turbulent coherent structures are suppressed by SWO and the drag is reduced. Although the velocity and temperature statistics are disturbed by SWO differently, the turbulence transports of momentum and heat are simultaneously suppressed. The Reynolds analogy and the strong Reynolds analogy are also preserved in all the controlled flows, proving the consistent mechanisms of momentum transport and heat transport in the turbulent boundary layer with SWO. Despite the extra dissipation and heat induced by SWO, a net wall heat flux reduction can be achieved with the proper selected SWO parameters. The consistent mechanism of momentum and heat transports supports the application of turbulent drag reduction technologies to wall heat flux controls in high-speed vehicles.
International Nuclear Information System (INIS)
Highlights: • We study the stably thermally-stratified turbulent boundary layer by means of DNS. • The counter diffusion phenomenon is discovered in both the velocity and thermal fields in our DNS. • The detailed turbulent statistics and structures in stably thermally-stratified turbulent boundary layer are discussed. • The anisotropy tensor, turbulent heat flux tensor, vortex structure, and fluctuation Reynolds shear stress are indicated. - Abstract: The objectives of this study are to investigate the counter diffusion phenomenon (CDP) in a stably thermally-stratified turbulent boundary layer by means of direct numerical simulation (DNS). In this study, four cases of stably thermally-stratified turbulent boundary layers are simulated to reproduce the CDP, in which two Reynolds numbers and four Richardson numbers are set. The CDP is discovered in both the velocity and thermal fields in three cases. DNS clearly shows the CDP, which indicates the negative sign of the Reynolds shear stress and the wall-normal turbulent heat flux with the positive sign of mean velocity and temperature gradients. The turbulent heat flux tensor is also shown in order to indicate the variation of the thermal field, in which the streamwise turbulent heat flux tensor maintains a high value even in the case of strong CDP occurrence. The relation between the vortex structure and the Reynolds shear stress fluctuation is shown, where the negative value of Reynolds shear stress fluctuation frequently appears around the vortex structure in the case of CDP occurrence
The Modelling of Particle Resuspension in a Turbulent Boundary Layer
International Nuclear Information System (INIS)
lift and drag forces in turbulent boundary layers, the lift and drag we have con sidered a
Boundary-layer turbulence in experiments of quasi-Keplerian flows
Lopez, Jose M
2016-01-01
Most flows in nature and engineering are turbulent because of their large velocities and spatial scales. Laboratory experiments of rotating quasi-Keplerian flows, for which the angular velocity decreases radially but the angular momentum increases, are however laminar at Reynolds numbers exceeding one million. This is in apparent contradiction to direct numerical simulations showing that in these experiments turbulence transition is triggered by the axial boundaries. We here show numerically that as the Reynolds number increases turbulence becomes progressively confined to the boundary layers and the flow in the bulk fully relaminarizes. Our findings support that hydrodynamic turbulence cannot drive accretion in astrophysical disks.
Turbulent Statistics of the Turbulent Boundary Layer over a Cube-Roughened Wall
International Nuclear Information System (INIS)
Direct numerical simulation (DNS) of a spatially developing turbulent boundary layer (TBL) with regularly arrayed cubical roughness elements was performed to investigate the effects of three-dimensional (3D) surface elements. The staggered cubes downstream were periodically arranged in the streamwise and spanwise directions with pitches of ρx/κ=8 and ρz/κ=2, where ρx and ρz are the streamwise and spanwise spacings of the cubes; the roughness height (κ) was κ=1.5θin, where θin is the momentum thickness at the inlet. Spatially developing characteristics over the 3D cubical roughness were compared with the data obtained from the DNS over the two-dimensional (2D) rod roughened wall and smooth wall. Introduction of the cubical roughness on the TBL affected the turbulent Reynolds stresses not only in the roughness sublayer but also in the outer layer; and these effects are consistent with those observed over the 2D rough wall
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Mazur, Oton; Uruba, Václav
Stockholm: KTH Royal Institute of Technology + Congrex Sweden AB, 2009. s. 194-195 [IUTAM Symposium on Laminar-Turbulent Transition /7./. 23.06.2009-26.06.2009, Stockholm] R&D Projects: GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * laminar-turbulent transition * effect of free stream turbulence * effect of surface roughness Subject RIV: BK - Fluid Dynamics
A turbulent burst model for boundary layer flows with pressure gradient
Thomas, L. C.; Benton, D. J.
The object of this paper is to develop a surface renewal model of the turbulent burst phenomenon for momentum and energy transfer in the wall region for turbulent boundary layer flows with pressure gradient. In addition to obtaining inner laws for the distributions in velocity and temperature, predictions are obtained for the effect of pressure gradient on the mean burst frequency and on the turbulent Prandtl number within the wall region for slight favorable and mild adverse pressure gradients.
A turbulence model for steady and unsteady boundary layers in strong pressure gradients
Hytopoulos, Evangelos
1994-01-01
A new turbulence model designed for two-dimensional, steady and unsteady boundary layers in strong adverse pressure gradients is described. The model is developed in a rational way based on an understanding of the flow physics obtained from recent experimental observations. The turbulent shear stress is given by a mixing length model, but the variation of the mixing length in the outer region is not constant; it varies according to an integral form of the turbulence kinetic-energy equation. T...
Review of wave-turbulence interactions in the stable atmospheric boundary layer
Yagüe Anguis, Carlos; otros, ...
2015-01-01
Flow in a stably stratified environment is characterized by anisotropic and intermittent turbulence and wavelike motions of varying amplitudes and periods. Understanding turbulence intermittency and wave-turbulence interactions in a stably stratified flow remains a challenging issue in geosciences including planetary atmospheres and oceans. The stable atmospheric boundary layer (SABL) commonly occurs when the ground surface is cooled by longwave radiation emission such as at night over land s...
Loitsianskii. L. G.
1956-01-01
The fundamental, practically the most important branch of the modern mechanics of a viscous fluid or a gas, is that branch which concerns itself with the study of the boundary layer. The presence of a boundary layer accounts for the origin of the resistance and lift force, the breakdown of the smooth flow about bodies, and other phenomena that are associated with the motion of a body in a real fluid. The concept of boundary layer was clearly formulated by the founder of aerodynamics, N. E. Joukowsky, in his well-known work "On the Form of Ships" published as early as 1890. In his book "Theoretical Foundations of Air Navigation," Joukowsky gave an account of the most important properties of the boundary layer and pointed out the part played by it in the production of the resistance of bodies to motion. The fundamental differential equations of the motion of a fluid in a laminar boundary layer were given by Prandtl in 1904; the first solutions of these equations date from 1907 to 1910. As regards the turbulent boundary layer, there does not exist even to this day any rigorous formulation of this problem because there is no closed system of equations for the turbulent motion of a fluid. Soviet scientists have done much toward developing a general theory of the boundary layer, and in that branch of the theory which is of greatest practical importance at the present time, namely the study of the boundary layer at large velocities of the body in a compressed gas, the efforts of the scientists of our country have borne fruit in the creation of a new theory which leaves far behind all that has been done previously in this direction. We shall herein enumerate the most important results by Soviet scientists in the development of the theory of the boundary layer.
Diaz Daniel, Carlos; Laizet, Sylvain; Vassilicos, John Christos
2015-11-01
The Townsend-Perry hypothesis of wall-attached eddies relates the friction velocity uτ at the wall to velocity fluctuations at a position y from the wall, resulting in a wavenumber range where the streamwise fluctuating velocity spectrum scales as E (k) ~k-1 and the corresponding structure function scales as uτ2 in the corresponding length-scale range. However, this model does not take in account the fluctuations of the skin friction velocity, which are in fact strongly intermittent. A DNS of zero-pressure gradient turbulent boundary layer suggests a 10 to 15 degree angle from the lag of the peak in the cross-correlations between the fluctuations of the shear stress and streamwise fluctuating velocities at different heights in the boundary layer. Using this result, it is possible to refine the definition of the attached eddy range of scales, and our DNS suggests that, in this range, the second order structure function depends on filtered skin friction fluctuations in a way which is about the same at different distances from the wall and different local Reynolds numbers.
Calculation of Turbulent Boundary Layers Using the Dissipation Integral Method
Institute of Scientific and Technical Information of China (English)
MatthiasBuschmann
1999-01-01
This paper gives an introduction into the dissipation integral method.The general integral equations for the three-dimensional case are derved.It is found that for a practical calculation algorithm the integral monentum equation and the integral energy equation are msot useful.Using Two different sets of mean velocity profiles the hyperbolical character of a dissipation integral method is shown.Test cases for two-and three-dimensional boundary layers are analysed and discussed.The paper concludes with a discussion of the advantages and limits of dissipation integral methods.
Statistics of the turbulent boundary layers over 3D cube-roughened walls
International Nuclear Information System (INIS)
Highlights: • To simulate turbulent boundary layers over 3D cube-roughened walls and to see turbulence in the inner and outer fluid layers. • To compare turbulence statistics with those affected by different wall conditions. • To propose a suitable geometrical parameter for estimation of turbulence statistics in the inner and outer layers. -- Abstract: Direct numerical simulations (DNSs) of turbulent boundary layers (TBLs) over three-dimensional (3D) cube-roughened walls were performed and the turbulent characteristics in the inner and outer layers were statistically analyzed. The spanwise spacing was varied over pz/k = 2, 3, 4, and 6 (pz is the spanwise spacing between cubes and k is the height of the roughness) to examine the effects of the roughness spacing on the TBLs. The form drag (Cp) reached a maximum at pz/k = 3, whereas the skin-friction drag (Cf) reached a minimum at the same extent. The Reynolds stresses in the outer region were shown to increase with increasing pz/k, and similar behavior was observed in the wall-normal velocity fluctuations at the roughness crest (vw+). The properties of the turbulence in the inner and outer layers were found to be well represented by the roughness density (λp)
Two-phase wall function for modeling of turbulent boundary layer in subcooled boiling flow
International Nuclear Information System (INIS)
Full text of publication follows: The heat transfer and phase-change mechanisms in the subcooled flow boiling are governed mainly by local multidimensional mechanisms near the heated wall, where bubbles are generated. The structure of such 'wall boiling flow' is inherently non-homogeneous and is further influenced by the two-phase flow turbulence, phase-change effects in the bulk, interfacial forces and bubble interactions (collisions, coalescence, break-up). In this work the effect of two-phase flow turbulence on the development of subcooled boiling flow is considered. Recently, the modeling of two-phase flow turbulence has been extensively investigated. A notable progress has been made towards deriving reliable models for description of turbulent behaviour of continuous (liquid) and dispersed phase (bubbles) in the bulk flow. However, there is a lack of investigation considering the modeling of two-phase flow boundary layer. In most Eulerian two-fluid models standard single-phase wall functions are used for description of turbulent boundary layer of continuous phase. That might be a good approximation at adiabatic flows, but their use for boundary layers with high concentration of dispersed phase is questionable. In this work, the turbulent boundary layer near the heated wall will be modeled with the so-called 'two-phase' wall function, which is based on the assumption of additional turbulence due to bubble-induced stirring in the boundary layer. In the two-phase turbulent boundary layer the wall function coefficients strongly depend on the void fraction. Moreover, in the turbulent boundary layer with nucleating bubbles, the bubble size variation also has a significant impact on the liquid phase. As a basis, the wall function of Troshko and Hassan (2001), developed for adiabatic bubbly flows will be used. The simulations will be performed by a general-purpose CFD code CFX-4.4 using additional models provided by authors. The results will be compared to the boiling
Geostrophic convective turbulence: The effect of boundary layers
Ostilla-Mónico, Rodolfo; Kunnen, Rudie P J; Verzicco, Roberto; Lohse, Detlef
2014-01-01
This Letter presents results of the first direct numerical simulations of rotating Rayleigh--B\\'enard convection in the so-called geostrophic regime, (hence very small Ekman numbers $\\mathcal{O}(10^{-7})$ and high Rayleigh numbers~$Ra=10^{10}$ and~$5\\cdot 10^{10}$), employing the \\emph{full} Navier--Stokes equations. In the geostrophic regime the criteria of very strong rotation and large supercriticality are met simultaneously, which is true for many geophysical and astrophysical flows. Until now, numerical approaches of this regime have been based on \\emph{reduced} versions of the Navier--Stokes equations (cf. Sprague \\emph{et al.} J. Fluid Mech., \\textbf{551}, 141 (2006)), omitting the effect of the viscous (Ekman) boundary layers. By using different velocity boundary conditions at the plates, we study the effect of these Ekman layers. We find that the formation of large-scale structures (Rubio \\emph{et al.} (Phys. Rev. Lett. \\textbf{112} (2014)), which indicates the presence of an inverse energy cascade, ...
DEFF Research Database (Denmark)
Cavar, Dalibor; Meyer, Knud Erik
2011-01-01
A large eddy simulation (LES) study of turbulent non-equilibrium boundary layer flow over 2 D Bump, at comparatively low Reynolds number Reh = U∞h/ν = 1950, was conducted. A well-known LES issue of obtaining and sustaining turbulent flow inside the computational domain at such low Re, is addressed...... on a local estimate of the subgrid scale turbulent kinetic energy ksgs and implicit damping of turbulent SGS viscosity νt(sgs) in the near-wall region, was selected as a suitable basis for the present LES computations due to the fact that block structured MPI parallelized CFD code used in the current...
Numerical simulations of two-fluid boundary layers beneath free-stream turbulence
Jung, Seo Yoon; Zaki, Tamer
2011-11-01
In two-fluid boundary layers, a wall-film is sheared by an external stream with different density and viscosity. As a result, the flow becomes prone to both shear and interfacial instabilities. In this study, the evolution of two-fluid boundary layers beneath free-stream vortical forcing is investigated using DNS. The simulations employ a conservative level-set technique in conjunction with a ghost fluid approach in order to capture a sharp interface. The wall film is less viscous than the outer flow, and its thickness is 10 % of that of the boundary layer at the inlet. The choice of viscosity ratio influences the spatial development of disturbances within the boundary layer. The spatial growth of instabilities is examined into the non-linear regime, which includes the region of breakdown to turbulence. We demonstrate that, at moderate levels of free-stream turbulence intensities, appropriate choice of the viscosity ratio can yield considerable transition delay.
Characterization of an incipiently separated shock wave/turbulent boundary layer interaction
Schreyer, A.-M.; Dussauge, J.-P.; Krämer, E.
2016-05-01
The turbulence structure in a shock wave/turbulent boundary layer interaction at incipient separation was investigated in order to get insight into turbulence generation and amplification mechanisms in such flow fields. The flow along a two-dimensional 11.5° compression corner was studied experimentally at a Mach number of M=2.53 and with a momentum-thickness Reynolds number of Re_{θ }=5370 . From hot-wire boundary layer traverses and surface heat-flux density fluctuation measurements with the fast-response atomic layer thermopile, the turbulence structure and amplification was described. Space-time correlations of the mass-flux fluctuations across the boundary layer and the surface heat-flux density fluctuations were measured to further characterize the development of the turbulence structure across the interaction. The large-scale boundary layer structures are concealed by shock-related effects in the strongly disturbed shock-foot region. Shortly downstream, however, large-scale structures dominate the signal again, just as in the incoming flow. A mechanism explaining this behavior is suggested.
Turbulent Transport of 222-Rn and its Short-lived Daughters in Convective Boundary Layers
VINUESA JEAN; GALMARINI STEFANO
2006-01-01
222Rn is a natural radioactive compound with a half-life of 3.8 days. Because of its noble gas nature, it is a suitable tracer in studies of atmospheric boundary layers. Ground-based measurements and vertical distributions of 222Rn and its daughters have been extensively studied in the past, e.g., to characterize the turbulent properties of the atmospheric boundary layer, to perform regional and global circulation model benchmarking and to estimate regional surface flu...
A high-resolution code for large eddy simulation of incompressible turbulent boundary layer flows
Cheng, Wan
2014-03-01
We describe a framework for large eddy simulation (LES) of incompressible turbulent boundary layers over a flat plate. This framework uses a fractional-step method with fourth-order finite difference on a staggered mesh. We present several laminar examples to establish the fourth-order accuracy and energy conservation property of the code. Furthermore, we implement a recycling method to generate turbulent inflow. We use the stretched spiral vortex subgrid-scale model and virtual wall model to simulate the turbulent boundary layer flow. We find that the case with Reθ ≈ 2.5 × 105 agrees well with available experimental measurements of wall friction, streamwise velocity profiles and turbulent intensities. We demonstrate that for cases with extremely large Reynolds numbers (Reθ = 1012), the present LES can reasonably predict the flow with a coarse mesh. The parallel implementation of the LES code demonstrates reasonable scaling on O(103) cores. © 2013 Elsevier Ltd.
A Note Concerning the Turbulent Boundary Layer Drag at Large Reynolds Numbers
Barenblatt, G.I.; Chorin, A.J.; Prostokishin, V. M.
2000-01-01
A correlation is obtained for the drag coefficient $c '_f$ of the turbulent boundary layer as a function of the effective boundary layer Reynolds number $Re$ that we previously introduced. A comparison is performed also with another correlation for the drag coefficient as a function of the traditional Reynolds number $Re_{\\th}$, based on the momentum thickness of the boundary layer proposed recently by R.D.Watson, R.M.Hall and J.B.Anders (NASA Langley Research Center) on the basis of differen...
Representation of the grey zone of turbulence in the atmospheric boundary layer
Honnert, Rachel
2016-04-01
Numerical weather prediction model forecasts at horizontal grid lengths in the range of 100 to 1 km are now possible. This range of scales is the "grey zone of turbulence". Previous studies, based on large-eddy simulation (LES) analysis from the MésoNH model, showed that some assumptions of some turbulence schemes on boundary-layer structures are not valid. Indeed, boundary-layer thermals are now partly resolved, and the subgrid remaining part of the thermals is possibly largely or completely absent from the model columns. First, some modifications of the equations of the shallow convection scheme have been tested in the MésoNH model and in an idealized version of the operational AROME model at resolutions coarser than 500 m. Secondly, although the turbulence is mainly vertical at mesoscale (> 2 km resolution), it is isotropic in LES (production of turbulence cannot be neglected at resolutions finer than half of the boundary-layer height. Thus, in the grey zone, fully unidirectional turbulence scheme should become tridirectional around 500 m resolution. At Météo-France, the dynamical turbulence is modelled by a K-gradient in LES as well as at mesoscale in both MésoNH and AROME, which needs mixing lengths in the formulation. Vertical and horizontal mixing lengths have been calculated from LES of neutral and convective cases at resolutions in the grey zone.
Kawai, Soshi
2014-11-01
In this talk, we first propose a numerical strategy that is robust and high-order accurate for enabling to simulate transcritical flows at supercritical pressures under abrupt variations in thermodynamic properties due to the real fluid effects. The method is based on introducing artificial density diffusion in a physically-consistent manner in order to capture the steep variation of thermodynamic properties in transcritical conditions robustly, while solving a pressure evolution equation to achieve pressure equilibrium at the transcritical interfaces. We then discuss the direct numerical simulation (DNS) of transcritical heated turbulent boundary layers on a zero-pressure-gradient flat plate at supercritical pressures. To the best of my knowledge, the present DNS is the first DNS of zero-pressure-gradient flat-plate transcritical turbulent boundary layer. The turbulent kinetic budget indicates that the compressibility effects (especially, pressure-dilatation correlation) are not negligible at the transcritical conditions even if the flow is subsonic. The unique and interesting interactions between the real fluid effects and wall turbulence, and their turbulence statistics, which have never been seen in the ideal-fluid turbulent boundary layers, are also discussed. This work was supported in part by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Young Scientists (A) KAKENHI 26709066 and the JAXA International Top Young Fellowship Program.
Turbulent exchange processes of the planetary boundary layer - TUAREG
International Nuclear Information System (INIS)
A mobile groundstation with associated sovftware has been developed to measure fluxes of properties and constituents, and the vertical distribution of chemically reactive trace gases. The significance and accuracy of the derived fluxes have been investigated. Within the validity of the meteorological assumptions used, the error is less than 10%. The turbulent vertical transport has been investigated over homogeneous areas in mixed heterogeneous terrain during four field experiments. The following results were obtained: characteristics of the structure of the turbulence - diurnal variations of the fluxes of momentum and energy - the vertical distribution of NO, NO2 and O3 -diurnal variations of their flux and deposition velocity - balance of ozone and exchange processes in the convective PBL. Correlation and profile measurements at a fixed point in mixed heterogeneous terrain are representative of the surface type, if the upwind dimension of the homogeneous areas is at least 500 m. If this is not the case, anisotropic and organized turbulence develops. Then the formally calculated fluxes arise, in part, due to random numbers and cannot be attributed to a local site. A definitive conclusion would require measurements of the three dimensional structure of turbulence. There are no counter-gradient fluxes in the nondivergent PBL. They arise from the use of inadequate integration intervals in correlation and profile calculations. In contrast, they do occur in regions of divergence. Since the similarity theory is not valid in this case, fluxes can be neither measured nor calculated. Airborne measurements were carried out by the ''Institut fuer Physik der Atmosphaere'', DLR. The following results are attached: the mean structure of the PBL - the turbulent fluxes of meteorological variables - the horizontal variability of the fluxes near the ground - the turbulent flux of ozone and the ozone balance. Comparisons with model calculations show good agreement. (orig./KW). 116
First Signs of Flow Reversal Within a Separated Turbulent Boundary Layer
Hammerton, Jared; Lang, Amy
2015-11-01
A shark's skin is covered in millions of microscopic scales that have been shown to be able to bristle in a reversing flow. The motive of this project is to further explore a potential bio-inspired passive separation control mechanism which can reduce drag. To better understand this mechanism, a more complete understanding of flow reversal within the turbulent boundary layer is required. In order to capture this phenomenon, water tunnel testing at The University of Alabama was conducted. Using a long flat plate and a rotating cylinder, a large turbulent boundary layer and adverse pressure gradient were generated. Under our testing conditions the boundary layer had a Reynolds number of 200,000 and a boundary layer height in the testing window of 5.6 cm. The adverse pressure gradient causes the viscous length scale to increase and thus increase the size of the individual components of the turbulent boundary layer. This will make the low speed streaks approximately 1 cm in width and thus large enough to measure. Results will be presented that test our hypothesis that the first signs of flow reversal will occur within the section of lowest momentum located furthest from the wall, or within the low speed streaks. This Project was funded by NSF REU Site Award 1358991.
Embedded-LES and experiment of turbulent boundary layer flow around a floor-mounted cube
DEFF Research Database (Denmark)
Jørgensen, Nina Gall; Koss, Holger; Bennetsen, Jens Chr.
An Embedded LES approach is used to numerically simulate fluctuating surface pressures on a floor-mounted cube in a turbulent boundary layer flow and compared to wind tunnel experiments. The computation were performed with the CFD software ANSYS FLUENT at a Reynolds number at cube height of Reh = 1...
Calculation of the turbulent boundary layer in the initial section of pipe
Energy Technology Data Exchange (ETDEWEB)
Temirkhanov, A.M.; Spivak, V.M.
1987-11-01
This article constructs a flow model for the turbulent boundary layer in a pipe operating under conditions of pressure and gravitation encountered in a hydroelectric power plant. Pipe roughness and friction factors are taken into account as are hydraulic conductivity and pipe dimension considerations. Continuity equations are given and the accuracy of the model is compared against experimental data.
RANS-based simulation of turbulent wave boundary layer and sheet-flow sediment transport processes
DEFF Research Database (Denmark)
Fuhrman, David R.; Schløer, Signe; Sterner, Johanna
2013-01-01
with bed and suspended load descriptions, the latter based on an unsteady turbulent-diffusion equation, for simulation of sheet-flow sediment transport processes. In addition to standard features common within such RANS-based approaches, the present model includes: (1) hindered settling velocities at high......A numerical model coupling the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equationswith two-equation k−ω turbulence closure is presented and used to simulate a variety of turbulent wave boundary layer processes. The hydrodynamic model is additionally coupled...
Laminar-turbulent boundary layer transition modeling for turbomachinery flows
Czech Academy of Sciences Publication Activity Database
Straka, P.; Příhoda, Jaromír
-, č. 4 (2010), s. 10-12. ISSN 1211-877X R&D Projects: GA ČR(CZ) GAP101/10/1329; GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbomachinery flow * transitional flow * k-omega turbulence model Subject RIV: BK - Fluid Dynamics
Wind tunnel study of a vertical axis wind turbine in a turbulent boundary layer flow
Rolin, Vincent; Porté-Agel, Fernando
2015-04-01
Vertical axis wind turbines (VAWTs) are in a relatively infant state of development when compared to their cousins the horizontal axis wind turbines. Very few studies have been carried out to characterize the wake flow behind VAWTs, and virtually none to observe the influence of the atmospheric boundary layer. Here we present results from an experiment carried out at the EPFL-WIRE boundary-layer wind tunnel and designed to study the interaction between a turbulent boundary layer flow and a VAWT. Specifically we use stereoscopic particle image velocimetry to observe and quantify the influence of the boundary layer flow on the wake generated by a VAWT, as well as the effect the VAWT has on the boundary layer flow profile downstream. We find that the wake behind the VAWT is strongly asymmetric, due to the varying aerodynamic forces on the blades as they change their position around the rotor. We also find that the wake adds strong turbulence levels to the flow, particularly on the periphery of the wake where vortices and strong velocity gradients are present. The boundary layer is also shown to cause greater momentum to be entrained downwards rather than upwards into the wake.
RANS Modeling of Stably Stratified Turbulent Boundary Layer Flows in OpenFOAM®
Directory of Open Access Journals (Sweden)
Wilson Jordan M.
2015-01-01
Full Text Available Quantifying mixing processes relating to the transport of heat, momentum, and scalar quantities of stably stratified turbulent geophysical flows remains a substantial task. In a stably stratified flow, such as the stable atmospheric boundary layer (SABL, buoyancy forces have a significant impact on the flow characteristics. This study investigates constant and stability-dependent turbulent Prandtl number (Prt formulations linking the turbulent viscosity (νt and diffusivity (κt for modeling applications of boundary layer flows. Numerical simulations of plane Couette flow and pressure-driven channel flow are performed using the Reynolds-averaged Navier-Stokes (RANS framework with the standard k-ε turbulence model. Results are compared with DNS data to evaluate model efficacy for predicting mean velocity and density fields. In channel flow simulations, a Prandtl number formulation for wall-bounded flows is introduced to alleviate overmixing of the mean density field. This research reveals that appropriate specification of Prt can improve predictions of stably stratified turbulent boundary layer flows.
Vegetation Effects on Turbulent Boundary Layer Flows and their Role in Lotic Ecosystems
Neary, V. S.
2009-12-01
The effects of vegetation on fully developed turbulent boundary layer flows are profound and play an important role in lotic ecosystems. Recent experiments on flow past isolated plant stems (e.g. tree trunks in flood plains), alternating vegetation patches (e.g. Justicia americana in gravel bed rivers), and simulated emergent and submerged plant stem arrays in laboratory flumes are reviewed. Particular emphasis is given to fully developed turbulent flows through submerged vegetation modeled by large eddy simulation (LES), with a focus on understanding the role of the coherent structures on the momentum transfer across the water-plant interface. Comparisons are made with fully developed turbulent boundary layer flows in unobstructed (unvegetated) channels to show how the vegetation significantly changes the mean flow, Reynolds shear stress, turbulence intensities, turbulence event frequencies and the energy budget within and above the vegetation layer. The results demonstrate how vegetation in the lotic environment delineates ecotones with edge effects that are beneficial to mobile organisms (e.g. macroinvertebrates and fish), and how it alters mean flow and turbulence characteristics near the bed to promote desirable physical habitat conditions, e.g. substrate composition and stability, for benthic organisms such as mussels and crayfish. Vincent Neary, Ph.D., P.E. President, Springburn LLC Natural Engineering and Restoration
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
A direct numerical simulation of the shock/turbulent boundary layer interaction flow in a supersonic 24-degree compression ramp is conducted with the free stream Mach number 2.9.The blow-and-suction disturbance in the upstream wall boundary is used to trigger the transition.Both the mean wall pressure and the velocity profiles agree with those of the experimental data,which validates the simulation.The turbulent kinetic energy budget in the separation region is analyzed.Results show that the turbulent production term increases fast in the separation region,while the turbulent dissipation term reaches its peak in the near-wall region.The turbulent transport term contributes to the balance of the turbulent conduction and turbulent dissipation.Based on the analysis of instantaneous pressure in the downstream region of the mean shock and that in the separation bubble,the authors suggest that the low frequency oscillation of the shock is not caused by the upstream turbulent disturbance,but rather the instability of separation bubble.
Walker, G. J.; Solomon, W. J.
2007-01-01
Quantitative observations of transitional boundary layers in regions of strong flow deceleration on an axial compressor stator blade are reported. Measurements are obtained at a fixed chordwise position, and the blade incidence was varied by changing the compressor throughflow so as to move the transition region relative to the stationary probe. It was thus possible to observe typical boundary layer behavior at various stages of transition in the turbomachine environment. The range of observations covers separating laminar flow at transition onset, and reattachment of intermittently turbulent periodically separated shear layers.
DEFF Research Database (Denmark)
Keck, Rolf-Erik; Veldkamp, Dick; Wedel-Heinen, Jens Jakob;
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...... 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...
Institute of Scientific and Technical Information of China (English)
Alex R. Parfitt; Julian F.V. Vincent
2005-01-01
An area of protruding feathers found around the beak of many penguin species is thought to induce a turbulent boundary layer whilst swimming. Hydrodynamic tests on a model Humboldt penguin, Spheniscus humboldti, suggest that induced turbulence causes a significant reduction in boundary layer height, flow separation, and an average of 31% reduction in drag (1.0 m/s to 4.5 m/s). Visualisation of surface flow showed it to follow the body profile, over the feet and tail, before separating. Movement of the feet in swimming penguins correlates with steering of the bird. Induced turbulence may therefore further increase swimming efficiency by reducing the amount of foot movement required to direct the swimming bird.
Near Wall Investigation of Three Dimensional Turbulent Boundary Layers
Kuhl, David Derieg
2001-01-01
This report documents the experimental study for four different three-dimensional turbulent flows. The investigation focuses on near wall measurements in these flows. Several experimental techniques are used in the studies; however, the bulk of the investigation focuses on a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system. The control volume of the 3D-LDA is on the order of 50 micro-meter in size, or a y+ distance of around 2.3 units (using average v...
Directory of Open Access Journals (Sweden)
Chernoray Valery
2012-04-01
Full Text Available We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.
Response of a skewed turbulent boundary layer to favourable pressure gradient
Energy Technology Data Exchange (ETDEWEB)
Escudier, M.P.; Johnson, M.W. [Dept. of Engineering Mechanical Engineering, Liverpool Univ. (United Kingdom); Ramadan, A. [Dept. of Mechanical Engineering, Kings College, London (United Kingdom)
2001-06-01
Experimental results are reported for the response to a favourable pressure gradient of an initially turbulent boundary layer (Re{sub {theta}} {approx} 1600) developing on a flat plate with its leading edge skewed at 60 to the approach flow. The pressure gradient orthogonal to the leading edge is nominally the same as that which was shown by Escudier et al. [(1998) Exp Fluids 25: 491-502] to cause extreme thinning of a two-dimensional (2D) (i.e. unskewed) turbulent boundary layer and the intermittency in the immediate vicinity of the surface to fall to zero, i.e. an apparent laminarisation of the boundary layer. In the case of the skewed boundary layer, the responses of the turbulence and mean-flow structures are qualitatively similar to those for the 2D situation. However, the streamwise pressure gradient is much weaker than for the 2D experiment and the extent of the changes it produces is much reduced. Even so, the changes are considerably greater than would be expected from the magnitude of the streamwise pressure gradient. (orig.)
Sood, A.; Bange, J.
2009-09-01
The atmospheric boundary layer (ABL) flow is more complex at the land-sea transition zone due to the formation of coherent mesoscale land-sea breeze circulation triggered by abrupt changes in the surface roughness and thermal forcing. Since the structure of the boundary layer flow is closely related to the representation of the surface conditions as determined by e.g. orography, land use, surface roughness etc., we begin with investigating the sensitivity of the boundary layer flow to the surface forcing at the land-sea transition zone including the coastline, the islands, the near (offshore regions at the north-western German coast, the Borkum island and the offshore research platform FINO-1. The turbulent momentum, heat and moisture fluxes derived from in-situ airborne Helipod measurements are compared with results from the Mellor-Yamada-Janic (MYJ), Mellor-Yamada-Nakanishi-Niino (YMNN) and the Quasi Normal Scale Elimination (QSNE) boundary layer parameterization schemes implemented in the WRF (V3.1) mesoscale model. Since ground stations and measurement towers offer only isolated point measurements, and remote sensing methods rely strongly on assumptions on the turbulent structure of the lower part of the atmospheric boundary layer, the best strategy to obtain precise in-situ data are airborne measurements. Probably the most accurate airborne measurement platform offering highest spatial and temporal resolution of thermodynamic quantities is the helicopter-borne turbulence probe Helipod. The Helipod is attached to a 15 m rope and carried below a helicopter and outside the downwash area of the rotor blades at 40 m/s. At a sampling rate of 500 Hz, measurements of the wind vector, temperature and humidity resolve sub-meter turbulence but also large (e.g. convective) structures. Vertical profiles and horizontal legs can be flown between 1500 m and a few meters above the surface, although the latter is limited by local flight safety rules (settlements, power lines
A body-force based method to generate supersonic equilibrium turbulent boundary layer profiles
Waindim, M.; Gaitonde, D. V.
2016-01-01
We further develop a simple counterflow body force-based approach to generate an equilibrium spatially developing turbulent boundary layer suitable for Direct Numerical Simulations (DNS) or Large Eddy Simulations (LES) of viscous-inviscid interactions. The force essentially induces a small separated region in an incoming specified laminar boundary layer. The resulting unstable shear layer then transitions and breaks down to yield the desired unsteady profile. The effects of wall thermal conditions are explored to demonstrate the capability of the method for both fixed wall and adiabatic wall conditions. We then describe an efficient method to select parameters that ensure transition by examining precursor signatures using generalized stability variables. These precursors are shown to be evident in a computational domain spanning only a small region around the trip and can also be detected using 2D simulations. Finally, the method is tested for different Mach numbers ranging from 1.7 to 2.9, with emphasis on flow field surveys, Reynolds stresses, and energy spectra. These results provide guidance on boundary conditions for desired boundary layer thickness at each Mach number. The consequences of using a much lower Reynolds number in computation relative to experiment are evident at the higher Mach number, where a self sustaining turbulent boundary layer is more difficult to obtain.
Helicity and potential vorticity in the surface boundary layer turbulence
Chkhetiani, Otto; Kurgansky, Michael; Koprov, Boris; Koprov, Victor
2016-04-01
An experimental measurement of all three components of the velocity and vorticity vectors, as well as the temperature and its gradient, and potential vorticity, has been developed using four acoustic anemometers. Anemometers were placed at vertices of a tetrahedron, the horizontal base of which was a rectangular triangle with equal legs, and the upper point was exactly above the top of the right angle. The distance from the surface to the tetrahedron its base was 5.5 m, and the lengths of legs and a vertical edge were 5 m. The measurements were carried out of total duration near 100 hours both in stable and unstable stratification conditions (at the Tsimlyansk Scientific Station in a uniform area of virgin steppe 700 x 650 m, August 2012). A covariance-correlation matrix for turbulent variations in all measured values has been calculated. In the daytime horizontal and vertical components of the helicity are of the order of -0.03 and +0.01 m s-2, respectively. The nighttime signs remain unchanged, but the absolute values are several times smaller. It is confirmed also by statistics of a relative helicity. The cospectra and spectral correlation coefficients have been calculated for all helicity components. The time variations in the components of "instantaneous" relative helicity and potential vorticity are considered. Connections of helicity with Monin-Obukhov length and the wind vertical profile structure are discussed. This work was supported by the Russian Science Foundation (Project No 14-27-00134).
Laminar and turbulent boundary layer separation control of Mako shark skin
Afroz, Farhana
The Shortfin Mako shark (Isurus oxyrinchus) is one of the fastest swimmers in nature. They have an incredible turning agility and are estimated to achieve speeds as high as ten body lengths per second. Shark skin is known to contain flexible denticles or scales, capable of being actuated by the flow whereby a unique boundary layer control (BLC) method could reduce drag. It is hypothesized that shark scales bristle when the flow is reversed, and this bristling may serve to control flow separation by (1) inhibiting the localized flow reversal near the wall and (2) inducing mixing within the boundary layer by cavities formed between the scales that increases the momentum of the flow near the wall. To test this hypothesis, samples of Mako shark skin have been studied under various amounts of adverse pressure gradient (APG). These samples were collected from the flank region of a Shortfin Mako shark where the scales have the greatest potential for separation control due to the highest bristling angles. An easy technique for inducing boundary layer separation has been developed where an APG can be generated and varied using a rotating cylinder. Both the experimental and numerical studies showed that the amount of APG can be varied as a function of cylinder rotation speed or cylinder gap height for a wide range of Reynolds numbers. This method of generating an APG is used effectively for inducing both laminar and turbulent boundary layer separation over a flat plate. Laminar and turbulent boundary layer separation studies conducted over a smooth plate have been compared with the same setup repeated over shark skin. The time-averaged DPIV results showed that shark scale bristling controlled both laminar and turbulent boundary layer separation to a measurable extent. It shows that the shark scales cause an early transition to turbulence and reduce the degree of laminar separation. For turbulent separation, reverse flow near the wall and inside the boundary layer is
Separation of a turbulent supersonic boundary layer with heat supply ahead of a flat step
Larin, O. B.; Levin, V. A.
2015-05-01
The influence of an electric discharge in a supersonic gas flow modeled by a heat source with a specified intensity and configuration on the development of a turbulent boundary layer ahead of a flat step is numerically studied. If the discharge power is sufficiently large, it is demonstrated that heat transfer to the wall does not affect the position of separation, which arises due to a non-zero shear stress on the body surface and is caused by the development of a reverse flow in the core of the boundary layer.
Sun, Jielun; Mahrt, Larry; Nappo, Carmen; Lenschow, Donald
2015-04-01
We investigate atmospheric internal gravity waves (IGWs): their generation and induction of global intermittent turbulence in the nocturnal stable atmospheric boundary layer based on the new concept of turbulence generation discussed in Sun et al. (2012). The IGWs are generated by air lifted by convergence forced by the colliding background flow and cold currents near the ground. The buoyancy-forced IGWs enhance wind speed at the wind-speed wave crests such that the bulk shear instability generates large coherent eddies, which augment local turbulent mixing and vertically redistribute momentum and heat. The periodically enhanced turbulent mixing, in turn, modifies the air temperature and flow oscillations of the original IGWs. These turbulence-forced oscillations (TFOs) resemble waves and coherently transport momentum and sensible heat. The observed momentum and sensible heat fluxes at the IGW frequency, which are either due to the buoyancy-forced IGWs themselves or by the TFOs, are larger than turbulent fluxes near the surface. The IGWs enhance not only the bulk shear at the wave crests, but also local shear over the wind speed troughs of the surface IGWs. Temporal and spatial variations of turbulent mixing as a result of this wave-induced turbulent mixing change the mean air flow and the shape of the IGWs.
Institute of Scientific and Technical Information of China (English)
LIU Jian-hua; JIANG Nan; WANG Zhen-dong; SHU Wei
2005-01-01
The time sequence of longitudinal velocity component at different vertical locations in turbulent boundary layer was finely measured in a wind tunnel. The concept of coarse-grained velocity structure functions, which describes the relative motions of straining and compressing for multi-scale eddy structures in turbulent flows, was put forward based on the theory of locally multi-scale average. Based on the consistency between coarse-grained velocity structure function and Harr wavelet transformation, detecting method was presented,by which the coherent structures and their intermittency was identified by multi-scale flatness factor calculated by locally average structure function. Phase-averaged evolution course for multi-scale coherent eddy structures in wall turbulence were extracted by this conditional sampling to educe scheme. The dynamics course of multi-scale coherent eddy structures and their effects on statistics of turbulent flows were studied.
A scaling analysis of the turbulent boundary-layer in a shallow urban lake
Mezemate, Yacine; Fitton, George; Tchiguirinskaia, Ioulia; Schertzer, Daniel; Bonhomme, Céline; Soulignac, Frédéric; Lemaire, Bruno; Vinçon Leite, Brigitte
2014-05-01
The turbulent boundary-layer (TBL) has been the focus of countless experimental and numerical studies. Due to its complex nature the dynamics of the TBL are still far from being understood. Thus, to study, in particular the scaling properties of a TBL, we use a three-dimensional velocity time-series measured from an Acoustic Doppler Current Profiler(ADCP). The ADCP is particularly useful for analysing the TBL as it is able to measure the 3D velocity in the vertical, 127 cells over 3 meters. The ADCP is positioned next to a storm water discharge point at the bottom of a shallow urban lake in Créteil, a region in Paris. The positioning of the ADCP, in a stable, stratified lake, with a strong turbulent flow occurring close to the surface has given us a unique situation in which a turbulent bounded-layer can be analysed. Vertical profiles measured in the atmospheric boundary-layer are typically intrusive due to the requirement of masts and other complex measuring structures. Moreover atmospheric profilers are normally coarsely spaced in the vertical. In order to analyse the scaling properties of the velocity we compute its energy spectrum. In a log- log plot, if the velocity is scaling, the spectral exponent is its slope. It frequently that in the presence of a boundary-layer, a -1 spectral exponent is observed. Dimensional arguments suggest a -1 spectral exponent when the energy flux becomes dependent on the friction velocity instead of the length-scale. Due to the fine vertical spacing of the measurements we are not only able to observe a -1 spectral exponent, but observe a smooth transition from a free-stream turbulent regime (spectral exponent close to -5/3) to a boundary-layer -1 exponent. Because the transition shows such a strong a depth dependence we are able to propose a general model based on dynamical equations for the scaling exponent as a function of height. This generalised scaling boundary-layer model allows one to easily reproduce the turbulent
Diagnostic analysis of turbulent boundary layer data by a trivariate Lagrangian partitioning method
Energy Technology Data Exchange (ETDEWEB)
Welsh, P.T. [Florida State Univ., Tallahassee, FL (United States)
1994-12-31
The rapid scientific and technological advances in meteorological theory and modeling predominantly have occurred on the large (or synoptic) scale flow characterized by the extratropical cyclone. Turbulent boundary layer flows, in contrast, have been slower in developing both theoretically and in accuracy for several reasons. There are many existing problems in boundary layer models, among them are limits to computational power available, the inability to handle countergradient fluxes, poor growth matching to real boundary layers, and inaccuracy in calculating the diffusion of scalar concentrations. Such transport errors exist within the boundary layer as well as into the free atmosphere above. This research uses a new method, which can provide insight into these problems, and ultimately improve boundary layer models. There are several potential applications of the insights provided by this approach, among them are estimation of cloud contamination of satellite remotely sensed surface parameters, improved flux and vertical transport calculations, and better understanding of the diurnal boundary layer growth process and its hysteresis cycle.
Chakroun, Walid M.; Taylor, Robert P.
1996-01-01
The objective of this research was to experimentally investigate the combined effects of freestream acceleration and surface roughness on heat transfer and fluid flow in the turbulent boundary layer. The experiments included a variety of flow conditions ranging from aerodynamically smooth to transitionally rough to fully rough boundary layers with accelerations ranging from moderate to moderately strong. The test surfaces used were a smooth-wall test surface and two rough-wall surfaces which were roughened with 1.27 mm diameter hemispheres spaced 2 and 4 base diameters apart in a staggered array. The measurements consisted of Stanton number distributions, mean temperature profiles, skin friction distributions, mean velocity profiles, turbulence intensity profiles, and Reynolds stress profiles. The Stanton numbers for the rough-wall experiments increased with acceleration. For aerodynamically smooth and transitionally rough boundary layers, the effect of roughness is not seen immediately at the beginning of the accelerated region as it is for fully rough boundary layers; however, as the boundery layer thins under acceleration, the surface becomes relatively rougher resulting in a sharp increase in Stanton number.
A law of the wall for turbulent boundary layers with suction: Stevenson's formula revisited
Vigdorovich, Igor
2016-08-01
The turbulent velocity field in the viscous sublayer of the boundary layer with suction to a first approximation is homogeneous in any direction parallel to the wall and is determined by only three constant quantities — the wall shear stress, the suction velocity, and the fluid viscosity. This means that there exists a finite algebraic relation between the turbulent shear stress and the longitudinal mean-velocity gradient, using which as a closure condition for the equations of motion, we establish an exact asymptotic behavior of the velocity profile at the outer edge of the viscous sublayer. The obtained relationship provides a generalization of the logarithmic law to the case of wall suction.
Acoustic Radiation from High-Speed Turbulent Boundary Layers in a Tunnel-Like Environment
Duan, Lian; Choudhari, Meelan M.; Zhang, Chao
2015-01-01
Direct numerical simulation of acoustic radiation from a turbulent boundary layer in a cylindrical domain will be conducted under the flow conditions corresponding to those at the nozzle exit of the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) operated under noisy-flow conditions with a total pressure p(sub t) of 225 kPa and a total temperature of T(sub t) equal to 430 K. Simulations of acoustic radiation from a turbulent boundary layer over a flat surface are used as a reference configuration to illustrate the effects of the cylindrical enclosure. A detailed analysis of acoustic freestream disturbances in the cylindrical domain will be reported in the final paper along with a discussion pertaining to the significance of the flat-plate acoustic simulations and guidelines concerning the modeling of the effects of an axisymmetric tunnel wall on the noise field.
Jin, C; Reeks, M W
2014-01-01
We present a simple stochastic quadrant model for calculating the transport and deposition of heavy particles in a fully developed turbulent boundary layer based on the statistics of wall-normal fluid velocity fluctuations obtained from a fully developed channel flow. Individual particles are tracked through the boundary layer via their interactions with a succession of random eddies found in each of the quadrants of the fluid Reynolds shear stress domain in a homogeneous Markov chain process. Deposition rates for a range of heavy particles predicted by the model compare well with benchmark experimental measurements. In addition deposition rates are compared with those obtained from continuous random walk (CRW) models including those based on a Langevin equation for the turbulent fluctuations. Various statistics related to the particle near wall behavior are also presented.
Reeks, Michael; Jin, Chunyu; Potts, Ian
2015-11-01
We present a simple stochastic quadrant model for calculating the transport and deposition of heavy particles in a fully developed turbulent boundary layer based on the statistics of wall-normal fluid velocity fluctuations obtained from a fully developed channel flow. Individual particles are tracked through the boundary layer via their interactions with a succession of random eddies found in each of the quadrants of the fluid Reynolds shear stress domain in a homogeneous Markov chain process. Deposition rates for a range of heavy particles predicted by the model compare well with benchmark experimental measurements. In addition deposition rates are compared with those obtained continuous random walk (CRW) models including those based on the Langevin equation for the turbulent fluctuations. In addition, various statistics related to the particle near wall behavior are also presented.
Effect of low-frequency tones and turbulent-boundary-layer noise on annoyance
Shepherd, K. P.; Leatherwood, J. D.; Clevenson, S. A.
1983-01-01
A laboratory study was conducted to examine annoyance to combinations of low-frequency tones and turbulent-boundary-layer noise. A total of 240 sounds, containing tones in the range from 80 to 315 Hz, were rated by 108 test subjects in an anechoic chamber. The results indicated that tone penalties (defines as the failure of a noise metric to account for the presence of pure tones) are highly dependent on the choice of noise metric. A-weighted sound pressure level underpredicted annoyance by as much as the equivalent of 5 db and unweighted sound pressure level overpredicted by as much as the equivalent of db. Tone penalties were observed to be dependent on the shape of the turbulent boundary-layer noise spectrum.
Institute of Scientific and Technical Information of China (English)
GE Huiliang; HE Zuoyong; BAO Xuemei
2001-01-01
The point power spectrum density and the wavenumber frequency spectrum density of turbulent-boundary-layer fluctuation pressure were measured in water-tunnel by use of a φ8 mm hydrophone and a 20-element array, respectively. The non-dimensional representation of measured point power spectrum coincides with the measured results by Bull M. K. et. al. in wind tunnel. The convection peak can be seen clearly in the measured wavenumber frequencyspectrum and the convection velocity can be calculated from the location of the convection peak.The response spectrum of a polyvinylidence fluoride (PVDF) hydrophone, which receiving area is 100 mm × 60 mm, was also measured. By comparing it with the response spectrum of the φ8 mm hydrophone, it is shown that the PVDF hyrdophone has a strong wavenumber filtering effect on turbulent-boundary-layer pressure fluctuation.
On determining characteristic length scales in pressure-gradient turbulent boundary layers
Vinuesa, R.; Bobke, A.; Örlü, R.; Schlatter, P.
2016-05-01
In the present work, we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. ["Criteria for assessing experiments in zero pressure gradient boundary layers," Fluid Dyn. Res. 41, 021404 (2009)] and the one by Nickels ["Inner scaling for wall-bounded flows subject to large pressure gradients," J. Fluid Mech. 521, 217-239 (2004)], and the other one based on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. ["A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the `outer' peak," Phys. Fluids 23, 041702 (2011)]. The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a direct numerical simulation at chord Reynolds number Rec = 400 000, are used as the test case, besides other numerical and experimental data from favorable, zero, and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta-Clauser pressure-gradient parameter β larger than around 7) lead to inconsistent results in all the techniques except the diagnostic plot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlights the robustness of the diagnostic plot method to determine the
Effect of the wall roughness and external flow turbulence on boundary layer development
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Mazur, Oton; Uruba, Václav
München: European Mechanics Society, 2010. s. 22-22. ISBN N. [Euromech Fluid Mechanics Conference /8./. 13.09.2010-16.09.2010, Bad Reichenhall] R&D Projects: GA AV ČR(CZ) IAA200760614 Institutional research plan: CEZ:AV0Z20760514 Keywords : rough surface boundary layer * by-pass transition * effect of external turbulence scales Subject RIV: BK - Fluid Dynamics
Flat Plate Boundary Layer Transition Affected by the External Turbulence and Surface Roughness
Czech Academy of Sciences Publication Activity Database
Hladík, Ondřej; Jonáš, Pavel; Mazur, Oton; Uruba, Václav
Karlsruhe , 2010. s. 9-9. ISBN N. [GAMM annual meeting 2010 /81./. 22.03.2010-26.03.2010, Karlsruhe ] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer transition * external turbulence * surface roughness Subject RIV: BK - Fluid Dynamics
Stability and Dynamics of Flow in a Turbulent Boundary Layer Separation Region
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Bertinoro : TU Darmstadt, 2010 - (Oberlack, M.). s. 52-53 ISBN N. [iTi 2010 Conference in Turbulence. 19.09.2010-22.09.2010, Bertinoro] R&D Projects: GA ČR GAP101/10/1230; GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * separation * dynamics Subject RIV: BK - Fluid Dynamics
The behaviour of a compressible turbulent boundary layer under incipient separation conditions
Muck, K. C.; Smits, A. J.
1984-01-01
This paper presents an experimental study of a turbulent boundary-layer/shock-wave interaction. The interaction was generated by a two-dimensional compression corner, and the flow was on the point of separating. Measurements were made using both normal and inclined hot wires, and the data include measurements of the longitudinal mass-flow fluctuation intensity and the mass-weighted Reynolds shear stress.
Czech Academy of Sciences Publication Activity Database
Kellnerová, Radka; Jaňour, Zbyněk
Holany-Litice: Institut of Hydrodynamics AS CR, v. v. i., 2009 - (Chára, Z.; Klaboch, L.), s. 53-60 ISBN 978-80-87117-06-4. [Symposium on Anemometry /23./. Holany -Litice (CZ), 02.06.2009-03.06.2009] R&D Projects: GA MŠk OC 113 Institutional research plan: CEZ:AV0Z20760514 Keywords : very rough boundary layer * turbulence intensity * integral lengthscale Subject RIV: DG - Athmosphere Sciences, Meteorology
Weyburne, David
2015-01-01
The use of the defect profile instead of the experimentally observed velocity profile for the search for similarity parameters has become firmly imbedded in the turbulent boundary layer literature. However, a search of the literature reveals that there are no theoretical reasons for this defect profile preference over the more traditional velocity profile. In the report herein, we use the flow governing equation approach to develop similarity criteria for the two profiles. Results show that t...
Comparison of PIV and hot-wire statistics of turbulent boundary layer
Czech Academy of Sciences Publication Activity Database
Drózdz, A.; Uruba, Václav
Vol. 1. Bristol: Institute of Physics Publishing, 2014, 012044-012044. (530). ISSN 1742-6588. [Fluid Mechanics Conference : FMC 2014 /21./. Krakov (PL), 15.06.2014-18.06.2014] R&D Projects: GA ČR GAP101/10/1230; GA MŠk 7AMB13PL003 Institutional support: RVO:61388998 Keywords : turbulent boundary layer * PIV * hot wire Subject RIV: BK - Fluid Dynamics
DEFF Research Database (Denmark)
Fuhrman, David R.; Fredsøe, Jørgen; Sumer, B. Mutlu
2009-01-01
streaming is conceptually explained using analogies from steady converging and diffuser flows. A parametric study is undertaken to assess both the peak and time-averaged bed shear stresses in converging and diverging half periods under rough-turbulent conditions. The results are presented as friction factor......A numerical model solving incompressible Reynolds-averaged Navier-Stokes equations, combined with a two-equation k-omega turbulence closure, is used to study converging-diverging effects from a sloping bed on turbulent (oscillatory) wave boundary layers. Bed shear stresses from the numerical model...... diagrams. A local similarity condition is derived for relating oscillatory flow in a convergent-divergent tunnel, as considered herein, to shoaling shallow-water waves by matching spatial gradients in the free stream orbital length....
The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence
Directory of Open Access Journals (Sweden)
M. Lothon
2014-04-01
Full Text Available Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective to the night-time stable boundary layer, still raises several scientific issues. This phase of the diurnal cycle is challenging from both modeling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective regime, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of integrated instrument platforms including full-size aircraft, remotely piloted aircraft systems (RPAS, remote sensing instruments, radiosoundings, tethered balloons, surface flux stations, and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observations from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, like new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the residual layer of the previous day, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence
Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer
Zhang, Wei; Markfort, Corey D.; Porté-Agel, Fernando
2012-05-01
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical stream-wise planes ( x- z) and vertical span-wise planes ( y- z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and
The evolution of the boundary layer in turbulent Rayleigh-Bénard convection in air
du Puits, R.; Willert, C.
2016-04-01
We report measurements of the near-wall flow field in turbulent Rayleigh-Bénard convection in air (Pr = 0.7) using particle image velocimetry. The measurements were performed in a thin, rectangular sample at fixed Rayleigh number Ra = 1.45 × 1010. In particular, we focus on the evolution of the boundary layer that a single convection roll generates along its path at the lower horizontal plate. We identify three specific flow regions along this path: (i) a region of wall-normal impingement of the down flow close to one corner of the sample, (ii) a region where a shear layer with almost constant thickness evolves, and (iii) a region in which this boundary layer grows and eventually detaches from the plate surface at the opposite corner of the sample. Our measurements with a spatial resolution better than 1/500 of the total thickness of the boundary layer show that the typical velocity field as well as its statistics qualitatively varies between the three flow regions. In particular, it could be verified that the shear layer region covering about 75% of the total area of the plate is in transition to turbulence at the Rayleigh number as low as investigated in the present work.
Lyons, G. W.; Murray, N. E.
2015-12-01
Turbulence in the atmospheric boundary layer (ABL) produces fluctuations in the static pressure. The instantaneous pressure at a point depends on an integral over the entire flow; therefore, the effects from turbulence far aloft may be felt at the earth's surface. The statistics of fluctuating pressure at the surface have been studied extensively in the context of wall-bounded engineering-type flows. At best, these neutral flows are a special case of the thermally-stratified ABL, but relatively few experimental studies have considered pressure at the ground under various stability conditions. Here the scaling of pressure statistics at the surface, particularly the spectral density, is reported over a range of convective and stable conditions for both inner and outer turbulence parameters. Measurements of turbulent surface pressure were made using low-frequency microphones buried flush to the ground in a field near Laramie, Wyoming. Simultaneous measurements from three near-surface sonic anemometers and a 50-meter wind tower give estimates of the mean surface-layer parameters. The normalization of the pressure spectrum with the inner scales collapses the spectra along the high-frequency viscous power-law band. The wall shear stress, Obukhov length, L, and horizontal integral scale, λ, are identified as outer scaling parameters for the surface pressure spectrum from an integral solution employing a Monin-Obukhov-similar profile and a simple model of inhomogeneous surface-layer turbulence. Normalization with the outer scales collapses the spectra at low frequencies. Spectral scaling also reveals trends with λ/L in the low-frequency region for both convective and stable boundary layers.
DNS of laminar-turbulent boundary layer transition induced by solid obstacles
Orlandi, Paolo; Bernardini, Matteo
2015-01-01
Results of numerical simulations obtained by a staggered finite difference scheme together with an efficient immersed boundary method are presented to understand the effects of the shape of three-dimensional obstacles on the transition of a boundary layer from a laminar to a turbulent regime. Fully resolved Direct Numerical Simulations (DNS), highlight that the closer to the obstacle the symmetry is disrupted the smaller is the transitional Reynolds number. It has been also found that the transition can not be related to the critical roughness Reynolds number used in the past. The simulations highlight the differences between wake and inflectional instabilities, proving that two-dimensional tripping devices are more efficient in promoting the transition. Simulations at high Reynolds number demonstrate that the reproduction of a real experiment with a solid obstacle at the inlet is an efficient tool to generate numerical data bases for understanding the physics of boundary layers. The quality of the numerical ...
National Aeronautics and Space Administration — Large amplitude, unsteady heating loads and steep flow gradients produced in regions of shock-wave/turbulent boundary-layer interaction (SWTBLI) pose a serious and...
Coherent structures in direct numerical simulation of turbulent boundary layers at Mach 3
Ringuette, Matthew J.; Wu, Minwei; Mart?N, M. Pino
We demonstrate that data from direct numerical simulation of turbulent boundary layers at Mach 3 exhibit the same large-scale coherent structures that are found in supersonic and subsonic experiments, namely elongated, low-speed features in the logarithmic region and hairpin vortex packets. Contour plots of the streamwise mass flux show very long low-momentum structures in the logarithmic layer. These low-momentum features carry about one-third of the turbulent kinetic energy. Using Taylor's hypothesis, we find that these structures prevail and meander for very long streamwise distances. Structure lengths on the order of 100 boundary layer thicknesses are observed. Length scales obtained from correlations of the streamwise mass flux severely underpredict the extent of these structures, most likely because of their significant meandering in the spanwise direction. A hairpin-packet-finding algorithm is employed to determine the average packet properties, and we find that the Mach 3 packets are similar to those observed at subsonic conditions. A connection between the wall shear stress and hairpin packets is observed. Visualization of the instantaneous turbulence structure shows that groups of hairpin packets are frequently located above the long low-momentum structures. This finding is consistent with the very large-scale motion model of Kim & Adrian (1999).
Krypton tagging velocimetry in a turbulent Mach 2.7 boundary layer
Zahradka, D.; Parziale, N. J.; Smith, M. S.; Marineau, E. C.
2016-05-01
The krypton tagging velocimetry (KTV) technique is applied to the turbulent boundary layer on the wall of the "Mach 3 Calibration Tunnel" at Arnold Engineering Development Complex (AEDC) White Oak. Profiles of velocity were measured with KTV and Pitot-pressure probes in the Mach 2.7 turbulent boundary layer comprised of 99 % {N}2/1 % Kr at momentum-thickness Reynolds numbers of {Re}_{\\varTheta }= 800, 1400, and 2400. Agreement between the KTV- and Pitot-derived velocity profiles is excellent. The KTV and Pitot velocity data follow the law of the wall in the logarithmic region with application of the Van Driest I transformation. The velocity data are analyzed in the outer region of the boundary layer with the law of the wake and a velocity-defect law. KTV-derived streamwise velocity fluctuation measurements are reported and are consistent with data from the literature. To enable near-wall measurement with KTV (y/δ ≈ 0.1-0.2), an 800-nm longpass filter was used to block the 760.2-nm read-laser pulse. With the longpass filter, the 819.0-nm emission from the re-excited Kr can be imaged to track the displacement of the metastable tracer without imaging the reflection and scatter from the read-laser off of solid surfaces. To operate the Mach 3 AEDC Calibration Tunnel at several discrete unit Reynolds numbers, a modification was required and is described herein.
Extraction of very-large scale structures in turbulent boundary layer
Roux, Stéphane; Kerhervé, Franck; Stanislas, Michel; Marc Foucaut, Jean; Delville, Joel; Team
2012-11-01
The examined flow is a zero-pressure gradient turbulent boundary layer. The data used are taken from the joined experimental campaign conducted during the european WALLTURB program in the large wind tunnel at Laboratoire de Mécanique de Lille (LML). The free-stream velocity is 10 m/s. At the investigated position, the boundary layer thickness is 30 cm and the Reynolds number based on the momentum thickness is 19100. A methodology for eduction of super-structures is presented. These structures are characterised by a large degree of persistance and are thought to participate actively to the turbulence regeneration in the near-wall region (Marusic et al. 2010). A time-resolved estimate of the three-dimensionnal structures is obtained by combining low-speed two-dimensional stereo-PIV at 4 Hz and a two-dimensionnal rake of 143 single hot-wire probes at 30 kHz. The very large scale structures are clearly reconstructed which exhibit a streamwise extent an order of magnitude larger than the boundary layer thickness. Interest is particulary focused on the low-speed species of these structures. Associated coounter-rotating vortices are also evidenced in good agreement with the litterature.
Tomographic PIV investigation of coherent structures in a turbulent boundary layer flow
Institute of Scientific and Technical Information of China (English)
Zhan-Qi Tang; Nan Jiang; Andreas Schr(ǒ)der; Reinhard Geisler
2012-01-01
Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in the logarithmic region of the turbulent boundary layer in a water tunnel.The Reynolds number based on momentum thickness is Reθ =2 460.The instantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low speed fluid,which is flanked on either side by highspeed ones.Statistical support for the existence of hairpins is given by conditional averaged eddy within an increasing spanwise width as the distance from the wall increases,and the main vortex characteristic in different wall-normal regions can be reflected by comparing the proportion of ejection and its contribution to Reynolds stress with that of sweep event.The pre-multiplied power spectra and two-point correlations indicate the presence of large-scale motions in the boundary layer,which are consistent with what have been termed very large scale motions (VLSMs).The three dimensional spatial correlations of three components of velocity further indicate that the elongated low-speed and highspeed regions will be accompanied by a counter-rotating roll modes,as the statistical imprint of hairpin packet structures,all of which together make up the characteristic of coherent structures in the logarithmic region of the turbulent boundary layer (TBL).
Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models
Lazeroms, W. M. J.; Svensson, G.; Bazile, E.; Brethouwer, G.; Wallin, S.; Johansson, A. V.
2016-08-01
We test a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the atmospheric boundary layer. First of all, we consider a stable boundary layer used as the well-known first test case from the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS1). The model is shown to agree well with data from large-eddy simulations (LES), and this agreement is significantly better than for a standard operational scheme with a prognostic equation for turbulent kinetic energy. Furthermore, we apply the model to a case with a (idealized) diurnal cycle and make a qualitative comparison with a simpler first-order model. Some interesting features of the model are highlighted, pertaining to its stronger foundation on physical principles. In particular, the use of more prognostic equations in the model is shown to give a more realistic dynamical behaviour. This qualitative study is the first step towards a more detailed comparison, for which additional LES data are needed.
Global effect of local skin friction drag reduction in spatially developing turbulent boundary layer
Stroh, A; Schlatter, P; Frohnapfel, B
2016-01-01
A numerical investigation of two locally applied drag reducing control schemes is carried out in the configuration of a spatially developing turbulent boundary layer (TBL). One control is designed to damp near-wall turbulence and the other induces constant mass flux in the wall-normal direction. Both control schemes yield similar local drag reduction rates within the control region. However, the flow development downstream of the control significantly differs: persistent drag reduction is found for the uniform blowing case whereas drag increase is found for the turbulence damping case. In order to account for this difference the formulation of a global drag reduction rate is suggested. It represents the reduction of the streamwise force exerted by the fluid on a finite length plate. Furthermore, it is shown that the far downstream development of the TBL after the control region can be described by a single quantity, namely a streamwise shift of the uncontrolled boundary layer, i.e. a changed virtual origin. B...
Influence of pressure gradient on streamwise skewness factor in turbulent boundary layer
International Nuclear Information System (INIS)
The paper shows an effect of favourable and adverse pressure gradients on turbulent boundary layer. The skewness factor of streamwise velocity component was chosen as a measure of the pressure gradient impact. It appears that skewness factor is an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the skewness factor decreases in the flow region subjected to FPG and increases in the APG conditions. The changes of convection velocity and skewness factor are caused by influence of large-scale motion through the mechanism called amplitude modulation. The large-scale motion is less active in FPG and more active in APG, therefore in FPG the production of vortices is random (there are no high and low speed regions), while in the APG the large-scale motion drives the production of vortices. Namely, the vortices appear only in the high-speed regions, therefore have convection velocity higher than local mean velocity. The convection velocity affects directly the turbulent sweep and ejection events. The more flow is dominated by large-scale motion the higher values takes both the convection velocity of small-scale structures and sweep events induced by them.
Numerical simulation of a shock wave/turbulent boundary layer interaction in a duct
Yang, Wei-Li; Greber, Isaac
1993-01-01
A numerical investigation of the interaction of an incident oblique shock wave with a turbulent duct flow is presented. The investigation consists of solving the three-dimensional, unsteady, compressible, mass averaged Navier-Stokes equations, using an implicit finite volume, lower-upper time marching code and incorporates the three-dimensional Baldwin-Lomax turbulence model. Computed results are obtained Mach number 2.9 for a turning angle of 13 degrees and Reynolds number based on duct width of 1.36 x 10 exp 7. Under various inlet conditions, the results clearly depict the flow characteristics, including the shock geometry, the separated flow region, the wall pressure distribution, and the skin friction distribution. The findings provide a physical understanding of the three-dimensional vortex structure of the flow in a duct in which a shock wave interacts with a turbulent boundary layer. The results show that the ratio of the boundary layer thickness to the duct width is the critical parameter in determining the separation structure.
Large-eddy simulation of a solid-particles suspension in a turbulent boundary layer
Rahman, Mustafa; Samtaney, Ravi
2014-11-01
We decribe a framework for the large-eddy simulation of solid particles suspended and transported within an incompressible turbulent boundary layer. The underlying approach to simulate the solid-particle laden flow is Eulerian-Eulerian in which the particles are characterized by statistical descriptors. For the fluid phase, the large-eddy simulation (LES) of incompressible turbulent boundary layer employs stretched spiral vortex subgrid-scale model and a virtual wall model similar to the work of Inoue & Pullin (J. Fluid Mech. 2011). Furthermore, a recycling method to generate turbulent inflow is implemented. For the particle phase, the direct quadrature method of moments (DQMOM) is chosen in which the weights and abscissas of the quadrature approximation are tracked directly rather than the moments themselves. The numerical method in this framework is based on a fractional-step method with an energy-conservative fourth-order finite difference scheme on a staggered mesh. It is proposed to utilize this framework to examine transport of sand in desert sandstorms. Supported by KAUST OCRF funded CRG project on simulation of sandstorms.
Advances of drag-reducing surface technologies in turbulence based on boundary layer control
Institute of Scientific and Technical Information of China (English)
LUO Yuehao; WANG Liguo; GREEN Lork; SONG Kenan; WANG Liang; SMITH Robert
2015-01-01
Our living environment is surrounded by turbulence, which is also a concern of the global energy consumption and the greenhouse gas emission, and the viscous force on the solid-liquid/solid-gas interface is an important part of the turbulence. Reducing friction force in turbulence to the greatest extent is becoming an urgent issue to be resolved at present. In this paper, the various state-of-the-art approaches of drag-reducing and energy-saving technologies based on the boundary layer control are reviewed, focusing on the polymer drag reduction additives, the micro-morphology, the super-hydrophobic surface, the micro air bubbles, the heating wall, the vibrant flexible wall and the composite drag reduction methods. In addition, the mechanisms of different drag reductions based on the boundary layer control and the potential applications in fluid engineering are discussed. This paper aims not only to contribute to a better understanding of drag reduction mechanisms, but also to offer new perspectives to improve the current drag-reducing and energy saving technologies.
Breaking the boundary layer symmetry in turbulent convection using wall geometry
Toppaladoddi, Srikanth; Wettlaufer, John S
2014-01-01
We systematically probe the interaction of the boundary layer with the core flow during two-dimensional turbulent Rayleigh-B\\'{e}nard convection using numerical simulations and scaling theory. The boundary layer/core flow interaction is manipulated by configuring the top plate with a sinusoidal geometry and breaking the symmetry between the top and bottom thermal boundary layers. At long wavelength the planar results are recovered. However, at intermediate wavelengths, and for Rayleigh numbers ($Ra$) such that the amplitude of the roughness elements is larger than the boundary layer thickness, there is enhanced cold plume production at the tips of the elements. It is found that, while the interior of the flow is well mixed as in the classical theory of Malkus, the mean temperature is lower than that in the planar case. For a Prandtl number of unity and $Ra = 10^6$ to $2.5 \\times 10^9$ we find a Nusselt number ($Nu$) scaling law of $Nu = 0.052 \\times Ra^{0.34}$, in good agreement with recent experiments. The c...
Direct simulation of turbulent supersonic boundary layers by an extended temporal approach
Maeder, Thierry; Adams, Nikolaus A.; Kleiser, Leonhard
2001-02-01
The present paper addresses the direct numerical simulation of turbulent zero-pressure-gradient boundary layers on a flat plate at Mach numbers 3, 4.5 and 6 with momentum-thickness Reynolds numbers of about 3000. Simulations are performed with an extended temporal direct numerical simulation (ETDNS) method. Assuming that the slow streamwise variation of the mean boundary layer is governed by parabolized Navier Stokes equations, the equations solved locally in time with a temporal DNS are modified by a distributed forcing term so that the parabolized Navier Stokes equations are recovered for the spatial average. The correct mean flow is obtained without a priori knowledge, the streamwise mean-flow evolution being approximated from its upstream history. ETDNS reduces the computational effort by up to two orders of magnitude compared to a fully spatial simulation.
A measure of scale-dependent asymmetry in turbulent boundary layer flows
Guala, Michele; Singh, Arvind
2015-11-01
The distribution of scale-dependent, streamwise velocity increments is investigated in turbulent boundary layer flows at laboratory and atmospheric Reynolds number, using the SAFL wind tunnel (Singh et al. Phys. of Fluids 2014) and the SLTEST data (Metzger et al. Phil. Trans Royal Soc. A 2007). The third order moments of velocity increments, or asymmetry index As(a,z), is computed for varying wall distance z and scale separation a, where it was observed to leave a robust, distinct signature in the form of a hump, independent of Reynolds number and located across the inertial subrange. The hump is observed for z + intermittency is also enhanced in the same flow region. The combination of asymmetry and intermittency is inferred to point at non-local energy transfer across a range of scales and may thus be used to quantify interactions between structural types in boundary layer flows.
DNS of a spatially developing turbulent boundary layer with passive scalar transport
International Nuclear Information System (INIS)
A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier-Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U∞ and momentum thickness θ is Reθ=830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors' knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community.
DNS of a spatially developing turbulent boundary layer with passive scalar transport
Energy Technology Data Exchange (ETDEWEB)
Li Qiang [Linne Flow Centre, KTH Mechanics, Osquars Backe 18, SE-100 44 Stockholm (Sweden)], E-mail: qiang@mech.kth.se; Schlatter, Philipp; Brandt, Luca; Henningson, Dan S. [Linne Flow Centre, KTH Mechanics, Osquars Backe 18, SE-100 44 Stockholm (Sweden)
2009-10-15
A direct numerical simulation (DNS) of a spatially developing turbulent boundary layer over a flat plate under zero pressure gradient (ZPG) has been carried out. The evolution of several passive scalars with both isoscalar and isoflux wall boundary condition are computed during the simulation. The Navier-Stokes equations as well as the scalar transport equation are solved using a fully spectral method. The highest Reynolds number based on the free-stream velocity U{sub {infinity}} and momentum thickness {theta} is Re{sub {theta}}=830, and the molecular Prandtl numbers are 0.2, 0.71 and 2. To the authors' knowledge, this Reynolds number is to date the highest with such a variety of scalars. A large number of turbulence statistics for both flow and scalar fields are obtained and compared when possible to existing experimental and numerical simulations at comparable Reynolds number. The main focus of the present paper is on the statistical behaviour of the scalars in the outer region of the boundary layer, distinctly different from the channel-flow simulations. Agreements as well as discrepancies are discussed while the influence of the molecular Prandtl number and wall boundary conditions is also highlighted. A Pr scaling for various quantities is proposed in outer scalings. In addition, spanwise two-point correlation and instantaneous fields are employed to investigate the near-wall streak spacing and the coherence between the velocity and the scalar fields. Probability density functions (PDF) and joint probability density functions (JPDF) are shown to identify the intermittency both near the wall and in the outer region of the boundary layer. The present simulation data will be available online for the research community.
Henderson, Stephen M.
2016-04-01
Internal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10-24 h). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4 m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10-3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1-4.5 m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/d frequency to 0.7 at 2.4 cycles/d (cf. near-unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8-2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency dependence of the seiche reflection coefficient. For flat-bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation.
International Nuclear Information System (INIS)
Highlights: ► We study the turbulent boundary layer with separation and reattachment by DNS. ► Turbulent boundary layer with heat transfer over 2-D block is observed. ► Counter gradient diffusion phenomenon (CDP) can be found in thermal field. ► It can be found that interaction events affect the occurrence of CDP. ► The correlations among instantaneous quantities are clearly shown in the DNS. - Abstract: This paper presents observations and investigations of the detailed structure and mechanism of turbulent heat transfer in the turbulent boundary layer with separation and reattachment by means of direct numerical simulation (DNS). In order to observe turbulent heat transfer in a boundary layer with reattachment and separation, a DNS of the boundary layer with heat transfer over a 2-dimensional block (2DB) is carried out, in which the effects of Reynolds number and block size are observed. The lengths of reattachment and maximum Stanton number points behind 2DB become longer with an increase in Reynolds number in the case of similar block size with one exception. On the other hand, these points become shorter with an increase in the width of the 2DB. Moreover, the counter gradient diffusion phenomenon (CDP) of the thermal field can be found on the 2DB. A quadrant analysis is carried out to investigate the turbulence motion which decides Reynolds shear stress and the wall-normal turbulent heat flux in the turbulent boundary layer with heat transfer over 2DB, in which it can be found that Q1 and Q3 events (i.e., interactions) affect the occurrence of CDP on 2DB. Also, the correlations among instantaneous fluctuating temperature, temperature gradient and vorticities to observe the turbulent structures of heat transfer around 2DB are clearly shown in the present DNS.
Directory of Open Access Journals (Sweden)
Jonáš P.
2008-12-01
Full Text Available This paper considers the knowledge of the individual action and joint action of surface roughness and external flow turbulence on the mean flow in boundary layer. The experimental evidence of this problem has been reviewed. A lack of results has been ascertain of the investigation on the joint action of the mentioned influences on the development of a boundary layer from the state with laminar flow up to a turbulent boundary layer. The knowledge on the actions of individual effects has been gathered with the regard to the improvement of the evaluation and analysis of the mean flow characteristics of the zero pressure gradient boundary layer developing under the joint action of the uniform roughness of the surface and homogeneous, close to isotropy, free stream turbulence.
Buono, Armand C.
The numerical method presented in this study attempts to predict the mean, non-uniform flow field upstream of a propeller partially immersed in a thick turbulent boundary layer with an actuator disk using CFD based on RANS in ANSYS FLUENT. Three different configurations, involving an infinitely thin actuator disk in the freestream (Configuration 1), an actuator disk near a wall with a turbulent boundary layer (Configuration 2), and an actuator disk with a hub near a wall with a turbulent boundary layer (Configuration 3), were analyzed for a variety of advance ratios ranging from J = 0.48 to J =1.44. CFD results are shown to be in agreement with previous works and validated with experimental data of reverse flow occurring within the boundary layer above the flat plate upstream of a rotor in the Virginia Tech's Stability Wind Tunnel facility. Results from Configuration 3 will be used in future aero-acoustic computations.
Stability and Dynamics of Flow in a Turbulent Boundary Layer Separation Region
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Berlin, Heidelberg : Springer-Verlag, 2012, s. 105-108. ISBN 978-3-642-28967-5. ISSN 0930-8989. - (Springer Proceedings in Physics. 141). [iTi 2010 Conference in Turbulence. Bertinoro (IT), 19.09.2010-22.09.2010] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Institutional support: RVO:61388998 Keywords : boundary layer separation * stability * dynamics Subject RIV: BK - Fluid Dynamics
Numerical simulation of quasi-streamwise hairpin-like vortex generation in turbulent boundary layer
Institute of Scientific and Technical Information of China (English)
ZHANG Nan; LU Li-peng; DUAN Zhen-zhen; YUAN Xiang-jiang
2008-01-01
A mechanism for generation of near wall quasi-streamwise hairpin-like vortex (QHV) and secondary quasi-streamwise vortices (SQV) is presented. The conceptual model of resonant triad in the theory of hydrodynamic instability and direct numerical simulation of a turbulent boundary layer were applied to reveal the formation of QHV and SQV. The generation procedures and the characteristics of the vortex structures are obtained, which share some similarities with previous numerical simulations. The research using resonant triad conceptual model and numerical simulation provides a possibility for investigating and controling the vortex structures, which play a dominant role in the evolution of coherent structures in the near-wall region.
International Nuclear Information System (INIS)
We report high-resolution LDA and HWA measurements of the streamwise velocity component of a flat-plate turbulent boundary layer (ZPG TBL) over a range of momentum thickness Reynolds number from 1,170 to 3,720. The primary objective of this work is to investigate the near-wall behavior and the scaling of high-order statistics. In particular, we are interested in certain Kármán number dependencies. The obtained data are in excellent agreement with most recent DNS-results, which allows direct comparison of detailed results such as peak value and position of streamwise stress, wall-values of skewness and flatness factors, and turbulence dissipation rate. The experimental data clearly reveal the failure of classical scaling. An alternative mixed scaling based on uτ3/2ue1/2 removes these discrepancies.
Buschmann, M. H.; Keirsbulck, L.; Fourrié, G.; Labraga, L.; Gad-el-Hak, M.
2011-12-01
We report high-resolution LDA and HWA measurements of the streamwise velocity component of a flat-plate turbulent boundary layer (ZPG TBL) over a range of momentum thickness Reynolds number from 1,170 to 3,720. The primary objective of this work is to investigate the near-wall behavior and the scaling of high-order statistics. In particular, we are interested in certain Kármán number dependencies. The obtained data are in excellent agreement with most recent DNS-results, which allows direct comparison of detailed results such as peak value and position of streamwise stress, wall-values of skewness and flatness factors, and turbulence dissipation rate. The experimental data clearly reveal the failure of classical scaling. An alternative mixed scaling based on uτ3/2ue1/2 removes these discrepancies.
Double large field stereoscopic PIV in a high Reynolds number turbulent boundary layer
Coudert, S.; Foucaut, J. M.; Kostas, J.; Stanislas, M.; Braud, P.; Fourment, C.; Delville, J.; Tutkun, M.; Mehdi, F.; Johansson, P.; George, W. K.
2011-01-01
An experiment on a flat plate turbulent boundary layer at high Reynolds number has been carried out in the Laboratoire de Mecanique de Lille (LML, UMR CNRS 8107) wind tunnel. This experiment was performed jointly with LEA (UMR CNRS 6609) in Poitiers (France) and Chalmers University of Technology (Sweden), in the frame of the WALLTURB European project. The simultaneous recording of 143 hot wires in one transverse plane and of two perpendicular stereoscopic PIV fields was performed successfully. The first SPIV plane is 1 cm upstream of the hot wire rake and the second is both orthogonal to the first one and to the wall. The first PIV results show a blockage effect which based on both statistical results (i.e. mean, RMS and spatial correlation) and a potential model does not seem to affect the turbulence organization.
Coupled Mesoscale-Large-Eddy Modeling of Realistic Stable Boundary Layer Turbulence
Wang, Yao; Manuel, Lance
2013-01-01
Site-specific flow and turbulence information are needed for various practical applications, ranging from aerodynamic/aeroelastic modeling for wind turbine design to optical diffraction calculations. Even though highly desirable, collecting on-site meteorological measurements can be an expensive, time-consuming, and sometimes a challenging task. In this work, we propose a coupled mesoscale-large-eddy modeling framework to synthetically generate site-specific flow and turbulence data. The workhorses behind our framework are a state-of-the-art, open-source atmospheric model called the Weather Research and Forecasting (WRF) model and a tuning-free large-eddy simulation (LES) model. Using this coupled framework, we simulate a nighttime stable boundary layer (SBL) case from the well-known CASES-99 field campaign. One of the unique aspects of this work is the usage of a diverse range of observations for characterization and validation. The coupled models reproduce certain characteristics of observed low-level jets....
Large-scale motions in a supersonic turbulent boundary layer on a curved surface
Donovan, J. F.; Smits, A. J.
1990-01-01
This paper presents measurements in a Mach 3 turbulent boundary layer which was subjected to a short region of concave surface curvature. Mean velocity and time-averaged turbulence measurements are presented to indicate the severity of the distortion, although the focus of this study is on how the large-scale organized motions are affected by the distortion. A 12 deg increase in the inclination angle of organized motions is observed, and correlation measurements indicate an increase in the streamwise length scale. VITA conditional sampling is used to identify strong positive streamwise mass-flux gradients as the large-scale structure responsible for the measured structure angle and to determine the ensemble-averaged flowfield associated with the mass-flux gradient structures.
Edge states for the turbulence transition in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M; Eckhardt, Bruno
2013-01-01
We demonstrate the existence of an exact invariant solution to the Navier-Stokes equations for the asymptotic suction boundary layer. The identified periodic orbit with a very long period of several thousand advective time units is found as a local dynamical attractor embedded in the stability boundary between laminar and turbulent dynamics. Its dynamics captures both the interplay of downstream oriented vortex pairs and streaks observed in numerous shear flows as well as the energetic bursting that is characteristic for boundary layers. By embedding the flow into a family of flows that interpolates between plane Couette flow and the boundary layer we demonstrate that the periodic orbit emerges in a saddle-node infinite-period (SNIPER) bifurcation of two symmetry-related travelling wave solutions of plane Couette flow. Physically, the long period is due to a slow streak instability which leads to a violent breakup of a streak associated with the bursting and the reformation of the streak at a different spanwi...
Structuring of turbulence and its impact on basic features of Ekman boundary layers
Directory of Open Access Journals (Sweden)
I. Esau
2013-08-01
Full Text Available The turbulent Ekman boundary layer (EBL has been studied in a large number of theoretical, laboratory and modeling works since F. Nansen's observations during the Norwegian Polar Expedition 1893–1896. Nevertheless, the proposed analytical models, analysis of the EBL instabilities, and turbulence-resolving numerical simulations are not fully consistent. In particular, the role of turbulence self-organization into longitudinal roll vortices in the EBL and its dependence on the meridional component of the Coriolis force remain unclear. A new set of large-eddy simulations (LES are presented in this study. LES were performed for eight different latitudes (from 1° N to 90° N in the domain spanning 144 km in the meridional direction. Geostrophic winds from the west and from the east were used to drive the development of EBL turbulence. The emergence and growth of longitudinal rolls in the EBL was simulated. The simulated rolls are in good agreement with EBL stability analysis given in Dubos et al. (2008. The destruction of rolls in the westerly flow at low latitude was observed in simulations, which agrees well with the action of secondary instability on the rolls in the EBL. This study quantifies the effect of the meridional component of the Coriolis force and the effect of rolls in the EBL on the internal EBL parameters such as friction velocity, cross-isobaric angle, parameters of the EBL depth and resistance laws. A large impact of the roll development or destruction is found. The depth of the EBL in the westerly flow is about five times less than it is in the easterly flow at low latitudes. The EBL parameters, which depend on the depth, also exhibit large difference in these two types of the EBL. Thus, this study supports the need to include the horizontal component of the Coriolis force into theoretical constructions and parameterizations of the boundary layer in models.
Markovian properties of velocity increments in a high Reynolds number turbulent boundary layer
Fredbo, Maren; Tutkun, Murat
2010-11-01
Statistics of velocity increments in a flat plate turbulent boundary layer are investigated using the theory of Markov processes (J. Fluid Mech., Vol. 433, pp. 383-409, 2001). The database analyzed here is a subset of data taken in the 20 m long wind tunnel of Laboratoire de M'ecanique de Lille (LML) using a hot-wire rake of 143 single wire probes. The Reynolds number based on momentum thickness, Reθ, tested in this study was 19:100. The freestream velocity of the tunnel and the boundary layer thickness at the measurement location were 10 m s-1 and 30 cm respectively. Our analysis on the increments of longitudinal velocities at different wall-normal positions show that the flow exhibits Markovian properties when the separation (δr) between different scales is on the order of the Taylor microscale, λ. Initial results indicate that smallest δr/λ, where the process can be defined as Markovian, decreases from wall to the inertial layer. As the probe moves inside the inertial layer, however, a constant δr/λ is observed. The ratio starts growing in the outer layer once the probe leaves the inertial layer.
On the Formation Mechanisms of Artificially Generated High Reynolds Number Turbulent Boundary Layers
Rodríguez-López, Eduardo; Bruce, Paul J. K.; Buxton, Oliver R. H.
2016-08-01
We investigate the evolution of an artificially thick turbulent boundary layer generated by two families of small obstacles (divided into uniform and non-uniform wall normal distributions of blockage). One- and two-point velocity measurements using constant temperature anemometry show that the canonical behaviour of a boundary layer is recovered after an adaptation region downstream of the trips presenting 150~% higher momentum thickness (or equivalently, Reynolds number) than the natural case for the same downstream distance (x≈ 3 m). The effect of the degree of immersion of the trips for h/δ ≳ 1 is shown to play a secondary role. The one-point diagnostic quantities used to assess the degree of recovery of the canonical properties are the friction coefficient (representative of the inner motions), the shape factor and wake parameter (representative of the wake regions); they provide a severe test to be applied to artificially generated boundary layers. Simultaneous two-point velocity measurements of both spanwise and wall-normal correlations and the modulation of inner velocity by the outer structures show that there are two different formation mechanisms for the boundary layer. The trips with high aspect ratio and uniform distributed blockage leave the inner motions of the boundary layer relatively undisturbed, which subsequently drive the mixing of the obstacles' wake with the wall-bounded flow (wall-driven). In contrast, the low aspect-ratio trips with non-uniform blockage destroy the inner structures, which are then re-formed further downstream under the influence of the wake of the trips (wake-driven).
Jens, Elizabeth T.; Miller, Victor A.; Cantwell, Brian J.
2016-03-01
Combustion in a turbulent boundary layer over a solid fuel is studied using simultaneous schlieren and OH* chemiluminescence imaging. The flow configuration is representative of a hybrid rocket motor combustor. Six different hydrocarbon fuels, including both classical hybrid rocket fuels and a high regression rate fuel (paraffin wax), are burned in an undiluted oxygen free-stream at pressures ranging from atmospheric to 1524.2 kPa (221.1 psi). A detailed explanation of methods for registering the schlieren and OH* chemiluminescence images to one another is presented, and additionally, details of the routines used to extract flow features of interest (like the boundary layer height and flame location) are provided. At atmospheric pressure, the boundary layer location is consistent between all fuels; however, the flame location varies for each fuel. The flame zone appears to be smoothly distributed over the fuel surface at atmospheric pressure. At elevated pressures and correspondingly increased Dahmköhler number (but at constant Reynolds number), flame morphology is markedly different, exhibiting large rollers in a shear layer above the fuel grain and finer structures in the flame. The chemiluminescence intensity is found to be roughly proportional to the fuel burn rate at both atmospheric and elevated chamber pressures.
Cluster observation of magnetohydrodynamic turbulence in the plasma sheet boundary layer
Narita, Y.
2016-04-01
Measurement of turbulent magnetic field is presented from the Earth magnetotail crossing of the Cluster spacecraft on August 25, 2006, as an ideal case study of magnetohydrodynamic turbulence in the plasma sheet boundary layer on a spatial scale of about 10,000 km. The fluctuation energy of the magnetic field is evaluated in both the frequency and wavevector domains. The observed plasma sheet turbulence event shows anisotropy in the wavevector domain with a spectral extension perpendicular to the mean magnetic field. The analyses of the dispersion relation and phase speed diagrams indicate that the coherent wave components should be regarded as a set of the linear-mode waves and the other fluctuation components in magnetohydrodynamics. Although the magnetic field fluctuation amplitudes are sufficiently small compared to the large-scale field strength, there is no clear indication of the linear-mode dominance in the plasma sheet. As a lesson, magnetohydrodynamic turbulence must be modeled by including both linear-mode waves and nonlinear wave components such as sideband waves.
International Nuclear Information System (INIS)
Highlights: • We study the turbulent boundary layer with heat transfer by DNS. • Turbulent boundary layers with suddenly changing wall thermal conditions are observed. • The detailed turbulent statistics and structures in turbulent thermal boundary layer are discussed. • Turbulence models in LES and RANS are evaluated using DNS results. • LES and RANS are almost in good agreement with DNS results. -- Abstract: The objectives of this study are to investigate a thermal field in a turbulent boundary layer with suddenly changing wall thermal conditions by means of direct numerical simulation (DNS), and to evaluate predictions of a turbulence model in such a thermal field, in which DNS of spatially developing boundary layers with heat transfer can be conducted using the generation of turbulent inflow data as a method. In this study, two types of wall thermal condition are investigated using DNS and predicted by large eddy simulation (LES) and Reynolds-averaged Navier–Stokes equation simulation (RANS). In the first case, the velocity boundary layer only develops in the entrance of simulation, and the flat plate is heated from the halfway point, i.e., the adiabatic wall condition is adopted in the entrance, and the entrance region of thermal field in turbulence is simulated. Then, the thermal boundary layer develops along a constant temperature wall followed by adiabatic wall. In the second case, velocity and thermal boundary layers simultaneously develop, and the wall thermal condition is changed from a constant temperature to an adiabatic wall in the downstream region. DNS results clearly show the statistics and structure of turbulent heat transfer in a constant temperature wall followed by an adiabatic wall. In the first case, the entrance region of thermal field in turbulence can be also observed. Thus, both the development and the entrance regions in thermal fields can be explored, and the effects upstream of the thermal field on the adiabatic region are
DNS of Laminar-Turbulent Transition in Swept-Wing Boundary Layers
Duan, L.; Choudhari, M.; Li, F.
2014-01-01
Direct numerical simulation (DNS) is performed to examine laminar to turbulent transition due to high-frequency secondary instability of stationary crossflow vortices in a subsonic swept-wing boundary layer for a realistic natural-laminar-flow airfoil configuration. The secondary instability is introduced via inflow forcing and the mode selected for forcing corresponds to the most amplified secondary instability mode that, in this case, derives a majority of its growth from energy production mechanisms associated with the wall-normal shear of the stationary basic state. An inlet boundary condition is carefully designed to allow for accurate injection of instability wave modes and minimize acoustic reflections at numerical boundaries. Nonlinear parabolized stability equation (PSE) predictions compare well with the DNS in terms of modal amplitudes and modal shape during the strongly nonlinear phase of the secondary instability mode. During the transition process, the skin friction coefficient rises rather rapidly and the wall-shear distribution shows a sawtooth pattern that is analogous to the previously documented surface flow visualizations of transition due to stationary crossflow instability. Fully turbulent features are observed in the downstream region of the flow.
Energy Technology Data Exchange (ETDEWEB)
Jin, C.; Potts, I.; Reeks, M. W., E-mail: mike.reeks@ncl.ac.uk [School of Mechanical and Systems Engineering, Newcastle University, Stephenson Building, Claremont Road, Newcastle upon Tyne NE1 7RU (United Kingdom)
2015-05-15
We present a simple stochastic quadrant model for calculating the transport and deposition of heavy particles in a fully developed turbulent boundary layer based on the statistics of wall-normal fluid velocity fluctuations obtained from a fully developed channel flow. Individual particles are tracked through the boundary layer via their interactions with a succession of random eddies found in each of the quadrants of the fluid Reynolds shear stress domain in a homogeneous Markov chain process. In this way, we are able to account directly for the influence of ejection and sweeping events as others have done but without resorting to the use of adjustable parameters. Deposition rate predictions for a wide range of heavy particles predicted by the model compare well with benchmark experimental measurements. In addition, deposition rates are compared with those obtained from continuous random walk models and Langevin equation based ejection and sweep models which noticeably give significantly lower deposition rates. Various statistics related to the particle near wall behavior are also presented. Finally, we consider the model limitations in using the model to calculate deposition in more complex flows where the near wall turbulence may be significantly different.
Computation of turbulent boundary layers employing the defect wall-function method. M.S. Thesis
Brown, Douglas L.
1994-01-01
In order to decrease overall computational time requirements of spatially-marching parabolized Navier-Stokes finite-difference computer code when applied to turbulent fluid flow, a wall-function methodology, originally proposed by R. Barnwell, was implemented. This numerical effort increases computational speed and calculates reasonably accurate wall shear stress spatial distributions and boundary-layer profiles. Since the wall shear stress is analytically determined from the wall-function model, the computational grid near the wall is not required to spatially resolve the laminar-viscous sublayer. Consequently, a substantially increased computational integration step size is achieved resulting in a considerable decrease in net computational time. This wall-function technique is demonstrated for adiabatic flat plate test cases from Mach 2 to Mach 8. These test cases are analytically verified employing: (1) Eckert reference method solutions, (2) experimental turbulent boundary-layer data of Mabey, and (3) finite-difference computational code solutions with fully resolved laminar-viscous sublayers. Additionally, results have been obtained for two pressure-gradient cases: (1) an adiabatic expansion corner and (2) an adiabatic compression corner.
Bermejo-Moreno, Ivan; Campo, Laura; Larsson, Johan; Emory, Mike; Bodart, Julien; Palacios, Francisco; Iaccarino, Gianluca; Eaton, John
2013-11-01
We study the interaction between an oblique shock wave and the turbulent boundary layers inside a nearly-square duct by combining wall-modeled LES, 2D and 3D RANS simulations, targeting the experiment of Campo, Helmer & Eaton, 2012 (nominal conditions: M = 2 . 05 , Reθ = 6 , 500). A primary objective is to quantify the effect of aleatory and epistemic uncertainties on the STBLI. Aleatory uncertainties considered include the inflow conditions (Mach number of the incoming air stream and thickness of the boundary layers) and perturbations of the duct geometry upstream of the interaction. The epistemic uncertainty under consideration focuses on the RANS turbulence model form by injecting perturbations in the Reynolds stress anisotropy in regions of the flow where the model assumptions (in particular, the Boussinesq eddy-viscosity hypothesis) may be invalid. These perturbations are then propagated through the flow solver into the solution. The uncertainty quantification (UQ) analysis is done through 2D and 3D RANS simulations, assessing the importance of the three-dimensional effects imposed by the nearly-square duct geometry. Wall-modeled LES are used to verify elements of the UQ methodology and to explore the flow features and physics of the STBLI for multiple shock strengths. Financial support from the United States Department of Energy under the PSAAP program is gratefully acknowledged.
International Nuclear Information System (INIS)
We present a simple stochastic quadrant model for calculating the transport and deposition of heavy particles in a fully developed turbulent boundary layer based on the statistics of wall-normal fluid velocity fluctuations obtained from a fully developed channel flow. Individual particles are tracked through the boundary layer via their interactions with a succession of random eddies found in each of the quadrants of the fluid Reynolds shear stress domain in a homogeneous Markov chain process. In this way, we are able to account directly for the influence of ejection and sweeping events as others have done but without resorting to the use of adjustable parameters. Deposition rate predictions for a wide range of heavy particles predicted by the model compare well with benchmark experimental measurements. In addition, deposition rates are compared with those obtained from continuous random walk models and Langevin equation based ejection and sweep models which noticeably give significantly lower deposition rates. Various statistics related to the particle near wall behavior are also presented. Finally, we consider the model limitations in using the model to calculate deposition in more complex flows where the near wall turbulence may be significantly different
Large Civil Tiltrotor (LCTR2) Interior Noise Predictions due to Turbulent Boundary Layer Excitation
Grosveld, Ferdinand W.
2013-01-01
The Large Civil Tiltrotor (LCTR2) is a conceptual vehicle that has a design goal to transport 90 passengers over a distance of 1800 km at a speed of 556 km/hr. In this study noise predictions were made in the notional LCTR2 cabin due to Cockburn/Robertson and Efimtsov turbulent boundary layer (TBL) excitation models. A narrowband hybrid Finite Element (FE) analysis was performed for the low frequencies (6-141 Hz) and a Statistical Energy Analysis (SEA) was conducted for the high frequency one-third octave bands (125- 8000 Hz). It is shown that the interior sound pressure level distribution in the low frequencies is governed by interactions between individual structural and acoustic modes. The spatially averaged predicted interior sound pressure levels for the low frequency hybrid FE and the high frequency SEA analyses, due to the Efimtsov turbulent boundary layer excitation, were within 1 dB in the common 125 Hz one-third octave band. The averaged interior noise levels for the LCTR2 cabin were predicted lower than the levels in a comparable Bombardier Q400 aircraft cabin during cruise flight due to the higher cruise altitude and lower Mach number of the LCTR2. LCTR2 cabin noise due to TBL excitation during cruise flight was found not unacceptable for crew or passengers when predictions were compared to an acoustic survey on a Q400 aircraft.
Shockwave-turbulent boundary layer interaction control using magnetically driven surface discharges
Kalra, Chiranjeev S.; Zaidi, Sohail H.; Miles, Richard B.; Macheret, Sergey O.
2011-03-01
This study demonstrates the potential for shockwave-turbulent boundary layer interaction control in air using low current DC constricted surface discharges forced by moderate strength magnetic fields. An analytical model describing the physics of magnetic field forced discharge interaction with boundary layer flow is developed and compared to experiments. Experiments are conducted in a Mach 2.6 indraft air tunnel with discharge currents up to 300 mA and magnetic field strengths up to 5 Tesla. Separation- and non-separation-inducing shocks are generated with diamond-shaped shockwave generators located on the wall opposite to the surface electrodes, and flow properties are measured with schlieren imaging, static wall pressure probes and acetone flow visualization. The effect of plasma control on boundary layer separation depends on the direction of the Lorentz force ( j × B). It is observed that by using a Lorentz force that pushes the discharge upstream, separation can be induced or further strengthened even with discharge currents as low as 30 mA in a 3-Tesla magnetic field. If shock-induced separation is present, it is observed that by using Lorentz force that pushes the discharge downstream, separation can be suppressed, but this required higher currents, greater than 80 mA. Acetone planar laser scattering is used to image the flow structure in the test section and the reduction in the size of recirculation bubble and its elimination are observed experimentally as a function of actuation current and magnetic field strength.
Jelinek, Tomas; Straka, Petr; Uruba, Vaclav
2016-06-01
The article deals with the effects of the inlet flow parameters on the flow field structures in axial turbine stage. The experiment was performed on the axial turbine stage rig with an air as a working medium. The variable inlet channel produced the different inlet turbulence intensity and different inlet end-wall boundary layer thickness, resp. different inlet velocity distribution was applied. The turbulence was measured by CTA probes. The measured parameters of the inlet velocity distribution and turbulence intensity across the inlet channel height are presented. Based on the experimental inlet parameters the CFD fully turbulent calculation of the flow field was made. The differences in outlet kinetic energy loss, outlet vane angle and the turbulence distribution in the vane mid-span section are depicted. Changes of secondary flow structures with the different inlet end-wall boundary layer thickness were observed on the vane outlet parameters.
Scaling laws of turbulence intermittency in the atmospheric boundary layer: the role of stability
Paradisi, Paolo; Cesari, Rita; Allegrini, Paolo
2015-09-01
Bursting and intermittent behavior is a fundamental feature of turbulence, especially in the vicinity of solid obstacles. This is associated with the dynamics of turbulent energy production and dissipation, which can be described in terms of coherent motion structures. These structures are generated at random times and remain stable for long times, after which they become suddenly unstable and undergo a rapid decay event. This intermittent behavior is described as a birth-death point process of self-organization, i.e., a sequence of critical events. The Inter-Event Time (IET) distribution, associated with intermittent self-organization, is typically a power-law decay, whose power exponent is known as complexity index and characterizes the complexity of the system, i.e., the ability to develop self-organized, metastable motion structures. We use a method, based on diffusion scaling, for the estimation of system's complexity. The method is applied to turbulence velocity data in the atmospheric boundary layer. A neutral condition is compared with a stable one, finding that the complexity index is lower in the neutral case with respect to the stable one. As a consequence, the crucial birth-death events are more rare in the stable case, and this could be associated with a less efficient transport dynamics.
Influence of grid aspect ratio on planetary boundary layer turbulence in large-eddy simulations
Nishizawa, S.; Yashiro, H.; Sato, Y.; Miyamoto, Y.; Tomita, H.
2015-10-01
We examine the influence of the grid aspect ratio of horizontal to vertical grid spacing on turbulence in the planetary boundary layer (PBL) in a large-eddy simulation (LES). In order to clarify and distinguish them from other artificial effects caused by numerical schemes, we used a fully compressible meteorological LES model with a fully explicit scheme of temporal integration. The influences are investigated with a series of sensitivity tests with parameter sweeps of spatial resolution and grid aspect ratio. We confirmed that the mixing length of the eddy viscosity and diffusion due to sub-grid-scale turbulence plays an essential role in reproducing the theoretical -5/3 slope of the energy spectrum. If we define the filter length in LES modeling based on consideration of the numerical scheme, and introduce a corrective factor for the grid aspect ratio into the mixing length, the theoretical slope of the energy spectrum can be obtained; otherwise, spurious energy piling appears at high wave numbers. We also found that the grid aspect ratio has influence on the turbulent statistics, especially the skewness of the vertical velocity near the top of the PBL, which becomes spuriously large with large aspect ratio, even if a reasonable spectrum is obtained.
Saxton-Fox, Theresa; Gordeyev, Stanislav; Smith, Adam; McKeon, Beverley
2015-11-01
Strong density gradients associated with turbulent structure were measured in a mildly heated turbulent boundary layer using an optical sensor (Malley probe). The Malley probe measured index of refraction gradients integrated along the wall-normal direction, which, due to the proportionality of index of refraction and density in air, was equivalently an integral measure of density gradients. The integral output was observed to be dominated by strong, localized density gradients. Conditional averaging and Pearson correlations identified connections between the streamwise gradient of density and the streamwise gradient of wall-normal velocity. The trends were suggestive of a process of pick-up and transport of heat away from the wall. Additionally, by considering the density field as a passive marker of structure, the role of the wall-normal velocity in shaping turbulent structure in a sheared flow was examined. Connections were developed between sharp gradients in the density and flow fields and strong vertical velocity fluctuations. This research is made possible by the Department of Defense through the National Defense & Engineering Graduate Fellowship (NDSEG) Program and by the Air Force Office of Scientific Research Grant # FA9550-12-1-0060.
A CFD model for particle dispersion in turbulent boundary layer flows
International Nuclear Information System (INIS)
In Lagrangian particle dispersion modeling, the assumption that turbulence is isotropic everywhere yields erroneous predictions of particle deposition rates on walls, even in simple geometries. In this investigation, the stochastic particle tracking model in Fluent 6.2 is modified to include a better treatment of particle-turbulence interactions close to walls where anisotropic effects are significant. The fluid rms velocities in the boundary layer are computed using fits of DNS data obtained in channel flow. The new model is tested against correlations for particle removal rates in turbulent pipe flow and 90o bends. Comparison with experimental data is much better than with the default model. The model is also assessed against data of particle removal in the human mouth-throat geometry where the flow is decidedly three-dimensional. Here, the agreement with the data is reasonable, especially in view of the fact that the DNS fits used are those of channel flows, for lack of better alternatives. The CFD Best Practice Guidelines are followed to a large extent, in particular by using multiple grid resolutions and at least second order discretization schemes
Rahman, Mustafa; Samtaney, Ravi
2015-11-01
We present results of solid particles suspension and transport in a fully-developed turbulent boundary layer flow using large-eddy simulation of the incompressible Navier-Stokes equations. We adopt the Eulerian-Eulerian approach to simulating particle laden flow with a large number of particles, in which the particles are characterized by statistical descriptors. For the particulate phase, the direct quadrature method of moments (DQMOM) is chosen in which the weights and abscissas of the quadrature approximation are tracked directly rather than the moments themselves. The underlying approach in modeling the turbulence of fluid phase utilizes the stretched spiral vortex subgrid-scale model and a virtual wall model similar to the work proposed by Inoue & Pullin (J. Fluid Mech. 2011). The solver is verified against simple analytical solutions and the computational results are found to be in a good agreement with these. The capability of the new numerical solver will be exercised to investigate turbulent transport of sand in sandstorms. Finally, the adequacy and limitations of the solver will be discussed. Supported by the KAUST Office of Competitive Research Funds under Award No. URF/1/1704-01.
Washuta, Nathan; Duncan, James H
2016-01-01
The complex interactions between turbulence and the free surface, including air entrainment processes, in boundary layer shear flows created by vertical surface-piercing plates are considered. A laboratory-scale device was built that utilizes a surface-piercing stainless steel belt that travels in a loop around two vertical rollers, with one length of the belt between the rollers acting as a horizontally-moving flat wall. The belt is operated both as a suddenly-started plate to reproduce boundary layer flow or at steady state in the presence of a stationary flat plate positioned parallel to the belt to create a Couette flow with a free surface. Surface profiles are measured with a cinematic laser-induced fluorescence system in both experiments and air entrainment events and bubble motions are observed with stereo underwater white-light movies in the suddenly started belt experiment. It is found that the RMS surface height fluctuations, $\\eta$, peak near the boundaries of the flows and increase approximately l...
Direct Numerical Simulation of Two Shock Wave/Turbulent Boundary Layer Interactions
Priebe, Stephan
Direct numerical simulations (DNSs) of two shock wave/turbulent boundary layer interactions (STBLIs) are presented in this thesis. The first interaction is a 24° compression ramp at Mach 2.9, and the second interaction is an 8° compression ramp at Mach 7.2. The large-scale low-frequency unsteadiness in the Mach 2.9 DNS is investigated with the aim of shedding some light on its physical origin. Previous experimental and computational works have linked the unsteadiness either to fluctuations in the incoming boundary layer or to a mechanism in the downstream separated flow. Consistent with experimental observations, the shock in the DNS is found to undergo streamwise oscillations, which are broadband and occur at frequencies that are about two orders of magnitude lower than the characteristic frequency of the energy-containing turbulent scales in the incoming boundary layer. Based on a coherence and phase analysis of signals at the wall and in the flow field, it is found that the low frequency shock unsteadiness is statistically linked to pulsations of the downstream separated flow. The statistical link with fluctuations in the upstream boundary layer is also investigated. A weak link is observed: the value of the low-frequency coherence with the upstream flow is found to lie just above the limit of statistical significance, which is determined by means of a Monte Carlo study. The dynamics of the downstream separated flow are characterized further based on low-pass filtered DNS fields. The results suggest that structural changes occur in the downstream separated flow during the low-frequency motions, including the breaking-up of the separation bubble, which is observed when the shock moves downstream. The structural changes are described based on the Cf distribution through the interaction, as well as the velocity and vorticity fields. The possible link between the low-frequency dynamics observed in the DNS and results from global instability theory is explored. It
Numerical study of oscillating boundary layer flow over a flat plate using k–kL–ω turbulence model
International Nuclear Information System (INIS)
Highlights: • Oscillating boundary layer flow over infinite flat plate at rest simulated using k–kL–ω turbulence model. • Simulation conducted for flow regimes ranging from fully laminar flow to fully turbulent flow. • Results predicted by k–kL–ω model compare very well with experimental and LES model results. • The k–kL–ω model is able to accurately predict the onset of transition for intermittently turbulent flow regime. -- Abstract: Oscillating boundary layer flow over an infinite flat plate at rest was simulated using the k–kL–ω turbulence model for a Reynolds number range of 32 ⩽ Reδ ⩽ 10,000 ranging from fully laminar flow to fully turbulent flow. The k–kL–ω model was validated by comparing the predictions with LES results and experimental results for intermittently turbulent and fully turbulent flow regimes. The good agreement obtained between the k–kL–ω model prediction with the experimental and LES results indicate that the k–kL–ω model is able to accurately simulate transient intermittently turbulent flow and as well as accurately predict the onset of turbulence for such oscillatory flows
Institute of Scientific and Technical Information of China (English)
Yang Shaoqiong杨绍琼; Li Shan李山; Tian Haiping田海平; Wang Qingyi王清毅; Jiang Nan姜楠
2015-01-01
The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peak-to-peak spacing being approximately s+≈21)and 0.19 m/s( s+≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified Kelvin-Helmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been dis-cussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y+≈20 in both cases of s+≈21 and s+≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the inter-actions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.
Kornilov, V. I.; Boiko, A. V.; Kavun, I. N.
2015-11-01
The characteristics of an incompressible turbulent boundary layer on a flat plate with air blown in though a finely perforated surface from an external confined flow through an input device, located on the "idle" side of the plate, have been investigated experimentally and numerically. A stable decrease in the local values of the coefficient of surface friction along the plate length that attains 85% at the end of the perforated portion is shown. The experimental and calculated data obtained point to the possibility of modeling, under earth conditions, the process of controlling a turbulent boundary layer with air injection by using the resources of an external confined flow.
Renard, N.; Deck, S.
2014-01-01
Je n'ai pas les mots clés en anglais donc j'ai mis ceux en français International audience Mean quantities in attached turbulent boundary layers are provided by RANS simulations, but resolving the most energetic turbulent fluctuations is sometimes needed (e.g. mild flow separation sensitive to the upstream history of the boundary layer, dynamic loading and noise predictions). A DNS and even a wall-resolved LES at the highest Reynolds numbers relevant to the industry is extremely expensi...
Directory of Open Access Journals (Sweden)
Kiran Bhaganagar
2014-09-01
Full Text Available Turbulence structure in the wake behind a full-scale horizontal-axis wind turbine under the influence of real-time atmospheric inflow conditions has been investigated using actuator-line-model based large-eddy-simulations. Precursor atmospheric boundary layer (ABL simulations have been performed to obtain mean and turbulence states of the atmosphere under stable stratification subjected to two different cooling rates. Wind turbine simulations have revealed that, in addition to wind shear and ABL turbulence, height-varying wind angle and low-level jets are ABL metrics that influence the structure of the turbine wake. Increasing stability results in shallower boundary layers with stronger wind shear, steeper vertical wind angle gradients, lower turbulence, and suppressed vertical motions. A turbulent mixing layer forms downstream of the wind turbines, the strength and size of which decreases with increasing stability. Height dependent wind angle and turbulence are the ABL metrics influencing the lateral wake expansion. Further, ABL metrics strongly impact the evolution of tip and root vortices formed behind the rotor. Two factors play an important role in wake meandering: tip vortex merging due to the mutual inductance form of instability and the corresponding instability of the turbulent mixing layer.
On the universality of inertial energy in the log layer of turbulent boundary layer and pipe flows
Chung, D.; Marusic, I.; Monty, J. P.; Vallikivi, M.; Smits, A. J.
2015-07-01
Recent experiments in high Reynolds number pipe flow have shown the apparent obfuscation of the behaviour in spectra of streamwise velocity fluctuations (Rosenberg et al. in J Fluid Mech 731:46-63, 2013). These data are further analysed here from the perspective of the behaviour in second-order structure functions, which have been suggested as a more robust diagnostic to assess scaling behaviour. A detailed comparison between pipe flows and boundary layers at friction Reynolds numbers of 5000-20,000 reveals subtle differences. In particular, the slope of the pipe flow structure function decreases with increasing wall distance, departing from the expected slope in a manner that is different to boundary layers. Here, , the slope of the log law in the streamwise turbulence intensity profile at high Reynolds numbers. Nevertheless, the structure functions for both flows recover the slope in the log layer sufficiently close to the wall, provided the Reynolds number is also high enough to remain in the log layer. This universality is further confirmed in very high Reynolds number data from measurements in the neutrally stratified atmospheric surface layer. A simple model that accounts for the `crowding' effect near the pipe axis is proposed in order to interpret the aforementioned differences.
Abedina, Mohammad Zoynal; Islam, Mohammed Moinul; Hanif, Md. Abu; Alam, Md. Jahangir
2016-07-01
A numerical investigation is performed in the turbulent combined-convection boundary layer with aiding flows in air along a heated vertical flat plate at a higher freestream velocity (Reδ0 = 600) by time-developing direct numerical simulation (DNS). At higher freestream velocity, the transition from laminar to turbulent delays for aiding flows and relatively a lower and higher heat transfer rates are observed, respectively, in the laminar and turbulent region compared to that of lower freestream velocity. The wall shear stresses are higher in the laminar region compared to that in the turbulent region, and at higher freestream velocity, the wall shear stress in the transition region shows a higher peak value. The intensity of velocity and temperature fluctuations for aiding flows with higher freestream velocity become appreciably lower than that for lower freestream velocity due to the laminarization of the boundary layer.
Directory of Open Access Journals (Sweden)
Haiping Tian
2015-01-01
Full Text Available This study aims at the mechanism of drag reduction in turbulent boundary layer (TBL with superhydrophobic surface. Comparing the time-resolved particle image velocimetry (TRPIV measurement results with that of hydrophilic surface, the drag reduction rate over a superhydrophobic surface is approximately 10%. To investigate the characteristics of coherent structure in a drag-reduced TBL with superhydrophobic surface, a modified multi-scale spatial locally-averaged structure function is proposed for detecting coherent structure. The conditional sampling and spatial phase-lock average methods are employed to obtain the topology of physical quantities like the velocity fluctuation, spanwise vorticity, and Reynolds stress during eject and sweep process. The results indicate that the suppression of coherent structure burst in the near-wall region is the key mechanism in reducing the skin friction drag for TBL over superhydrophobic surface.
Turbulent flow over a house in a simulated hurricane boundary layer
Taylor, Zachary; Gurka, Roi; Kopp, Gregory
2009-01-01
Every year hurricanes and other extreme wind storms cause billions of dollars in damage worldwide. For residential construction, such failures are usually associated with roofs, which see the largest aerodynamic loading. However, determining aerodynamic loads on different portions of North American houses is complicated by the lack of clear load paths and non-linear load sharing in wood frame roofs. This problem of fluid-structure interaction requires both wind tunnel testing and full-scale structural testing. A series of wind tunnel tests have been performed on a house in a simulated atmospheric boundary layer (ABL), with the resulting wind-induced pressures applied to the full-scale structure. The ABL was simulated for flow over open country terrain where both velocity and turbulence intensity profiles, as well as spectra, were matched with available full scale measurements for this type of terrain. The first set of measurements was 600 simultaneous surface pressure measurements over the entire house. A key...
Drag Reduction in Turbulent Boundary Layers with Half Wave Wall Oscillations
Directory of Open Access Journals (Sweden)
Maneesh Mishra
2015-01-01
Full Text Available Spatial square waves with positive cycle are used as steady forcing technique to study drag reduction effects on a turbulent boundary layer flow. Pseudospectral method is used for performing direct numerical simulations on very high resolution grids. A smooth step function is employed to prevent Gibbs phenomenon at the sharp discontinuities of a square wave. The idea behind keeping only the positive cycle of the spatial forcing is to reduce the power consumption to boost net power savings. For some spatial frequency of the oscillations with half waves, it is possible to prevent recovery of skin friction back to the reference case values. A set of wall oscillation parameters is numerically simulated to study its effect on the power budget.
Dombroski, Daniel Edward
In aquatic benthic environments, hydrodynamic transport of mass and momentum have shaped the evolution of form-function relationships. Animals whose life cycle depends on success in such environments have developed the biological structure and behavioral mechanisms to sustain dynamic stresses and complex chemical signals. It has become increasingly clear that understanding the ecology of these organisms is dependent on examining the complexities of the turbulent environment. In this dissertation, hydrodynamics and the structure of chemical signals within turbulent boundary layer flows are examined in the context of natural and biological systems. Experiments were conducted in the benthic region of a water flume using a combination of point-measurement and full-field imaging techniques. There are three areas of focus within the complete body of work: (1) The accuracy of an acoustic measurement technique commonly used in natural flows was evaluated. Errors in the technique, primarily attributed to a sampling volume that is large relative to the scales of motion in turbulent flows, were found to be larger than and extend farther from the bed than previously reported. (2) A three-dimensional laser-based imaging system was developed for quantifying turbulent scalar structure. The system was employed to study the topology and orientation of structure within a bed-level, passively released scalar plume. (3) Hydrodynamic stresses were measured near marine fouling communities in a study aimed at predicting larval settlement probabilities. Turbulent stresses, and by extension, the suitability of microhabitats, were found to be highly dependent on local topography and outer-scale flow conditions. This body of work advances the field of experimental fluid mechanics by contributing to the development of methods for quantifying turbulent flows, as well as furthering current understanding of the capabilities and limitations associated with new and existing techniques. Statistical
Turbulent boundary layer flow with a step change from smooth to rough surface
International Nuclear Information System (INIS)
Highlights: • Evidence for mean flow universality for turbulent boundary layer with 2-D roughness is provided. • Characteristics of overshooting behavior for the statistics are presented. • It is shown direct evidence for predominance of hairpin vortices over the rough wall. • A possible cause for spanwise scale growth of structures over the rough wall is examined. - Abstract: A direct numerical simulation (DNS) dataset of a turbulent boundary layer (TBL) with a step change from a smooth to a rough surface is analyzed to examine the characteristics of a spatially developing flow. The roughness elements are periodically arranged two-dimensional (2-D) spanwise rods, with the first rod placed 80θin downstream from the inlet, where θin denotes the inlet momentum thickness. Based on an accurate estimation of relevant parameters, clear evidence for mean flow universality is provided when scaled properly, even for the present roughness configuration, which is believed to have one of the strongest impacts on the flow. Compared to previous studies, it is shown that overshooting behavior is present in the first- and second-order statistics and is locally created either within the cavity or at the leading edge of the roughness depending on the type of statistics and the wall-normal measurement location. Inspection of spatial two-point correlations of the streamwise velocity fluctuations shows a continuous increase of spanwise length scales of structures over the rough wall after the step change at a greater growth rate than that over smooth wall TBL flow. This is expected because spanwise energy spectrum shows presence of much energetic wider structures over the rough wall. Full images of the DNS data are presented to describe not only predominance of hairpin vortices but also a possible spanwise scale growth mechanism via merging over the rough wall
Pressure estimation from single-snapshot tomographic PIV in a turbulent boundary layer
Schneiders, Jan F. G.; Pröbsting, Stefan; Dwight, Richard P.; van Oudheusden, Bas W.; Scarano, Fulvio
2016-04-01
A method is proposed to determine the instantaneous pressure field from a single tomographic PIV velocity snapshot and is applied to a flat-plate turbulent boundary layer. The main concept behind the single-snapshot pressure evaluation method is to approximate the flow acceleration using the vorticity transport equation. The vorticity field calculated from the measured instantaneous velocity is advanced over a single integration time step using the vortex-in-cell (VIC) technique to update the vorticity field, after which the temporal derivative and material derivative of velocity are evaluated. The pressure in the measurement volume is subsequently evaluated by solving a Poisson equation. The procedure is validated considering data from a turbulent boundary layer experiment, obtained with time-resolved tomographic PIV at 10 kHz, where an independent surface pressure fluctuation measurement is made by a microphone. The cross-correlation coefficient of the surface pressure fluctuations calculated by the single-snapshot pressure method with respect to the microphone measurements is calculated and compared to that obtained using time-resolved pressure-from-PIV, which is regarded as benchmark. The single-snapshot procedure returns a cross-correlation comparable to the best result obtained by time-resolved PIV, which uses a nine-point time kernel. When the kernel of the time-resolved approach is reduced to three measurements, the single-snapshot method yields approximately 30 % higher correlation. Use of the method should be cautioned when the contributions to fluctuating pressure from outside the measurement volume are significant. The study illustrates the potential for simplifying the hardware configurations (e.g. high-speed PIV or dual PIV) required to determine instantaneous pressure from tomographic PIV.
International Nuclear Information System (INIS)
A novel methodology is presented for the numerical treatment of multi-dimensional pdf (probability density function) models used to study particle transport in turbulent boundary layers. A system of coupled Fokker–Planck type equations is constructed to describe the transport of phase-space conditioned moments of particle and fluid velocities, both streamwise and wall-normal. This system, unlike conventional moment-based transport equations, allows for an exact treatment of particle deposition at the flow boundary and provides an efficient way to handle the 5-dimensional phase-space domain. Moreover, the equations in the system are linear and can be solved in a sequential fashion; there is no closure problem to address. A hybrid Hermite-Discontinuous Galerkin scheme is developed to treat the system. The choice of Hermite basis functions in combination with an iterative scaling approach permits the efficient computation of solutions to high accuracy. Results demonstrate the effectiveness of the methodology in resolving the extreme gradients characteristic of distributions near an absorbing boundary.
Streamwise vortices originating from synthetic jet–turbulent boundary layer interaction
International Nuclear Information System (INIS)
The interaction between a flat plate turbulent boundary layer and a synthetic jet issuing from a rectangular slot slanted with respect to the free stream was studied experimentally using digital particle image velocimetry. Instantaneous flow fields were sampled in a cross-plane downstream of the slot. Results concerning the effects of varying the synthetic jet velocity ratio at fixed stroke length L0 and yaw angle, and the effects of varying the orifice yaw angle β at a fixed frequency are presented. The formation of a pair of counter-rotating vortical structures, completely embedded in the boundary layer, was observed in the mean flow field when the slot was aligned with the cross-flow. As the slot yaw angle was increased, the leeward vortex intensified while the other became weaker. These vortical structures are the traces of streamwise vortices forming upstream, at the slot exit, during the blowing phases. As the jet velocity ratio and the slot yaw angle were increased the vortices grew in size and intensity. The vortex identification technique showed that these vortical structures are intermittently present in the instantaneous flow fields with a percentage growing with the frequency but not influenced by the yaw angle. Conditional averages showed that while the rotational core of the identified vortices is nearly unaffected, their outer region is greatly modified and grows in size and intensity as the jet velocity ratio and the yaw angle increases. (paper)
Vortex packet recovery in a turbulent boundary layer perturbed by an array of cylinders
Tan, Yan Ming; Longmire, Ellen
2015-11-01
PIV measurements were acquired in a zero pressure gradient turbulent boundary layer (Reτ = 2500) perturbed by a narrowly spaced (0.2 δ) array of cylinders. Two array heights were considered with one extending to the top of the log region and the other to the top of the boundary layer. Wall-parallel measurements were obtained at three locations in the log region by fixed and flying PIV. The measurement system for flying PIV moves with the flow to track the evolution of structures upstream and downstream of the array. Initially, both arrays disrupt the packets such that none are apparent. Then, packets appear either to recover or re-initiate at some distance downstream. A packet signature was denoted by a low momentum region bounded by counter rotating swirling structures. A low momentum region identification algorithm was applied to both fixed and flying PIV data to quantify packet recovery downstream of the array. The results indicate that packets reappear sooner further from the wall and later closer to the wall for the shorter array supporting the top down notion of packet reorganization proposed by Zheng & Longmire (JFM, 2014). The opposite trend was observed for the taller array whereby packets recovered earlier closer to the wall and later further from the wall.
Directory of Open Access Journals (Sweden)
J.-F. Miao
2009-06-01
Full Text Available This paper investigates the sensitivity of sea breeze (SB simulations to combinations of boundary-layer turbulence and land-surface process parameterizations implemented in the MM5 mesoscale meteorological mode for an observed SB case over the Swedish west coast. Various combinations from four different planetary boundary layer (PBL schemes [Blackadar, Gayno-Seaman (GS, Eta, MRF], and two land surface model (LSM schemes (SLAB, Noah with different complexity are designed to simulate a typical SB case over the Swedish west coast. The simulations are conducted using two-way interactively nested grids. Simulated 10-m winds are compared against observed near-surface wind data from the GÖTE2001 campaign to examine the diurnal cycle of wind direction and speed for SB timing. The SB (vertical circulation is also compared in the different experiments. The results show that the different combinations of PBL and LSM parameterization schemes result in different SB timing and vertical circulation characteristics. All experiments predict a delayed SB. The vertical component of the SB circulation varies in the experiments, among which the GS PBL scheme produces the strongest SB circulation. Evident differences between the SLAB and Noah LSMs are also found, especially in maximum of updraft and downdraft velocities of the SB vertical circulation. The results have significant implications for convective initiation, air quality studies and other environmental problems in coastal areas.
Yuan, Jing
2016-04-01
A full-scale experimental study of turbulent boundary layer flows under irregular waves and currents is conducted with the primary objective to investigate the equivalent-wave concept by Madsen (1994). Irregular oscillatory flows following the bottom-velocity spectrum under realistic surface irregular waves are produced over two fixed rough bottoms in an oscillatory water tunnel, and flow velocities are measured using a Particle Image Velocimetry. The root-mean-square (RMS) value and representative phase lead of wave velocities have vertical variations very similar to those of the first-harmonic velocity of periodic wave boundary layers, e.g., the RMS wave velocity follows a logarithmic distribution controlled by the physical bottom roughness in the very near-bottom region. The RMS wave bottom shear stress and the associated representative phase lead can be accurately predicted using the equivalent-wave approach. The spectra of wave bottom shear stress and boundary layer velocity are found to be proportional to the spectrum of free-stream velocity. Currents in the presence of irregular waves exhibit the classic two-log-profile structure with the lower log-profile controlled by the physical bottom roughness and the upper log-profile controlled by a much larger apparent roughness. Replacing the irregular waves by their equivalent sinusoidal waves virtually makes no difference for the coexisting currents. These observations, together with the excellent agreement between measurements and model predictions, suggest that the equivalent-wave representation adequately characterizes the basic wave-current interaction under irregular waves.
Young, Steven D.; Brungart, Timothy A.; Lauchle, Gerald C.; Howe, Michael S.
2005-12-01
An analytical and experimental investigation is made of the effect of a 2-D ventilated gas cavity on the spectrum of turbulent boundary layer wall pressure fluctuations upstream of a gas cavity on a plane rigid surface. The analytical model predicts the ratio of the wall pressure spectrum in the presence of the cavity to the blocked wall pressure spectrum that would exist if the cavity were absent. The ratio is found to oscillate in amplitude with upstream distance (-x) from the edge of the cavity. It approaches unity as -ωx/Uc-->∞, where ω is the radian frequency and Uc is the upstream turbulence convection velocity. To validate these predictions an experiment was performed in a water tunnel over a range of mean flow velocities. Dynamic wall pressure sensors were flush mounted to a flat plate at various distances upstream from a backward facing step. The cavity was formed downstream of the step by injecting carbon dioxide gas. The water tunnel measurements confirm the predicted oscillatory behavior of the spectral ratio, as well as its relaxation to unity as -ωx/Uc-->∞. For -ωx/Uc>7 the cavity has a negligible influence on the upstream wall pressure fluctuations.
Aerodynamic wake study: oscillating model wind turbine within a turbulent boundary layer
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..
Profiles of Wind and Turbulence in the Coastal Atmospheric Boundary Layer of Lake Erie
Wang, H
2014-06-16
Prediction of wind resource in coastal zones is difficult due to the complexity of flow in the coastal atmospheric boundary layer (CABL). A three week campaign was conducted over Lake Erie in May 2013 to investigate wind characteristics and improve model parameterizations in the CABL. Vertical profiles of wind speed up to 200 m were measured onshore and offshore by lidar wind profilers, and horizontal gradients of wind speed by a 3-D scanning lidar. Turbulence data were collected from sonic anemometers deployed onshore and offshore. Numerical simulations were conducted with the Weather Research Forecasting (WRF) model with 2 nested domains down to a resolution of 1-km over the lake. Initial data analyses presented in this paper investigate complex flow patterns across the coast. Acceleration was observed up to 200 m above the surface for flow coming from the land to the water. However, by 7 km off the coast the wind field had not yet reached equilibrium with the new surface (water) conditions. The surface turbulence parameters over the water derived from the sonic data could not predict wind profiles observed by the ZephlR lidar located offshore. Horizontal wind speed gradients near the coast show the influence of atmospheric stability on flow dynamics. Wind profiles retrieved from the 3-D scanning lidar show evidence of nocturnal low level jets (LLJs). The WRF model was able to capture the occurrence of LLJ events, but its performance varied in predicting their intensity, duration, and the location of the jet core.
Modeling turbulent mixing and sand distribution in the bottom boundary layer
Absi, Rafik
2011-01-01
For the calculation of turbulent mixing in the bottom boundary layer, we present simple analytical tools for the mixing velocity wm and the mixing length lm. Based on observations of turbulence intensity measurements, the mixing velocity wm is represented by an exponential function decaying with z. We suggest two theoretical functions for the mixing length, a first lm1 obtained from the k-equation written as a constant modeled fluctuating kinetic energy flux and a second lm2 based on von K\\'arm\\'an's similarity hypothesis. These analytical tools were used in the finite-mixing-length model of Nielsen and Teakle (2004). The modeling of time-mean sediment concentration profiles C(z) over wave ripples shows that at the opposite of the second equation lm2 which increases the upward convexity of C(z), the first equation lm1 increases the upward concavity of C(z) and is able to reproduce the shape of the measured concentrations for coarse sand.
Reduced-order FSI simulation of NREL 5 MW wind turbine in atmospheric boundary layer turbulence
Motta-Mena, Javier; Campbell, Robert; Lavely, Adam; Jha, Pankaj
2015-11-01
A partitioned fluid-structure interaction (FSI) solver based on an actuator-line method solver and a finite-element modal-dynamic structural solver is used to evaluate the effect of blade deformation in the presence of a day-time, moderately convective atmospheric boundary layer (ABL). The solver components were validated separately and the integrated solver was partially validated against FAST. An overview of the solver is provided in addition to results of the validation study. A finite element model of the NREL 5 MW rotor was developed for use in the present simulations. The effect of blade pitching moment and the inherent bend/twist coupling of the rotor blades are assessed for both uniform inflow and the ABL turbulence cases. The results suggest that blade twisting in response to pitching moment and the bend/twist coupling can have a significant impact on rotor out-of-plane bending moment and power generated for both the uniform inflow and the ABL turbulence cases.
Profiles of Wind and Turbulence in the Coastal Atmospheric Boundary Layer of Lake Erie
International Nuclear Information System (INIS)
Prediction of wind resource in coastal zones is difficult due to the complexity of flow in the coastal atmospheric boundary layer (CABL). A three week campaign was conducted over Lake Erie in May 2013 to investigate wind characteristics and improve model parameterizations in the CABL. Vertical profiles of wind speed up to 200 m were measured onshore and offshore by lidar wind profilers, and horizontal gradients of wind speed by a 3-D scanning lidar. Turbulence data were collected from sonic anemometers deployed onshore and offshore. Numerical simulations were conducted with the Weather Research Forecasting (WRF) model with 2 nested domains down to a resolution of 1-km over the lake. Initial data analyses presented in this paper investigate complex flow patterns across the coast. Acceleration was observed up to 200 m above the surface for flow coming from the land to the water. However, by 7 km off the coast the wind field had not yet reached equilibrium with the new surface (water) conditions. The surface turbulence parameters over the water derived from the sonic data could not predict wind profiles observed by the ZephlR lidar located offshore. Horizontal wind speed gradients near the coast show the influence of atmospheric stability on flow dynamics. Wind profiles retrieved from the 3-D scanning lidar show evidence of nocturnal low level jets (LLJs). The WRF model was able to capture the occurrence of LLJ events, but its performance varied in predicting their intensity, duration, and the location of the jet core
Impact of planetary boundary layer turbulence on model climate and tracer transport
McGrath-Spangler, E. L.; Molod, A.; Ott, L. E.; Pawson, S.
2015-07-01
Planetary boundary layer (PBL) processes are important for weather, climate, and tracer transport and concentration. One measure of the strength of these processes is the PBL depth. However, no single PBL depth definition exists and several studies have found that the estimated depth can vary substantially based on the definition used. In the Goddard Earth Observing System (GEOS-5) atmospheric general circulation model, the PBL depth is particularly important because it is used to calculate the turbulent length scale that is used in the estimation of turbulent mixing. This study analyzes the impact of using three different PBL depth definitions in this calculation. Two definitions are based on the scalar eddy diffusion coefficient and the third is based on the bulk Richardson number. Over land, the bulk Richardson number definition estimates shallower nocturnal PBLs than the other estimates while over water this definition generally produces deeper PBLs. The near-surface wind velocity, temperature, and specific humidity responses to the change in turbulence are spatially and temporally heterogeneous, resulting in changes to tracer transport and concentrations. Near-surface wind speed increases in the bulk Richardson number experiment cause Saharan dust increases on the order of 1 × 10-4 kg m-2 downwind over the Atlantic Ocean. Carbon monoxide (CO) surface concentrations are modified over Africa during boreal summer, producing differences on the order of 20 ppb, due to the model's treatment of emissions from biomass burning. While differences in carbon dioxide (CO2) are small in the time mean, instantaneous differences are on the order of 10 ppm and these are especially prevalent at high latitude during boreal winter. Understanding the sensitivity of trace gas and aerosol concentration estimates to PBL depth is important for studies seeking to calculate surface fluxes based on near-surface concentrations and for studies projecting future concentrations.
Impact of planetary boundary layer turbulence on model climate and tracer transport
Directory of Open Access Journals (Sweden)
E. L. McGrath-Spangler
2014-12-01
Full Text Available Planetary boundary layer (PBL processes are important for weather, climate, and tracer transport and concentration. One measure of the strength of these processes is the PBL depth. However, no single PBL depth definition exists and several studies have found that the estimated depth can vary substantially based on the definition used. In the Goddard Earth Observing System (GEOS-5 atmospheric general circulation model, the PBL depth is particularly important because it is used to calculate the turbulent length scale that is used in the estimation of turbulent mixing. This study analyzes the impact of using three different PBL depth definitions in this calculation. Two definitions are based on the scalar eddy diffusion coefficient and the third is based on the bulk Richardson number. Over land, the bulk Richardson number definition estimates shallower nocturnal PBLs than the other estimates while over water this definition generally produces deeper PBLs. The near surface wind velocity, temperature, and specific humidity responses to the change in turbulence are spatially and temporally heterogeneous, resulting in changes to tracer transport and concentrations. Near surface wind speed increases in the bulk Richardson number experiment cause Saharan dust increases on the order of 1 × 10−4 kg m−2 downwind over the Atlantic Ocean. Carbon monoxide (CO surface concentrations are modified over Africa during boreal summer, producing differences on the order of 20 ppb, due to the model's treatment of emissions from biomass burning. While differences in carbon dioxide (CO2 are small in the time mean, instantaneous differences are on the order of 10 ppm and these are especially prevalent at high latitude during boreal winter. Understanding the sensitivity of trace gas and aerosol concentration estimates to PBL depth is important for studies seeking to calculate surface fluxes based on near-surface concentrations and to studies projecting future
Bhattacharya, Ritthik; Stevens, Bjorn
2016-03-01
A two Turbulence Kinetic Energy (2TKE) model is developed to address the boundary layer "grey zone" problem. The model combines ideas from local and nonlocal models into a single energetically consistent framework. By applying the Reynolds averaging to the large eddy simulation (LES) equations that employ Deardorff's subgrid TKE, we arrive at a system of equations for the boundary layer quantities and two turbulence kinetic energies: one which encapsulates the TKE of large boundary-layer-scale eddies and another which represents the energy of eddies subgrid to the vertical grid size of a typical large-scale model. These two energies are linked via the turbulent cascade of energy from larger to smaller scales and are used to model the mixing in the boundary layer. The model is evaluated for three dry test cases and found to compare favorably to large eddy simulations. The usage of two TKEs for mixing helps reduce the dependency of the model on the vertical grid scale as well as on the free tropospheric stability and facilitates a smoother transition from convective to stable regimes. The usage of two TKEs representing two ranges of scales satisfies the prerequisite for modeling the boundary layer in the "grey zone": an idea that is explored further in a companion paper.
Directory of Open Access Journals (Sweden)
Xiao Hong
2013-08-01
large deviation. The hypersonic flat-plate laminar flow was also compared with CP and st calculated from the three turbulence models for the three grids. Evidently, the grids near the wall must be encrypted to an appropriate extent to simulate more accurately the boundary laminar flow as well as obtain proper surface friction and heat flow. The calculation in the present study showed that the Reynolds number in the first layer of the grid was more reasonable when it was about 20. The simulation result for the hypersonic isothermal two-dimensional turning wall flow showed that the calculation and experiment results from the different turbulence model were consistent. There was little difference between the location of the simulated heat flow peak and the position given by experiment. However, the peak, the curve trend after the peak and the experimental result widely differed. The curve and experimental results for pressure distribution greatly varied because of the existence of an isolated area in the calculation of the laminar flow. The calculation and experimental results from different turbulence models were close. The curve trend, the peak and the experimental result basically matched.
Institute of Scientific and Technical Information of China (English)
Sun-Hee SHIN; Kyung-Ja HA
2009-01-01
The effect of a vertical diffusion scheme over a stratocumulus topped boundary layer (STBL) was investigated using the YONU AGCM (Yonsei University Atmospheric General Circulation Model).To consider the impact of clouds on the turbulence production,the turbulence mixing term,driven by radiative cooling at the cloud top,is implemented as an extended non-local diffusion scheme.In the model with this new scheme,the STBL parameterization significantly influences the lower atmosphere over the tropical and subtropical regions.Consideration of the turbulent mixing within the cloud layer leads to continuous stratocumulus formation.The cloud-top radiative cooling tends to favor more rapid entrainment and produces top-down turbulent mixing.This cooling develops a mixed layer without initiation of deep convection by surface fluxes.Variations in thermodynamical and dynamical features are produced by planetary boundary layer (PBL)cloud development.The simulated stratocumulus induces more mixing of heat and moisture due to the cloud forcing.Over STBL regions,the lower boundary layer bccomes warmer and drier.It also weakens vertical motion and zonal trade winds in the eastern Pacific,which indicates that stratocumulus cloud cover plays a role in weakening the Walker circulation;that is,cloud cover damps the tropical circulation.
Characteristics of the turbulent flow in the boundary layer of a Tropical Glacier
Litt, M.; Sicart, J.
2012-12-01
An extensive micro-meteorological experiment has been deployed within the atmospheric boundary layer over the ablation zone of the tropical Zongo glacier, Bolivia, during the dry season from July to August, 2007. It included two complete eddy correlation systems (Campbell CSAT and LICOR7500) at a 2-m mean level and a 6-m mast measuring the mean profiles of air temperature (type-T artificially ventilated thermocouples) and of wind speed (Vector A100R). Weakly stable conditions prevailed in the first meters above the ice or snow surface. With weak large scale forcing, a katabatic downslope flow with a wind maximum at about 2-m height usually appeared in the middle of the afternoon and maintained itself during most of the night. Characteristics and structure of the turbulent flow were studied using spectral and quadrant analysis, along with the study of statistical moments of high frequency wind speed and temperature data. The wind regime was found to be highly gusty and irregular: more than 50% of the flux was exchanged during less than 10% of the time. Stationary conditions were rarely encountered. The spectral analysis shows that the observed turbulence cannot be generated only by local shear, and that some outside layer perturbations must transport kinetic energy in the vicinity of the surface. Flux exchanges are thus found to be greater than predicted by aerodynamic approaches which use mean temperature and wind speed measurements and stability-correction functions based on the Monin-Obukhov similarity theory. The net surface energy balance is quantified during selected periods using fusion measurements derived from height variations of the ice surface (measured with an ultrasonic depth gauge). It is compared to the energy balance computed from radiative balance along with mean wind speed and temperature or eddy covariance fluxes.This data helps us to quantify errors made with classical similarity methods, and their variation regarding to meteorological forcings.
Directory of Open Access Journals (Sweden)
Ma Li
2014-04-01
Full Text Available It is of great significance to improve the accuracy of turbulence models in shock-wave/boundary layer interaction flow. The relationship between the pressure gradient, as well as the shear layer, and the development of turbulent kinetic energy in impinging shock-wave/turbulent boundary layer interaction flow at Mach 2.25 is analyzed based on the data of direct numerical simulation (DNS. It is found that the turbulent kinetic energy is amplified by strong shear in the separation zone and the adverse pressure gradient near the separation point. The pressure gradient was non-dimensionalised with local density, velocity, and viscosity. Spalart–Allmaras (S–A model is modified by introducing the non-dimensional pressure gradient into the production term of the eddy viscosity transportation equation. Simulation results show that the production and dissipation of eddy viscosity are strongly enhanced by the modification of S–A model. Compared with DNS and experimental data, the wall pressure and the wall skin friction coefficient as well as the velocity profile of the modified S–A model are obviously improved. Thus it can be concluded that the modification of S–A model with the pressure gradient can improve the predictive accuracy for simulating the shock-wave/turbulent boundary layer interaction.
Sun, Jielun; Lenschow, Donald; LeMone, Margaret; Mahrt, Larry
2015-04-01
Turbulent fluxes from the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES-99) field experiment are further analyzed for both day- and nighttime as a follow-on to the investigation of the nighttime turbulence in Sun et al. (2012). The behavior of momentum and heat fluxes is investigated as functions of wind speed and the bulk temperature difference between observation heights and the surface. Vertical variations of momentum and heat flux at a given height z are correlated and are explained in terms of the energy and heat balance in a layer above the ground surface in which the surface heating/cooling and momentum sink need to be included. In addition, the surface also plays an important role in constraining the size of the dominant turbulent eddies, which is directly related to turbulence strength and the length scale of turbulence generation. The turbulence generation is not related to local vertical gradients especially under neutral condition as assumed in Monin-Obukhov similarity theory. Based on the observed relationships between momentum and heat fluxes, a new bulk formula for turbulence parameterization is developed to mainly examine the above-mentioned surface effects on vertical variation of turbulent momentum and heat fluxes. The new understanding of the observed relationships between these turbulent variables and mean variables explains the observed nighttime turbulence regime change observed in Sun et al. (2012) as well as the daytime momentum and heat flux variations with height up to the maximum observation height of 55 m.
Boundary Layer under Oscillatory Wave
Mohammad Bagus Adityawan; Hitoshi Tanaka
2011-01-01
Turbulence due to wave motion and propagation is a very important aspect in sediment transport modeling. The boundary layer characteristic during the process will highly influence the sediment transport mechanism at the bottom. 1D model approach has been widely used to assess the turbulent boundary layer. However, the need for a more detailed model leads to the development of a more sophisticated models. This study presents a 2D turbulent model using k-ω equation to approach the turbulent bou...
International Nuclear Information System (INIS)
Highlights: • Parametric study of turbulent boundary layers over a converging–diverging riblet-type surface. • This unique surface roughness induces large-scale spanwise periodicity that distort the layer thickness. • Large-scale low and high speed regions form above converging and diverging regions respectively. • The converging and diverging regions also exhibit increased and reduced turbulent intensity. • These highly directional rough surfaces seem to induce large counter-rotating roll-modes. -- Abstract: The effect of converging–diverging riblet-type surface roughness (riblets arranged in a ‘herringbone’ pattern) are investigated experimentally in a zero pressure gradient turbulent boundary layer. For this initial parametric investigation three different parameters of the surface roughness are analysed in detail; the converging–diverging riblet yaw angle α, the streamwise fetch or development length over the rough surface Fx and the viscous-scaled riblet height h+. It is observed that this highly directional surface roughness pattern induces a large-scale spanwise periodicity onto the boundary layer, resulting in a pronounced spanwise modification of the boundary layer thickness. Hot-wire measurements reveal that above the diverging region, the local mean velocity increases while the turbulent intensity decreases, resulting in a thinner overall boundary layer thickness in these locations. The opposite situation occurs over the converging region, where the local mean velocity is decreased and the turbulent intensity increases, producing a locally thicker boundary layer. Increasing the converging–diverging angle or the viscous-scaled riblet height results in stronger spanwise perturbations. For the strongest convergent–divergent angle, the spanwise variation of the boundary layer thickness between the diverging and converging region is almost a factor of two. Such a large variation is remarkable considering that the riblet height is only
Talpos, Simona; Apostol, Marian
2015-12-01
It is shown that the Reynolds equations for a turbulent flow over an unbounded flat surface in the presence of a constant pressure-gradient lead to a displaced logarithmic profile of the velocity distribution; the displaced logarithmic profile is obtained by assuming a constant production rate of turbulence energy. The displacement height measured on the (vertical) axis perpendicular to the surface is either positive or negative. For a positive displacement height the boundary layer exhibits an inversion, while for a negative displacement height the boundary layer is a direct one. In an inversion boundary layer the logarithmic velocity profile is disrupted into two distinct branches separated by a logarithmic singularity. The viscosity transforms this logarithmic singularity into a sharp edge, governed by a generalized Reynolds number. The associated temperature distribution is calculated, and the results are discussed in relation to meteorological boundary-layer jets and stratified layers. The effects of gravitation and atmospheric thermal or fluid-mixture concentration gradients ("external forcings") are also considered; it is shown that such circumstances may lead to various modifications of the boundary layers. A brief presentation of a similar situation is described for a circular pipe.
Foufoula-Georgiou, E.
2002-12-01
Deepening our understanding of the space-time variability of atmospheric/hydrologic processes and their interactions over a range of scales has important implications for improving model parameterizations and increasing the accuracy of predictive models. At the same time, the inherent nonlinear and chaotic character of some of these processes imposes limits on their predictability, and therefore provides upper bounds on the expected prediction accuracy from numerical models. This paper will address questions of scaling, nonlinearity and predictability in processes active at two major interfaces of the hydrologic system: the land-atmosphere interface, and the land-water interface. Specifically, recent findings and their practical implications will be presented on: (a) multiscale interactions in turbulent boundary layers and implications for boundary condition formulations; (b) predictability assessment of turbulent velocities in a boundary layer as a function of scale; and (c) nonlinear dynamics of basin hydrologic response as a function of spatio-temporally varying forcing and basin geomorphological organization.
Diurnal variation in the turbulent structure of the cloudy marine boundary layer during FIRE 1987
Hignett, Phillip
1990-01-01
During the 1987 FIRE marine stratocumulus experiment the U.K. Meteorological Office operated a set of turbulence probes attached to the tether cable of a balloon based on San Nicolas Island. Typically six probes were used; each probe is fitted with Gill propeller anemometers, a platinum resistance thermometer and wet and dry thermistors, to permit measurements of the fluxes of momentum, heat, and humidity. The orientation of each probe is determined from a pair of inclinometers and a three-axis magnetometer. Sufficient information is available to allow the measured wind velocities to be corrected for the motion of the balloon. On the 14 to 15 July measurements were made over the period 1530 to 1200 UTC and again, after a short break for battery recharging and topping-up the balloon, between 0400 to 0900 UTC. Data were therefore recorded from morning to early evening, and again for a period overnight. Six probes were available for the daytime measurements, five for the night. Data were recorded at 4 Hz for individual periods of a little over an hour. The intention was to keep a minimum of one probe at or just above cloud top; small changes in balloon height were necessary to accommodate changes in inversion height. The ability of the balloon system to make simultaneous measurements at several levels allows the vertical structure of the boundary layer to be displayed without resort to composites. Turbulent statistics were calculated from 2 hour periods, one straddling local noon and one at night. These were subdivided into half-hour averaging intervals for the evaluation of variances and fluxes.
Realtime Surface Shear Stress Control with MEMS Sensors/Actuators in Turbulent Boundary Layers
Huang, Adam; Lew, James; Ho, Chih-Ming; Xu, Yong; Tai, Yu-Chong
2003-11-01
High-speed surface streaks in turbulent boundary layers have been attributed to approximately 40friction drag. A real-time control system for reducing surface shear stress has being developed. The system consists of two linear arrays of MEMS surface shear stress imagers for providing control and feedback measurements and a recently developed, micro-machined flap-type actuator for interaction with the streak structures. Driven by a constant temperature anemometry circuit with an overheat ratio of 12sensitivity of 100 mV/Pa and frequency response of 20 kHz. The micro-machined bubble-flap actuator is essentially a thin silicon cantilever beam which hangs/sits on top of a silicone diaphragm molded into a bulk etched silicon cavity. The flap shape used is a 3mm long (streamwise) by 1mm wide rectangular beam, with a thickness of 40 um. Actuation is achieved by pneumatically inflating the silicone diaphragm, which then pushes up the silicon beam. The current flap can achieve off-plane deflections of over 130 um at frequencies up to 150 Hz, with a rise time of 2ms and a fall time of 4ms. Experiments are carried out with the system installed onto the wall of a 2-D turbulent wind tunnel. At Re 10k, corresponding to flow velocity of 10 m/s, time-averaged reduction of 4achieved continuous actuation at 130 um and 150 Hz. Furthermore, in offline data processing, it has been found that the actuator interacting with the streak structures has reduce the peak shear stress of a streak by an additional 0.2 Pa, or about 50
Effects of micro-ramps on a shock wave/turbulent boundary layer interaction
Blinde, P.L.; Humble, R.A.; Van Oudheusden, B.W.; Scarano, F.
2009-01-01
Stereoscopic particle image velocimetry is used to investigate the effects of micro-ramp sub-boundary layer vortex generators, on an incident shock wave/boundary layer interaction at Mach 1.84. Single- and double-row arrangements of micro-ramps are considered. The micro-ramps have a height of 20% of
Couvreux, Fleur; Bazile, Eric; Canut, Guylaine; Seity, Yann; Lothon, Marie; Lohou, Fabienne; Guichard, Françoise; Nilsson, Erik
2016-07-01
This study evaluates the ability of three operational models, with resolution varying from 2.5 to 16 km, to predict the boundary-layer turbulent processes and mesoscale variability observed during the Boundary Layer Late-Afternoon and Sunset Turbulence (BLLAST) field campaign. We analyse the representation of the vertical profiles of temperature and humidity and the time evolution of near-surface atmospheric variables and the radiative and turbulent fluxes over a total of 12 intensive observing periods (IOPs), each lasting 24 h. Special attention is paid to the evolution of the turbulent kinetic energy (TKE), which was sampled by a combination of independent instruments. For the first time, this variable, a central one in the turbulence scheme used in AROME and ARPEGE, is evaluated with observations.In general, the 24 h forecasts succeed in reproducing the variability from one day to another in terms of cloud cover, temperature and boundary-layer depth. However, they exhibit some systematic biases, in particular a cold bias within the daytime boundary layer for all models. An overestimation of the sensible heat flux is noted for two points in ARPEGE and is found to be partly related to an inaccurate simplification of surface characteristics. AROME shows a moist bias within the daytime boundary layer, which is consistent with overestimated latent heat fluxes. ECMWF presents a dry bias at 2 m above the surface and also overestimates the sensible heat flux. The high-resolution model AROME resolves the vertical structures better, in particular the strong daytime inversion and the thin evening stable boundary layer. This model is also able to capture some specific observed features, such as the orographically driven subsidence and a well-defined maximum that arises during the evening of the water vapour mixing ratio in the upper part of the residual layer due to fine-scale advection. The model reproduces the order of magnitude of spatial variability observed at
Free-stream Turbulence Effects on the Boundary Layer of a High-lift Low-Pressure-Turbine Blade
Institute of Scientific and Technical Information of China (English)
Simoni D.; Ubaldi M.; Zunino P.; Ampellio E.
2016-01-01
The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally investigated at low and high free-stream turbulence intensity conditions.Measurements have been carried out in order to analyze the boundary layer transition and separation processes at a low Reynolds number,under both steady and unsteady inflows.Static pressure distributions along the blade surfaces as well as total pressure distributions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions.Particle Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields.The flow field has been surveyed in two orthogonal planes (a blade-to-blade plane and a wall-parallel one).These measurements allow the identification of the Kelvin-Helmholtz large scale coherent structures shed as a consequence of the boundary layer laminar separation under steady inflow,as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks.A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state,thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.
Free-stream turbulence effects on the boundary layer of a high-lift low-pressure-turbine blade
Simoni, D.; Ubaldi, M.; Zunino, P.; Ampellio, E.
2016-06-01
The suction side boundary layer evolution of a high-lift low-pressure turbine cascade has been experimentally investigated at low and high free-stream turbulence intensity conditions. Measurements have been carried out in order to analyze the boundary layer transition and separation processes at a low Reynolds number, under both steady and unsteady inflows. Static pressure distributions along the blade surfaces as well as total pressure distributions in a downstream tangential plane have been measured to evaluate the overall aerodynamic efficiency of the blade for the different conditions. Particle Image Velocimetry has been adopted to analyze the time-mean and time-varying velocity fields. The flow field has been surveyed in two orthogonal planes (a blade-to-blade plane and a wall-parallel one). These measurements allow the identification of the Kelvin-Helmholtz large scale coherent structures shed as a consequence of the boundary layer laminar separation under steady inflow, as well as the investigation of the three-dimensional effects induced by the intermittent passage of low and high speed streaks. A close inspection of the time-mean velocity profiles as well as of the boundary layer integral parameters helps to characterize the suction side boundary layer state, thus justifying the influence of free-stream turbulence intensity on the blade aerodynamic losses measured under steady and unsteady inflows.
On the routes to inertial mean dynamics in smooth- and rough-wall turbulent boundary layers
Klewicki, Joseph; Mehdi, Faraz
2012-11-01
Connections between the structure of smooth- and rough-wall turbulent boundary layers are established within the context of the order of magnitude properties exhibited by the terms in the mean momentum equation. These properties are shown to be associated with the processes by which inertial mean dynamics emerge with distance from the wall. A key element is the process by which the vorticity field becomes three-dimensional. In the smooth-wall case, vorticity stretching leads to the three-dimensionalization of the vorticity field in the region where the mean viscous force retains leading order. This underlies the well-established Reynolds number scaling behaviors exhibited by smooth-wall flows. Roughness modifies (generally augments) the process by which the vorticity field becomes three dimensional, rendering scalings for the route to inertial mean dynamics that depend on the relative scale separations between the inner, roughness, and outer scales. Evidence (from existing and recent experiments) of these combined scaling regimes is presented. The present analyses provide a basis for predicting where and physically why Townsend's similarity hypothesis should hold, as well as under what conditions outer similarity loses validity. The support of the ONR (N000140810836, grant monitor Ronald Joslin) is gratefully acknowledged.
Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions
Bernardini, Matteo; Pirozzoli, Sergio; Grasso, Francesco
2016-01-01
Direct numerical simulations are carried out to investigate the effect of the wall temperature on the behavior of oblique shock-wave/turbulent boundary layer interactions at freestream Mach number $2.28$ and shock angle of the wedge generator $\\varphi = 8^{\\circ}$. Five values of the wall-to-recovery-temperature ratio ($T_w/T_r$) are considered, corresponding to cold, adiabatic and hot wall thermal conditions. We show that the main effect of cooling is to decrease the characteristic scales of the interaction in terms of upstream influence and extent of the separation bubble. The opposite behavior is observed in the case of heating, that produces a marked dilatation of the interaction region. The distribution of the Stanton number shows that a strong amplification of the heat transfer occurs across the interaction, and the maximum values of thermal and dynamic loads are found in the case of cold wall. The analysis reveals that the fluctuating heat flux exhibits a strong intermittent behavior, characterized by ...
A Cautionary Note on the Zagarola and Smits Similarity Parameter for the Turbulent Boundary Layer
Weyburne, David
2015-01-01
Zagarola and Smits developed an empirical velocity parameter for scaling the outer region of the turbulent boundary layer velocity profile that has been widely applied and has resulted in similarity in many of those datasets. In all the cases studied thus far claims for similarity involving the Zagarola and Smits scaling parameter have been based on examining plots of the defect profile. In the work herein it is shown that the common practice of finding similarity behavior using the defect profile has often been incomplete in the sense that not all of the criteria for similarity have been checked for compliance. When full compliance is checked it is found that some datasets displaying defect similarity do not satisfy all the criteria for similarity. The nature of this contradiction and noncompliance is described in detail. It is shown that the original datasets used by Zagarola and Smits display this flawed similarity behavior. Hence, a careful reassessment of any claims in the literature is required for thos...
International Nuclear Information System (INIS)
In the present work our focus is to improve the performance of a wind farm by coordinated control of all turbines with the aim to increase the overall energy extraction by the farm. To this end, we couple flow simulations performed using Large Eddy Simulations (LES) with gradient based optimization to control individual turbines in a farm. The control parameters are the disk-based thrust coefficient of individual turbines as a function of time. They indirectly represent the effect of control actions that would correspond to blade-pitching of the turbines. We employ a receding-horizon predictive control setting and solve the optimization problem iteratively at each time horizon based on the gradient information obtained from the evolution of the flow field and the adjoint computation. We find that the extracted farm power increases by approximately 16% for a cost functional that is based on total energy extraction. However, this energy is gained from a slow deceleration of the boundary layer which is sustained for approximately 1 hour. We further analyze the turbulent stresses and compare to wind farms without optimal control
Wind Tunnel Measurements of Turbulent Boundary Layer over Hypothetical Urban Roughness Elements
Ho, Y. K.; Liu, C. H.
2012-04-01
Urban morphology affects the near-ground atmospheric boundary layer that in turn modifies the wind flows and pollutant dispersion over urban areas. A number of numerical models (large-eddy simulation, LES and k-ɛ turbulence models) have been developed to elucidate the transport processes in and above urban street canyons. To complement the modelling results, we initiated a wind tunnel study to examine the influence of idealized urban roughness on the flow characteristics and pollutant dispersion mechanism over 2D idealized street canyons placed in cross flows. Hot-wire anemometry (HWA) was employed in this study to measure the flows over 2D street canyons in the wind tunnel in our university. Particular focus in the beginning stage was on the fabrication of hot-wire probes, data acquisition system, and signal processing technique. Employing the commonly adopted hot-wire universal function, we investigated the relationship in between and developed a scaling factor which could generalize the output of our hot-wire probes to the standardized one as each hot-wire probes has its unique behaviour. Preliminary experiments were performed to measure the wind flows over street canyons of unity aspect ratio. Vertical profiles of the ensemble average velocity and fluctuations at three different segments over the street canyons were collected. The results were then compared with our LES that show a good argument with each other. Additional experiments are undertaken to collect more data in order to formulate the pollutant dispersion mechanism of street canyons and urban areas.
Temporal and spatial transients in turbulent boundary layer flow over an oscillating wall
International Nuclear Information System (INIS)
Highlights: ► Drag reduction is 18% and 29% respectively for two different strengths of forcing. ► The Reynolds number dependency is investigated. ► Spanwise shear follows the non-harmonic solution to the laminar equations. ► Temporal and spatial transients are related through a coordinate transformation. ► Spanwise Reynolds stress behaves almost exactly as in a channel flow. - Abstract: Direct numerical simulations have been performed to study the effect of an oscillating segment of the wall on a turbulent boundary layer flow. Two different oscillation amplitudes with equal oscillation period have been used, which allows a direct comparison between a relatively weak and strong forcing of the flow. The weaker forcing results in 18% drag reduction while the stronger forcing, with twice the amplitude, yields 29% drag reduction. The downstream development of the drag reduction is compared with earlier simulations and experiments. In addition, a simulation with identical oscillation parameters as in previous numerical and experimental investigations allows for an estimation of the effect of the Reynolds number on the drag reduction. Reductions in the Reynolds stresses and the important role that the edge of the Stokes layer has is explained. An estimation of the idealized power consumption shows that a positive energy budget is only possible for the weaker wall velocity case. Spatial and temporal transients are investigated and a transformation between spatial and temporal coordinates via a convection velocity is shown to facilitate a comparison between the two transients in a consistent manner. The streamwise shear exhibits a similar monotonic behavior in the spatial and temporal transients, while the non-monotinic temporal transient of the longitudinal Reynolds stress has no counterpart in the spatial development. Furthermore, the evolution in time of the spanwise Reynolds stress is very similar to previously reported channel flow data. The instantaneous
Energy Technology Data Exchange (ETDEWEB)
Kadja, P. [Universite de Constantine (Algeria)
1993-12-31
Thanks to a numerical code to solve boundary layer equations, natural air convection along a hot and vertical plate was predicted with different turbulence models in order to choose the best one suitable for the calculation of this kind of heat transfer. The turbulence models compared are: the Cebeci Smith algebraic model, the k- standard model with wall functions for k and, and three low Reynolds number k- models: Lam and Bremhorst, de Chien and Jones and Launder. (Authors). 8 refs., 3 figs., 1 tab.
Czech Academy of Sciences Publication Activity Database
Hladík, Ondřej; Jonáš, Pavel; Mazur, Oton; Uruba, Václav
Berlin, Heidelberg : Springer-Verlag, 2012, s. 205-208. ISBN 978-3-642-28967-5. ISSN 0930-8989. - (Springer Proceedings in Physics. 141). [iTi 2010 Conference in Turbulence. Bertinoro (IT), 19.09.2010-22.09.2010] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Institutional support: RVO:61388998 Keywords : boundary layer * laminar - turbulent transition * bypass transition Subject RIV: BK - Fluid Dynamics
Institute of Scientific and Technical Information of China (English)
HUANG ZhangFeng; ZHOU Heng; LUO JiSheng
2007-01-01
Through temporal mode direct numerical simulation, flow field database of a fully developed turbulent boundary layer on a flat plate with Mach number 4.5 and Reynolds number Reθ=1094 has been obtained. Commonly used detection methods in experiments are applied to detecting coherent structures in the flow field,and it is found that coherent structures do exist in the wall region of a supersonic turbulent boundary layer. The detected results show that a low-speed streak is detected by using the Mu-level method, the rising parts of this streak are detected by using the second quadrant method, and the crossing regions from a low-speed streak to the high-speed one are detected by using the VITA method respectively.Notwithstanding that different regions are detected by different methods, they are all accompanied by quasi-stream-wise vortex structures.
Institute of Scientific and Technical Information of China (English)
2007-01-01
Through temporal mode direct numerical simulation, flow field database of a fully developed turbulent boundary layer on a flat plate with Mach number 4.5 and Reynolds number Reθ =1094 has been obtained. Commonly used detection meth- ods in experiments are applied to detecting coherent structures in the flow field, and it is found that coherent structures do exist in the wall region of a supersonic turbulent boundary layer. The detected results show that a low-speed streak is de- tected by using the Mu-level method, the rising parts of this streak are detected by using the second quadrant method, and the crossing regions from a low-speed streak to the high-speed one are detected by using the VITA method respectively. Notwithstanding that different regions are detected by different methods, they are all accompanied by quasi-stream-wise vortex structures.
Marine boundary layer and turbulent fluxes over the Baltic Sea: Measurements and modelling
DEFF Research Database (Denmark)
Gryning, Sven-Erik; Batchvarova, E.
2002-01-01
Two weeks of measurements of the boundary-layer height over a small island (Christianso) in the Baltic Sea are discussed. The meteorological conditions are characterised by positive heat flux over the sea. The boundary-layer height was simulated with two models, a simple applied high-resolution (2...... km x 2 km) model, and the operational numerical weather prediction model HIRLAM (grid resolution of 22.5 km x 22.5 km). For southwesterly winds it was found that a relatively large island (Bornholm) lying 20-km upwind of the measuring site influences the boundary-layer height. In this situation the...... high-resolution simple applied model reproduces the characteristics of the boundary-layer height over the measuring site. Richardson-number based methods using data from simulations with the HIRLAM model fail, most likely because the island and the water fetch to the measuring site are about the size...
International Nuclear Information System (INIS)
Transition to turbulence occurring in a flat-plate boundary-layer flow subjected to high levels of free-stream turbulence is considered. This scenario, denoted bypass transition, is characterised by the non-modal growth of streamwise elongated disturbances. These so-called streaks are regions of positive and negative streamwise velocity alternating in the spanwise direction inside the boundary layer. When they reach large enough amplitudes, breakdown into turbulent spots occurs via their secondary instability. In this work, the bypass-transition process is simulated using direct numerical simulations (DNS) and large-eddy simulations (LES). The ADM-RT subgrid-scale model turned out to be particularly suited for transitional flows after a thorough validation. Linear feedback control is applied in order to reduce the perturbation energy and consequently delay transition. This case represents therefore an extension of the linear approach (Chevalier, M., Hoepffner, J., Akervik, E., Henningson, D.S., 2007a. Linear feedback control and estimation applied to instabilities in spatially developing boundary layers. J. Fluid Mech. 588, 163-187, 167-187.) to flows characterised by strong nonlinearities. Control is applied by blowing and suction at the wall and it is both based on the full knowledge of the instantaneous velocity field (i.e. full information control) and on the velocity field estimated from wall measurements. The results show that the control is able to delay the growth of the streaks in the region where it is active, which implies a delay of the whole transition process. The flow field can be estimated from wall measurements alone: The structures occurring in the 'real' flow are reproduced correctly in the region where the measurements are taken. Downstream of this region the estimated field gradually diverges from the 'real' flow, revealing the importance of the continuous excitation of the boundary layer by the external free-stream turbulence. Control based on
Chefranov, Sergey G
2010-01-01
For Gagen-Poiseuille flow, we show that exponential instability (to extremely small, axially symmetric disturbances represented by Galerkin's approximation) is possible only if there exists conditionally periodic variability of the disturbances along the pipe axis when the threshold Reynolds number depends on the ratio of two longitudinal periods. Absolute minimum (for) is obtained that corresponds to the observed conditions of transition from the laminar resistance law to the turbulent one and Tollmien-Schlichting waves exciting in the boundary layer.
Rottner, L.; Baehr, C.
2014-12-01
Turbulent phenomena in the atmospheric boundary layer (ABL) are characterized by small spatial and temporal scales which make them difficult to observe and to model.New remote sensing instruments, like Doppler Lidar, give access to fine and high-frequency observations of wind in the ABL. This study suggests to use a method of nonlinear estimation based on these observations to reconstruct 3D wind in a hemispheric volume, and to estimate atmospheric turbulent parameters. The wind observations are associated to particle systems which are driven by a local turbulence model. The particles have both fluid and stochastic properties. Therefore, spatial averages and covariances may be deduced from the particles. Among the innovative aspects, we point out the absence of the common hypothesis of stationary-ergodic turbulence and the non-use of particle model closure hypothesis. Every time observations are available, 3D wind is reconstructed and turbulent parameters such as turbulent kinectic energy, dissipation rate, and Turbulent Intensity (TI) are provided. This study presents some results obtained using real wind measurements provided by a five lines of sight Lidar. Compared with classical methods (e.g. eddy covariance) our technic renders equivalent long time results. Moreover it provides finer and real time turbulence estimations. To assess this new method, we suggest computing independently TI using different observation types. First anemometer data are used to have TI reference.Then raw and filtered Lidar observations have also been compared. The TI obtained from raw data is significantly higher than the reference one, whereas the TI estimated with the new algorithm has the same order.In this study we have presented a new class of algorithm to reconstruct local random media. It offers a new way to understand turbulence in the ABL, in both stable or convective conditions. Later, it could be used to refine turbulence parametrization in meteorological meso-scale models.
Jha, Pankaj Kumar
Wind energy is becoming one of the most significant sources of renewable energy. With its growing use, and social and political awareness, efforts are being made to harness it in the most efficient manner. However, a number of challenges preclude efficient and optimum operation of wind farms. Wind resource forecasting over a long operation window of a wind farm, development of wind farms over a complex terrain on-shore, and air/wave interaction off-shore all pose difficulties in materializing the goal of the efficient harnessing of wind energy. These difficulties are further amplified when wind turbine wakes interact directly with turbines located downstream and in adjacent rows in a turbulent atmospheric boundary layer (ABL). In the present study, an ABL solver is used to simulate different atmospheric stability states over a diurnal cycle. The effect of the turbines is modeled by using actuator methods, in particular the state-of-the-art actuator line method (ALM) and an improved ALM are used for the simulation of the turbine arrays. The two ALM approaches are used either with uniform inflow or are coupled with the ABL solver. In the latter case, a precursor simulation is first obtained and data saved at the inflow planes for the duration the turbines are anticipated to be simulated. The coupled ABL-ALM solver is then used to simulate the turbine arrays operating in atmospheric turbulence. A detailed accuracy assessment of the state-of-the-art ALM is performed by applying it to different rotors. A discrepancy regarding over-prediction of tip loads and an artificial tip correction is identified. A new proposed ALM* is developed and validated for the NREL Phase VI rotor. This is also applied to the NREL 5-MW turbine, and guidelines to obtain consistent results with ALM* are developed. Both the ALM approaches are then applied to study a turbine-turbine interaction problem consisting of two NREL 5-MW turbines. The simulations are performed for two ABL stability
Rodríguez-López, Eduardo; Bruce, Paul J. K.; Buxton, Oliver R. H.
2015-04-01
The present paper describes a method to extrapolate the mean wall shear stress, , and the accurate relative position of a velocity probe with respect to the wall, , from an experimentally measured mean velocity profile in a turbulent boundary layer. Validation is made between experimental and direct numerical simulation data of turbulent boundary layer flows with independent measurement of the shear stress. The set of parameters which minimize the residual error with respect to the canonical description of the boundary layer profile is taken as the solution. Several methods are compared, testing different descriptions of the canonical mean velocity profile (with and without overshoot over the logarithmic law) and different definitions of the residual function of the optimization. The von Kármán constant is used as a parameter of the fitting process in order to avoid any hypothesis regarding its value that may be affected by different initial or boundary conditions of the flow. Results show that the best method provides an accuracy of for the estimation of the friction velocity and for the position of the wall. The robustness of the method is tested including unconverged near-wall measurements, pressure gradient, and reduced number of points; the importance of the location of the first point is also tested, and it is shown that the method presents a high robustness even in highly distorted flows, keeping the aforementioned accuracies if one acquires at least one data point in . The wake component and the thickness of the boundary layer are also simultaneously extrapolated from the mean velocity profile. This results in the first study, to the knowledge of the authors, where a five-parameter fitting is carried out without any assumption on the von Kármán constant and the limits of the logarithmic layer further from its existence.
Nilsson, E.; Lohou, F.; M. Lothon; Pardyjak, E.; Mahrt, L.; C. Darbieu
2015-01-01
The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from mid-day until zero buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 Intensive Observation Period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics a...
Afzal, Bushra; Noor Afzal Team; Bushra Afzal Team
2014-11-01
The momentum and thermal turbulent boundary layers over a continuous moving sheet subjected to a free stream have been analyzed in two layers (inner wall and outer wake) theory at large Reynolds number. The present work is based on open Reynolds equations of momentum and heat transfer without any closure model say, like eddy viscosity or mixing length etc. The matching of inner and outer layers has been carried out by Izakson-Millikan-Kolmogorov hypothesis. The matching for velocity and temperature profiles yields the logarithmic laws and power laws in overlap region of inner and outer layers, along with friction factor and heat transfer laws. The uniformly valid solution for velocity, Reynolds shear stress, temperature and thermal Reynolds heat flux have been proposed by introducing the outer wake functions due to momentum and thermal boundary layers. The comparison with experimental data for velocity profile, temperature profile, skin friction and heat transfer are presented. In outer non-linear layers, the lowest order momentum and thermal boundary layer equations have also been analyses by using eddy viscosity closure model, and results are compared with experimental data. Retired Professor, Embassy Hotel, Rasal Ganj, Aligarh 202001 India.
Wake Turbulence of Two NREL 5-MW Wind Turbines Immersed in a Neutral Atmospheric Boundary-Layer Flow
Bashioum, Jessica L; Schmitz, Sven; Duque, Earl P N
2013-01-01
The fluid dynamics video considers an array of two NREL 5-MW turbines separated by seven rotor diameters in a neutral atmospheric boundary layer (ABL). The neutral atmospheric boundary-layer flow data were obtained from a precursor ABL simulation using a Large-Eddy Simulation (LES) framework within OpenFOAM. The mean wind speed at hub height is 8m/s, and the surface roughness is 0.2m. The actuator line method (ALM) is used to model the wind turbine blades by means of body forces added to the momentum equation. The fluid dynamics video shows the root and tip vortices emanating from the blades from various viewpoints. The vortices become unstable and break down into large-scale turbulent structures. As the wakes of the wind turbines advect further downstream, smaller-scale turbulence is generated. It is apparent that vortices generated by the blades of the downstream wind turbine break down faster due to increased turbulence levels generated by the wake of the upstream wind turbine.
Energy Technology Data Exchange (ETDEWEB)
Ames, Forrest E. [University of North Dakota; Kingery, Joseph E. [University of North Dakota
2015-06-17
Full coverage shaped-hole film cooling and downstream heat transfer measurements have been acquired in the accelerating flows over a large cylindrical leading edge test surface. The shaped holes had an 8° lateral expansion angled at 30° to the surface with spanwise and streamwise spacings of 3 diameters. Measurements were conducted at four blowing ratios, two Reynolds numbers and six well documented turbulence conditions. Film cooling measurements were acquired over a four to one range in blowing ratio at the lower Reynolds number and at the two lower blowing ratios for the higher Reynolds number. The film cooling measurements were acquired at a coolant to free-stream density ratio of approximately 1.04. The flows were subjected to a low turbulence condition (Tu = 0.7%), two levels of turbulence for a smaller sized grid (Tu = 3.5%, and 7.9%), one turbulence level for a larger grid (8.1%), and two levels of turbulence generated using a mock aero-combustor (Tu = 9.3% and 13.7%). Turbulence level is shown to have a significant influence in mixing away film cooling coverage progressively as the flow develops in the streamwise direction. Effectiveness levels for the aero-combustor turbulence condition are reduced to as low as 20% of low turbulence values by the furthest downstream region. The film cooling discharge is located close to the leading edge with very thin and accelerating upstream boundary layers. Film cooling data at the lower Reynolds number, show that transitional flows have significantly improved effectiveness levels compared with turbulent flows. Downstream effectiveness levels are very similar to slot film cooling data taken at the same coolant flow rates over the same cylindrical test surface. However, slots perform significantly better in the near discharge region. These data are expected to be very useful in grounding computational predictions of full coverage shaped hole film cooling with elevated turbulence levels and acceleration. IR
Vijayakumar, Ganesh
Modern commercial megawatt-scale wind turbines occupy the lower 15-20% of the atmospheric boundary layer (ABL), the atmospheric surface layer (ASL). The current trend of increasing wind turbine diameter and hub height increases the interaction of the wind turbines with the upper ASL which contains spatio-temporal velocity variations over a wide range of length and time scales. Our interest is the interaction of the wind turbine with the energetic integral-scale eddies, since these cause the largest temporal variations in blade loadings. The rotation of a wind turbine blade through the ABL causes fluctuations in the local velocity magnitude and angle of attack at different sections along the blade. The blade boundary layer responds to these fluctuations and in turn causes temporal transients in local sectional loads and integrated blade and shaft bending moments. While the integral scales of the atmospheric boundary layer are ˜ O(10--100m) in the horizontal with advection time scales of order tens of seconds, the viscous surface layer of the blade boundary layer is ˜ O(10 -- 100 mum) with time scales of order milliseconds. Thus, the response of wind turbine blade loadings to atmospheric turbulence is the result of the interaction between two turbulence dynamical systems at extremely disparate ranges of length and time scales. A deeper understanding of this interaction can impact future approaches to improve the reliability of wind turbines in wind farms, and can underlie future improvements. My thesis centers on the development of a computational framework to simulate the interaction between the atmospheric and wind turbine blade turbulence dynamical systems using a two step one-way coupled approach. Pseudo-spectral large eddy simulation (LES) is used to generate a true (equilibrium) atmospheric boundary layer over a flat land with specified surface roughness and heating consistent with the stability state of the daytime lower troposphere. Using the data from the
Cheng, Wan
2015-06-30
We describe large-eddy simulations of turbulent boundary-layer flow over a flat plate at high Reynolds number in the presence of an unsteady, three-dimensional flow separation/reattachment bubble. The stretched-vortex subgrid-scale model is used in the main flow domain combined with a wall-model that is a two-dimensional extension of that developed by Chung & Pullin (2009). Flow separation and re-attachment of the incoming boundary layer is induced by prescribing wall-normal velocity distribution on the upper boundary of the flow domain that produces an adverse-favorable stream-wise pressure distribution at the wall. The LES predicts the distribution of mean shear stress along the wall including the interior of the separation bubble. Several properties of the separation/reattachment flow are discussed.
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel; Hladík, Ondřej; Mazur, Oton; Uruba, Václav
2012-01-01
Roč. 12, č. 1 (2012), s. 467-468. ISSN 1617-7061. [Annual Meeting of the International Association of Applied Mathematics and Mechanics /83./. Darmstadt, 26.03.2012-30.03.2012] R&D Projects: GA ČR GAP101/10/1230; GA ČR GAP101/12/1271 Institutional research plan: CEZ:AV0Z20760514 Keywords : laminar/turbulent transition * wall friction * rough wall * boundary layer Subject RIV: BK - Fluid Dynamics http://onlinelibrary.wiley.com/doi/10.1002/pamm.201210222/abstract
DEFF Research Database (Denmark)
Fuhrman, David R.; Fredsøe, Jørgen; Sumer, B. Mutlu
2009-01-01
contributions believed to play a prominent role in cross-shore boundary layer and sediment transport processes: (1) converging-diverging effects from bed slope, (2) wave skewness, (3) wave asymmetry, and (4) waves combined with superposed negative currents (intended to loosely represent, for example, return...... currents or undertow). The effects from each of the four components are isolated and quantified using a standard set of bed shear stress quantities, allowing their easy comparison. For conditions representing large shallow-water waves on steep slopes, the results suggest that converging-diverging effects...... from beach slope may make a significant onshore bed load contribution. Generally, however, the results suggest wave skewness (in addition to conventional steady streaming) as the most important onshore contribution outside the surf zone. Streaming induced within the wave boundary layer is also...
Yang, X. I. A.; Marusic, I.; Meneveau, C.
2016-06-01
Townsend [Townsend, The Structure of Turbulent Shear Flow (Cambridge University Press, Cambridge, UK, 1976)] hypothesized that the logarithmic region in high-Reynolds-number wall-bounded flows consists of space-filling, self-similar attached eddies. Invoking this hypothesis, we express streamwise velocity fluctuations in the inertial layer in high-Reynolds-number wall-bounded flows as a hierarchical random additive process (HRAP): uz+=∑i=1Nzai . Here u is the streamwise velocity fluctuation, + indicates normalization in wall units, z is the wall normal distance, and ai's are independently, identically distributed random additives, each of which is associated with an attached eddy in the wall-attached hierarchy. The number of random additives is Nz˜ln(δ /z ) where δ is the boundary layer thickness and ln is natural log. Due to its simplified structure, such a process leads to predictions of the scaling behaviors for various turbulence statistics in the logarithmic layer. Besides reproducing known logarithmic scaling of moments, structure functions, and correlation function [" close="]3/2 uz(x ) uz(x +r ) >, new logarithmic laws in two-point statistics such as uz4(x ) > 1 /2, 1/3, etc. can be derived using the HRAP formalism. Supporting empirical evidence for the logarithmic scaling in such statistics is found from the Melbourne High Reynolds Number Boundary Layer Wind Tunnel measurements. We also show that, at high Reynolds numbers, the above mentioned new logarithmic laws can be derived by assuming the arrival of an attached eddy at a generic point in the flow field to be a Poisson process [Woodcock and Marusic, Phys. Fluids 27, 015104 (2015), 10.1063/1.4905301]. Taken together, the results provide new evidence supporting the essential ingredients of the attached eddy hypothesis to describe streamwise velocity fluctuations of large, momentum transporting eddies in wall-bounded turbulence, while observed deviations suggest the need for further extensions of the
Impact of planetary boundary layer turbulence on model climate and tracer transport
E. L. McGrath-Spangler; A. Molod; Ott, L. E.; Pawson, S.
2014-01-01
Planetary boundary layer (PBL) processes are important for weather, climate, and tracer transport and concentration. One measure of the strength of these processes is the PBL depth. However, no single PBL depth definition exists and several studies have found that the estimated depth can vary substantially based on the definition used. In the Goddard Earth Observing System (GEOS-5) atmospheric general circulation model, the PBL depth is particularly important ...
Impact of planetary boundary layer turbulence on model climate and tracer transport
E. L. McGrath-Spangler; A. Molod; Ott, L. E.; Pawson, S.
2015-01-01
Planetary boundary layer (PBL) processes are important for weather, climate, and tracer transport and concentration. One measure of the strength of these processes is the PBL depth. However, no single PBL depth definition exists and several studies have found that the estimated depth can vary substantially based on the definition used. In the Goddard Earth Observing System (GEOS-5) atmospheric general circulation model, the PBL depth is particularly important because it is u...
Marginally stable and turbulent boundary layers in low-curvature Taylor-Couette flow
Brauckmann, Hannes J
2016-01-01
Marginal stability arguments are used to describe the rotation-number dependence of torque in Taylor-Couette (TC) flow for radius ratios $\\eta \\geq 0.9$ and shear Reynolds number $Re_S=2\\times 10^4$. With an approximate representation of the mean profile by piecewise linear functions, characterized by the boundary-layer thicknesses at the inner and outer cylinder and the angular momentum in the center, profiles and torques are extracted from the requirement that the boundary layers represent marginally stable TC subsystems and that the torque at the inner and outer cylinder coincide. This model then explains the broad shoulder in the torque as a function of rotation number near $R_\\Omega\\approx 0.2$. For rotation numbers $R_\\Omega < 0.07$ the TC stability conditions predict boundary layers in which shear Reynolds numbers are very large. Assuming that the TC instability is bypassed by some shear instability, a second maximum in torque appears, in very good agreement with numerical simulations. The results s...
Li, Lixiao; Kareem, Ahsan; Hunt, Julian; Xiao, Yiqing; Zhou, Chaoying; Song, Lili
2015-02-01
A conceptual model is proposed for the characteristic sub-ranges in the velocity and temperature spectra in the boundary layer of tropical cyclones (hurricanes or typhoons). The model is based on observations and computation of radial and vertical profiles of the mean flow and turbulence, and on the interpretation of eddy mechanisms determined by shear (namely roll and streak structures near the surface), convection, rotation, blocking and sheltering effects at the ground/sea surface and in internal shear layers. The significant sub-ranges, as the frequency increases, are associated with larger energy containing eddies, shear and blocking, inertial transfer between large and small scales, and intense small-scale eddies generated near the surface caused by waves, coastal roughness change, and the buoyancy force associated with the evaporation of spray droplets. These sub-ranges vary with the locations at which the spectra are measured, i.e. the level in relation to the height of the peak mean velocity and the depth of the boundary layer, and the radius in relation to the eyewall radius and the outer-vortex radius . For two tropical cyclones (Nuri and Hagupit), experimental data were analyzed. Spectra were measured where is near to and using four 1-h long datasets at coastal towers, at 10- and 60-m heights for tropical cyclone Nuri, and at 60-m height for tropical cyclone Hagupit at the south China coast. The field measurements of spectra within the boundary layer show significant sub-ranges of self-similar energy spectra (lying between the length scale 1,000 m and the smallest scales less than 40 m) that are consistent with the above conceptual model of the surface layer. However, with very high wind speeds near the eyewall, the energy of the independently generated intense surface eddy motions, associated with surface waves and water droplets in the airflow, greatly exceeds the energies of the small scales in the inertial sub-range of the boundary layer, over
International Nuclear Information System (INIS)
Further development of large-eddy simulation (LES) faces as major obstacles the strong coupling between subgrid-scale (SGS) modeling and the truncation error of the numerical discretization. One can exploit this link by developing discretization methods where the truncation error itself functions as an implicit SGS model. The adaptive local deconvolution method (ALDM) is an approach to LES of turbulent flows that represents a full coupling of SGS model and discretization scheme. To provide evidence for the validity of this new SGS model, well resolved large-eddy simulations of a fully turbulent flat-plate boundary-layer flow subjected to a constant adverse pressure gradient are conducted. Flow parameters are adapted to an available experiment. The Reynolds number based on the local free-stream velocity and momentum thickness is 670 at the inflow and 5100 at the separation point. Clauser's pressure-gradient parameter increases monotonically from 0 up to approximately 100 since a constant pressure gradient is prescribed. The adverse pressure gradient leads to a highly unsteady and massive separation of the boundary layer. The numerical predictions agree well with theory and experimental data
Goger, Brigitta; Rotach, Mathias W.; Gohm, Alexander; Fuhrer, Oliver; Stiperski, Ivana
2016-04-01
Atmospheric processes associated with complex terrain include various phenomena on the meso- and microscale, which contribute significantly to the local weather in mountainous areas of the Earth. One of the most prominent and well-known boundary-layer phenomena in mountainous terrain is the daytime valley wind circulation, which is very pronounced on clear-sky days with weak synoptic forcing. We use several chosen "valley wind days" in the Inn Valley, Austria, as case studies for the evaluation of the performance of the NWP model COSMO on a horizontal resolution of 1.1 km with a focus on boundary-layer processes and turbulent exchange. The overall goal is to evaluate the model setup and to investigate whether the model's physics schemes (initially developed for horizontally homogeneous and flat surroundings) are suitable for truly complex terrain. We evaluate the model by using measurements from the so-called "i-Box" located in the Inn Valley. The i-Box consists of six core sites that are located at representative locations in the Inn Valley, and two remote sensing systems (wind Lidar and HATPRO passive T/RH profiler) in the city of Innsbruck. The long-term data set provides a data pool of high-resolution velocity variances, turbulence variables, radiation, soil moisture, and vertical profiles of temperature, humidity, and wind in the lower troposphere, which allows a process-oriented analysis. A special focus is laid on the daytime valley boundary layer and its interaction with the developing up-valley wind. Vertical cross-sections show that the valley wind has an asymmetric structure, hence, the i-Box stations show a high spatial variability. While the station on the valley bottom and on the south-facing slope are clearly under the strong influence of the valley wind, the two stations on the north-facing slope are rather dominated by slope flows. We find that the valley wind has a strong (indirect) influence on the development of the local turbulence kinetic
Tumin, Anatoli
2015-11-01
Zavol'skii and Reutov (1983), Luchini (2008, 2010), Fedorov (2010, 2012, 2014) explored potential role of kinetic fluctuations (KF) in incompressible and calorically perfect gas boundary layer flows. The results indicate that role of KF is comparable with other disturbance sources in flight experiments and in quiet wind tunnels. The analysis is based on the Landau and Lifshitz (1957) concept of fluctuating hydrodynamics representing the dissipative fluxes as an average and fluctuating parts. We are extending analysis of the receptivity problem to the fluctuating dissipative fluxes in chemically reacting nonequilibrium boundary layer flows of binary mixtures. There are new terms in the energy, and the species equations. The species conservation equation includes the dissipative diffusion flux and the species generation due to dissociation. The momentum equation includes fluctuating stress tensor. The energy equation includes fluctuating heat flux, energy flux due to diffusion of the species, and fluctuating dissipative flux due to viscosity. The effects are compared for the cases stemming from constraints of the HTV project (Klentzman and Tumin, AIAA Paper 2013-2882). Supported by AFOSR.
International Nuclear Information System (INIS)
The natural occurrence of Tollmien-Schlichting (TS) waves has so far only been observed in boundary layers subjected to moderate levels of free stream turbulence (Tu <1%), due to the difficulty in detecting small-amplitude waves in highly perturbed boundary layers. By introducing controlled oscillations with a vibrating ribbon, it is possible to study small amplitude waves using phase-selective filtering techniques. In the present work, the effect of TS-waves on the transition is studied at Tu = 1.5%. It is demonstrated that TS-waves can exist and develop in a similar way as in an undisturbed boundary layer. It is also found that TS-waves with quite small amplitudes are involved in nonlinear interactions which lead to a regeneration of TS-waves in the whole unstable frequency band. This results in a significant increase in the number of turbulent spots, which promote the onset of turbulence. 28 refs
Jordan, Stephen A.
2016-05-01
Long thin circular cylinders commonly serve as towed sonar tracking devices, where the radius-of-curvature along the longitudinal axis is quite low [ρr = O(10-4)]. Because no understanding presently exists about the direct impact of longitudinal curvature on the turbulent statistics, the long cylinder is simply viewed as a chain of straight segments at various (increasing then decreasing) small inclinations to the freestream direction. Realistically, even our statistical evidence along straight thin cylinders at low incidence angles is inadequate to build solid evidence towards forming reliable empirical models. In the present study, we address these shortcomings by executing Large-Eddy Simulations (LESs) of straight and longitudinally curved thin cylinders at low to moderate turbulent radius-based Reynolds numbers (500 ≤ Rea ≤ 3500) and small angles-of-incidence (α = 0° → 9°). Coupled with the previous experimental measurements and numerical results, the new expanded database (311 ≤ Rea ≤ 56 500) delivered sufficient means to propose power-law expressions for the longitudinal evolution of the skin friction, normal drag, and turbulent boundary layer (TBL) length scales. Surprisingly, the LES computations of the curved cylinders at analogous geometric and kinematic conditions as the straight cylinder showed similar character in terms of the longitudinal skin friction. Beyond incidence 1°-3° (upper end corresponds to the highest simulated Rea), the skin friction was directly proportional to the yaw angle and monotonically shifted downward with higher Rea. Conversely, the flow structure, normal drag, TBL length scales, Reynolds stresses, and the separation state of the transverse shear layers towards regular vortex shedding for the curved cylinder were highly dissimilar than the straight one at equivalent incidence angles.
Donovan, J. F.; Smits, A. J.
1987-01-01
An experimental investigation concentrating on the change in the large-scale organized motions of a supersonic turbulent boundary layer subjected to a short region of concave surface curvature is presented. Two flows were studied, a relatively weak and a relatively strong perturbation. In both flows the freestream Mach number was 2.89 and the Reynolds number based on momentum thickness was 81,900. Hot-wire anemometry and schlieren photography provided the principle tools for this investigation. Hot-wire data were analyzed using spectral and correlation methods and conditional sampling techniques. Changes in structure angle and results of conditionally sampled velocity signals are discussed. The strong perturbation is found to increase the structure angle, and in both flows the length scales of the organized motions are altered.
Polar spacecraft observations of the turbulent outer cusp/magnetopause boundary layer of Earth
Directory of Open Access Journals (Sweden)
J. S. Pickett
1999-01-01
Full Text Available The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 RE. The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theoretical values of the electromagnetic lower hybrid and whistler modes and may be composed of both modes in concert with kinetic Alfvén waves and/or fast magnetosonic waves. Fourier spectra of the higher frequency wave data also show the electromagnetic turbulence at frequencies up to and near the electron cyclotron frequency. This higher frequency electromagnetic turbulence is most likely associated with whistler mode waves. The lower hybrid drift and current gradient instabilities are suggested as possible mechanisms for producing the turbulence. The plasma and field environment of this turbulent region is examined and found to be extremely complex. Some of the wave activity is associated with processes occurring locally, such as changes in the DC magnetic field, while others are associated with solar wind and interplanetary magnetic field changes.
Wind Energy and the Turbulent Nature of the Atmospheric Boundary Layer
Wächter, Matthias; Hölling, Michael; Morales, Allan; Milan, Patrick; Mücke, Tanja; Peinke, Joachim; Reinke, Nico; Rinn, Philip
2012-01-01
The challenge of developing a sustainable and renewable energy supply within the next decades requires collaborative efforts as well as new concepts in the fields of science and engineering. Here we give an overview on the impact of small-scale properties of atmospheric turbulence on the wind energy conversion process. Special emphasis is given to the noisy and intermittent structure of turbulence and its outcome for wind energy conversion and utilization. Experimental, theoretical, analytical, and numerical concepts and methods are presented. In particular we report on new aspects resulting from the combination of basic research, especially in the field of turbulence and complex stochastic systems, with engineering applications.
Polar spacecraft observations of the turbulent outer cusp/magnetopause boundary layer of Earth
J. S. Pickett; Menietti, J. D.; J. H. Dowell; Gurnett, D.A.; Scudder, J. D.
1999-01-01
The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 RE. The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theore...
Wall-drag measurements of smooth- and rough-wall turbulent boundary layers using a floating element
Baars, W. J.; Squire, D. T.; Talluru, K. M.; Abbassi, M. R.; Hutchins, N.; Marusic, I.
2016-05-01
The mean wall shear stress, overline{τ }_w, is a fundamental variable for characterizing turbulent boundary layers. Ideally, overline{τ }_w is measured by a direct means and the use of floating elements has long been proposed. However, previous such devices have proven to be problematic due to low signal-to-noise ratios. In this paper, we present new direct measurements of overline{τ }_w where high signal-to-noise ratios are achieved using a new design of a large-scale floating element with a surface area of 3 m (streamwise) × 1 m (spanwise). These dimensions ensure a strong measurement signal, while any error associated with an integral measurement of overline{τ }_w is negligible in Melbourne's large-scale turbulent boundary layer facility. Wall-drag induced by both smooth- and rough-wall zero-pressure-gradient flows are considered. Results for the smooth-wall friction coefficient, C_f ≡ overline{τ }_w/q_{∞}, follow a Coles-Fernholz relation C_f = [ 1/κ ln ( Re_{θ }) + C] ^{-2} to within 3 % (κ = 0.38 and C = 3.7) for a momentum thickness-based Reynolds number, Re_{θ } > 15{,}000. The agreement improves for higher Reynolds numbers to 38{,}000. This smooth-wall benchmark verification of the experimental apparatus is critical before attempting any rough-wall studies. For a rough-wall configuration with P36 grit sandpaper, measurements were performed for 10{,}500< Re_{θ } < 88{,}500, for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime.
Kuhn, G. D.
1971-01-01
A computer program was developed to do the calculations for two-dimensional or axisymmetric configurations from low speeds to hypersonic speeds with arbitrary streamwise pressure, temperature, and Mach number distributions. Options are provided for obtaining initial conditions either from experimental information or from a theoretical similarity solution. The transition region can be described either by an arbitrary distribution of intermittency or by a function based on Emmons' probability theory. Correlations were developed for use in estimating the parameters of the theoretical intermittency function. Correlations obtained from other sources are used for estimating the transition point. Comparisons were made between calculated and measured boundary layer quantities for laminar, transitional, and turbulent flows on flat plates, cones, cone flares, and a waisted body of revolution. Excellent agreement was obtained between the present theory and two other theories based on the method of finite differences. The intermittency required to reproduce some experimental heat transfer results in hypersonic flow was found to be quite different from the theoretical function. It is suggested that the simple probability theory of Emmons may not be valid for representing the intermittency of hypersonic transitional boundary layers and that the program could be useful as a tool for detailed study of the intermittency of the transition region.
Doppler lidar measurements of winds and turbulence in the boundary layer
International Nuclear Information System (INIS)
Coherent Doppler lidar is ideally suited for accurate measurements of the profiles of wind and turbulence using various beam scanning and processing techniques. The most attractive scanning pattern is a three dimensional pattern which samples the largest number of independent samples of the turbulent field. Careful processing of the radial velocity estimates is required to correct for the spatial filtering by the lidar pulse and to remove the bias introduced by the estimation error. The various processing techniques and fundamental assumptions will be presented. Lidar derived profiles of wind and turbulence will be compared with in situ data, when available. Results will also be presented from a field campaign focused on wind energy applications
International Nuclear Information System (INIS)
Highlights: • Numerically simulated forced-convection turbulent flow over convex-curved surface. • Analyzed effects of curvature and supercritical thermal state on heat transfer. • Near-wall streaks spaced farther apart due to curvature-induced radial equilibrium. • Curvature and supercritical effects on turbulence are of comparable magnitude. -- Abstract: Direct numerical simulation (DNS) results are used to establish the effect of convex streamwise curvature on the development of turbulent boundary layers, and the effect of such curvature on the forced-convection heat transfer variations observed at certain supercritical thermodynamic states. The results illustrate the stabilizing effects of this flow geometry through modification of the structure and distribution of hairpin-like vortical flow structures in the boundary layer. Furthermore, enhancement of convective heat transfer realized at a particular heat flux-to-mass flux ratio with the working fluid at a supercritical state is observed to be reduced by the stabilizing effect of convex surface curvature
Sjöholm, Mikael; Kapp, Stefan; Kristensen, Leif; Mikkelsen, Torben
2011-11-01
Affordable coherent wind lidars based on modern telecom components have recently emerged on the wind energy market spurred by high demand of the industry for compact and accurate remote sensing wind and turbulence profilers. Today, hundreds of ground based wind lidars that achieve the range resolution by either focusing a continuous-wave laser beam or by gating a pulsed laser beam are used for measuring mean wind and turbulence profiles in the lower atmospheric boundary-layer. However, detailed understanding of the influence of the spatial filtering of the lidars on their precise assessment of turbulence is still a challenge. For assessment of the fine structure turbulence, and in particular for the easy and fast assessment of the dissipation rate of turbulent kinetic energy from measurements in the Kolmogorov inertial subrange, we havemodeled the atmospheric velocity structure functions and spectra obtainable from fixed-orientation along-beam wind measurements by these lidars. The dissipation rate retrieval model is experimentally evaluated with data obtained with a pulsed lidar pointing horizontally into horizontally homogeneous turbulence encountered at the top level of a 125 m tall meteorological tower, equipped with an in-situ turbulence measurement device (a three-dimensional sonic anemometer) for intercomparison. Our experimental study has revealed that the easily manageable analytical model accounts well for the observed fine structure turbulent spectra and their dependence on the pointing direction of the lidar beam relative to the mean wind direction. The results demonstrate that turbulence dissipation rates, and hence boundary-layer turbulence, can easily be obtained from wind lidar-based fine structure measurements.
International Nuclear Information System (INIS)
The γ-Reθ transition model of Menter et al. [Menter, F.R., Langtry, R.B., Volker, S., Huang, P.G., 2005. Transition modelling for general purpose CFD codes. ERCOFTAC International Symposium Engineering Turbulence Modelling and Measurements] is a highly generalized transport equation model in which it has been developed based on the concept of local variables compatible with modern CFD methods where the unstructured grid and the parallel computing technique are usually integrated in. To perform the prediction with this model, two essential parameters, Flength which is used to control the length of the transition region and Reθc which is used to control the onset of the transition location, must be specified to close the model. At present, both parameters are proprietary and their formulations are unpublished. For the first time here, the relations for both parameters are formulated by means of numerical experiments and analysis under the assumption of Reθc = Reθt corresponding with the bypass transition behavior. Based on this analysis, the optimized values of the parameters are found and their relations can be constructed as follows: Reθc = 803.73(Tu∞,le + 0.6067)-1.027 and Flength = 163 ln(Tu∞,le) + 3.625. The performance of this transition model is assessed by testing with the experimental cases of T3AM, T3A, and T3B. Detailed comparisons with the predicted results by the transition models of Suzen and Huang [Suzen, Y.B., Huang, P.G., 2000. Modeling of flow transition using an intermittency transport equation. J. Fluids Eng. 122, 273-284] and Lodefier et al. [Lodefier, K., Merci, B., De Langhe, C., Dick, E., 2003. Transition modelling with the SST turbulence model and intermittency transport equation. ASME Turbo Expo, Atlanta, GA, USA, June 16-19], and also with the predicted results by the k-ε model of Launder and Sharma [Launder, B.E., Sharma, B., 1974. Application of the energy dissipation model of turbulence to the calculation of flow near a
Transition to Turbulence and Separation of the Boundary Layer in the Context of Airfoil Design
Czech Academy of Sciences Publication Activity Database
Popelka, Lukáš; Matějka, M.; Schmirler, M.
Žilina: Žilinská universita, 2006, s. 221-224. ISBN 80-8070-533-X. [Aplikácia experimentálnych a numerických metód v mechanike tekutín. Strečno (SK), 26.04.2006-28.04.2006] R&D Projects: GA AV ČR IAA2076403 Institutional research plan: CEZ:AV0Z20760514 Keywords : Transition to Turbulence * Separation * Airfoil Subject RIV: JU - Aeronautics, Aerodynamics, Aircrafts
Transport of ozone by turbulence and clouds in an urban boundary layer
International Nuclear Information System (INIS)
The turbulent fluxes of ozone and latent and sensible heat are computed from fast-response measurements made abroard a National Oceanic and Atmospheric Administration aircraft over downtown Philadelphia and the surrounding suburbs during the afternoon and evening of August 22, 1979. In the afternoon the ozone flux at a height of 200 m is downward throughout the region with the largest magnitude (-2(ppb)m s-1) occurring over the urban center. During the afternoon at both 200 m and a few hundred meters below cloud base, the horizontal profile of mean ozone concentration peaks at 130 ppb over the urban core with values of the order of 90 ppb to the southeast and northwest. The urban ozone condentration at 200 m decreases to 35 ppb by early evening. The normalized variances and spectra of vertical velocity, temperature, and ozone show little change with height or location in the urban center and northwest suburbs during the afternoon in good agreement with normalized statistics obtained over rural terrain (Kaimal et al., 1976); Lenshow et al., 1980). Data from a cloud penetration by the aircraft is used to estimate a mean updraft velocity of 4 m s-1 and an updraft area of approximately 1 km3. The flux of ozone due to the mean motion in the updraft is 2 orders of magnitude larger than the turbulent eddy fluxes within the cloud
Particle Resuspension in Turbulent Boundary Layers and the Influence of Non-Gaussian Removal Forces
Zhang, F; Kissane, M
2012-01-01
The work presented is concerned with the way very small micron-size particles attached to a surface are resuspended when exposed to a turbulent flow. Of particular concern is the remobilization of radioactive particles as a consequence of potential nuclear accidents. In this particular case the focus is on small particles, < 5 microns in diameter, where the principal force holding such particles onto a surface arises from van der Waals inter-molecular adhesive forces. Here an improved version of the "Rock n Roll" model (Reeks & Hall, 2001) is developed where this model employs a stochastic approach to resuspension involving the rocking and rolling of a particle about surface asperities arising from the moments of the fluctuating drag forces acting on the particle close to the surface. In this work the model is significantly improved by using values of both the stream-wise fluid velocity and acceleration close to the wall obtained from Direct Numerical Simulation (DNS) of turbulent channelflow. Using an...
Benhachmi, Driss; Greber, Isaac; Hingst, Warren R.
1988-01-01
A combined experimental and numerical study of the interaction of an incident oblique shock wave with a turbulent boundary layer on a rough plate and on a porous plate with suction is presented. The experimental phase involved the acquisition of mean data upstream of, within, and downstream of the interaction region at Mach numbers 2.5 and 3.0. Data were taken at unit Reynolds numbers of 1.66 E7 and 1.85 E7 m respectively, and for flow deflection angles of 0, 4, 6 and 8 degs. Measured data include wall static pressure, pitot pressure profiles, and local bleed distributions on the porous plate. On the rough plate, with no suction, the boundary layer profiles were modified near the wall, but not separated for the 4 deg flow deflection angle. For the higher deflection angles of 6 and 8 degs, the boundary layer was separated. Suction increases the strength of the incident shock required to separate the turbulent boundary layer; for all shock strengths tested, separation is completely eliminated. The pitot pressure profiles are affected throughout the whole boundary layer; they are fuller than the ones obtained on the rough plate. It is also found that the combination of suction and roughness introduces spatial perturbations.
Marvin, Joseph G.; Brown, James L.; Gnoffo, Peter A.
2013-01-01
A database compilation of hypersonic shock-wave/turbulent boundary layer experiments is provided. The experiments selected for the database are either 2D or axisymmetric, and include both compression corner and impinging type SWTBL interactions. The strength of the interactions range from attached to incipient separation to fully separated flows. The experiments were chosen based on criterion to ensure quality of the datasets, to be relevant to NASA's missions and to be useful for validation and uncertainty assessment of CFD Navier-Stokes predictive methods, both now and in the future. An emphasis on datasets selected was on surface pressures and surface heating throughout the interaction, but include some wall shear stress distributions and flowfield profiles. Included, for selected cases, are example CFD grids and setup information, along with surface pressure and wall heating results from simulations using current NASA real-gas Navier-Stokes codes by which future CFD investigators can compare and evaluate physics modeling improvements and validation and uncertainty assessments of future CFD code developments. The experimental database is presented tabulated in the Appendices describing each experiment. The database is also provided in computer-readable ASCII files located on a companion DVD.
Maxit, Laurent
2016-08-01
This paper investigates the modeling of a vibrating structure excited by a turbulent boundary layer (TBL). Although the wall pressure field (WPF) of the TBL constitutes a random excitation, the element-based methods generally used for describing complex mechanical structures consider deterministic loads. The response of such structures to a random excitation like TBL is generally deduced from calculations of numerous Frequency Response Functions. Consequently, the process is computationally expansive. To tackle this issue, an efficient process is proposed for generating realizations of the WPF corresponding to the TBL. This process is based on a formulation of the problem in the wavenumber space and the interpretation of the WPF as uncorrelated wall plane waves. Once the WPF has been synthesized, the local vibroacoustic responses are calculated for the different realizations and averaged together in the last step. A numerical application of this process to a plate located beneath a TBL is used to verify its efficiency and ability to reproduce the partial space correlation of the excitation. To further illustrate the proposed method, a stiffened panel modeled using the finite element method is finally examined. PMID:27586754
Ali, Naseem; Tutkun, Murat; Cal, Rau'l.
2015-11-01
Low order decompositions and Lagrangian coherent structures are used to identify structures in a high-Reynolds-number turbulent boundary layer flow. Data are collected in Laboratoire de Mécanique de Lille (LML) wind tunnel using time resolved stereo particle image velocimetry. Low-order descriptors are based on proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) frameworks to obtain energy content and frequency information of the flow, respectively. Repelling and attracting Lagrangian coherent structures (LCS)s reveal complex patterns within the flow field containing a hyperbolic behavior and the shapes of the attracting and repelling vary with advection time as result of the temporal coherence. The attracting and repelling LCSs are matched with POD and DMD modes to understand the relationship between the frameworks and respective representations. The POD is used as a low pass filtering of kinetic energy and then mode-dependent velocity reconstructions provide, firstly, the most coherent features of the flow and second are employed to generate new mode-based LCSs. This representations then provide clarity as to the organization of the LCS based on the energy contained in them and the dynamic coherence.
International Nuclear Information System (INIS)
To capture the full spectrum of the fluctuating wall pressure beneath a turbulent boundary layer (TBL) provides a unique challenge in transducer design. This paper discusses the design, construction and testing of an array of surface-mounted piezoelectric ceramic elements with the goal of having both the spatial resolution and the frequency bandwidth to accurately sense the low-frequency, low-wavenumber events beneath a TBL at moderately low Reynolds numbers. The array is constructed from twenty 1.27 cm tall prismatic rods with 0.18 cm × 0.16 cm cross-section made of Navy type II piezoelectric ceramic material. Calibration was performed by comparing the response of a Navy H56 precision-calibrated hydrophone to the outputs of each element on the array for a given input from a Navy J9 projector. The elements show an average sensitivity of −184 dB (re: 1 V μPa−1) and are assembled with a centre-to-centre spacing of 0.2 cm. Measurements of the fluctuating wall pressure below a 2d TBL with Reynolds numbers (based on momentum thickness) ranging from 2100 to 4300 show that the dimensions of the elements are between 64 and 107 viscous length units, respectively. A spatial and temporal footprint of the fluctuating wall pressure reveals convective speeds averaging 75% of the free stream velocity. (paper)
Baars, Woutijn J.; Hutchins, Nicholas; Marusic, Ivan
2015-11-01
Interactions between small- and large-scale motions are inherent in the near-wall dynamics of wall-bounded flows. We here examine the scale-interaction embedded within the streamwise velocity component. Data were acquired using hot-wire anemometry in ZPG turbulent boundary layers, for Reynolds numbers ranging from Reτ ≡ δUτ / ν ~ 2800 to 22800. After first decomposing velocity signals into contributions from small- and large-scales, we then represent the time-varying small-scale energy with time series of its instantaneous amplitude and instantaneous frequency, via a wavelet-based method. Features of the scale-interaction are inferred from isocorrelation maps, formed by correlating the large-scale velocity with its concurrent small-scale amplitude and frequency. Below the onset of the log-region, the physics constitutes aspects of amplitude modulation and frequency modulation. Time shifts, associated with the correlation extrema--representing the lead/lag of the small-scale signatures relative to the large-scales--are shown to be governed by inner-scaling. Wall-normal trends of time shifts are explained by considering the arrangement of scales in the log- and intermittent-regions, and how they relate to stochastic top-down and bottom-up processes.
A Study of the Mean Force Structure of Rough-Wall Turbulent Boundary Layers
Mehdi, Faraz; Klewicki, Joseph
2011-11-01
Analysis of existing data by Mehdi, Klewicki & White [Physica D 239(2010)] provides evidence that the traditional classifications do not fully account for the combined effects of roughness and Reynolds number. We continue to explore this further, and in the present talk report on experiments that used 24-grit sandpaper and pea gravel for roughness over an 8m fetch. Two-component LDV measurements are used to acquire well-resolved mean velocity and Reynolds stress profiles over a modest range of Reynolds numbers. These data are used to estimate the terms in the appropriate mean statement of dynamics, which directly reveals the operative time-averaged balance of forces. The present results further reinforce the previous observation that the mean viscous force retains dominant order above (and often well-above) the roughness elements. Force balance data are shown to be usefully organized relative to the length scale that defines the region from the wall to where the leading order mean dynamics are described by a balance between mean advection and the mean effect of turbulent inertia. In the smooth-wall flow, this length scale is only a function of Reynolds number. In rough-wall flows, the data indicate it to be a function of roughness and Reynolds number. The support of the ONR (N000140810836, grant monitor Ronald Joslin) is gratefully acknowledged.
International Nuclear Information System (INIS)
Highlights: • The boundary layer developing on the suction side of a LPT is surveyed by PIV. • POD is adopted to post-process data obtained in two orthogonal planes. • Coherent structures driving transition at high and low turbulence level are discussed. • 2-D Kelvin–Helmholtz rolls are observed in the low FS turbulence (separated) case. • At high FS turbulence the instability of streaky structures drives the transition. - Abstract: Particle Image Velocimetry (PIV) has been adopted to analyze the instantaneous flow field developing on a high-lift turbine blade profile operating under low and elevated free-stream turbulence conditions (FSTI). Results reported in the paper allow us to analyze the dynamics leading to transition and separation of the suction side boundary layer, looking to generation, propagation and breakdown of coherent structures observed in the two different FSTI cases. To this end, measurements have been performed in two orthogonal planes. Results obtained in the blade-to-blade plane allow the detailed characterization of the propagation of Kelvin–Helmholtz (KH) rolls generating, at low FSTI condition, as a consequence of a non-reattaching separation. Otherwise, data in the wall-parallel plane allow recognizing the presence of three-dimensional disuniformities induced at high FSTI by low and high speed streaks (Klebanoff mode). The sinuous breakdown of boundary layer streaks generates other complex three-dimensional coherent structures such as hairpin or cane-like vortices that induce transition. Proper Orthogonal Decomposition (POD) has been adopted to in depth characterize these structures, thus further explaining the mechanisms through which the free-stream turbulence intensity modify the transition/separation processes of the suction side boundary layer of an highly loaded low pressure turbine blade
Czech Academy of Sciences Publication Activity Database
Kellnerová, Radka; Kukačka, Libor; Jurčáková, Klára; Uruba, Václav; Jaňour, Zbyněk
Warsaw: University of Warsaw Poland, 2011 - (Bajer, K.; Kopeć, J.; Podziemski, P.), s. 1-10 [European Turbulence Conference /13./. Warsaw (PL), 12.09.2011-15.09.2011] Institutional research plan: CEZ:AV0Z20760514 Keywords : wavelet analysis * street canyon * POD Subject RIV: DG - Athmosphere Sciences, Meteorology www.etc13.fuw.edu.pl
Czech Academy of Sciences Publication Activity Database
Kellnerová, Radka; Kukačka, Libor; Jurčáková, Klára; Uruba, Václav; Jaňour, Zbyněk
2011-01-01
Roč. 318, - (2011), s. 1-10. E-ISSN 1742-6596. [European Turbulence Conference /13./. Warsaw, 12.09.2011-15.09.2011] Institutional research plan: CEZ:AV0Z20760514 Keywords : Wavelet analysis * street canyon * POD Subject RIV: DG - Athmosphere Sciences, Meteorology http://iopscience.iop.org/1742-6596/318/6/062012?fromSearchPage=true
Hamilton, H. Harris, II; Millman, Daniel R.; Greendyke, Robert B.
1992-01-01
A computer code was developed that uses an implicit finite-difference technique to solve nonsimilar, axisymmetric boundary layer equations for both laminar and turbulent flow. The code can treat ideal gases, air in chemical equilibrium, and carbon tetrafluoride (CF4), which is a useful gas for hypersonic blunt-body simulations. This is the only known boundary layer code that can treat CF4. Comparisons with experimental data have demonstrated that accurate solutions are obtained. The method should prove useful as an analysis tool for comparing calculations with wind tunnel experiments and for making calculations about flight vehicles where equilibrium air chemistry assumptions are valid.
Pal, Sandip
2016-06-01
The convective boundary layer (CBL) turbulence is the key process for exchanging heat, momentum, moisture and trace gases between the earth's surface and the lower part of the troposphere. The turbulence parameterization of the CBL is a challenging but important component in numerical models. In particular, correct estimation of CBL turbulence features, parameterization, and the determination of the contribution of eddy diffusivity are important for simulating convection initiation, and the dispersion of health hazardous air pollutants and Greenhouse gases. In general, measurements of higher-order moments of water vapor mixing ratio (q) variability yield unique estimates of turbulence in the CBL. Using the high-resolution lidar-derived profiles of q variance, third-order moment, and skewness and analyzing concurrent profiles of vertical velocity, potential temperature, horizontal wind and time series of near-surface measurements of surface flux and meteorological parameters, a conceptual framework based on bottom up approach is proposed here for the first time for a robust characterization of the turbulent structure of CBL over land so that our understanding on the processes governing CBL q turbulence could be improved. Finally, principal component analyses will be applied on the lidar-derived long-term data sets of q turbulence statistics to identify the meteorological factors and the dominant physical mechanisms governing the CBL turbulence features. PMID:26950615
Energy Technology Data Exchange (ETDEWEB)
El Kabiri, M.; Paranthoen, P.; Rosset, L.; Lecordier, J.C. [Rouen Univ., 76 - Mont-Saint-Aignan (France)
1997-12-31
An experimental study of heat transport downstream of a linear source installed in a turbulent boundary layer is performed. Second and third order momenta of velocity and temperature fields are presented and compared to gradient-type modeling. (J.S.) 7 refs.
McPhee, M. G.; Stevens, C. L.; Smith, I. J.; Robinson, N. J.
2015-11-01
Late winter measurements of turbulent quantities in tidally modulated flow under land-fast sea ice near the Erebus Glacier Tongue, McMurdo Sound, identified processes that influence growth at the interface of an ice surface in contact with supercool seawater. The data suggest that turbulent heat exchange at the ocean-ice boundary is characterized by the product of friction velocity and (negative) water temperature departure from freezing, analogous to similar results for moderate melting rates in seawater above freezing. Platelet ice growth appears to increase the hydraulic roughness (drag) of fast ice compared with undeformed fast ice without platelets. We hypothesize that platelet growth in supercool water under thick ice is rate-limited by turbulent heat transfer and that this is a significant factor to be considered in mass transfer at the under-side of ice shelves and sea ice in the vicinity of ice shelves.
McPhee, Miles G.; Stevens, Craig L.; Smith, Inga J.; Robinson, Natalie J.
2016-04-01
Late winter measurements of turbulent quantities in tidally modulated flow under land-fast sea ice near the Erebus Glacier Tongue, McMurdo Sound, Antarctica, identified processes that influence growth at the interface of an ice surface in contact with supercooled seawater. The data show that turbulent heat exchange at the ocean-ice boundary is characterized by the product of friction velocity and (negative) water temperature departure from freezing, analogous to similar results for moderate melting rates in seawater above freezing. Platelet ice growth appears to increase the hydraulic roughness (drag) of fast ice compared with undeformed fast ice without platelets. Platelet growth in supercooled water under thick ice appears to be rate-limited by turbulent heat transfer and that this is a significant factor to be considered in mass transfer at the underside of ice shelves and sea ice in the vicinity of ice shelves.
Zong, Zhaowang
A novel electrochromic thin film transistor (EC-TFT) was fabricated and characterized in this work. This concept relies on ion transport to control gating. The channel material is tungsten oxide (WOx), produced by reactive magnetron sputtering. In its oxidized state WO3 is a transparent, wide band gap insulator (> 3 eV). However upon intercalation of light ions (H+, Li+) the material becomes both electrically conducting and opaque to visible light. This allows the EC-TFT to generate a complementary optical response. We optimized the fabrication of individual layers of the EC-TFT, and found that controlling the stoichiometry of WOx is a key step. Using RF magnetron sputtering, it was found that there is a narrow window to obtain material capable of reversible switching. Fully oxidized films proved it is difficult to intercalate ions efficiently. In contrast, insufficient oxygen produced films that were always in a metallic like state. Best results were obtained with the sputter power set at 200 W using O2 fractions of 42%-46% in argon. In the preliminary studies, the device was tested in a two-step fashion. First, devices were placed in solution and cyclic voltammetry was used to set the level of ion intercalation. Samples were then removed from the electrolyte, dried, and the source/drain current was measured on a probe station. This demonstrated the concept of the EC-TFT, showing that the transistor could be turned on and off reversely with a current ratio of Ion/Ioff ~ 1000, and showed significant color change during the intercalation. However, the conductivity in off state was too high to be a promising transistor, the two step approach made the data rather noisy, it was difficult to make good contacts and this was not an in situ measurement. In order to get the in situ measurement, macrodevices with large electrodes were fabricated and characterized, which made it easier to make electrical contacts. However, after the initial cycle the macrodevices were always
Brown, James L.
2014-01-01
Examined is sensitivity of separation extent, wall pressure and heating to variation of primary input flow parameters, such as Mach and Reynolds numbers and shock strength, for 2D and Axisymmetric Hypersonic Shock Wave Turbulent Boundary Layer interactions obtained by Navier-Stokes methods using the SST turbulence model. Baseline parametric sensitivity response is provided in part by comparison with vetted experiments, and in part through updated correlations based on free interaction theory concepts. A recent database compilation of hypersonic 2D shock-wave/turbulent boundary layer experiments extensively used in a prior related uncertainty analysis provides the foundation for this updated correlation approach, as well as for more conventional validation. The primary CFD method for this work is DPLR, one of NASA's real-gas aerothermodynamic production RANS codes. Comparisons are also made with CFL3D, one of NASA's mature perfect-gas RANS codes. Deficiencies in predicted separation response of RANS/SST solutions to parametric variations of test conditions are summarized, along with recommendations as to future turbulence approach.
Sergeev, Daniil; Troitskaya, Yuliya; Vdovin, Maxim
2015-04-01
the spray of droplets generation, especially heat transfer. The work was supported by RFBR grants (14-05-91767, 14-08-31740, 15-35-20953) and RSF grant 14-17-00667 and by President grant for young scientists MK-3550.2014.5 References: 1. Emanuel, K. A. Sensitivity of tropical cyclones to surface exchange coefficients and a revised steady-state model incorporating eye dynamics // J. Atmos. Sci., 52(22), 3969-3976,1995. 2. Brian K. Haus, Dahai Jeong, Mark A. Donelan, Jun A. Zhang, and Ivan Savelyev Relative rates of sea-air heat transfer and frictional drag in very high winds // GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L07802, doi:10.1029/2009GL042206, 2010 3. Yu. I. Troitskaya, D.A. Sergeev, A.A. Kandaurov, G.A Baidakov, M.A. Vdovin, V.I. Kazakov Laboratory and theoretical modeling of air-sea momentum transfer under severe wind conditions // JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C00J21, 13 PP., 2012 doi:10.1029/2011JC007778 4. Yu.I.Troitskaya, D.A.Sergeev, A.A.Kandaurov, M.I. Vdovin, A.A. Kandaurov, E.V.Ezhova, S.S.Zilitinkevich Momentum and buoyancy exchange in a turbulent air boundary layer over a wavy water surface. Part 2. Wind wave spectra // Nonlinear. Geoph. Processes, Vol. 20, P. 841-856, 2013.
Zhu, Ping; Wang, Yuting; Chen, Shuyi S.; Curcic, Milan; Gao, Cen
2016-01-01
Roll vortices in the atmospheric boundary layer (ABL) are important to oil operation and oil spill transport. This study investigates the impact of storm-induced sea surface temperature (SST) cooling on the roll vortices generated by the convective and dynamic instability in the ABL of Hurricane Isaac (2012) and the roll induced transport using hindcasting large eddy simulations (LESs) configured from the multiply nested Weather Research & Forecasting model. Two experiments are performed: one forced by the Unified Wave INterface - Coupled Model and the other with the SST replaced by the NCEP FNL analysis that does not include the storm-induced SST cooling. The simulations show that the roll vortices are the prevalent eddy circulations in the ABL of Isaac. The storm-induced SST cooling causes the ABL stability falls in a range that satisfies the empirical criterion of roll generation by dynamic instability, whereas the ABL stability without considering the storm-induced SST cooling meets the criterion of roll generation by convective instability. The ABL roll is skewed and the increase of convective instability enhances the skewness. Large convective instability leads to large vertical transport of heat and moisture; whereas the dominant dynamic instability results in large turbulent kinetic energy but relatively weak heat and moisture transport. This study suggests that failure to consider roll vortices or incorrect initiation of dynamic and convective instability of rolls in simulations may substantially affect the transport of momentum, energy, and pollutants in the ABL and the dispersion/advection of oil spill fume at the ocean surface.
Boundary Plasma Turbulence Simulations for Tokamaks
Energy Technology Data Exchange (ETDEWEB)
Xu, X; Umansky, M; Dudson, B; Snyder, P
2008-05-15
The boundary plasma turbulence code BOUT models tokamak boundary-plasma turbulence in a realistic divertor geometry using modified Braginskii equations for plasma vorticity, density (ni), electron and ion temperature (T{sub e}; T{sub i}) and parallel momenta. The BOUT code solves for the plasma fluid equations in a three dimensional (3D) toroidal segment (or a toroidal wedge), including the region somewhat inside the separatrix and extending into the scrape-off layer; the private flux region is also included. In this paper, a description is given of the sophisticated physical models, innovative numerical algorithms, and modern software design used to simulate edge-plasmas in magnetic fusion energy devices. The BOUT code's unique capabilities and functionality are exemplified via simulations of the impact of plasma density on tokamak edge turbulence and blob dynamics.
Simulation of atmospheric turbulence layers with phase screens by JAVA
Zhang, Xiaofang; Chen, Wenqin; Yu, Xin; Yan, Jixiang
2008-03-01
In multiconjugate Adaptive Optics (MCAO), the phase screens are used to simulate atmospheric turbulence layers to study the optimal turbulence delamination and the determination of layer boundary position. In this paper, the method of power spectrum inversion and sub-harmonic compensation were used to simulate atmospheric turbulence layers and results can be shown by grey map. The simulation results showed that, with the increase of turbulence layers, the RMS of adaptive system decreased, but the amplitude diminished. So the atmospheric turbulence can be split into 2-3 layers and be modeled by phase screens. Otherwise, a small simulation atmospheric turbulence delamination system was realized by JAVA.
Rued, Klaus
1987-01-01
The requirements for fundamental experimental studies of the influence of free stream turbulence, pressure gradients and wall cooling are discussed. Under turbine-like free stream conditions, comprehensive tests of transitional boundary layers with laminar, reversing and turbulent flow increments were performed to decouple the effects of the parameters and to determine the effects during mutual interaction.
International Nuclear Information System (INIS)
A comparative analysis of the fundamental properties of fluctuations in the vicinity of boundaries in fusion plasmas and in plasmas of magnetospheric turbulent boundary layers (TBLs) shows the similarity of their basic statistical characteristics, including the scaling of the structure functions and mutifractal parameters. Important features observed include intermittent fluctuations and anomalous mass and momentum transport, due to sporadic plasma flow injections with large flow amplitudes occuring with a much higher probability than predicted for classical Gaussian diffusion. Turbulence in edge fusion plasmas and in TBLs exhibits general self-similarity in a wide range of scales extending to the dissipation scale. Experimental scalings obtained for plasma TBLs are compared with neutral fluid results, revealing the universal properties of developed turbulence. TBL scalings are described within the log-Poisson model, which takes quasi-one-dimensional dissipative structures into account. The time (τ) dependence of the mean-square displacement 2> obtained from the experimental parameters of the log-Poisson distribution takes the form 2> ∝τα with α∼ 1.2 - 1.8 and indicates the presence of superdiffusion in the TBLs studied. Determining the nature of the generalized diffusion process from available regular data is a necessary step toward the quantitative description of TBL transport. (reviews of topical problems)
Local scaling characteristics of Antarctic surface layer turbulence
Directory of Open Access Journals (Sweden)
S. Basu
2010-03-01
Full Text Available Over the past years, several studies have validated Nieuwstadt's local scaling hypothesis by utilizing turbulence observations from the mid-latitude, nocturnal stable boundary layers. In this work, we probe into the local scaling characteristics of polar, long-lived stable boundary layers by analyzing turbulence data from the South Pole region of the Antarctic Plateau.
High frequency ground temperature fluctuation in a Convective Boundary Layer
Garai, A.; Kleissl, J.; Lothon, M.; Lohou, F.; Pardyjak, E.; Saïd, F.; Cuxart, J.; Steeneveld, G.J.; Yaguë, C.; Derrien, S.; Alexander, D.; Villagrasa, D.M.
2012-01-01
To study influence of the turbulent structures in the convective boundary layer (CBL) on the ground temperature, during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) observational campaign, high frequency ground temperature was recorded through infra-red imagery from 13 June - 8 J
Mechanics of inhomogeneous turbulence and interfacial layers
Hunt, J.C.R; Eames, I; Westerweel, J.
2006-01-01
The mechanics of inhomogeneous turbulence in and adjacent to interfacial layers bounding turbulent and non-turbulent regions are analysed. Different mechanisms are identified according to the straining by the turbulent eddies in relation to the strength of the mean shear adjacent to, or across, the interfacial layer. How the turbulence is initiated and the topology of the region of turbulence are also significant factors. Specifically the cases of a layer of turbulence bounded on one, or two,...
Nilsson, E.; Lohou, F.; Lothon, M.; Pardyjak, E.; Mahrt, L.; Darbieu, C.
2015-11-01
The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from mid-day until zero buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 Intensive Observation Period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and meso-scale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near surface production of TKE is compensated by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of -0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with
Directory of Open Access Journals (Sweden)
E. Nilsson
2015-11-01
Full Text Available The decay of turbulence kinetic energy (TKE and its budget in the afternoon period from mid-day until zero buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST field campaign for 10 Intensive Observation Period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and meso-scale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near surface production of TKE is compensated by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of −0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are
Nilsson, Erik; Lohou, Fabienne; Lothon, Marie; Pardyjak, Eric; Mahrt, Larry; Darbieu, Clara
2016-07-01
The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zero-buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of -0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with
Willert, Christian E
2015-01-01
This study reports on experimentally observed near-wall reverse flow events in a fully developed flat plate boundary layer at zero pressure gradient with Reynolds numbers between $Re_\\tau = 1000$ and $Re_\\tau = 2700$. The reverse flow events are captured using high magnification particle image velocimetry sequences with record lengths varying from 50,000 to 126,000 samples. Time resolved particle image sequences allow singular reverse flow events to be followed over several time steps whereas long records of nearly statistically independent samples provide a variety of single snapshots at a higher spatial resolution. The probability of occurrence lies in the range of 0.01% to 0.1% which matches predictions made with direct numerical simulations (DNS). The self-similar size of the reverse flow bubble is about 30-50 wall units in length and 5 wall units in height which also agrees well to DNS data provided by Lenaers et al. (ETC13, Journal of Physics: Conference Series 318 (2011) 022013).
Directory of Open Access Journals (Sweden)
T.-S. El-Madany
2014-03-01
Full Text Available SODAR (SOund Detection And Ranging, eddy-covariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyse their importance in nocturnal atmosphere–biosphere exchange as quantified with eddy-covariance measurements. If intermittency occurs frequently at a measurement site this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production. Fourteen events of intermittent turbulence were identified and classified into above canopy drainage flows (ACDF and low-level jets (LLJ according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 min and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases upslope flows occurred, counteracting the general flow regime at night time. The observations suggest that the LLJ and ACDF penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds. Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis and stationary tests. Significantly higher fluxes of sensible heat, latent heat and shear stress occurred during these periods. During LLJ and ACDF, fluxes of sensible heat, latent heat, and CO2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes were mostly positive during LLJ and ACDF with a median value of 34 W m−2, while outside these periods the median was 2 W m−2. In conclusion, intermittent
Green House Gases Flux Model in Boundary Layer
Nurgaliev, Ildus
Analytical dynamic model of the turbulent flux in the three-layer boundary system is presented. Turbulence is described as a presence of the non-zero vorticity. The generalized advection-diffusion-reaction equation is derived for an arbitrary number of components in the flux. The fluxes in the layers are objects for matching requirements on the boundaries between the layers. Different types of transport mechanisms are dominant on the different levels of the layers.
Directory of Open Access Journals (Sweden)
Yu. I. Troitskaya
2013-10-01
Full Text Available The surface-drag and mass-transfer coefficients are determined within a self-consistent problem of wave-induced perturbations and mean fields of velocity and density in the air, using a quasi-linear model based on the Reynolds equations with down-gradient turbulence closure. Investigation of a harmonic wave propagating along the wind has disclosed that the surface drag is generally larger for shorter waves. This effect is more pronounced in the unstable and neutral stratification. The stable stratification suppresses turbulence, which leads to weakening of the momentum and mass transfer.
Xia, Geng; Zhou, Liming; Freedman, Jeffrey M.; Roy, Somnath Baidya; Harris, Ronald A.; Cervarich, Matthew Charles
2016-04-01
Recent studies using satellite observations show that operational wind farms in west-central Texas increase local nighttime land surface temperature (LST) by 0.31-0.70 °C, but no noticeable impact is detected during daytime, and that the diurnal and seasonal variations in the magnitude of this warming are likely determined by those in the magnitude of wind speed. This paper further explores these findings by using the data from a year-long field campaign and nearby radiosonde observations to investigate how thermodynamic profiles and surface-atmosphere exchange processes work in tandem with the presence of wind farms to affect the local climate. Combined with satellite data analyses, we find that wind farm impacts on LST are predominantly determined by the relative ratio of turbulence kinetic energy (TKE) induced by the wind turbines compared to the background TKE. This ratio explains not only the day-night contrast of the wind farm impact and the warming magnitude of nighttime LST over the wind farms, but also most of the seasonal variations in the nighttime LST changes. These results indicate that the diurnal and seasonal variations in the turbine-induced turbulence relative to the background TKE play an essential role in determining those in the magnitude of LST changes over the wind farms. In addition, atmospheric stability determines the sign and strength of the net downward heat transport as well as the magnitude of the background TKE. The study highlights the need for better understanding of atmospheric boundary layer and wind farm interactions, and for better parameterizations of sub-grid scale turbulent mixing in numerical weather prediction and climate models.
Druzhinin, Oleg; Troitskaya, Yliya; Zilitinkevich, Sergej
2015-04-01
Detailed knowledge of the interaction of surface water waves with the wind flow is of primary importance for correct parameterization of turbulent momentum and heat fluxes which define the energy and momentum transfer between the atmosphere and hydrosphere. The objective of the present study is to investigate the properties of the stably stratified turbulent boundary-layer (BL) air-flow over waved water surface by direct numerical simulation (DNS) at a bulk Reynolds number varying from 15000 to 80000 and the surface-wave slope up to ka = 0.2. The DNS results show that the BL-flow remains in the statistically stationary, turbulent regime if the Reynolds number (ReL) based on the Obukhov length scale and friction velocity is sufficiently large (ReL > 100). In this case, mean velocity and temperature vertical profiles are well predicted by log-linear asymptotic solutions following from the Monin-Obukhov similarity theory provided the velocity and temperature roughness parameters, z0U and z0T, are appropriately prescribed. Both z0U and z0T increase for larger surface-wave slope. DNS results also show that turbulent momentum and heat fluxes and turbulent velocity and temperature fluctuations are increased for larger wave slope (ka) whereas the mean velocity and temperature derivatives remain practically the same for different ka. Thus, we conclude that the source of turbulence enhancement in BL-flow are perturbations induced by the surface wave, and not the shear instability of the bulk flow. On the other hand, if stratification is sufficiently strong, and the surface-wave slope is sufficiently small, the BL-flow over waved surface relaminarizes in the bulk of the domain. However, if the surface-wave slope exceeds a threshold value, the velocity and temperature fluctuations remain finite in the vicinity of the critical-layer level, where the surface-wave phase velocity coincides with the mean flow velocity. We call this new stably-stratified BL-flow regime observed in
El-Madany, T. S.; Duarte, H. F.; Durden, D. J.; Paas, B.; Deventer, M. J.; Juang, J.-Y.; Leclerc, M. Y.; Klemm, O.
2014-08-01
Sodar (SOund Detection And Ranging), eddy-covariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyze their importance in nocturnal atmosphere-biosphere exchange as quantified with eddy-covariance measurements. If intermittency occurs frequently at a measurement site, then this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production. Fourteen events of intermittent turbulence were identified and classified into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases, upslope flows occurred, counteracting the general flow regime at nighttime. The observations suggest that the LLJs and ACDFs penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds. Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis of the vertical wind velocity and the steady-state test for the time series of the vertical wind velocity in combination with the horizontal wind component, the temperature, and carbon dioxide. Significantly higher fluxes of sensible heat, latent heat, and shear stress occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat, latent heat, and CO2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes
Troitskaya, Yu. I.; Ezhova, E. V.; Zilitinkevich, S. S.
2013-01-01
The surface-drag and mass-transfer coefficients are determined within a self-consistent problem of wave-induced perturbations and mean fields of velocity and density in the air, using a quasi-linear model based on the Reynolds equations with down-gradient turbulence closure. Investigation of a harmonic wave propagating along the wind has disclosed that the surface drag is generally larger for shorter waves. This effect is more pronounced in the unstable and neutral stratification. The stable ...
Cassiani, Massimo; Stohl, Andreas; Brioude, Jerome
2015-03-01
A correction for the vertical gradient of air density has been incorporated into a skewed probability density function formulation for turbulence in the convective boundary layer. The related drift term for Lagrangian stochastic dispersion modelling has been derived based on the well-mixed condition. Furthermore, the formulation has been extended to include unsteady turbulence statistics and the related additional component of the drift term obtained. These formulations are an extension of the drift formulation reported by Luhar et al. (Atmos Environ 30:1407-1418, 1996) following the well-mixed condition proposed by Thomson (J Fluid Mech 180:529-556, 1987). Comprehensive tests were carried out to validate the formulations including consistency between forward and backward simulations and preservation of a well-mixed state with unsteady conditions. The stationary state CBL drift term with density correction was incorporated into the FLEXPART and FLEXPART-WRF Lagrangian models, and included the use of an ad hoc transition function that modulates the third moment of the vertical velocity based on stability parameters. Due to the current implementation of the FLEXPART models, only a steady-state horizontally homogeneous drift term could be included. To avoid numerical instability, in the presence of non-stationary and horizontally inhomogeneous conditions, a re-initialization procedure for particle velocity was used. The criteria for re-initialization and resulting errors were assessed for the case of non-stationary conditions by comparing a reference numerical solution in simplified unsteady conditions, obtained using the non-stationary drift term, and a solution based on the steady drift with re-initialization. Two examples of "real-world" numerical simulations were performed under different convective conditions to demonstrate the effect of the vertical gradient in density on the particle dispersion in the CBL.
Intermittent Turbulence in the Very Stable Ekman Layer
Energy Technology Data Exchange (ETDEWEB)
Barnard, James C.
2001-01-05
INTERMITTENT TURBULENCE IN THE VERY STABLE EKMAN LAYER This study describes a Direct Numerical Simulation (DNS) of a very stable Ekman layer in which a constant downward heat flux is applied at the lower boundary, thus cooling the fluid above. Numerical experiments were performed in which the strength of the imposed heat flux was varied. For downward heat fluxes above a certain critical value the turbulence becomes intermittent and, as the heat flux increases beyond this value, the flow tends to relaminarize because of the very strong ambient stratification. We adopt Mahrt?s (1999) definition of the very stable boundary layer as a boundary layer in which intermittent, rather than continuous turbulence, is observed. Numerical experiments were used to test various hypothesis of where in ?stability parameter space? the very stable boundary layer is found. These experiments support the findings of Howell and Sun (1999) that the boundary layer will exhibit intermittency and therefore be categorized as ?very stable?, when the stability parameter, z/L, exceeds unity. Another marker for the very stable boundary layer, Derbyshire?s (1990) maximum heat flux criterion, was also examined. Using a case study drawn from the simulations where turbulence intermittency was observed, the mechanism that causes the intermittence was investigated. It was found that patchy turbulence originates from a vigorous inflectional, Ekman-like instability -- a roll cell -- that lifts colder air over warmer air. The resulting convective instability causes an intense burst of turbulence. This turbulence is short-lived because the lifting motion of the roll cell, as well as the roll cell itself, is partially destroyed after the patchy turbulence is generated. Examples of intermittent turbulence obtained from the simulations appear to be consistent with observations of intermittency even though the Reynolds number of the DNS is relatively low (400).
Jain, Akash; Mehdi, Faraz; Sheng, Jian
2014-11-01
The near-wake field, a short region characterized by the physical specifications of a turbine, is of particular interest for flow-structure interactions responsible for asymmetric loadings, premature structural breakdown, noise generation etc. Helical tip vortices constitute a distinctive feature of this region and are dependent not only on the turbine geometry but also on the incoming flow profile. High-spatial resolution PIV measurements are made in the wake of a horizontal-axis model wind turbine embedded in a neutrally stratified turbulent boundary layer. The data is acquired over consecutive locations up to 10 diameters downstream of the turbine but the focus here is on the tip vortices identified in the instantaneous fields. Contrary to previous studies, both top and bottom tip vortices are clearly distinguishable in either ensemble fields or instantaneous realizations. The streamwise extent of these vortices stretches from the turbine till they merge into the expanding mid-span wake. The similarities and differences in the top and bottom tip vortices are explored through the evolution of their statistics. In particular, the distributions of the loci of vortex cores and their circulations are compared. The information will improve our understanding of near wake vortical dynamics, provide data for model validation, and aid in the devise of flow control strategies.
Rackl, Robert; Weston, Adam
2005-01-01
The literature on turbulent boundary layer pressure fluctuations provides several empirical models which were compared to the measured TU-144 data. The Efimtsov model showed the best agreement. Adjustments were made to improve its agreement further, consisting of the addition of a broad band peak in the mid frequencies, and a minor modification to the high frequency rolloff. The adjusted Efimtsov predicted and measured results are compared for both subsonic and supersonic flight conditions. Measurements in the forward and middle portions of the fuselage have better agreement with the model than those from the aft portion. For High Speed Civil Transport supersonic cruise, interior levels predicted by use of this model are expected to increase by 1-3 dB due to the adjustments to the Efimtsov model. The space-time cross-correlations and cross-spectra of the fluctuating surface pressure were also investigated. This analysis is an important ingredient in structural acoustic models of aircraft interior noise. Once again the measured data were compared to the predicted levels from the Efimtsov model.
Wendt, Bruce J.; Greber, Isaac; Hingst, Warren R.
1991-01-01
An investigation of the structure and development of streamwise vortices embedded in a turbulent boundary layer was conducted. The vortices were generated by a single spanwise row of rectangular vortex generator blades. A single embedded vortex was examined, as well as arrays of embedded counter rotating vortices produced by equally spaced vortex generators. Measurements of the secondary velocity field in the crossplane provided the basis for characterization of vortex structure. Vortex structure was characterized by four descriptors. The center of each vortex core was located at the spanwise and normal position of peak streamwise vorticity. Vortex concentration was characterized by the magnitude of the peak streamwise vorticity, and the vortex strength by its circulation. Measurements of the secondary velocity field were conducted at two crossplane locations to examine the streamwise development of the vortex arrays. Large initial spacings of the vortex generators produced pairs of strong vortices which tended to move away from the wall region while smaller spacings produced tight arrays of weak vortices close to the wall. A model of vortex interaction and development is constructed using the experimental results. The model is based on the structure of the Oseen Vortex. Vortex trajectories are modelled by including the convective effects of neighbors.
Costigliola, V.
2010-09-01
It has long been known that specific atmospheric processes, such as weather and longer-term climatic fluctuations, affect human health. The biometeorological literature refers to this relationship as meteorotropism, defined as a change in an organism that is correlated with a change in atmospheric conditions. Plenty of (patho)physiological functions are affected by those conditions - like the respiratory diseases - and currently it is difficult to put any limits for pathologies developed in reply. Nowadays the importance of atmospheric boundary layer and health is increasingly recognised. A number of epidemiologic studies have reported associations between ambient concentrations of air pollution, specifically particulate pollution, and adverse health effects, even at the relatively low concentrations of pollution found. Since 1995 there have been over twenty-one studies from four continents that have explicitly examined the association between ambient air pollutant mixes and daily mortality. Statistically significant and positive associations have been reported in data from various locations around the world, all with varying air pollutant concentrations, weather conditions, population characteristics and public health policies. Particular role has been given to atmospheric boundary layer processes, the impact of which for specific patient-cohort is, however, not well understood till now. Assessing and monitoring air quality are thus fundamental to improve Europe's welfare. One of current projects run by the "European Medical Association" - PASODOBLE will develop and demonstrate user-driven downstream information services for the regional and local air quality sectors by combining space-based and in-situ data with models in 4 thematic service lines: - Health community support for hospitals, pharmacies, doctors and people at risk - Public information for regions, cities, tourist industry and sporting event organizers - Compliance monitoring support on particulate
Hartogensis, O.K.; Debruin, H.A.R.
2005-01-01
The Monin-Obukhov similarity theory (MOST) functions fepsi; and fT, of the dissipation rate of turbulent kinetic energy (TKE), ¿, and the structure parameter of temperature, CT2, were determined for the stable atmospheric surface layer using data gathered in the context of CASES-99. These data cover
Boundary-layer linear stability theory
Mack, L. M.
1984-06-01
Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer
Boundary-layer linear stability theory
Mack, L. M.
1984-01-01
Most fluid flows are turbulent rather than laminar and the reason for this was studied. One of the earliest explanations was that laminar flow is unstable, and the linear instability theory was first developed to explore this possibility. A series of early papers by Rayleigh produced many notable results concerning the instability of inviscid flows, such as the discovery of inflectional instability. Viscosity was commonly thought to act only to stabilize the flow, and flows with convex velocity profiles appeared to be stable. The investigations that led to a viscous theory of boundary layer instability was reported. The earliest application of linear stability theory to transition prediction calculated the amplitude ratio of the most amplified frequency as a function of Reynolds number for a Blasius boundary layer, and found that this quantity had values between five and nine at the observed Ret. The experiment of Schubauer and Skramstad (1947) completely reversed the prevailing option and fully vindicated the Gottingen proponents of the theory. This experiment demonstrated the existence of instability waves in a boundary layer, their connection with transition, and the quantitative description of their behavior by the theory of Tollmien and Schlichting. It is generally accepted that flow parameters such as pressure gradient, suction and heat transfer qualitatively affect transition in the manner predicted by the linear theory, and in particular that a flow predicted to be stable by the theory should remain laminar. The linear theory, in the form of the e9, or N-factor is today in routine use in engineering studies of laminar flow. The stability theory to boundary layers with pressure gradients and suction was applied. The only large body of numerical results for exact boundary layer solutions before the advent of the computer age by calculating the stability characteristics of the Falkner-Skan family of velocity profiles are given. When the digital computer
Instabilities and transition in boundary layers
Indian Academy of Sciences (India)
N Vinod; Rama Govindarajan
2005-03-01
Some recent developments in boundary layer instabilities and transition are reviewed. Background disturbance levels determine the instability mechanism that ultimately leads to turbulence. At low noise levels, the traditional Tollmien–Schlichting route is followed, while at high levels, a `by-pass' route is more likely. Our recent work shows that spot birth is related to the pattern of secondary instability in either route.
Boundary layer physics over snow and ice
Directory of Open Access Journals (Sweden)
P. S. Anderson
2007-06-01
Full Text Available A general understanding of the physics of advection and turbulent mixing within the near surface atmosphere assists the interpretation and predictive power of air chemistry theory. The theory of the physical processes involved in diffusion of trace gas reactants in the near surface atmosphere is still incomplete. Such boundary layer theory is least understood over snow and ice covered surfaces, due in part to the thermo-optical properties of the surface. Polar boundary layers have additional aspects to consider, due to the possibility of long periods without diurnal forcing and enhanced Coriolis effects.
This paper provides a review of present concepts in polar boundary layer meteorology, which will generally apply to atmospheric flow over snow and ice surfaces. It forms a companion paper to the chemistry review papers in this special issue of ACP.
Bypass transition and spot nucleation in boundary layers
Kreilos, Tobias; Schlatter, Philipp; Duguet, Yohann; Henningson, Dan S; Eckhardt, Bruno
2016-01-01
The spatio-temporal aspects of the transition to turbulence are considered in the case of a boundary layer flow developing above a flat plate exposed to free-stream turbulence. Combining results on the receptivity to free-stream turbulence with the nonlinear concept of a transition threshold, a physically motivated model suggests a spatial distribution of spot nucleation events. To describe the evolution of turbulent spots a probabilistic cellular automaton is introduced, with all parameters directly fitted from numerical simulations of the boundary layer. The nucleation rates are then combined with the cellular automaton model, yielding excellent quantitative agreement with the statistical characteristics for different free-stream turbulence levels. We thus show how the recent theoretical progress on transitional wall-bounded flows can be extended to the much wider class of spatially developing boundary-layer flows.
Behrendt, A.; Wulfmeyer, V.; Hammann, E.; Muppa, S. K.; Pal, S.
2015-05-01
The rotational Raman lidar (RRL) of the University of Hohenheim (UHOH) measures atmospheric temperature profiles with high resolution (10 s, 109 m). The data contain low-noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz) and a very efficient interference-filter-based polychromator. In this paper, the first profiling of the second- to fourth-order moments of turbulent temperature fluctuations is presented. Furthermore, skewness profiles and kurtosis profiles in the convective planetary boundary layer (CBL) including the interfacial layer (IL) are discussed. The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56'' N, 6°27'50.39'' E; 110 m a.s.l.) on 24 April 2013 during the Intensive Observations Period (IOP) 6 of the HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction) Observational Prototype Experiment (HOPE). We used the data between 11:00 and 12:00 UTC corresponding to 1 h around local noon (the highest position of the Sun was at 11:33 UTC). First, we investigated profiles of the total noise error of the temperature measurements and compared them with estimates of the temperature measurement uncertainty due to shot noise derived with Poisson statistics. The comparison confirms that the major contribution to the total statistical uncertainty of the temperature measurements originates from shot noise. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. (above ground level) at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1020 m a.g.l. Autocovariance and spectral analyses of the atmospheric temperature fluctuations confirm that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the integral scale of
Modeling the urban boundary layer
Bergstrom, R. W., Jr.
1976-01-01
A summary and evaluation is given of the Workshop on Modeling the Urban Boundary Layer; held in Las Vegas on May 5, 1975. Edited summaries from each of the session chairpersons are also given. The sessions were: (1) formulation and solution techniques, (2) K-theory versus higher order closure, (3) surface heat and moisture balance, (4) initialization and boundary problems, (5) nocturnal boundary layer, and (6) verification of models.
Nature, theory and modelling of geophysical convective planetary boundary layers
Zilitinkevich, Sergej
2015-04-01
Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in
Modeling the summertime Arctic cloudy boundary layer
Energy Technology Data Exchange (ETDEWEB)
Curry, J.A.; Pinto, J.O. [Univ. of Colorado, Boulder, CO (United States); McInnes, K.L. [CSIRO Division of Atmospheric Research, Mordialloc (Australia)
1996-04-01
Global climate models have particular difficulty in simulating the low-level clouds during the Arctic summer. Model problems are exacerbated in the polar regions by the complicated vertical structure of the Arctic boundary layer. The presence of multiple cloud layers, a humidity inversion above cloud top, and vertical fluxes in the cloud that are decoupled from the surface fluxes, identified in Curry et al. (1988), suggest that models containing sophisticated physical parameterizations would be required to accurately model this region. Accurate modeling of the vertical structure of multiple cloud layers in climate models is important for determination of the surface radiative fluxes. This study focuses on the problem of modeling the layered structure of the Arctic summertime boundary-layer clouds and in particular, the representation of the more complex boundary layer type consisting of a stable foggy surface layer surmounted by a cloud-topped mixed layer. A hierarchical modeling/diagnosis approach is used. A case study from the summertime Arctic Stratus Experiment is examined. A high-resolution, one-dimensional model of turbulence and radiation is tested against the observations and is then used in sensitivity studies to infer the optimal conditions for maintaining two separate layers in the Arctic summertime boundary layer. A three-dimensional mesoscale atmospheric model is then used to simulate the interaction of this cloud deck with the large-scale atmospheric dynamics. An assessment of the improvements needed to the parameterizations of the boundary layer, cloud microphysics, and radiation in the 3-D model is made.
Role of residual layer and large-scale phenomena on the evolution of the boundary layer
Blay, E.; D. Pino; Vilà-Guerau de Arellano, J.; Boer; Coster, van, R.; I. Faloona; Garrouste, O.; Hartogensis, O. K.
2012-01-01
Mixed-layer theory and large-eddy simulations are used to analyze the dynamics of the boundary layer on two intensive operational periods during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign: 1st and 2nd of July 2011, when convective boundary layers (CBLs) were observed. Continuous measurements made by several remote sensing and in situ instruments in combination with radiosoundings, and measurements done by unmanned aerial vehicles and an aircraft probed the verti...
Atmospheric boundary layer over steep surface waves
Troitskaya, Yuliya; Sergeev, Daniil A.; Druzhinin, Oleg; Kandaurov, Alexander A.; Ermakova, Olga S.; Ezhova, Ekaterina V.; Esau, Igor; Zilitinkevich, Sergej
2014-08-01
Turbulent air-sea interactions coupled with the surface wave dynamics remain a challenging problem. The needs to include this kind of interaction into the coupled environmental, weather and climate models motivate the development of a simplified approximation of the complex and strongly nonlinear interaction processes. This study proposes a quasi-linear model of wind-wave coupling. It formulates the approach and derives the model equations. The model is verified through a set of laboratory (direct measurements of an airflow by the particle image velocimetry (PIV) technique) and numerical (a direct numerical simulation (DNS) technique) experiments. The experiments support the central model assumption that the flow velocity field averaged over an ensemble of turbulent fluctuations is smooth and does not demonstrate flow separation from the crests of the waves. The proposed quasi-linear model correctly recovers the measured characteristics of the turbulent boundary layer over the waved water surface.
Behrendt, A.; Wulfmeyer, V.; Hammann, E.; Muppa, S. K.; Pal, S.
2014-11-01
The rotational Raman lidar of the University of Hohenheim (UHOH) measures atmospheric temperature profiles during daytime with high resolution (10 s, 109 m). The data contain low noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz) and a very efficient interference-filter-based polychromator. In this paper, we present the first profiling of the second- to forth-order moments of turbulent temperature fluctuations as well as of skewness and kurtosis in the convective boundary layer (CBL) including the interfacial layer (IL). The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56'' N, 6°27'50.39'' E, 110 m a.s.l.) within one hour around local noon on 24 April 2013 during the Intensive Observations Period (IOP) 6 of the HD(CP)2 Observational Prototype Experiment (HOPE), which is embedded in the German project HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction). First, we investigated profiles of the noise variance and compared it with estimates of the statistical temperature measurement uncertainty Δ T based on Poisson statistics. The agreement confirms that photon count numbers obtained from extrapolated analog signal intensities provide a lower estimate of the statistical errors. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1000 m a.g.l.. Then we confirmed by autocovariance and spectral analyses of the atmospheric temperature fluctuations that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the profile of the integral scale of the temperature fluctuations, which was in the range of 40 to 120 s in the CBL. Analyzing then profiles of the second
Directory of Open Access Journals (Sweden)
A. Behrendt
2014-11-01
Full Text Available The rotational Raman lidar of the University of Hohenheim (UHOH measures atmospheric temperature profiles during daytime with high resolution (10 s, 109 m. The data contain low noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz and a very efficient interference-filter-based polychromator. In this paper, we present the first profiling of the second- to forth-order moments of turbulent temperature fluctuations as well as of skewness and kurtosis in the convective boundary layer (CBL including the interfacial layer (IL. The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56′′ N, 6°27'50.39′′ E, 110 m a.s.l. within one hour around local noon on 24 April 2013 during the Intensive Observations Period (IOP 6 of the HD(CP2 Observational Prototype Experiment (HOPE, which is embedded in the German project HD(CP2 (High-Definition Clouds and Precipitation for advancing Climate Prediction. First, we investigated profiles of the noise variance and compared it with estimates of the statistical temperature measurement uncertainty Δ T based on Poisson statistics. The agreement confirms that photon count numbers obtained from extrapolated analog signal intensities provide a lower estimate of the statistical errors. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1000 m a.g.l.. Then we confirmed by autocovariance and spectral analyses of the atmospheric temperature fluctuations that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the profile of the integral scale of the temperature fluctuations, which was in the range of 40 to 120 s in the CBL. Analyzing then
The Boundary Layer Interaction with Shock Wave and Expansion Fan
Institute of Scientific and Technical Information of China (English)
MaratA.Goldfeld; RomanV.Nestoulia; 等
2000-01-01
The results of experimental investigation of a turbulent boundary layer on compression and expansion surfaces are presented.They include the study of the shock wave and /or expansion fan action upon the boundary layer,boundary layer sepqartion and its relaxation.Complex events of paired interactions and the flow on compression convex-concave surfaces were studied.The posibility and conditions of the boundary layer relaminarization behind the expansion fan and its effect on the relaxation length are presented.Different model configurations for wide range conditions were investigated.Comparison of results for different interactions was carried out.
Elbing, Brian R.; Perlin, Marc; Dowling, David R.; Ceccio, Steven L.
2013-08-01
The current study explores the influence of polymer drag reduction on the near-wall velocity distribution in a turbulent boundary layer (TBL) and its dependence on Reynolds number. Recent moderate Reynolds number direct numerical simulation and experimental studies presented in White et al. [Phys. Fluids 24, 021701 (2012)], 10.1063/1.3681862 have challenged the classical representation of the logarithmic dependence of the velocity profile for drag-reduced flows, especially at drag reduction levels above 40%. In the present study, high Reynolds number data from a drag reduced TBL is presented and compared to the observations of White et al. [Phys. Fluids 24, 021701 (2012)], 10.1063/1.3681862. Data presented here were acquired in the TBL flow on a 12.9-m-long flat plate at speeds to 20.3 m s-1, achieving momentum thickness based Reynolds number to 1.5 × 105, which is an order of magnitude greater than that available in the literature. Polyethylene oxide solutions with an average molecular weight of 3.9 × 106 g mol-1 were injected into the flow at various concentrations and volumetric fluxes to achieve a particular level of drag reduction. The resulting mean near-wall velocity profiles show distinctly different behavior depending on whether they fall in the low drag reduction (LDR) or the high drag reduction (HDR) regimes, which are nominally divided at 40% drag reduction. In the LDR regime, the classical view that the logarithmic slope remains constant at the Newtonian value and the intercept constant increases with increasing drag reduction appears to be valid. However, in the HDR regime the behavior is no longer universal. The intercept constant continues to increase linearly in proportion to the drag reduction level until a Reynolds-number-dependent threshold is achieved, at which point the intercept constant rapidly decreases to that predicted by the ultimate profile. The rapid decrease in the intercept constant is due to the corresponding increase in the
Dynamics of turbulent western boundary currents at low latitude in a shallow water model
Directory of Open Access Journals (Sweden)
C. Q. C. Akuetevi
2014-03-01
Full Text Available The dynamics of low latitude turbulent western boundary currents, subject to two different types of idealized wind forcing, Monsoon Wind and Trade Wind, is considered using numerical results from integrations of a reduced gravity shallow-water model. For viscosity values of 1000 m2 s−1 and above, the boundary layer dynamics compares well to the analytical solutions of the Munk-layer and the inertial-layer, derived from quasigeostrophic theory. Modifications due to variations in the layer thickness (vortex stretching are only important close to the boundary. When the viscosity is reduced the boundary layer becomes turbulent and coherent structures in form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer and dipoles appear. Three distinct boundary layers emerge, the viscous sub-layer, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity. The second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. A pragmatic approach to determine the eddy viscosity diagnostically for coarse resolution numerical models is proposed.
DNS Study on Physics of Late Boundary Layer Transition
Liu, Chaoqun
2014-01-01
This paper serves as a review of our recent new DNS study on physics of late boundary layer transition. This includes mechanism of the large coherent vortex structure formation, small length scale generation and flow randomization. The widely spread concept vortex breakdown to turbulence,which was considered as the last stage of flow transition, is not observed and is found theoretically incorrect. The classical theory on boundary layer transition is challenged and we proposed a new theory with five steps, i.e. receptivity, linear instability, large vortex formation, small length scale generation, loss of symmetry and randomization to turbulence. We have also proposed a new theory about turbulence generation. The new theory shows that all small length scales (turbulence) are generated by shear layer instability which is produced by large vortex structure with multiple level vortex rings, multiple level sweeps and ejections, and multiple level negative and positive spikes near the laminar sub-layers.Therefore,...
Active control of ionized boundary layers
Mendes, R V
1997-01-01
The challenging problems, in the field of control of chaos or of transition to chaos, lie in the domain of infinite-dimensional systems. Access to all variables being impossible in this case and the controlling action being limited to a few collective variables, it will not in general be possible to drive the whole system to the desired behaviour. A paradigmatic problem of this type is the control of the transition to turbulence in the boundary layer of fluid motion. By analysing a boundary layer flow for an ionized fluid near an airfoil, one concludes that active control of the transition amounts to the resolution of an generalized integro-differential eigenvalue problem. To cope with the required response times and phase accuracy, electromagnetic control, whenever possible, seems more appropriate than mechanical control by microactuators.
A global boundary-layer height climatology
Energy Technology Data Exchange (ETDEWEB)
Dop, H. van; Krol, M.; Holtslag, B. [Inst. for Marine and Atmospheric Research Utrecht, IMAU, Utrecht (Netherlands)
1997-10-01
In principle the ABL (atmospheric boundary layer) height can be retrieved from atmospheric global circulation models since they contain algorithms which determine the intensity of the turbulence as a function of height. However, these data are not routinely available, or on a (vertical) resolution which is too crude in view of the application. This justifies the development of a separate algorithm in order to define the ABL. The algorithm should include the generation of turbulence by both shear and buoyancy and should be based on readily available atmospheric parameters. There is obviously a wide application for boundary heights in off-line global and regional chemistry and transport modelling. It is also a much used parameter in air pollution meteorology. In this article we shall present a theory which is based on current insights in ABL dynamics. The theory is applicable over land and sea surfaces in all seasons. The theory is (for various reasons) not valid in mountainous areas. In areas where boundary-layer clouds or deep cumulus convection are present the theory does not apply. However, the same global atmospheric circulation models contain parameterizations for shallow and deep convection from which separate estimates can be obtained for the extent of vertical mixing. (au)
Asymptotic analysis and boundary layers
Cousteix, Jean
2007-01-01
This book presents a new method of asymptotic analysis of boundary-layer problems, the Successive Complementary Expansion Method (SCEM). The first part is devoted to a general comprehensive presentation of the tools of asymptotic analysis. It gives the keys to understand a boundary-layer problem and explains the methods to construct an approximation. The second part is devoted to SCEM and its applications in fluid mechanics, including external and internal flows. The advantages of SCEM are discussed in comparison with the standard Method of Matched Asymptotic Expansions. In particular, for the first time, the theory of Interactive Boundary Layer is fully justified. With its chapter summaries, detailed derivations of results, discussed examples and fully worked out problems and solutions, the book is self-contained. It is written on a mathematical level accessible to graduate and post-graduate students of engineering and physics with a good knowledge in fluid mechanics. Researchers and practitioners will estee...
Numerical simulation of tsunami-scale wave boundary layers
DEFF Research Database (Denmark)
Williams, Isaac A.; Fuhrman, David R.
2016-01-01
, boundary layer thickness, turbulence, and bed shear stresses induced are systematically monitored and parameterised, under both hydraulically smooth and roughbed conditions. The results generally support a notion that the boundary layers induced by tsunami-scalewaves are both current-like, due...... layer properties beneath wind-waves maintain reasonable accuracy when extrapolated to full tsunami scales. Boundary layers driven by actual field-measured tsunami signals are likewise simulated, stemming from both the 2004 Indian Ocean as well as the 2011 Tohoku events. These results are reconciled...
Modelling of the Evolving Stable Boundary Layer
Sorbjan, Zbigniew
2014-06-01
A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of "horse-shoe" shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing.
Dynamics of turbulent western boundary currents at low latitude in a shallow water model
Directory of Open Access Journals (Sweden)
C. Q. C. Akuetevi
2014-11-01
Full Text Available The dynamics of low latitude turbulent western boundary currents crossing the equator is considered using numerical results from integrations of a reduced gravity shallow-water model. For viscosity values of 1000 m2 s−1 and more, the boundary layer dynamics compares well to the analytical Munk-layer solution. When the viscosity is reduced, the boundary layer becomes turbulent and coherent structures in form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer and dipoles appear. Three distinct boundary layers emerge, the viscous sub-layer, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity. The second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. Characteristic scales of the dynamics and dissipation are determined. A pragmatic approach to determine the eddy viscosity diagnostically for coarse resolution numerical models is proposed.
Propeller slipstream/wing boundary layer effects at low Reynolds numbers
Miley, S. J.; Howard, R. M.; Holmes, B. J.
1985-01-01
The effects of propeller slipstream on the wing laminar boundary are being investigated. Hot-wire velocity sensor measurements have been performed in flight and in a wind tunnel. It is shown that the boundary layer cycles between a laminar state and a turbulent state at the propeller blade passage rate. The cyclic length of the turbulent state increases with decreasing laminar stability. Analyses of the time varying velocity profiles show the turbulent state to lie in a transition region between fully laminar and fully turbulent. The observed cyclic boundary layer has characteristics similar to relaminarizing flow and laminar flow with external turbulence.
Dense gas boundary layer experiments: Visualization, pressure measurements, concentration evaluation
Energy Technology Data Exchange (ETDEWEB)
Reichenbach, H.; Neuwald, P. [Ernst-Mach-Institut, Freiburg (DE); Kuhl, A.L. [R and D Associates, Los Angeles, CA (United States)
1992-11-01
This technical report describes methods that were applied to investigate turbulent boundary layers generated by inviscid, baroclinic effects. The Cranz-Schardin 24-sparks camera was used to visualize the interactions of a planar shock wave with a Freon R12-layer. The shock propagates more slowly in the Freon layer than in air because of its smaller sound speed. This causes the shock front to be curved and to be reflected between the wall and the layer interface. As a consequence of the reflection process, a series of compression and expansion waves radiate from the layer. Large fluctuations in the streamwise velocity and in pressure develop for about 1 ms. These waves strongly perturb the interface shear layer, which rapidly transitions to a turbulent boundary flow. Pressure measurements showed that the fluctuations in the Freon layer reach a peak pressure 4 times higher than in the turbulent boundary flow. To characterize the preshock Freon boundary layer, concentration measurements were performed with a differential interferometry technique. The refraction index of Freon R12 is so high that Mach-Zehnder interferometry was not successful in these experiments. The evaluation of the concentration profile is described here in detail. Method and results of corresponding LDV measurements under the same conditions are presented in a different report, EMI Report T 9/92. The authors plan to continue the dense gas layer investigations with the gas combination helium/Freon.
Velocity fields and optical turbulence near the boundary in a strongly convective laboratory flow
Matt, Silvia; Hou, Weilin; Goode, Wesley; Hellman, Samuel
2016-05-01
Boundary layers around moving underwater vehicles or other platforms can be a limiting factor for optical communication. Turbulence in the boundary layer of a body moving through a stratified medium can lead to small variations in the index of refraction, which impede optical signals. As a first step towards investigating this boundary layer effect on underwater optics, we study the flow near the boundary in the Rayleigh-Bénard laboratory tank at the Naval Research Laboratory Stennis Space Center. The tank is set up to generate temperature-driven, i.e., convective turbulence, and allows control of the turbulence intensity. This controlled turbulence environment is complemented by computational fluid dynamics simulations to visualize and quantify multi-scale flow patterns. The boundary layer dynamics in the laboratory tank are quantified using a state-of-the-art Particle Image Velocimetry (PIV) system to examine the boundary layer velocities and turbulence parameters. The velocity fields and flow dynamics from the PIV are compared to the numerical model and show the model to accurately reproduce the velocity range and flow dynamics. The temperature variations and thus optical turbulence effects can then be inferred from the model temperature data. Optical turbulence is also visible in the raw data from the PIV system. The newly collected data are consistent with previously reported measurements from high-resolution Acoustic Doppler Velocimeter profilers (Nortek Vectrino), as well as fast thermistor probes and novel next-generation fiber-optics temperature sensors. This multi-level approach to studying optical turbulence near a boundary, combining in-situ measurements, optical techniques, and numerical simulations, can provide new insight and aid in mitigating turbulence impacts on underwater optical signal transmission.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Hanzelka, Pavel; Králík, Tomáš; Musilová, Věra; Srnka, Aleš; Skrbek, L.
2013-01-01
Roč. 110, 8 May (2013), 199402:1-1. ISSN 0031-9007 Institutional support: RVO:68081731 Keywords : cryogenic turbulence * turbulent convection * Rayleigh-Bénard convection Subject RIV: BJ - Thermodynamics Impact factor: 7.728, year: 2013
Directory of Open Access Journals (Sweden)
Yee-Lin Wu
2010-01-01
Full Text Available Linyuan (LY is a coastal station located down wind of the industrial city of Kaohsiung in southern Taiwan. This station is often affected by severe ozone pollution during sea breeze events. Intensive tethered ozone soundings were per formed at this station during a 4-day ozone episode in November, 2005. Back air trajectories were also calculated to track the origins of air masses arriving at the station during the experiment. The investigation revealed complicated ozone pro files in the lower at mo sphere (be low 1300 m both day and night. At night, industrial plumes forming no-ozone air layers were frequently distributed at 400 - 800 m. Mixing layers rapidly decreased from 800 - 1100 m down to 200 - 350 m in the late morning hours when sea breezes and thermal internal boundary layers (TIBLs developed. Recirculation of polluted in land air masses over the sea, the development of TIBLs, and the late development of sea-breeze events all are likely responsible for severe ozone pollution at the LY station. Elevated industrial plumes or ozone aloft above TIBLs revealed only aminor contribution to ozone pollution via a downward mixing process. Elevated ozone levels (140 - 170 ppb were of ten trapped within transitional layers of sea-breeze circulations at 600 - 800 m and were accompanied by ambient northerly flows parallel to the coast line, suggesting that an ozone pollution core likely formed over the west coast of Taiwan on ozone-episodic days when sea-breeze circulations developed.
Measurement of turbulence in the oceanic mixed layer using Synthetic Aperture Radar (SAR)
George, S.G.; Tatnall, A. R. L.
2012-01-01
Turbulence in the surface layer of the ocean contributes to the transfer of heat, gas and momentum across the air-sea boundary. As such, study of turbulence in the ocean surface layer is becoming increasingly important for understanding its effects on climate change. Direct Numerical Simulation (DNS) techniques were implemented to examine the interaction of small-scale wake turbulence in the upper ocean layer with incident electromagnetic radar waves. Hydrodynamic-electromag...
Bailly, Christophe
2015-01-01
This book covers the major problems of turbulence and turbulent processes, including physical phenomena, their modeling and their simulation. After a general introduction in Chapter 1 illustrating many aspects dealing with turbulent flows, averaged equations and kinetic energy budgets are provided in Chapter 2. The concept of turbulent viscosity as a closure of the Reynolds stress is also introduced. Wall-bounded flows are presented in Chapter 3, and aspects specific to boundary layers and channel or pipe flows are also pointed out. Free shear flows, namely free jets and wakes, are considered in Chapter 4. Chapter 5 deals with vortex dynamics. Homogeneous turbulence, isotropy, and dynamics of isotropic turbulence are presented in Chapters 6 and 7. Turbulence is then described both in the physical space and in the wave number space. Time dependent numerical simulations are presented in Chapter 8, where an introduction to large eddy simulation is offered. The last three chapters of the book summarize remarka...
Turbulence and intermittent transport at the boundary of magnetized plasmas
DEFF Research Database (Denmark)
Garcia, O.E.; Naulin, V.; Nielsen, A.H.;
2005-01-01
Numerical fluid simulations of interchange turbulence for geometry and parameters relevant to the boundary region of magnetically confined plasmas are shown to result in intermittent transport qualitatively similar to recent experimental measurements. The two-dimensional simulation domain features...... a forcing region with spatially localized sources of particles and heat outside which losses due to the motion along open magnetic-field lines dominate, corresponding to the edge region and the scrape-off layer, respectively. Turbulent states reveal intermittent eruptions of hot plasma from the edge...... formation of blob structures is thus related to profile variations, which are here triggered in a quasiperiodic manner by a global dynamical regulation due to the self-sustained sheared flows. (C) 2005 American Institute of Physics....
Stabilization of boundary layer streaks by plasma actuators
International Nuclear Information System (INIS)
A flow's transition from laminar to turbulent leads to increased levels of skin friction. In recent years, dielectric barrier discharge actuators have been shown to be able to delay the onset of turbulence in boundary layers. While the laminar to turbulent transition process can be initiated by several different instability mechanisms, so far, only stabilization of the Tollmien–Schlichting path to transition has received significant attention, leaving the stabilization of other transition paths using these actuators less explored. To fill that void, a bi-global stability analysis is used here to examine the stabilization of boundary layer streaks in a laminar boundary layer. These streaks, which are important to both transient and by-pass instability mechanisms, are damped by the addition of a flow-wise oriented plasma body force to the boundary layer. Depending on the magnitude of the plasma actuation, this damping can be up to 25% of the perturbation's kinetic energy. The damping mechanism appears to be due to highly localized effects in the immediate vicinity of the body force, and when examined using a linearized Reynolds-averaged Navier–Stokes energy balance, indicate negative production of the perturbation's kinetic energy. Parametric studies of the stabilization have also been performed, varying the magnitude of the plasma actuator's body force and the spanwise wavenumber of the actuation. Based on these parametric studies, the damping of the boundary layer streaks appears to be linear with respect to the total amount of body force applied to the flow. (paper)
Agostini, Lionel; Leschziner, Michael
2016-04-01
The "quasi-steady hypothesis," as understood in the context of large-scale/small-scale interactions in near-wall turbulence, rests on the assumption that the small scales near the wall react within very short time scales to changes imposed on them by energetic large scales whose length scales differ by at least one order of magnitude and whose energy reaches a maximum in the middle to the outer portion of the log-law layer. A key statistical manifestation of this assumption is that scaling the small-scale motions with the large-scale wall-friction-velocity footprints renders the small-scale statistics universal. This hypothesis is examined here by reference to direct numerical simulation (DNS) data for channel flow at Reτ ≈ 4200, subjected to a large-scale/small-scale separation by the empirical mode decomposition method. Flow properties examined include the mean velocity, second moments, joint probability density functions, and skewness. It is shown that the validity of the hypothesis depends on the particular property being considered and on the range of length scales of structures included within the large-scale spectrum. The quasi-steady hypothesis is found to be well justified for the mean velocity and streamwise energy of the small scales up to y + ˜ O ( 80 ) , but only up to y + ˜ O ( 30 ) for other properties.
CISM Course on Recent Advances in Boundary Layer Theory
1998-01-01
Recent advances in boundary-layer theory have shown how modern analytical and computational techniques can and should be combined to deepen the understanding of high Reynolds number flows and to design effective calculation strategies. This is the unifying theme of the present volume which addresses laminar as well as turbulent flows.
Fluid Mechanics and Heat Transfer in Transitional Boundary Layers
Wang, Ting
2007-01-01
Experiments have been performed to investigate the effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers. The free-stream turbulence ranges from 0.5 to 6.4% and the streamwise acceleration ranges from K = 0 to 0.8 x 10(exp -6). The onset of transition, transition length and the turbulent spot formation rate are determined. The statistical results and conditionally sampled results of th streamwise and cross-stream velocity fluctuations, temperature fluctuations, Reynolds stress and Reynolds heat fluxes are presented.
Surface layer similarity in the nocturnal boundary layer: the application of Hilbert-Huang transform
Directory of Open Access Journals (Sweden)
J. Hong
2009-10-01
Full Text Available Turbulence statistics such as flux-variance relationship is critical information in measuring and modeling carbon, water, energy, and momentum exchanges at the biosphere-atmosphere interface. Using a recently proposed mathematical technique, the Hilbert-Huang transform (HHT, this study highlights its possibility to quantify impacts of non-turbulent flows on turbulence statistics in the stable surface layer. The HHT is suitable for the analysis of non-stationary and intermittent data and thus very useful for better understanding of the interplay of the surface layer similarity with complex nocturnal environment. Our analysis showed that the HHT can successfully sift non-turbulent components and be used as a tool to estimate the relationships between turbulence statistics and atmospheric stability in complex environment such as nocturnal stable boundary layer.
Surface layer similarity in the nocturnal boundary layer: the application of Hilbert-Huang transform
Directory of Open Access Journals (Sweden)
J. Hong
2010-04-01
Full Text Available Turbulence statistics such as flux-variance relationship are critical information in measuring and modeling ecosystem exchanges of carbon, water, energy, and momentum at the biosphere-atmosphere interface. Using a recently proposed mathematical technique, the Hilbert-Huang transform (HHT, this study highlights its possibility to quantify impacts of non-turbulent flows on turbulence statistics in the stable surface layer. The HHT is suitable for the analysis of non-stationary and intermittent data and thus very useful for better understanding the interplay of the surface layer similarity with complex nocturnal environment. Our analysis showed that the HHT can successfully sift non-turbulent components and be used as a tool to estimate the relationships between turbulence statistics and atmospheric stability in complex environments such as nocturnal stable boundary layer.
Localized travelling waves in the asymptotic suction boundary layer
Kreilos, Tobias; Schneider, Tobias M
2016-01-01
We present two spanwise-localized travelling wave solutions in the asymptotic suction boundary layer, obtained by continuation of solutions of plane Couette flow. One of the solutions has the vortical structures located close to the wall, similar to spanwise-localized edge states previously found for this system. The vortical structures of the second solution are located in the free stream far above the laminar boundary layer and are supported by a secondary shear gradient that is created by a large-scale low-speed streak. The dynamically relevant eigenmodes of this solution are concentrated in the free stream, and the departure into turbulence from this solution evolves in the free stream towards the walls. For invariant solutions in free-stream turbulence, this solution thus shows that that the source of energy of the vortical structures can be a dynamical structure of the solution itself, instead of the laminar boundary layer.
Wind Tunnel Simulation of the Atmospheric Boundary Layer
Hohman, Tristen; Smits, Alexander; Martinelli, Luigi
2013-11-01
To simulate the interaction of large Vertical Axis Wind Turbines (VAWT) with the Atmospheric Boundary Layer (ABL) in the laboratory, we implement a variant of Counihan's technique [Counihan 1969] in which a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements is used to create an artificial ABL profile in a standard closed loop wind tunnel. To examine the development and formation of the artificial ABL hotwire and SPIV measurements were taken at various downstream locations with changes in wall roughness, wall type, and vortex generator arrangements. It was found possible to generate a boundary layer at Reθ ~106 , with a mean velocity that followed the 1/7 power law of a neutral ABL over rural terrain and longitudinal turbulence intensities and power spectra that compare well with the data obtained for high Reynolds number flat plate turbulent boundary layers [Hultmark et al. 2010]. Supported by Hopewell Wind Power Ltd., and the Princeton Grand Challenges Program.
Numerical Modeling of the Evolving Stable Boundary Layer
Sorbjan, Z.
2013-12-01
A single-column model of the evolving stable boundary layer is tested for the consistency of turbulence parameterization, self-similar properties of the flow, and effects of ambient forcing. The turbulence closure of the model is based on the K-theory approach, with stability functions based on empirical data, and a semi-empirical form of the mixing length. The model has one internal, governing stability parameter, the Richardson number Ri, which dynamically adjusts to the boundary conditions and to external forcing. Model results, expressed in terms of local similarity scales, are universal functions of the Richardson number, i.e. they are satisfied in the entire stable boundary layer, for all instants of time, and all kinds of external forcing. Based on similarity expression, a realizability condition is derived for the minimum turbulent heat flux in the stable boundary layer. Numerical experiments show that the development of 'horse-shoe' shaped, 'fixed-elevation' wind hodographs in the interior of the stable boundary layer are solely caused by effects imposed by surface thermal forcing, and are not related to the inertial oscillation mechanism.
Boundary Layer Heights from CALIOP
Kuehn, R.; Ackerman, S. A.; Holz, R.; Roubert, L.
2012-12-01
This work is focused on the development of a planetary boundary layer (PBL) height retrieval algorithm for CALIOP and validation studies. Our current approach uses a wavelet covariance transform analysis technique to find the top of the boundary layer. We use the methodology similar to that found in Davis et. al. 2000, ours has been developed to work with the lower SNR data provided by CALIOP, and is intended to work autonomously. Concurrently developed with the CALIOP algorithm we will show results from a PBL height retrieval algorithm from profiles of potential temperature, these are derived from Aircraft Meteorological DAta Relay (AMDAR) observations. Results from 5 years of collocated AMDAR - CALIOP retrievals near O'Hare airport demonstrate good agreement between the CALIOP - AMDAR retrievals. In addition, because we are able to make daily retrievals from the AMDAR measurements, we are able to observe the seasonal and annual variation in the PBL height at airports that have sufficient instrumented-aircraft traffic. Also, a comparison has been done between the CALIOP retrievals and the NASA Langley airborne High Spectral Resolution Lidar (HSRL) PBL height retrievals acquired during the GoMACCS experiment. Results of this comparison, like the AMDAR comparison are favorable. Our current work also involves the analysis and verification of the CALIOP PBL height retrieval from the 6 year CALIOP global data set. Results from this analysis will also be presented.
Interaction between surface and atmosphere in a convective boundary layer /
Garai, Anirban
2013-01-01
Solar heating of the surface causes the near surface air to warm up and with sufficient buoyancy it ascends through the atmosphere as surface-layer plumes and thermals. The cold fluid from the upper part of the boundary layer descends as downdrafts. The downdrafts and thermals form streamwise roll vortices. All these turbulent coherent structures are important because they contribute most of the momentum and heat transport. While these structures have been studied in depth, their imprint on t...
A Note on the bottom shear stress in oscillatory planetary boundary layer flow
Directory of Open Access Journals (Sweden)
Dag Myrhaug
1988-07-01
Full Text Available A simple analytical theory is presented, which describes the motion in a turbulent oscillatory planetary boundary layer near a rough seabed using a two-layer, time-invariant eddy viscosity model. The bottom shear stress is outlined, and comparison is made with Pingree and Griffiths' (1974 measurements of turbulent tidal planetary boundary layer flow on the continental shelf south-west of Lands End, England.
A Note on the bottom shear stress in oscillatory planetary boundary layer flow
Dag Myrhaug
1988-01-01
A simple analytical theory is presented, which describes the motion in a turbulent oscillatory planetary boundary layer near a rough seabed using a two-layer, time-invariant eddy viscosity model. The bottom shear stress is outlined, and comparison is made with Pingree and Griffiths' (1974) measurements of turbulent tidal planetary boundary layer flow on the continental shelf south-west of Lands End, England.
Moist turbulent Rayleigh-Benard convection with Neumann and Dirichlet boundary conditions
Weidauer, Thomas
2012-01-01
Turbulent Rayleigh-Benard convection with phase changes in an extended layer between two parallel impermeable planes is studied by means of three-dimensional direct numerical simulations for Rayleigh numbers between 10^4 and 1.5\\times 10^7 and for Prandtl number Pr=0.7. Two different sets of boundary conditions of temperature and total water content are compared: imposed constant amplitudes which translate into Dirichlet boundary conditions for the scalar field fluctuations about the quiescent diffusive equilibrium and constant imposed flux boundary conditions that result in Neumann boundary conditions. Moist turbulent convection is in the conditionally unstable regime throughout this study for which unsaturated air parcels are stably and saturated air parcels unstably stratified. A direct comparison of both sets of boundary conditions with the same parameters requires to start the turbulence simulations out of differently saturated equilibrium states. Similar to dry Rayleigh-Benard convection the differences...
New insights into adverse pressure gradient boundary layers
George, William K.; Stanislas, Michel; Laval, Jean-Philippe
2010-11-01
In a recent paper Shah et al. 2010 (Proc. of the WALLTURB Meeting, 2009), Lille, FR, Springer, in press) documented a number of adverse pressure gradient flows (APG's), with and without wall curvature, where the turbulence intensity peak moved quite sharply away from the wall with increasing distance. They further suggested that this peak was triggered by the adverse pressure gradient and had its origin in an instability hidden in the turbulent boundary layer, developing soon after the change of sign of the pressure gradient. They then offered that this may explain the difficulties encountered up to now in finding a universal scaling for turbulent boundary layers. We build on these observations, and show that in fact there is clear evidence in the literature (in most experiments, both old and new) for such a development downstream of the imposition of an adverse pressure gradient. The exact nature of the evolution and the distance over which it occurs depends on the upstream boundary layer and the manner in which the APG is imposed. But far enough downstream the mean velocity profile in all cases becomes an inflectional point profile with the location of the inflection point corresponding quite closely to the observed peak in the streamwise turbulence intensity. This does not seem to have been previously noticed.
Boundary-layer control by electric fields A feasibility study
Mendes, R V
1998-01-01
A problem of great concern in aviation and submarine propulsion is the control of the boundary layer and, in particular, the methods to extend the laminar region as a means to decrease noise and fuel consumption. In this paper we study the flow of air along an airfoil when a layer of ionized gas and a longitudinal electric field are created in the boundary layer region. By deriving scaling solutions and more accurate numerical solutions we discuss the possibility of achieving significant boundary layer control for realistic physical parameters. Practical design formulas and criteria are obtained. We also discuss the perspectives for active control of the laminar-to-turbulent transition fluctuations by electromagnetic field modulation.
The theoretical model of atmospheric turbulence spectrum in surface layer
Liu, Shida; Liu, Shikuo; Xin, Guojun; Liang, Fuming
1994-12-01
It is shown that the slope of energy spectrum obtained from the velocity solution of Kdv—Burgers equation lies between —5/3 and—2 in the dilogarithmic coordinates paper. The spectrum is very close to one of Kolmogorov's isotropic turbulence and Frisch's intermittent turbulence in inertial region. In this paper, the Kdv-Burgers equation to describe atmospheric boundary layer turbulence is obtained. In the equation, the 1 / R e corresponds to dissipative coefficient v, R /2 t to dispersive coefficient β, then ( v/2 β)2 corresponds to 1 / R 2 e • Ri. We prove that the wave number corresponding to maximum energy spectrum S(k) decreases with the decrease of stability (i.e., the increase of ( v / 2 β)2 in eddy—containing region. And the spectrim amplitude decreases with the increase of ( v / 2 β)2 (i.e., the decrease of stability). These results are consistent with actual turbulence spectrum of atmospheric surface layer from turbulence data.
Microgravity Effects on Plant Boundary Layers
Stutte, Gary; Monje, Oscar
2005-01-01
The goal of these series of experiment was to determine the effects of microgravity conditions on the developmental boundary layers in roots and leaves and to determine the effects of air flow on boundary layer development. It is hypothesized that microgravity induces larger boundary layers around plant organs because of the absence of buoyancy-driven convection. These larger boundary layers may affect normal metabolic function because they may reduce the fluxes of heat and metabolically active gases (e.g., oxygen, water vapor, and carbon dioxide. These experiments are to test whether there is a change in boundary layer associated with microgravity, quantify the change if it exists, and determine influence of air velocity on boundary layer thickness under different gravity conditions.
Johnson, Perry L.; Shyam, Vikram
2012-01-01
A Large Eddy Simulation (LES) is performed of a high blowing ratio (M = 1.7) film cooling flow with density ratio of unity. Mean results are compared with experimental data to show the degree of fidelity achieved in the simulation. While the trends in the LES prediction are a noticeable improvement over Reynolds-Averaged Navier-Stokes (RANS) predictions, there is still a lack a spreading on the underside of the lifted jet. This is likely due to the inability of the LES to capture the full range of influential eddies on the underside of the jet due to their smaller structure. The unsteady structures in the turbulent coolant jet are also explored and related to turbulent mixing characteristics
Characteristics of the Boundary Layer Structure of Sea Fog on the Coast of Southern China
Institute of Scientific and Technical Information of China (English)
HUANG Huijun; LIU Hongnian; JIANG Weimei; HUANG Jian; MAO Weikang
2011-01-01
Using boundary layer data with regard to sea fog observed at the Science Experiment Base for Marine Meteorology at Bohe,Guangdong Province,the structure of the atmospheric boundary layer and the characteristics of the tops of the fog and the clouds were analyzed.In addition,the effects of advection,radiation,and turbulence during sea fog were also investigated.According to the stability definition of saturated,wet air,the gradient of the potential pseudo-equivalent temperature equal to zero was defined as the thermal turbulence interface.There is evidence to suggest that two layers of turbulence exist in sea fog.Thermal turbulence produced by long-wave radiation is prevalent above the thermal turbulence interface,whereas mechanical turbulence aroused by wind shear is predominant below the interface.The height of the thermal turbulence interface was observed between 180 m and 380 m.Three important factors are closely related to the development of the top of the sea fog:(1) the horizontal advection of the water vapor,(2) the long-wave radiation of the fog top,and (3) the movement of the vertical turbulence.Formation,development,and dissipation are the three possible phases of the evolution of the boundary-layer structure during the sea fog season.In addition,the thermal turbulence interface is the most significant turbulence interface during the formation and development periods; it is maintained after sea fog rises into the stratus layer.
An investigation of the effects of the propeller slipstream of a laminar wing boundary layer
Howard, R. M.; Miley, S. J.; Holmes, B. J.
1985-01-01
A research program is in progress to study the effects of the propeller slipstream on natural laminar flow. Flight and wind tunnel measurements of the wing boundary layer have been made using hot-film velocity sensor probes. The results show the boundary layer, at any given point, to alternate between laminar and turbulent states. This cyclic behavior is due to periodic external flow turbulence originating from the viscous wake of the propeller blades. Analytic studies show the cyclic laminar/turbulent boundary layer to result in a significantly lower wing section drag than a fully turbulent boundary layer. The application of natural laminar flow design philosophy yields drag reduction benefits in the slipstream affected regions of the airframe, as well as the unaffected regions.
Characterization of the Martian Convective Boundary Layer
Martínez, Germán; Valero Rodríguez, Francisco; Vázquez Martínez, Luis
2009-01-01
The authors have carried out an extensive characterization of the Martian mixed layer formed under convective conditions. The values of the mixed layer height, convective velocity scale, convective temperature scale, mean temperature standard deviation, mean horizontal and vertical velocity standard deviations, and mean turbulent viscous dissipation rate have been obtained during the strongest convective hours for the mixed layer. In addition, the existing database of the surface layer has be...
Role of the vertical pressure gradient in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård; Sumer, B. Mutlu; Vittori, Giovanna;
2014-01-01
By direct numerical simulation (DNS) of the flow in an oscillatory boundary layer, it is possible to obtain the pressure field. From the latter, the vertical pressure gradient is determined. Turbulent spots are detected by a criterion involving the vertical pressure gradient. The vertical pressure...... gradient is also treated as any other turbulence quantity like velocity fluctuations and statistical properties of the vertical pressure gradient are calculated from the DNS data. The presence of a vertical pressure gradient in the near bed region has significant implications for sediment transport....
Transition in Hypersonic Boundary Layers: Role of Dilatational Waves
Zhu, Yiding; Yuan, Huijing; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2015-01-01
Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 quiet wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. It is found that the second instability acoustic mode is the key modulator of the transition process. The second mode experiences a rapid growth and a very fast annihilation due to the effect of bulk viscosity. The second mode interacts strongly with the first vorticity mode to directly promote a fast growth of the latter and leads to immediate transition to turbulence.
A Thermal Plume Model for the Martian Convective Boundary Layer
Colaïtis, Arnaud; Hourdin, Frédéric; Rio, Catherine; Forget, François; Millour, Ehouarn
2013-01-01
The Martian Planetary Boundary Layer [PBL] is a crucial component of the Martian climate system. Global Climate Models [GCMs] and Mesoscale Models [MMs] lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically-based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in Large-Eddy Simulations [LESs]. We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking ...
Shifted periodic boundary conditions for simulations of wall-bounded turbulent flows
Munters, Wim; Meneveau, Charles; Meyers, Johan
2016-02-01
In wall-bounded turbulent flow simulations, periodic boundary conditions combined with insufficiently long domains lead to persistent spanwise locking of large-scale turbulent structures. This leads to statistical inhomogeneities of 10%-15% that persist in time averages of 60 eddy turnover times and more. We propose a shifted periodic boundary condition that eliminates this effect without the need for excessive streamwise domain lengths. The method is tested based on a set of direct numerical simulations of a turbulent channel flow, and large-eddy simulations of a high Reynolds number rough-wall half-channel flow. The method is very useful for precursor simulations that generate inlet conditions for simulations that are spatially inhomogeneous, but require statistically homogeneous inlet boundary conditions in the spanwise direction. The method's advantages are illustrated for the simulation of a developing wind-farm boundary layer.
Behaviour of tracer diffusion in simple atmospheric boundary layer models
Directory of Open Access Journals (Sweden)
P. S. Anderson
2007-10-01
Full Text Available 1-D profiles and time series from an idealised atmospheric boundary layer model are presented, which show agreement with boundary layer measurements of polar NO_{x}. Diffusion models are increasingly being used as the framework for studying tropospheric air chemistry dynamics. Models based on standard boundary layer diffusivity profiles have an intrinsic behaviour that is not necessarily intuitive, due to the variation of turbulent diffusivity with height. The simple model presented captures the essence of the evolution of a trace gas released at the surface, and thereby provides both a programming and a conceptual tool in the analysis of observed trace gas evolution. A time scale inherent in the model can be tuned by fitting model time series to observations. This scale is then applicable to the more physically simple but chemically complex zeroth order or box models of chemical interactions.
Large Eddy Simulation of the ventilated wave boundary layer
DEFF Research Database (Denmark)
Lohmann, Iris P.; Fredsøe, Jørgen; Sumer, B. Mutlu; Christensen, Erik Damgaard
2006-01-01
A Large Eddy Simulation (LES) of (1) a fully developed turbulent wave boundary layer and (2) case 1 subject to ventilation (i.e., suction and injection varying alternately in phase) has been performed, using the Smagorinsky subgrid-scale model to express the subgrid viscosity. The model was found...... overall (local) grid size. The results indicate that the large eddies develop in the resolved scale, corresponding to fluid with an effective viscosity decided by the sum of the kinematic and subgrid viscosity. Regarding case 2, the results are qualitatively in accordance with experimental findings....... Injection generally slows down the flow in the full vertical extent of the boundary layer, destabilizes the flow and decreases the mean bed shear stress significantly; whereas suction generally speeds up the flow in the full vertical extent of the boundary layer, stabilizes the flow and increases the mean...
EFFECT OF COOLED BOUNDARY ON THE TURBULENT STRUCTURE
Institute of Scientific and Technical Information of China (English)
Li Guo-xiang; Mao Hua-yong; Li Na
2003-01-01
The flow field in the cooled channel of a heat exchanger was measured using the X-type film probes of Hot Wire/Firm Anemotheter, and the turbulent mechanism was discussed. It is concluded that the airflow is cooled in the flow process, the distribution of the turbulent intensity is relatively convergent near the centerline and the boundary, the constriction action produced due to heat release at the foot of the fins causes u to decrease and w to increase near the root downstream. It is concluded that the turbulent flow with cooled boundary results from the balance of production, dissipation and intermittency caused by constriction action.
A thermal plume model for the Martian convective boundary layer
Colaïtis, A.; Spiga, A.; Hourdin, F.; Rio, C.; Forget, F.; Millour, E.
2013-07-01
The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the "thermal plume" model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single-column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large-eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near-surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface-atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near-surface lander measurements. Using a thermal plume model moreover enables a first-order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.
Application of Viscothermal Wave Propagation Theory for Reduction of Boundary Layer Induced Noise
Wijnant, Y.H.; Hannink, M.H.C.; Boer, de A.
2003-01-01
Boundary layer induced noise, i.e. noise inside the aircraft resulting from the turbulent boundary layer enclosing the fuselage, is known to dominate air-cabin noise at cruise conditions. In this paper a method is described to design trim panels containing a large number of coupled tubes to effectiv
BUBBLE - an urban boundary layer meteorology project
DEFF Research Database (Denmark)
Rotach, M.W.; Vogt, R.; Bernhofer, C.;
2005-01-01
The Basel urban Boundary Layer Experiment (BUBBLE) was a year-long experimental effort to investigate in detail the boundary layer structure in the City of Basel, Switzerland. At several sites over different surface types (urban, sub-urban and rural reference) towers up to at least twice the main...
Experimental investigation of wave boundary layer
DEFF Research Database (Denmark)
Sumer, B. Mutlu
2003-01-01
A review is presented of experimental investigation of wave boundary layer. The review is organized in six main sections. The first section describes the wave boundary layer in a real-life environment and its simulation in the laboratory in an oscillating water tunnel and in a water tank with an ...
Magnetohydrodynamic cross-field boundary layer flow
Directory of Open Access Journals (Sweden)
D. B. Ingham
1982-01-01
Full Text Available The Blasius boundary layer on a flat plate in the presence of a constant ambient magnetic field is examined. A numerical integration of the MHD boundary layer equations from the leading edge is presented showing how the asymptotic solution described by Sears is approached.
On the Effects of Surface Roughness on Boundary Layer Transition
Choudhari, Meelan M.; Li, Fei; Chang, Chau-Lyan; Edwards, Jack
2009-01-01
Surface roughness can influence laminar-turbulent transition in many different ways. This paper outlines selected analyses performed at the NASA Langley Research Center, ranging in speed from subsonic to hypersonic Mach numbers and highlighting the beneficial as well as adverse roles of the surface roughness in technological applications. The first theme pertains to boundary-layer tripping on the forebody of a hypersonic airbreathing configuration via a spanwise periodic array of trip elements, with the goal of understanding the physical mechanisms underlying roughness-induced transition in a high-speed boundary layer. The effect of an isolated, finite amplitude roughness element on a supersonic boundary layer is considered next. The other set of flow configurations examined herein corresponds to roughness based laminar flow control in subsonic and supersonic swept wing boundary layers. A common theme to all of the above configurations is the need to apply higher fidelity, physics based techniques to develop reliable predictions of roughness effects on laminar-turbulent transition.
Turbulent transport in the atmospheric surface layer
Energy Technology Data Exchange (ETDEWEB)
Tagesson, Torbern [Dept. of Physical Geography and Ecosystem Science, Lund Univ., Lund (Sweden)
2012-04-15
In the modelling of transport and accumulation of the radioactive isotope carbon-14 (C-14) in the case of a potential release from a future repository of radioactive waste, it is important to describe the transport of the isotope in the atmosphere. This report aims to describe the turbulent transport within the lower part of the atmosphere; the inertial surface layer and the roughness sublayer. Transport in the inertial surface layer is dependent on several factors, whereof some can be neglected under certain circumstances. Under steady state conditions, fully developed turbulent conditions, in flat and horizontal homogeneous areas, it is possible to apply an eddy diffusivity approach for estimating vertical transport of C. The eddy diffusivity model assumes that there is proportionality between the vertical gradient and the transport of C. The eddy diffusivity is depending on the atmospheric turbulence, which is affected by the interaction between mean wind and friction of the ground surface and of the sensible heat flux in the atmosphere. In this report, it is described how eddy diffusivity of the inertial surface layer can be estimated from 3-d wind measurements and measurements of sensible heat fluxes. It is also described how to estimate the eddy diffusivity in the inertial surface layer from profile measurements of temperature and wind speed. Close to the canopy, wind and C profiles are influenced by effects of the surface roughness; this section of the atmosphere is called the roughness sublayer. Its height is up to {approx}3 times the height of the plant canopy. When the mean wind interacts with the canopy, turbulence is not only produced by shear stress and buoyancy, it is additionally created by wakes, which are formed behind the plants. Turbulence is higher than it would be over a flat surface, and the turbulent transport is hereby more efficient. Above the plant canopy, but still within the roughness sublayer, a function that compensates for the effect
Plasma boundary layer and magnetopause layer of the earth's magnetosphere
International Nuclear Information System (INIS)
IMP 6 observations of the plasma boundary layer (PBL) and magnetopause layer (MPL) of the earth's magnetosphere indicate that plasma in the low-latitude portion of the PBL is supplied primarily by direct transport of magnetosheath plasma across the MPL and that this transport process is relatively widespread over the entire sunward magnetospheric boundary
Optimizing EDMF parameterization for stratocumulus-topped boundary layer
Jones, C. R.; Bretherton, C. S.; Witek, M. L.; Suselj, K.
2014-12-01
We present progress in the development of an Eddy Diffusion / Mass Flux (EDMF) turbulence parameterization, with the goal of improving the representation of the cloudy boundary layer in NCEP's Global Forecast System (GFS), as part of a multi-institution Climate Process Team (CPT). Current GFS versions substantially under-predict cloud amount and cloud radiative impact over much of the globe, leading to large biases in the surface and top of atmosphere energy budgets. As part of the effort to correct these biases, the CPT is developing a new EDMF turbulence scheme for GFS, in which local turbulent mixing is represented by an eddy diffusion term while nonlocal shallow convection is represented by a mass flux term. The sum of both contributions provides the total turbulent flux. Our goal is for this scheme to more skillfully simulate cloud radiative properties without negatively impacting other measures of weather forecast skill. One particular challenge faced by an EDMF parameterization is to be able to handle stratocumulus regimes as well as shallow cumulus regimes. In order to isolate the behavior of the proposed EDMF parameterization and aid in its further development, we have implemented the scheme in a portable MATLAB single column model (SCM). We use this SCM framework to optimize the simulation of stratocumulus cloud top entrainment and boundary layer decoupling.
Calculation of transitional boundary layer under pressure gradient
International Nuclear Information System (INIS)
A modified κ-ε model is proposed for calculation of transitional boundary-layer flows under pressure gradient with high freestream turbulence intensity. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in a stream-wise intermittency factor, bridging the eddy-viscosities between in the pre-transition region and in the fully turbulent region. From intermittency data in various flows, Narashima's intermittency function, F(γ), has been found to be proportional to xn according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition under pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted
Boundary Layer Flow Over a Moving Wavy Surface
Hendin, Gali; Toledo, Yaron
2016-04-01
Boundary Layer Flow Over a Moving Wavy Surface Gali Hendin(1), Yaron Toledo(1) January 13, 2016 (1)School of Mechanical Engineering, Tel-Aviv University, Israel Understanding the boundary layer flow over surface gravity waves is of great importance as various atmosphere-ocean processes are essentially coupled through these waves. Nevertheless, there are still significant gaps in our understanding of this complex flow behaviour. The present work investigates the fundamentals of the boundary layer air flow over progressive, small-amplitude waves. It aims to extend the well-known Blasius solution for a boundary layer over a flat plate to one over a moving wavy surface. The current analysis pro- claims the importance of the small curvature and the time-dependency as second order effects, with a meaningful impact on the similarity pattern in the first order. The air flow over the ocean surface is modelled using an outer, inviscid half-infinite flow, overlaying the viscous boundary layer above the wavy surface. The assumption of a uniform flow in the outer layer, used in former studies, is now replaced with a precise analytical solution of the potential flow over a moving wavy surface with a known celerity, wavelength and amplitude. This results in a conceptual change from former models as it shows that the pressure variations within the boundary layer cannot be neglected. In the boundary layer, time-dependent Navier-Stokes equations are formulated in a curvilinear, orthogonal coordinate system. The formulation is done in an elaborate way that presents additional, formerly neglected first-order effects, resulting from the time-varying coordinate system. The suggested time-dependent curvilinear orthogonal coordinate system introduces a platform that can also support the formulation of turbulent problems for any surface shape. In order to produce a self-similar Blasius-type solution, a small wave-steepness is assumed and a perturbation method is applied. Consequently, a
Defining the Entrainment Zone in Stratocumulus-topped Boundary Layers
Wang, Q.; Zhou, M.; Kalogiros, J. A.; Lenschow, D. H.; Dai, C.; Wang, S.
2010-12-01
The presence of an entrainment zone near the top of the stratocumulus-topped boundary layers has been identified by many early studies. However, the definition of the entrainment zone was rather vague. We have examined the fine vertical variations of cloud liquid water content, wind, temperature and humidity near the stratocumulus top and developed a new method to identify the entrainment zone objectively. Aircraft measurements from various field projects in stratocumulus-topped boundary layers are used, taking advantage of the fast sampling capability of many of the aircraft sensors. Because of the inhomogeneous mixing of two air masses with distinctively different thermodynamic properties, the magnitude of temperature perturbations within the entrainment zone is significantly larger than those above or below. This characteristics is used to define the upper and lower boundaries of the entrainment zone using a wavelet spectra analyses. The definition of the entrainment zone is further evaluated by the presence of a linear mixing line through mixing line analyses. Various other interfaces at the cloud top are also examined, including the cloud interface, temperature interface (inversion), and moisture interface. The heights of these interfaces are examined relative to the height of the entrainment zone. This study also systematically revealed the presence of turbulence above the local cloud top and/or above the entrainment zone. Wind shear near the cloud top is one possible source that generated local turbulence. Other potential sources of turbulence will also be discussed.
Two-phase boundary layer prediction in upward boiling flow
International Nuclear Information System (INIS)
In the present work, the numerical modelling of the two-phase turbulent boundary layer in upward boiling flow was investigated. First, non-dimensional liquid velocity and temperature profiles in the two-phase boundary layer were validated on the one-dimensional section of a pipe with prescribed radial void fraction profiles. Simulations were performed on a fine grid with a commercial code CFX-5 using the k-ω turbulence model. A significant deviation of results from the analytical single-phase and two-phase wall functions from the literature was observed. Second, a wall boiling model in a vertical heated pipe was simulated (CFX-5) on the coarse grid. Here the prediction of the two-phase thermal boudary layer was compared to the experimental data, k-ω calculation on the fine grid and against the singlephase analytical wall function. Again a major deviation against single-phase temperature wall function was obtained. Presented analyses suggest that the existing analytical velocity and temperature wall functions cannot be valid for the boiling boundary layer with the high void fraction on the wall. (author)
Grey zone simulations of the morning convective boundary layer development
Efstathiou, G. A.; Beare, R. J.; Osborne, S.; Lock, A. P.
2016-05-01
Numerical simulations of two cases of morning boundary layer development are conducted to investigate the impact of grid resolution on mean profiles and turbulent kinetic energy (TKE) partitioning from the large eddy simulation (LES) to the mesoscale limit. Idealized LES, using the 3-D Smagorinsky scheme, is shown to be capable of reproducing the boundary layer evolution when compared against measurements. However, increasing grid spacing results in the damping of resolved TKE and the production of superadiabatic temperature profiles in the boundary layer. Turbulence initiation is significantly delayed, exhibiting an abrupt onset at intermediate resolutions. Two approaches, the bounding of vertical diffusion coefficient and the blending of the 3-D Smagorinsky with a nonlocal 1D scheme, are used to model subgrid diffusion at grey zone resolutions. Simulations are compared against the coarse-grained fields from the validated LES results for each case. Both methods exhibit particular strengths and weaknesses, indicating the compromise that needs to be made currently in high-resolution numerical weather prediction. The blending scheme is able to reproduce the adiabatic profiles although turbulence is underestimated in favor of the parametrized heat flux, and the spin-up of TKE remains delayed. In contrast, the bounding approach gives an evolution of TKE that follows the coarse-grained LES very well, relying on the resolved motions for the nonlocal heat flux. However, bounding gives unrealistic static instability in the early morning temperature profiles (similar to the 3-D Smagorinsky scheme) because model dynamics are unable to resolve TKE when the boundary layer is too shallow compared to the grid spacing.
Exploring the possible role of small scale terrain drag on stable boundary layers over land
Steeneveld, G. J.; Holtslag, A.A.M.; C. J. Nappo; Wiel, van de, C.C.M.; Mahrt, L.
2008-01-01
This paper addresses the possible role of unresolved terrain drag, relative to the turbulent drag on the development of the stable atmospheric boundary layer over land. Adding a first-order estimate for terrain drag to the turbulent drag appears to provide drag that is similar to the enhanced turbulent drag obtained with the so-called long-tail mixing functions. These functions are currently used in many operational models for weather and climate, although they lack a clear physical basis. Co...
Vortex statistics for turbulence in a container with rigid boundaries
DEFF Research Database (Denmark)
Clercx, H.J.H.; Nielsen, A.H.
2000-01-01
The evolution of vortex statistics for decaying two-dimensional turbulence in a square container with rigid no-slip walls is compared with a few available experimental results and with the scaling theory of two-dimensional turbulent decay as proposed by Carnevale et al. Power-law exponents, compu......, computed from an ensemble average of several numerical runs, coincide with some experimentally obtained values, but not with data obtained from numerical simulations of decaying two-dimensional turbulence with periodic boundary conditions....
Boundary layer physics over snow and ice
Directory of Open Access Journals (Sweden)
P. S. Anderson
2008-07-01
Full Text Available Observations of the unique chemical environment over snow and ice in recent decades, particularly in the polar regions, have stimulated increasing interest in the boundary layer processes that mediate exchanges between the ice/snow interface and the atmosphere. This paper provides a review of the underlying concepts and examples from recent field studies in polar boundary layer meteorology, which will generally apply to atmospheric flow over snow and ice surfaces. It forms a companion paper to the chemistry review papers in this special issue of ACP that focus on processes linking halogens to the depletion of boundary layer ozone in coastal environments, mercury transport and deposition, snow photochemistry, and related snow physics. In this context, observational approaches, stable boundary layer behavior, the effects of a weak or absent diurnal cycle, and transport and mixing over the heterogeneous surfaces characteristic of coastal ocean environments are of particular relevance.
Clear-air radar observations of the atmospheric boundary layer
Ince, Turker
2001-10-01
This dissertation presents the design and operation of a high-resolution frequency-modulated continuous-wave (FM- CW) radar system to study the structure and dynamics of clear-air turbulence in the atmospheric boundary layer (ABL). This sensitive radar can image the vertical structure of the ABL with both high spatial and temporal resolutions, and provide both qualitative information about the morphology of clear-air structures and quantitative information on the intensity of fluctuations in refractive-index of air. The principles of operation and the hardware and data acquisition characteristics of the radar are described in the dissertation. In October 1999, the radar participated in the Cooperative Atmosphere-Surface Exchange Study (CASES'99) Experiment to characterize the temporal structure and evolution of the boundary-layer features in both convective and stable conditions. The observed structures include clear-air convection, boundary layer evolution, gravity waves, Kelvin-Helmholtz instabilities, stably stratified layers, and clear-air turbulence. Many of the S-band radar images also show high- reflectivity returns from Rayleigh scatterers such as insects. An adaptive median filtering technique based on local statistics has, therefore, been developed to discriminate between Bragg and Rayleigh scattering in clear-air radar observations. The filter is tested on radar observations of clear air convection with comparison to two commonly used image processing techniques. The dissertation also examines the statistical mean of the radar-measured C2n for clear-air convection, and compares it with the theoretical predictions. The study also shows that the inversion height, local thickness of the inversion layer, and the height of the elevated atmospheric layers can be estimated from the radar reflectivity measurements. In addition, comparisons to the radiosonde-based height estimates are made. To examine the temporal and spatial structure of C2n , the dissertation
Characterization of internal boundary layer capacitors
International Nuclear Information System (INIS)
Internal boundary layer capacitors were characterized by scanning transmission electron microscopy and by microscale electrical measurements. Data are given for the chemical and physical characteristics of the individual grains and boundaries, and their associated electric and dielectric properties. Segregated internal boundary layers were identified with resistivities of 1012-1013 Ω-cm. Bulk apparent dielectric constants were 10,000-60,000. A model is proposed to explain the dielectric behavior in terms of an equivalent n-c-i-c-n representation of ceramic microstructure, which is substantiated by capacitance-voltage analysis
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
2012-01-01
simulation and the boundary layer shape will be modified due to the interaction of the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [1] while having the advantage of resolving the wall layer with a coarser grid......Large eddy simulation (LES) of flow in a wind farm is studied in neutral as well as thermally stratified atmospheric boundary layer (ABL). An approach has been practiced to simulate the flow in a fully developed wind farm boundary layer. The approach is based on the Immersed Boundary Method (IBM...
The height of the atmospheric boundary layer during unstable conditions
Energy Technology Data Exchange (ETDEWEB)
Gryning, S.E.
2005-11-01
The height of the convective atmospheric boundary layer, also called the mixed-layer, is one of the fundamental parameters that characterise the structure of the atmosphere near the ground. It has many theoretical and practical applications such as the prediction of air pollution concentrations, surface temperature and the scaling of turbulence. However, as pointed out by Builtjes (2001) in a review paper on Major Twentieth Century Milestones in Air Pollution Modelling and Its Application, the weakest point in meteorology data is still the determination of the height of the mixed-layer, the so-called mixing height. A simple applied model for the height of the mixed-layer over homogeneous terrain is suggested in chapter 2. It is based on a parameterised budget for the turbulent kinetic energy. In the model basically three terms - the spin-up term and the production of mechanical and convective turbulent kinetic energy - control the growth of the mixed layer. The interplay between the three terms is related to the meteorological conditions and the height of the mixed layer. A stable layer, the so-called entrainment zone, which is confined between the mixed layer and the free air above, caps the mixed layer. A parameterisation of the depth of the entrainment zone is also suggested, and used to devise a combined model for the height of the mixed layer and the entrainment zone. Another important aspect of the mixed layer development exists in coastal areas where an internal boundary layer forms downwind from the coastline. A model for the growth of the internal boundary layer is developed in analogy with the model for mixed layer development over homogeneous terrain. The strength of this model is that it can operate on a very fine spatial resolution with minor computer resources. Chapter 3 deals with the validation of the models. It is based in parts on data from the literature, and on own measurements. For the validation of the formation of the internal boundary layer
Behaviour of tracer diffusion in simple atmospheric boundary layer models
Directory of Open Access Journals (Sweden)
P. S. Anderson
2006-12-01
Full Text Available 1-D profiles and time series from an idealised atmospheric boundary layer model are presented, which show agreement with measurements of polar photogenic NO and NO_{2}. Diffusion models are increasingly being used as the framework for studying tropospheric air chemistry dynamics. Models based on standard boundary layer diffusivity profiles have an intrinsic behaviour that is not necessarily intuitive, due to the variation of turbulent diffusivity with height. The relatively simple model provides both a programming and a conceptual tool in the analysis of observed trace gas evolution. A time scale inherent in the model can be tuned by fitting model time series to observations. This scale is then applicable to the more physically simple but chemically complex zeroth order or box models of chemical interactions.
Numerical modeling of the transitional boundary layer over a flat plate
Ivanov, Dimitry; Chorny, Andrei
2015-11-01
Our example is connected with fundamental research on understanding how an initially laminar boundary layer becomes turbulent. We have chosen the flow over a flat plate as a prototype for boundary-layer flows around bodies. Special attention was paid to the near-wall region in order to capture all levels of the boundary layer. In this study, the numerical software package OpenFOAM has been used in order to solve the flow field. The results were used in a comparative study with data obtained from Large Eddy Simulation (LES). The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. Presently this model is extended to the LES of the zero-pressure gradient, flat-plate turbulent boundary layer. In current study the time discretization is based on a second order Crank-Nicolson/Adams-Bashforth method. LES solver using Smagorinsky and the one-equation LES turbulence models. The transition models significantly improve the prediction of the onset location compared to the fully turbulent models.LES methods appear to be the most promising new tool for the design and analysis of flow devices including transition regions of the turbulent flow.
Role of the basin boundary conditions in gravity wave turbulence
Deike, Luc; Gutiérrez-Matus, Pablo; Jamin, Timothée; Semin, Benoit; Aumaitre, Sébastien; Berhanu, Michael; Falcon, Eric; BONNEFOY, Félicien
2014-01-01
Gravity wave turbulence is studied experimentally in a large wave basin where irregular waves are generated unidirectionally. The role of the basin boundary conditions (absorbing or reflecting) and of the forcing properties are investigated. To that purpose, an absorbing sloping beach opposite to the wavemaker can be replaced by a reflecting vertical wall. We observe that the wave field properties depend strongly on these boundary conditions. Quasi-one dimensional field of nonlinear waves propagate before to be damped by the beach whereas a more multidirectional wave field is observed with the wall. In both cases, the wave spectrum scales as a frequency-power law with an exponent that increases continuously with the forcing amplitude up to a value close to -4, which is the value predicted by the weak turbulence theory. The physical mechanisms involved are probably different according to the boundary condition used, but cannot be easily discriminated with only temporal measurements. We have also studied freely...
Problems of matter-antimatter boundary layers
International Nuclear Information System (INIS)
This paper outlines the problems of the quasi-steady matter-antimatter boundary layers discussed in Klein-Alfven's cosmological theory, and a crude model of the corresponding ambiplasma balance is presented: (i) at interstellar particle densities, no well-defined boundary layer can exist in presence of neutral gas, nor can such a layer be sustained in an unmagnetized fully ionized ambiplasma. (ii) Within the limits of applicability of the present model, sharply defined boundary layers are under certain conditions found to exist in a magnetized ambiplasma. Thus, at beta values less than unity, a steep pressure drop of the low-energy components of matter and antimatter can be balanced by a magnetic field and the electric currents in the ambiplasma. (iii) The boundary layer thickness is of the order of 2x0 approximately 10/BT0sup(1/4) meters, where B is the magnetic field strength in MKS units and T0 the characteristic temperature of the low-energy components in the layer. (Auth.)
On Hairpin Vortices in a Transitional Boundary Layer
Czech Academy of Sciences Publication Activity Database
Hladík, Ondřej; Jonáš, Pavel; Uruba, Václav
Liberec : Technical University of Liberec, 2011 - (Vít, T.; Dančová, P.; Novotný, P.), s. 163-170 ISBN 978-80-7372-784-0. - (Vol. 2). [Experimental Fluid Mechanics 2011. Jičín (CZ), 22.11.2011-25.11.2011] R&D Projects: GA ČR GA101/08/1112; GA ČR GAP101/10/1230 Institutional research plan: CEZ:AV0Z20760514 Keywords : turbulence transition * boundary layer * hairpin vortex Subject RIV: BK - Fluid Dynamics http:// orion .kez.tul.cz/efm/
Large Eddy Simulation and Study of the Urban Boundary Layer
Institute of Scientific and Technical Information of China (English)
苗世光; 蒋维楣
2004-01-01
Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5-3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.
Boundary layer heights derived from velocity spectra
Energy Technology Data Exchange (ETDEWEB)
Hoejstrup, J.; Barthelmie, R.J. [Risoe National Lab., Roskilde (Denmark); Kaellstrand, B. [Univ. of Uppsala, Uppsala (Sweden)
1997-10-01
It is a well-known fact that the height of the mixed layer determines the size of the largest and most energetic eddies that can be observed in the unstable boundary layer, and consequently a peak can be observed in the power spectra of the along-wind velocity component at scales comparable to the mixed layer depth. We will now show how the mixed layer depth can be derived from the u-specta and the results will be compared with direct measurements using pibal and tethersonde measurements. (au)
A Qualitative Description of Boundary Layer Wind Speed Records
Kavasseri, R G; Nagarajan, Radhakrishnan
2006-01-01
The complexity of the atmosphere endows it with the property of turbulence by virtue of which, wind speed variations in the atmospheric boundary layer (ABL) exhibit highly irregular fluctuations that persist over a wide range of temporal and spatial scales. Despite the large and significant body of work on microscale turbulence, understanding the statistics of atmospheric wind speed variations has proved to be elusive and challenging. Knowledge about the nature of wind speed at ABL has far reaching impact on several fields of research such as meteorology, hydrology, agriculture, pollutant dispersion, and more importantly wind energy generation. In the present study, temporal wind speed records from twenty eight stations distributed through out the state of North Dakota (ND, USA), ($\\sim$ 70,000 square-miles) and spanning a period of nearly eight years are analyzed. We show that these records exhibit a characteristic broad multifractal spectrum irrespective of the geographical location and topography. The rapi...
Vertical pressure gradient and particle motions in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård
is a function of phase. Therefore the particle will settle towards the end of each half period, and after flow reversal, when the turbulent intensity becomes large enough it can be suspended. If the particle is light enough it can be maintained in suspension, otherwise it will settle before it is....... This is in contrast to velocity fluctuations that are diffusive, so they can also contain residual turbulence from the previous half cycle until they are dissipated. Furthermore, the magnitude of the mean value of conditionally averaged vertical pressure gradient (for −∂p∗/∂x∗ 2 > 0) is compared to the...... submerged weight of sediment. This revels that the upward directed vertical pressure gradient on average has a magnitude that yields in a contribution to the force needed to overcome the submerged weight of the water-sediment mixture. Secondly particle motion in the oscillatory boundary layer is...
Self-similar magnetohydrodynamic boundary layers
Energy Technology Data Exchange (ETDEWEB)
Nunez, Manuel; Lastra, Alberto, E-mail: mnjmhd@am.uva.e [Departamento de Analisis Matematico, Universidad de Valladolid, 47005 Valladolid (Spain)
2010-10-15
The boundary layer created by parallel flow in a magnetized fluid of high conductivity is considered in this paper. Under appropriate boundary conditions, self-similar solutions analogous to the ones studied by Blasius for the hydrodynamic problem may be found. It is proved that for these to be stable, the size of the Alfven velocity at the outer flow must be smaller than the flow velocity, a fact that has a ready physical explanation. The process by which the transverse velocity and the thickness of the layer grow with the size of the Alfven velocity is detailed.
Kamardin, A. P.; Nevzorova, I. V.; Odintsov, S. L.
2015-11-01
In the work, we consider estimates of the height of layer of intense turbulent heat exchange in stably stratified atmospheric boundary layer, made with the use of meteorological acoustic radar (sodar). Dependence of this height on temperature gradient is analyzed. Current temperature stratification of the atmosphere in the layer with height up to 1 000 m was determined with the help of MTP-5 meteorological temperature profiler.
Directory of Open Access Journals (Sweden)
H. Pietersen
2014-07-01
Full Text Available We study the disturbances of CBL dynamics due to large-scale atmospheric contributions for a representative day observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST campaign. We first reproduce the observed boundary-layer dynamics by combining the Dutch Atmospheric Large-Eddy Simulation (DALES model with a mixed-layer theory based model. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70%. If we constrain our numerical experiments with the BLLAST comprehensive data set, we are able to quantify the contributions of advection of heat and moisture, and subsidence. We find that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. Finally, we study whether the vertical and temporal evolution of turbulent variables are influenced by these large-scale forcings. Our model results show good correspondence of the vertical structure of turbulent variables with observations. Our findings further indicate that when large-scale advection and subsidence are applied, the values for turbulent kinetic are lower than without these large-scale forcings. We conclude that the prototypical CBL can still be used as a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are well characterized.
Turbulent transport across shear layers in magnetically confined plasmas
International Nuclear Information System (INIS)
Shear layers modify the turbulence in diverse ways and do not only suppress it. A spatial-temporal investigation of gyrofluid simulations in comparison with experiments allows to identify further details of the transport process across shear layers. Blobs in and outside a shear layer merge, thereby exchange particles and heat and subsequently break up. Via this mechanism particles and heat are transported radially across shear layers. Turbulence spreading is the immanent mechanism behind this process
Statistics and scaling of turbulence in a spatially developing mixing layer at Reλ = 250
Attili, Antonio
2012-03-21
The turbulent flow originating from the interaction between two parallel streams with different velocities is studied by means of direct numerical simulation. Rather than the more common temporal evolving layer, a spatially evolving configuration, with perturbed laminar inlet conditions is considered. The streamwise evolution and the self-similar state of turbulence statistics are reported and compared to results available in the literature. The characteristics of the transitional region agree with those observed in other simulations and experiments of mixing layers originating from laminar inlets. The present results indicate that the transitional region depends strongly on the inlet flow. Conversely, the self-similar state of turbulent kinetic energy and dissipation agrees quantitatively with those in a temporal mixing layer developing from turbulent initial conditions [M. M. Rogers and R. D. Moser, “Direct simulation of a self-similar turbulent mixing layer,” Phys. Fluids6, 903 (1994)]. The statistical features of turbulence in the self-similar region have been analysed in terms of longitudinal velocity structure functions, and scaling exponents are estimated by applying the extended self-similarity concept. In the small scale range (60 < r/η < 250), the scaling exponents display the universal anomalous scaling observed in homogeneous isotropic turbulence. The hypothesis of isotropy recovery holds in the turbulent mixing layer despite the presence of strong shear and large-scale structures, independently of the means of turbulence generation. At larger scales (r/η > 400), the mean shear and large coherent structures result in a significant deviation from predictions based on homogeneous isotropic turbulence theory. In this second scaling range, the numerical values of the exponents agree quantitatively with those reported for a variety of other flows characterized by strong shear, such as boundary layers, as well as channel and wake flows.
Boundary-layer theory for blast waves
Kim, K. B.; Berger, S. A.; Kamel, M. M.; Korobeinikov, V. P.; Oppenheim, A. K.
1975-01-01
It is profitable to consider the blast wave as a flow field consisting of two regions: the outer, which retains the properties of the inviscid solution, and the inner, which is governed by flow equations including terms expressing the effects of heat transfer and, concomitantly, viscosity. The latter region thus plays the role of a boundary layer. Reported here is an analytical method developed for the study of such layers, based on the matched asymptotic expansion technique combined with patched solutions.
Bypass transition of the bottom boundary layer under solitary wave
Sadek, Mahmoud; Diamessis, Peter; Parras, Luis; Liu, Philip
2015-11-01
The transition to turbulence in the bottom boundary layer (BBL) flow driven by a soliton-like pressure gradient in an oscillating water tunnel (an approximation for the BBL under solitary waves) is investigated using hydrodynamic linear stability theory and DNS. As observed in the laboratory experiment by Sumer et al. (2010), two possible transition scenarios exist. The first scenario is associated with the classical transition resulting from the breakdown of the exponentially growing 2-D Tollmien-Schlichting waves. The alternative scenario; i.e., bypass transition; takes place through formation of localized turbulent spots. The investigation of the latter transition scenario is performed in two steps. The first step consists of reformulating the linear stability analysis in the non-modal framework for the purpose of finding the optimum disturbance characteristics which lead to the formation of those turbulent spots. In the second step, the computed optimum noise structure is inserted in the 3D DNS in order to induce the formation of the turbulent spots and effectively simulate the bypass transition observed experimentally.
Allaerts, Dries; Meyers, Johan
2014-06-01
In this study we consider large wind farms in a conventionally neutral atmospheric boundary layer. In large wind farms the energy extracted by the turbines is dominated by downward vertical turbulent transport of kinetic energy from the airflow above the farm. However, atmospheric boundary layers are almost always capped by an inversion layer which slows down the entrainment rate and counteracts boundary layer growth. In a suite of large eddy simulations the effect of the strength of the capping inversion on the boundary layer and on the performance of a large wind farm is investigated. For simulations with and without wind turbines the results indicate that the boundary layer growth is effectively limited by the capping inversion and that the entrainment rate depends strongly on the inversion strength. The power output of wind farms is shown to decrease for increasing inversions.
Statistical characterization of turbulence in the boundary plasma of EAST
DEFF Research Database (Denmark)
Yan, Ning; Nielsen, Anders Henry; Xu, G.S.;
2013-01-01
In Ohmic heated low confinement mode (L-mode) discharges, the intermittent statistical characteristics of turbulent fluctuations have been investigated in the edge and the scrape-off layer (SOL) plasma on EAST (the experimental advanced superconducting tokamak) by fast reciprocating Langmuir probe...
Lagrangian properties of the entrainment across turbulent/non-turbulent interface layers
Watanabe, Tomoaki; da Silva, Carlos B.; Sakai, Yasuhiko; Nagata, Koji; Hayase, Toshiyuki
2016-03-01
Lagrangian statistics obtained from direct numerical simulations of turbulent planar jets and mixing layers are reported for the separation distance between the tracer particles at the outer edge of the turbulent/non-turbulent interface layer, and the entrained fluid particles. In the viscous superlayer (VSL) the mean square particle distance exhibits a ballistic evolution, while the Richardson-like scaling for relative dispersion prevails inside the turbulent sublayer (TSL). The results further support the existence of two different regimes within the interface layer, where small-scale outward enstrophy diffusion governs the entrained particles in the VSL, while inviscid small-scale motions govern the TSL.
UK-ADMS - a new approach to modelling dispersion in the earth's atmospheric boundary layer
International Nuclear Information System (INIS)
The UK Atmospheric Dispersion Modelling System is described in considerable detail. The principle modules are dealt with. A key to the methodology is that vertical profiles of mean velocity, temperature and turbulence in the boundary layer depend on the relative values of the height above the ground, the height of the boundary layer, and a length scale determined by the friction velocity and the heat flux and air temperature at the surface. The models can be used at any location. (AB) (15 refs.)
Boundary Layer Measurements of the NACA0015 and Implications for Noise Modeling
Bertagnolio, Franck
2011-01-01
A NACA0015 airfoil section instrumented with an array of high frequency microphones flush-mounted beneath its surface was measured in the wind tunnel at LM Wind Power in Lunderskov. Various inflow speeds and angles of attack were investigated. In addition, a hot-wire device system was used to measure the velocity profiles and turbulence characteristics in the boundary layer near the trailing edge of the airfoil. The measured boundary layer data are presented in this report and compared with C...
Thick diffusion limit boundary layer test problems
International Nuclear Information System (INIS)
We develop two simple test problems that quantify the behavior of computational transport solutions in the presence of boundary layers that are not resolved by the spatial grid. In particular we study the quantitative effects of 'contamination' terms that, according to previous asymptotic analyses, may have a detrimental effect on the solutions obtained by both discontinuous finite element (DFEM) and characteristic-method (CM) spatial discretizations, at least for boundary layers caused by azimuthally asymmetric incident intensities. Few numerical results have illustrated the effects of this contamination, and none have quantified it to our knowledge. Our test problems use leading-order analytic solutions that should be equal to zero in the problem interior, which means the observed interior solution is the error introduced by the contamination terms. Results from DFEM solutions demonstrate that the contamination terms can cause error propagation into the problem interior for both orthogonal and non-orthogonal grids, and that this error is much worse for non-orthogonal grids. This behavior is consistent with the predictions of previous analyses. We conclude that these boundary layer test problems and their variants are useful tools for the study of errors that are introduced by unresolved boundary layers in diffusive transport problems. (authors)
DYNAMICS OF A BOUNDARY LAYER SEPARATION
Czech Academy of Sciences Publication Activity Database
Uruba, Václav; Knob, Martin
2009-01-01
Roč. 16, č. 1 (2009), s. 29-38. ISSN 1802-1484 R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * triple-deck theory * Time-Resolved PIV Subject RIV: BK - Fluid Dynamics
Analysis of Laminar Boundary Layer Equations
Directory of Open Access Journals (Sweden)
R. Yesman
2012-01-01
Full Text Available The paper proposes methodology for analysis and calculation of laminar fluid flow processes in a boundary layer.The presented dependences can be used for practical calculations while power carriers of various application are moving in the channels of heat and power devices.
Global stability analysis of axisymmetric boundary layers
Vinod, N
2016-01-01
This paper presents the linear global stability analysis of the incompressible axisymmetric boundary layer on a circular cylinder. The base flow is parallel to the axis of the cylinder at inlet. The pressure gradient is zero in the streamwise direction. The base flow velocity profile is fully non-parallel and non-similar in nature. The boundary layer grows continuously in the spatial directions. Linearized Navier-Stokes(LNS) equations are derived for the disturbance flow quantities in the cylindrical polar coordinates. The LNS equations along with homogeneous boundary conditions forms a generalized eigenvalues problem. Since the base flow is axisymmetric, the disturbances are periodic in azimuthal direction. Chebyshev spectral collocation method and Arnoldi's iterative algorithm is used for the solution of the general eigenvalues problem. The global temporal modes are computed for the range of Reynolds numbers and different azimuthal wave numbers. The largest imaginary part of the computed eigenmodes are nega...
Measurement of turbulence in the oceanic mixed layer using Synthetic Aperture Radar (SAR
Directory of Open Access Journals (Sweden)
S. G. George
2012-09-01
Full Text Available Turbulence in the surface layer of the ocean contributes to the transfer of heat, gas and momentum across the air-sea boundary. As such, study of turbulence in the ocean surface layer is becoming increasingly important for understanding its effects on climate change. Direct Numerical Simulation (DNS techniques were implemented to examine the interaction of small-scale wake turbulence in the upper ocean layer with incident electromagnetic radar waves. Hydrodynamic-electromagnetic wave interaction models were invoked to demonstrate the ability of Synthetic Aperture Radar (SAR to observe and characterise surface turbulent wake flows. A range of simulated radar images are presented for a turbulent surface current field behind a moving surface vessel, and compared with the surface flow fields to investigate the impact of turbulent currents on simulated radar backscatter. This has yielded insights into the feasibility of resolving small-scale turbulence with remote-sensing radar and highlights the potential for extracting details of the flow structure and characteristics of turbulence using SAR.
Simulations of edge and scrape off layer turbulence in mega ampere spherical tokamak plasmas
DEFF Research Database (Denmark)
Militello, F; Fundamenski, W; Naulin, Volker; Nielsen, Anders Henry
2012-01-01
The L-mode interchange turbulence in the edge and scrape-off-layer (SOL) of the tight aspect ratio tokamak MAST is investigated numerically. The dynamics of the boundary plasma are studied using the 2D drift-fluid code ESEL, which has previously shown good agreement with large aspect ratio machines...
Significant Atmospheric Boundary Layer Change Observed above an Agulhas Current Warm Cored Eddy
Directory of Open Access Journals (Sweden)
C. Messager
2016-01-01
Full Text Available The air-sea impact of a warm cored eddy ejected from the Agulhas Retroflection region south of Africa was assessed through both ocean and atmospheric profiling measurements during the austral summer. The presence of the eddy causes dramatic atmospheric boundary layer deepening, exceeding what was measured previously over such a feature in the region. This deepening seems mainly due to the turbulent heat flux anomaly above the warm eddy inducing extensive deep and persistent changes in the atmospheric boundary layer thermodynamics. The loss of heat by turbulent processes suggests that this kind of oceanic feature is an important and persistent source of heat for the atmosphere.
Delaying natural transition of a boundary layer using smooth steps
Xu, Hui; Sherwin, Spencer J
2015-01-01
The boundary layer flow over a smooth forward-facing stepped plate is studied with particular emphasis on the delay of the transition to turbulence. The interaction between the Tollmien-Schlichting (T-S) waves and the base flow over a single/two forward facing smooth steps is conducted by linear analysis indicating the amplitude of the T-S waves are attenuated in the boundary layer over a single smooth plate. Furthermore, we show that two smooth forward facing steps give rise to a further reduction of the amplitude of the T-S waves. A direct numerical simulation (DNS) is performed for the two smooth forward steps correlating favourably with the linear analysis and showing that for the investigated parameters, the K-type transition is inhibited whereas the turbulence onset of the H-type transition is postponed albeit not suppressed. Transition is indeed delayed and drag reduced for both these transition scenarios suggesting smooth forward facing steps could be leveraged as a passive flow control strategy to de...
On boundary layer modelling using the ASTEC code
International Nuclear Information System (INIS)
The modelling of fluid boundary layers adjacent to non-slip, heated surface using the ASTEC code is described. The pricipal boundary layer characteristics are derived using simple dimensional arguments and these are developed into criteria for optimum placement of the computational mesh to achieve realistic simulation. In particular, the need for externally-imposed drag and heat transfer correlations as a function of the local mesh concentration is discussed in the context of both laminar and turbulent flow conditions. Special emphasis is placed in the latter case on the (k-ε) turbulence model, which is standard in the code. As far as possible, the analyses are pursued from first principles, so that no comprehensive knowledge of the history of the subject is required for the general ASTEC user to derive practical advice from the document. Some attention is paid to the use of heat transfer correlations for internal solid/fluid surfaces, whose treatment is not straightforward in ASTEC. It is shown that three formulations are possible to effect the heat transfer, called Explicit, Jacobian and Implicit. The particular advantages and disadvantages of each are discussed with regard to numerical stability and computational efficiency. (author) 18 figs., 1 tab., 39 refs
Internal wave energy radiated from a turbulent mixed layer
Energy Technology Data Exchange (ETDEWEB)
Munroe, James R., E-mail: jmunroe@mun.ca [Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John' s, Newfoundland A1B 3X7 (Canada); Sutherland, Bruce R., E-mail: bsuther@ualberta.ca [Departments of Physics and Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2R3 (Canada)
2014-09-15
We examine mixed-layer deepening and the generation of internal waves in stratified fluid resulting from turbulence that develops in response to an applied surface stress. In laboratory experiments the stress is applied over the breadth of a finite-length tank by a moving roughened conveyor belt. The turbulence in the shear layer is characterized using particle image velocimetry to measure the kinetic energy density. The internal waves are measured using synthetic schlieren to determine their amplitudes, frequencies, and energy density. We also perform fully nonlinear numerical simulations restricted to two dimensions but in a horizontally periodic domain. These clearly demonstrate that internal waves are generated by transient eddies at the integral length scale of turbulence and which translate with the background shear along the base of the mixed layer. In both experiments and simulations we find that the energy density of the generated waves is 1%–3% of the turbulent kinetic energy density of the turbulent layer.
Measurements of Reynolds stress and turbulence in the boundary plasma of the HT-7 tokamak
Institute of Scientific and Technical Information of China (English)
Song Mei; Wan Bao-Nian; Xu Guo-Sheng
2004-01-01
Measurements of electric field fluctuations, Reynolds stress and poloidal flow have been performed in the boundary region of the HT-7 tokamak using a Langmuir probe array. Sheared radial electric field and poloidal flow have been found in the vicinity of the limiter and the turbulence has been clearly modified in this region. Furthermore, the electrostatic Reynolds stress component shows a radial gradient close to the velocity shear layer location. All results here indicate that the radial gradient of Reynolds stress may play an important role in the driving of poloidal flows in the plasma boundary region.
Simulation of a 5MW wind turbine in an atmospheric boundary layer
International Nuclear Information System (INIS)
This article presents detached eddy simulation (DES) results of a 5MW wind turbine in an unsteady atmospheric boundary layer. The evaluation performed in this article focuses on turbine blade loads as well as on the influence of atmospheric turbulence and tower on blade loads. Therefore, the turbulence transport of the atmospheric boundary layer to the turbine position is analyzed. To determine the influence of atmospheric turbulence on wind turbines the blade load spectrum is evaluated and compared to wind turbine simulation results with uniform inflow. Moreover, the influences of different frequency regimes and the tower on the blade loads are discussed. Finally, the normal force coefficient spectrum is analyzed at three different radial positions and the influence of tower and atmospheric turbulence is shown
Double layer formed by beam driven ion-acoustic turbulence
International Nuclear Information System (INIS)
Small amplitudes steady-state ion-acoustic double layers are observed to form in a plasma transversed by a beam of cold electrons. The importance of turbulence in maintaining the double layer is demonstrated. The measured wave spectrum is in approximate agreement with models deriveted from renornalized turbulence theory. The general features of the double layer are compared with results from particle simulation studies. (author)
The Impact of the Winter Monsoon on Marine Surface-Layer Turbulence
Nishida, Tomoya; Waseda, Takuji
2015-10-01
During winter the East-Asia monsoon is the dominant climatological phenomenon in the north-west Pacific, especially around the Japan Islands. Previous studies based on satellite and radiosonde observations revealed that cold and dry strong flow from Siberia over the warm Kuroshio Extension enhances instability of the marine boundary layer and reduces vertical wind shear. Since active convection manifests as intensification of near-surface turbulence, in situ meteorological buoy data were analyzed, confirming that turbulence intensity or equivalently the gust factor increases during the winter north-westerly monsoon. Concurrently, a hindcast atmospheric simulation was conducted to quantify the thermal and vapour fluxes in the marine boundary layer. When large gust factors or turbulence intensity are observed, the buoyancy production plays an important role in the turbulent kinetic energy budget. To further clarify the respective roles of the sensible and latent heat fluxes at the sea surface, atmospheric model experiments showed that the sensible heat flux predominantly contributes to the increase in turbulence intensity. On the other hand, the latent heat flux mainly affects the generation of clouds at the top of the marine boundary layer.
DEFF Research Database (Denmark)
Fischer, Andreas
2011-01-01
and to improve it, because the predictions gave in general too low far field noise levels. Our main finding is that the acoustic formulations to relate the fluctuating surface pressure field close to the trailing edge of airfoil to the radiated far field sound give excellent results when compared to far field......The present work aims at the characterization of aerodynamic noise from wind turbines. There is a consensus among scientists that the dominant aerodynamic noise mechanism is turbulent boundary trailing edge noise. In almost all operational conditions the boundary layer flow over the wind turbine...... blades makes a transition from laminar to turbulent. In the turbulent boundary layer eddies are created which are a potential noise sources. They are ineffective as noise source on the airfoil surface or in free flow, but when convecting past the trailing edge of the airfoil their efficiency is much...
Magnetic activity in accretion disc boundary layers
Armitage, Philip J.
2002-03-01
We use three-dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high-density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle, the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.
An investigation of the effects of the propeller slipstream on a wing boundary layer
Howard, Richard Moore
1987-12-01
The behavior of a wing boundary layer immersed in a propeller slipstream has been studied experimentally. Airfoil surface static pressure measurements were made for time-averaged effects, and time-dependent measurements were made with hot-film anemometer sensors for the determination of instantaneous velocities. Vertical boundary layer traverses were made at fixed chord locations for the determination of velocity profiles and for values of the turbulence intensity. The boundary layer has a coherent, time-dependent cycle of transitional behavior, varying from laminar to turbulent. This layer shows similarities to those disturbed by high levels of external flow turbulence and to those in a relaminarizing environment. Profile drag coefficients determined from the time-dependent ensemble-average velocity profiles for the freewheeling propeller case show the drag in the propeller slipstream varies from the undisturbed laminar value to a value less than that predicted for fully turbulent flow. Drag values determined from the low Reynolds number thrusting propeller case in the wind tunnel show that the effects of the slipstream are to enhance the stability of the boundary layer and to reduce the drag coefficient in the laminar portion of the slipstream cycle below its undisturbed value.
DYNAMICS OF A BOUNDARY LAYER SEPARATION
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Budapest : University of Technology and Economics , 2009, s. 268-275. ISBN 978-963-420-985-0. [Conference on Modelling Fluid Flow CMFF'09. Budapest (HU), 09.09.2009-12.09.2009] R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * dynamics * separation * POPs Subject RIV: BK - Fluid Dynamics
Numerical Simulation of the Atmospheric Boundary Layer
Czech Academy of Sciences Publication Activity Database
Bauer, Petr
Praha : Česká technika - nakladatelství ČVUT, 2006 - (Ambrož, P.; Masáková, Z.), s. 11-18 [Doktorandské dny 2006. Katedra matematiky FJFI ČVUT, Praha (CZ), 10.11.2006-24.11.2006] Institutional research plan: CEZ:AV0Z20760514 Keywords : atmospheric boundary layer * numerical simulation * finite element method Subject RIV: DI - Air Pollution ; Quality
Dynamical analysis of separated boundary layer flow
Czech Academy of Sciences Publication Activity Database
Uruba, Václav
Berlin : Technische Universität Berlin, 2009. s. 1-2 ISBN N. [Nonlinear Normal Modes, Dimension Reduction and Localization in Vibrating Systems. 27.09.2009-02.10.2009, Frascati (Rome)] R&D Projects: GA ČR GA101/08/1112 Institutional research plan: CEZ:AV0Z20760514 Keywords : boundary layer * separation * dynamics Subject RIV: BK - Fluid Dynamics
Submarine design optimization using boundary layer control
Christopher L Warren
1997-01-01
Several hull designs are studied with parametric based volume and area estimates to obtain preliminary hull forms. The volume and area study includes the effects of technologies which manifest themselves in the parametric study through stack length requirements. Subsequently, the hull forms are studied using a Reynolds Averaged Navier Stokes analysis coupled with a vortex lattice propeller design code. Optimization is done through boundary layer control analysis and through studies on the eff...
Energy Technology Data Exchange (ETDEWEB)
Chandra, A S; Kollias, P; Giangrande, S E; Klein, S A
2009-08-20
A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004-08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes-thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.
Coupled wake boundary layer model of windfarms
Stevens, Richard; Gayme, Dennice; Meneveau, Charles
2014-11-01
We present a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a windfarm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall windfarm boundary layer structure. Wake models capture the effect of turbine positioning, while the top-down approach represents the interaction between the windturbine wakes and the atmospheric boundary layer. Each portion of the CWBL model requires specification of a parameter that is unknown a-priori. The wake model requires the wake expansion rate, whereas the top-down model requires the effective spanwise turbine spacing within which the model's momentum balance is relevant. The wake expansion rate is obtained by matching the mean velocity at the turbine from both approaches, while the effective spanwise turbine spacing is determined from the wake model. Coupling of the constitutive components of the CWBL model is achieved by iterating these parameters until convergence is reached. We show that the CWBL model predictions compare more favorably with large eddy simulation results than those made with either the wake or top-down model in isolation and that the model can be applied successfully to the Horns Rev and Nysted windfarms. The `Fellowships for Young Energy Scientists' (YES!) of the Foundation for Fundamental Research on Matter supported by NWO, and NSF Grant #1243482.
Assessment of boundary layer profiling formulas using tower, sodar and balloon data
Energy Technology Data Exchange (ETDEWEB)
Paine, R.J. [ENSR Consulting and Engineering, Inc., Acton, MA (United States); Kendall, S.B. [Phelps Dodge Corp., Phoenix, AZ (United States)
1994-12-31
The accuracy of an air quality dispersion model is largely dependent upon the availability of representative meteorological data for the simulation of plume rise, transport, and dispersion. In many cases where tall stacks and/or buoyant plumes are involved, the available meteorological measurements do not extend to plume height. Air quality models contend with these situations by either assuming no change of meteorological variables with elevation or by applying a profiling relationship based upon theoretical or empirical relationships. The latter treatment is employed in recently-developed models such as CTDMPLUS, and HPDM, and OML. In the well-mixed convective boundary layer, meteorological variables such as wind direction, wind speed, and turbulence do not vary substantially above the surface layer (about 0.1 z{sub i}, the mixed-layer height). Above the surface layer, behavior on an hourly average basis is fairly well parameterized by boundary-layer formulations. However, models are sensitive to the height of the convective boundary layer, z{sub i}, which affects the magnitude of the convective velocity scale, w., and is important for simulating plume trapping and plume penetration into the stable layer aloft. In the stable boundary layer, plumes are often released at heights above the stable boundary layer, the height of which is often hard to define. Models are sensitive to the manner in which wind direction, wind speed, temperature and turbulence are profiled with height in stable conditions.
Remote sensing of the nocturnal boundary layer for wind energy applications
International Nuclear Information System (INIS)
The fine temporal and spatial resolution of Doppler lidar observations has been highly effective in the study of wind and turbulence dynamic in the nocturnal boundary layer during Lamar Low-Level Project in 2003. The High-Resolution Doppler Lidar (HRDL), designed and developed at the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL), measures range-resolved profiles of line-of sight (LOS) Doppler velocity and aerosol backscatter with a pulse repetition frequency of 200 Hz, velocity precision about 10 cm s-1, and a very narrow beam width. The majority of the lidar-measured wind speed and variance profiles were derived using a vertical-scan mode and the application of a vertical binning technique. The profile data were used to calculate quantities important for wind energy applications, including turbulence intensity, wind and directional shear through the layer of the turbine rotor. Profiles of all quantities show a strong variation with height. The mean wind fields, the turbulence, and turbulence intensities show a good agreement with sonic anemometer sodar high confidence (high SNR) measurements. The ability of HRDL to provide continuous information about wind and turbulence conditions at the turbine height and above the range of the tower measurements made HRDL as a powerful instrument for studies of the nighttime boundary layer features. Such information is needed as turbine rotors continue to rise higher into the boundary layer
Shock Wave-Boundary Layer Interaction in Forced Shock Oscillations
Institute of Scientific and Technical Information of China (English)
Piotr Doerffer; Oskar Szulc; Franco Magagnato
2003-01-01
The flow in transonic diffusers as well as in supersonic air intakes becomes often unsteady due to shock wave boundary layer interaction. The oscillations may be induced by natural separation unsteadiness or may be forced by boundary conditions. Significant improvement of CFD tools, increase of computer resources as well as development of experimental methods have again.drawn the attention of researchers to this topic.To investigate the problem forced oscillations of transonic turbulent flow in asymmetric two-dimensional Laval nozzle were considered. A viscous, perfect gas flow, was numerically simulated using the Reynolds-averaged compressible Navier-Stokes solver SPARC, employing a two-equation, eddy viscosity, turbulence closure in the URANS approach.For time-dependent and stationary flow simulations, Mach numbers upstream of the shock between 1.2 and 1.4 were considered. Comparison of computed and experimental data for steady states generally gave acceptable agreement. In the case of forced oscillations, a harmonic pressure variation was prescribed at the exit plane resulting in shock wave motion. Excitation frequencies between 0 Hz and 1024 Hz were investigated at the same pressure amplitude.The main result of the work carried out is the relation between the amplitude of the shock wave motion and the excitation frequency in the investigated range. Increasing excitation frequency resulted in decreasing amplitude of the shock movement. At high frequencies a natural mode of shock oscillation (of small amplitude) was observed which is not sensitive to forced excitement.
International Nuclear Information System (INIS)
The exchange of pollutants between the soil and the near-surface boundary layer is considered taking into account the effects of dry deposition, sedimentation and resuspension. The time taken to reach steady-state conditions has been evaluated. The exact solutions of conventional vertical turbulent diffusion with a given lower boundary condition have been derived and analysed. (author)
Reynolds number effects in DNS of pipe flow and comparison with channels and boundary layers
International Nuclear Information System (INIS)
Highlights: • New direct numerical simulations of turbulent pipe flow up to Reτ=2003. • Streamwise and spanwise variances do not exhibit inner scaling due to large-scale. • Transverse velocity statistics show differences between pipes and boundary layers. • Difference in variance of transverse velocities due to higher turbulence production. -- Abstract: Direct numerical simulations of turbulent pipe flow were performed at four Reynolds numbers: Reτ=180,500,1002and2003. Beyond Reτ=1000 viscous scaling holds near the wall for the mean velocity, Reynolds shear stress and wall-normal velocity variance. Streamwise and spanwise velocity variances do not exhibit inner (viscous) scaling due to increasing large-scale energy contributions. A comparison with channel and boundary layer DNS data shows negligible statistical differences between pipes and channels, whereas the transverse velocities for pipes/channels are significantly different when compared with boundary layers. A further comparison displays that the boundary layer pressure fluctuations is greater than pipes/channels. In addition, is it shown that the higher pressure fluctuations in the boundary layer is not the sole mechanism responsible for a stronger wake region in the flow
DEFF Research Database (Denmark)
Kristensen, Leif; Lenschow, Donald H.; Gurarie, David; Jensen, Niels Otto
2010-01-01
We have developed a simple, steady-state, one-dimensional second-order closure model to obtain continuous profiles of turbulent fluxes and mean concentrations of non-conserved scalars in a convective boundary layer without shear. As a basic tool we first set up a model for conserved species with ...
An Observational Study of the Structure of the Nocturnal Boundary Layer
DEFF Research Database (Denmark)
Mahrt, Larry; Heald, R. C.; Lenschow, D. H.; Stankov, B. B.; Troen, Ib
1980-01-01
In an effort to describe the basic vertical structure of the nocturnal boundary layer, observations from four experiments are analyzed. During the night, the depth of significant cooling appears to increase with time while the depth of the turbulence and height of the low level wind maximum tend ...
Dispersion of a passive tracer in buoyancy- and shear-driven boundary layers
Dosio, A.; Vilà-Guerau de Arellano, J.; Holtslag, A.A.M.; Builtjes, P.J.H.
2003-01-01
By means of finescale modeling [large-eddy simulation (LES)], the combined effect of thermal and mechanical forcing on the dispersion of a plume in a convective boundary layer is investigated. Dispersion of a passive tracer is studied in various atmospheric turbulent flows, from pure convective to a
The aim of this research program is to improve our knowledge and predictive capability of buoyant plume dispersion in the convective boundary layer (CBL) with emphasis on the mean (C) and root-mean-square (?c) concentration fields. The CBL turbulence leads to large random fluc...
Tomas, J.M.; Pourquie, M.J.B.M.; Jonker, H.J.J.
2016-01-01
Large-eddy simulations (LES) are used to investigate the effect of stable stratification on rural-to-urban roughness transitions. Smooth-wall turbulent boundary layers are subjected to a generic urban roughness consisting of cubes in an in-line arrangement. Two line sources of pollutant are added to
Kranenburg, W.M.; Ribberink, J.S.; Uittenbogaard, R.E.; Hulscher, S.J.M.H.
2012-01-01
The net current (streaming) in a turbulent bottom boundary layer under waves above a flat bed, identified as potentially relevant for sediment transport, is mainly determined by two competing mechanisms: an onshore streaming resulting from the horizontal non-uniformity of the velocity field under pr
Experimental study of the boundary layer over an airfoil in plunging motion
Institute of Scientific and Technical Information of China (English)
F. Rasi Marzabadi; M. R. Soltani
2012-01-01
This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions.It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer.The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake.The experiments were conducted at Reynolds numbers of 0.42 × 106 to 0.84 × 106 and the reduced frequency was varied from 0.01 to 0.1 1.The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases.For the static tests,boundary layer transition occurred through a laminar separation bubble.By increasing the angle of attack,disturbances and the transition location moved toward the leading edge.For the dynamic tests,earlier transition occurred with increasing rather than decreasing effective angle of attack.The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer.By increasing the reduced frequency,the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack,but the quasi skin friction coefficient was decreased.
Experimental study of the boundary layer over an airfoil in plunging motion
Marzabadi, F. Rasi; Soltani, M. R.
2012-04-01
This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions. It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer. The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake. The experiments were conducted at Reynolds numbers of 0.42×106 to 0.84 × 106 and the reduced frequency was varied from 0.01 to 0.11. The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases. For the static tests, boundary layer transition occurred through a laminar separation bubble. By increasing the angle of attack, disturbances and the transition location moved toward the leading edge. For the dynamic tests, earlier transition occurred with increasing rather than decreasing effective angle of attack. The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer. By increasing the reduced frequency, the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack, but the quasi skin friction coefficient was decreased.
Institute of Scientific and Technical Information of China (English)
Fiancesca Satta; Daniele Simoni; Marina Ubaldi; Pietro Zunino
2008-01-01
The present work is part of an extensive experimental activity carried out by the authors in recent years aimed at investigating the boundary layer transition phenomenon in turbine blades. The large scale of the cascade and the use of advanced LDV instrumentation and precision probe traversing mechanism resulted in high degree of spa-tial resolution and high accuracy of measurements. The main dissipation mechanism determining the profile losses in turbomachinery blades is the work of deformation of the mean motion within the boundary layer oper-ated by both viscous and turbulent shear stresses. In the present paper, the local viscous and turbulent deformation works have been directly evaluated from the detailed measurements of boundary layer mean velocity and Rey-nolds shear stress. The results show the distributions and the relative importance of the viscous and turbulent con-tributions to the loss production, in relation with the boundary layer states occurring along the turbine profile.
Effect of wall boundary condition on scalar transfer in a fully developed turbulent flume
Tiselj, Iztok; Pogrebnyak, Elena; Li, Changfeng; Mosyak, Albert; Hetsroni, Gad
2001-04-01
We performed direct numerical simulation of fully developed turbulent velocity and temperature fields in a flume, for Reynolds number, based on the wall shear velocity and the height of the flume, Re=171 and Prandtl numbers Pr=1.0 and Pr=5.4. To elucidate exactly the role of the wall boundary condition for passive scalar, the system considered was the flow at constant properties of the fluid. Two types of thermal wall boundary conditions (BCs) for the dimensionless temperature equation were studied: isothermal wall boundary condition—H1, and isoflux wall boundary condition—H2. The profile of the mean temperature was not affected by the type of BC. However, the type of BC has a profound effect on the statistics of the temperature fluctuations in the near-wall region y+<10. Comparison of near-wall statistics of temperature fluctuations shows that at Pr=1 the buffer part of the turbulent boundary layer significantly influences the scalar transfer in the conductive sublayer, whereas at Pr=5.4 the near-wall temperature field may be associated with predominant motion in the viscous sublayer.
Directory of Open Access Journals (Sweden)
J. Lauros
2010-08-01
Full Text Available We carried out column model simulations to study particle fluxes and deposition and to evaluate different particle formation mechanisms at a boreal forest site in Finland. We show that kinetic nucleation of sulphuric acid cannot be responsible for new particle formation alone as the vertical profile of particle number distribution does not correspond to observations. Instead organic induced nucleation leads to good agreement confirming the relevance of the aerosol formation mechanism including organic compounds emitted by biosphere.
Simulation of aerosol concentration inside the atmospheric boundary layer during nucleation days shows highly dynamical picture, where particle formation is coupled with chemistry and turbulent transport. We have demonstrated suitability of our turbulent mixing scheme in reproducing most important characteristics of particle dynamics inside the atmospheric boundary layer. Deposition and particle flux simulations show that deposition affects noticeably only the smallest particles at the lowest part of the atmospheric boundary layer.
Chitta, Subhashini; Steinhoff, John
2015-11-01
This paper describes the use of Vorticity Confinement (VC) to efficiently treat complex blunt bodies with thin shed vortex sheets and attached boundary layers. Because these flows involve turbulence in the vortical regions, there is currently no ab initio method to treat them on current or foreseeable computers. In fact, in spite of years of turbulence modeling efforts (such as LES or RANS), serious flaws in aerodynamic design involving vortex shedding may still be left undetected until the expensive prototype or production stage. Our basic premise is that, for a class of real-world problems requiring simulating ensembles of flow conditions for overall accuracy, conventional turbulence models suffer cost constraints. For these reasons, VC is used to rapidly simulate many operating conditions, as is often done in expensive testing programs for flying prototypes, and in realistic simulations. To achieve dramatically lower computational cost, VC treats the entire flow in a uniform, coarse grid with solid surfaces ``immersed'' in the grid so that they can be quickly generated for many configurations with no requirement for adaptive or conforming fine grids. Also, the VC method has the efficiency of panel methods, but the generality and ease of use of Euler equation methods. We would like to thank Dr. Frank Caradonna for his suggestions and support.
Wind-Turbine Wakes in a Convective Boundary Layer: A Wind-Tunnel Study
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
Two Dimensional Boundary Layer Growth with Suction
Directory of Open Access Journals (Sweden)
Krishna Lal
1970-07-01
Full Text Available The boundary layer equations for the unsteady fluid flow with constant suction velocity have been worked out for the impulsive motion of a circular cylinder in the form V(t=A exp (Ct where A and C are certain constants. The stream function has been expanded in terms of some functions X/sub 0/(s where s is a function of y coordinate. The phase angles for various terms have been calculated, and variations shown graphically for large and small frequency of oscillations, where the oscillatory motion is obtained on replacing C by iw.
Unsteady Phenomena in Shock Wave/Boundary Layer Interaction
Dolling, D. S.
1993-01-01
A brief review is given of the unsteadiness of shock wave/turbulent boundary layer interaction. The focus is on interactions generated by swept and unswept compression ramps, by flares, steps and incident shock waves, by cylinders and blunt fins, and by glancing shock waves. The effects of Mach number, Reynolds number, and separated flow scale are discussed as are the physical causes of the unsteadiness. The implications that the unsteadiness has for interpreting time-average surface and flowfield data, and for comparisons of such experimental data with computation, is also briefly discussed. Finally, some suggestions for future work are given. It is clear that there are large gaps in the data base and that many aspects of such phenomena are poorly understood. Much work remains to be done.
Segregation in the Atmospheric Boundary Layer - A Discussion
Dlugi, Ralph; Berger, Martina; Zelger, Michael; Hofzumahaus, Andreas; Rohrer, Franz; Holland, Frank; Lu, Keding; Tsokankunku, Anywhere; Sörgel, Matthias; Kramm, Gerhard; Mölders, Nicole
2016-04-01
Segregation is a well known topic in technical chemistry and means an incomplete mixing of the reactants. Incomplete mixing reduces the rate of reaction which is of utmost importance in technical chemistry but has been payed less attention in atmospheric chemistry. Different observational and modelling studies on chemical reactions in the turbulent and convective atmospheric boundary layer are analysed for the influences of segregation in the systems NO ‑ NO2 ‑ O3 and OH + V OCs (with main focus on isoprene). Also some estimates on reactions like HO2 + NO (an important recycling mechanism for OH) will be given. Especially, different terms of the intensity of segregation IS (correlation coefficients, standard deviations of mixing ratios) are compared and are related to characteristics of the flow regimes, such as mixing conditions and Damköhler numbers. Also influences of fluctuations of actinic fluxes are discussed which influence the mostly photo chemically driven reactions that were investigated.
On Hydromagnetic Stresses in Accretion Disk Boundary Layers
DEFF Research Database (Denmark)
Pessah, Martin Elias; Chan, Chi-kwan
2012-01-01
viscosity satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability (MRI) is inefficient in disk regions where, as...... angular frequencies that increase outward in the shearing-sheet framework. We isolate the modes that are unrelated to the standard MRI and provide analytic solutions for the long-term evolution of the resulting shearing MHD waves. We show that, although the energy density of these waves can be amplified...... significantly, their associated stresses oscillate around zero, rendering them an inefficient mechanism to transport significant angular momentum (inward). These findings are consistent with the results obtained in numerical simulations of MHD accretion disk boundary layers and challenge the standard assumption...
Large-eddy simulation of an infinitely large wind farm in a stable atmospheric boundary layer
Lu, H.; Porté-Agel, F.
2010-09-01
When deployed as large arrays, wind turbines interact among themselves and with atmospheric boundary layer. To optimize their geometric arrangements, accurate knowledge of wind-turbine array boundary layer is of great importance. In this study, we integrated large eddy simulation with an actuator line technique, and used it to study the characteristics of wind-turbine wake in an idealized wind farm inside a stably stratified atmospheric boundary layer (SBL). The wind turbines, with a rotor diameter of 112m and a tower height of 119m, were placed in a well-known SBL turbulent case that has a boundary layer height of approximately 180m. The super-geostrophic nocturnal jet near the top of the boundary layer was eliminated due to the energy extraction and the enhanced mixing of momentum. Non-axisymmetric behavior of wake structure was observed in response to the non-uniform incoming turbulence, the Coriolis effects, and the rotational effects induced by blade motions. The turbulence intensity in the simulated turbine wakes was found to reach a maximum at the top-tip level and a downwind distance of approximately 3-5 rotor diameters from the turbines. The Coriolis effects caused a skewed spatial structure and drove certain amount of turbulent energy away from the center of the wake. The SBL height was increased, while the magnitudes of the surface momentum flux and the surface buoyancy flux were reduced by approximately 30%. The wind farm was also found to have a strong effect on area-averaged vertical turbulent fluxes of momentum and heat, which highlights the potential impact of wind farms on local meteorology.
Unsteady boundary layer studies on ultra-high-lift low-pressure turbine blades
Energy Technology Data Exchange (ETDEWEB)
Zhang, X.; Hodson, H. [Cambridge Univ. (United Kingdom). Whittle Lab.; Harvey, N. [Rolls-Royce plc., Derby (United Kingdom). Turbine Systems
2005-09-15
The unsteady boundary layer development on the suction surface of two ultra-high-lift low-pressure (LP) turbine blades, known as T106C and U2, was investigated to further understand the loss reduction mechanism, which is integral to optimize the blade design in unsteady flow. The T106C profile is a mid-loaded ultra-high-lift LP turbine blade. Owing to the strong local adverse pressure gradient, the laminar boundary layer on the suction surface separates shortly after the suction side peak velocity. The turbulence within the incoming wakes cannot induce transition around the separation point because of the low receptivity of the laminar boundary layer. This allows the wake's negative jet to induce roll-up vortex, which reduces the benefits of the wake-induced transition. The wake-turbulence induced transition, which occurs downstream of the separation point, helps the separated boundary layer to reattach earlier. Owing to its mid-loaded nature, a large portion of the suction surface is covered by the reattached turbulent flow, which also contributes to the high profile loss. Therefore, LP turbine blades designed to make best use of unsteady flow effects should be aft-loaded. The ultra-high-lift blade U2 is an aft-loaded profile. Furthermore, the mild local adverse pressure gradient after the suction side peak velocity allows the laminar boundary layer to further develop before separation and results in a local Re {theta} of about 250 at separation. Therefore, the turbulence in the passing wake is able to induce transition close to the separation point. The earlier wake-induced transition and calmed region significantly reduce the size of the separation bubble. Furthermore, the earlier occurrence of wake-turbulence induced transition prevents the wake's negative jet from generating roll-up vortex, which also leads to lower losses. (author)
Angular momentum transport in accretion disk boundary layers around weakly magnetized stars
DEFF Research Database (Denmark)
Pessah, M.E.; Chan, C.-K.
2013-01-01
The standard model for turbulent shear viscosity in accretion disks is based on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. This implies that the turbulent stress must be negative and thus transport angular momentum inwards, in...... the boundary layer where the accretion disk meets the surface of a weakly magnetized star. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability...... (MRI) is inefficient in disk regions where, as expected in boundary layers, the angular frequency increases with radius. Motivated by the need of a deeper understanding of the behavior of an MHD fluid in a differentially rotating background that deviates from a Keplerian profile, we study the dynamics...
Heat transfer and fluid mechanics measurements in transitional boundary layer flows
Wang, T.; Simon, T. W.; Buddhavarapu, J.
1985-01-01
Experimental results are presented to document hydrodynamic and thermal development of flat-plate boundary layers undergoing natural transition. Local heat transfer coefficients, skin friction coefficients and profiles of velocity, temperature and Reynolds normal and shear stresses are presented. A case with no transition and transitional cases with 0.68 percent and 2.0 percent free-stream disturbance intensities were investigated. The locations of transition are consistent with earlier data. A late-laminar state with significant levels of turbulence is documented. In late-transitional and early-turbulent flows, turbulent Prandtl number and conduction layer thickness values exceed, and the Reynolds analogy factor is less than, values previously measured in fully turbulent flows.
“Lidar Investigations of Aerosol, Cloud, and Boundary Layer Properties Over the ARM ACRF Sites”
Energy Technology Data Exchange (ETDEWEB)
Ferrare, Richard [NASA Langley Research Center, Hampton, VA (United States); Turner, David [National Oceanic and Atmospheric Administration (NOAA) National Severe Storms Lab., Norman, OK (United States)
2015-01-13
Project goals; Characterize the aerosol and ice vertical distributions over the ARM NSA site, and in particular to discriminate between elevated aerosol layers and ice clouds in optically thin scattering layers; Characterize the water vapor and aerosol vertical distributions over the ARM Darwin site, how these distributions vary seasonally, and quantify the amount of water vapor and aerosol that is above the boundary layer; Use the high temporal resolution Raman lidar data to examine how aerosol properties vary near clouds; Use the high temporal resolution Raman lidar and Atmospheric Emitted Radiance Interferometer (AERI) data to quantify entrainment in optically thin continental cumulus clouds; and Use the high temporal Raman lidar data to continue to characterize the turbulence within the convective boundary layer and how the turbulence statistics (e.g., variance, skewness) is correlated with larger scale variables predicted by models.
International Nuclear Information System (INIS)
Results of the experimental investigation on the development of boundary layers on flat plates with the smooth surface and with the surfaces covered by sandpapers 60-grit, 80-grit and 100-grit under external turbulent flows of various grid turbulence scales are presented. The displacement thickness Reynolds number was at the most 2000 during experiments. The investigated boundary layers belong to the class of layers close to the lower limit of admissible roughness region, k+ = 4.6, 5.7 and 8.7 respectively. It was certified that both the wall roughness and the free stream turbulence accelerate individually the boundary layer development from the laminar state of boundary layer to turbulence. Next it was ascertained that their joint effect amplifies the development of boundary layers so, that the surface roughness impact is predominating but the actions of intensity and length scale of the free stream turbulence disturbances are also significant. With the increasing roughness number the initial region with a pseudo-laminar flow structure and the transitional region become shorter.
Application of Arnoldi method to boundary layer instability
Zhang, Yong-Ming; Luo, Ji-Sheng
2015-12-01
The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory (LST) for two-dimensional (2D) disturbance on one-dimensional (1D) basic flow, comparison with LST for three-dimensional (3D) disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence (FST). Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien-Schlichting (TS) waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous’ in secondary instability. Project supported by the National Natural Science Foundation of China (Grant Nos. 11202147, 11332007, 11172203, and 91216111) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120032120007).
Strong, Stuart L.; Meade, Andrew J., Jr.
1992-01-01
Preliminary results are presented of a finite element/finite difference method (semidiscrete Galerkin method) used to calculate compressible boundary layer flow about airfoils, in which the group finite element scheme is applied to the Dorodnitsyn formulation of the boundary layer equations. The semidiscrete Galerkin (SDG) method promises to be fast, accurate and computationally efficient. The SDG method can also be applied to any smoothly connected airfoil shape without modification and possesses the potential capability of calculating boundary layer solutions beyond flow separation. Results are presented for low speed laminar flow past a circular cylinder and past a NACA 0012 airfoil at zero angle of attack at a Mach number of 0.5. Also shown are results for compressible flow past a flat plate for a Mach number range of 0 to 10 and results for incompressible turbulent flow past a flat plate. All numerical solutions assume an attached boundary layer.
International Nuclear Information System (INIS)
An analytical model for the dispersion of particulates and finely divided material released into the atmosphere near the ground is presented. The possible precipitation when the particles are dense enough and large enough to have deposition velocity, is taken into consideration. The model is derived analytically in the mixing layer or Ekman boundary layer where the mixing process is a direct consequence of turbulent and convective motions generated in the boundary layer. (author)
Velocities, turbulence, and skin friction in a deep-sea logarithmic layer
Gust, Giselher; Weatherly, Georges L.
1985-05-01
Speed, turbulence, skin friction, and drag measurements made with metal-clad hot wires, epoxy-coated hot films, and Savonius rotors are reported for a deep-sea boundary layer at a water depth of ˜5000 m. They include data from heights z < 30 cm, a region hitherto only investigated in detail by Chriss and Caldwell (1982) for a shelf site. A mean speed logarithmic layer was observed at 3 < z < 200 cm. The difference between the friction velocity u*log determined from the speed profiles and the skin friction u*skin measured by flush-mounted hot films was statistically significant at the 95% level in five out of eight analyzed burst intervals. This result suggests form-drag influence on the vertical mean flow profile. Although identified from the mean speed data as a hydrodynamically rough boundary layer, the turbulence and bottom stress intensities at the deep-sea site were found to be reduced by more than 40% compared to smooth-wall open-channel flow and planetary boundary layers. Applicability of the universal law of the wall has not been confirmed for this deep-sea boundary layer.
FOREWORD: International Conference on Planetary Boundary Layer and Climate Change
Djolov, G.; Esau, I.
2010-05-01
One of the greatest achievements of climate science has been the establisment of the concept of climate change on a multitude of time scales. The Earth's complex climate system does not allow a straightforward interpretation of dependences between the external parameter perturbation, internal stochastic system dynamics and the long-term system response. The latter is usually referred to as climate change in a narrow sense (IPCC, 2007). The focused international conference "Planetary Boundary Layers and Climate Change" has addressed only time scales and dynamical aspects of climate change with possible links to the turbulent processes in the Planetary Boundary Layer (PBL). Although limited, the conference topic is by no means singular. One should clearly understand that the PBL is the layer where 99% of biosphere and human activity are concentrated. The PBL is the layer where the energy fluxes, which are followed by changes in cryosphere and other known feedbacks, are maximized. At the same time, the PBL processes are of a naturally small scale. What is the averaged long-term effect of the small-scale processes on the long-term climate dynamics? Can this effect be recognized in existing long-term paleo-climate data records? Can it be modeled? What is the current status of our theoretical understanding of this effect? What is the sensitivity of the climate model projections to the representation of small-scale processes? Are there significant indirect effects, e.g. through transport of chemical components, of the PBL processes on climate? These and other linked questions have been addressed during the conference. The Earth's climate has changed many times during the planet's history, with events ranging from ice ages to long periods of warmth. Historically, natural factors such as the amount of energy released from the Sun, volcanic eruptions and changes in the Earth's orbit have affected the Earth's climate. Beginning late in the 18th century, human activities
Wave breaking on turbulent energy budget in the ocean surface mixed layer
Institute of Scientific and Technical Information of China (English)
SUN Qun; GUAN Changlong; SONG Jinbao
2008-01-01
As an important physical process at the air-sea interface.wave movement and breaking have a significant effect on the ocean surface mixed layer (OSML).When breaking waves occur at the ocean surface,turbulent kinetic energy (TKE) is input downwards,and a sublayer is formed near the surface and turbulence vertical mixing is intensively enhanced.A one-dimensional ocean model including the Mellor-Yamada level 2.5 turbulence closure equations was employed in our research on variations in turbulent energy budget wimin OSML.The influence of wave breaking could be introduced into the model by modifying an existing surface boundary condition of the TKE equation and specifying its input.The vertical diffusion and dissipation of TKE were effectively enhanced in the sublayer when wave breaking was considered.Turbulent energy dissipated in the sublayer was about 92.0% of the total depth-integrated dissipated TKE,which is twice higher than that of non-wave breaking.The shear production of TKE decreased bv 3.5% because the mean flow fields tended to be uniform due to wave-enhanced turbulent mixing.As a result.a new local equilibrium between diffusion and dissipation of TKE was reached in the wave-enhanced layer.Below the sublayer,the local equilibrium between shear production and dissipation of TKE agreed with the conclusion drawn from the classical law-of-the-wall (Craig and Banner,1994).
Indian Academy of Sciences (India)
A N V Satyanarayana; U C Mohanty; N V Sam; Swati Basu; V N Lykossov
2000-06-01
An attempt has been made to study the marine boundary layer characteristics over Bay of Bengal using BOBMEX (Bay of Bengal and Monsoon Experiment) pilot experiment data sets, which was conducted between 23rd October and 12th November 1998 on board ORV Sagar Kanya. A one-dimensional multi- level atmospheric boundary layer with TKE- closure scheme is employed to study the marine boundary layer characteristics. In this study two synoptic situations are chosen: one represents an active convection case and the other a suppressed convection. In the present article the marine boundary layer charac- teristics such as temporal evolution of turbulent kinetic energy, height of the boundary layer and the air- sea exchange processes such as sensible and latent heat fluxes, drag coefficient for momentum are simulated during both active and suppressed convection. Marine boundary layer height is estimated from the vertical profiles of potential temperature using the stability criterion. The model simulations are compared with the available observations.
Intermittent phenomena in the boiling two-phase boundary layer
International Nuclear Information System (INIS)
In order to investigate statistical properties of temperature fluctuation in a boiling two-phase boundary layer the corresponding intermittency functions, which describe liquid, vapour and interface region at an individual fixed point, have been defined. In water boiling on a horizontal surface the temperature fluctuation was measured with a microthermocouple and the signal was processed through the digital computer with the detector function specified for liquid, vapor and interface region. The results obtained confirm that the temperature fluctuation in the boiling two-phase layer can be divided into three parts corresponding to individual regions and that its statistical distribution depends on the properties of respective systems. It has also been shown that the temperature fluctuation in the interface region is determinative and corresponds to the temperature changes in the liquid layer surrounding vapor bubble growth. Amplitude distribution in the liquid region changes its form with the distance from the wall as a result of the change in intensity of turbulence at different distances. The probability density distribution in the vapor region shows very small amplitude fluctuation and is almost constant for all distances. (author)
Time-dependent boundary-layer response in a propeller slipstream
Howard, Richard M.; Miley, Stan J.
1989-01-01
The time-dependent behavior of a wing boundary layer immersed in a propeller slipstream has been studied experimentally in wind-tunnel tests and in flight. Hot-wire anemometer measurements were made through the boundary layer for time-dependent, ensemble-average velocity and turbulence-intensity profiles at various chord locations. The boundary layer has a coherent, time-dependent cycle of transitional behavior, varying from a laminar to a turbulent-transitional state. Local drag coefficients determined from the velocity profiles for the freewheeling propeller case in flight show that the time-dependent drag in the propeller slipstream varies from the undisturbed laminar value to a value less than that predicted for fully turbulent flow. Local drag coefficients determined from the thrusting propeller case in the wind tunnel indicate that the effects of the slipstream are to enhance the stability of the boundary layer and to reduce the drag coefficient in the laminar portion of the cycle below its undisturbed laminar value.
Structure Identification Within a Transitioning Swept-Wing Boundary Layer
Chapman, Keith; Glauser, Mark
1996-01-01
Extensive measurements are made in a transitioning swept-wing boundary layer using hot-film, hot-wire and cross-wire anemometry. The crossflow-dominated flow contains stationary vortices that breakdown near mid-chord. The most amplified vortex wavelength is forced by the use of artificial roughness elements near the leading edge. Two-component velocity and spanwise surface shear-stress correlation measurements are made at two constant chord locations, before and after transition. Streamwise surface shear stresses are also measured through the entire transition region. Correlation techniques are used to identify stationary structures in the laminar regime and coherent structures in the turbulent regime. Basic techniques include observation of the spatial correlations and the spatially distributed auto-spectra. The primary and secondary instability mechanisms are identified in the spectra in all measured fields. The primary mechanism is seen to grow, cause transition and produce large-scale turbulence. The secondary mechanism grows through the entire transition region and produces the small-scale turbulence. Advanced techniques use Linear Stochastic Estimation (LSE) and Proper Orthogonal Decomposition (POD) to identify the spatio-temporal evolutions of structures in the boundary layer. LSE is used to estimate the instantaneous velocity fields using temporal data from just two spatial locations and the spatial correlations. Reference locations are selected using maximum RMS values to provide the best available estimates. POD is used to objectively determine modes characteristic of the measured flow based on energy. The stationary vortices are identified in the first laminar modes of each velocity component and shear component. Experimental evidence suggests that neighboring vortices interact and produce large coherent structures with spanwise periodicity at double the stationary vortex wavelength. An objective transition region detection method is developed using
PERIODIC BOUNDARY CONDITION IN SIMULATION OF TURBULENT FLOW
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
In this paper, the simulations of the three-di-mensional turbulent flows through hydraulic turbine compo-nents[1] were conducted based on the standard k-ε turbulentmodel with body-fitted coordinates and staggering grid sys-tem. The SIMPLEC algorithm was adopted in the numericalprocedure. A new method to treat the periodic boundary con-dition was used. The calculated results of the new methodwere compared with those of traditional ones. These resultsindicate that the new method can give much better results,and can be used in simulating flow through rotational impel-lers. The presented method can be combined with alternativeturbulent model or employed in large eddy simulation.
Institute of Scientific and Technical Information of China (English)
WANG Liang; FU Song
2009-01-01
Based on Reynolds-averaged Navier-Stokes approach, a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows. The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy, ωthe specific dissipation rate and the intermittency factor γ.The particular features of the model are that: 1) k includes the non-turbulent, as well as turbulent fluctuations; 2) a transport equation for the intermittency factor γis proposed here with a source term set to trigger the transition onset; 3) through the introduction of a new length scale normal to wall, the present model employs the local variables only avoiding the use of the integral parameters, like the boundary layer thickness δ,which are often cost-ineffective with the modern CFD (Computational Fluid Dynamics) methods; 4) in the fully turbulent region, the model retreats to the well-known k-ωSST (Shear Stress Transport) model. This model is validated with a number of available experiments on boundary layer transitions including the incompressible, supersonic and hypersonic flows past flat plates, straight/flared cones at zero incidences, etc. It is demonstrated that the present model can be successfully applied to the engineering calculations of a variety of aerodynamic flow transition.
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
Based on Reynolds-averaged Navier-Stokes approach,a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows.The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy,ωthe specific dissipation rate and the intermittency factorγ.The particular features of the model are that:1)k includes the non-turbulent,as well as turbulent fluctuations;2)a transport equation for the intermittency factorγis proposed here with a source term set to trigger the transition onset;3)through the introduction of a new length scale normal to wall,the present model employs the local variables only avoiding the use of the integral parameters,like the boundary layer thicknessδ,which are often cost-ineffective with the modern CFD(Computational Fluid Dynamics)methods;4)in the fully turbulent region,the model retreats to the well-known k-ωSST(Shear Stress Transport)model.This model is validated with a number of available experiments on boundary layer transitions including the incompressible,supersonic and hypersonic flows past flat plates,straight/flared cones at zero incidences,etc.It is demonstrated that the present model can be successfully applied to the engineering calculations of a variety of aerodynamic flow transition.
A numerical study of shock wave/boundary layer interaction in a supersonic compressor cascade
International Nuclear Information System (INIS)
A numerical analysis of shock wave boundary layer interaction in transonic/supersonic axial flow compressor cascade has been performed by using a characteristic upwind Navier-Stokes method with various turbulence models. Two equation turbulence models were applied to transonic/supersonic flows over a NACA 0012 airfoil. The results are superion to those from an algebraic turbulence model. High order TVD schemes predicted shock wave/boundary layer interactions reasonably well. However, the prediction of SWBLI depends more on turbulence models than high order schemes. In a supersonic axial flow cascade at M=1.59 and exit/inlet static pressure ratio of 2.21, k-ω and Shear Stress Transport (SST) models were numerically stables. However, the k-ω model predicted thicker shock waves in the flow passage. Losses due to shock/shock and shock/boundary layer interactions in transonic/supersonic compressor flowfields can be higher losses than viscous losses due to flow separation and viscous dissipation
Buoyant production and consumption of turbulence kinetic energy in cloud-topped mixed layers
Randall, D. A.
1984-01-01
It is pointed out that studies of the entraining planetary boundary layer (PBL) have generally emphasized the role of buoyancy fluxes in driving entrainment. The buoyancy flux is proportional to the rate of conversion of the potential energy of the mean flow into the kinetic energy of the turbulence. It is not unusual for conversion to proceed in both directions simultaneously. This occurs, for instance, in both clear and cloudy convective mixed layers which are capped by inversions. A partitioning of the net conversion into positive parts, generating turbulence kinetic energy (TKE), and negative parts (TKE-consuming), would make it possible to include the positive part in the gross production rate, and closure would be achieved. Three different approaches to partitioning have been proposed. The present investigation is concerned with a comparison of the three partitioning theories. Particular attention is given to the cloud-topped mixed layer because in this case the differences between two partitioning approaches are most apparent.
Atmospheric boundary layers in storms: advanced theory and modelling applications
Directory of Open Access Journals (Sweden)
S. S. Zilitinkevich
2005-01-01
Full Text Available Turbulent planetary boundary layers (PBLs control the exchange processes between the atmosphere and the ocean/land. The key problems of PBL physics are to determine the PBL height, the momentum, energy and matter fluxes at the surface and the mean wind and scalar profiles throughout the layer in a range of regimes from stable and neutral to convective. Until present, the PBLs typical of stormy weather were always considered as neutrally stratified. Recent works have disclosed that such PBLs are in fact very strongly affected by the static stability of the free atmosphere and must be treated as factually stable (we call this type of the PBL "conventionally neutral" in contract to the "truly neutral" PBLs developed against the neutrally stratified free flow. It is common knowledge that basic features of PBLs exhibit a noticeable dependence on the free-flow static stability and baroclinicity. However, the concern of the traditional theory of neural and stable PBLs was almost without exception the barotropic nocturnal PBL, which develops at mid latitudes during a few hours in the night, on the background of a neutral or slightly stable residual layer. The latter separates this type of the PBL from the free atmosphere. It is not surprising that the nature of turbulence in such regimes is basically local and does not depend on the properties of the free atmosphere. Alternatively, long-lived neutral (in fact only conditionally neutral or stable PBLs, which have much more time to grow up, are placed immediately below the stably stratified free flow. Under these conditions, the turbulent transports of momentum and scalars even in the surface layer - far away from the PBL outer boundary - depend on the free-flow Brunt-Väisälä frequency, N. Furthermore, integral measures of the long-lived PBLs (their depths and the resistance law functions depend on N and also on the baroclinic shear, S. In the traditional PBL models both non-local parameters N and S
Bathel, Brett F.; Johansen, Craig T.; Danehy, Paul M.; Inman, Jennifer A.; Jones, Stephen B.; Goyne, Christopher P.
2011-01-01
Measurements of instantaneous and mean streamwise velocity profiles in a hypersonic laminar boundary layer as well as a boundary layer undergoing laminar-to-turbulent transition were obtained over a 10-degree half-angle wedge model. A molecular tagging velocimetry technique consisting of a NO2 approaches?NO photo-dissociation reaction and two subsequent excitations of NO was used. The measurement of the transitional boundary layer velocity profiles was made downstream of a 1-mm tall, 4-mm diameter cylindrical trip along several lines lying within a streamwise measurement plane normal to the model surface and offset 6-mm from the model centerline. For laminar and transitional boundary layer measurements, the magnitudes of streamwise velocity fluctuations are compared. In the transitional boundary layer the fluctuations were, in general, 2-4 times larger than those in the laminar boundary layer. Of particular interest were fluctuations corresponding to a height of approximately 50% of the laminar boundary layer thickness having a magnitude of nearly 30% of the mean measured velocity. For comparison, the measured fluctuations in the laminar boundary layer were approximately 5% of the mean measured velocity at the same location. For the highest 10% signal-to-noise ratio data, average single-shot uncertainties using a 1 ?Es and 50 ?Es interframe delay were 115 m/s and 3 m/s, respectively. By averaging single-shot measurements of the transitional boundary layer, uncertainties in mean velocity as low as 39 m/s were obtained in the wind tunnel. The wall-normal and streamwise spatial resolutions were 0.14-mm (2 pixel) and 0.82-mm (11 pixels), respectively. These measurements were performed in the 31-inch Mach 10 Air Wind Tunnel at the NASA Langley Research Center.
Mean motion and trajectories of heavy particles falling through a boundary layer
Energy Technology Data Exchange (ETDEWEB)
Stout, J.E.; Arya, S.P. [North Carolina State Univ., Raleigh, NC (United States)
1994-12-31
As particles fall through a turbulent boundary layer they experience a rather complex and unique time series of aerodynamic forces and, thus, each individual particle follows a rather complex and unique trajectory to the surface. For sufficiently large and heavy particles, the turbulence induced particle motion can be thought of as a small perturbation superimposed on the mean trajectory. By ignoring the turbulent contribution to particle motion it is possible to calculate the trajectory of a particle due to the mean flow alone. The mean trajectory provides an estimate of the ensemble-averaged path of a set of particles released from a given point in the atmosphere. The effect of turbulence on individual particle trajectories, the distribution of particle displacements from the mean trajectory, and their deposition patterns on the surface will be investigated in a separate study, using a random walk model.
Turbulence-double-layer synergetic auroral electron acceleration
International Nuclear Information System (INIS)
In this letter we present a theoretical discussion on the problem of the auroral electron acceleration that supports the conjecture of wave-particle interaction (turbulence) assisting the auroral electron acceleration due to a dc magnetic field aligned electric field, created by a double layer, working as to enhance the electron flux
Institute of Scientific and Technical Information of China (English)
CHEN Zhipeng; SUN Yue; WANG Zhijiang; YANG Zhoujun; DING Yonghua; ZHUANG Ge; J-TEXT team
2012-01-01
As the basic of a deeper investigation on the turbulent transport, the fluctuation property in the boundary of the newly-reconstructed Joint Texas Experimental Tokamak (J- TEXT) is studied experimentally using the reciprocating Langmuir four-tip probe, which has been built and operated as the primary diagnostic tool in the boundary of J-TEXT tokamak. In this paper, spatial profiles of the plasma-edge parameters are obtained, such as electron temperature, plasma density, plasma potential, poloidal electric field and their fluctuations. The results indicate the existence of a Er ×BT shear layer at the vicinity of last closed flux surface (LCFS), with the fluctuations suppressed in varying degrees. The turbulence-induced particle and energy fluxes can be calculated by the local plasma parameters above. Convection dominates the cross-field turbulent transport in boundary plasma. Electrostatic fluctuations properties are also studied in detail with the help of numerical analysis. Statistical analysis on density fluctuation shows that, the intermittency can affect the turbulence in the scrape-off layer (SOL).
Magnus Force of Common Projectile Bodies with Turbulent Layers
Institute of Scientific and Technical Information of China (English)
CHEN Jun
2005-01-01
Calculating formulae of Magnus force on common projectile bodies (cone-shaped and parabola-shaped) with turbulent layers were built based on Magnus theory. The effects of temperature exponential were considered, and curve-fitting approaches were adopted in the research that could give more exact result data. Both flow layer constants and shape constants are presented in Magnus force formulae, which are useful to evaluate Magnus force in different states.
A boundary-layer cloud study using Southern Great Plains Cloud and radiation testbed (CART) data
Energy Technology Data Exchange (ETDEWEB)
Albrecht, B.; Mace, G.; Dong, X.; Syrett, W. [Pennsylvania State Univ., University Park, PA (United States)] [and others
1996-04-01
Boundary layer clouds-stratus and fairweather cumulus - are closely coupled involves the radiative impact of the clouds on the surface energy budget and the strong dependence of cloud formation and maintenance on the turbulent fluxes of heat and moisture in the boundary layer. The continuous data collection at the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site provides a unique opportunity to study components of the coupling processes associated with boundary layer clouds and to provide descriptions of cloud and boundary layer structure that can be used to test parameterizations used in climate models. But before the CART data can be used for process studies and parameterization testing, it is necessary to evaluate and validate data and to develop techniques for effectively combining the data to provide meaningful descriptions of cloud and boundary layer characteristics. In this study we use measurements made during an intensive observing period we consider a case where low-level stratus were observed at the site for about 18 hours. This case is being used to examine the temporal evolution of cloud base, cloud top, cloud liquid water content, surface radiative fluxes, and boundary layer structure. A method for inferring cloud microphysics from these parameters is currently being evaluated.
Flow around new wind fence with multi-scale fractal structure in an atmospheric boundary layer
McClure, Sarah; Lee, Sang-Joon; Zhang, Wei
2015-11-01
Understanding and controlling atmospheric boundary-layer flows with engineered structures, such as porous wind fences or windbreaks, has been of great interest to the fluid mechanics and wind engineering community. Previous studies found that the regular mono-scale grid fence of 50% porosity and a bottom gap of 10% of the fence height are considered to be optimal over a flat surface. Significant differences in turbulent flow structure have recently been noted behind multi-scale fractal wind fences, even with the same porosity. In this study, wind-tunnel tests on the turbulent flow and the turbulence kinetic energy transport of 1D and 2D multi-scale fractal fences under atmospheric boundary-layer were conducted. Velocity fields around the fractal fences were systematically measured using Particle Image Velocimetry to uncover effects of key parameters on turbulent flows around the fences at a Reynolds number of approximately 3.6x104 based on the free-stream speed and fence height. The turbulent flow structures induced by specific 1D/2D multi-scale fractal wind fences were compared to those of a conventional grid fence. The present results would contribute to the design of new-generation wind fences to reduce snow/sand deposition on critical infrastructure such as roads and bridges.
Reduced-order representation of near-wall structures in the late transitional boundary layer
Sayadi, Taraneh; Schmid, Peter J.; Nichols, Joseph W.; Moin, Parviz
2014-01-01
Direct numerical simulations (DNS) of controlled H- and K-type transitions to turbulence in an M=0.2 (where M is the Mach number) nominally zero-pressure-gradient and spatially developing flat-plate boundary layer are considered. Sayadi, Hamman & Moin (J. Fluid Mech., vol. 724, 2013, pp. 480-509) showed that with the start of the transition process, the skin-friction profiles of these controlled transitions diverge abruptly from the laminar value and overshoot the turbulent estimation. The ob...
Reduction of horizontal wind speed in a boundary layer with obstacles
DEFF Research Database (Denmark)
Emeis, S.; Frandsen, S.
1993-01-01
The reduction of horizontal wind speed at hub height in an infinite cluster of wind turbines is computed from a balance between a loss of horizontal momentum due to the drag and replenishment from above by turbulent fluxes. This reduction is derived without assumptions concerning the vertical wind...... profile above or below hub height, only some basic assumptions on turbulent exchange have been made. Two applications of the result are presented, one considering wind turbines and one pressure drag on orographic obstacles in the atmospheric boundary layer. Both applications are basically governed by the...
Simulation of Wind turbines in the atmospheric boundary layer
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
Large eddy simulation of an arbitrary wind farm is studied in the neutral and thermally stratified atmospheric boundary Layer. Large eddy simulations of industrial flows usually requires full resolution of the flow near the wall and this is believed to be one of the main deficiencies of LES because...... in the boundary layer. In the current study, another approach has been implemented to simulate the flow in a fully developed wind farm boundary layer. The approach is based on Immersed Boundary Method and involves implementation of an arbitrary prescribed initial boundary layer. An initial boundary...... based on the turbine wakes and buoyancy contributions. The implemented method is capable of capturing the most important features of wakes of wind farms [2] while having the advantage of resolving the wall layer with a coarser grid than a typical required grid size for such problems. LES simulations are...
Characteristics of the boundary layer of magnetic clouds and a new definition of the cloud boundary
Institute of Scientific and Technical Information of China (English)
魏奉思; 刘睿; 范全林; 冯学尚
2003-01-01
Based on the analysis of the boundaries of 70 magnetic clouds from 1967 to 1998, and relatively complete spacecraft observations, it is indicated that the magnetic cloud boundaries are boundary layers formed through the interaction between the magnetic clouds and the ambient medium. Most of the outer boundaries of the layers, with relatively high proton temperature, density and plasma β, are magnetic reconnection boundaries, while the inner boundaries, with low proton temperature, proton density and plasma β, separate the main body of magnetic clouds, which has not been affected by the interaction, from the boundary layers. The average time scale of the front boundary layer is 1.7 h and that of the tail boundary layer 3.1 h. It is also found that the magnetic probability distribution function undergoes significant changes across the boundary layers. This new definition, supported by the preliminary numerical simulation in principle, could qualitatively explain the observations of interplanetary magnetic clouds, and could help resolve the controversy in identifying the boundaries of magnetic clouds. Our concept of the boundary layer may provide some understanding of what underlies the observations, and a fresh train of thought in the interplanetary dynamics research.
Effects of stable stratification on turbulent/nonturbulent interfaces in turbulent mixing layers
Watanabe, T.; Riley, J. J.; Nagata, K.
2016-08-01
Direct numerical simulations are used for investigating the effects of stable stratification on the turbulent/nonturbulent (T/NT) interface in stably stratified mixing layers whose buoyancy Reynolds number Reb on the centerline is large enough for small-scale three-dimensional turbulence to exist. The stratification changes the interface geometry, and a large part of the interface is oriented with normal in the vertical direction in the stratified flows. The structures of the T/NT interface layer are similar between the nonstratified and stratified flows, and the T/NT interface consists of the viscous superlayer and the turbulent sublayer. The stratification is locally strengthened near the T/NT interface as evidenced by the large vertical density gradient, resulting in the decrease in Reb in the T/NT interface layer. Thus, even the small-scale dissipation range is directly affected by the buoyancy near the T/NT interface, although the small scales are somewhat free from the direct effects of the buoyancy in the turbulent core region. The production rates of enstrophy and scalar dissipation, which arise from the strain/vorticity and strain/density-gradient interactions, are decreased near the T/NT interface because the stratification modifies the alignments among the vorticity, density gradient, and strain-rate eigenvectors near the T/NT interface. This influence on the small-scale turbulence dynamics is not observed in the turbulent core region because of the large Reb. A possible explanation is given for the influence of buoyancy on the alignment statistics based on the suppression of the vertical turbulent motions by buoyancy.
Numerical model of a non-steady atmospheric planetary boundary layer, based on similarity theory
DEFF Research Database (Denmark)
Zilitinkevich, S.S.; Fedorovich, E.E.; Shabalova, M.V.
1992-01-01
A numerical model of a non-stationary atmospheric planetary boundary layer (PBL) over a horizontally homogeneous flat surface is derived on the basis of similarity theory. The two most typical turbulence regimes are reproduced: one corresponding to a convectively growing PBL and another correspon......A numerical model of a non-stationary atmospheric planetary boundary layer (PBL) over a horizontally homogeneous flat surface is derived on the basis of similarity theory. The two most typical turbulence regimes are reproduced: one corresponding to a convectively growing PBL and another......-surface values of heat, water vapor and momentum fluxes. The internal structure of the PBL is considered self-similar. This allows one to represent the interaction between the air flow and the underlying surface by means of universal heat/mass transfer and resistance laws. Numerical experiments on the diurnal...
Application of a transitional boundary-layer theory in the low hypersonic Mach number regime
Shamroth, S. J.; Mcdonald, H.
1975-01-01
An investigation is made to assess the capability of a finite-difference boundary-layer procedure to predict the mean profile development across a transition from laminar to turbulent flow in the low hypersonic Mach-number regime. The boundary-layer procedure uses an integral form of the turbulence kinetic-energy equation to govern the development of the Reynolds apparent shear stress. The present investigation shows the ability of this procedure to predict Stanton number, velocity profiles, and density profiles through the transition region and, in addition, to predict the effect of wall cooling and Mach number on transition Reynolds number. The contribution of the pressure-dilatation term to the energy balance is examined and it is suggested that transition can be initiated by the direct absorption of acoustic energy even if only a small amount (1 per cent) of the incident acoustic energy is absorbed.
Edge states as mediators of bypass transition in boundary-layer flows
Khapko, Taras; Schlatter, Philipp; Duguet, Yohann; Eckhardt, Bruno; Henningson, Dan S
2016-01-01
The concept of edge state is investigated in the asymptotic suction boundary layer in relation with the receptivity process to noisy perturbations and the nucleation of turbulent spots. Edge tracking is first performed numerically, without imposing any discrete symmetry, in a large computational domain allowing for full spatial localisation of the perturbation velocity. The edge state is a three-dimensional localised structure recurrently characterised by a single low-speed streak that experiences erratic bursts and planar shifts. This recurrent streaky structure is then compared with predecessors of individual spot nucleation events, triggered by non-localised initial noise. The present results suggest a nonlinear picture, rooted in dynamical systems theory, of the nucleation process of turbulent spots in boundary-layer flows, in which the localised edge state play the role of state-space mediator.
Wang, T.; Simon, T. W.
1987-01-01
The test section of the present experiment to ascertain the effects of convex curvature and freestream turbulence on boundary layer momentum and heat transfer during natural transition provided a two-dimensional boundary layer flow on a uniformly heated curved surface, with bending to various curvature radii, R. Attention is given to results for the cases of R = infinity, 180 cm, and 90 cm, each with two freestream turbulence intensity levels. While the mild convex curvature of R = 180 cm delays transition, further bending to R = 90 cm leads to no signifucant further delay of transition. Cases with both curvature and higher freestream disturbance effects exhibit the latter's pronounced dominance. These data are pertinent to the development of transition prediction models for gas turbine blade design.
Coherent structures in a zero-pressure-gradient and a strongly decelerated boundary layer
Simens, Mark P.; Gungor, Ayse G.; Maciel, Yvan
2016-04-01
Coherent structures in a strongly decelerated large-velocity-defect turbulent boundary layer (TBL) and a zero pressure gradient (ZPG) boundary layer are analysed by direct numerical simulation (DNS). The characteristics of the one-point velocity stastistics are also considered. The adverse pressure gradient (APG) TBL simulation is a new one carried out by the present authors. The APG TBL begins as a zero pressure gradient boundary layer, decelerates under a strong adverse pressure gradient, and separates near the end of the domain in the form of a very thin separation bubble. The one-point velocity statistics in the outer region of this large-defect boundary layer are compared to those of two other large-velocity-defect APG TBLs (one in dynamic equilibrium, the other in disequilibrium) and a mixing layer. In the upper half of the large-defect boundary layers, the velocity statistics are similar to those of the mixing layer. The dominant peaks of turbulence production and Reynolds stresses are located in the middle of the boundary layers. Three-dimensional spatial correlations of (u, u) and (u, v) show that coherence is lost in the streamwise and spanwise directions as the velocity defect increases. Near-wall streaks tend to disappear in the large-defect zone of the flow to be replaced by more disorganized u motions. Near-wall sweeps and ejections are also less numerous. In the outer region, the u structures tend to be shorter, less streaky, and more inclined with respect to the wall than in the ZPG TBL. The sweeps and ejections are generally bigger with respect to the boundary layer thickness in the large-defect boundary layer, even if the biggest structures are found in the ZPG TBL. Large sweeps and ejections that reach the wall region (wall-attached) are less streamwise elongated and they occupy less space than in the ZPG boundary layer. The distinction between wall-attached and wall-detached structures is not as pronounced in the large-defect TBL.
On the parametrization of the planetary boundary layer of the atmosphere
Energy Technology Data Exchange (ETDEWEB)
Yordanov, D. [Bulgarian Academy of Sciences, Geophysical Inst., Sofia (Bulgaria); Syrakov, D.; Kolarova, M. [Bulgarian Academy of Sciences, National Inst. of Meteorology and Hydrology, Sofia (United Kingdom)
1997-10-01
The investigation of the dynamic processes in the planetary boundary layer presents a definite theoretical challenge and plays a growing role for the solution of a number of practical tasks. The improvement of large-scale atmospheric weather forecast depends, to a certain degree, on the proper inclusion of the planetary boundary layer dynamics in the numerical models. The modeling of the transport and the diffusion of air pollutants is connected with estimation of the different processes in the Planetary Boundary Layer (PBL) and needs also a proper PBL parametrization. For the solution of these practical tasks the following PBL models;(i) a baroclinic PBL model with its barotropic version, and (ii) a convective PBL model were developed. Both models are one dimensional and are based on the similarity theory and the resistance lows extended for the whole PBL. Two different PBL parametrizations under stable and under convective conditions are proposed, on the basis of which the turbulent surface heat and momentum fluxes are estimated using generalized similarity theory. By the proposed parametrizations the internal parameters are calculated from the synoptic scale parameters as geostrophyc wind, potential temperature and humidity given at two levels (ground level and at 850 hPa) and from them - the PBL profiles. The models consists of two layers: a surface layer (SL) with a variable height and a second (Ekman layer) over it with a constant with height turbulent exchange coefficient. (au) 14 refs.
General mechanisms of thin layers in high Reynolds number turbulent flows
Hunt, Julian; Ishihara, Takashi; Morishita, Koji
2015-11-01
Mechanisms and computation are presented for the three types of thin, high vorticity, randomly moving shear layers at high Reynolds number. They decorrelate eddy motions on each side and, in the first two types, have an internal micro-scale, dissipative structure. Their form also depends on the mean strain/shear outside the layer, and the proximity of any resistive boundaries. The first type (T/NT) lie between regions of sheared turbulence and external non-turbulent motions. Depending on whether the inflection points of the conditional mean shear profile, yi) > , relative to the interface coordinate yi, are on the outside or inside edges of the layer, the forms of the interface are ``nibbling'' motions on the scale of the layer thickness or large ``engulfing'' motions, which affect the overall flow structure. In the second type (T/In), which occurs in the interior of turbulent flows, because the interface instabilities are suppressed, the stretching increases more than in T/NT, causing the micro-scale vorticity, velocity and dissipation to greatly exceed Kolmogorov's theory. The third type (T/W) within the buffer wall layer, by blocking outer eddies, determines the displaced form of the mean logarithmic profile, and fluctuations of wall shear stress.
Czech Academy of Sciences Publication Activity Database
Jonáš, Pavel
Praha : Ústav termomechaniky AV ČR, v. v. i., 2014 - (Jonáš, P.; Uruba, V.), s. 19-20 ISBN 978-80-87012-53-6. [Colloquium FLUID DYNAMICS 2014. Praha (CZ), 22.10.2014-24.10.2014] R&D Projects: GA ČR GAP101/12/1271 Institutional support: RVO:61388998 Keywords : boundary layer * laminar-turbulent transition * flow control by suction * porous surface Subject RIV: BK - Fluid Dynamics