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 oscillating seabed. A brief account is given of measured quantities, measurement techniques (LDA, PIV, flow visualization) and limitations/constraints in the experimental investigation of the wave boundary layer in the laboratory. The second section concentrates on uniform oscillating boundary layers...... with a smooth bed. The boundary layer process is described over the entire range of the Reynolds number (Re from practically nil to Re = O(107)), from the laminar regime to the transitional regime and to the fully developed turbulent regime. The third section focuses on the effect of the boundary roughness...
Study of interaction between shock wave and unsteady boundary layer
Institute of Scientific and Technical Information of China (English)
董志勇; 韩肇元
2003-01-01
This paper reports theoretical and experimental study of a new type of interaction of a moving shock wave with an unsteady boundary layer. This type of shock wave-boundary layer interaction describes a moving shock wave interaction with an unsteady boundary layer induced by another shock wave and a rarefaction wave. So it is different from the interaction of a stationary shock wave with steady boundary layer, also different from the interaction of a reflected moving shock wave at the end of a shock tube with unsteady boundary layer induced by an incident shock. Geometrical shock dynamics is used for the theoretical analysis of the shock wave-unsteady boundary layer interaction, and a double-driver shock tube with a rarefaction wave bursting diaphragm is used for the experimental investigation in this work.
Leaky waves in boundary layer flow
Pralits, Jan
2005-11-01
Linear stability analysis of boundary layer flow is traditionally performed by solving the Orr-Sommerfeld equation (OSE), either in a temporal or a spatial framework. The mode structure of the OSE is in both cases composed of a finite number of discrete modes which decay at infinity in the wall- normal direction y, and a continuous spectrum of propagating modes behaving as (±ik y) when y->∞, with real k. A peculiarity of this structure is that the number of discrete modes changes with the Reynolds number, Re. They indeed seem to disappear behind the continuous spectrum at certain Re. This phenomenon is here investigated by studying the response of the Blasius boundary layer forced instantaneously in space and time. Since the solution of the forced and homogeneous Laplace-transformed problem both depend on the free-stream boundary conditions, it is shown here that a suitable change of variables can remove the branch cut in the Laplace plane. As a result, integration of the inverse Laplace transform along the two sides of the branch cut, which gives rise to the continuous spectrum, can be replaced by a sum of residues corresponding to an additional set of discrete eigenvalues. These new modes grow at infinity in the y direction, and are analogous to the leaky waves found in the theory of optical waveguides, i.e. optical fibers, which are attenuated in the direction of the waveguide but grow unbounded in the direction perpendicular to it.
Wave boundary layer hydrodynamics during onshore bar migration
Henriquez, M.; Reniers, A.; Ruessink, G.; Stive, M.J.F.
2010-01-01
To study onshore bar migration and the accompanying intra-wave sediment transport a wave flume experiment was conducted. The wave flume had a rigid bottom with a single bar profile. The focus of the experiment was to measure the hydrodynamics in the wave bottom boundary layer. The results show that
Shock-Wave Boundary Layer Interactions
1986-02-01
proprietes de la couche limite subissent au cours de I’interaction; les methodes integrales ou aux differences finies qui permettent le calcul continu de...interesse par la recherche d’une plus ample information. CONTENTS Page PREFACE »’ INTRODUCTION 1 PART I: A PHYSICAL DESCRIPTION OF SHOCK-WAVE/BOUNDARY...References 105 109 PART II: METHODS OF CALCULATION GLOBAL METHODS 1.1 Introductory Remarks 109 1.2 Two-Dimensional Interactions HO 1.2.1
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.
The use of a wave boundary layer model in SWAN
DEFF Research Database (Denmark)
Du, Jianting; Bolaños, Rodolfo; Larsén, Xiaoli Guo
2017-01-01
A Wave Boundary Layer Model (WBLM) is implemented in the third-generation ocean wave model SWAN to improve the wind-input source function under idealized, fetch-limited condition. Accordingly, the white capping dissipation parameters are re-calibrated to fit the new wind-input source function...
Numerical simulation of tsunami-scale wave boundary layers
DEFF Research Database (Denmark)
Williams, Isaac A.; Fuhrman, David R.
2016-01-01
demonstrating the ability to reproduce accurate velocity profiles, turbulence, and bed shear stresses on both smooth and rough beds.The validated model is then employed for the study of transient wave boundary layers at full tsunami scales,covering a wide and realistic geophysical range in terms of the flow......, is newly extended to incorporate a transitional variant of the standard two-equation k–ω turbulence closure. The developed numerical model is successfully validated against recent experimental measurements involving transient solitary wave boundary layers as well as for oscillatory flows, collectively......This paper presents a numerical study of the boundary layer flow and properties induced by tsunami-scalewaves. For this purpose, an existing one-dimensional vertical (1DV) boundary layer model, based on the horizontal component of the incompressible Reynolds-averaged Navier–Stokes (RANS) equations...
Effect of externally generated turbulence on wave boundary layer
DEFF Research Database (Denmark)
Fredsøe, Jørgen; Sumer, B. Mutlu; Kozakiewicz, A.
2003-01-01
This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel...... with a smooth bottom. Turbulence was generated ´externally´ as the flow in the oscillator was passed through a series of grids, that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity...... results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore...
Transition in Hypersonic Boundary Layers: Role of Dilatational Waves
Zhu, Yiding; Zhang, Chuanhong; Tang, Qing; Yuan, Huijing; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-El-Hak, Mohamed
2015-11-01
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-mode instability is a key modulator of the transition process. Although the second mode is primarily an acoustic wave, it causes the formation of high-frequency vortical waves. While the growing acoustic wave itself is rapidly annihilated due to its large and sharp dissipation peak that is enhanced by the bulk viscosity, the acoustically generated high-frequency vortical wave keeps growing and triggers a fast transition to turbulence.
Modelling wave-boundary layer interaction for wind power applications
Jenkins, A. D.; Barstad, I.; Gupta, A.; Adakudlu, M.
2012-04-01
Marine wind power production facilities are subjected to direct and indirect effects of ocean waves. Direct effects include forces due to wave orbital motions and slamming of the water surface under breaking wave conditions, corrosion and icing due to sea spray, and the effects of wave-generated air bubbles. Indirect effects include include the influence of waves on the aerodynamic sea-surface roughness, air turbulence, the wind velocity profile, and air velocity oscillations, wave-induced currents and sediment transport. Field observations within the boundary layers from floating measurement may have to be corrected to account for biases induced as a result of wave-induced platform motions. To estimate the effect of waves on the atmospheric boundary layer we employ the WRF non-hydrostatic mesoscale atmosphere model, using the default YSU planetary boundary layer (PBL) scheme and the WAM spectral wave model, running simultaneously and coupled using the open-source coupler MCEL which can interpolate between different model grids and timesteps. The model is driven by the WRF wind velocity at 10 m above the surface. The WRF model receives from WAM updated air-sea stress fields computed from the wind input source term, and computes new fields for the Charnock parameter and marine surface aerodynamic roughness. Results from a North Atlantic and Nordic Seas simulation indicate that the two-way coupling scheme alters the 10 metre wind predicted by WRF by up to 10 per cent in comparison with a simulation using a constant Charnock parameter. The changes are greatest in developing situations with passages of fronts, moving depressions and squalls. This may be directly due to roughness length changes, or may be due to changes in the timing of front/depression/squall passages. Ongoing work includes investigating the effect of grid refinement/nesting, employing different PBL schemes, and allowing the wave field to change the direction of the total air-sea stress.
Nonlinear interaction of two waves in boundary-layer flows
Nayfeh, A. H.; Bozatli, A. N.
1980-01-01
First-order nonlinear interactions of Tollmien-Schlichting waves of different frequencies and initial amplitudes in boundary-layer flows are analyzed using the method of multiple scales. Numerical results for flow past a flat plate show that the spatial detuning wipes out resonant interactions unless the initial amplitudes are very large. Thus, a wave having a moderate amplitude has little influence on its subharmonic although it has a strong influence on its second harmonic. Moreover, two waves having moderate amplitudes have a strong influence on their difference frequency. The results show that the difference frequency can be very unstable when generated by the nonlinear interaction, even though it may be stable when introduced by itself in the 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.
Wave analysis of the evolution of a single wave packet in supersonic boundary layer
Yermolaev, Yury G.; Yatskikh, Aleksey A.; Kosinov, Alexander D.; Semionov, Nickolay V.
2016-10-01
The evolution of the artificial wave packet in laminar flat-plate boundary layer was experimentally studied by hot-wire measurements at M=2. The localized disturbances were generated by pulse glow discharge. The wave analysis of evolution of wave packet was provided. It was found, that the most unstable waves are oblique, that consistent with results of linear theory.
Simulation of hypersonic shock wave - laminar boundary layer interactions
Kianvashrad, N.; Knight, D.
2017-06-01
The capability of the Navier-Stokes equations with a perfect gas model for simulation of hypersonic shock wave - laminar boundary layer interactions is assessed. The configuration is a hollow cylinder flare. The experimental data were obtained by Calspan-University of Buffalo (CUBRC) for total enthalpies ranging from 5.07 to 21.85 MJ/kg. Comparison of the computed and experimental surface pressure and heat transfer is performed and the computed §ow¦eld structure is analyzed.
Large Eddy Simulation of the ventilated wave boundary layer
DEFF Research Database (Denmark)
Lohmann, Iris P.; Fredsøe, Jørgen; Sumer, B. Mutlu
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...... 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...... significantly. Ventilation therefore results in a net current, even in symmetric waves....
STUDY OF SWEPT SHOCK WAVE AND BOUNDARY LAYER INTERACTIONS
Institute of Scientific and Technical Information of China (English)
1998-01-01
This paper presents briefly the recent progress on study of swept shock wave/boundary layer interactions with emphasis on application of zonalanalysis and correlation analysis to them. Based on the zonal analysis an overall framework of complicated interaction flow structure including both surface flowfield and space flowfield is discussed. Based on correlation analysis the conical interactions induced by four families of shock wave generators have been discussedin detail. Some control parameter and physical mechanism of conical interaction have been revealed. Finally some aspects of the problem and the prospects for future work are suggested.
Three-Dimensional Waves in Tilt Thermal Boundary Layers
Institute of Scientific and Technical Information of China (English)
TAO Jian-Jun; YUAN Xiang-Jiang
2009-01-01
We numerically and theoretically study the stabilities of tilt thermal boundary layers immersed in stratified air. An interesting phenomenon is revealed: the stationary longitudinal-roll mode becomes unstable to some oscillating state even when the Grashof number is smaller than its corresponding critical value. By stability analysis, this phenomenon is explained in terms of a new three-dimensional wave mode. The effect of the tilt angle on the stability of the boundary flows is investigated. Since the new three-dimensional wave is found to be the most unstable mode when the title angle is between 30° and 64°, it is expected to play an important role in the transition to turbulence.
Aerodynamic Heating in Hypersonic Boundary Layers:\\ Role of Dilatational Waves
Zhu, Yiding; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2016-01-01
The evolution of multi-mode instabilities in a hypersonic boundary layer and their effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization, fast-response pressure sensors, fluorescent temperature-sensitive paint (TSP), and particle image velocimetry (PIV). Calculations are also performed based on both parabolized stability equations (PSE) and direct numerical simulations (DNS). It is found that second-mode dilatational waves, accompanied by high-frequency alternating fluid compression and expansion, produce intense aerodynamic heating in a small region that rapidly heats the fluid passing through it. As a result, the surface temperature rapidly increases and results in an overshoot over the nominal transitional value. When the dilatation waves decay downstream, the surface temperature decreases gradually until transition is completed. A theoretical analysis is provided to interpret the temperature distribution affected by ...
Nonlinear interaction of waves in boundary-layer flows
Nayfeh, A. H.; Bozatli, A. N.
1979-01-01
First-order nonlinear interactions of Tollmien-Schlichting waves of different frequencies and initial amplitudes in boundary-layer flows are analyzed by using the method of multiple scales. For the case of two waves, a strong nonlinear interaction exists if one of the frequencies w2 is twice the other frequency w1. Numerical results for flow past a flat plate show that this interaction mechanism is strongly destabilizing even in regions where either the fundamental or its harmonic is damped in the absence of the interaction. For the case of three waves, a strong nonlinear interaction exists when w3 = w2- w1. This combination resonance causes the amplitude of the wave with the difference frequency w3 to multiply many times in magnitude in a short distance even if it is damped in the absence of the interaction. The initial amplitudes play a dominant role in determining the changes in the amplitudes of the waves in both of these mechanisms.
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.
Wave boundary layer over a stone-covered bed
DEFF Research Database (Denmark)
Dixen, Martin; Hatipoglu, Figen; Sumer, B. Mutlu
2008-01-01
This paper summarizes the results of an experimental investigation on wave boundary layers over a bed with large roughness, simulating stone/rock/armour block cover on the sea bottom. The roughness elements used in the experiments were stones the size of 1.4cm and 3.85cm in one group of experiments...... and regular ping-pong balls the size 3.6cm in the other. The orbital-motion-amplitude-to-roughness ratio at the bed was rather small, in the range a/ks=0.6-3. The mean and turbulence properties of the boundary-layer flow were measured. Various configurations of the roughness elements were used in the ping......-pong ball experiments to study the influence of packing pattern, packing density, number of layers and surface roughness of the roughness elements. The results show that the friction factor seems to be not extremely sensitive to these factors. The results also show that the friction factor for small values...
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.
Perfectly-matched-layer boundary integral equation method for wave scattering in a layered medium
Lu, Wangtao; Qian, Jianliang
2016-01-01
For scattering problems of time-harmonic waves, the boundary integral equation (BIE) methods are highly competitive, since they are formulated on lower-dimension boundaries or interfaces, and can automatically satisfy outgoing radiation conditions. For scattering problems in a layered medium, standard BIE methods based on the Green's function of the background medium must evaluate the expensive Sommefeld integrals. Alternative BIE methods based on the free-space Green's function give rise to integral equations on unbounded interfaces which are not easy to truncate, since the wave fields on these interfaces decay very slowly. We develop a BIE method based on the perfectly matched layer (PML) technique. The PMLs are widely used to suppress outgoing waves in numerical methods that directly discretize the physical space. Our PML-based BIE method uses the Green's function of the PML-transformed free space to define the boundary integral operators. The method is efficient, since the Green's function of the PML-tran...
Sensitivity of African easterly waves to boundary layer conditions
Directory of Open Access Journals (Sweden)
A. Lenouo
2008-06-01
Full Text Available A linearized version of the quasi-geostrophic model (QGM with an explicit Ekman layer and observed static stability parameter and profile of the African easterly jet (AEJ, is used to study the instability properties of the environment of the West African wave disturbances. It is found that the growth rate, the propagation velocity and the structure of the African easterly waves (AEW can be well simulated. Two different lower boundary conditions are applied. One assumes a lack of vertical gradient of perturbation stream function and the other assumes zero wind perturbation at the surface. The first case gives more realistic results since in the absence of horizontal diffusion, growth rate, phase speed and period have values of 0.5 day^{−1}, 10.83 m s^{−1} and 3.1 day, respectively. The zero wind perturbation at the surface case leads to values of these parameters that are 50 percent lower. The analysis of the sensitivity to diffusion shows that the magnitude of the growth rate decreases with this parameter. Modelled total relative vorticity has its low level maximum around 900 hPa under no-slip, and 700 hPa under free slip condition.
Kranenburg, Wouter; Ribberink, Jan S.; Uittenbogaard, R.E.; Hulscher, Suzanne 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
Kranenburg, Wouter; Ribberink, Jan S.; Uittenbogaard, R.E.; Hulscher, Suzanne 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
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
Coherent structures in wave boundary layers. Part 1. Oscillatory motion
DEFF Research Database (Denmark)
Carstensen, Stefan; Sumer, B. Mutlu; Fredsøe, Jørgen
2010-01-01
This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i......) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent...
Spike-like solitary waves in incompressible boundary layers driven by a travelling wave.
Feng, Peihua; Zhang, Jiazhong; Wang, Wei
2016-06-01
Nonlinear waves produced in an incompressible boundary layer driven by a travelling wave are investigated, with damping considered as well. As one of the typical nonlinear waves, the spike-like wave is governed by the driven-damped Benjamin-Ono equation. The wave field enters a completely irregular state beyond a critical time, increasing the amplitude of the driving wave continuously. On the other hand, the number of spikes of solitary waves increases through multiplication of the wave pattern. The wave energy grows in a sequence of sharp steps, and hysteresis loops are found in the system. The wave energy jumps to different levels with multiplication of the wave, which is described by winding number bifurcation of phase trajectories. Also, the phenomenon of multiplication and hysteresis steps is found when varying the speed of driving wave as well. Moreover, the nature of the change of wave pattern and its energy is the stability loss of the wave caused by saddle-node bifurcation.
Vertical pressure gradient and particle motions in wave boundary layers
DEFF Research Database (Denmark)
Jensen, Karsten Lindegård
The present study covers both a numerical and experimental investigation of the processes in the oscillatory boundary layer. In the first part a direct numerical simulation (DNS) is conducted to study the vertical pressure gradient, and its role in relation to laminar to turbulent transition...... and its role in the fully turbulent boundary layer. The pressure in the flow is obtained from the flow fields of the oscillatory boundary layer. What differs, the vertical pressure gradient, from other turbulent quantities, like e.g. velocity fluctuations is that it can detect newly generated turbulence....... 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...
Coherent structures in wave boundary layers. Part 2. Solitary motion
DEFF Research Database (Denmark)
Sumer, B. Mutlu; Jensen, Palle Martin; Sørensen, Lone B.;
2010-01-01
in an oscillating water tunnel. Two kinds of measurements were made: bed shear stress measurements and velocity measurements. The experiments show that the solitary-motion boundary layer experiences three kinds of flow regimes as the Reynolds number is increased: (i) laminar regime; (ii) laminar regime where...... the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces...
Combined Wave and Current Bottom Boundary Layers: A Review
2016-03-01
viscoelastic surface (Dalrymple and Liu 1978; Foda et al. 1993; Jain and Mehta 2009; Mallard and Dalrymple 1977; Mei et al. 2010; Ng and Zhang 2007...parameters. They noted good agreement between the damping coefficient and wave number as a function of layer thickness with available measurements. Foda et...attenuation tended to increase as a function of the layer thickness and bed stiffness. Foda et al. (1993) further surmised that the mud behaved like
Space and Astrophysical Plasmas : Sun–Earth connection: Boundary layer waves and auroras
Indian Academy of Sciences (India)
G S Lakhina; B T Tsurutani; J K Arballo; C Galvan
2000-11-01
Boundary layers are the sites where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The main characteristics of the broadband plasma waves (with frequencies > 1 Hz) observed in the magnetopause, polar cap, and plasma sheet boundary layers are described. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufﬁcient precipitated energy ﬂux to the ionosphere to create the diffused auroral oval. The broadband plasma waves may also play an important role in the processes of local heating/acceleration of the boundary layer plasma.
2014-12-16
Shock Wave /Turbulent Boundary Layer Interaction in Conical Flows FA9550-11-1-0203 Dr. Charles E. Tinney, Aerospace Engineering and Engineering...Low-Dimensional Dynamical Characteristics of Shock Wave /Turbulent Boundary Layer Interaction in Conical Flows Contract/Grant Number: FA9550-11-1-0203...driven by transonic resonance (Zaman et al, 2002). What is common about many of these planar nozzle studies is that there is just one single
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.
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....
Wave-Particle Interactions in the Turbulent Plasmaspheric Boundary Layer
Mishin, Evgeny
2012-10-01
We present in situ satellite observations of plasmaspheric lower hybrid/fast magnetosonic turbulence and broadband hiss-like VLF emissions related with substorm subauroral ion drifts/polarization streams (SAID/SAPS) in the magnetosphere and topside ionosphere. SAID/SAPS appear in ˜10 min after the substorm onset consistent with the fast propagation of substorm injection fronts. The SAID channel follows the dispersionless cutoff of the energetic electron flux at the plasmapause. This indicates that the cold plasma maintains charge neutrality within the channel, thereby short-circuiting the injected plasmoid (injection front) over the plasmasphere. As with the well-documented plasmoid-magnetic barrier problem, plasma turbulence ensures the circuit resistivity and magnetic diffusion as well as significant electron heating and acceleration. The SAID/SAPS-related VLF emissions were used to simulate interactions with the outer zone electrons. These emissions appear to constitute a distinctive subset of substorm/storm-related VLF activity in the region co-located with freshly injected energetic ions equatorward of the plasma sheet boundary. Significant pitch-angle diffusion coefficients suggest that substorm SAID/SAPS-related VLF waves could be responsible for the alteration of the outer radiation belt boundary during (sub)storms.
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
suspended sediment concentrations, (2) turbulence suppression due to density gradients in the water–sand mixture, (3) boundary layer streaming due to convective terms, and (4) converging–diverging effects due to a sloping bed. The present model therefore provides a framework for simultaneous inclusion...... of a number of local factors important within cross-shore wave boundary layer and sediment transport dynamics. The hydrodynamic model is validated for both hydraulically smooth and rough conditions, based on wave friction factor diagrams and boundary layer streaming profiles, with the results in excellent...... to investigate the importance of boundary layer streaming effects on sediment transport in selected velocity-skewed conditions. For the medium sand grain conditions considered, the model results suggest that streaming effects can enhance onshore sediment transport rates by asmuch as a factor of two...
Wave mediated angular momentum transport in astrophysical boundary layers
Hertfelder, Marius
2015-01-01
Context. Disk accretion onto weakly magnetized stars leads to the formation of a boundary layer (BL) where the gas loses its excess kinetic energy and settles onto the star. There are still many open questions concerning the BL, for instance the transport of angular momentum (AM) or the vertical structure. Aims. It is the aim of this work to investigate the AM transport in the BL where the magneto-rotational instability (MRI) is not operating owing to the increasing angular velocity $\\Omega(r)$ with radius. We will therefore search for an appropriate mechanism and examine its efficiency and implications. Methods. We perform 2D numerical hydrodynamical simulations in a cylindrical coordinate system $(r, \\varphi)$ for a thin, vertically inte- grated accretion disk around a young star. We employ a realistic equation of state and include both cooling from the disk surfaces and radiation transport in radial and azimuthal direction. The viscosity in the disk is treated by the {\\alpha}-model; in the BL there is no v...
PIV measurements of the bottom boundary layer under nonlinear surface waves
Henriquez, M.; Reniers, A. J H M; Ruessink, B. G.; Stive, M. J F
2014-01-01
Sediment in the nearshore is largely mobilized in the wave bottom boundary layer (wbbl) hereby emphasizing the importance of this relatively thin layer to nearshore morphology. This paper presents a laboratory experiment where hydrodynamic properties of the wbbl were quantified by measuring flow vel
Ultra low frequency waves observed by Double Star TC-1 in the plasmasphere boundary layer
Institute of Scientific and Technical Information of China (English)
2008-01-01
The characteristic and properties of ULF waves in the plasmasphere boundary layer during two very quiet periods are present. The ULF waves were detected by Double Star TC-1 when the spacecraft passed through the plasmasphere in an outbound and inbound trajectories, respectively. A clear association between the ULF waves and periodic variations of energetic ions fluxes was observed. The ob-servations showed that the wave frequency was higher inside the plasmasphere than outside. The mechanism generating these ULF waves and possible diagnos-ing of the "classical plasmapause" location with the ULF wave were discussed.
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...
High-resolution PIV measurements of a transitional shock wave-boundary layer interaction
Giepman, R. H. M.; Schrijer, F. F. J.; van Oudheusden, B. W.
2015-06-01
This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m-1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96. The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80-90 downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43), and transition is found to be accelerated over the separation bubble.
Vortex tubes in the wave bottom boundary layer
Henriquez, M.; Reniers, A.J.H.M.; Ruessink, B.G.; Stive, M.J.F.
2012-01-01
The cause of sediment suspension events during flow reversal under waves in the nearshore is not well understood. Vortex tubes and horizontal pressure gradients have been suggested to be the cause of the suspension events. A medium sized wave flume experiment has been conducted to give insight in th
Directory of Open Access Journals (Sweden)
G. Stenberg
2007-11-01
Full Text Available We use whistler waves observed close to the magnetopause as an instrument to investigate the internal structure of the magnetopause-magnetosheath boundary layer. We find that this region is characterized by tube-like structures with dimensions less than or comparable with an ion inertial length in the direction perpendicular to the ambient magnetic field. The tubes are revealed as they constitute regions where whistler waves are generated and propagate. We believe that the region containing tube-like structures extend several Earth radii along the magnetopause in the boundary layer. Within the presumed wave generating regions we find current structures moving at the whistler wave group velocity in the same direction as the waves.
Ishihara, S.; Tamura, S.; Ishii, K.; Kataoka, H.
2016-09-01
To study the effects of the boundary layer on the deflagration to detonation transition (DDT) process, the mixture behind an incident shock wave was ignited using laser breakdown. Ignition timing was controlled so that the interaction of the resulting flame with a laminar or turbulent boundary layer could be examined. In the case of the interaction with a laminar boundary layer, wrinkling of the flame was observed after the flame reached the corner of the channel. On the other hand, interaction with the turbulent boundary layer distorted the flame front and increased the spreading rate of the flame followed by prompt DDT. The inner structure of the turbulent boundary layer plays an important role in the DDT process. The region that distorted the flame within the turbulent boundary layer was found to be the intermediate region 0.01DDT was independent of the ignition position. The effect of the boundary layer on the propagating flame, thus, became relatively small after the accelerating flame was generated.
Budget of Turbulent Kinetic Energy in a Shock Wave Boundary-Layer Interaction
Vyas, Manan A.; Waindim, Mbu; Gaitonde, Datta V.
2016-01-01
Implicit large-eddy simulation (ILES) of a shock wave/boundary-layer interaction (SBLI) was performed. Quantities present in the exact equation of the turbulent kinetic energy transport were accumulated and used to calculate terms like production, dissipation, molecular diffusion, and turbulent transport. The present results for a turbulent boundary layer were validated by comparison with direct numerical simulation data. It was found that a longer development domain was necessary for the boundary layer to reach an equilibrium state and a finer mesh resolution would improve the predictions. In spite of these findings, trends of the present budget match closely with that of the direct numerical simulation. Budgets for the SBLI region are presented at key axial stations. These budgets showed interesting dynamics as the incoming boundary layer transforms and the terms of the turbulent kinetic energy budget change behavior within the interaction region.
An analytical model for the amplitude of lee waves forming on the boundary layer inversion
Sachsperger, Johannes; Serafin, Stefano; Stiperski, Ivana; Grubišić, Vanda
2016-04-01
Lee waves are horizontally propagating gravity waves with a typical wavelength of 5-15 km that may be generated when stratified flow is lifted over a mountain. A frequently observed type of such waves is that of interfacial lee waves. Those develop, similar to surface waves on a free water surface, when the upstream flow features a density discontinuity. Such conditions are often present for example at the capping inversion in boundary layer flow. The dynamics of interfacial lee waves can be described concisely with linear interfacial gravity wave theory. However, while this theoretical framework accurately describes the wavelength, it fails to properly predict the amplitude of lee waves. It is well known that large amplitude lee waves may lead to low-level turbulence, which poses a potential hazard for aviation. Therefore, this property of interfacial lee waves deserves further attention. In this study, we develop a simple analytical model for the amplitude of lee waves forming on the boundary layer inversion. This model is based on the energetics of two-layer flow. We obtain an expression for the wave amplitude by equating the energy loss across an internal jump with the energy radiation through lee waves. The verification of the result with water tank experiments of density-stratified two-layer flow over two-dimensional topography from the HYDRALAB campaign shows good agreement between theory and observations. This new analytical model may be useful in determining potential hazards of interfacial lee waves with negligible computational cost as compared to numerical weather prediction models.
Turbulent boundary-layer control with spanwise travelling waves
Energy Technology Data Exchange (ETDEWEB)
Whalley, Richard D; Choi, Kwing-So, E-mail: Richard.Whalley@nottingham.ac.uk, E-mail: Kwing-So.Choi@nottingham.ac.uk [Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD (United Kingdom)
2011-12-22
It has been demonstrated through numerical simulations using Lorentz forcing that spanwise travelling waves on turbulent wall flows can lead to a skin-friction drag reduction on the order of 30%. As an aeronautical application of this innovative flow control technique, we have investigated into the use of Dielectric-Barrier-Discharge (DBD) plasma actuators to generate spanwise travelling waves in air. The near-wall structures modified by the spanwise travelling waves were studied using the PIV technique in a wind tunnel, while the associated turbulence statistics were carefully documented using hot-wire anemometry. We observed the spreading of low-speed fluid by the spanwise travelling streamwise vortices, which seems to have greatly attenuated the turbulence production process. This is very much in line with the finding of DNS studies, where wide low-speed ribbons replaced the low-speed streaks.
Effect of air jet vortex generators on a shock wave boundary layer interaction
Souverein, L.J.; Debiève, J.-F.
2010-01-01
The effect of upstream injection by means of continuous air jet vortex generators (AJVGs) on a shock wave turbulent boundary layer interaction is experimentally investigated. The baseline interaction is of the impinging type, with a flow deflection angle of 9.5degrees and a Mach number Me = 2.3. Con
Effect on a shock wave boundary layer interaction of air jet vortex generators
Souverein, L.J.; Debieve, J.F.
2013-01-01
The effect of upstream injection by means of continuous Air Jet Vortex Generators (AJVGs) on a shock wave turbulent boundary layer interaction is experimentally investigated. The baseline interaction is of the impinging type, with a flow deflection angle of 9.5◦, a Mach number Me = 2.3, and a moment
Unsteady Flow Organization of a Shock Wave/Boundary Layer Interaction
Humble, R.A.
2009-01-01
A fundamental experimental study is carried out to investigate the unsteady flow organization of an incident shock wave/turbulent boundary layer interaction at Mach 2.1. Planar and tomographic particle image velocimetry (PIV) are used in combination with data processing using the proper orthogonal d
An Optimized Combined Wave and Current Bottom Boundary Layer Model for Arbitrary Bed Roughness
2017-06-30
studies of shelf circulation patterns that incorporate wave-current effects in the bottom boundary layer have been conducted in the past (e.g...presence of surface waves reemphasizes the fact that wave-current effects are important on storm- dominated continental shelves. In addition to these...can be defined for / rz z1 (i.e., */ rrzz ακR1 ), where * * / )(r cw rR u z ω . The two expressions are related by * */ /r r zR zR 0 , where
Broadband plasma waves observed in the polar cap boundary layer: Polar
Tsurutani, B. T.; Lakhina, G. S.; Ho, C. M.; Arballo, J. K.; Galvan, C.; Boonsiriseth, A.; Pickett, J. S.; Gurnett, D. A.; Peterson, W. K.; Thorne, R. M.
1998-08-01
Polar observations indicate the presence of intense broadband plasma waves nearly all of the time (96% occurrence frequency in this study) near the apogee of the Polar trajectory (~6-8RE). The region of wave activity bounds the dayside (0500 to 1800 LT) polar cap magnetic fields, and we thus call these waves polar cap boundary layer (PCBL) waves. The waves are spiky signals spanning a broad frequency range from ~101 to 2×104Hz. The waves have a rough power law spectral shape. The wave magnetic component has on average a f-2.7 frequency dependence and appears to have an upper frequency cutoff of ~(6-7)×103Hz, which is the electron cyclotron frequency. The electric component has on average a f-2.2 frequency dependence and extends up to ~2×104Hz. The frequency dependences of the waves and the amplitude ratios of B'/E' indicate a possible mixture of obliquely propagating electromagnetic whistler mode waves plus electrostatic waves. There are no clear intensity peaks in either the magnetic or electric spectra which can identify the plasma instability responsible for the generation of the PCBL waves. The wave character (spiky nature, frequency dependence and admixture of electromagnetic and electrostatic components) and intensity are quite similar to those of the low-latitude boundary layer (LLBL) waves detected at and inside the low-latitude dayside magnetopause. Because of the location of the PCBL waves just inside the polar cap magnetic field lines, it is natural to assume that these waves are occurring on the same magnetic field lines as the LLBL waves, but at lower altitudes. Because of the similar wave intensities at both locations and the occurrence at all local times, we rule out an ionospheric source. We also find a magnetosheath origin improbable. The most likely scenario is that the waves are locally generated by field-aligned currents or current gradients. We find a strong relationship between the presence of ionospheric and magnetosheath ions and the
Influence of rigid boundary on the propagation of torsional surface wave in an inhomogeneous layer
Indian Academy of Sciences (India)
Shishir Gupta; Rehena Sultana; Santimoy Kundu
2015-02-01
The present work illustrates a theoretical study on the effect of rigid boundary for the propagation of torsional surface wave in an inhomogeneous crustal layer over an inhomogeneous half space. It is believed that the inhomogeneity in the half space arises due to hyperbolic variation in shear modulus and density whereas the layer has linear variation in shear modulus and density. The dispersion equation has been obtained in a closed form by using Whittaker’s function, which shows the variation of phase velocity with corresponding wave number. Numerical results show the dispersion equations, which are discussed and presented by means of graphs. Results in some special cases are also compared with existing solutions available from analytical methods, which show a close resemblance. It is also observed that, for a layer over a homogeneous half space, the velocity of torsional waves does not coincide with that of Love waves in the presence of the rigid boundary, whereas it does at the free boundary. Graphical user interface (GUI) software has been developed using MATLAB 7.5 to generalize the effect of various parameter discussed.
Institute of Scientific and Technical Information of China (English)
唐登斌; 夏浩
2002-01-01
The nonlinear evolution problem in nonparallel boundary layer stability was studied. The relative parabolized stability equations of nonlinear nonparallel boundary layer were derived. The developed numerical method, which is very effective, was used to study the nonlinear evolution of T-S disturbance wave at finite amplitudes. Solving nonlinear equations of different modes by using predictor-corrector and iterative approach, which is uncoupled between modes, improving computational accuracy by using high order compact differential scheme, satisfying normalization condition, determining tables of nonlinear terms at different modes, and implementing stably the spatial marching, were included in this method. With different initial amplitudes, the nonlinear evolution of T-S wave was studied. The nonlinear nonparallel results of examples compare with data of direct numerical simulations (DNS) using full Navier- Stokes equations.
Martis, R. R.; Misra, A.
2017-03-01
A numerical study is conducted to determine the effectiveness of six different microvortex generator geometries in controlling swept shock wave/boundary-layer interactions. The geometries considered are base ramp, base ramp with declining angle of 45°, blunt ramp, split ramp, thick vanes, and ramped vanes. Microvortex generators with a gap were found to be better suited for delaying the separation. Thick vanes showed the largest delay in separation among the devices studied.
Martis, R. R.; Misra, A.
2017-09-01
A numerical study is conducted to determine the effectiveness of six different microvortex generator geometries in controlling swept shock wave/boundary-layer interactions. The geometries considered are base ramp, base ramp with declining angle of 45°, blunt ramp, split ramp, thick vanes, and ramped vanes. Microvortex generators with a gap were found to be better suited for delaying the separation. Thick vanes showed the largest delay in separation among the devices studied.
Characterization of an incipiently separated shock wave/turbulent boundary layer interaction
Schreyer, A.-M.; Dussauge, J.-P.; Krämer, E.
2017-03-01
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.
Effect of the acoustic boundary layer on the wave propagation in ducts
Nayfeh, A. H.
1973-01-01
An analysis is presented for the wave propagation in two-dimensional and circular lined ducts taking into account the effects of viscosity in both the mean and the acoustic problems. The method of composite expansions is used to express each acoustic flow quantity as the sum of an inviscid part and a boundary layer part insignificant outside a thin layer next to the wall. The problem is reduced to solving a second-order ordinary differential equation for the pressure perturbation as in the inviscid acoustic case but with a modified specific wall admittance. An analytic expression is presented for the variation of the modified admittance with the wall and flow parameters, such as the acoustic boundary layer thickness, the mean velocity and temperature gradients at the wall, the frequency of oscillation, and the wavelength.
WIND observations of plasma waves inside the magnetic cloud boundary layers
Institute of Scientific and Technical Information of China (English)
WEI Fengsi; ZHONG Dingkun; FENG Xueshang; YANG Fang; LIU Rui
2005-01-01
Based on the WIND observational data for the plasma waves from thermal noise receptor (TNR) working on the frequency 4―256 kHz and the solar wind and the magnetic fields, we analyze the plasma wave activities in the 60 magnetic cloud's boundary layers (BLs) and find that there are often various plasma wave activities in the BLs, which are different from those in the adjacent solar wind (SW) and the magnetic clouds (MC). The basic characteristics are that: (1) the enhancement of the Langmuir wave near the electronic plasma frequency (fpe) is a dominant wave activity, which occupies 75% investigated samples; (2) the events enhanced both in the langmuir and ion acustic (f < fpe) waves are about 60% of investigated samples; (3) broadband, continuous enhancement events in the plasma wave activities were observed in the whole frequency band of TNR, and about 30% of the 60 samples, however, were not observed in the SW and the MC investigated events; (4) although the ratio of the temperatures between the electon and proton, Te/Tp≤1, the ion caustic wave enhancement activities are still often observed in the BLs, which makes it difficult to ex-plain them by the traditional plasma theory. New results reported in this paper further show that the magnetic cloud's BL is an important dynamic structure, which could provide useful diagnosis for understanding the cloud's BL physics and could expand a space developing space plasma wave theory.
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.
Alfven Waves in a Plasma Sheet Boundary Layer Associated with Near-Tail Magnetic Reconnection
Institute of Scientific and Technical Information of China (English)
YUAN Zhi-Gang; DENG Xiao-Hua; PANG Ye; LI Shi-You; WANG Jing-Fang
2007-01-01
We report observations from Geotail satellite showing that large Poynting fluxes associated with Alfven waves in the plasma sheet boundary layer(PSBL) occur in the vicinity of the near-tail reconnection region on 10 December 1996.During the period of large Poynting fluxex,Geotail also observed strong tailward plasma flws.These observations demonstrate the importance of near-tail reconnection process as the energy source of Alfven waves in the PSBL.Strong tailward(Earthward)plasma flows ought to be an important candidate in generating Alfven waves.Furthermore,the strong pertutbations not only of the magnetic field but also of the electric field observed in the PSBL indicate that the PSBL plays an important role in the generation and propagation of the energy flux associated with Alfven waves.
An LDA investigation of the normal shock wave boundary layer interaction
Chriss, R. M.; Hingst, W. R.; Strazisar, A. J.; Keith, T. G.
1990-01-01
Nonintrusive measurements have been made of two normal shock wave-boundary layer interactions. Two-dimensional measurements were made throughout the interaction region while three-dimensional measurements were made in the vicinity of the shock wave. The measurements were made in the corner of the test section of a continuous flow supersonic wind tunnel in which a normal shock wave had been stabilized. LDA, surface pressure measurement and flow visualization techniques were employed for two freestream Mach number test cases: 1.6 and 1.3. The former contained separated flow regions and a system of shock waves. The latter was found to be far less complicated. The reported results define the flowfield structure in detail for each case.
Diagnosing Lee Wave Rotor Onset Using a Linear Model Including a Boundary Layer
Directory of Open Access Journals (Sweden)
Miguel A. C. Teixeira
2017-01-01
Full Text Available A linear model is used to diagnose the onset of rotors in flow over 2D hills, for atmospheres that are neutrally stratified near the surface and stably stratified aloft, with a sharp temperature inversion in between, where trapped lee waves may propagate. This is achieved by coupling an inviscid two-layer mountain-wave model and a bulk boundary-layer model. The full model shows some ability to diagnose flow stagnation associated with rotors as a function of key input parameters, such as the Froude number and the height of the inversion, in numerical simulations and laboratory experiments carried out by previous authors. While calculations including only the effects of mean flow attenuation and velocity perturbation amplification within the surface layer represent flow stagnation fairly well in the more non-hydrostatic cases, only the full model, taking into account the feedback of the surface layer on the inviscid flow, satisfactorily predicts flow stagnation in the most hydrostatic case, although the corresponding condition is unable to discriminate between rotors and hydraulic jumps. Versions of the model not including this feedback severely underestimate the amplitude of trapped lee waves in that case, where the Fourier transform of the hill has zeros, showing that those waves are not forced directly by the orography.
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...
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...
Interaction between a shock wave and a turbulent boundary layer in transonic flow
Adamson, T. C., Jr.; Feo, A.
1975-01-01
Interaction between a shock wave and an unseparated turbulent boundary layer is considered. The method of matched asymptotic expansions is used, with solutions valid in the double limit as Reynolds number tends to infinity and Mach number tends to unity. The shock is weak enough that interaction effects can be considered as perturbations to the undisturbed flow; the case considered is that where the sonic line is near the outer edge of the boundary layer. It is shown that, with order estimates for Reynolds stress perturbations, the induced wall pressure distribution can be calculated using only the two outer interaction regions, independent of a specific closure condition and that this solution is in fact a turbulent free interaction solution. A detailed analysis of the inner regions, for which an eddy viscosity model for the Reynolds shear stress is used, provides a description of the variations in velocity, temperature and density near and at the wall.
Numerical investigations of shock wave interaction with laminar boundary layer on compressor profile
Piotrowicz, M.; Flaszyński, P.
2016-10-01
The investigation of shockwave boundary layer interaction on suction side of transonic compressor blade is one of main objectives of TFAST project (Transition Location Effect on Shock Wave Boundary Layer Interaction). In order to look more closely into the flow structure on suction side of a profile, a design of generic test section in linear transonic wind tunnel was proposed. The experimental and numerical results of flow structure on a suction side of the compressor profile investigations are presented. The numerical simulations are carried out for EARSM (Explicit Algebraic Reynolds Stress Model) turbulence model with transition model. The result are compared with oil flow visualisation, schlieren pictures, Pressure Sensitive Paint (PSP) and static pressure.
Multiple harmonic ULF waves in the plasma sheet boundary layer observed by Cluster
Engebretson, M. J.; Kahlstorf, C. R. G.; Posch, J. L.; Keiling, A.; Walsh, A. P.; Denton, R. E.; Broughton, M. C.; Owen, C. J.; FornaçOn, K.-H.; RèMe, H.
2010-12-01
The passage of the Cluster satellites in a polar orbit through Earth's magnetotail has provided numerous observations of harmonically related Pc 1-2 ULF wave events, with the fundamental near the local proton cyclotron frequency Ωcp. Broughton et al. (2008) reported observations by Cluster of three such events in the plasma sheet boundary layer, and used the wave telescope technique to determine that their wave vectors k were nearly perpendicular to B. This paper reports the results of a search for such waves throughout the 2003 Cluster tail passage. During the 4 month period of July-October 2003, 35 multiple-harmonic wave events were observed, all in the plasma sheet boundary layer (PSBL). From the first observed event (22 July) to the last (28 October), 13 of Cluster's 42 tail passes had at least one event. The wave events were rather evenly distributed from XGSE = -7 RE out to the Cluster apogee distance of -18 RE, with one event observed at -4 RE. ZGSE for these events ranged from -10 to -3 RE and +3 to +7 RE (i.e., there were no events for ∣Z∣ elevated fluxes of counterstreaming ions with energies ranging from ˜3 to 30 keV, and elevated fluxes of electrons with energies ranging from 0.25 to ˜5 keV. Analysis of plasma parameters suggests that although waves occurred only when the ion beta exceeded 0.1 (somewhat larger than typical for the PSBL), ion particle pressure may be of more physical importance in controlling wave occurrence. Electron distributions were more isotropic in pitch angles than the ion distributions, but some evidence of counterstreaming electrons was detected in 83% of the events. The ions also showed clear signatures of shell-like or ring-like distributions; i.e., with reduced fluxes below the energy of maximum flux. The suprathermal ion fluxes were asymmetric in all events studied, with more ions streaming earthward (for events both north and south of the central plasma sheet). Good agreement between the observed frequency of the
Separation control in a hypersonic shock wave / turbulent boundary-layer interaction
Schreyer, Anne-Marie; Bermejo-Moreno, Ivan; Kim, Jeonglae; Urzay, Javier
2016-11-01
Hypersonic vehicles play a key role for affordable access to space. The associated flow fields are strongly affected by shock wave/turbulent boundary-layer interactions, and the inherent separation causes flow distortion and low-frequency unsteadiness. Microramp sub-boundary layer vortex generators are a promising means to control separation and diminish associated detrimental effects. We investigate the effect of a microramp on the low-frequency unsteadiness in a fully separated interaction. A large eddy simulation of a 33 ∘ -compression-ramp interaction was performed for an inflow Mach number of 7.2 and a Reynolds number based on momentum thickness of Reθ = 3500 , matching the experiment of Schreyer et al. (2011). For the control case, we introduced a counter-rotating vortex pair, as induced by a single microramp, into the boundary layer through the inflow conditions. We applied a dynamic mode decomposition (DMD) on both cases to identify coherent structures that are responsible for the dynamic behavior. Based on the DMD, we discuss the reduction of the separation zone and the stabilization of the shock motion achieved by the microramp, and contribute to the description of the governing mechanisms. Pursued during the 2016 CTR Summer Program at Stanford University.
A computational study on oblique shock wave-turbulent boundary layer interaction
Joy, Md. Saddam Hossain; Rahman, Saeedur; Hasan, A. B. M. Toufique; Ali, M.; Mitsutake, Y.; Matsuo, S.; Setoguchi, T.
2016-07-01
A numerical computation of an oblique shock wave incident on a turbulent boundary layer was performed for free stream flow of air at M∞ = 2.0 and Re1 = 10.5×106 m-1. The oblique shock wave was generated from a 8° wedge. Reynolds averaged Navier-Stokes (RANS) simulation with k-ω SST turbulence model was first utilized for two dimensional (2D) steady case. The results were compared with the experiment at the same flow conditions. Further, to capture the unsteadiness, a 2D Large Eddy Simulation (LES) with sub-grid scale model WMLES was performed which showed the unsteady effects. The frequency of the shock oscillation was computed and was found to be comparable with that of experimental measurement.
Messiter, A. F.
1980-01-01
Asymptotic solutions are derived for the pressure distribution in the interaction of a weak normal shock wave with a turbulent boundary layer. The undisturbed boundary layer is characterized by the law of the wall and the law of the wake for compressible flow. In the limiting case considered, for 'high' transonic speeds, the sonic line is very close to the wall. Comparisons with experiment are shown, with corrections included for the effect of longitudinal wall curvature and for the boundary-layer displacement effect in a circular pipe.
Multiple harmonic ULF waves in the plasma sheet boundary layer: Instability analysis
Denton, R. E.; Engebretson, M. J.; Keiling, A.; Walsh, A. P.; Gary, S. P.; DéCréAu, P. M. E.; Cattell, C. A.; RèMe, H.
2010-12-01
Multiple-harmonic electromagnetic waves in the ULF band have occasionally been observed in Earth's magnetosphere, both near the magnetic equator in the outer plasmasphere and in the plasma sheet boundary layer (PSBL) in Earth's magnetotail. Observations by the Cluster spacecraft of multiple-harmonic electromagnetic waves with fundamental frequency near the local proton cyclotron frequency, Ωcp, were recently reported in the plasma sheet boundary layer by Broughton et al. (2008). A companion paper surveys the entire magnetotail passage of Cluster during 2003, and reports 35 such events, all in the PSBL, and all associated with elevated fluxes of counterstreaming ions and electrons. In this study we use observed pitch angle distributions of ions and electrons during a wave event observed by Cluster on 9 September 2003 to perform an instability analysis. We use a semiautomatic procedure for developing model distributions composed of bi-Maxwellian components that minimizes the difference between modeled and observed distribution functions. Analysis of wave instability using the WHAMP electromagnetic plasma wave dispersion code and these model distributions reveals an instability near Ωcp and its harmonics. The observed and model ion distributions exhibit both beam-like and ring-like features which might lead to instability. Further instability analysis with simple beam-like and ring-like model distribution functions indicates that the instability is due to the ring-like feature. Our analysis indicates that this instability persists over an enormous range in the effective ion beta (based on a best fit for the observed distribution function using a single Maxwellian distribution), β', but that the character of the instability changes with β'. For β' of order unity (for instance, the observed case with β' ˜ 0.4), the instability is predominantly electromagnetic; the fluctuating magnetic field has components in both the perpendicular and parallel directions, but the
Kelvin-Helmholtz wave at the subsolar magnetopause boundary layer under radial IMF
Grygorov, K.; Němeček, Z.; Šafránková, J.; Přech, L.; Pi, G.; Shue, J.-H.
2016-10-01
We present the first observation of the Kelvin-Helmholtz (KH) rolled-up vortex at the dayside magnetopause layers under a radial interplanetary magnetic field (IMF). The study uses measurements of four Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes aligned along the YGSE axis about 10 RE upstream of the Earth and located in different regions of the near-Earth environment. THEMIS C and A serve as monitors of the quiet solar wind and fluctuating magnetosheath conditions, respectively, and THEMIS D and E observe the magnetopause and low-latitude boundary layer (LLBL) crossings. The analysis shows the following: (1) a radial IMF changes to the southward pointing magnetosheath magnetic field; (2) dayside reconnection forms the thin but dense LLBL; (3) a large velocity shear at the LLBL inner edge excites a train of KH waves; and (4) in spite of a short path from the subsolar point (≈5 RE), one of the KH waves exhibits all features of a fully developed rolled-up vortex.
Hall, P.
1985-01-01
The Taylor-Gortler vortex instability equations are formulated for steady and unsteady interacting boundary layer flows of the type which arise in triple-deck theory. The effective Gortler number is shown to be a function of the all shape in the boundary layer and the possibility of both steady and unsteady Taylor-Gortler modes exists. As an example the steady flow in a symmetrically constricted channel is considered and it is shown that unstable Gortler vortices exist before the boundary layers at the wall develop the Goldstein singularity. As an example of an unsteady spatially varying basic state the instability of high frequency large amplitude Tollmien-Schlichting waves in a curved channel were considered. It is shown that they are unstable in the first Stokes layer stage of the hierarchy of nonlinear states. The Tollmien-Schlichting waves are shown to be unstable in the presence of both convex and concave curvature.
Liou, M. S.; Adamson, T. C., Jr.
1980-01-01
Asymptotic methods are used to calculate the shear stress at the wall for the interaction between a normal shock wave and a turbulent boundary layer on a flat plate. A mixing length model is used for the eddy viscosity. The shock wave is taken to be strong enough that the sonic line is deep in the boundary layer and the upstream influence is thus very small. It is shown that unlike the result found for laminar flow an asymptotic criterion for separation is not found; however, conditions for incipient separation are computed numerically using the derived solution for the shear stress at the wall. Results are compared with available experimental measurements.
A NEW MEASURE FOR DIRECT MEASUREMENT OF THE BED SHEAR STRESS OF WAVE BOUNDARY LAYER IN WAVE FLUME
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
In this article, a shear plate was mounted on the bottom in a wave flume and direct measurements of the smooth and rough bed shear stress under regular and irregular waves were conducted with the horizontal force exerted on the shear plates by the bottom shear stress in the wave boundary layer. Under immobile bed condition, grains of sand were glued uniformly and tightly onto the shear plate, being prevented from motion with the fluid flow and generation of sand ripples. The distribution of the bottom mean shear stress varying with time was measured by examining the interaction between the shear plate and shear transducers. The relation between the force measured by the shear transducers and its voltage is a linear one. Simultaneous measurements of the bottom velocity were carried out by an Acoustic Doppler Velocimeter (ADV), while the whole process was completely controlled by computers, bottom shear stress and velocity were synchronously measured. Based on the experimental results, it can be concluded that (1) the friction coefficient groews considerably with the increase of the Reynolds number, (2) the shear stress is a function varying with time and linearly proportional to the velocity. Compared with theoretical results and previous experimental data, it is shown that the experimental method is feasible and effective, A further study on the bed shear stress under regular or irregular waves can be carried out. And applicability to the laboratory studies on the initiation of sediments and the measurement of the shear stress after sediment imigration.
Study of kinetic Alfven wave (KAW) in plasma - sheet-boundary- layer
Energy Technology Data Exchange (ETDEWEB)
Shukla, Nidhi; Varma, P; Tiwari, M S, E-mail: tiwarims@rediffmail.co, E-mail: poornimavarma@yahoo.co, E-mail: nidhiphy.shukla@gmail.co [Department of Physics and Electronics, Dr. H. S. Gour University, Sagar (M.P.), 470003 (India)
2010-02-01
The effect of parallel electric field with general loss-cone distribution function on the dispersion relation and damping rate/growth rate of the kinetic Alfven wave (KAW) is evaluated by kinetic approach. The generation of KAW by the combined effect of parallel electric field and loss-cone distribution indices (J) at a particular range of k{sub p}erpendicular{rho}{sub i} (k{sub p}erpendicular{rho}{sub i} <1 and k{sub p}erpendicular{rho}{sub i} >1) is noticed, where k{sub p}erpendicular is perpendicular wave number and {rho}{sub i} is the ion-gyro radius. Thus the propagation of KAW and loss of the Poynting flux from plasma sheet boundary layer (PSBL) to the ionosphere can be explained on the basis of present investigation. It is found that the present study also shows that the loss-cone distribution index is an important parameter to study KAW in the PSBL.
Zhang, M. Y.; Li, Y. S.
1997-08-01
A third-generation wind wave model based on the energy balance equation taking into account the effects of time-varying currents and coupled dynamically with a semi-implicit three-dimensional hydrodynamic model incorporating the influences of time- and space-varying vertical eddy viscosity, bottom topography and wave-current interactions is presented in this paper. The wave model is synchronously coupled with the three-dimensional hydrodynamic model through the surface atmospheric turbulent boundary layer and the bottom boundary layer. The theory of Janssen (1991) (in Journal of Physical Oceanography21, 1631-1642) is used to incorporate the effects of waves on the surface boundary layer, while the theory of Grant and Maddsen (1979) [in Journal of Geophysical Research (Oceans)84, 1797-1808], which was used by Signell et al. (1990) (in Journal of Geophysical Research95, 9671-9678) on the bottom boundary layer for constant waves, is modified for the inclusion of time-varying waves. The mutual influences between waves and currents are investigated through an idealized continental shelf case and hindcastings of storm events in the sea area adjacent to Hong Kong in the northern South China Sea. Calculations are compared with other computed results and observations. Calculations show that the wave-dependent surface stress incorporated in the three-dimensional hydrodynamic model has significant impact on water surface velocities and surface elevations (over 10% higher). The inclusion of wave-dependent bottom stress also shows some effects; however, in the presence of the wave-dependent surface stress, its effect on surge levels becomes negligible. The effect of currents on waves amounts to the reduction of the significant wave height by about 8% and less for wave mean periods. However, the inclusion of the wave-dependent bottom stress in the three-dimensional hydrodynamic model has little effect on wave characteristics whether or not the wave-dependent surface stress is
DEFF Research Database (Denmark)
Fuhrman, David R.; Fredsøe, Jørgen; Sumer, B. Mutlu
2009-01-01
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...
Boundary layer transition studies
Watmuff, Jonathan H.
1995-01-01
A small-scale wind tunnel previously used for turbulent boundary layer experiments was modified for two sets of boundary layer transition studies. The first study concerns a laminar separation/turbulent reattachment. The pressure gradient and unit Reynolds number are the same as the fully turbulent flow of Spalart and Watmuff. Without the trip wire, a laminar layer asymptotes to a Falkner & Skan similarity solution in the FPG. Application of the APG causes the layer to separate and a highly turbulent and approximately 2D mean flow reattachment occurs downstream. In an effort to gain some physical insight into the flow processes a small impulsive disturbance was introduced at the C(sub p) minimum. The facility is totally automated and phase-averaged data are measured on a point-by-point basis using unprecedently large grids. The evolution of the disturbance has been tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. At first, the amplitude decays exponentially with streamwise distance in the APG region, where the layer remains attached, i.e. the layer is viscously stable. After separation, the rate of decay slows, and a point of minimum amplitude is reached where the contours of the wave packet exhibit dispersive characteristics. From this point, exponential growth of the amplitude of the disturbance is observed in the detached shear layer, i.e. the dominant instability mechanism is inviscid. A group of large-scale 3D vortex loops emerges in the vicinity of the reattachment. Remarkably, the second loop retains its identify far downstream in the turbulent boundary layer. The results provide a level of detail usually associated with CFD. Substantial modifications were made to the facility for the second study concerning disturbances generated by Suction Holes for laminar flow Control (LFC). The test section incorporates suction through interchangeable porous test surfaces. Detailed studies have been made using isolated
Hirt, Stefanie M.
2015-01-01
A test was conducted in the 15 cm x 15 cm supersonic wind tunnel at NASA Glenn Research Center that focused on corner effects of an oblique shock-wave/boundary-layer interaction. In an attempt to control the interaction in the corner region, eight corner fillet configurations were tested. Three parameters were considered for the fillet configurations: the radius, the fillet length, and the taper length from the square corner to the fillet radius. Fillets effectively reduced the boundary-layer thickness in the corner; however, there was an associated penalty in the form of increased boundary-layer thickness at the tunnel centerline. Larger fillet radii caused greater reductions in boundary-layer thickness along the corner bisector. To a lesser, but measureable, extent, shorter fillet lengths resulted in thinner corner boundary layers. Overall, of the configurations tested, the largest radius resulted in the best combination of control in the corner, evidenced by a reduction in boundary-layer thickness, coupled with minimal impacts at the tunnel centerline.
Kim, Hyun Dae; Felder, James L.
2011-01-01
The performance benefit of boundary layer or wake ingestion on marine and air vehicles has been well documented and explored. In this article, a quasi-one-dimensional boundary layer ingestion (BLI) benefit analysis for subsonic and transonic propulsion systems is performed using a control volume of a ducted propulsion system that ingests the boundary layer developed by the external airframe surface. To illustrate the BLI benefit, a relationship between the amount of BLI and the net thrust is established and analyzed for two propulsor types. One propulsor is an electric fan, and the other is a pure turbojet. These engines can be modeled as a turbofan with an infinite bypass ratio for the electric fan, and with a zero bypass ratio for the pure turbojet. The analysis considers two flow processes: a boundary layer being ingested by an aircraft inlet and a shock wave sitting in front of the inlet. Though the two processes are completely unrelated, both represent a loss of total pressure and velocity. In real applications, it is possible to have both processes occurring in front of the inlet of a transonic vehicle. Preliminary analysis indicates that the electrically driven propulsion system benefits most from the boundary layer ingestion and the presence of transonic shock waves, whereas the benefit for the turbojet engine is near zero or negative depending on the amount of total temperature rise across the engine.
Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions
Bernardini, M.; Asproulias, I.; Larsson, J.; Pirozzoli, S.; Grasso, F.
2016-12-01
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 free-stream Mach number 2.28 and shock angle of the wedge generator φ =8∘ . Five values of the wall-to-recovery-temperature ratio (Tw/Tr ) 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, which 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, with the maximum thermal and dynamic loads found for the case of the cold wall. The analysis reveals that the fluctuating heat flux exhibits a strong intermittent behavior, characterized by scattered spots with extremely high values compared to the mean. Furthermore, the analogy between momentum and heat transfer, typical of compressible, wall-bounded, equilibrium turbulent flows, does not apply for most of the interaction domain. The premultiplied spectra of the wall heat flux do not show any evidence of the influence of the low-frequency shock motion, and the primary mechanism for the generation of peak heating is found to be linked with the turbulence amplification in the interaction region.
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 ...
LES of shock wave/turbulent boundary layer interaction affected by microramp vortex generators
Joly, Laurent; Grebert, Arnaud; Jamme, Stéphane; Bodart, Julien; Aerodynamics, Energetics; Propulsion Dep. Team
2016-11-01
At large Mach numbers, the interaction of an oblique shock wave with a turbulent boundary layer (SWTBLI) developing over a flat plate gives rise to a separation bubble known to exhibit low-frequency streamwise oscillations around StL = 0 . 03 (a Strouhal number based on the separated region length). Because these oscillations yield wall pressure or load fluctuations, efforts are made to reduce their amplitude. We perform large eddy simulations to reproduce the experiments by Wang etal (2012) where a rake of microramp vortex generators (MVGs) were inserted upstream the SWTBLI with consequences yet to be fully understood. There is no consensus on the flow structure downstream MVGs and this is first clarified in the case of MVGs protruding by 0 . 47 δ in a TBL at Mach number M = 2 . 7 and Reynolds number Reθ = 3600 . Large-scale vortices intermittently shed downstream the MVGs are characterized by a streamwise period close to twice the TBL thickness and a frequency f 0 . 5Ue / δ , two orders of magnitude higher than the one of the uncontrolled SWTBLI. We then characterize the interaction between the unsteady wake of the MVGs with the SWTBLI resulting in the reduction of the interaction length and the high-frequency modulation of the shock feet motions.
Kampinga, W.R.; Wijnant, Ysbrand H.; de Boer, Andries; Sas, P; Munck, de M.
2006-01-01
The so called low reduced frequency model has been shown to be both an accurate and a relatively simple description of wave propagation in narrow tubes or layers, under small signal conditions. In this paper, the low reduced frequency model will be applied on a circular layer between a fixed surface
Turbulent Flow and Large Surface Wave Events in the Marine Boundary Layers
2013-08-22
equilibrium with wave age ∼ 1.4. The wave spectrum at the bot-827 tom of the PBLs is the same. Notice the range of the color bar is different828 between...and can interact with gravity waves and trigger Kelvin-Helmholtz insta- bilities leading to intermittent loads that fatigue turbine components ( Kelley ...model with a flat lower bot- tom , outlined in Section 2.1, to an atmospheric PBL flow with a varying boundary shape we apply a conventional grid
Institute of Scientific and Technical Information of China (English)
Yang Guang; Yao Yufeng; Fang Jian; Gan Tian; Li Qiushi; Lu Lipeng
2016-01-01
The efficiency and mechanism of an active control device‘‘SparkJet”and its application in shock-induced separation control are studied using large-eddy simulation in this paper. The base flow is the interaction of an oblique shock-wave generated by 8? wedge and a spatially-developing Ma=2.3 turbulent boundary layer. The Reynolds number based on the incoming flow property and the boundary layer displacement thickness at the impinging point without shock-wave is 20000. The detailed numerical approaches were presented. The inflow turbulence was generated using the digital filter method to avoid artificial temporal or streamwise periodicity. The numerical results including velocity profile, Reynolds stress profile, skin friction, and wall pressure were sys-tematically validated against the available wind tunnel particle image velocimetry (PIV) measure-ments of the same flow condition. Further study on the control of flow separation due to the strong shock-viscous interaction using an active control actuator ‘‘SparkJet” was conducted. The single-pulsed characteristic of the device was obtained and compared with the experiment. Both instantaneous and time-averaged flow fields have shown that the jet flow issuing from the actuator cavity enhances the flow mixing inside the boundary layer, making the boundary layer more resis-tant to flow separation. Skin friction coefficient distribution shows that the separation bubble length is reduced by about 35%with control exerted.
Directory of Open Access Journals (Sweden)
Yang Guang
2016-06-01
Full Text Available The efficiency and mechanism of an active control device “SparkJet” and its application in shock-induced separation control are studied using large-eddy simulation in this paper. The base flow is the interaction of an oblique shock-wave generated by 8° wedge and a spatially-developing Ma = 2.3 turbulent boundary layer. The Reynolds number based on the incoming flow property and the boundary layer displacement thickness at the impinging point without shock-wave is 20000. The detailed numerical approaches were presented. The inflow turbulence was generated using the digital filter method to avoid artificial temporal or streamwise periodicity. The numerical results including velocity profile, Reynolds stress profile, skin friction, and wall pressure were systematically validated against the available wind tunnel particle image velocimetry (PIV measurements of the same flow condition. Further study on the control of flow separation due to the strong shock-viscous interaction using an active control actuator “SparkJet” was conducted. The single-pulsed characteristic of the device was obtained and compared with the experiment. Both instantaneous and time-averaged flow fields have shown that the jet flow issuing from the actuator cavity enhances the flow mixing inside the boundary layer, making the boundary layer more resistant to flow separation. Skin friction coefficient distribution shows that the separation bubble length is reduced by about 35% with control exerted.
Towards a climatology of orographic induced wave drag in the stable boundary layer over real terrain
Kleczek, M.A.; Steeneveld, G.J.; Nappo, C.J.; Holtslag, A.A.M.
2012-01-01
The stable boundary layer (SBL) is of particular interest for numerous environmental issues as air quality, aviation, fog forecasting, wind energy engineering, and climate modelling. Unfortunately the current understanding of the SBL is still rather poor, and progress is slow. The relatively poor un
Meyers, Johan; Allaerts, Dries
2016-11-01
Conventionally neutral boundary layers (CNBL) often arise in offshore conditions. In these situations the neutral boundary layer is capped by a strong inversion layer and a stably stratified free atmosphere aloft. We use large-eddy simulations to investigate the interaction between a CNBL and a large wind farm. Following the approach of Allaerts & Meyers (2015), a set of equilibrium CNBLs are produced in a precursor simulation, with a height of approx. 300, 500, and 1000m, respectively. These are used at the inlet of a large wind-farm with a fetch of 15 km, and 20 rows of turbines. We find that above the farm, an internal boundary layer (IBL) develops. For the two lower CNBL cases, the IBL growth is stopped by the overlying capping inversion. Moreover, the upward displacement of the CNBL excites gravity waves in the inversion layer and the free atmosphere above. For the lower CNBL cases, these waves induce significant pressure gradients in the farm. A detailed energy budget analysis of the CNBL is further presented. The authors acknowledge support from the European Research Council (FP7-Ideas, Grant No. 306471).
Study on Shock Wave and Turbulent Boundary Layer Interactions in a Square Duct at Mach 2 and 4
Institute of Scientific and Technical Information of China (English)
Hiromu SUGIYAMA; Ryojiro MINATO; Kazuhide MIZOBATA; Akira TOJO; Yohei MUTO
2006-01-01
In this paper, the outline of the Mach 4 supersonic wind runnel for the investigation of the supersonic internal flows in ducts was firstly described. Secondly, the location, structure and characteristics of the Mach 2 and Mach 4 pseudo-shock waves in a square duct were investigated by color schlieren photographs and duct wall pressure fluctuation measurements. Finally, the wall shear stress distributions on the side, top and bottom walls of the square duct with the Mach 4 pseudo-shock wave were investigated qualitatively by the shear stress-sensitive liquid crystal visualization method. The side wall boundary layer separation region under the first shock is narrow near the top wall, while the side wall boundary layer separation region under the first shock is very wide near the bottom wall.
Boundary layer transition studies
Watmuff, Jonathan H.
1995-02-01
A small-scale wind tunnel previously used for turbulent boundary layer experiments was modified for two sets of boundary layer transition studies. The first study concerns a laminar separation/turbulent reattachment. The pressure gradient and unit Reynolds number are the same as the fully turbulent flow of Spalart and Watmuff. Without the trip wire, a laminar layer asymptotes to a Falkner & Skan similarity solution in the FPG. Application of the APG causes the layer to separate and a highly turbulent and approximately 2D mean flow reattachment occurs downstream. In an effort to gain some physical insight into the flow processes a small impulsive disturbance was introduced at the C(sub p) minimum. The facility is totally automated and phase-averaged data are measured on a point-by-point basis using unprecedently large grids. The evolution of the disturbance has been tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. At first, the amplitude decays exponentially with streamwise distance in the APG region, where the layer remains attached, i.e. the layer is viscously stable. After separation, the rate of decay slows, and a point of minimum amplitude is reached where the contours of the wave packet exhibit dispersive characteristics. From this point, exponential growth of the amplitude of the disturbance is observed in the detached shear layer, i.e. the dominant instability mechanism is inviscid. A group of large-scale 3D vortex loops emerges in the vicinity of the reattachment. Remarkably, the second loop retains its identify far downstream in the turbulent boundary layer. The results provide a level of detail usually associated with CFD. Substantial modifications were made to the facility for the second study concerning disturbances generated by Suction Holes for laminar flow Control (LFC). The test section incorporates suction through interchangeable porous test surfaces. Detailed studies have been made using isolated
Kegerise, Michael A.; Rufer, Shann J.
2016-08-01
In this paper, we report on the application of the atomic layer thermopile (ALTP) heat-flux sensor to the measurement of laminar-to-turbulent transition in a hypersonic flat-plate boundary layer. The centerline of the flat-plate model was instrumented with a streamwise array of ALTP sensors, and the flat-plate model was exposed to a Mach 6 freestream over a range of unit Reynolds numbers. Here, we observed an unstable band of frequencies that are associated with second-mode instability waves in the laminar boundary layer that forms on the flat-plate surface. The measured frequencies, group velocities, phase speeds, and wavelengths of these instability waves are consistent with data previously reported in the literature. Heat flux time series, and the Morlet wavelet transforms of them, revealed the wave-packet nature of the second-mode instability waves. In addition, a laser-based radiative heating system was used to measure the frequency response functions (FRF) of the ALTP sensors used in the wind tunnel test. These measurements were used to assess the stability of the sensor FRFs over time and to correct spectral estimates for any attenuation caused by the finite sensor bandwidth.
Hall, Philip; Bennett, James
1986-01-01
The Taylor-Goertler vortex instability equations are formulated for steady and unsteady interacting boundary-layer flows. The effective Goertler number is shown to be a function of the wall shape in the boundary layer and the possibility of both steady and unsteady Taylor-Goertler modes exists. As an example the steady flow in a symmetrically constricted channel is considered and it is shown that unstable Goertler vortices exist before the boundary layers at the wall develop the Goldstein singularity discussed by Smith and Daniels (1981). As an example of an unsteady spatially varying basic state, it is considered the instability of high-frequency large-amplitude two- and three-dimensional Tollmien-Schlichting waves in a curved channel. It is shown that they are unstable in the first 'Stokes-layer stage' of the hierarchy of nonlinear states discussed by Smith and Burggraf (1985). This instability of Tollmien-Schlichting waves in an internal flow can occur in the presence of either convex or concave curvature. Some discussion of this instability in external flows is given.
Chriss, R. M.; Hingst, W. R.; Strazisar, A. J.; Keith, T. G., Jr.
1989-01-01
Nonintrusive measurements were made of a normal shock wave/boundary layer interaction. Two dimensional measurements were made throughout the interaction region while 3-D measurements were made in the vicinity of the shock wave. The measurements were made in the corner of the test section of a continuous supersonic wind tunnel in which a normal shock wave had been stabilized. Laser Doppler Anemometry, surface pressure measurement and flow visualization techniques were employed for two freestream Mach number test cases: 1.6 and 1.3. The former contained separated flow regions and a system of shock waves. The latter was found to be far less complicated. The results define the flow field structure in detail for each case.
Stability of compressible boundary layers
Nayfeh, Ali H.
1989-01-01
The stability of compressible 2-D and 3-D boundary layers is reviewed. The stability of 2-D compressible flows differs from that of incompressible flows in two important features: There is more than one mode of instability contributing to the growth of disturbances in supersonic laminar boundary layers and the most unstable first mode wave is 3-D. Whereas viscosity has a destabilizing effect on incompressible flows, it is stabilizing for high supersonic Mach numbers. Whereas cooling stabilizes first mode waves, it destabilizes second mode waves. However, second order waves can be stabilized by suction and favorable pressure gradients. The influence of the nonparallelism on the spatial growth rate of disturbances is evaluated. The growth rate depends on the flow variable as well as the distance from the body. Floquet theory is used to investigate the subharmonic secondary instability.
Shockwave-boundary layer interactions
Glepman, R.
2014-01-01
Shock wave-boundary layer interactions are a very common feature in both transonic and supersonic flows. They can be encountered on compressor and turbine blades, in supersonic jet inlets, on transonic wings, on the stabilization fins of missiles and in many more situations. Because of their major i
Transition in hypersonic boundary layers
Zhang, Chuanhong; Zhu, Yiding; Chen, Xi; Yuan, Huijing; Wu, Jiezhi; Chen, Shiyi; Lee, Cunbiao; Gad-el-Hak, Mohamed
2015-10-01
Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. It is found that the second-mode instability is a key modulator of the transition process. Although the second-mode is primarily an acoustic wave, it causes the formation of high-frequency vortical waves, which triggers a fast transition to turbulence.
Transition in hypersonic boundary layers
Directory of Open Access Journals (Sweden)
Chuanhong Zhang
2015-10-01
Full Text Available Transition and turbulence production in a hypersonic boundary layer is investigated in a Mach 6 wind tunnel using Rayleigh-scattering visualization, fast-response pressure measurements, and particle image velocimetry. It is found that the second-mode instability is a key modulator of the transition process. Although the second-mode is primarily an acoustic wave, it causes the formation of high-frequency vortical waves, which triggers a fast transition to turbulence.
Advances in CFD Prediction of Shock Wave Turbulent Boundary Layer Interactions
2006-01-01
8◦. The wall is adiabatic. Experimental data of Deleuze [103] and Laurent [104] is available. The flow conditions are summarized in Table 10. The...Eddy Simulation of Shock Boundary Layer Interaction. In Third AFOSR International Conference on DNS and LES, Arlington, TX, August 2001. [103] Deleuze J...Conditions Reference Data M∞ Reδ × 10−4 Garnier et al[101, 102] LES 2.3 6.0 Deleuze [103], Laurent[104] E 2.3 6.0 Advances in CFD Prediction of Shock
The Plasmasphere Boundary Layer
Directory of Open Access Journals (Sweden)
D. L. Carpenter
2004-12-01
Full Text Available As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere.
Key words. Magnetospheric physics (plasmasphere; plasma convection; MHD waves and instabilities
In-flight active wave cancelation with delayed-x-LMS control algorithm in a laminar boundary layer
Simon, Bernhard; Fabbiane, Nicolò; Nemitz, Timotheus; Bagheri, Shervin; Henningson, Dan S.; Grundmann, Sven
2016-10-01
This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broadband Tollmien-Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a `black-box' system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere.
In-Flight Active Wave Cancelation with Delayed-x-LMS Control Algorithm in a Laminar Boundary Layer
Simon, Bernhard; Fabbiane, Nicolo; Nemitz, Timotheus; Bagheri, Shervin; Henningson, Dan; Grundmann, Sven
2016-11-01
This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broad-band Tollmien-Schlichting (TS) disturbances is performed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a "black box" system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS controller is operated without a model and is able to adapt to varying conditions that may occur during flight in atmosphere. DFG No.GR3524/4-1.
The aqueous thermal boundary layer
Katsaros, Kristina B.
1980-02-01
This article reviews the available data, measurement techniques, and present understanding of the millimeter thick aqueous thermal boundary layer. A temperature difference between the surface and lower strata, δT, of the order of a few tenths to -1 °C have been observed. Techniques ranging from miniature mercury thermometers and electrical point sensors to optical interferometry and infrared radiometry have been employed. Many processes influence the temperature structure in this thin boundary layer. Among them are: the net upward heat flux due to evaporation and sensible heat transfer; infrared and solar radiation; and the turbulence near the interface due to wind mixing, wave breaking and current shear. Presence of solute and surface-active materials stimulate or dampen these mixing processes thereby influencing boundary-layer thickness and temperature structure.
Frank, D. P.; Foster, D. L.; Chou, P.; Kao, Y.
2012-12-01
In-situ measurements of incipient motion within the mobile bed layer were conducted with state-of-the-art micro-electronic machines (MEMs). These devices were embedded in coarse-gravel sized Delrin enclosures, which have been scaled with the mobility criteria for small-scale wave flumes. The role of shear stress and pressure gradient on incipient motion of an intermittently mobile sediment bed was investigated under various oscillatory flows. Experiments conducted in a large-scale wave flume demonstrated a rocking motion before the ePebble rolled at incipient motion. The underwater video camera recorded the movement of the balls and the sensors resolved the accelerations at incipient motion. Complementary measurements with acoustic Doppler velocimeters were made to determine the hydrodynamics in the test section. The results suggest evidence of pressure gradient influenced incipient motion; in contrast with the more commonly used threshold for sediment motion based on the bed shear stress. The motion of the ePebbles correspond temporally to peaks in the pressure gradient. Calculated values of the Sleath parameter, used to quantify the effects of the pressure gradients, were comparable with field observations of pressure gradient induced plug flow by Foster et al (2006). The current configuration of the ePebble helps to identify the characteristics of incipient motion and determine orientation. These mobile nodes make a significant step towards resolving the Lagrangian dynamics of individual coarse gravel-sized particles within the mobile bed layer in the nearshore. On a larger scale, they will reduce the effects of beach erosion by improving beach nourishment design.
Troitskaya, Yuliya; Papko, Vladislav; Baidakov, Georgy; Vdovin, Maxim; Kandaurov, Alexander; Sergeev, Daniil
2013-04-01
This paper presents the results of field experiments conducted at the Gorky Reservoir to test a quasi-linear model of the atmospheric boundary layer [1]. In the course of the experiment we simultaneously measured profiles of wind speed and surface wave spectra using instruments placed on the Froude buoy, which measures the following parameters: i) the module and the direction of the wind speed using ultrasonic wind sensor WindSonic Gill instruments, located on the 4 - levels from 0.1 x 5 m long; ii) profile of the surface waves with 3-channel string wave-gauge with a base of 5 cm, iii) the temperature of the water and air with a resistive sensor. From the measured profiles of wind speed, we calculated basic parameters of the atmospheric boundary layer: the friction velocity u*, the wind speed at the standard height of 10 m U10 and the drag coefficient CD. Data on CD(U10), obtained at the Gorky Reservoir, were compared with similar data obtained on Lake George in Australia during the Australian Shallow Water Experiment (AUSWEX) conducted in 1997 - 1999 [2,3]. A good agreement was obtained between measured data at two different on the parameters of inland waters: deep Gorky reservoir and shallow Lake George.To elucidate the reasons for this coincidence of the drag coefficients under strongly different conditions an analysis of surface waves was conducted.Measurements have shown that in both water bodies the surface wave spectra have almost the same asymptotics (spatial spectrum - k-3, the frequency spectrum -5), corresponding to the Phillips saturation spectrum.These spectra are typically observed for the steep surface waves, for which the basic dissipation mechanism is wave breaking. The similarity of the short-wave parts of the spectra can be regarded as a probable cause of coincidence of dependency of drag coefficient of the water surface on wind speed. Quantitative verification of this hypothesis was carried out in the framework of quasi-linear model of the wind
Charge density wave order in 1D mirror twin boundaries of single-layer MoSe2
Barja, Sara; Wickenburg, Sebastian; Liu, Zhen-Fei; Zhang, Yi; Ryu, Hyejin; Ugeda, Miguel M.; Hussain, Zahid; Shen, Zhi-Xun; Mo, Sung-Kwan; Wong, Ed; Salmeron, Miquel B.; Wang, Feng; Crommie, Michael F.; Ogletree, D. Frank; Neaton, Jeffrey B.; Weber-Bargioni, Alexander
2016-08-01
We provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs) of single-layer semiconducting MoSe2. Such MTBs have been previously observed by transmission electron microscopy and have been predicted to be metallic in MoSe2 and MoS2. Our low-temperature scanning tunnelling microscopy/spectroscopy measurements revealed a substantial bandgap of 100 meV opening at the Fermi energy in the otherwise metallic one-dimensional structures. We found a periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. In addition to mapping the energy-dependent density of states, we determined the atomic structure and bonding of the MTB through simultaneous high-resolution non-contact atomic force microscopy. Density functional theory calculations based on the observed structure reproduced both the gap opening and the spatially resolved density of states.
Schlichting (Deceased), Hermann
2017-01-01
This new edition of the near-legendary textbook by Schlichting and revised by Gersten presents a comprehensive overview of boundary-layer theory and its application to all areas of fluid mechanics, with particular emphasis on the flow past bodies (e.g. aircraft aerodynamics). The new edition features an updated reference list and over 100 additional changes throughout the book, reflecting the latest advances on the subject.
Stagg, G. W.; Parker, N. G.; Barenghi, C. F.
2017-03-01
We model the superfluid flow of liquid helium over the rough surface of a wire (used to experimentally generate turbulence) profiled by atomic force microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that the sharpest features in the surface induce vortex nucleation both intrinsically (due to the raised local fluid velocity) and extrinsically (providing pinning sites to vortex lines aligned with the flow). Vortex interactions and reconnections contribute to form a dense turbulent layer of vortices with a nonclassical average velocity profile which continually sheds small vortex rings into the bulk. We characterize this layer for various imposed flows. As boundary layers conventionally arise from viscous forces, this result opens up new insight into the nature of superflows.
Wang, Lu; Li, Tim
2017-04-01
Mechanisms for an in-phase relationship between convection and low-level zonal wind and the slow propagation of the convectively coupled Kelvin wave (CCKW) are investigated by analyzing satellite-based brightness temperature and reanalysis data and by constructing a simple theoretical model. Observational data analysis reveals an eastward shift of the low-level convergence and moisture relative to the CCKW convective center. The composite vertical structures show that the low-level convergence lies in the planetary boundary layer (PBL) (below 800 hPa), and is induced by the pressure trough above the top of PBL through an Ekman-pumping process. A traditional view of a slower eastward propagation speed compared to the dry Kelvin waves is attributed to the reduction of atmospheric static stability in mid-troposphere due to the convective heating effect. The authors' quantitative assessment of the heating effect shows that this effect alone cannot explain the observed CCKW phase speed. We hypothesize that additional slowing process arises from the effect of zonally asymmetric PBL moisture. A simple theoretical model is constructed to understand the relative role of the heating induced effective static stability effect and the PBL moisture effect. The result demonstrates the important role of the both effects. Thus, PBL-free atmosphere interaction is important in explaining the observed structure and propagation of CCKW.
Calantoni, Joseph; Holland, K Todd; Drake, Thomas G
2004-09-15
Sediment transport in oscillatory boundary layers is a process that drives coastal geomorphological change. Most formulae for bed-load transport in nearshore regions subsume the smallest-scale physics of the phenomena by parametrizing interactions amongst particles. In contrast, we directly simulate granular physics in the wave-bottom boundary layer using a discrete-element model comprised of a three-dimensional particle phase coupled to a one-dimensional fluid phase via Newton's third law through forces of buoyancy, drag and added mass. The particulate sediment phase is modelled using discrete particles formed to approximate natural grains by overlapping two spheres. Both the size of each sphere and the degree of overlap can be varied for these composite particles to generate a range of non-spherical grains. Simulations of particles having a range of shapes showed that the critical angle--the angle at which a grain pile will fail when tilted slowly from rest--increases from approximately 26 degrees for spherical particles to nearly 39 degrees for highly non-spherical composite particles having a dumbbell shape. Simulations of oscillatory sheet flow were conducted using composite particles with an angle of repose of approximately 33 degrees and a Corey shape factor greater than about 0.8, similar to the properties of beach sand. The results from the sheet-flow simulations with composite particles agreed more closely with laboratory measurements than similar simulations conducted using spherical particles. The findings suggest that particle shape may be an important factor for determining bed-load flux, particularly for larger bed slopes.
Irshad, Ranah; Bowles, N. E.; Calcutt, S. B.; Hurley, J.
2010-10-01
The Boundary Layer Radiometer is a small, low mass (<1kg) radiometer with only a single moving part - a scan/calibration mirror. The instrument consists of a three mirror telescope system incorporating an intermediate focus for use with miniature infrared and visible filters. It also has an integrated low power blackbody calibration target to provide long-term calibration stability The instrument may be used as an upward looking boundary layer radiometer for both the terrestrial and Martian atmospheres with appropriate filters for the mid-infrared carbon dioxide band, as well as a visible channel for the detection of aerosol components such as dust. The scan mirror may be used to step through different positions from the local horizon to the zenith, allowing the vertical temperature profile of the atmosphere to be retrieved. The radiometer uses miniature infrared filter assemblies developed for previous space-based instruments by Oxford, Cardiff and Reading Universities. The intermediate focus allows for the use of upstream blocking filters and baffles, which not only simplifies the design of the filters and focal plane assembly, but also reduces the risk of problems due to stray light. Combined with the calibration target this means it has significant advantages over previous generations of small radiometers.
Calculation of Viscous Effects on Ship Wave Resistance Using Axisymmetric Boundary Layer Approaches
1985-05-13
6" 7.6" BOW RADIUS DIMENSIONS ARE FOR 400 ’ SHIP LBP 20’ STATION SPACING I 19 1/2 19 1 SERIES 60 STEM AND STERN CONTOURS f2’ - 10-13/16’ r -3...Layers in Pressure Gradients," NSRDC Report 3308, April 1970. 38. Garcia, J.M. and Zazurca, J.A.A., " Calculo de la Resistencia Viscosa de un Buque a
Directory of Open Access Journals (Sweden)
A. Olsson
2004-06-01
Full Text Available Ion shell distributions are hollow spherical shells in velocity space that can be formed by many processes and occur in several regions of geospace. They are interesting because they have free energy that can, in principle, be transmitted to ions and electrons. Recently, a technique has been developed to estimate the original free energy available in shell distributions from in-situ data, where some of the energy has already been lost (or consumed. We report a systematic survey of three years of data from the Polar satellite. We present an estimate of the free energy available from ion shell distributions on auroral field lines sampled by the Polar satellite below 6 R_{E} geocentric radius. At these altitudes the type of ion shells that we are especially interested in is most common on auroral field lines close to the polar cap (i.e. field lines mapping to the plasma sheet boundary layer, PSBL. Our analysis shows that ion shell distributions that have lost some of their free energy are commonly found not only in the PSBL, but also on auroral field lines mapping to the boundary plasma sheet (BPS, especially in the evening sector auroral field lines. We suggest that the PSBL ion shell distributions are formed during the so-called Velocity Dispersed Ion Signatures (VDIS events. Furthermore, we find that the partly consumed shells often occur in association with enhanced wave activity and middle-energy electron anisotropies. The maximum downward ion energy flux associated with a shell distribution is often 10mWm^{-2} and sometimes exceeds 40mWm^{-2} when mapped to the ionosphere and thus may be enough to power many auroral processes. Earlier simulation studies have shown that ion shell distributions can excite ion Bernstein waves which, in turn, energise electrons in the parallel direction. It is possible that ion shell distributions are the link between the X-line and the auroral wave activity and electron
Institute of Scientific and Technical Information of China (English)
Dilip Das
2015-01-01
There is a large class of problems in the field of fluid structure interaction where higher-order boundary conditions arise for a second-order partial differential equation. Various methods are being used to tackle these kind of mixed boundary-value problems associated with the Laplace’s equation (or Helmholtz equation) arising in the study of waves propagating through solids or fluids. One of the widely used methods in wave structure interaction is the multipole expansion method. This expansion involves a general combination of a regular wave, a wave source, a wave dipole and a regular wave-free part. The wave-free part can be further expanded in terms of wave-free multipoles which are termed as wave-free potentials. These are singular solutions of Laplace’s equation or two-dimensional Helmholz equation. Construction of these wave-free potentials and multipoles are presented here in a systematic manner for a number of situations such as two-dimensional non-oblique and oblique waves, three dimensional waves in two-layer fluid with free surface condition with higher order partial derivative are considered. In particular, these are obtained taking into account of the effect of the presence of surface tension at the free surface and also in the presence of an ice-cover modelled as a thin elastic plate. Also for limiting case, it can be shown that the multipoles and wave-free potential functions go over to the single layer multipoles and wave-free potential.
Analysis of turbulent boundary layers
Cebeci, Tuncer
1974-01-01
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
Murthy, V. S.; Rose, W. C.
1977-01-01
Detailed measurements of wall shear stress (skin friction) were made with specially developed buried wire gages in the interaction regions of a Mach 2.9 turbulent boundary layer with externally generated shocks. Separation and reattachment points inferred by these measurements support the findings of earlier experiments which used a surface oil flow technique and pitot profile measurements. The measurements further indicate that the boundary layer tends to attain significantly higher skin-friction values downstream of the interaction region as compared to upstream. Comparisons between measured wall shear stress and published results of some theoretical calculation schemes show that the general, but not detailed, behavior is predicted well by such schemes.
Institute of Scientific and Technical Information of China (English)
徐云滨; 郑连存
2008-01-01
A class of singular nonlinear boundary value problems arising in the boundary layer behind expansion wave are studied. Sufficient conditions for the existence and uniqueness of positive solutions to the problems are established by utilizing the monotonic approaching technique. And a theoretical estimate formula for skin friction coefficient is presented. The numerical solution is presented by using the shoot method. The reliability and efficiency of the theoretical prediction are verified by numerical results.
Fairfield, D. H.; Farrugia, C. J.; Gratton, F. T.; Mukai, T.; Nagai, T.
2005-01-01
On August 1, 1998, the Geotail spacecraft made an inbound passage perpendicular to the dusk magnetopause at the dusk terminator when the interplanetary magnetic field had been very northward for more than 10 hours. Typical 3-minute-period Kelvin-Helmholtz waves were observed and the density in the boundary layer and magnetopause was observed to have an unusually high value near 5 /cc. Compressible MHD calculations using the measured values at Geotail yield substantial growth rates that support the idea that the magnetopause was Kelvin-Helmholtz unstable. In contrast to many previous events where a spacecraft remained in the boundary layer, this passage allowed study of how the waves varied with distance inward from the magnetopause. In a layer adjacent to the magnetosheath, rapid magnetic field fluctuations were seen with variations of at least 50 nT/s. Initially the boundary waves led to transitions between the magnetosheath and the fluctuating region with magnetosheath-like densities and tailward velocities, but as the spacecraft moved inward, the transitions were more likely to be between the fluctuating region and a hotter region with magnetosphere-like densities of 5kc. Gradually the velocity perturbations began to exhibit 360 degree rotations. Such rotations are similar to the vortices seen earlier by the ISEE spacecraft throughout the magnetotail which were suspected of being caused by Kelvin-Helmholtz instability of the boundary.
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
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.
Directory of Open Access Journals (Sweden)
R. M. Worthington
2005-06-01
Full Text Available Thin stable atmospheric layers cause VHF radars to receive increased echo power from near zenith. Layers can be tilted from horizontal, for instance by gravity waves, and the direction of VHF "glinting" is measurable by spatial domain interferometry or many-beam Doppler beam swinging (DBS. This paper uses the Middle and Upper atmosphere (MU radar, Shigaraki, Japan as a volume-imaging radar with 64-beam DBS, to show tilting of layers and air flow in mountain waves. Tilt of aspect-sensitive echo power from horizontal is nearly parallel to air flow, as assumed in earlier measurements of mountain-wave alignment. Vertical-wind measurements are self-consistent from different beam zenith angles, despite the combined effects of aspect sensitivity and horizontal-wind gradients.
The Ocean Boundary Layer beneath Hurricane Frances
Dasaro, E. A.; Sanford, T. B.; Terrill, E.; Price, J.
2006-12-01
The upper ocean beneath the peak winds of Hurricane Frances (57 m/s) was measured using several varieties of air-deployed floats as part of CBLAST. A multilayer structure was observed as the boundary layer deepened from 20m to 120m in about 12 hours. Bubbles generated by breaking waves create a 10m thick surface layer with a density anomaly, due to the bubbles, of about 1 kg/m3. This acts to lubricate the near surface layer. A turbulent boundary layer extends beneath this to about 40 m depth. This is characterized by large turbulent eddies spanning the boundary layer. A stratified boundary layer grows beneath this reaching 120m depth. This is characterized by a gradient Richardson number of 1/4, which is maintained by strong inertial currents generated by the hurricane, and smaller turbulent eddies driven by the shear instead of the wind and waves. There is little evidence of mixing beneath this layer. Heat budgets reveal the boundary layer to be nearly one dimensional through much of the deepening, with horizontal and vertical heat advection becoming important only after the storm had passed. Turbulent kinetic energy measurements support the idea of reduced surface drag at high wind speeds. The PWP model correctly predicts the degree of mixed layer deepening if the surface drag is reduced at high wind speed. Overall, the greatest uncertainty in understanding the ocean boundary layer at these extreme wind speeds is a characterization of the near- surface processes which govern the air-sea fluxes and surface wave properties.
Hurricane Boundary-Layer Theory
2010-01-01
2501. Kundu PK. 1990. Fluid Mechanics . Academic Press: San Diego, USA. Kuo HL. 1982. Vortex boundary layer under quadratic surface stress. Boundary...identification of two mechanisms for the spin-up of the mean tangential circulation of a hurricane. The first involves convergence of absolute angular...momentum above the boundary layer, where this quantity is approximately conserved. This mechanism acts to spin up the outer circulation at radii
Energy Technology Data Exchange (ETDEWEB)
Kostylev, M. [School of Physics, M013, University of Western Australia, Crawley, Perth 6009, Western Australia (Australia)
2014-06-21
In this work, we derive the interface exchange boundary conditions for the classical linear dynamics of magnetization in ferromagnetic layers with the interface Dzyaloshinskii-Moriya interaction (IDMI). We show that IDMI leads to pinning of dynamic magnetization at the interface. An unusual peculiarity of the IDMI-based pinning is that its scales as the spin-wave wave number. We incorporate these boundary conditions into an existing numerical model for the dynamics of the Damon-Eshbach spin wave in ferromagnetic films. IDMI affects the dispersion and the frequency non-reciprocity of the travelling Damon-Eshbach spin wave. For a broad range of film thicknesses L and wave numbers, the results of the numerical simulations of the spin wave dispersion are in a good agreement with a simple analytical expression, which shows that the contribution of IDMI to the dispersion scales as 1/L, similarly to the effect of other types of interfacial anisotropy. Suggestions to experimentalists how to detect the presence of IDMI in a spin wave experiment are given.
50 kHz PIV of a Swept-Ramp Shock-Wave Boundary-Layer Interaction at Mach 2
Vanstone, Leon; Musta, Mustafa Nail; Seckin, Serdar; Saleem, Mohammad; Clemens, Noel
2016-11-01
The interaction from a 30° sweep, 22.5° compression ramp in a Mach 2 flow is examined using wide-field 5Hz and 50 kHz PIV. The high-speed PIV is fast enough to resolve the large-scale unsteady motions of the SWBLI and can be band-pass filtered to investigate the driving mechanisms of unsteadiness and the widefield PIV allows comparisons with mean flow-fields. Preliminary investigation looked at three distinct frequency bands: 10-50 kHz (0.025-0.25 U∞ /δ99), 1-10 kHz (0.025-0.25 U∞ /δ99), and 0-1 kHz (0-0.025 U∞ /δ99). The unsteadiness associated with 10-50 kHz shows no correlation with the upstream boundary layer and accounts for 40% of the amplitude. The unsteadiness associated with 1-10 kHz is correlated with the upstream boundary-layer and also accounts for 40% of unsteadiness. This frequency is similar to those of boundary-layer superstructures. The unsteadiness associated with 0-1 kHz shows the strongest correlation with the upstream boundary-layer but accounts for only 20% of the amplitude. Clearly a range of unsteadiness mechanisms are present, with significant amplitude associated with higher frequencies. Future work will focus on expanding these findings with surface pressure and additional PIV. This work is sponsored by the AFOSR under Grant FA9550-14-1-0167 with Ivett Leyva as the program manager. This source of support is gratefully acknowledged. Further, Mustafa Musta thanks the Scientific and Technological research Council of Turkey.
Boundary layers in stochastic thermodynamics.
Aurell, Erik; Mejía-Monasterio, Carlos; Muratore-Ginanneschi, Paolo
2012-02-01
We study the problem of optimizing released heat or dissipated work in stochastic thermodynamics. In the overdamped limit these functionals have singular solutions, previously interpreted as protocol jumps. We show that a regularization, penalizing a properly defined acceleration, changes the jumps into boundary layers of finite width. We show that in the limit of vanishing boundary layer width no heat is dissipated in the boundary layer, while work can be done. We further give an alternative interpretation of the fact that the optimal protocols in the overdamped limit are given by optimal deterministic transport (Burgers equation).
Wang, C. R.; Hingst, W. R.; Porro, A. R.
1991-01-01
The properties of 2-D shock wave/turbulent boundary layer interaction flows were calculated by using a compressible turbulent Navier-Stokes numerical computational code. Interaction flows caused by oblique shock wave impingement on the turbulent boundary layer flow were considered. The oblique shock waves were induced with shock generators at angles of attack less than 10 degs in supersonic flows. The surface temperatures were kept at near-adiabatic (ratio of wall static temperature to free stream total temperature) and cold wall (ratio of wall static temperature to free stream total temperature) conditions. The computational results were studied for the surface heat transfer, velocity temperature correlation, and turbulent shear stress in the interaction flow fields. Comparisons of the computational results with existing measurements indicated that (1) the surface heat transfer rates and surface pressures could be correlated with Holden's relationship, (2) the mean flow streamwise velocity components and static temperatures could be correlated with Crocco's relationship if flow separation did not occur, and (3) the Baldwin-Lomax turbulence model should be modified for turbulent shear stress computations in the interaction flows.
Stability of separating subsonic boundary layers
Masad, Jamal A.; Nayfeh, Ali H.
1994-01-01
The primary and subharmonic instabilities of separating compressible subsonic two-dimensional boundary layers in the presence of a two-dimensional roughness element on a flat plate are investigated. The roughness elements considered are humps and forward- and backward-facing steps. The use of cooling and suction to control these instabilities is studied. The similarities and differences between the instability characteristics of separating boundary layers and those of the boundary layer over a flat plate with a zero pressure gradient are pointed out and discussed. The theoretical results agree qualitatively and quantitatively with the experimental data of Dovgal and Kozlov. Cooling and suction decrease the growth rates of primary and subharmonic waves in the attached-flow regions but increase them in the separated-flow regions.
Pfaff, R. F.
2009-01-01
On December 14,2002, a NASA Black Brant X sounding rocket was launched equatorward from Ny Alesund, Spitzbergen (79 N) into the dayside cusp and subsequently cut across the open/closed field line boundary, reaching an apogee of771 km. The launch occurred during Bz negative conditions with strong By negative that was changing during the flight. SuperDarn (CUTLASS) radar and subsequent model patterns reveal a strong westward/poleward convection, indicating that the rocket traversed a rotational reversal in the afternoon merging cell. The payload returned DC electric and magnetic fields, plasma waves, energetic particle, suprathermal electron and ion, and thermal plasma data. We provide an overview of the main observations and focus on the DC electric field results, comparing the measured E x B plasma drifts in detail with the CUTLASS radar observations of plasma drifts gathered simultaneously in the same volume. The in situ DC electric fields reveal steady poleward flows within the cusp with strong shears at the interface of the closed/open field lines and within the boundary layer. We use the observations to discuss ionospheric signatures of the open/closed character of the cusp/low latitude boundary layer as a function of the IMF. The electric field and plasma density data also reveal the presence of very strong plasma irregularities with a large range of scales (10 m to 10 km) that exist within the open field line cusp region yet disappear when the payload was equatorward of the cusp on closed field lines. These intense low frequency wave observations are consistent with strong scintillations observed on the ground at Ny Alesund during the flight. We present detailed wave characteristics and discuss them in terms of Alfven waves and static irregularities that pervade the cusp region at all altitudes.
Boundary layer control for airships
Pake, F. A.; Pipitone, S. J.
1975-01-01
An investigation is summarized of the aerodynamic principle of boundary layer control for nonrigid LTA craft. The project included a wind tunnel test on a BLC body of revolution at zero angle of attack. Theoretical analysis is shown to be in excellent agreement with the test data. Methods are evolved for predicting the boundary layer development on a body of revolution and the suction pumping and propulsive power requirements. These methods are used to predict the performance characteristics of a full-scale airship. The analysis indicates that propulsive power reductions of 15 to 25 percent and endurance improvements of 20 to 40 percent may be realized in employing boundary-layer control to nonrigid airships.
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...
Astrophysical Boundary Layers: A New Picture
Belyaev, Mikhail; Rafikov, Roman R.; Mclellan Stone, James
2016-04-01
Accretion is a ubiquitous process in astrophysics. In cases when the magnetic field is not too strong and a disk is formed, accretion can proceed through the mid plane all the way to the surface of the central compact object. Unless that compact object is a black hole, a boundary layer will be formed where the accretion disk touches its surfaces. The boundary layer is both dynamically and observationally significant as up to half of the accretion energy is dissipated there.Using a combination of analytical theory and computer simulations we show that angular momentum transport and accretion in the boundary layer is mediated by waves. This breaks with the standard astrophysical paradigm of an anomalous turbulent viscosity that drives accretion. However, wave-mediated angular momentum transport is a natural consequence of "sonic instability." The sonic instability, which we describe analytically and observe in our simulations, is a close cousin of the Papaloizou-Pringle instability. However, it is very vigorous in the boundary layer due to the immense radial velocity shear present at the equator.Our results are applicable to accreting neutron stars, white dwarfs, protostars, and protoplanets.
Nonparallel stability of boundary layers
Nayfeh, Ali H.
1987-01-01
The asymptotic formulations of the nonparallel linear stability of incompressible growing boundary layers are critically reviewed. These formulations can be divided into two approaches. The first approach combines a numerical method with either the method of multiple scales, or the method of averaging, of the Wentzel-Kramers-Brillouin (WKB) approximation; all these methods yield the same result. The second approach combined a multi-structure theory with the method of multiple scales. The first approach yields results that are in excellent agreement with all available experimental data, including the growth rates as well as the neutral stability curve. The derivation of the linear stability of the incompressible growing boundary layers is explained.
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.
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...
the Martian atmospheric boundary layer
DEFF Research Database (Denmark)
Petrosyan, A.; Galperin, B.; Larsen, Søren Ejling
2011-01-01
The planetary boundary layer (PBL) represents the part of the atmosphere that is strongly influenced by the presence of the underlying surface and mediates the key interactions between the atmosphere and the surface. On Mars, this represents the lowest 10 km of the atmosphere during the daytime...
Stability of Boundary Layer Flow.
1980-03-01
and Teske (1975). We can conclude (as in the case of ducting) that theoretical models of boundary layer structure and associated radar structure...FI33 (Secret). Hitney, (1978) "Surface Duct Effects," Naval Ocean Systems Center, San Diego, Calif., Report No. TD144. Lewellen, W. S., and M. E. Teske
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
Cyclone with boundary layer displacement
Energy Technology Data Exchange (ETDEWEB)
Gorton-Huelgerth, A.; Hoffmann, D.; Staudinger, G. [Technische Universitaet Graz, Graz (Austria). Inst. fuer Verfahrenstechnik, Abt. fuer Apparatebau und Mechanische Verfahrenstechnik
1998-12-31
In a cyclone the boundary layers at the cover plate and outside of the vortex finder are considered to affect the separation efficiency of a cyclone. To improve separation efficiency, the boundary layers at and the space between vortex finder and cover plate were investigated. Two identical cyclones, 400 mm diameter and 990 mm long were manufactured with high precision from stainless steel. One was equipped with openings for insertion of velocity probes; the opening could be closed with glass windows to allow Laser Doppler Anemometry. The other cyclone was used for testing separation efficiency. Velocity measurements by both conventional pressure probes and Laser Doppler Anemometry revealed that only a minor part of the boundary layer at the outside of the vortex finder reaches the lower end of it, because the axial velocity is slowed down drastically. Light sheet visualization showed that there exists a heavily particle loaded boundary layer at the cover plate. There are no particles at the lower part of the vortex finder. This can be explained by the radial mass exchange. The effect of vortex finder length on separation efficiency was investigated by varying the length of the vortex finder and measuring the separation efficiency using a limestone powder as test material. It was found that the separation efficiency is not sensitive to the length of the vortex finder. Particle slip increases only where the vortex finder is definitely shorter than its diameter. It was tried to improve the separation efficiency by introducing an air curtain along the vortex finder. The positive effect on separation efficiency and particle cut size is minimal. Results from velocity measurements with cylinder pressure probes in the boundary layer do not fit the results from other measurement techniques. Many older measurements were made with these probes. The conclusions drawn and the design rules deduced are to be questioned. 17 refs., 21 figs., 2 tabs.
Stability of three-dimensional boundary layers
Nayfeh, A. H.
1979-01-01
A theory is presented for the three-dimensional stability of boundary layers. Equations are derived for the evolution of a disturbance having a given frequency and originating at a given curve. These equations are used to determine the rays along which the disturbance energy propagates. It is shown that the results can be obtained by using the saddle-point method, or kinematic wave theory, or the method of multiple scales. Extension of the theory to the case of a wave packet is also presented.
Suarez Mullins, Astrid
Terrain-induced gravity waves and rotor circulations have been hypothesized to enhance the generation of submeso motions (i.e., nonstationary shear events with spatial and temporal scales greater than the turbulence scale and smaller than the meso-gamma scale) and to modulate low-level intermittency in the stable boundary layer (SBL). Intermittent turbulence, generated by submeso motions and/or the waves, can affect the atmospheric transport and dispersion of pollutants and hazardous materials. Thus, the study of these motions and the mechanisms through which they impact the weakly to very stable SBL is crucial for improving air quality modeling and hazard predictions. In this thesis, the effects of waves and rotor circulations on submeso and turbulence variability within the SBL is investigated over the moderate terrain of central Pennsylvania using special observations from a network deployed at Rock Springs, PA and high-resolution Weather Research and Forecasting (WRF) model forecasts. The investigation of waves and rotors over central PA is important because 1) the moderate topography of this region is common to most of the eastern US and thus the knowledge acquired from this study can be of significance to a large population, 2) there have been little evidence of complex wave structures and rotors reported for this region, and 3) little is known about the waves and rotors generated by smaller and more moderate topographies. Six case studies exhibiting an array of wave and rotor structures are analyzed. Observational evidence of the presence of complex wave structures, resembling nonstationary trapped gravity waves and downslope windstorms, and complex rotor circulations, resembling trapped and jump-type rotors, is presented. These motions and the mechanisms through which they modulate the SBL are further investigated using high-resolution WRF forecasts. First, the efficacy of the 0.444-km horizontal grid spacing WRF model to reproduce submeso and meso
Energy Technology Data Exchange (ETDEWEB)
Asai, M.; Aiba, K. [Tokyo Metropolitan Institute of Technology, Tokyo (Japan)
1995-09-01
Low-frequency Tollmien-Schlichting (T-S) waves may be thought generated as a result of high-frequency disturbance between two proximity frequency modes grown unstably in a separation shear layer causing secondary nonlinear interference to occur. This fact has been verified by a numerical simulation. A non-compression Navier-Stokes equation was used for the fundamental equation, a tertiary windward difference for the convection term, and a secondary central difference for other differential calculus. The Reynolds number was 200, and the disturbance was introduced by applying `v` variation continuously on the wall face. Non-introduction of the disturbance results in a steady flow. Disturbance frequencies of 0.15 and 0.20 were selected as disturbance frequencies from the relationship between the spatial amplification and the frequency dependency. The structure of the excited disturbance agreed with the intrinsic mode. The difference mode due to nonlinear interference grows as the basic mode was amplified. The basic mode decays sharply in the boundary layer after reattachment, while the difference mode decays slowly. Distribution of the difference mode is a distribution of viscous T-S waves, which may be converted into the intrinsic mode. 8 refs., 7 figs.
Liou, M. S.; Adamson, T. C., Jr.
1979-01-01
An analysis is presented of the flow in the two inner layers, the Reynolds stress sublayer and the wall layer. Included is the calculation of the shear stress at the wall in the interaction region. The limit processes considered are those used for an inviscid flow.
Directory of Open Access Journals (Sweden)
Yu. I. Troitskaya
2013-10-01
Full Text Available Drag and mass exchange coefficients are calculated within a self-consistent problem for the wave-induced air perturbations and mean velocity and density fields using a quasi-linear model based on the Reynolds equations with down-gradient turbulence closure. This second part of the report is devoted to specification of the model elements: turbulent transfer coefficients and wave number-frequency spectra. It is shown that the theory agrees with laboratory and field experimental data well when turbulent mass and momentum transfer coefficients do not depend on the wave parameters. Among several model spectra better agreement of the theoretically calculated drag coefficients with TOGA (Tropical Ocean Global Atmosphere COARE (Coupled Ocean–Atmosphere Response Experiment data is achieved for the Hwang spectrum (Hwang, 2005 with the high frequency part completed by the Romeiser spectrum (Romeiser et al., 1997.
Troitskaya, Yu. I.; Ezhova, E. V.; Sergeev, D. A.; Kandaurov, A. A.; Baidakov, G. A.; Vdovin, M. I.; Zilitinkevich, S. S.
2013-10-01
Drag and mass exchange coefficients are calculated within a self-consistent problem for the wave-induced air perturbations and mean velocity and density fields using a quasi-linear model based on the Reynolds equations with down-gradient turbulence closure. This second part of the report is devoted to specification of the model elements: turbulent transfer coefficients and wave number-frequency spectra. It is shown that the theory agrees with laboratory and field experimental data well when turbulent mass and momentum transfer coefficients do not depend on the wave parameters. Among several model spectra better agreement of the theoretically calculated drag coefficients with TOGA (Tropical Ocean Global Atmosphere) COARE (Coupled Ocean-Atmosphere Response Experiment) data is achieved for the Hwang spectrum (Hwang, 2005) with the high frequency part completed by the Romeiser spectrum (Romeiser et al., 1997).
Troitskaya, Yuliya; Kandaurov, Alexander; Sergeev, Daniil; Bopp, Maximilian; Caulliez, Guillemette
2017-04-01
Air-sea coupling in general is important for weather, climate, fluxes. Wind wave source is crucially important for surface waves' modeling. But the wind-wave growth rate is strongly uncertain. Using direct measurements of pressure by wave-following Elliott probe [1] showed, weak and indefinite dependence of wind-wave growth rate on the wave steepness, while Grare et.al. [2] discuss the limitations of direct measurements of pressure associated with the inability to measure the pressure close to the surface by contact methods. Recently non-invasive methods for determining the pressure on the basis of technology of time-resolved PIV are actively developed [3]. Retrieving air flow velocities by 2D PIV techniques was started from Reul et al [4]. The first attempt for retrieving wind pressure field of waves in the laboratory tank from the time-resolved PIV measurements was done in [5]. The experiments were performed at the Large Air-Sea Interaction Facility (LASIF) - MIO/Luminy (length 40 m, cross section of air channel 3.2 x 1.6 m). For 18 regimes with wind speed up to 14 m/s including presence of puddle waves, a combination of time resolved PIV technique and optical measurements of water surface form was applied to detailed investigation of the characteristics of the wind flow over the water surface. Ammonium chloride smoke was used for flow visualization illuminated by two 6 Wt blue diode lasers combined into a vertical laser plane. Particle movement was captured with high-speed camera using Scheimpflug technique (up to 20 kHz frame rate with 4-frame bursts, spatial resolution about 190 μm, field of view 314x12 mm). Velocity air flow field was retrieved by PIV images processing with adaptive cross-correlation method on the curvilinear grid following surface wave form. The resulting time resolved instantaneous velocity fields on regular grid allowed us to obtain momentum fluxes directly from measured air velocity fluctuations. The average wind velocity patterns were
Turbulent Plasmaspheric Boundary Layer: Observables and Consequences
Mishin, Evgeny
2014-10-01
In situ satellite observations reveal strong lower hybrid/fast magnetosonic turbulence and broadband hiss-like VLF waves in the substorm subauroral geospace at and earthward of the electron plasmasheet boundary. These coincide with subauroral ion drifts/polarization streams (SAID/SAPS) in the plasmasphere and topside ionosphere. SAID/SAPS appear in ~10 min after the substorm onset consistent with the fast propagation of substorm injection fronts. The SAID channel follows the dispersionless cutoff of the energetic electron flux at the plasmapause. This indicates that the cold plasma maintains charge neutrality within the channel, thereby short-circuiting the injected plasma jet (injection fronts over the plasmasphere. Plasma turbulence leads to the circuit resistivity and magnetic diffusion as well as significant electron heating and acceleration. As a result, a turbulent boundary layer forms between the inner edge of the electron plasmasheet and plasmasphere. The SAID/SAPS-related VLF emissions appear to constitute a distinctive subset of substorm/storm-related VLF activity in the region co-located with freshly injected energetic ions inside the plasmasphere. Significant pitch-angle diffusion coefficients suggest that substorm SAID/SAPS-related VLF waves could be responsible for the alteration of the outer radiation belt boundary during (sub)storms. Supported by the Air Force Office of Scientific Research.
Multiple paths to subharmonic laminar breakdown in a boundary layer
Zang, Thomas A.; Hussaini, M. Yousuff
1990-01-01
Numerical simulations demonstrate that laminar breakdown in a boundary layer induced by the secondary instability of two-dimensional Tollmien-Schlichting waves to three-dimensional subharmonic disturbancews need not take the conventional lambda vortex/high-shear layer path.
Outer layer effects in wind-farm boundary layers: Coriolis forces and boundary layer height
Allaerts, Dries; Meyers, Johan
2015-11-01
In LES studies of wind-farm boundary layers, scale separation between the inner and outer region of the atmospheric boundary layer (ABL) is frequently assumed, i.e., wind turbines are presumed to fall within the inner layer and are not affected by outer layer effects. However, modern wind turbine and wind farm design tends towards larger rotor diameters and farm sizes, which means that outer layer effects will become more important. In a prior study, it was already shown for fully-developed wind farms that the ABL height influences the power performance. In this study, we use the in-house LES code SP-Wind to investigate the importance of outer layer effects on wind-farm boundary layers. In a suite of LES cases, the ABL height is varied by imposing a capping inversion with varying inversion strengths. Results indicate the growth of an internal boundary layer (IBL), which is limited in cases with low inversion layers. We further find that flow deceleration combined with Coriolis effects causes a change in wind direction throughout the farm. This effect increases with decreasing boundary layer height, and can result in considerable turbine wake deflection near the end of the farm. The authors are supported by the ERC (ActiveWindFarms, grant no: 306471). Computations were performed on VSC infrastructiure (Flemish Supercomputer Center), funded by the Hercules Foundation and the Flemish Government-department EWI.
Reineman, B. D.; Lenain, L.; Melville, W. K.
2014-12-01
We present measurements obtained during the October 2012 EquatorMix experiment (0N, 140W), in which we deployed ship-launched and recovered Boeing-Insitu ScanEagle unmanned aerial vehicles (UAVs) to measure momentum and energy fluxes, ocean surface processes, and the marine atmospheric boundary layer (MABL). The UAV dataset is complemented by measurements from a suite of ship-based instrumentation, including a foremast MABL eddy covariance system, scanning and point lidar altimeters, a laser Doppler wind profiler, and a digitized X-band radar system (WaMoS). The combination of the unmanned aircraft and the ship instrumentation provides a novel and valuable dataset of many air-sea interaction phenomena, extending from 100s of meters below the surface to 1500 m above. Ocean surface displacements observed with the UAV lidar altimeter (coupled with a GPS/IMU) give evidence of high-frequency equatorial internal waves, with measurements consistent and coherent with those from ship-based X-band radar, the Hydrographic Doppler Sonar System (HDSS), and a theoretical model. UAV-based flux measurements at low altitudes (down to 30 meters) are consistent with ship-based eddy covariance measurements, but reveal differences between along- and crosswind sampling flight legs associated with longitudinal roll structures that are not captured by the ship measurements from tracks mainly in the upwind-downwind directions.
The ULF wave foreshock boundary: Cluster observations
Andres, Nahuel; Mazelle, Christian; Bertucci, Cesar; Gomez, Daniel
2014-01-01
The interaction of backstreaming ions with the incoming solar wind in the upstream region of the bow shock gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. Because of their finite growth rate, the ULF waves are spatially localized in the foreshock region. Previous studies have reported observational evidences of the existence of a ULF wave foreshock boundary, which geometrical characteristics are very sensitive to the interplanetary magnetic field (IMF) cone angle. The statistical properties of the ULF wave foreshock boundary is examined in detail using Cluster data. A new identification of the ULF wave foreshock boundary is presented using specific and accurate criterion for a precises determination of boundary crossings. The criterion is based on the degree of IMF rotation as Cluster crosses the boundary. The obtained ULF wave foreshock boundary is compared with previous results reported in the literature as well as with theoretical predictions. Also, we ex...
Introduction to computational techniques for boundary layers
Energy Technology Data Exchange (ETDEWEB)
Blottner, F.G.
1979-09-01
Finite-difference procedures to solve boundary layer flows in fluid mechanics are explained. The governing equations and the transformations utilized are described. Basic solution techniques are illustrated with the similar boundary layer equations. Nonsimilar solutions are developed for the incompressible equations. Various example problems are solved, and the numerical results in the Fortran listing of the computer codes are presented.
Modelling stable atmospheric boundary layers over snow
Sterk, H.A.M.
2015-01-01
Thesis entitled: Modelling Stable Atmospheric Boundary Layers over Snow H.A.M. Sterk Wageningen, 29th of April, 2015 Summary The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs typically form at night and in polar re
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.
Modelling stable atmospheric boundary layers over snow
Sterk, H.A.M.
2015-01-01
Thesis entitled: Modelling Stable Atmospheric Boundary Layers over Snow H.A.M. Sterk Wageningen, 29th of April, 2015 Summary The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs typically form at night and in polar re
Modelling stable atmospheric boundary layers over snow
Sterk, H.A.M.
2015-01-01
Thesis entitled: Modelling Stable Atmospheric Boundary Layers over Snow H.A.M. Sterk Wageningen, 29th of April, 2015 Summary The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs typically form at night and in polar
Structure of relaminarizing turbulent boundary layers
Ramesh, O.; Patwardhan, Saurabh
2014-11-01
Relaminarization of a turbulent boundary layer in a strongly accelerated flow has received a great attention in recent times. It has been found that such relaminarization is a general and regularly occurring phenomenon in the leading-edge region of a swept wing of an airplane (van Dam et al., 1993). In this work, we investigate the effect of initial Reynolds number on the process of relaminarization in turbulent boundary layers. The experimental and numerical investigation of relaminarizing turbulent boundary layers undergoing same history reveals that the boundary layer with higher initial Reynolds number relaminarizes at a lower pressure gradient value compared to the one with lower Reynolds number. This effect can be explained on the inviscid theory proposed earlier in the literature. Further, various parameter criteria proposed to predict relaminarization, are assessed and the structure of relaminarizing boundary layers is investigated. A mechanism for stabilization of near-wall low speed streaks is proposed.
Boundary Layers in Laminar Vortex Flows.
Baker, Glenn Leslie
A detailed experimental study of the flow in an intense, laminar, axisymmetric vortex has been conducted in the Purdue Tornado Vortex Simulator. The complicated nature of the flow in the boundary layer of laboratory vortices and presumably on that encountered in full-scale tornadoes has been examined. After completing a number of modifications to the existing facility to improve the quality of the flow in the simulator, hot-film anemometry was employed for making velocity-component and turbulence-intensity measurements of both the free-stream and boundary layer portions of the flow. The measurements represent the first experimental boundary layer investigation of a well-defined vortex flow to appear in the literature. These results were compared with recent theoretical work by Burggraf, Stewartson and Belcher (1971) and with an exact similarity solution for line-sink boundary layers developed by the author. A comparison is also made with the numerical simulation of Wilson (1981) in which the boundary conditions were matched to those of the present experimental investigation. Expressions for the vortex core radius, the maximum tangential velocity and the maximum pressure drop are given in terms of dimensionless modeling parameters. References. Burggraf, O. R., K. Stewartson and R. Belcher, Boundary layer. induced by a potential vortex. Phys. Fluids 14, 1821-1833 (1971). Wilson, T., M. S. thesis, Vortex Boundary Layer Dynamics, Univ. Calif. Davis (1981).
Tests and Applications of An Approach to Absorbing Reflected Waves Towards Incident Boundary
Institute of Scientific and Technical Information of China (English)
张洪生; 王炎; 许春辉; 商辉; 于小伟
2013-01-01
If the upstream boundary conditions are prescribed based on the incident wave only, the time-dependent numerical models cannot effectively simulate the wave field when the physical or spurious reflected waves become significant. This paper describes carefully an approach to specifying the incident wave boundary conditions combined with a set sponge layer to absorb the reflected waves towards the incident boundary. Incorporated into a time-dependent numerical model, whose governing equations are the Boussinesq-type ones, the effectiveness of the approach is studied in detail. The general boundary conditions, describing the down-wave boundary conditions are also generalized to the case of random waves. The numerical model is in detail examined. The test cases include both the normal one-dimensional incident regular or random waves and the two-dimensional oblique incident regular waves. The calculated results show that the present approach is effective on damping the reflected waves towards the incident wave boundary.
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...... a very detailed physical scale-model in a wind tunnel. In the present paper details of all these activities are presented together with first results....
Dynamics of boundary layer electrons around a laser wakefield bubble
Luo, J.; Chen, M.; Zhang, G.-B.; Yuan, T.; Yu, J.-Y.; Shen, Z.-C.; Yu, L.-L.; Weng, S.-M.; Schroeder, C. B.; Esarey, E.
2016-10-01
The dynamics of electrons forming the boundary layer of a highly nonlinear laser wakefield driven in the so called bubble or blowout regime is investigated using particle-in-cell simulations. It is shown that when the driver pulse intensity increases or the focal spot size decreases, a significant amount of electrons initially pushed by the laser pulse can detach from the bubble structure at its tail, middle, or front and form particular classes of waves locally with high densities, referred to as the tail wave, lateral wave, and bow wave. The tail wave and bow wave correspond to real electron trajectories, while the lateral wave does not. The detached electrons can be ejected transversely, containing considerable energy, and reducing the efficiency of the laser wakefield accelerator. Some of the transversely emitted electrons may obtain MeV level energy. These electrons can be used for wake evolution diagnosis and producing high frequency radiation.
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
2012-01-01
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......) and involves implementation of an arbitrary prescribed initial boundary layer (See [1]). A prescribed initial boundary layer profile is enforced through the computational domain using body forces to maintain a desired flow field. The body forces are then stored and applied on the domain through the 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 than typically...
Control of the Transitional Boundary Layer
Belson, Brandt A.
This work makes advances in the delay of boundary layer transition from laminar to turbulent flow via feedback control. The applications include the reduction of drag over streamline bodies (e.g., airplane wings) and the decrease of mixing and heat transfer (e.g., over turbine blades in jet engines). A difficulty in many fields is designing feedback controllers for high-dimensional systems, be they experiments or high-fidelity simulations, because the required time and resources are too large. A cheaper alternative is to approximate the high-dimensional system with a reduced-order model and design a controller for the model. We implement several model reduction algorithms in "modred", an open source and publicly available library that is applicable to a wide range of problems. We use this library to study the role of sensors and actuators in feedback control of transition in the 2D boundary layer. Previous work uses a feedforward configuration in which the sensor is upstream of the actuator, but we show that the actuator-sensor pair is unsuitable for feedback control due to an inability to sense the exponentially-growing Tollmien-Schlichting waves. A new actuator-sensor pair is chosen that more directly affects and measures the TS waves, and as a result it is effective in a feedback configuration. Lastly, the feedback controller is shown to outperform feedforward controllers in the presence of unmodeled disturbances. Next, we focus on a specific type of actuator, the single dielectric barrier discharge (SDBD) plasma actuator. An array of these plasma actuators is oriented to produce stream-wise vorticity and thus directly cancel the structures with the largest transient growth (so-called stream-wise streaks). We design a feedback controller using only experimental data by first developing an empirical input-output quasi-steady model. Then, we design feedback controllers for the model such that the controllers perform well when applied to the experiment. Lastly, we
Directory of Open Access Journals (Sweden)
A. Keiling
2006-10-01
Full Text Available Recent Cluster studies reported properties of multiple energy-dispersed ion structures in the plasma sheet boundary layer (PSBL that showed substructure with several well separated ion beamlets, covering energies from 3 keV up to 100 keV (Keiling et al., 2004a, b. Here we report observations from two PSBL crossings, which show a number of identified one-to-one correlations between this beamlet substructure and several plasma-field characteristics: (a bimodal ion conics (<1 keV, (b field-aligned electron flow (<1 keV, (c perpendicular electric field spikes (~20 mV/m, (d broadband electrostatic ELF wave packets (<12.5 Hz, and (e enhanced broadband electromagnetic waves (<4 kHz. The one-to-one correlations strongly suggest that these phenomena were energetically driven by the ion beamlets, also noting that the energy flux of the ion beamlets was 1–2 orders of magnitude larger than, for example, the energy flux of the ion outflow. In addition, several more loosely associated correspondences were observed within the extended region containing the beamlets: (f electrostatic waves (BEN (up to 4 kHz, (g traveling and standing ULF Alfvén waves, (h field-aligned currents (FAC, and (i auroral emissions on conjugate magnetic field lines. Possible generation scenarios for these phenomena are discussed. In conclusion, it is argued that the free energy of magnetotail ion beamlets drove a variety of phenomena and that the spatial fine structure of the beamlets dictated the locations of where some of these phenomena occurred. This emphasizes the notion that PSBL ion beams are important for magnetosphere-ionosphere coupling. However, it is also shown that the dissipation of electromagnetic energy flux (at altitudes below Cluster of the simultaneously occurring Alfvén waves and FAC was larger (FAC being the largest than the dissipation of beam kinetic energy flux, and thus these two energy carriers contributed more to the energy transport on PSBL field lines
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.
Alpha models and boundary-layer turbulence
Cheskidov, Alexey
We study boundary-layer turbulence using the Navier-Stokes-alpha model obtaining an extension of the Prandtl equations for the averaged flow in a turbulent boundary layer. In the case of a zero pressure gradient flow along a flat plate, we derive a nonlinear fifth-order ordinary differential equation, an extension of the Blasius equation. We study it analytically and prove the existence of a two-parameter family of solutions satisfying physical boundary conditions. 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 the skin-friction coefficient in the turbulent boundary layer. The two-parameter family of solutions to the equation matches experimental data in the transitional boundary layers with different free stream turbulence intensity. A one-parameter sub-family of solutions, obtained using our skin-friction coefficient law, matches experimental data in the turbulent boundary layer for moderately large Reynolds numbers.
THERMAL BOUNDARY LAYER IN CFB BOILER RISER
Institute of Scientific and Technical Information of China (English)
Jinwei; Wang; Xinmu; Zhao; Yu; Wang; Xing; Xing; Jiansheng; Zhang; Guangxi; Yue
2006-01-01
Measurement of temperature profiles of gas-solid two-phase flow at different heights in commercial-scale circulating fluidized bed (CFB) boilers was carried out. Experimental results showed that the thickness of thermal boundary layer was generally independent of the distance from the air distributor, except when close to the riser outlet. Through analysis of flow and combustion characteristics in the riser, it was found that the main reasons for the phenomena were: 1) the hydrodynamic boundary layer was thinner than the thermal layer and hardly changed along the CFB boiler height, and 2) both radial and axial mass and heat exchanges were strong in the CFB boiler. Numerical simulation of gas flow in the outlet zone confirmed that the distribution of the thermal boundary layer was dominated by the flow field characteristics.
The ULF wave foreshock boundary: Cluster observations
Andres, N.; Meziane, K.; Mazelle, C. X.; Bertucci, C.; Gomez, D. O.
2013-12-01
In the upstream region of the bow shock, the interaction of backstreaming ions with the incoming solar wind gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. Due to the finite growth rate, it is expected that the region of ULF wave activity is spatially localized in the ion foreshock. Observational evidence of the ULF wave foreshock boundary has accumulated over the last three decades. Among other things, it has been shown that the geometrical characteristics of the boundary are very sensitive to the interplanetary magnetic field (IMF) cone angle. In the present work, we aimed at revisiting the properties of the ULF wave foreshock boundary. For this purpose, we use the first three years of magnetic field data from the flux gate magnetometer (FGM), and the plasma densities and velocities from the hot ion analyzer (HIA) low-geometry factor side on board RUMBA (SC 1). We use a specific and accurate criterion for the determination of boundary crossings, and a 3-D structure bow shock model to reconstruct the foreshock geometry. In particular, our criterion is used to qualitatively measure the differences between the magnetic field in the wave and no-wave zones, taking into account possible rotations of the IMF. A new identification of the ULF wave foreshock boundary is presented and it is compared with previous results reported in the literature as well as with theoretical predictions.
1975-10-01
Moore and Erdos (Ref. 28) in solving the boundary layer equations for dissoci- tion and ionizing air in a nonequilibrium flow. * 21 Another powerful...8217 =o g =o ’ =o(4.31b) fit = 0 g" = 0 Zt =0 fi’ = 0 git = 0 where, the prime denotes differentiation with respect to 1. The edge of the sheath is now the...i LX ] (4.37e) where, C = pR/Pee and the prime denotes differentiation with respect to T. Equations 4.31 and 4.37 produce seven, seven, and five
Scattering of Acoustic Waves from Ocean Boundaries
2015-09-30
of Acoustic Waves from Ocean Boundaries Marcia...J. Isakson Applied Research Laboratories The University of Texas at Austin, TX 78713-8029 phone: (512)835-3790 fax: (512)835-3259 email...plane wave integral transform method which assumes invariance in one spatial dimension of the waveguide. In this case, the dimension is
Institute of Scientific and Technical Information of China (English)
S·古普塔; A·卡托帕德亚; D·K·玛里; 吴承平
2011-01-01
根据介质的力学性能,正如Cowin及Nunziato一样,导出多孔弹性层覆盖在多孔弹性半空间上时,研究其刚性边界对扭转表面波传播的影响.导出了速度方程并对其结果进行了讨论.发现介质中可能存在两类扭转表面波阵面,而Dey等(Tamkang Journal of Science and Engineering,2003,6(4):241-249.)给出的没有刚性边界面时,存在3类扭转表面波阵面.研究还揭示,多孔弹性层中Love波也可能随同扭转表面波一起存在.值得注意的是,刚性边界面多孔弹性层中Love波的相速度,不同于自由边界面多孔弹性层中的相速度.实际观察到扭转波的色散性,以及速度随着振荡频率的增大而减小.%The effect of rigid boundary on the propagation of torsional surface waves in a porous elastic layer over a porous elastic half space was presented using the mechanics of the medium as derived by Cowin and Nunziato. The velocity equation was derived and the results were discussed. It is observed that there may be two torsional surface wave fronts in the medium whereas there exists three wave fronts of torsional surface waves in the absence of rigid boundary plane given by Dey et al( Tamkang Journal of Science and Engineering, 2003, 6(4 ): 241-249. ). The results also reveals that in the porous layer, the Love wave is also available along with the torsional surface waves. It is remarkable that phase speed of Love wave in a porous layer with rigid surface is different from that in a porous layer with a free surface. The torsional waves are observed to be dispersive in nature, and the velocity decreases as the frequency of oscillation increases.
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.
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)
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.
Experimental studies on the stability and transition of 3-dimensional boundary layers
Nitschke-Kowsky, P.
1987-01-01
Three-dimensional unstable boundary layers were investigated as to their characteristic instabilities, leading to turbulence. Standing cross-flow instabilities and traveling waves preceding the transition were visualized with the hydrogen bubble technique in the boundary layer above the wall of a swept cylinder. With the sublimation method and hot film technique, a model consisting of a swept flat plate with a pressure-inducing displacement body in the 1 m wind tunnel was studied. Standing waves and traveling waves in a broad frequency are observed. The boundary layer of this model is close to the assumptions of the theory.
An effective absorbing boundary algorithm for acoustical wave propagator
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
In this paper, Berenger's perfectly matched layer (PML) absorbing boundary condition for electromagnetic waves is introduced as the truncation area of the computational domain to absorb one-dimensional acoustic wave for the scheme of acoustical wave propagator (AWP). To guarantee the efficiency of the AWP algorithm, a regulated propagator matrix is derived in the PML medium.Numerical simulations of a Gaussian wave packet propagating in one-dimensional duct are carried out to illustraze the efficiency of the combination of PML and AWP. Compared with the traditional smoothing truncation windows technique of AWP, this scheme shows high computational accuracy in absorbing acoustic wave when the acoustical wave arrives at the computational edges. Optimal coefficients of the PML configurations are also discussed.
Studies of stability of blade cascade suction surface boundary layer
Institute of Scientific and Technical Information of China (English)
DONG Xue-zhi; YAN Pei-gang; HAN Wan-jin
2007-01-01
Compressible boundary layers stability on blade cascade suction surface was discussed by wind tunnel experiment and numerical solution. Three dimensional disturbance wave Parabolized Stability Equations(PSE) of orthogonal Curvilinear Coordinates in compressible flow was deducted. The surface pressure of blade in wind tunnel experiment was measured. The Falkner-Skan equation was solved under the boundary conditions of experiment result, and velocity, pressure and temperature of average flow were obtained. Substituted this result for discretization of the PSE Eigenvalue Problem, the stability problem can be solved.
Turbulent Boundary Layers - Experiments, Theory and Modelling
1980-01-01
DEVELOPMENT (ORGANISATION DU TRAITE DE L’ATLANTIQUE NORD ) AGARD Conference Proceedings No.271 TURBULENT BOUNDARY LAYERS - EXPERIMENTS, THEORY AND...photographs of Figures 21 and 22. In this case, the photographs are taken with a single flash strobe and thus yield the instantaneous positions of the
Comments on Hypersonic Boundary-Layer Transition
1990-09-01
laver transition results from instabilities as described by linear stability theory, then the disturbance growth historias follow a prescribed...mechanism by which boundary-layer disturbance growth is generally initiated and establishes the initial distur- banca amplitude at the onset of disturbance
Boundary layer control device for duct silencers
Schmitz, Fredric H. (Inventor); Soderman, Paul T. (Inventor)
1993-01-01
A boundary layer control device includes a porous cover plate, an acoustic absorber disposed under the porous cover plate, and a porous flow resistive membrane interposed between the porous cover plate and the acoustic absorber. The porous flow resistive membrane has a flow resistance low enough to permit sound to enter the acoustic absorber and high enough to damp unsteady flow oscillations.
Atmospheric Boundary Layers: Modeling and Parameterization
Holtslag, A.A.M.
2015-01-01
In this contribution we deal with the representation of the atmospheric boundary layer (ABL) for modeling studies of weather, climate, and air quality. As such we review the major characteristics of the ABL, and summarize the basic parameterizations for the description of atmospheric turbulence and
Response of a Hypersonic Boundary Layer to Freestream Pulse Acoustic Disturbance
Directory of Open Access Journals (Sweden)
Zhenqing Wang
2014-01-01
Full Text Available The response of hypersonic boundary layer over a blunt wedge to freestream pulse acoustic disturbance was investigated. The stability characteristics of boundary layer for freestream pulse wave and continuous wave were analyzed comparatively. Results show that freestream pulse disturbance changes the thermal conductivity characteristics of boundary layer. For pulse wave, the number of main disturbance clusters decreases and the frequency band narrows along streamwise. There are competition and disturbance energy transfer among different modes in boundary layer. The dominant mode of boundary layer has an inhibitory action on other modes. Under continuous wave, the disturbance modes are mainly distributed near fundamental and harmonic frequencies, while under pulse wave, the disturbance modes are widely distributed in different modes. For both pulse and continuous waves, most of disturbance modes slide into a lower-growth or decay state in downstream, which is tending towards stability. The amplitude of disturbance modes in boundary layer under continuous wave is considerably larger than pulse wave. The growth rate for the former is also considerably larger than the later the disturbance modes with higher growth are mainly distributed near fundamental and harmonic frequencies for the former, while the disturbance modes are widely distributed in different frequencies for the latter.
Response of a hypersonic boundary layer to freestream pulse acoustic disturbance.
Wang, Zhenqing; Tang, Xiaojun; Lv, Hongqing
2014-01-01
The response of hypersonic boundary layer over a blunt wedge to freestream pulse acoustic disturbance was investigated. The stability characteristics of boundary layer for freestream pulse wave and continuous wave were analyzed comparatively. Results show that freestream pulse disturbance changes the thermal conductivity characteristics of boundary layer. For pulse wave, the number of main disturbance clusters decreases and the frequency band narrows along streamwise. There are competition and disturbance energy transfer among different modes in boundary layer. The dominant mode of boundary layer has an inhibitory action on other modes. Under continuous wave, the disturbance modes are mainly distributed near fundamental and harmonic frequencies, while under pulse wave, the disturbance modes are widely distributed in different modes. For both pulse and continuous waves, most of disturbance modes slide into a lower-growth or decay state in downstream, which is tending towards stability. The amplitude of disturbance modes in boundary layer under continuous wave is considerably larger than pulse wave. The growth rate for the former is also considerably larger than the later the disturbance modes with higher growth are mainly distributed near fundamental and harmonic frequencies for the former, while the disturbance modes are widely distributed in different frequencies for the latter.
Controls on boundary layer ventilation: Boundary layer processes and large-scale dynamics
Sinclair, V. A.; Gray, S. L.; Belcher, S. E.
2010-06-01
Midlatitude cyclones are important contributors to boundary layer ventilation. However, it is uncertain how efficient such systems are at transporting pollutants out of the boundary layer, and variations between cyclones are unexplained. In this study 15 idealized baroclinic life cycles, with a passive tracer included, are simulated to identify the relative importance of two transport processes: horizontal divergence and convergence within the boundary layer and large-scale advection by the warm conveyor belt. Results show that the amount of ventilation is insensitive to surface drag over a realistic range of values. This indicates that although boundary layer processes are necessary for ventilation they do not control the magnitude of ventilation. A diagnostic for the mass flux out of the boundary layer has been developed to identify the synoptic-scale variables controlling the strength of ascent in the warm conveyor belt. A very high level of correlation (R2 values exceeding 0.98) is found between the diagnostic and the actual mass flux computed from the simulations. This demonstrates that the large-scale dynamics control the amount of ventilation, and the efficiency of midlatitude cyclones to ventilate the boundary layer can be estimated using the new mass flux diagnostic. We conclude that meteorological analyses, such as ERA-40, are sufficient to quantify boundary layer ventilation by the large-scale dynamics.
Numerical methods for hypersonic boundary layer stability
Malik, M. R.
1990-01-01
Four different schemes for solving compressible boundary layer stability equations are developed and compared, considering both the temporal and spatial stability for a global eigenvalue spectrum and a local eigenvalue search. The discretizations considered encompass: (1) a second-order-staggered finite-difference scheme; (2) a fourth-order accurate, two-point compact scheme; (3) a single-domain Chebychev spectral collocation scheme; and (4) a multidomain spectral collocation scheme. As Mach number increases, the performance of the single-domain collocation scheme deteriorates due to the outward movement of the critical layer; a multidomain spectral method is accordingly designed to furnish superior resolution of the critical layer.
Secondary instability in boundary-layer flows
Nayfeh, A. H.; Bozatli, A. N.
1979-01-01
The stability of a secondary Tollmien-Schlichting wave, whose wavenumber and frequency are nearly one half those of a fundamental Tollmien-Schlichting instability wave is analyzed using the method of multiple scales. Under these conditions, the fundamental wave acts as a parametric exciter for the secondary wave. The results show that the amplitude of the fundamental wave must exceed a critical value to trigger this parametric instability. This value is proportional to a detuning parameter which is the real part of k - 2K, where k and K are the wavenumbers of the fundamental and its subharmonic, respectively. For Blasius flow, the critical amplitude is approximately 29% of the mean flow, and hence many other secondary instabilities take place before this parametric instability becomes significant. For other flows where the detuning parameter is small, such as free-shear layer flows, the critical amplitude can be small, thus the parametric instability might play a greater role.
Adamson, T. C., Jr.; Liou, M. S.; Messiter, A. F.
1980-01-01
An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.
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
The Lowest Atmosphere: Atmospheric Boundary Layer Including Atmospheric Surface Layer.
1996-04-01
of motion of the atmosphere— "second order closure"—to such applications as the SCIPUFF -PC code for tracer dispersion (see Sykes, 1994). Now, for...Turbulence, Methuen, London, 2nd Ed., 1955. Sykes, R.I., "The SCIPUFF -PC Code," ARAP Draft Report, 1994. Tennekes, H., "The Atmospheric Boundary Layer
Turbulent dispersion in cloud-topped boundary layers
Verzijlbergh, R.A.; Jonker, H.J.J.; Heus, T.; Vilà-Guerau de Arellano, J.
2009-01-01
Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary
Bursting frequency prediction in turbulent boundary layers
Energy Technology Data Exchange (ETDEWEB)
LIOU,WILLIAM W.; FANG,YICHUNG
2000-02-01
The frequencies of the bursting events associated with the streamwise coherent structures of spatially developing incompressible turbulent boundary layers were predicted using global numerical solution of the Orr-Sommerfeld and the vertical vorticity equations of hydrodynamic stability problems. The structures were modeled as wavelike disturbances associated with the turbulent mean flow. The global method developed here involves the use of second and fourth order accurate finite difference formula for the differential equations as well as the boundary conditions. An automated prediction tool, BURFIT, was developed. The predicted resonance frequencies were found to agree very well with previous results using a local shooting technique and measured data.
Turbulent Boundary Layer Flow over Superhydrophobic Surfaces
2013-05-10
Figure 1 were a highly viscous fluid, such as honey , the boundary layer would be thick while if the fluid were water, a low-viscosity fluid, the boundary...drag has become even more important. In response to this need, and with the benefit of modern technology, the drag-reduction field is replete with...manufactured with “riblets,” small ridges on the order of fractions of millimeters, built-into the hull or skin that seek to reduce frictional drag. The
Particulate plumes in boundary layers with obstacles
Petrosyan, Arakel; Karelsky, Kirill
2013-04-01
This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by non-slip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of big wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations.We deal with describing big field
Abstract wave equations with acoustic boundary conditions
Mugnolo, Delio
2010-01-01
We define an abstract setting to treat wave equations equipped with time-dependent acoustic boundary conditions on bounded domains of ${\\bf R}^n$. We prove a well-posedness result and develop a spectral theory which also allows to prove a conjecture proposed in (Gal-Goldstein-Goldstein, J. Evol. Equations 3 (2004), 623-636). Concrete problems are also discussed.
Steady water waves with multiple critical layers
Ehrnström, Mats; Wahlén, Erik
2010-01-01
We construct small-amplitude periodic water waves with multiple critical layers. In addition to waves with arbitrarily many critical layers and a single crest in each period, two-dimensional sets of waves with several crests and troughs in each period are found. The setting is that of steady two-dimensional finite-depth gravity water waves with vorticity.
Large-eddy simulation of the very stable boundary layer
Chinita, M. J.; Matheou, G.
2016-12-01
The stable boundary layer is ubiquitous and typically forms at night when the ground radiatively cools and in polar regions throughout the day. Stable stratification and the associated reduction in the energetic scales in combination with the large anisotropy of turbulent motions challenge numerical models. This modeling difficulty also affects large-eddy simulation (LES) methods leading to scarce LES results for very stable conditions. In contrast, the NWP of convective flows has greatly benefited from the ample availability of high quality LES data. In order to overcome these limitations, a novel LES model setup is developed to enable the modeling of very stable boundary layers. A series of Ekman layer-type boundary layers at various surface cooling rates, geotropic winds and latitudes (rotation rates) is presented. A temperature surface condition is applied in the LES. The surface heat flux is dynamically computed byresolving the surface layer since the often-used Monin-Obukhov similarity theory cannot represent very stable conditions. Depending on the conditions, the LES gracefully transitions to a direct numerical simulation (DNS) where the flow becomes fully resolved. Two stability regimes can be discerned based on vertical profiles of the Richardson number. Overall, the model predicts that turbulence is very resilient with respect to stability. Temperature and velocity fluctuations persist even at high Richardson numbers. The nature of the fluctuations, i.e., due to turbulence/overturning or waves, is discussed. Scaling relations and spectra are also presented and discussed.
Boundary layer control of rotating convection systems.
King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M
2009-01-15
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.
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.
MHD Turbulence in Accretion Disk Boundary Layers
Chan, Chi-kwan
2012-01-01
The physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, 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 is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations...
Supersonic Turbulent Boundary Layer: DNS and RANS
Institute of Scientific and Technical Information of China (English)
XU Jing-Lei; MA Hui-Yang
2007-01-01
We assess the performance of a few turbulence models for Reynolds averaged Navier-Stokes (RANS) simulation of supersonic boundary layers, compared to the direct numerical simulations (DNS) of supersonic flat-plate turbulent boundary layers, carried out by Gao et al. [Chin. Phys. Lett. 22 (2005) 1709] and Huang et al. [Sci.Chin. 48 (2005) 614], as well as some available experimental data. The assessment is made for two test cases, with incoming Mach numbers and Reynolds numbers M = 2.25, Re = 365, 000/in, and M = 4.5, Re - 1.7 × 107/m,respectively. It is found that in the first case the prediction of RANS models agrees well with the DNS and the experimental data, while for the second case the agreement of the DNS models with experiment is less satisfactory.The compressibility effect on the RANS models is discussed.
Turbulent boundary layer over flexible plates
Rostami, Parand; Ioppolo, Tindaro
2016-11-01
This research describes the structure of a turbulent boundary layer flow with a zero pressure gradient over elastic plates. The elastic plates made of a thin aluminum sheets with thickness between 50 and 500 microns were placed on the floor of a subsonic wind tunnel and exposed to a turbulent boundary layer flow with a free stream velocity between 20m/s and 100m/s. The ceiling of the test section of the wind tunnel is adjustable so that a nearly zero pressure gradient is obtained in the test section. Hot-wire anemometry was used to measure the velocity components. Mean, fluctuating velocities and Reynolds stresses will be presented and compared with the values of a rigid plate.
BOREAS AFM-6 Boundary Layer Height Data
Wilczak, James; Hall, Forrest G. (Editor); Newcomer, Jeffrey A. (Editor); Smith, David E. (Technical Monitor)
2000-01-01
The Boreal Ecosystem-Atmosphere Study (BOREAS) Airborne Fluxes and Meteorology (AFM)-6 team from National Oceanic and Atmospheric Adminsitration/Environment Technology Laboratory (NOAA/ETL) operated a 915-MHz wind/Radio Acoustic Sounding System (RASS) profiler system in the Southern Study Area (SSA) near the Old Jack Pine (OJP) site. This data set provides boundary layer height information over the site. The data were collected from 21 May 1994 to 20 Sep 1994 and are stored in tabular ASCII files. The boundary layer height data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The data files are available on a CD-ROM (see document number 20010000884).
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}\
RANS Modeling of Benchmark Shockwave / Boundary Layer Interaction Experiments
Georgiadis, Nick; Vyas, Manan; Yoder, Dennis
2010-01-01
This presentation summarizes the computations of a set of shock wave / turbulent boundary layer interaction (SWTBLI) test cases using the Wind-US code, as part of the 2010 American Institute of Aeronautics and Astronautics (AIAA) shock / boundary layer interaction workshop. The experiments involve supersonic flows in wind tunnels with a shock generator that directs an oblique shock wave toward the boundary layer along one of the walls of the wind tunnel. The Wind-US calculations utilized structured grid computations performed in Reynolds-averaged Navier-Stokes mode. Three turbulence models were investigated: the Spalart-Allmaras one-equation model, the Menter Shear Stress Transport wavenumber-angular frequency two-equation model, and an explicit algebraic stress wavenumber-angular frequency formulation. Effects of grid resolution and upwinding scheme were also considered. The results from the CFD calculations are compared to particle image velocimetry (PIV) data from the experiments. As expected, turbulence model effects dominated the accuracy of the solutions with upwinding scheme selection indicating minimal effects.!
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.
2007 Program of Study: Boundary Layers
2008-06-01
PM Coalescence of charged water droplets Andrew Belmonte, Pennsylvania State University August 9 - 10:30 AM Multiscale analysis of strongly...Properties of Helium Near the Liquid-Vapor Critical Point. J. low Temp. Phys. 46, 115-135. [15] Polezhaev, V. I. and Soboleva, E. B. 2004 Rayleigh...through which potassium permanganate was added in most of the experiments in order to detect a possible appearance of boundary layer separation and
Experimental studies on transitional separated boundary layers
Serna Serrano, José
2013-01-01
Separated transitional boundary layers appear on key aeronautical processes such as the flow around wings or turbomachinery blades. The aim of this thesis is the study of these flows in representative scenarios of technological applications, gaining knowledge about phenomenology and physical processes that occur there and, developing a simple model for scaling them. To achieve this goal, experimental measurements have been carried out in a low speed facility, ensuring the flow homogeneity and...
Three dimensional boundary layers in internal flows
Bodonyi, R. J.
1987-01-01
A numerical study of the effects of viscous-inviscid interactions in three-dimensional duct flows is presented. In particular interacting flows for which the oncoming flow is not fully-developed were considered. In this case there is a thin boundary layer still present upstream of the surface distortion, as opposed to the fully-developed pipe flow situation wherein the flow is viscous across the cross section.
Regional Wave Climates along Eastern Boundary Currents
Semedo, Alvaro; Soares, Pedro
2016-04-01
Two types of wind-generated gravity waves coexist at the ocean surface: wind sea and swell. Wind sea waves are waves under growing process. These young growing waves receive energy from the overlaying wind and are strongly coupled to the local wind field. Waves that propagate away from their generation area and no longer receive energy input from the local wind are called swell. Swell waves can travel long distances across entire ocean basins. A qualitative study of the ocean waves from a locally vs. remotely generation perspective is important, since the air sea interaction processes is strongly modulated by waves and vary accordingly to the prevalence of wind sea or swell waves in the area. A detailed climatology of wind sea and swell waves along eastern boundary currents (EBC; California Current, Canary Current, in the Northern Hemisphere, and Humboldt Current, Benguela Current, and Western Australia Current, in the Southern Hemisphere), based on the ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-Interim reanalysis will be presented. The wind regime along EBC varies significantly from winter to summer. The high summer wind speeds along EBC generate higher locally generated wind sea waves, whereas lower winter wind speeds in these areas, along with stronger winter extratropical storms far away, lead to a predominance of swell waves there. In summer, the coast parallel winds also interact with coastal headlands, increasing the wind speed through a process called "expansion fan", which leads to an increase in the height of locally generated waves downwind of capes and points. Hence the spatial patterns of the wind sea or swell regional wave fields are shown to be different from the open ocean along EBC, due to coastal geometry and fetch dimensions. Swell waves will be shown to be considerably more prevalent and to carry more energy in winter along EBC, while in summer locally generated wind sea waves are either more comparable to swell waves or
Turbulent Boundary Layer at Large Re
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Horia DUMITRESCU
2016-03-01
Full Text Available The fluids as deformable bodies without own shape, when starting from rest, experience interactions between the flowing fluid and the physical surfaces marking the bounds of flow. These interactions are a kind of impact process where there is a momentum exchange between two colliding bodies, i.e. the flow and its boundary surfaces. Within a short time of contact a post-impact shear flow occurs where two main effects are triggered off by the flow-induced collision: dramatic redistribution of the momentum and the boundary vorticity followed by the shear stress/viscosity change in the microstructure of the fluid which at the beginning behaves as linear reactive medium and latter as nonlinear dispersive medium. The disturbance of the starting flow induces the entanglement of the wall-bounded flow in the form of point-vortices or concentrated vorticity balls whence waves are emitted and propagated through flow field. The paper develops a wave mechanism for the transport of the concentrated boundary vorticity, directly related to the fascinating turbulence phenomenon, using the torsion concept of vorticity filaments associated with the hypothesis of thixotropic/nonlinear viscous fluid.
Analytical solutions to a compressible boundary layer problem with heat transfer
Institute of Scientific and Technical Information of China (English)
Liancun Zheng; Xinxin Zhang; Jicheng He
2004-01-01
The problem of momentum and heat transfer in a compressible boundary layer behind a thin expansion wave was solved by the application of the similarity transformation and the shooting technique. Utilizing the analytical expression of a two-point boundary value problem for momentum transfer, the energy boundary layer solution was represented as a function of the dimensionless velocity, and as the parameters of the Prandtl number, the velocity ratio, and the temperature ratio.
Three-dimensional stability of growing boundary layers
Nayfeh, A. H.
1980-01-01
A theory is developed for the linear stability of three-dimensional growing boundary layers. The method of multiple scales is used to derive partial-differential equations describing the temporal and spatial evolution of the complex amplitudes and wavenumbers of the disturbances. In general, these equations are elliptic unless certain conditions are satisfied. For a monochromatic disturbance, these conditions demand that the ratio of the components of the complex group velocity be real and thereby relate the direction of growth of the disturbance to the disturbance wave angle. For a nongrowing boundary layer, this condition reduces to d-alpha/d-beta being real, in agreement with the result obtained by using the saddle-point method. For a wavepacket, these conditions demand that the components of the group velocity be real.
Numerical study of the laminar shock boundary layer interaction
Katzer, E.
1985-02-01
The interaction of an oblique shock wave with a laminar boundary layer on an adiabatic flat plate was analyzed numerically with solutions of the two dimensional Navier-Stokes equations using McCormack's explicit finite volume method. The agreement between numerical calculations and experimental results is good. Local and global properties of the interaction region are discussed regarding shock strength, separation bubble length using a similarity law, and separation environment. The asymetrical structure inside the separation bubble produces an asymetrical shape of the wall shear stress distribution. The calculation speed was increased by algorithm vectorization on a CRAY 1S supercomputer. Further investigations for determination of a similarity law in interaction with turbulent boundary layer, of the physical mechanisms of the laminar interaction, and for study of the wall temperature transfer are recommended.
New Algebraic Approaches to Classical Boundary Layer Problems
Institute of Scientific and Technical Information of China (English)
Xiao Ping XU
2011-01-01
Classical non-steady boundary layer equations are fundamental nonlinear partial differential equations in the boundary layer theory of fluid dynamics. In this paper, we introduce various schemes with multiple parameter functions to solve these equations and obtain many families of new explicit exact solutions with multiple parameter functions. Moreover, symmetry transformations are used to simplify our arguments. The technique of moving frame is applied in the three-dimensional case in order to capture the rotational properties of the fluid. In particular, we obtain a family of solutions singular on any moving surface, which may be used to study turbulence. Many other solutions are analytic related to trigonometric and hyperbolic functions, which reflect various wave characteristics of the fluid. Our solutions may also help engineers to develop more effective algorithms to find physical numeric solutions to practical models.
Leading-edge effects on boundary-layer receptivity
Gatski, Thomas B.; Kerschen, Edward J.
1990-01-01
Numerical calculations are presented for the incompressible flow over a parabolic cylinder. The computational domain extends from a region upstream of the body downstream to the region where the Blasius boundary-layer solution holds. A steady mean flow solution is computed and the results for the scaled surface vorticity, surface pressure and displacement thickness are compared to previous studies. The unsteady problem is then formulated as a perturbation solution starting with and evolving from the mean flow. The response to irrotational time harmonic pulsation of the free-stream is examined. Results for the initial development of the velocity profile and displacement thickness are presented. These calculations will be extended to later times to investigate the initiation of instability waves within the boundary-layer.
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)
One-way Penetration of the Boundary Wave in Anisotropic Structures
Directory of Open Access Journals (Sweden)
A.D. Arkhipov
2012-06-01
Full Text Available One-way penetration of the boundary wave in single-layer and multilayer anisotropic structures is investigated. We show that the conditions for minimal wave reflection from one side and full wave reflection from another side of the structure are possible. No conversation of polarization can be obtained for the considered effect.
Aerosol fluxes in the marine boundary layer
Petelski, Tomasz; Zieliński, Tymon; Makuch, Przemysław; Kowalczyk, Jakub; Ponczkowska, Agnieszka; Drozdowska, Violetta; Piskozub, Jacek
2010-05-01
We present aerosol emission fluxes and concentrations calculated from in-situ measurement in the Nordic Sea from R/V Oceania. We compare vertical fluxes calculated with the eddy correlation and gradient methods. We use the results to test the hypothesis that marine aerosol emitted from the sea surface helps to clear the boundary layer from other aerosol particles. As the emitted droplets do not dry out in the highly humid surface layer air and because of their sizes most of them are deposited quickly at the sea surface. Therefore marine aerosol has many features of rain meaning that the deposition in the marine boundary layer in high wind events is controlled not only by the "dry" processes but also by the "wet" scavenging. We have estimated the effectiveness of the process using our own measurements of vertical aerosol fluxes in the Nordic Seas. This process could explain observed phenomenon of lower Arctic aerosol optical thickness (AOT) when the air masses moved over open sea than over sea-ice. We show a negative correlation between the sea-ice coverage in the seas adjacent to Svalbard and monthly AOT values in Ny Alesund.
A Coordinate Transformation for Unsteady Boundary Layer Equations
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Paul G. A. CIZMAS
2011-12-01
Full Text Available This paper presents a new coordinate transformation for unsteady, incompressible boundary layer equations that applies to both laminar and turbulent flows. A generalization of this coordinate transformation is also proposed. The unsteady boundary layer equations are subsequently derived. In addition, the boundary layer equations are derived using a time linearization approach and assuming harmonically varying small disturbances.
Kelvin-Helmholtz instability in the magnetopause-boundary layer region
Lee, L. C.; Albano, R. K.; Kan, J. R.
1981-01-01
The Kelvin-Helmholtz instability in the magnetopause-boundary layer region is studied on the basis of an idealized model which consists of three uniform plasma regions: the magnetosheath, the boundary layer, and the magnetosphere. There are two unstable modes in the magnetopause-boundary layer region: one is excited at the magnetopause (the magnetopause mode) and the other is excited at the inner surface of the boundary layer (the inner mode). The inner mode is found to be unstable most of the time, while the excitation of the magnetopause mode depends on the magnetic field in the magnetosheath. The observed variation of the boundary layer thickness can be attributed to the unstable inner mode. Possible relationships between the Pc 3-5 geomagnetic pulsations and the surface waves excited on the magnetospheric boundary are also discussed.
Dynamics of wave equations with moving boundary
Ma, To Fu; Marín-Rubio, Pedro; Surco Chuño, Christian Manuel
2017-03-01
This paper is concerned with long-time dynamics of weakly damped semilinear wave equations defined on domains with moving boundary. Since the boundary is a function of the time variable the problem is intrinsically non-autonomous. Under the hypothesis that the lateral boundary is time-like, the solution operator of the problem generates an evolution process U (t , τ) :Xτ →Xt, where Xt are time-dependent Sobolev spaces. Then, by assuming the domains are expanding, we establish the existence of minimal pullback attractors with respect to a universe of tempered sets defined by the forcing terms. Our assumptions allow nonlinear perturbations with critical growth and unbounded time-dependent external forces.
Effect of a bulge on the secondary instability of boundary layers
Nayfeh, Ali H.; Ragab, Saad A.
1987-01-01
The influence of a two-dimensional hump on the three-dimensional (3-D) subharmonic secondary instability on a flat plate is investigated. The mean flow is calculated using interacting boundary layers, thereby accounting for the inviscid/viscous interaction. The primary wave is taken in the form of a two-dimensional (2-D) Tollmien-Schlichting (T-S) wave. The secondary wave is taken in the form of a 3-D subharmonic T-S wave.
An analytical model of capped turbulent oscillatory bottom boundary layers
Shimizu, Kenji
2010-03-01
An analytical model of capped turbulent oscillatory bottom boundary layers (BBLs) is proposed using eddy viscosity of a quadratic form. The common definition of friction velocity based on maximum bottom shear stress is found unsatisfactory for BBLs under rotating flows, and a possible extension based on turbulent kinetic energy balance is proposed. The model solutions show that the flow may slip at the top of the boundary layer due to capping by the water surface or stratification, reducing the bottom shear stress, and that the Earth's rotation induces current and bottom shear stress components perpendicular to the interior flow with a phase lag (or lead). Comparisons with field and numerical experiments indicate that the model predicts the essential characteristics of the velocity profiles, although the agreement is rather qualitative due to assumptions of quadratic eddy viscosity with time-independent friction velocity and a well-mixed boundary layer. On the other hand, the predicted linear friction coefficients, phase lead, and veering angle at the bottom agreed with available data with an error of 3%-10%, 5°-10°, and 5°-10°, respectively. As an application of the model, the friction coefficients are used to calculate e-folding decay distances of progressive internal waves with a semidiurnal frequency.
Traveling Lamb wave in elastic metamaterial layer
Shu, Haisheng; Xu, Lihuan; Shi, Xiaona; Zhao, Lei; Zhu, Jie
2016-10-01
The propagation of traveling Lamb wave in single layer of elastic metamaterial is investigated in this paper. We first categorized the traveling Lamb wave modes inside an elastic metamaterial layer according to different combinations (positive or negative) of effective medium parameters. Then the impacts of the frequency dependence of effective parameters on dispersion characteristics of traveling Lamb wave were studied. Distinct differences could be observed when comparing the traveling Lamb wave along an elastic metamaterial layer with one inside the traditional elastic layer. We further examined in detail the traveling Lamb wave mode supported in elastic metamaterial layer, when the effective P and S wave velocities were simultaneously imaginary. It was found that the effective modulus ratio is the key factor for the existence of special traveling wave mode, and the main results were verified by FEM simulations from two levels: the level of effective medium and the level of microstructure unit cell.
Compressibility Effects in Turbulent Boundary Layers
Institute of Scientific and Technical Information of China (English)
CAO Yu-Hui; PEI Jie; CHEN Jun; SHE Zhen-Su
2008-01-01
Local cascade (LC) scheme and space-time correlations are used to study turbulent structures and their convection behaviour in the near-wall region of compressible boundary layers at Ma = 0.8 and 1.3. The convection velocities of fluctuating velocity components u (streamwise) and v (vertical) are investigated by statistically analysing scale-dependent ensembles of LC structures. The results suggest that u is convected with entropy perturbations while v with an isentropic process. An abnormal thin layer distinct from the conventional viscous sub-layer is discovered in the immediate vicinity of the wall (y+≤1) in supersonic flows. While in the region 1 ＜ y+ ＜ 30,streamwise streaks dominate velocity, density and temperature fluctuations, the abnormal thin layer is dominated by spanwise streaks in vertical velocity and density fluctuations, where pressure and density fluctuations are strongly correlated. The LC scheme is proven to be effective in studying the nature of supersonic flows and compressibility effects on wall-bounded motions.
Turbulent boundary layer over a chine.
Panchapakesan, N. R.; Joubert, P. N.
1999-11-01
The flow over an edge aligned with the streamwise direction is studied as a representative of the turbulent boundary layers developing over hard chines found on the hulls of ships and catamarans. We present results of a traditional experimental investigation of this geometry in a wind tunnel with pitot tubes and hot-wires. The chine model consisted of two surfaces made of varnished fibre boards with leading edges of airfoil sections and a 90 degree corner. The boundary layer was tripped with wires close to the leading edge. The model was housed in a test section of length 6.5 m in a closed circuit wind tunnel. The experiments were conducted at a unit Reynolds number of 680,000 /m corresponding to a nominal free stream velocity of 10 m/s. The mean velocity field and the associated integral parameters obtained with pitot tube measurements are presented for different streamwise locations from 0.2 to 4.7 m from the trip wire. The flow at the two farthest locations were also studied with single and 'x' hot-wires. The secondary mean flow and the turbulence field in the corner region are described with these measurements.
Receptivity of Boundary Layer over a Blunt Wedge due to Freestream Pulse Disturbances at Mach 6
Directory of Open Access Journals (Sweden)
Jianqiang Shi
2016-01-01
Full Text Available Direct numerical simulation (DNS of a hypersonic compressible flow over a blunt wedge with fast acoustic disturbances in freestream is performed. The receptivity characteristics of boundary layer to freestream pulse acoustic disturbances are numerically investigated at Mach 6, and the frequency effects of freestream pulse wave on boundary layer receptivity are discussed. Results show that there are several main disturbance mode clusters in boundary layer under acoustic pulse wave, and the number of main disturbance clusters decreases along the streamwise. As disturbance wave propagates from upstream to downstream direction, the component of the modes below fundamental frequency decreases, and the component of the modes above second harmonic components increases quickly in general. There are competition and disturbance energy transfer between different boundary layer modes. The nose boundary layer is dominated by the nearby mode of fundamental frequency. The number of the main disturbance mode clusters decreases as the freestream disturbance frequency increases. The frequency range with larger growth narrows along the streamwise. In general, the amplitudes of both fundamental mode and harmonics become larger with the decreasing of freestream disturbance frequency. High frequency freestream disturbance accelerates the decay of disturbance wave in downstream boundary layer.
Directory of Open Access Journals (Sweden)
Kazutaka Yanase
2016-12-01
Full Text Available The boundary layers of rainbow trout, Oncorhynchus mykiss [0.231±0.016 m total body length (L (mean±s.d.; N=6], swimming at 1.6±0.09 L s−1 (N=6 in an experimental flow channel (Reynolds number, Re=4×105 with medium turbulence (5.6% intensity were examined using the particle image velocimetry technique. The tangential flow velocity distributions in the pectoral and pelvic surface regions (arc length from the rostrum, lx=71±8 mm, N=3, and lx=110±13 mm, N=4, respectively were approximated by a laminar boundary layer model, the Falkner−Skan equation. The flow regime over the pectoral and pelvic surfaces was regarded as a laminar flow, which could create less skin-friction drag than would be the case with turbulent flow. Flow separation was postponed until vortex shedding occurred over the posterior surface (lx=163±22 mm, N=3. The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. This was consistent with the condition of the boundary layer laminarization that had been confirmed earlier using a mechanical model. These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment.
Velocity-vorticity correlation structures in compressible turbulent boundary layer
Chen, Jun; Li, Shi-Yao; She, Zhen-Su
2016-11-01
A velocity-vorticity correlation structure (VVCS) analysis is applied to analyze data of 3-dimensional (3-D) direct numerical simulations (DNS), to investigate the quantitative properties of the most correlated vortex structures in compressible turbulent boundary layer (CTBL) at Mach numbers, Ma = 2 . 25 and 6 . 0 . It is found that the geometry variation of the VVCS closely reflects the streamwise development of CTBL. In laminar region, the VVCS captures the instability wave number of the boundary layer. The transition region displays a distinct scaling change of the dimensions of VVCS. The developed turbulence region is characterized by a constant spatial extension of the VVCS. For various Mach numbers, the maximum correlation coefficient of the VVCS presents a clear multi-layer structure with the same scaling laws as a recent symmetry analysis proposed to quantifying the sublayer, the log-layer, and the wake flow. A surprising discovery is that the wall friction coefficient, Cf, holds a "-1"-power law of the wall normal distance of the VVCS, ys. This validates the speculation that the wall friction is determined by the near-wall coherent structure, which clarifies the correlation between statistical structures and the near-wall dynamics. Project 11452002 and 11172006 supported by National Natural Science Foundation of China.
Sergeev, Daniil; Troitskaya, Yuliya; Vdovin, Maxim; Ermoshkin, Alexey
2016-04-01
The effect of foam presence on the transfer processes and the parameters of the surface roughness within the laboratory simulation of wind-wave interaction was carried out on the Thermostratified Wind-Wave Tank (TSWiWaT) IAP, using a specially designed foam generator. The parameters of air flow profiles and waves elevation were measured with scanning Pitot gauge and wire wave gauges respectively in the range of equivalent wind speed U10 from 12 to 38 m/s (covering strong winds) on the clean water and with foam. It was shown that the foam reduces the amplitudes and slopes of the waves in comparison with the clean water in the hole range of wind speeds investigated, and the peak frequency and wave numbers remain almost constant. The drag coefficient calculating by profiling method demonstrated similar behavior (almost independent on U10) for case of foam and increased compared with clear water, particularly noticeable for low wind speeds. Simultaneously the investigations of influence of the foam on the peculiarity of the microwave radio back scattering of X-diapason was investigated. These measurements were carried for different sensing angles (30, 40 i 50 degrees from vertical) and for four polarizations: co-polarized HH and VV, and de-polarized HV and VH. It was shown that foam leads to decrease of specific radar cross section of the wavy surface in comparison with clean water. The work was supported by the Russian Foundation for Basic Research (grants No. 15-35-20953, 14-05-00367, 16-55-52022) and project ASIST of FP7. The experiment is supported by Russian Science Foundation (Agreement No. 15-17-20009), radilocation measurments are partially supported by Russian Science Foundation (Agreement No. 14-17-00667).
ON NONLINEAR STABILITY IN NONPARALLEL BOUNDARY LAYER FLOW
Institute of Scientific and Technical Information of China (English)
TANG Deng-bin; WANG Wei-zhi
2004-01-01
The nonlinear stability problem in nonparallel boundary layer flow for two-dimensional disturbances was studied by using a newly presented method called Parabolic Stability Equations (PSE). A series of new modes generated by the nonlinear interaction of disturbance waves were tabulately analyzed, and the Mean Flow Distortion (MFD) was numerically given. The computational techniques developed, including the higher-order spectral method and the more effective algebraic mapping, increased greatly the numerical accuracy and the rate of convergence. With the predictor-corrector approach in the marching procedure, the normalization condition was satisfied, and the stability of numerical calculation could be ensured. With different initial amplitudes, the nonlinear stability of disturbance wave was studied. The results of examples show good agreement with the data given by the DNS using the full Navier-Stokes equations.
On Hydromagnetic Stresses in Accretion Disk Boundary Layers
DEFF Research Database (Denmark)
Pessah, Martin Elias; Chan, Chi-kwan
2012-01-01
Detailed calculations of the physical structure of accretion disk boundary layers, and thus their inferred observational properties, rely on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear...... 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...... with 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...
Boundary Layer Instabilities Generated by Freestream Laser Perturbations
Chou, Amanda; Schneider, Steven P.
2015-01-01
A controlled, laser-generated, freestream perturbation was created in the freestream of the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT). The freestream perturbation convected downstream in the Mach-6 wind tunnel to interact with a flared cone model. The geometry of the flared cone is a body of revolution bounded by a circular arc with a 3-meter radius. Fourteen PCB 132A31 pressure transducers were used to measure a wave packet generated in the cone boundary layer by the freestream perturbation. This wave packet grew large and became nonlinear before experiencing natural transition in quiet flow. Breakdown of this wave packet occurred when the amplitude of the pressure fluctuations was approximately 10% of the surface pressure for a nominally sharp nosetip. The initial amplitude of the second mode instability on the blunt flared cone is estimated to be on the order of 10 -6 times the freestream static pressure. The freestream laser-generated perturbation was positioned upstream of the model in three different configurations: on the centerline, offset from the centerline by 1.5 mm, and offset from the centerline by 3.0 mm. When the perturbation was offset from the centerline of a blunt flared cone, a larger wave packet was generated on the side toward which the perturbation was offset. The offset perturbation did not show as much of an effect on the wave packet on a sharp flared cone as it did on a blunt flared cone.
Stability and modal analysis of shock/boundary layer interactions
Nichols, Joseph W.; Larsson, Johan; Bernardini, Matteo; Pirozzoli, Sergio
2016-06-01
The dynamics of oblique shock wave/turbulent boundary layer interactions is analyzed by mining a large-eddy simulation (LES) database for various strengths of the incoming shock. The flow dynamics is first analyzed by means of dynamic mode decomposition (DMD), which highlights the simultaneous occurrence of two types of flow modes, namely a low-frequency type associated with breathing motion of the separation bubble, accompanied by flapping motion of the reflected shock, and a high-frequency type associated with the propagation of instability waves past the interaction zone. Global linear stability analysis performed on the mean LES flow fields yields a single unstable zero-frequency mode, plus a variety of marginally stable low-frequency modes whose stability margin decreases with the strength of the interaction. The least stable linear modes are grouped into two classes, one of which bears striking resemblance to the breathing mode recovered from DMD and another class associated with revolving motion within the separation bubble. The results of the modal and linear stability analysis support the notion that low-frequency dynamics is intrinsic to the interaction zone, but some continuous forcing from the upstream boundary layer may be required to keep the system near a limit cycle. This can be modeled as a weakly damped oscillator with forcing, as in the early empirical model by Plotkin (AIAA J 13:1036-1040, 1975).
Stability and modal analysis of shock/boundary layer interactions
Nichols, Joseph W.; Larsson, Johan; Bernardini, Matteo; Pirozzoli, Sergio
2017-02-01
The dynamics of oblique shock wave/turbulent boundary layer interactions is analyzed by mining a large-eddy simulation (LES) database for various strengths of the incoming shock. The flow dynamics is first analyzed by means of dynamic mode decomposition (DMD), which highlights the simultaneous occurrence of two types of flow modes, namely a low-frequency type associated with breathing motion of the separation bubble, accompanied by flapping motion of the reflected shock, and a high-frequency type associated with the propagation of instability waves past the interaction zone. Global linear stability analysis performed on the mean LES flow fields yields a single unstable zero-frequency mode, plus a variety of marginally stable low-frequency modes whose stability margin decreases with the strength of the interaction. The least stable linear modes are grouped into two classes, one of which bears striking resemblance to the breathing mode recovered from DMD and another class associated with revolving motion within the separation bubble. The results of the modal and linear stability analysis support the notion that low-frequency dynamics is intrinsic to the interaction zone, but some continuous forcing from the upstream boundary layer may be required to keep the system near a limit cycle. This can be modeled as a weakly damped oscillator with forcing, as in the early empirical model by Plotkin (AIAA J 13:1036-1040, 1975).
Free surface simulation of a two-layer fluid by boundary element method
Directory of Open Access Journals (Sweden)
Weoncheol Koo
2010-09-01
Full Text Available A two-layer fluid with free surface is simulated in the time domain by a two-dimensional potential-based Numerical Wave Tank (NWT. The developed NWT is based on the boundary element method and a leap-frog time integration scheme. A whole domain scheme including interaction terms between two layers is applied to solve the boundary integral equation. The time histories of surface elevations on both fluid layers in the respective wave modes are verified with analytic results. The amplitude ratios of upper to lower elevation for various density ratios and water depths are also compared.
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...
Linear Controllers for Turbulent Boundary Layers
Lim, Junwoo; Kim, John; Kang, Sung-Moon; Speyer, Jason
2000-11-01
Several recent studies have shown that controllers based on a linear system theory work surprisingly well in turbulent flows, suggesting that a linear mechanism may play an important role even in turbulent flows. It has been also shown that non-normality of the linearized Navier-Stokes equations is an essential characteristic in the regeneration of near-wall turbulence structures in turbulent boundary layers. A few controllers designed to reduce the role of different linear mechanisms, including that to minimize the non-normality of the linearized Navier-Stokes equations, have been developed and applied to a low Reynolds nubmer turbulent channel flow. A reduced-order model containing the most controllable and observables modes is derived for each system. Other existing control schemes, such as Choi et al's opposition control, have been examined from the point of a linear system control. Further discussion on controller design, such as choice of cost function and other control parameters, will be presented.
Some measurements in synthetic turbulent boundary layers
Savas, O.
1980-01-01
Synthetic turbulent boundary layers are examined which were constructed on a flat plate by generating systematic moving patterns of turbulent spots in a laminar flow. The experiments were carried out in a wind tunnel at a Reynolds number based on plate length of 1,700,000. Spots were generated periodically in space and time near the leading edge to form a regular hexagonal pattern. The disturbance mechanism was a camshaft which displaced small pins momentarily into the laminar flow at frequencies up to 80 Hz. The main instrumentation was a rake of 24 hot wires placed across the flow in a line parallel to the surface. The main measured variable was local intermittency; i.e., the probability of observing turbulent flow at a particular point in space and time. The results are reported in x-t diagrams showing the evolution of various synthetic flows along the plate. The dimensionless celerity or phase velocity of the large eddies is found to be 0.88, independent of eddy scale. All patterns with sufficiently small scales eventually showed loss of coherence as they moved downstream. A novel phenomenon called eddy transposition was observed in several flows which contained appreciable laminar regions. The large eddies shifted in formation to new positions, intermediate to their original ones, while preserving their hexagonal pattern. The present results, together with some empirical properties of a turbulent spot, are used to estimate the best choice of scales for constructing a synthetic boundary layer suitable for detailed study. The values recommended are: spanwise scale/thickness = 2.5, streamwise scale/thickness = 8.
Large Eddy Simulation of Transitional Boundary Layer
Sayadi, Taraneh; Moin, Parviz
2009-11-01
A sixth order compact finite difference code is employed to investigate compressible Large Eddy Simulation (LES) of subharmonic transition of a spatially developing zero pressure gradient boundary layer, at Ma = 0.2. The computational domain extends from Rex= 10^5, where laminar blowing and suction excites the most unstable fundamental and sub-harmonic modes, to fully turbulent stage at Rex= 10.1x10^5. Numerical sponges are used in the neighborhood of external boundaries to provide non-reflective conditions. Our interest lies in the performance of the dynamic subgrid scale (SGS) model [1] in the transition process. It is observed that in early stages of transition the eddy viscosity is much smaller than the physical viscosity. As a result the amplitudes of selected harmonics are in very good agreement with the experimental data [2]. The model's contribution gradually increases during the last stages of transition process and the dynamic eddy viscosity becomes fully active and dominant in the turbulent region. Consistent with this trend the skin friction coefficient versus Rex diverges from its laminar profile and converges to the turbulent profile after an overshoot. 1. Moin P. et. al. Phys Fluids A, 3(11), 2746-2757, 1991. 2. Kachanov Yu. S. et. al. JFM, 138, 209-247, 1983.
Turbulent dispersion in cloud-topped boundary layers
Verzijlbergh, R. A.; Jonker, H. J. J.; Heus, T.; Vilöguerau de Arellano, J.
2009-02-01
Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion - a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.
Analytical solution for the convectively-mixed atmospheric boundary layer
Ouwersloot, H.G.; Vilà-Guerau de Arellano, J.
2013-01-01
Based on the prognostic equations of mixed-layer theory assuming a zeroth order jump at the entrainment zone, analytical solutions for the boundary-layer height evolution are derived with different degrees of accuracy. First, an exact implicit expression for the boundary-layer height for a situation
Gap Waves in Piezoelectric layered Medium
Directory of Open Access Journals (Sweden)
Danoyan Z.N.
2007-03-01
Full Text Available In the present paper the conditions of existence of shear electroelastic gap waves in piezoelectric-vacuum-dielectric layered system are found. It is shown that in the discontact layered system the gap electroelastic waves can be propagated. It is considered the limiting case when the thickness of vacuuming layer tends to zero. It is proved that the statement of the problem is true when there is no acoustic contact between piezoelectric and dielectric grounded media.
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.
Nonparallel stability of boundary layers with pressure gradients and suction
Saric, W. S.; Nayfeh, A. H.
1977-01-01
An analysis is presented for the linear nonparallel stability of boundary layer flows with pressure gradients and suction. The effect of the boundary layer growth is included by using the method of multiple scales. The present analysis is compared with those of Bouthier and Gaster and the roles of the different definitions of the amplification rates are discussed. The results of these theories are compared with experimental data for the Blasius boundary layer. Calculations are presented for stability characteristics of boundary layers with pressure gradients and nonsimilar suction distributions.
Wave energy input into the Ekman layer
Institute of Scientific and Technical Information of China (English)
无
2008-01-01
This paper is concerned with the wave energy input into the Ekman layer, based on 3 observational facts that surface waves could significantly affect the profile of the Ekman layer. Under the assumption of constant vertical diffusivity, the analytical form of wave energy input into the Ekman layer is derived. Analysis of the energy balance shows that the energy input to the Ekman layer through the wind stress and the interaction of the Stokes-drift with planetary vorticity can be divided into two kinds. One is the wind energy input, and the other is the wave energy input which is dependent on wind speed, wave characteristics and the wind direction relative to the wave direction. Estimates of wave energy input show that wave energy input can be up to 10% in high-latitude and high-wind speed areas and higher than 20% in the Antarctic Circumpolar Current, compared with the wind energy input into the classical Ekman layer. Results of this paper are of significance to the study of wave-induced large scale effects.
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)
WEI; Fengsi(魏奉思); LIU; Rui(刘睿); FAN; Quanlin(范全林); FENG; Xueshang(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.
Boundary-layer effects in droplet splashing
Riboux, Guillaume; Gordillo, José Manuel
2017-07-01
A drop falling onto a solid substrate will disintegrate into smaller parts when its impact velocity V exceeds the so-called critical velocity for splashing, i.e., when V >V* . Under these circumstances, the very thin liquid sheet, which is ejected tangentially to the solid after the drop touches the substrate, lifts off as a consequence of the aerodynamic forces exerted on it. Subsequently, the growth of capillary instabilities breaks the toroidal rim bordering the ejecta into smaller droplets, violently ejected radially outward, provoking the splash [G. Riboux and J. M. Gordillo, Phys. Rev. Lett. 113, 024507 (2014)], 10.1103/PhysRevLett.113.024507. In this contribution, the effect of the growth of the boundary layer is included in the splash model presented in Phys. Rev. Lett. 113, 024507 (2014), 10.1103/PhysRevLett.113.024507, obtaining very good agreement between the measured and the predicted values of V* for wide ranges of liquid and gas material properties, atmospheric pressures, and substrate wettabilities. Our description also modifies the way at when the liquid sheet is first ejected, which can now be determined in a much more straightforward manner than that proposed in Phys. Rev. Lett. 113, 024507 (2014), 10.1103/PhysRevLett.113.024507.
Turbulence in the Stable Atmospheric Boundary Layer
Fernando, Harindra; Kit, Eliezer; Conry, Patrick; Hocut, Christopher; Liberzon, Dan
2016-11-01
During the field campaigns of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, fine-scale measurements of turbulence in the atmospheric boundary layer (ABL) were made using a novel sonic and hot-film anemometer dyad (a combo probe). A swath of scales, from large down to Kolmogorov scales, was covered. The hot-film was located on a gimbal within the sonic probe volume, and was automated to rotate in the horizontal plane to align with the mean flow measured by sonic. This procedure not only helped satisfy the requirement of hot-film alignment with the mean flow, but also allowed in-situ calibration of hot-films. This paper analyzes a period of nocturnal flow that was similar to an idealized stratified parallel shear flow. Some new phenomena were identified, which included the occurrence of strong bursts in the velocity records indicative of turbulence generation at finer scales that are not captured by conventional sonic anemometers. The spectra showed bottleneck effect, but its manifestation did not fit into the framework of previous bottleneck-effect theories and was unequivocally related to bursts of turbulence. The measurements were also used to evaluate the energetics of stratified shear flows typical of the environment. ONR # N00014-11-1-0709; NSF # AGS-1528451; ISF 408/15.
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...... 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 layer is enforced...... height and the flow development is seen based on the temperature variations and wind turbine wake generations and interactions of wakes occurs as soon as the wakes of the upwind turbine reach the downwind turbines. References: [1] U. Piomelli, Wall-layer models for large-eddy simulations, Progress...
Breakup of spiral wave under different boundary conditions
Institute of Scientific and Technical Information of China (English)
Zhao Ying-Kui; Wang Guang-Rui; Chen Shi-Gang
2007-01-01
In this paper, we investigate the breakup of spiral wave under no-flux, periodic and Dirichlet boundary conditions respectively. When the parameter ε is close to a critical value for Doppler-induced wave breakup, the instability of the system caused by the boundary effect occurs in the last two cases, resulting in the breakup of spiral wave near the boundary. With our defined average order measure of spiral wave (AOMSW), we quantify the degree of order of the system when the boundary-induced breakup of spiral wave happens. By analysing the AOMSW and outer diameter R of the spiral tip orbit, it is easy to find that this boundary effect is correlated with large values of R, especially under the Dirichlet boundary condition. This correlation is nonlinear, so the AOMSW sometimes oscillates with the variation of ε.
An analysis of the stability of the compressible Ekman boundary layer
Spall, J. R.; Wood, H. G., III
1984-01-01
The linear stability problem for the compressible Ekman boundary layer common to rotating fluids is formulated and the stability properties determined numerically. Three classes of unstable waves are identified (called class A, B, and C), their properties are described. The class C waves have only recently been reported in the literature and are present only in compressible Ekman boundary layers. Most of the calculations presented here are for uranium hexafluoride gas; however, critical Reynolds numbers are also computed for air and ammonia gas. Compressibility is generally found to decrease the critical Reynolds number for each class of wave. A comparison of results for the three different gases shows the stability to be largely unaffected by changes in the gas properties. Maximum growth rate calculations for each wave show the class A and B waves to be the dominant instabilities.
Subharmonic Route to Boundary-Layer Transition - Critical Layer Nonlinearity
Mankbadi, Reda R.
1991-01-01
The linear and nonlinear dynamics of a triad of initially linear stability waves comprising a single plane wave at fundamental frequency and two symmetric oblique waves with half the frequency and streamwise wave number of the plane wave are presented. Analysis is performed for the initial nonlinear development of the waves where the order of the oblique waves' amplitude is equal to or less than that of the plane wave. Results show that the fundamental basically follows the linear theory, while the subharmonic follows an exponential-of-an-exponential growth.
Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration
Wijmans, J.G.; Nakao, S.; Berg, van den J.W.A.; Troelstra, F.R.; Smolders, C.A.
1985-01-01
The influence of concentration polarization on the permeate flux in the ultrafiltration of aqueous Dextran T70 solutions can be described by (i) the osmotic pressure model and (ii) the boundary layer resistance model. In the latter model the hydrodynamic resistance of the non-gelled boundary layer i
THE UNSTABLE MODES OF NATURAL CONVECTION BOUNDARY LAYER
Institute of Scientific and Technical Information of China (English)
Tao Jianjun; Zhuang Fenggan; Yan Dachun
2000-01-01
The instability of natural convection boundary layer around a vertical heated flat plate is analyzed theoretically in this paper. The results illustrate that the “loop” in the neutral curve is not a real loop but a twist of the curve is the frequencywave number-Grashof number space, and there is only one unstable mode at small Prandtl numbers. Specially, when the Prandtl number is large enough two unstable modes will be found in the “loop” region. Along the amplifying surface intersection the two unstable modes have the same Grashof number, wave number and frequency but different amplifying rates. Their instability characteristics are analyzed and the criterion for determining the existence of the multi-unstable modes is also discussed.
Boundary layers interactions in the plane parallel incompressible flows
Nguyen, Toan
2011-01-01
We study the inviscid limit problem of the incompressible flows in the presence of both impermeable regular boundaries and a hypersurface transversal to the boundary across which the inviscid flow has a discontinuity jump. In the former case, boundary layers have been introduced by Prandtl as correctors near the boundary between the inviscid and viscous flows. In the latter case, the viscosity smoothes out the discontinuity jump by creating a transition layer which has the same amplitude and thickness as the Prandtl layer. In the neighborhood of the intersection of the impermeable boundary and of the hypersurface, interactions between the boundary and the transition layers must then be considered. In this paper, we initiate a mathematical study of this interaction and carry out a strong convergence in the inviscid limit for the case of the plane parallel flows introduced by Di Perna and Majda in \\cite{DM}.
Greene, Benton; Clemens, Noel; Magari, Patrick; Micka, Daniel; Ueckermann, Mattheus
2015-11-01
Shock-induced turbulent boundary layer separation can have many detrimental effects in supersonic inlets including flow distortion and instability, structural fatigue, poor pressure recovery, and unstart. The current study investigates the effect of pulsed plasma jets on the recovering boundary layer downstream of a reflected shock wave-boundary layer interaction. The effects of pitch and skew angle of the jet as well as the heating parameter and discharge time scale are tested using several pulsing frequencies. In addition, the effect of the plasma jets on the undisturbed boundary layer at 6 mm and 11 mm downstream of the jets is measured. A pitot-static pressure probe is used to measure the velocity profile of the boundary layer 35 mm downstream of the plasma jets, and the degree of boundary layer distortion is compared between the different models and run conditions. Additionally, the effect of each actuator configuration on the shape of the mean separated region is investigated using surface oil flow visualization. Previous studies with lower energy showed a weak effect on the downstream boundary layer. The current investigation will attempt to increase this effect using a higher-energy discharge. Funded by AFRL through and SBIR in collaboration with Creare, LLC.
Boundary Layer to a System of Viscous Hyperbolic Conservation Laws
Institute of Scientific and Technical Information of China (English)
2008-01-01
In this paper, we investigate the large-time behavior of solutions to the initial-boundary value problem for nxn hyperbolic system of conservation laws with artificial viscosity in the half line (0, ∞). We first show that a boundary layer exists if the corresponding hyperbolic part contains at least one characteristic field with negative propagation speed. We further show that such boundary layer is nonlinearly stable under small initial perturbation. The proofs are given by an elementary energy method.
Excimer emission from cathode boundary layer discharges
Moselhy, Mohamed; Schoenbach, Karl H.
2004-02-01
The excimer emission from direct current glow discharges between a planar cathode and a ring-shaped anode of 0.75 and 1.5 mm diameter, respectively, separated by a gap of 250 μm, was studied in xenon and argon in a pressure range from 75 to 760 Torr. The thickness of the "cathode boundary layer" plasma, in the 100 μm range, and a discharge sustaining voltage of approximately 200 V, indicates that the discharge is restricted to the cathode fall and the negative glow. The radiant excimer emittance at 172 nm increases with pressure and reaches a value of 4 W/cm2 for atmospheric pressure operation in xenon. The maximum internal efficiency, however, decreases with pressure having highest values of 5% for 75 Torr operation. When the discharge current is reduced below a critical value, the discharge in xenon changes from an abnormal glow into a mode showing self-organization of the plasma. Also, the excimer spectrum changes from one with about equal contributions from the first and second continuum to one that is dominated by the second continuum emission. The xenon excimer emission intensity peaks at this discharge mode transition. In the case of argon, self-organization of the plasma was not seen, but the emission of the excimer radiation (128 nm) again shows a maximum at the transition from abnormal to normal glow. As was observed with xenon, the radiant emittance of argon increases with pressure, and the efficiency decreases. The maximum radiant emittance is 1.6 W/cm2 for argon at 600 Torr. The maximum internal efficiency is 2.5% at 200 Torr. The positive slope of the current-voltage characteristics at maximum excimer emission in both cases indicates the possibility of generating intense, large area, flat excimer lamps.
Control Parameters for Boundary-Layer Instabilities in Unsteady Shock Interactions
Directory of Open Access Journals (Sweden)
LaVar King Isaacson
2012-01-01
Full Text Available This article presents the computation of a set of control parameters for the deterministic prediction of laminar boundary-layer instabilities induced by an imposed unsteady shock interaction. The objective of the study is exploratory in nature by computing a supersonic flight environment for flow over a blunt body and the deterministic prediction of the spectral entropy rates for the boundary layer subjected to an unsteady pressure disturbance. The deterministic values for the spectral entropy rate within the instabilities are determined for each control parameter. Computational results imply that the instabilities are of a span-wise vortex form, that the maximum vertical velocity wave vector components are produced in the region nearest the wall and that extended transient coherent structures are produced in the boundary layer at a vertical location slightly below the mid-point of the boundary layer.
Performance of Numerical Boundary Condition based on Active Wave Absorption
DEFF Research Database (Denmark)
Troch, Peter; De Rouck, Julien; Frigaard, Peter
2001-01-01
The performance of a new active wave generating-absorbing boundary condition for a numerical model based on the Volume Of Fluid (VOF) method for tracking free surfaces is presented.......The performance of a new active wave generating-absorbing boundary condition for a numerical model based on the Volume Of Fluid (VOF) method for tracking free surfaces is presented....
Receptivity of Hypersonic Boundary Layers to Distributed Roughness and Acoustic Disturbances
Balakumar, P.
2013-01-01
Boundary-layer receptivity and stability of Mach 6 flows over smooth and rough seven-degree half-angle sharp-tipped cones are numerically investigated. The receptivity of the boundary layer to slow acoustic disturbances, fast acoustic disturbances, and vortical disturbances is considered. The effects of three-dimensional isolated roughness on the receptivity and stability are also simulated. The results for the smooth cone show that the instability waves are generated in the leading edge region and that the boundary layer is much more receptive to slow acoustic waves than to the fast acoustic waves. Vortical disturbances also generate unstable second modes, however the receptivity coefficients are smaller than that of the slow acoustic wave. Distributed roughness elements located near the nose region decreased the receptivity of the second mode generated by the slow acoustic wave by a small amount. Roughness elements distributed across the continuous spectrum increased the receptivity of the second mode generated by the slow and fast acoustic waves and the vorticity wave. The largest increase occurred for the vorticity wave. Roughness elements distributed across the synchronization point did not change the receptivity of the second modes generated by the acoustic waves. The receptivity of the second mode generated by the vorticity wave increased in this case, but the increase is lower than that occurred with the roughness elements located across the continuous spectrum. The simulations with an isolated roughness element showed that the second mode waves generated by the acoustic disturbances are not influenced by the small roughness element. Due to the interaction, a three-dimensional wave is generated. However, the amplitude is orders of magnitude smaller than the two-dimensional wave.
Diffusive boundary layers over varying topography
Dell, R. W.
2015-03-25
Diffusive bottom boundary layers can produce upslope flows in a stratified fluid. Accumulating observations suggest that these boundary layers may drive upwelling and mixing in mid-ocean ridge flank canyons. However, most studies of diffusive bottom boundary layers to date have concentrated on constant bottom slopes. We present a study of how diffusive boundary layers interact with various idealized topography, such as changes in bottom slope, slopes with corrugations and isolated sills. We use linear theory and numerical simulations in the regional ocean modeling system (ROMS) model to show changes in bottom slope can cause convergences and divergences within the boundary layer, in turn causing fluid exchanges that reach far into the overlying fluid and alter stratification far from the bottom. We also identify several different regimes of boundary-layer behaviour for topography with oceanographically relevant size and shape, including reversing flows and overflows, and we develop a simple theory that predicts the regime boundaries, including what topographies will generate overflows. As observations also suggest there may be overflows in deep canyons where the flow passes over isolated bumps and sills, this parameter range may be particularly significant for understanding the role of boundary layers in the deep ocean.
Phase modulated solitary waves controlled by bottom boundary condition
Mukherjee, Abhik
2014-01-01
A forced KdV equation is derived to describe weakly nonlinear, shallow water surface wave propagation over non trivial bottom boundary condition. We show that different functional forms of bottom boundary conditions self-consistently produce different forced kdV equations as the evolution equations for the free surface. Solitary wave solutions have been analytically obtained where phase gets modulated controlled by bottom boundary condition whereas amplitude remains constant.
Effect of slip boundary conditions on interfacial stability of two-layer viscous fluids under shear
Patlazhan, Stanislav
2015-01-01
The traditional approach in the study of hydrodynamic stability of stratified fluids includes the stick boundary conditions between layers. However, this rule may be violated in polymer systems and as a consequence various instabilities may arise. The main objective of this paper is to analyze theoretically the influence of slip boundary conditions on the hydrodynamic stability of the interface between two immiscible viscous layers subjected to simple shear flow. It is found that the growth rate of long-wave disturbances is fairly sensitive to the slip at the interface between layers as well as at the external boundary. These phenomena are shown to give different contributions to the stability of shear flow depending on viscosity, thickness, and density ratios of the layers. Particularly, the interfacial slip can increase the perturbation growth rate and lead to unstable flow. An important consequence of this effect is the violation of stability for sheared layers with equal viscosities and densities in a bro...
A Generic Length-scale Equation For Second-order Turbulence Models of Oceanic Boundary Layers
Umlauf, L.; Burchard, H.
A generic transport equation for a generalized length-scale in second-order turbulence closure models for geophysical boundary layers is suggested. This variable consists of the products of powers of the turbulent kinetic energy, k, and the integral length-scale, l. The new approach generalizes traditional second-order models used in geophysical boundary layer modelling, e.g. the Mellor-Yamada model and the k- model, which, however, can be recovered as special cases. It is demonstrated how this new model can be calibrated with measurements in some typical geophysical boundary layer flows. As an example, the generic model is applied to the uppermost oceanic boundary layer directly influenced by the effects of breaking surface waves. Recent measurements show that in this layer the classical law of the wall is invalid, since there turbulence is dominated by turbulent transport of TKE from above, and not by shear-production. A widely accepted approach to describe the wave-affected layer with a one-equation turbulence model was suggested by Craig and Banner (1994). Here, some deficien- cies of their solutions are pointed out and a generalization of their ideas for the case of two-equation models is suggested. Direct comparison with very recently obtained measurements of the dissipation rate, , in the wave-affected boundary layer with com- puted results clearly demonstrate that only the generic two-equation model yields cor- rect predictions for the profiles of and the turbulent length scale, l. Also, the pre- dicted velocity profiles in the wave-affected layer, important e.g. for the interpretation of surface drifter experiments, are reproduced correctly only by the generic model. Implementation and computational costs of the generic model are comparable with traditonal two-equation models.
Boundary Layer Ventilation Processes During a High Pressure Event
Gray, S. L.; Dacre, H. F.; Belcher, S. E.
2006-12-01
It is often assumed that ventilation of the atmospheric boundary layer is weak during high pressure events. But is this always true? Here we investigate the processes responsible for ventilation of the atmospheric boundary layer during a high pressure event that occured on the 9 May 2005 using the UK Met Office Unifed Model. Pollution sources are represented by the constant emission of a passive tracer everywhere over land. The ventilation processes observed include a sea breeze circulation, turbulent mixing across the top of the boundary layer followed by large-scale ascent, and shallow convection. Vertical distributions of tracer are validated with AMPEP (Aircraft Measurement of chemical Processing Export fluxes of Pollutants over the UK) CO aircraft measurements and are shown to agree impressively well. Budget calculations of tracers are performed in order to determine the relative importance of these ventilation processes. The sea breeze circulation was found to ventilate 26% of the boundary layer tracer by sunset of which 2% was above 2km. A combination of the sea breeze circulation and turbulent mixing ventilated 46% of the boundary layer tracer, of which 10% was above 2km. Finally, the sea breeze circulation, turbulent mixing and shallow convection processes together ventilated 52% of the tracer into the free troposphere, of which 26% was above 2km. Hence this study shows that signicant ventilation of the boundary layer can occur during high pressure events; turbulent mixing and convection processes can double the amount of pollution ventilated from the boundary layer.
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...
Bristled shark skin: a microgeometry for boundary layer control?
Energy Technology Data Exchange (ETDEWEB)
Lang, A W; Hidalgo, P; Westcott, M [Aerospace Engineering and Mechanics Department, University of Alabama, Box 870280, Tuscaloosa, AL 35487 (United States); Motta, P [Biology Department, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620 (United States)], E-mail: alang@eng.ua.edu
2008-12-01
There exists evidence that some fast-swimming shark species may have the ability to bristle their scales during fast swimming. Experimental work using a water tunnel facility has been performed to investigate the flow field over and within a bristled shark skin model submerged within a boundary layer to deduce the possible boundary layer control mechanisms being used by these fast-swimming sharks. Fluorescent dye flow visualization provides evidence of the formation of embedded cavity vortices within the scales. Digital particle image velocimetry (DPIV) data, used to evaluate the cavity vortex formation and boundary layer characteristics close to the surface, indicate increased momentum in the slip layer forming above the scales. This increase in flow velocity close to the shark's skin is indicative of boundary layer control mechanisms leading to separation control and possibly transition delay for the bristled shark skin microgeometry.
Love wave propagation in layered magneto- electro-elastic structures
Institute of Scientific and Technical Information of China (English)
2008-01-01
An analytical approach was taken to investigate Love wave propagation in a layered magneto-electro-elastic structure, where a piezomagnetic (or piezoelectric) mate-rial thin layer was bonded to a semi-infinite piezoelectric (or piezomagnetic) sub-strate. Both piezoelectric and piezomagnetic ceramics were polarized in the anti-plane (z-axis) direction. The analytical solution of dispersion relations was obtained for magneto-electrically open and short boundary conditions. The phase velocity, group velocity, magneto-electromechanical coupling factor, electric po-tential, and magnetic potential were calculated and discussed in detail. The nu-merical results show that the piezomagnetic effects have remarkable effect on the propagation of Love waves in the layered piezomagnetic/piezoelectric structures.
Love wave propagation in layered magneto-electro-elastic structures
Institute of Scientific and Technical Information of China (English)
DU JianKe; JIN XiaoYing; WANG Ji
2008-01-01
An analytical approach was taken to investigate Love wave propagation in a layered magneto-electro-elastic structure,where a piezomagnetic (or piezoelectric) mate-rial thin layer was bonded to a semi-infinite piezoelectric (or piezomagnetic) sub-strate.Both piezoelectric and piezomagnetic ceramics were polarized in the anti-plane (z-axis) direction.The analytical solution of dispersion relations was obtained for magneto-electrically open and short boundary conditions.The phase velocity,group velocity,magneto-electromechanical coupling factor,electric po-tential,and magnetic potential were calculated and discussed in detail.The nu-merical results show that the piezomagnetic effects have remarkable effect on the propagation of Love waves in the layered piezomagnetic/piezoelectric structures.
Micro Ramps in Supersonic Turbulent Boundary Layers: An experimental and numerical study
Sun, Z.
2014-01-01
The micro vortex generator (MVG) is used extensively in low speed aerodynamic problems and is now extended into the supersonic flow regime to solve undesired flow features that are associated with shock wave boundary layer interactions (SWBLI) such as flow separation and associated unsteadiness of t
Secondary instability of compressible boundary layer to subharmonic three-dimensional disturbances
El Hady, Nabil M.
1989-01-01
Three-dimensional linear secondary instability theory is extended for compressible boundary layers on a flat plate in the presence of finite amplitude Tollmien-Schlichting (T-S) waves. The focus is on principal parametric resonance responsible for the strong growth of harmonics in a low disturbance environment.
Stable Boundary Layer Education (STABLE) Final Campaign Summary
Energy Technology Data Exchange (ETDEWEB)
Turner, David D. [Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2016-03-01
The properties of, and the processes that occur in, the nocturnal stable boundary layer are not well understood, making it difficult to represent adequately in numerical models. The nocturnal boundary layer often is characterized by a temperature inversion and, in the Southern Great Plains region, a low-level jet. To advance our understanding of the nocturnal stable boundary layer, high temporal and vertical resolution data on the temperature and wind properties are needed, along with both large-eddy simulation and cloud-resolving modeling.
Boundary-layer predictions for small low-speed contractions
Mehta, Rabindra D.; Bell, James H.
1989-01-01
The present scheme for the prediction of boundary-layer development in small, low-speed wind tunnel contraction sections proceeds by calculating the wall pressure distributions, and hence the wall velocity distributions, by means of a three-dimensional potential-flow method. For the family of contractions presently treated, the assumption of a laminar boundary layer appears to be justified; the measured boundary layer momentum thicknesses at the exit of the four contractions were found to lie within 10 percent of predicted values.
Structure of turbulence in three-dimensional boundary layers
Subramanian, Chelakara S.
1993-01-01
This report provides an overview of the three dimensional turbulent boundary layer concepts and of the currently available experimental information for their turbulence modeling. It is found that more reliable turbulence data, especially of the Reynolds stress transport terms, is needed to improve the existing modeling capabilities. An experiment is proposed to study the three dimensional boundary layer formed by a 'sink flow' in a fully developed two dimensional turbulent boundary layer. Also, the mean and turbulence field measurement procedure using a three component laser Doppler velocimeter is described.
Diffuse Waves and Energy Densities Near Boundaries
Sanchez-Sesma, F. J.; Rodriguez-Castellanos, A.; Campillo, M.; Perton, M.; Luzon, F.; Perez-Ruiz, J. A.
2007-12-01
Green function can be retrieved from averaging cross correlations of motions within a diffuse field. In fact, it has been shown that for an elastic inhomogeneous, anisotropic medium under equipartitioned, isotropic illumination, the average cross correlations are proportional to the imaginary part of Green function. For instance coda waves are due to multiple scattering and their intensities follow diffusive regimes. Coda waves and the noise sample the medium and effectively carry information along their paths. In this work we explore the consequences of assuming both source and receiver at the same point. From the observable side, the autocorrelation is proportional to the energy density at a given point. On the other hand, the imaginary part of the Green function at the source itself is finite because the singularity of Green function is restricted to the real part. The energy density at a point is proportional with the trace of the imaginary part of Green function tensor at the source itself. The Green function availability may allow establishing the theoretical energy density of a seismic diffuse field generated by a background equipartitioned excitation. We study an elastic layer with free surface and overlaying a half space and compute the imaginary part of the Green function for various depths. We show that the resulting spectrum is indeed closely related to the layer dynamic response and the corresponding resonant frequencies are revealed. One implication of present findings lies in the fact that spatial variations may be useful in detecting the presence of a target by its signature in the distribution of diffuse energy. These results may be useful in assessing the seismic response of a given site if strong ground motions are scarce. It suffices having a reasonable illumination from micro earthquakes and noise. We consider that the imaginary part of Green function at the source is a spectral signature of the site. The relative importance of the peaks of
Size distributions of boundary-layer clouds
Energy Technology Data Exchange (ETDEWEB)
Stull, R.; Berg, L.; Modzelewski, H. [Univ. of Wisconsin, Madison, WI (United States)
1996-04-01
Scattered fair-weather clouds are triggered by thermals rising from the surface layer. Not all surface layer air is buoyant enough to rise. Also, each thermal has different humidities and temperatures, resulting in interthermal variability of their lifting condensation levels (LCL). For each air parcel in the surface layer, it`s virtual potential temperature and it`s LCL height can be computed.
Influences on the Height of the Stable Boundary Layer as seen in LES
Energy Technology Data Exchange (ETDEWEB)
Kosovic, B; Lundquist, J
2004-06-15
Climate models, numerical weather prediction (NWP) models, and atmospheric dispersion models often rely on parameterizations of planetary boundary layer height. In the case of a stable boundary layer, errors in boundary layer height estimation can result in gross errors in boundary-layer evolution and in prediction of turbulent mixing within the boundary layer.
Boundary Layer Ventilation by Convection and Coastal Processes
Dacre, H.
2008-12-01
Several observational studies measuring aerosol in the atmosphere have found multiple aerosol layers located above the marine boundary layer. It is hypothesized that the existence of these layers is influenced by the diurnal variation in the structure of the upwind continental boundary layer. Furthermore, collision between a sea breeze and the prevailing wind can result in enhanced convection at the coast which can also lead to elevated layers of pollution. In this study we investigate the processes responsible for ventilation of the atmospheric boundary layer near the coast using the UK Met Office Unified Model. Pollution sources are represented by the constant emission of a passive tracer everywhere over land. The ventilation processes observed include shallow convection, a sea breeze circulation and coastal outflow. Vertical distributions of tracer at the coast are validated qualitatively with AMPEP (Aircraft Measurement of chemical Processing Export fluxes of Pollutants over the UK) CO aircraft measurements and are shown to agree well.
Receptivity of the Boundary Layer to Vibrations of the Wing Surface
Bernots, Tomass; Ruban, Anatoly; Pryce, David; Laminar Flow Control UK Group Team
2014-11-01
In this work we study generation of Tollmien-Schlichting (T-S) waves in the boundary layer due to elastic vibrations of the wing surface. The flow is investigated based on the asymptotic analysis of the Navier-Stokes equations at large values of the Reynolds number. It is assumed that in the spectrum of the wing vibrations there is a harmonic which comes in resonance with the T-S wave on the lower branch of the stability curve. It was found that the vibrations of the wing surface produce pressure perturbations in the flow outside the boundary layer which can be calculated with the help of the piston theory. As the pressure perturbations penetrate into the boundary layer, a Stokes layer forms on the wing surface which appears to be influenced significantly by the compressibility of the flow, and is incapable of producing the T-S waves. The situation changes when the Stokes layer encounters an roughness; near which the flow is described using the triple-deck theory. The solution of the triple-deck problem can be found in an analytic form. Our main concern is with the flow behaviour downstream of the roughness and, in particular, with the amplitude of the generated Tollmien-Schlichting waves. This research was performed in the Laminar Flow Control Centre (LFC-UK) at Imperial College London. The centre is supported by EPSRC, Airbus UK and EADS Innovation Works.
Reactive boundary layers in metallic rolling contacts
Energy Technology Data Exchange (ETDEWEB)
Burbank, John
2016-05-01
more thorough investigation into the effects of residual austenite on the properties of this material. The high-performance alternative steels, 36NiCrMoV1-5-7 (hot working steel) and 45SiCrMo6 (spring steel), were heat treated as recommended by their respective manufacturers, and were not case-hardened. The selection of materials with and materials without case-hardening allows for an investigation into whether or not case-hardening is even necessary to deliver acceptable friction behaviour and wear performance. Elemental analyses were conducted by multiple methods to ensure accurate results. Residual austenite contents of the steels and the depth profiles of residual stresses were determined by X-Ray diffraction (XRD), for 20MnCr5 ranging from approximately 6 - 14 vol.%, and under 2 vol.% for the alternative alloys. Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or 100Cr6/102Cr6, than of a non-case-hardened 20MnCr5. Residual stresses from machining and case-hardening were measured directly at the sample surface. The high-performance steels fulfilled manufacturer expectations in terms of elemental content, with hardness values between 50 - 55 HRC and strongly martensitic microstructure character. With characterization of the chosen materials complete, the materials could then be subjected to pre-conditioning. The first pre-conditioning method involved targeted generation of cold work hardening as induced boundary layers to protect the contact zone against wear. Work hardening was identified both by variations in residual stress profiles, i.e. the introduction of beneficial compressive residual stresses, and hardness increases in the contact zone, providing enhanced wear resistance. Parameters for work hardening were further optimized to reduce damage to the surface substrates
Sensing the Stable Boundary Layer in a Towing Tank
Steeneveld, G. J.; Dobrovolschi, D.; Paci, A.; Eiff, O.; Lacaze, L.; Holtslag, A. A. M.
2010-09-01
Understanding and forecasting the stable atmospheric boundary layer (SBL) over land is a challenge for already several decades. Generally, the SBL covers two different regimes. The first regime is the weakly SBL, characterised by well defined wind driven turbulence. The second regime covers the very SBL with weak turbulence, and then additional processes become relevant, such as meandering motions, gravity waves, drainage flows, intermittent turbulence and radiation divergence. Especially in this regime this complexity limits the understanding of the SBL and its representation in numerical weather prediction, climate models and air pollution models. For calm conditions, these models typically overestimate near surface temperature and wind speed, with adverse effects for understanding polar climate and end users in agriculture, transportation, and air quality assessment. To improve our understanding of the SBL, we study SBL turbulence in the CNRM-GAME stratified water flume in Toulouse. This unique facility, particularly well suited for stratified flow and BL studies, provides novel laboratory observations that extend earlier efforts of field observations and wind tunnel studies. Among other things, laboratory observations have the advantage of statistical robustness due to repeatability of the experiment and provide access to an extensive set of data. Hence, a 3x3 m2 plate covered with LEGO of Lx=1.57 cm and Ly=3.57 cm, (roughness length = 0.0014 m, and roughness density =0.250, index of frontal area = 0.125) was towed at different velocities through the tank of 22 x 3 x 1.6 m. In this way we were able to achieve an SBL of ~10 cm with bulk Richardson numbers in the range between 0.05 and 0.25, and turbulence with a well-behaved inertial subrange. We focus on the estimation of the non-dimensional velocity and density profiles, on higher order turbulent statistics (important for plume dispersion), as well as on the turbulence spectral behaviour. Finally, we aim to
Magnetic Structure of the Magnetopause Boundary Layer for Open Magnetosphere
Ma, Yonghui; Shen, Chao; Zeng, Gang
2017-04-01
Using Cluster and Magnetospheric MultiScale (MMS) spacecraft 4 point magnetic field measurements, we analyzed the magnetic structure of magnetopause boundary layer of the open magnetosphere. It is indicated that the magnetopause boundary layer is very thin under strong magnetic shear and the thickness is usually 0.1 Re. We found that the Rotational Discontinuity (RD) is very important structure at magnetopause when the Interplanetary Magnetic Field (IMF) is southward. Within the boundary layer, the magnetic field has a large rotation. Using curvature calculation method, we got that the minimum curvature radius of magnetic field of RD is 0.02 - 0.1Re, implying that the magnetosphere is open when the IMF is southward. Advanced research showed that the field-aligned currents are common in the magnetopause boundary layer.
Microprobe of structure of crystal/liquid interface boundary layers
Institute of Scientific and Technical Information of China (English)
无
2001-01-01
The molecular structures and its evolutive regularities within the boundary layers in the crystal growth of KDP and DKDP have been studied in real time by using holography and Raman microprobe. The experiments show that the molecular structure of mother solution within the boundary layers is distinctly different from that of the solutions alone. In this paper, the effects of cations within the boundary layers on the structure of solution are considered. Within the characteristic boundary layers, the effects of cations cause the changes in O-P-O bond angle, electronic density redistribution of the phosphate groups, and significant changes in the bond intensity, thus leading to the breaking of partial hydrogen bonds of the phosphate associations, the readjustment of geometry of anionic phosphate groups and desolvation, and the forming of the smectic ordering structure of the anions_cations. Finally, the crystallization unit of anion_cation should be formed at the proximate interface.
Institute of Scientific and Technical Information of China (English)
FENG Bo; ZHENG Yong-hong; YOU Ya-ge; HE Zai-ming
2007-01-01
The two-dimensional problems concerning the interaction of linear water waves with cylinders of arbitrary shape in two-layer deep water are investigated by use of the Boundary Integral Equation Method (BIEM). Simpler new expressions for the Green functions are derived, and verified by comparison of results obtained by BIEM with those by an analytical method. Examined are the radiation and scattering of linear waves by two typical configurations of cylinders in two-layer deep water. Hydrodynamic behaviors including hydrodynamic coefficients, wave forces, reflection and transmission coefficients and energies are analyzed in detail, and some interesting physical phenomena are observed.
Controlling near shore nonlinear surging waves through bottom boundary conditions
Mukherjee, Abhik; Kundu, Anjan
2016-01-01
Instead of taking the usual passive view for warning of near shore surging waves including extreme waves like tsunamis, we aim to study the possibility of intervening and controlling nonlinear surface waves through the feedback boundary effect at the bottom. It has been shown through analytic result that the controlled leakage at the bottom may regulate the surface solitary wave amplitude opposing the hazardous variable depth effect. The theoretical results are applied to a real coastal bathymetry in India.
Tropical Cyclogenesis in a Tropical Wave Critical Layer: Easterly Waves
Dunkerton, T. J.; Montgomery, M. T.; Wang, Z.
2009-01-01
The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside.
A note on boundary-layer friction in baroclinic cyclones
Boutle, I A; Belcher, S E; Plant, R S
2008-01-01
The interaction between extratropical cyclones and the underlying boundary layer has been a topic of recent discussion in papers by Adamson et. al. (2006) and Beare (2007). Their results emphasise different mechanisms through which the boundary layer dynamics may modify the growth of a baroclinic cyclone. By using different sea-surface temperature distributions and comparing the low-level winds, the differences are exposed and both of the proposed mechanisms appear to be acting within a single simulation.
CONTINUOUS WAVELET TRANSFORM OF TURBULENT BOUNDARY LAYER FLOW
Institute of Scientific and Technical Information of China (English)
LIU Ying-zheng; KE Feng; CHEN Han-ping
2005-01-01
The spatio-temporal characteristics of the velocity fluctuations in a fully-developed turbulent boundary layer flow was investigated using hotwire. A low-speed wind tunnel was established. The experimental data was extensively analyzed in terms of continuous wavelet transform coefficients and their auto-correlation. The results yielded a potential wealth of information on inherent characteristics of coherent structures embedded in turbulent boundary layer flow. Spatial and temporal variations of the low- and high- frequency motions were revealed.
DNS of compressible turbulent boundary layer over a blunt wedge
Institute of Scientific and Technical Information of China (English)
LI Xinliang; FU Dexun; MA Yanwan
2005-01-01
Direct numerical simulation of spatially evolving compressible boundary layer over a blunt wedge is performed in this paper. The free-stream Mach number is 6 and the disturbance source produced by wall blowing and suction is located downstream of the sound-speed point. Statistics are studied and compared with the results in incompressible flat-plate boundary layer. The mean pressure gradient effects on the vortex structure are studied.
A Compilation of Unsteady Turbulent Boundary Layer Experimental Data,
1981-11-01
HIRSCH KITAet ai, GOSTELOW EHERENSBERGER LU HO & CHEN KOBASHI & HAYAKAWA MAINARDI & PANDAY MARVIN* LORBER & COVERT MIZUSHINA I SAXENA RAMAPRIAN & TU...Laminar Boundary Layer by a Moving Belt. AIAA Paj_2r 69-40, New York, N.Y., 1969. (LT) Mainardi , H. and Panday, P. K.: A Study of Turbulent Pulsating...Flow in a (-cular Pipe. Eurovisc 77 - Unsteady Turbulent Boundary Layers and Shear Flows, Toulouse, France, Jar,. 2977. (TE-D) Mainardi , H. and Panday
Turbulent oceanic western-boundary layers at low latitude
Quam Cyrille Akuetevi, Cataria; Wirth, Achim
2013-04-01
Low latitude oceanic western-boundary layers range within the most turbulent regions in the worlds ocean. The Somali current system with the Great Whirl and the Brazilian current system with its eddy shedding are the most prominent examples. Results from analytical calculations and integration of a one layer reduced-gravity fine resolution shallow water model is used to entangle this turbulent dynamics. Two types of wind-forcing are applied: a remote Trade wind forcing with maximum shear along the equator and a local Monsoon wind forcing with maximum shear in the vicinity of the boundary. For high values of the viscosity (> 1000m2s-1) the stationary solutions compare well to analytical predictions using Munk and inertial layer theory. When lowering the friction parameter time dependence results. The onset of instability is strongly influenced by inertial effects. The unstable boundary current proceeds as a succession of anti-cyclonic coherent eddies performing a chaotic dynamics in a turbulent flow. The dynamics is governed by the turbulent fluxes of mass and momentum. We determine these fluxes by analyzing the (potential) vorticity dynamics. We demonstrate that the boundary-layer can be separated in four sub-layers, which are (starting from the boundary): (1) the viscous sub-layer (2) the turbulent buffer-layer (3) the layer containing the coherent structures and (4) the extended boundary layer. The characteristics of each sub-layer and the corresponding turbulent fluxes are determined, as are the dependence on latitude and the type of forcing. A new pragmatic method of determining the eddy viscosity, based on Munk-layer theory, is proposed. Results are compared to observations and solutions of the multi-level primitive equation model (DRAKKAR).
BOUNDARY LAYER AND VANISHING DIFFUSION LIMIT FOR NONLINEAR EVOLUTION EQUATIONS
Institute of Scientific and Technical Information of China (English)
彭艳
2014-01-01
In this paper, we consider an initial-boundary value problem for some nonlinear evolution equations with damping and diffusion. The main purpose is to investigate the boundary layer effect and the convergence rates as the diffusion parameterαgoes to zero.
LES model intercomparisons for the stable atmospheric boundary layer
Moene, A.F.; Baas, P.; Bosveld, F.C.; Basu, S.
2011-01-01
Model intercomparisons are one possible method to gain confidence in Large-Eddy Simulation (LES) as a viable tool to study turbulence in the atmospheric boundary-layer. This paper discusses the setup and some results of two intercomparison cases focussing on the stably stratified nocturnal boundary-
Collapse of nonlinear electron plasma waves in a plasma layer
Grimalsky, V.; Koshevaya, S.; Rapoport, Yu; Kotsarenko, A.
2016-10-01
The excitation of nonlinear electron plasma waves in the plasma layer is investigated theoretically. This excitation is realized by means of initial oscillatory perturbations of the volume electron concentration or by initial oscillatory distributions of the longitudinal electron velocity. The amplitudes of the initial perturbations are small and the manifestation of the volume nonlinearity is absent. When the amplitudes of the initial perturbations exceed some thresholds, the values of the electron concentration near the plasma boundary increase catastrophically. The maxima of the electron concentration reach extremely high magnitudes, and sharp peaks in the electron concentration occur, which are localized both in the longitudinal and transverse directions. This effect is interpreted as wave collapse near the plasma boundary.
Boundary layer effects on liners for aircraft engines
Gabard, Gwénaël
2016-10-01
The performance of acoustic treatments installed on aircraft engines is strongly influenced by the boundary layer of the grazing flow on the surface of the liner. The parametric study presented in this paper illustrates the extent of this effect and identifies when it is significant. The acoustic modes of a circular duct with flow are calculated using a finite difference method. The parameters are representative of the flow conditions, liners and sound fields found in current turbofan engines. Both the intake and bypass ducts are considered. Results show that there is a complex interplay between the boundary layer thickness, the direction of propagation and the liner impedance and that the boundary layer can have a strong impact on liner performance for typical configurations (including changes of the order of 30 dB on the attenuation of modes associated with tonal fan noise). A modified impedance condition including the effect of a small but finite boundary layer thickness is considered and compared to the standard Myers condition based on an infinitely thin boundary layer. We show how this impedance condition can be implemented in a mode calculation method by introducing auxiliary variables. This condition is able to capture the trends associated with the boundary layer effects and in most cases provides improved predictions of liner performance.
Tropical cyclogenesis in a tropical wave critical layer: easterly waves
Directory of Open Access Journals (Sweden)
T. J. Dunkerton
2008-06-01
Full Text Available The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, resembles the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development within the critical layer is given by the intersection of the wave's critical latitude and trough axis, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally this "marsupial paradigm" one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. This translation requires an appropriate "gauge" that renders translating streamlines and isopleths of translating stream function approximately equivalent to flow trajectories. In the translating frame, the closed circulation is stationary, and a dividing streamline effectively separates air within the critical layer from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because it
Wave propagation in elastic layers with damping
DEFF Research Database (Denmark)
Sorokin, Sergey; Darula, Radoslav
2016-01-01
The conventional concepts of a loss factor and complex-valued elastic moduli are used to study wave attenuation in a visco-elastic layer. The hierarchy of reduced-order models is employed to assess attenuation levels in various situations. For the forcing problem, the attenuation levels are found...... for alternative excitation cases. The differences between two regimes, the low frequency one, when a waveguide supports only one propagating wave, and the high frequency one, when several waves are supported, are demonstrated and explained....
Mechanism of transition in a hypersonic sharp cone boundary layer with zero angle of attack
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
Firstly, the steady laminar flow field of a hypersonic sharp cone boundary layer with zero angle of attack was computed. Then, two groups of finite amplitude T-S wave disturbances were introduced at the entrance of the computational field, and the spatial mode transition process was studied by direct numerical simulation (DNS) method.The mechanism of the transition process was analyzed. It was found that the change of the stability characteristics of the mean flow profile was the key issue. Furthermore, the characteristics of evolution for the disturbances of different modes in the hypersonic sharp cone boundary layer were discussed.
EVOLUTION OF A 2-D DISTURBANCE IN A SUPERSONIC BOUNDARY LAYER AND THE GENERATION OF SHOCKLETS
Institute of Scientific and Technical Information of China (English)
黄章峰; 周恒
2004-01-01
Through direct numerical simulation, the evolution of a 2-D disturbance in a supersonic boundary layer has been investigated. At a chosen location, a small amplitude T-S wave was fed into the boundary layer to investigate its evolution. Characteristics of nonlinear evolution have been found. Two methods were applied for the detection of shocklets ,and it was found that when the amplitude of the disturbance reached a certain value,shocklets would be generated, which should be taken into consideration when nonlinear theory of hydrodynamic stability for compressible flows is to be established.
The Kelvin-Helmholtz instability at the magnetopause and inner boundary layer surface
Ogilvie, Keith W.; Fitzenreiter, Richard J.
1989-01-01
Observations by the vector electron spectrometer on the ISEE 1 spacecraft are used to directly check the incompressible hydromagnetic stability condition given by Hasegawa (1975), which indicates how stability is influenced by the velocity shear, density ratio, magnetic field, and the direction of the wave vector of the unstable wave mode. The magnetopause is generally found to be stable, by a large margin. In contrast, many cases of marginal stability or instability are found across density transitions in, and at the inner edge of, the boundary layer for wave vectors in plausible directions. It thus appears that the Kelvin-Helmholtz instability is of importance at the inner edge of the magnetospheric boundary layer, as predicted by MHP theory.
Stability Analysis of Hypersonic Boundary Layer over a Cone at Small Angle of Attack
Directory of Open Access Journals (Sweden)
Feng Ji
2014-04-01
Full Text Available An investigation on the stability of hypersonic boundary layer over a cone at small angle of attack has been performed. After obtaining the steady base flow, linear stability theory (LST analysis has been made with local parallel assumption. The growth rates of the first mode and second mode waves at different streamwise locations and different azimuthal angles are obtained. The results show that the boundary layer stability was greatly influenced by small angles of attack. The maximum growth rate of the most unstable wave on the leeward is larger than that on the windward. Moreover, dominating second mode wave starts earlier on the leeward than that on the windward. The LST result also shows that there is a “valley” region around 120°~150° meridian in the maximum growth rates curve.
Tropical cyclogenesis in a tropical wave critical layer: easterly waves
Directory of Open Access Journals (Sweden)
T. J. Dunkerton
2009-08-01
Full Text Available The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i a region of
Towards Natural Transition in Compressible Boundary Layers
2016-06-29
UNIT NUMBER 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 16. SECURITY CLASSIFICATION OF: 19b. TELEPHONE NUMBER (Include area code) The public reporting...30-09-2011 to 29-03-2016, with Dr. James M. Fillerup serving as program manager. In this project, a DNS code was developed to investigate problems on...8 1.3.1 DNS simulations of wave packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 Objectives of this
Nonlinear stability of non-stationary cross-flow vortices in compressible boundary layers
Gajjar, J. S. B.
1995-01-01
The nonlinear evolution of long wavelength non-stationary cross-flow vortices in a compressible boundary layer is investigated and the work extends that of Gajjar (1994) to flows involving multiple critical layers. The basic flow profile considered in this paper is that appropriate for a fully three-dimensional boundary layer with O(1) Mach number and with wall heating or cooling. The governing equations for the evolution of the cross-flow vortex are obtained and some special cases are discussed. One special case includes linear theory where exact analytic expressions for the growth rate of the vortices are obtained. Another special case is a generalization of the Bassom & Gajjar (1988) results for neutral waves to compressible flows. The viscous correction to the growth rate is derived and it is shown how the unsteady nonlinear critical layer structure merges with that for a Haberman type of viscous critical layer.
Enhancing the prediction of turbulent kinetic energy in the marine atmospheric boundary layer
Foreman, R. J.; Emeis, S.
2010-09-01
A recent study by Shaikh and Siddiqui (2010) has shown definitively that the turbulent structure of boundary layer flows over water is fundamentally different compared with that over a smooth surface and with that over a solid wavy surface whose wave amplitude is similar to that of dynamically wind-generated waves. In light of this new information, the constants of the Mellor-Yamada boundary layer model, which are based on laboratory data over solid walls, are re-evaluated to suit the turbulent dynamics of a dynamic, wavy surface. The constants are based on the principal that the enhanced turbulent production in the vicinity of waves is redistributed among the normal stress components by virtue of the enhanced pressure-velocity covariances also found in the vicinity of waves. There is then a feedback mechanism whereby enhanced normal stresses modify the dynamic surface. The net effect of this is that in the marine boundary layer, one can expect an enhancement of turbulent kinetic energy due to the enhancement of normal stresses at the expense of shear stresses. The constants in the Mellor-Yamada-Janjic planetary boundary layer scheme within the Weather Research and Forecasting (WRF) model are changed to fit this principal. Simulations are then performed and compared with data (wind speed and turbulent kinetic energy) from the FINO1 platform in the North Sea. It is found that while predictions of the wind speed are barely changed, the magnitude of the tke error (RMS) is reduced by up to 50%. This is expected to be practically relevant for the estimation of blade fatigue of wind energy converters, where the tke is an important parameter in this assessment. It could also be relevant for pollution dispersion in marine boundary layers.
Wave propagation in elastic layers with damping
DEFF Research Database (Denmark)
Sorokin, Sergey; Darula, Radoslav
2016-01-01
The conventional concepts of a loss factor and complex-valued elastic moduli are used to study wave attenuation in a visco-elastic layer. The hierarchy of reduced-order models is employed to assess attenuation levels in various situations. For the forcing problem, the attenuation levels are found...
Plasma boundary layer and magnetopause layer of the earth's magnetosphere
Energy Technology Data Exchange (ETDEWEB)
Eastman, T.E.
1979-06-01
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.
THE WAVE-MAKING CHARACTERISTICS OF A MOVING BODY IN A TWO-LAYER FLUID
Institute of Scientific and Technical Information of China (English)
ZHU Wei
2005-01-01
The Wave-making characteristics of a moving body in a two-layer fluid with free surface is investigated numerically and experimentally. The numerical analysis is based on the modified layered boundary integral equation system. The wave characteristics on the free surface and interface generated by a moving sphere and an ellipsoid is numerically simulated in both finite depth and infinite depth of lower layer model. The numerical results of the sphere are compared with the analytical results for a dipole with the same velocity in a two-layer fluid of finite depth. The dependence of the wave systems and structures on the characteristic quantities is discussed. Three kinds of measurement techniques are used in model experiments on the internal waves generated by a sphere advancing in a two-layer fluid. The effects of the varying velocity and stratification on the wavelength, wave amplitudes and the maximum half angles of internal waves are analyzed qualitatively.
The Damage To The Armour Layer Due To Extreme Waves
Oztunali Ozbahceci, Berguzar; Ergin, Aysen; Takayama, Tomotsuka
2010-05-01
computation of wave time series, Deterministic Spectral Amplitude (DSA) model with FFT algorithm was used. It is possible to get thousands of time series which have different wave statistics in DSA model by setting up the target spectrum and using random numbers for phase angles (Tuah et.al. 1982). Multi-reflection in the wave channel was minimized by the absorption mode of wave generator. Incident wave energy spectrum was obtained by using the separation method introduced by Goda and Suzuki (1976). Three wave gauges in front of the model were used for the separation. Individual wave heights were determined by zero-up crossing method after obtaining incident wave train. After each test, damage of the breakwater was calculated. Van der Meer's (1988) definition of damage level, S, was used in the calculations as: S= Ae/Dn502 (1) where; Ae= Eroded area, Dn50: nominal diameter of armour stone In order to get eroded area, the profile of armour layer was measured by laser equipment through nine lines along the section. Results of the experiments indicate that the higher the extreme waves are, the more destructive the wave train is, even the data is scattered. The damage was also calculated by using Van der Meer's formulae (1988) and compared with the experimental results. The comparison shows that the damages are more than the expected results in the cases where at least one wave height in the train is higher than the twice of H1-3. In fact, the damage results calculated by Van der Meer's formulae form the lower boundary for the higher extreme wave cases. It is also found that the damage is highly correlated to the ratios of characteristic waves like H1-10/H1-3 or H1-20/H1-3. Therefore, the parameter αextreme covering the effect of all extreme waves is proposed. References Goda, Y. and Suzuki, Y. (1976) .' Estimation of Incident and Reflected Waves in Random wave experiments.' Proc. 15th. Int. Conf. Coastal Engg., Hawai,1976, pp.828-845. Goda Y. (1998), 'An Overview of Coastal
Analysis of diabatic flow modification in the internal boundary layer
DEFF Research Database (Denmark)
Floors, Rogier; Gryning, Sven-Erik; Pena Diaz, Alfredo
2011-01-01
is controlled by a combination of both downstream and upstream stability and surface roughness conditions. A model based on a diffusion analogy is able to predict the internal boundary layer height well. Modeling the neutral and long-term wind profile with a 3 layer linear interpolation scheme gives good......Measurements at two meteorological masts in Denmark, Horns Rev in the sea and Høvsøre near the coastline on land, are used to analyze the behaviour of the flow after a smooth-to-rough change in surface conditions. The study shows that the wind profile within the internal boundary layer...... results at Høvsøre. Based on a comparison with a numerical model and the measurements, the constants in the interpolation scheme are slightly adjusted, which yields an improvement for the description of the wind profile in the internal boundary layer....
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.
Definition of Turbulent Boundary-Layer with Entropy Concept
Directory of Open Access Journals (Sweden)
Zhao Rui
2016-01-01
Full Text Available The relationship between the entropy increment and the viscosity dissipation in turbulent boundary-layer is systematically investigated. Through theoretical analysis and direct numerical simulation (DNS, an entropy function fs is proposed to distinguish the turbulent boundary-layer from the external flow. This approach is proved to be reliable after comparing its performance in the following complex flows, namely, low-speed airfoil flows with different wall temperature, supersonic cavity-ramp flow dominated by the combination of free-shear layer, larger recirculation and shocks, and the hypersonic flow past an aeroplane configuration. Moreover, fs is deduced from the point of energy, independent of any particular turbulent quantities. That is, this entropy concept could be utilized by other engineering applications related with turbulent boundary-layer, such as turbulence modelling transition prediction and engineering thermal protection.
Real no-boundary wave function in Lorentzian quantum cosmology
Dorronsoro, J. Diaz; Halliwell, J. J.; Hartle, J. B.; Hertog, T.; Janssen, O.
2017-08-01
It is shown that the standard no-boundary wave function has a natural expression in terms of a Lorentzian path integral with its contour defined by Picard-Lefschetz theory. The wave function is real, satisfies the Wheeler-DeWitt equation and predicts an ensemble of asymptotically classical, inflationary universes with nearly-Gaussian fluctuations and with a smooth semiclassical origin.
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
Effect of a bulge on the subharmonic instability of boundary layers
Nayfeh, Ali H.; Ragab, Saad A.; Masad, Jamal A.
1990-01-01
The influence of a two-dimensional hump on the three-dimensional (3-D) subharmonic secondary instability on a flat plate is investigated. The mean flow is calculated by using interacting boundary layers, thereby accounting for the viscid/inviscid interaction. The primary wave is taken in the form of a two-dimensional (2-D) Tollmien-Schlichting (T-S) wave. The secondary wave is taken in the form of a 3-D subharmonic wave. The results show that increasing the hump height results in an increase in the amplification factors of the primary and subharmonic waves. When the hump causes separation, the growth rates of both the primary and subharmonic waves are considerably larger than those obtained in the case of no separation.
Boundary-Layer Effects on Acoustic Transmission Through Narrow Slit Cavities
Ward, G. P.; Lovelock, R. K.; Murray, A. R. J.; Hibbins, A. P.; Sambles, J. R.; Smith, J. D.
2015-07-01
We explore the slit-width dependence of the resonant transmission of sound in air through both a slit array formed of aluminum slats and a single open-ended slit cavity in an aluminum plate. Our experimental results accord well with Lord Rayleigh's theory concerning how thin viscous and thermal boundary layers at a slit's walls affect the acoustic wave across the whole slit cavity. By measuring accurately the frequencies of the Fabry-Perot-like cavity resonances, we find a significant 5% reduction in the effective speed of sound through the slits when an individual viscous boundary layer occupies only 5% of the total slit width. Importantly, this effect is true for any airborne slit cavity, with the reduction being achieved despite the slit width being on a far larger scale than an individual boundary layer's thickness. This work demonstrates that the recent prevalent loss-free treatment of narrow slit cavities within acoustic metamaterials is unrealistic.
Indian Academy of Sciences (India)
M Tajuddin; G Narayan Reddy
2005-08-01
A transcendental equation is derived relating wave number and phase velocity of propagation of waves in a partially saturated layered half-space under plane strain conditions. The period equation is derived and discussed for two types of boundaries, permeable and impermeable. In the limiting cases, the problem reduces to more simpliﬁed forms as discussed by earlier researchers.
Vortex Generators to Control Boundary Layer Interactions
Babinsky, Holger (Inventor); Loth, Eric (Inventor); Lee, Sang (Inventor)
2014-01-01
Devices for generating streamwise vorticity in a boundary includes various forms of vortex generators. One form of a split-ramp vortex generator includes a first ramp element and a second ramp element with front ends and back ends, ramp surfaces extending between the front ends and the back ends, and vertical surfaces extending between the front ends and the back ends adjacent the ramp surfaces. A flow channel is between the first ramp element and the second ramp element. The back ends of the ramp elements have a height greater than a height of the front ends, and the front ends of the ramp elements have a width greater than a width of the back ends.
Surface modes in sheared boundary layers over impedance linings
Brambley, E. J.
2013-08-01
Surface modes, being duct modes localized close to the duct wall, are analysed within a lined cylindrical duct with uniform flow apart from a thin boundary layer. As well as full numerical solutions of the Pridmore-Brown equation, simplified mathematical models are given where the duct lining and boundary layer are lumped together and modelled using a single boundary condition (a modification of the Myers boundary condition previously proposed by the author), from which a surface mode dispersion relation is derived. For a given frequency, up to six surface modes are shown to exist, rather than the maximum of four for uniform slipping flow. Not only is the different number and behaviour of surface modes important for frequency-domain mode-matching techniques, which depend on having found all relevant modes during matching, but the thin boundary layer is also shown to lead to different convective and absolute stability than for uniform slipping flow. Numerical examples are given comparing the predictions of the surface mode dispersion relation to full solutions of the Pridmore-Brown equation, and the accuracy with which surface modes are predicted is shown to be significantly increased compared with the uniform slipping flow assumption. The importance of not only the boundary layer thickness but also its profile (tanh or linear) is demonstrated. A Briggs-Bers stability analysis is also performed under the assumption of a mass-spring-damper or Helmholtz resonator impedance model.
Reflection and refraction of flexural waves at geometric boundaries.
Evans, Arthur A; Levine, Alex J
2013-07-19
We present a theory of flexural wave propagation on elastic shells having nontrivial geometry and develop an analogy to geometric optics. The transport of momentum within the shell itself is anisotropic due to the curvature, and as such complex classical effects such as birefringence are generically found. We determine the equations of reflection and refraction of such waves at boundaries between different local geometries, showing that waves are totally internally reflected, especially at boundaries between regions of positive and negative Gaussian curvature. We verify these effects by using finite element simulations and discuss the ramifications of these effects for the statistical mechanics of thin curved materials.
Artificial Boundary Method for Calculating Ship Wave Resistance
Institute of Scientific and Technical Information of China (English)
文新; 韩厚德
2003-01-01
The calculation of wave resistance for a ship moving at constant speed near a free surface is considered. This wave resistance is calculated with a linearized steady potential model. To deal with the unboundedness of the physical domain in the potential flow problem, we introduce one vertical side as an artificial upstream boundary and two vertical sides as the artificial downstream boundaries. On the artificial boundaries, a sequence of high-order global artificial boundary conditions are given. Then the potential flow problem is reduced to a problem defined on a finite computational domain, which is equivalent to a variational problem. The solution of the variational problem by the finite element method gives the numerical approximation of the potential flow around the ship, which was used to calculate the wave resistance. The numerical examples show the accuracy and efficiency of the proposed numerical scheme.
Institute of Scientific and Technical Information of China (English)
LI Ning; XIE Li-li; ZHAI Chang-hai
2007-01-01
The theory of perfectly matched layer (PML) artificial boundary condition (ABC), which is characterized by absorption any wave motions with arbitrary frequency and arbitrarily incident angle, is introduced. The construction process of PML boundary based on elastodynamic partial differential equation (PDE) system is developed.Combining with velocity-stress hybrid finite element formulation, the applicability of PML boundary is investigated and the numerical reflection of PML boundary is estimated. The reflectivity of PML and multi-transmitting formula (MTF) boundary is then compared based on body wave and surface wave simulations. The results show that although PML boundary yields some reflection, its absorption performance is superior to MTF boundary in the numerical simulations of near-fault wave propagation, especially in corner and large angle grazing incidence situations. The PML boundary does not arise any unstable phenomenon and the stability of PML boundary is better than MTF boundary in hybrid finite element method. For a specified problem and analysis tolerance, the computational efficiency of PML boundary is only a little lower than MTF boundary.
The inner core thermodynamics of the tropical cyclone boundary layer
Williams, Gabriel J.
2016-10-01
Although considerable progress has been made in understanding the inner-core dynamics of the tropical cyclone boundary layer (TCBL), our knowledge of the inner-core thermodynamics of the TCBL remains limited. In this study, the inner-core budgets of potential temperature (θ), specific humidity ( q), and reversible equivalent potential temperature (θ _e) are examined using a high-resolution multilevel boundary layer model. The potential temperature budgets show that the heat energy is dominated by latent heat release in the eyewall, evaporative cooling along the outer edge of the eyewall, and upward surface fluxes of sensible and latent heat from the underlying warm ocean. It is shown that the vertical θ advection overcompensates the sum of radial advective warming from the boundary layer outflow jet and latent heating for the development of cooling in the eyewall within the TCBL. The moisture budgets show the dominant upward transport of moisture in the eyewall updrafts, partly by the boundary-layer outflow jet from the bottom eye region, so that the eyewall remains nearly saturated. The θ _e budgets reveal that the TCBL is maintained thermodynamically by the upward surface flux of higher-θ _e air from the underlying warm ocean, the radial transport of low-θ _e air from the outer regions of the TCBL, and the dry adiabatic cooling associated by eyewall updrafts. These results underscore the significance of vertical motion and the location of the boundary layer outflow jet in maintaining the inner core thermal structure of the TCBL.
An investigation of streaklike instabilities in laminar boundary layer flames
Miller, Colin; Finney, Mark; Forthofer, Jason; McAllister, Sara; Gollner, Michael
2016-11-01
Observations of coherent structures in boundary layer flames, particularly wildland fires, motivated an investigation on flame instabilities within a boundary layer. This experimental study examined streaklike structures in a stationary diffusion flame stabilized within a laminar boundary layer. Flame streaks were found to align with pre-existing velocity perturbations, enabling stabilization of these coherent structures. Thermocouple measurements were used to quantify streamwise amplification of flame streaks. Temperature mapping indicated a temperature rise in the flame streaks, while the region in between these streaks, the trough, decreased in temperature. The heat flux to the surface was measured with a total heat flux gauge, and the heat flux below the troughs was found to be higher at all measurement locations. This was likely a function of the flame standoff distance, and indicated that the flame streaks were acting to modify the spanwise distribution of heat flux. Instabilities in boundary layer combustion can have an effect on the spanwise distribution of heat transfer. This finding has significant implications for boundary layer combustion, indicating that instantaneous properties can vary significantly in a three-dimensional flow field.
OBLIQUE WATER WAVES IMPACTING ON A THIN POROUS WALL WITH A PARTIAL-SLIPPING BOUNDARY CONDITION
Institute of Scientific and Technical Information of China (English)
HSU Hao-Jen; HUANG Liang-Hsiung
2011-01-01
When an incoming water wave is parallel to a porous breakwater, a paradoxical phenomenon exists in that by strictly following the potential flow boundary condition of normal flux continuity on the interfaces, the water wave permeates the wall completely, regardless of breakwater porosity. To account for this paradoxical phenomenon when solving the problem of water waves obliquely impacting on a thin porous wall, a new partial-slipping boundary condition on the thin porous wall for potential flow is proposed. Analytical results show that when the water wave is parallel to a thin porous wall (i.e., the incident angle equals to 90°),the transmitted wave side remains quiescent, i.e., the transmitted wave side does not capture any wave energy when no viscous effect exists. This reveals that the above-mentioned paradoxical phenomenon disappears. The viscous boundary layer effect is also investigated in this study, which provides proper boundary conditions on a thin porous wall for viscous flows and detailed flow information.
2016-06-07
turbulent boundary layer pressure fluctuation transmitted into a layer of viscoelastic material. The theoretical model used here is a plane elastomer...Spring 1985. The objective of this paper is to develop a model for calculating the turbulent boundary layer pressure fluctuation transmitted into a...the noise level calculated in terms of decibels. FIGURE 4 (CORCOS MODEL ) This is a model cross-spectrum of turbulent wall pressure , frequently
Boundary conditions on internal three-body wave functions
Energy Technology Data Exchange (ETDEWEB)
Mitchell, Kevin A.; Littlejohn, Robert G.
1999-10-01
For a three-body system, a quantum wave function {Psi}{sub m}{sup {ell}} with definite {ell} and m quantum numbers may be expressed in terms of an internal wave function {chi}{sub k}{sup {ell}} which is a function of three internal coordinates. This article provides necessary and sufficient constraints on {chi}{sub k}{sup {ell}} to ensure that the external wave function {Psi}{sub k}{sup {ell}} is analytic. These constraints effectively amount to boundary conditions on {chi}{sub k}{sup {ell}} and its derivatives at the boundary of the internal space. Such conditions find similarities in the (planar) two-body problem where the wave function (to lowest order) has the form r{sup |m|} at the origin. We expect the boundary conditions to prove useful for constructing singularity free three-body basis sets for the case of nonvanishing angular momentum.
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
Atmospheric boundary layers in storms: advanced theory and modelling applications
Zilitinkevich, S. S.; Esau, I. N.; Baklanov, A.
2005-03-01
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 were overlooked
ON FREE WAVE PROPAGATION IN ANISOTROPIC LAYERED MEDIA
Institute of Scientific and Technical Information of China (English)
Yongqiang Guo; Weiqiu Chen
2008-01-01
The method of reverberation-ray matrix (MRRM) is extended and modified for the analysis of free wave propagation in anisotropic layered elastic media. A general, numerically stable formulation is established within the state space framework. The compatibility of physical variables in local dual coordinates gives the phase relation, from which exponentially growing functions are excluded. The interface and boundary conditions lead to the scattering relation,which avoids matrix inversion operation. Numerical examples are given to show the high accuracy of the present MRRM.
Receptivity of Hypersonic Boundary Layers to Acoustic and Vortical Disturbances (Invited)
Balakumar, P.
2015-01-01
Boundary-layer receptivity to two-dimensional acoustic and vortical disturbances for hypersonic flows over two-dimensional and axi-symmetric geometries were numerically investigated. The role of bluntness, wall cooling, and pressure gradients on the receptivity and stability were analyzed and compared with the sharp nose cases. It was found that for flows over sharp nose geometries in adiabatic wall conditions the instability waves are generated in the leading-edge region and that the boundary layer is much more receptive to slow acoustic waves as compared to the fast waves. The computations confirmed the stabilizing effect of nose bluntness and the role of the entropy layer in the delay of boundary layer transition. The receptivity coefficients in flows over blunt bodies are orders of magnitude smaller than that for the sharp cone cases. Wall cooling stabilizes the first mode strongly and destabilizes the second mode. However, the receptivity coefficients are also much smaller compared to the adiabatic case. The adverse pressure gradients increased the unstable second mode regions.
Chou, D. C.; Lee, H. C.; Luidens, R. W.; Stockman, N. O.
1978-01-01
A semi-empirical scheme for the prediction of transonic pressure distribution on the surface of V/STOL inlets at high incidence angles has been developed. The investigation is intended to improve the boundary layer calculation and separation prediction by including the effects of shock wave-boundary layer interaction into the Lewis Inlet Viscous Computer Program. Wind-tunnel results and theoretical pressure calculation for critical cases are used in constructing the transonic pressure distribution. The program, which describes the development of the boundary layer and predicts the possible flow separation, can handle the cases of inlets at high incidence angles where local supersonic region may occur in the flow.
Optical measurements of degradation in aircraft boundary layers
Kelsall, D.
1980-01-01
Visible wavelength measurements of the degradation of optical beams when transmitted through the thin aerodynamic boundary layers around an aircraft are reviewed. The measured results indicated degradation levels for the KC-135 airplanes between 0.10 to 0.13 lambda increasing to 0.18 lambda (rms wavefront distortion). For the Lear Jet, degradation with a 25 mm diameter optics was roughly 0.07 lambda. The corresponding infinite aperture degradation levels are also calculated. The corresponding measured correlation lengths of roughly 12 mm for the KC-135 aircraft and 6 mm for the Lear Jet scale to roughly 20 and 25 mm, respectively, for infinite apertures. These boundary layer correlation lengths do not appear to reflect the different boundary layer thicknesses on the two different aircraft.
On the interaction between turbulence grids and boundary layers
Directory of Open Access Journals (Sweden)
Irps Thomas
2016-01-01
Full Text Available Turbulence grids are widely used in wind tunnels to produce representative turbulence levels when testing aerodynamic phenomena around models. Although the purpose of the grid is to introduce a desired turbulence level in the freestream flow, the wall boundary layers of the tunnel are subjected to modification due to the presence of such grids. This could have major implications to the flow around the models to be tested and hence there is a need to further understand this interaction. The study described in this paper examines wind tunnel wall boundary layer modification by turbulence grids of different mesh sizes and porosities to understand the effect of these parameters on such interaction. Experimental results are presented in the form of pressure loss coefficients, boundary layer velocity profiles and the statistics of turbulence modification.
Sound from boundary layer flow over steps and gaps
Ryan Catlett, M.; Devenport, William; Glegg, Stewart A. L.
2014-09-01
This study is concerned with the radiated sound from boundary layer flows over small forward and backward steps and gap configurations of similar dimension. These measurements were performed in the Virginia Tech Anechoic Wall Jet Facility for step heights that ranged from approximately 10 percent to 100 percent of the incoming boundary layer height. The results show the influence of step height and boundary layer edge velocity on the far-field sound from forward and backward steps. Neither source shows clear dipole directivity and at least the larger step heights considered in this study are shown to not be acoustically compact. A new mixed scaling normalization is proposed for the far-field spectra from both types of step. Backward steps are shown to be much weaker producers of far-field sound than similarly sized forward steps. The implications of this behavior are discussed with respect to the far-field sound measured from various gap flows.
Boundary-layer temperatures in high accretion rate cataclysmic variables
Energy Technology Data Exchange (ETDEWEB)
Hoare, M.G.; Drew, J.E. (Oxford Univ. (UK). Dept. of Physics Oxford Univ. (UK). Dept. of Astrophysics)
1991-04-01
We use the Zanstra method to derive limits on boundary-layer temperatures in eclipsing dwarf novae during outburst and nova-like variables, using the observed He II {lambda}1640 and {lambda}4686 recombination lines. It is assumed that all the emission is produced in the wind rather than the accretion disc. This method constrains the boundary-layer temperatures to between 50 000 and 100 000 K depending on the degree of wind bipolarity. These estimates are lower than the T>or approx200 000 K predicted theoretically. Possible explanations include rapid rotation of the white dwarf and spreading of the boundary layer over the entire white-dwarf surface. (author).
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.
Bypass transition and spot nucleation in boundary layers
Kreilos, Tobias; Khapko, Taras; Schlatter, Philipp; Duguet, Yohann; Henningson, Dan S.; Eckhardt, Bruno
2016-08-01
The spatiotemporal 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 obtained 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.
Slow Growth Formulation for DNS of Temporally Evolving Boundary Layers
Topalian, Victor; Sahni, Onkar; Oliver, Todd; Moser, Robert
2011-11-01
A formulation for DNS of temporally evolving boundary layers is developed and demonstrated. The formulation relies on a multiscale approach to account separately for the slow time evolution of statistical averages, and the fast time evolution of turbulent fluctuations. The source terms that arise from the multiscale analysis are modeled assuming a self-similar evolution of the averages. The performance of the formulation is evaluated using DNS of spatially evolving compressible boundary layers. This formulation was developed to provide data for the calibration of turbulence model parameters and enable the quantification of uncertainty due to the models. The extension of this formulation to homogenize spatially evolving boundary layers will also be discussed. This work is supported by the Department of Energy [National Nuclear Security Administration] under Award Number [DE-FC52-08NA28615]. Current Affiliation: Rensselaer Polytechnic Institute.
Coupled vs. decoupled boundary layers in VOCALS-REx
Directory of Open Access Journals (Sweden)
C. R. Jones
2011-03-01
Full Text Available We analyze the extent of subtropical stratocumulus-capped boundary layer decoupling and its relation to other boundary-layer characteristics and forcings using aircraft observations from VOCALS-REx along a swath of the subtropical southeast Pacific Ocean running west 1600 km from the coast of Northern Chile. We develop two complementary and consistent measures of decoupling. The first is based on boundary layer moisture stratification in flight profiles from near the surface to above the capping inversion, and the second is based the difference between the lifted condensation level (LCL and a mean lidar-derived cloud base measured on flight legs at 150m altitude. Most flights took place during early-mid morning, well before the peak in insolation-induced decoupling.
We find that the boundary layer is typically shallower, drier, and well mixed near the shore, and tends to deepen, decouple, and produce more drizzle further offshore to the west. Decoupling is strongly correlated to the “well-mixed cloud thickness”, defined as the difference between the capping inversion height and the LCL; other factors such as wind speed, cloud droplet concentration, and inversion thermodynamic jumps have little additional explanatory power. The results are broadly consistent with the deepening-warming theory of decoupling. In the deeper boundary layers observed well offshore, there was frequently nearly 100% boundary-layer cloud cover despite pronounced decoupling. The cloud cover was more strongly correlated to a κ parameter related to the inversion jumps of humidity and temperature, though the exact functional relation is slightly different than found in prior large-eddy simulation studies.
Bleustein-Gulyaev waves in a functionally graded piezoelectric material layered structure
Institute of Scientific and Technical Information of China (English)
CAO Xiaoshan; JIN Feng; WANG ZiKun; LU TianJian
2009-01-01
This work presents a theoretical study of the propagation behavior of Bleustein-Gulyaev waves in a layered structure consisting of a functionally graded piezoelectric material (FGPM) layer and a trans-versely isotropic piezoelectric substrate. The influence of the graded variation of FGPM coefficients on the dispersion relations of Bleustein-Gulyaev waves in the layered structure is investigated. It is dem-onstrated that, for a certain frequency range of Bleustein-Gulyaev waves, the mechanical perturbations of the particles are restricted in the FPGM layer and the phase velocity is independent of the electrical boundary conditions at the free surface. Results presented in this study can not only provide further Insight on the electromechanical coupling behavior of surface waves in FGPM layered structures, but also lend a theoretical basis for the design of high-performance surface acoustic wave (SAW) devices.
Bleustein-Gulyaev waves in a functionally graded piezoelectric material layered structure
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
This work presents a theoretical study of the propagation behavior of Bleustein-Gulyaev waves in a layered structure consisting of a functionally graded piezoelectric material(FGPM) layer and a transversely isotropic piezoelectric substrate. The influence of the graded variation of FGPM coefficients on the dispersion relations of Bleustein-Gulyaev waves in the layered structure is investigated. It is demonstrated that,for a certain frequency range of Bleustein-Gulyaev waves,the mechanical perturbations of the particles are restricted in the FPGM layer and the phase velocity is independent of the electrical boundary conditions at the free surface. Results presented in this study can not only provide further insight on the electromechanical coupling behavior of surface waves in FGPM layered structures,but also lend a theoretical basis for the design of high-performance surface acoustic wave(SAW) devices.
Axisymmetric fundamental solutions for a finite layer with impeded boundaries
Institute of Scientific and Technical Information of China (English)
程泽海; 陈云敏; 凌道盛; 唐晓武
2003-01-01
Axisymmetrie fundamental solutions that are applied in the consolidation calculations of a finite clay layer with impeded boundaries were derived. Laplace and Hankel integral transforms were utilized with respect to time and radial coordinates, respectively in the analysis. The derivation of fundamental solutions considers two boundary-value problems involving unit point loading and ring loading in the vertical. The solut-ions are extended to circular distributed and strip distributed normal load. The computation and analysis of set-tlements, vertical total stress and excess pore pressure in the consolidation layer subject to circular loading are presented.
Axisymmetric fundamental solutions for a finite layer with impeded boundaries
Institute of Scientific and Technical Information of China (English)
程泽海; 陈云敏; 凌道盛; 唐晓武
2003-01-01
Axisymmetric fundamental solutions that are applied in the consolidation calculations of a finite clay layer with impeded boundaries were derived. Laplace and Hankel integral transforms were utilized with respect to time and radial coordinates, respectively in the analysis. The derivation of fundamental solutions considers two boundary-value problems involving unit point loading and ring loading in the vertical. The solutions are extended to circular distributed and strip distributed normal load. The computation and analysis of settlements, vertical total stress and excess pore pressure in the consolidation layer subject to circular loading are presented.
Lower Atmospheric Boundary Layer Experiment (LABLE) Final Campaign Report
Energy Technology Data Exchange (ETDEWEB)
Klein, P [University of Oklahoma - School of Meteorology; Bonin, TA; Newman, JF [National Renewable Energy Laboratory; Turner, DD [National Oceanic and Atmospheric Administration; Chilson, P [University of Oklahoma; Blumberg, WG [University of Oklahoma; Mishra, S; Wainwright, CE; Carney, M [University of Oklahoma - School of Meteorology; Jacobsen, EP [University of Oklahoma; Wharton, S [Lawrence Livermore National Laboratory
2015-11-01
The Lower Atmospheric Boundary Layer Experiment (LABLE) included two measurement campaigns conducted at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site in Oklahoma during 2012 and 2013. LABLE was designed as a multi-phase, low-cost collaboration among the University of Oklahoma, the National Severe Storms Laboratory, Lawrence Livermore National Laboratory, and the ARM program. A unique aspect was the role of graduate students in LABLE. They served as principal investigators and took the lead in designing and conducting experiments using different sampling strategies to best resolve boundary-layer phenomena.
Rough-wall turbulent boundary layers with constant skin friction
Sridhar, A.
2017-03-28
A semi-empirical model is presented that describes the development of a fully developed turbulent boundary layer in the presence of surface roughness with length scale ks that varies with streamwise distance x . Interest is centred on flows for which all terms of the von Kármán integral relation, including the ratio of outer velocity to friction velocity U+∞≡U∞/uτ , are streamwise constant. For Rex assumed large, use is made of a simple log-wake model of the local turbulent mean-velocity profile that contains a standard mean-velocity correction for the asymptotic fully rough regime and with assumed constant parameter values. It is then shown that, for a general power-law external velocity variation U∞∼xm , all measures of the boundary-layer thickness must be proportional to x and that the surface sand-grain roughness scale variation must be the linear form ks(x)=αx , where x is the distance from the boundary layer of zero thickness and α is a dimensionless constant. This is shown to give a two-parameter (m,α) family of solutions, for which U+∞ (or equivalently Cf ) and boundary-layer thicknesses can be simply calculated. These correspond to perfectly self-similar boundary-layer growth in the streamwise direction with similarity variable z/(αx) , where z is the wall-normal coordinate. Results from this model over a range of α are discussed for several cases, including the zero-pressure-gradient ( m=0 ) and sink-flow ( m=−1 ) boundary layers. Trends observed in the model are supported by wall-modelled large-eddy simulation of the zero-pressure-gradient case for Rex in the range 108−1010 and for four values of α . Linear streamwise growth of the displacement, momentum and nominal boundary-layer thicknesses is confirmed, while, for each α , the mean-velocity profiles and streamwise turbulent variances are found to collapse reasonably well onto z/(αx) . For given α , calculations of U+∞ obtained from large-eddy simulations are streamwise
Calculation of a boundary layer with phase transformations
Dorosh, N. D.; Kharitonov, A. A.
A method for the analysis of a laminar boundary layer with phase transformations is developed. It is noted that volume gas condensation can occur in the case of flow past a cooled surface, drops becoming aggregated in groups in the process of condensation. The concept of group density and concentration is proposed, and this approach is used to investigate a boundary layer near the stagnation point of a two-dimensional blunt body in a flow of molecular oxygen. Profiles of temperature, stream function, and concentration of liquid-oxygen droplet groups are determined for various values of the condensation rate.
Turbulent boundary layer on perforated surfaces with vector injection
Eroshenko, V. M.; Zaichik, L. I.; Klimov, A. A.; Ianovskii, L. S.; Kondratev, V. I.
1980-10-01
The paper presents an experimental investigation of a turbulent boundary layer on perforated plates with uniform vector injection at various angles to gas flow. It was shown that with strong injection at angles oriented in the flow direction the intensity of turbulent pulsation is decreased, while injection at angles in the opposite direction increase the intensity. A relationship was established between the critical parameters of the boundary layer injection angles; it was concluded that the asymptotic theory of Kutateladze and Leontiev can be used for determining the coefficient of friction of vector injection.
Non-Equilibrium Effects on Hypersonic Turbulent Boundary Layers
Kim, Pilbum
Understanding non-equilibrium effects of hypersonic turbulent boundary layers is essential in order to build cost efficient and reliable hypersonic vehicles. It is well known that non-equilibrium effects on the boundary layers are notable, but our understanding of the effects are limited. The overall goal of this study is to improve the understanding of non-equilibrium effects on hypersonic turbulent boundary layers. A new code has been developed for direct numerical simulations of spatially developing hypersonic turbulent boundary layers over a flat plate with finite-rate reactions. A fifth-order hybrid weighted essentially non-oscillatory scheme with a low dissipation finite-difference scheme is utilized in order to capture stiff gradients while resolving small motions in turbulent boundary layers. The code has been validated by qualitative and quantitative comparisons of two different simulations of a non-equilibrium flow and a spatially developing turbulent boundary layer. With the validated code, direct numerical simulations of four different hypersonic turbulent boundary layers, perfect gas and non-equilibrium flows of pure oxygen and nitrogen, have been performed. In order to rule out uncertainties in comparisons, the same inlet conditions are imposed for each species, and then mean and turbulence statistics as well as near-wall turbulence structures are compared at a downstream location. Based on those comparisons, it is shown that there is no direct energy exchanges between internal and turbulent kinetic energies due to thermal and chemical non-equilibrium processes in the flow field. Instead, these non-equilibria affect turbulent boundary layers by changing the temperature without changing the main characteristics of near-wall turbulence structures. This change in the temperature induces the changes in the density and viscosity and the mean flow fields are then adjusted to satisfy the conservation laws. The perturbation fields are modified according to
Conference on Boundary and Interior Layers : Computational and Asymptotic Methods
2015-01-01
This volume offers contributions reflecting a selection of the lectures presented at the international conference BAIL 2014, which was held from 15th to 19th September 2014 at the Charles University in Prague, Czech Republic. These are devoted to the theoretical and/or numerical analysis of problems involving boundary and interior layers and methods for solving these problems numerically. The authors are both mathematicians (pure and applied) and engineers, and bring together a large number of interesting ideas. The wide variety of topics treated in the contributions provides an excellent overview of current research into the theory and numerical solution of problems involving boundary and interior layers. .
Boundary stabilization of wave equations with variable coefficients
Institute of Scientific and Technical Information of China (English)
FENG; Shaoji
2001-01-01
［1］Chen, G., Energy decay estimates and exact boundary value controllability for the wave equation in a bounded domain, J. Math. Pures. & Appl., 1979, 58: 249.［2］Komornik, V., Exact controllability and stabilization, Research in Applied Mathematics (Series Editors: Ciarlet, P. G., Lions, J.), New York: Masson/John Wiley copublication, 1994.［3］Komornik, V., Zuazua, E., A direct method for the boundary stabilization of the wave equation, J. Math. Pures. & Appl., 1990, 69: 33.［4］Lagnese, J., Decay of solutions of wave equations in a bounded region with boundary dissipation, J. Differential Equations, 1983, 50: 163.［5］Lasiecka, I., Triggiani, R., Uniform stabilization of the wave equation with Dirichlet or Neumann feedback control without geometrical conditions, Appl. Math. & Optim., 1992, 25: 189.［6］Wyler, A., Stability of wave equations with dissipative boundary conditions in a bounded domain, Differential and Integral Equations,1994, 7: 345.［7］Yao, P. F., On the observability inequality for exact controllability of wave equations with variable coefficients, SIAM J. Control & Optimization, 1999, 37, 5: 1568.［8］Wu, H., Shen, C. L., Yu, Y. L., Introduction to Riemannian Geometry (in Chinese), Beijing: Peking University Press, 1989.
A bursting phenomenon in a vortex-gas boundary layer
Sekaran, Aarthi; Narasimha, Roddam; Govindarajan, Rama
2014-11-01
Bursts are a central phenomenon in turbulent boundary layers as they are an integral part of turbulent energy and stress production. They have consequently been a continuing area of interest since the 1970s following the detailed investigations of Kline et al. (1967). Despite several attempts to understand their dynamics, it has been difficult to arrive at a consensus even on the scaling of the burst frequency. The present investigation simulates the outer part of a plane turbulent boundary layer using the vortex-gas model, in a first step towards understanding the role of the outer layer in boundary layer dynamics. Preliminary results indicate the formation of regions of concentrated vorticity near the wall, at a frequency that is independent of the initial vortex configuration but a function of the mean velocity profile. Further, comparisons with existing experimental data indicate a burst frequency which when scaled on outer variables, is within the range of scatter among different studies. Quadrant occupancy statistics are also related to those in conventional boundary layers. It appears as if a bursting phenomenon of some kind may be a general feature of an inviscid, wall-bounded shear flow, and does not necessitate inclusion of either viscosity or three-dimensionality.
Analysis of differential infrared thermography for boundary layer transition detection
Gardner, A. D.; Eder, C.; Wolf, C. C.; Raffel, M.
2017-09-01
This paper presents an analysis of the differential infrared thermography (DIT) technique, a contactless method of measuring the unsteady movement of the boundary layer transition position on an unprepared surface. DIT has been shown to measure boundary layer transition positions which correlate well with those from other measurement methods. In this paper unsteady aerodynamics from a 2D URANS solution are used and the resulting wall temperatures computed. It is shown that the peak of the temperature difference signal correlates well with the boundary layer transition position, but that the start and end of boundary layer transition cannot be extracted. A small systematic time-lag cannot be reduced by using different surface materials, but the signal strength can be improved by reducing the heat capacity and heat transfer of the surface layer, for example by using a thin plastic coating. Reducing the image time separation used to produce the difference images reduces the time-lag and also the signal level, thus the optimum is when the signal to noise ratio is at the minimum which can be evaluated.
Boundary-reflected waves and ultrasonic coda waves in rock physics experiments
Fu, Bo-Ye; Fu, Li-Yun; Wei, Wei; Zhang, Yan
2016-12-01
Ultrasonic coda waves are widely used to study high-frequency scattering. However, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-reflected waves, we performed ultrasonic experiments using aluminum and shale samples, and the rotating staggered-mesh finite-difference method to simulate the wavefield. We analyzed the wavefield characteristics at the different receiving points and the interference characteristics of the boundary-reflected waves with the ultrasonic coda wave, and the effect of sample geometry on the ultrasonic coda waves. The increase in the aspect ratio of the samples delays the interference effect of the laterally reflected waves and reduces the effect on the ultrasonic coda waves. The main waves interfering with the ultrasonic coda waves are laterally reflected PP-, PS-, PPP-, and PPS-waves. The scattering and attenuation of the high-frequency energy in actual rocks can weaken the interference of laterally reflected waves with the ultrasonic coda waves.
Energy Technology Data Exchange (ETDEWEB)
Vijayakumar, Ganesh [National Renewable Energy Lab. (NREL), Golden, CO (United States); Pennsylvania State Univ., University Park, PA (United States); Brasseur, James [Pennsylvania State Univ., University Park, PA (United States); Univ. of Colorado, Boulder, CO (United States); Lavely, Adam; Jayaraman, Balaji; Craven, Brent
2016-01-04
We describe the response of the NREL 5 MW wind turbine blade boundary layer to the passage of atmospheric turbulence using blade-boundary-layer-resolved computational fluid dynamics with hybrid URANS-LES modeling.
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.
Sergeev, Daniil; Kandaurov, Alexander; Troitskaya, Yuliya; Vdovin, Maxim
2016-11-01
The processes of momentum and heat transfer between ocean and atmosphere in the boundary layer were investigated within laboratory modeling for a wide range of wind speed and surface wave including hurricane conditions. Experiments were carried out on the Wind-Wave Flume of the Large Thermostratified Tank of IAP RAS. A special net located under the surface at different depths allows to vary parameters of surface waves independently on wind parameters. Theory of self-similarity of air flow parameters in the flume was used to calculate values aerodynamic and heat transfer coefficients from the measured velocity and temperature profiles by Pito and hotfilm gauges respectively. Simultaneous measurements of surface elevation with system wire allow to obtain spectra and integral parameters of waves. It was demonstrated that in contrast to the drag coefficient, heat transfer coefficient is virtually independent of wind speed and wave parameters to the moment of the beginning of spray generation and then increases rapidly.
Directory of Open Access Journals (Sweden)
Sergeev Daniil
2017-01-01
Full Text Available The processes of momentum and heat transfer between ocean and atmosphere in the boundary layer were investigated within laboratory modeling for a wide range of wind speed and surface wave including hurricane conditions. Experiments were carried out on the Wind-Wave Flume of the Large Thermostratified Tank of IAP RAS. A special net located under the surface at different depths allows to vary parameters of surface waves independently on wind parameters. Theory of self-similarity of air flow parameters in the flume was used to calculate values aerodynamic and heat transfer coefficients from the measured velocity and temperature profiles by Pito and hotfilm gauges respectively. Simultaneous measurements of surface elevation with system wire allow to obtain spectra and integral parameters of waves. It was demonstrated that in contrast to the drag coefficient, heat transfer coefficient is virtually independent of wind speed and wave parameters to the moment of the beginning of spray generation and then increases rapidly.
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,...
On the impact of adverse pressure gradient on the supersonic turbulent boundary layer
Wang, Qian-Cheng; Wang, Zhen-Guo; Zhao, Yu-Xin
2016-11-01
By employing the particle image velocimetry, the mean and turbulent characteristics of a Mach 2.95 turbulent boundary layer are experimentally investigated without the impact of curvature. The physical mechanism with which the streamwise adverse pressure gradient affects the supersonic boundary layer is revealed. The data are compared to that of the concave boundary layer with similar streamwise distributions of wall static pressure to clarify the separate impacts of the adverse pressure gradient and the concave curvature. The logarithmic law is observed to be well preserved for both of the cases. The dip below the logarithmic law is not observed in present investigation. Theoretical analysis indicates that it could be the result of compromise between the opposite impacts of the compression wave and the increased turbulent intensity. Compared to the zero pressure gradient boundary layer, the principal strain rate and the turbulent intensities are increased by the adverse pressure gradient. The shear layer formed due the hairpin packets could be sharpened by the compression wave, which leads to higher principal strain rate and the associated turbulent level. Due to the additional impact of the centrifugal instability brought by the concave wall, even higher turbulent intensities than that of the adverse pressure gradient case are introduced. The existence of velocity modes within the zero pressure gradient boundary layer suggests that the large scale motions are statistically well organized. The generation of new velocity modes due to the adverse pressure gradient indicates that the turbulent structure is changed by the adverse pressure gradient, through which more turbulence production that cannot be effectively predicted by the Reynolds-stress transport equations could be brought.
Effects of boundary layer and liquid viscosity and compressible air on sloshing characteristics
Zou, Chang-Fang; Wang, De-Yu; Cai, Zhong-Hua
2015-07-01
In this paper, numerical investigations for tank sloshing, based on commercial CFD package FLUENT, are performed to study effects of boundary layer grid, liquid viscosity and compressible air on sloshing pressure, wave height and rising time of impact pressure. Also, sloshing experiments for liquids of different viscosity are carried out to validate the numerical results. Through comparison of numerical and experimental results, a computational model including boundary layer grid can predict the sloshing pressure more accurately. Energy dissipation due to viscous friction leads to reduction of sloshing pressure and wave elevation. Sloshing pressure is also reduced because of cushion effect of compressible air. Due to high viscosity damping effect and compressible air effect, the rising time of impact pressure becomes longer. It is also found that liquid viscosity and compressible air influence distribution of dynamic pressure along the vertical tank wall.
Reflection of acoustic wave from the elastic seabed with an overlying gassy poroelastic layer
Chen, Weiyun; Wang, Zhihua; Zhao, Kai; Chen, Guoxing; Li, Xiaojun
2015-10-01
Based on the multiphase poroelasticity theory, the reflection characteristics of an obliquely incident acoustic wave upon a plane interface between overlying water and a gassy marine sediment layer with underlying elastic solid seabed are investigated. The sandwiched gassy layer is modelled as a porous material with finite thickness, which is saturated by two compressible and viscous fluids (liquid and gas). The closed-form expression for the amplitude ratio of the reflected wave, called reflection coefficient, is derived theoretically according to the boundary conditions at the upper and lower interfaces in our proposed model. Using numerical calculation, the influences of layer thickness, incident angle, wave frequency and liquid saturation of sandwiched porous layer on the reflection coefficient are analysed, respectively. It is revealed that the reflection coefficient is closely associated with incident angle and sandwiched layer thickness. Moreover, in different frequency ranges, the dependence of the wave reflection characteristics on moisture (or gas) variations in the intermediate marine sediment layer is distinguishing.
The implementation of an improved NPML absorbing boundary condition in elastic wave modeling
Institute of Scientific and Technical Information of China (English)
Qin Zhen; Lu Minghui; Zheng Xiaodong; Yao Yao; Zhang Cai; Song Jianyong
2009-01-01
In elastic wave forward modeling, absorbing boundary conditions (ABC) are used to mitigate undesired reflections from the model truncation boundaries. The perfectly matched layer (PML) has proved to be the best available ABC. However, the traditional splitting PML (SPML) ABC has some serious disadvantages: for example, global SPML ABCs require much more computing memory, although the implementation is easy. The implementation of local SPML ABCs also has some difficulties, since edges and comers must be considered. The traditional non-splitting perfectly matched layer (NPML) ABC has complex computation because of the convolution. In this paper, based on non-splitting perfectly matched layer (NPML) ABCs combined with the complex frequency-shifted stretching function (CFS), we introduce a novel numerical implementation method for PML absorbing boundary conditions with simple calculation equations, small memory requirement, and easy programming.
Routes to turbulence in the rotating disk boundary-layer of a rotor-stator cavity
Yim, Eunok; Serre, Eric; Martinand, Denis; Chomaz, Jean-Marc
2016-11-01
The rotating disk is an important classical problem, due to the similarities between the 3D boundary layers on a disk and a swept aircraft wing. It is nowadays admitted that a direct transition to turbulence may exist through a steep-fronted nonlinear global mode located at the boundary between the locally connectively and absolutely unstable regions (Pier 2003; Viaud et al. 2008, 2011; Imayama et al. 2014 and others). However, recent studies (Healey 2010; Harris et al. 2012; Imayama et al. 2013) suggest that there may be an alternative route starting at lower critical Reynolds number, based on convective travelling waves but this scenario is still not fully validated and proven. To better characterize such transition, direct numerical simulations are performed in a closed cylindrical rotor-stator cavity (without hub) up to Re = O (105) . All boundaries are no slip and for the stable region around the rotation axis prevents the disturbances coming from the very unstable stator boundary to disturb the rotor boundary layer. Different transition scenarii to turbulence are investigated when the rotor boundary layer is forced at different positions and forcing amplitude. The associated dynamics of coherent structures in various flow regions are also investigated when increasing Re .
On the Stability of Nonlinear Viscous Vortices in Three-Dimensional Boundary Layers
1992-04-01
wave disturbances in stable and unsta- ble parallel flows , Part 2. The development of a solution for plane Poiseuille and plane Couette flow . J. Fluid...unstable parallel flows , Part 1. The basic behaviour in plane Poiseuille flow . J. Fluid Mech. 9, 353-370. Watson, J. 1960 On the nonlinear mechanics of...vortices which a particular boundary layer may support. According to a linearised theory vortices within a high G6rtler number flow can take one of
Secondary subharmonic instability of boundary layers with pressure gradient and suction
El-Hady, Nabil M.
1988-01-01
Three-dimensional linear secondary instability is investigated for boundary layers with pressure gradient and suction in the presence of a finite amplitude TS wave. The focus is on principal parametric resonance responsible for a strong growth of subharmonics in a low disturbance environment. Calculations are presented for the effect of pressure gradients and suction on controlling the onset and amplification of the secondary instability.
Turbulent dusty boundary layer in an ANFO surface-burst explosion
Kuhl, A. L.; Ferguson, R. E.; Chien, K. Y.; Collins, J. P.
1992-01-01
This paper describes the results of numerical simulations of the dusty, turbulent boundary layer created by a surface burst explosion. The blast wave was generated by the detonation of a 600-T hemisphere of ANFO, similar to those used in large-scale field tests. The surface was assumed to be ideally noncratering but contained an initial loose layer of dust. The dust-air mixture in this fluidized bed was modeled as a dense gas (i.e., an equilibrium model, valid for very small-diameter dust particles). The evolution of the flow was calculated by a high-order Godunov code that solves the nonsteady conservation laws. Shock interactions with dense layer generated vorticity near the wall, a result that is similar to viscous, no-slip effects found in clean flows. The resulting wall shear layer was unstable, and rolled up into large-scale rotational structures. These structures entrained dense material from the wall layer and created a chaotically striated flow. The boundary layer grew due to merging of the large-scale structures and due to local entrainment of the dense material from the fluidized bed. The chaotic flow was averaged along similarity lines (i.e., lines of constant values of x = r/Rs and y = z/Rs where R(sub s) = ct(exp alpha)) to establish the mean-flow profiles and the r.m.s. fluctuating-flow profiles of the boundary layer.
Directory of Open Access Journals (Sweden)
Rajneesh Kumar
2014-01-01
Full Text Available The present investigation deals with the propagation of Rayleigh type surface waves in an isotropic microstretch thermoelastic diffusion solid half space under a layer of inviscid liquid. The secular equation for surface waves in compact form is derived after developing the mathematical model. The dispersion curves giving the phase velocity and attenuation coefficients with wave number are plotted graphically to depict the effect of an imperfect boundary alongwith the relaxation times in a microstretch thermoelastic diffusion solid half space under a homogeneous inviscid liquid layer for thermally insulated, impermeable boundaries and isothermal, isoconcentrated boundaries, respectively. In addition, normal velocity component is also plotted in the liquid layer. Several cases of interest under different conditions are also deduced and discussed.
Irregularity excitation associated with charged dust cloud boundary layers
Mahmoudian, A.; Scales, W. A.
2012-02-01
Irregularity generation associated with dust cloud expansion through a background plasma along a magnetic field is investigated. Because of the dust charging process, a boundary layer is produced, separating the dusty plasma generated and the background plasma. It is observed that under appropriate conditions, localized plasma irregularities may be generated in this boundary layer. Theoretical and computational models are used to study the evolution of relevant plasma instabilities thought to play a dominant role in irregularity production. An electron flow develops along the boundary layer of the dust cloud, and plasma irregularities are generated in response to this flow. Several aspects of the cloud's structure (thickness of the boundary layer, average particle size and density, collisional processes, and cloud expansion speed) and the ambient plasma are varied to determine the effect of these quantities on the resulting irregularities. The relevance of these results to past experimental observations in space and the laboratory for applications to the expansion of naturally or artificially created dust clouds is discussed.
Flow visualization of swept wing boundary layer transition
Serpieri, J.; Kotsonis, M.
2015-01-01
In this work the flow visualization of the transition pattern occurring on a swept wing in a subsonic flow is presented. This is done by means of fluorescent oil flow technique and boundary layer hot-wire scans. The experiment was performed at Reynolds number of 2:15 . 106 and at angle of attack of
Body surface adaptations to boundary-layer dynamics
Videler, J.J.
1995-01-01
Evolutionary processes have adapted nektonic animals to interact efficiently with the water that surrounds them. Not all these adaptations serve the same purpose. This paper concentrates on reduction of drag due to friction in the boundary layer close to the body surface. Mucus, compliant skins, sca
DNS of stratified spatially-developing turbulent thermal boundary layers
Araya, Guillermo; Castillo, Luciano; Jansen, Kenneth
2012-11-01
Direct numerical simulations (DNS) of spatially-developing turbulent thermal boundary layers under stratification are performed. It is well known that the transport phenomena of the flow is significantly affected by buoyancy, particularly in urban environments where stable and unstable atmospheric boundary layers are encountered. In the present investigation, the Dynamic Multi-scale approach by Araya et al. (JFM, 670, 2011) for turbulent inflow generation is extended to thermally stratified boundary layers. Furthermore, the proposed Dynamic Multi-scale approach is based on the original rescaling-recycling method by Lund et al. (1998). The two major improvements are: (i) the utilization of two different scaling laws in the inner and outer parts of the boundary layer to better absorb external conditions such as inlet Reynolds numbers, streamwise pressure gradients, buoyancy effects, etc., (ii) the implementation of a Dynamic approach to compute scaling parameters from the flow solution without the need of empirical correlations as in Lund et al. (1998). Numerical results are shown for ZPG flows at high momentum thickness Reynolds numbers (~ 3,000) and a comparison with experimental data is also carried out.
Boundary layer structure in turbulent Rayleigh-Benard convection
Shi, Nan; Schumacher, Joerg
2012-01-01
The structure of the boundary layers in turbulent Rayleigh-Benard convection is studied by means of three-dimensional direct numerical simulations. We consider convection in a cylindrical cell at an aspect ratio one for Rayleigh numbers of Ra=3e+9 and 3e+10 at fixed Prandtl number Pr=0.7. Similar to the experimental results in the same setup and for the same Prandtl number, the structure of the laminar boundary layers of the velocity and temperature fields is found to deviate from the prediction of the Prandtl-Blasius-Pohlhausen theory. Deviations decrease when a dynamical rescaling of the data with an instantaneously defined boundary layer thickness is performed and the analysis plane is aligned with the instantaneous direction of the large-scale circulation in the closed cell. Our numerical results demonstrate that important assumptions which enter existing classical laminar boundary layer theories for forced and natural convection are violated, such as the strict two-dimensionality of the dynamics or the s...
Turbulent boundary layer measurements over high-porosity surfaces
Efstathiou, Christoph; Luhar, Mitul
2016-11-01
Porous surfaces are ubiquitous across a variety of turbulent boundary layer flows of scientific and engineering interest. While turbulent flows over smooth and rough walls have been studied extensively, experimental measurements over porous walls have thus far focused on packed beds, which are limited in porosity (Φ = 0 . 3 - 0 . 5) by their geometry. The current project seeks to address this limitation. A two-component laser doppler velocimeter (LDV) is used to generate velocity measurements in turbulent boundary layer flows over commercially available reticulated foams and 3D-printed porous media at Reynolds number Reθ 3000 - 4000 . Smooth wall profiles for mean and turbulent quantities are compared to data over substrates with porosity Φ > 0 . 8 and average pore sizes in the range 0.4-2.5mm (corresponding to 8 - 50 viscous units). Previous analytical and simulation efforts indicate that the effects of porous substrates on boundary layer flows depend on a modified Reynolds number defined using the length scale √{ κ}, where κ is substrate permeability. A custom permeameter is currently being developed to estimate κ for the substrates tested in the boundary layer experiments.
Stability of the laminar boundary layer for an imperfect gas
Gasperas, G.
The linear perturbation equations are derived for the general case of a compressible imperfect gas characterized by an equation of state utilizing a compressibility factor. The specific case of the Beattie-Bridgeman gas is chosen for calculation. Amplification curves calculated using the Beattie-Bridgeman equation of state for two representative flat plate boundary layers are presented.
Boundary Layer Flows in Porous Media with Lateral Mass Flux
DEFF Research Database (Denmark)
Nemati, H; H, Bararnia; Noori, F;
2015-01-01
Solutions for free convection boundary layers on a heated vertical plate with lateral mass flux embedded in a saturated porous medium are presented using the Homotopy Analysis Method and Shooting Numerical Method. Homotopy Analysis Method yields an analytic solution in the form of a rapidly...
Vortex Generator Induced Flow in a High Re Boundary Layer
DEFF Research Database (Denmark)
Velte, Clara Marika; Braud, C.; Coudert, S.
2012-01-01
Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow...
Vortex Generator Induced Flow in a High Re Boundary Layer
DEFF Research Database (Denmark)
Velte, Clara Marika; Braud, C.; Coudert, S.
2014-01-01
Stereoscopic Particle Image Velocimetry measurements have been conducted in cross-planes behind three different geometries of Vortex Generators (VGs) in a high Reynolds number boundary layer. The VGs have been mounted in a cascade producing counter-rotating vortices and the downstream flow...
Boundary-Layer Wind Structure in a Landfalling Tropical Cyclone
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
In this study, a slab boundary layer model with a constant depth is used to analyze the boundary-layer wind structure in a landfalling tropical cyclone. Asymmetry is found in both the tangential and radial components of horizontal wind in the tropical cyclone boundary layer at landfall. For a steady tropical cyclone on a straight coastline at landfall, the magnitude of the radial component is greater in the offshoreflow side and the tangential component is greater over the sea, slightly offshore, therefore the greater total wind speed occurs in the offshore-flow side over the sea. The budget analysis suggests that: (1) a greater surface friction over land produces a greater inflow and the nonlinear effect advects the maximum inflow downstream, and (2) a smaller surface friction over the sea makes the decrease of the tangential wind component less than that over land. Moreover, the boundary layer wind structures in a tropical cyclone are related to the locations of the tropical cyclone relative to the coastline due to the different surface frictions. During tropical cyclone landfall, the impact of rough terrain on the cyclone increases, so the magnitude of the radial component of wind speed increases in the offshore-flow side and the tangential component outside the radius of maximum wind speed decreases gradually.
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...
The boundary layer growth in an urban area
Pino, D.; Vilà-Guerau de Arellano, J.; Comerón, A.; Rocadenbosch, F.
2004-01-01
The development and maintenance of the atmospheric boundary layer (ABL) plays a key role in the distribution of atmospheric constituents, especially in a polluted urban area. In particular, the ABL has a direct impact on the concentration and transformation of pollutants. In this work, in order to a
Response of neutral boundary-layers to changes of roughness
DEFF Research Database (Denmark)
Sempreviva, Anna Maria; Larsen, Søren Ejling; Mortensen, Niels Gylling
1990-01-01
When air blows across a change in surface roughness, an internal boundary layer (IBL) develops within which the wind adapts to the new surface. This process is well described for short fetches, > 1 km. However, few data exist for large fetches on how the IBL grows to become a new equilibrium boun...
Drizzle and Turbulence Variability in Stratocumulus-topped Boundary Layers
Kollias, P.; Luke, E. P.; Szyrmer, W.
2015-12-01
Marine stratocumulus clouds frequently produce light precipitation in the form of drizzle. The drizzle rate at the cloud base (RCB) dictates the impact of drizzle on the boundary layer turbulence and cloud organization. Here, synergistic observations from the US Department of Energy Atmospheric Radiation Measurement (ARM) program Eastern North Atlantic (ENA) site located on Graciosa Island in the Azores are used to investigate the relationship between RCB, and boundary layer turbulence and dynamics. The ARM ENA site is a heavily instrumented ground-based facility that offers new measurement capabilities in stratocumulus-topped boundary layers (STBL). The RCB is retrieved using a radar-lidar algorithm. The STBL turbulent structure is characterized using the Doppler lidar and radar observations. The profiling radar/lidar/radiometer observations are used to describe the cloud fraction and morphology. Finally, surface-based aerosol number concentration measurements are used to investigate the connection between the boundary layer turbulence, cloud morphology and aerosol loading. Preliminary correlative relationships between the aforementioned variables will be shown.
Body surface adaptations to boundary-layer dynamics
Videler, J.J.
1995-01-01
Evolutionary processes have adapted nektonic animals to interact efficiently with the water that surrounds them. Not all these adaptations serve the same purpose. This paper concentrates on reduction of drag due to friction in the boundary layer close to the body surface. Mucus, compliant skins,
Convective boundary layers driven by nonstationary surface heat fluxes
Van Driel, R.; Jonker, H.J.J.
2011-01-01
In this study the response of dry convective boundary layers to nonstationary surface heat fluxes is systematically investigated. This is relevant not only during sunset and sunrise but also, for example, when clouds modulate incoming solar radiation. Because the time scale of the associated change
Boundary Layer Simulation and Control in Wind Tunnels
1988-04-01
Vol. 1, No. 4, April 1963, pp. 931-933. AIAA 11. Maybe, Dennis G.: Some Remarks on Dynamic Aeroelastic Model Tests in Cryogenic Wind Tunnels...distribution). 42. Lindhout, J.P.F, Moek , C., Boer, E. de and Berg, B. van den: A Method for the Calculation of 3D Boundary Layers on Practical Wing
Turbulent Boundary Layer on a Cylinder in Axial Flow
1988-09-29
wall- norma 6caling or Rao’s wall-normal scaling. Other measurements of the mean velocity in a cylindrical boundary layer should be mentioned for...located near the wall at three azimuthal locations that wre 900 apa ,-t and at several streamwise spacings for flow conditions resulting in 8/a=8
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.
Entrainment process of carbon dioxide in the atmospheric boundary layer
Vilà-Guerau de Arellano, J.; Gioli, B.; Miglietta, F.; Jonker, H.J.J.; Klein Baltink, H.; Hutjes, R.W.A.; Holtslag, A.A.M.
2004-01-01
Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dio
Hair receptor sensitivity to changes in laminar boundary layer shape
Energy Technology Data Exchange (ETDEWEB)
Dickinson, B T, E-mail: btdickinson@lifetime.oregonstate.ed [Air Force Research Laboratory, Munitions Directorate, Eglin Air Force Base, FL 32542 (United States)
2010-03-15
Biologists have shown that bat wings contain distributed arrays of flow-sensitive hair receptors. The hair receptors are hypothesized to feedback information on airflows over the bat wing for enhanced stability or maneuverability during flight. Here, we study the geometric specialization of hair-like structures for the detection of changes in boundary layer velocity profiles (shapes). A quasi-steady model that relates the flow velocity profile incident on the longitudinal axis of a hair to the resultant moment and shear force at the hair base is developed. The hair length relative to the boundary layer momentum thickness that maximizes the resultant moment and shear-force sensitivity to changes in boundary layer shape is determined. The sensitivity of the resultant moment and shear force is shown to be highly dependent on hair length. Hairs that linearly taper to a point are shown to provide greater output sensitivity than hairs of uniform cross-section. On an order of magnitude basis, the computed optimal hair lengths are in agreement with the range of hair receptor lengths measured on individual bat species. These results support the hypothesis that bats use hair receptors for detecting changes in boundary layer shape and provide geometric guidelines for artificial hair sensor design and application.
Hair receptor sensitivity to changes in laminar boundary layer shape.
Dickinson, B T
2010-03-01
Biologists have shown that bat wings contain distributed arrays of flow-sensitive hair receptors. The hair receptors are hypothesized to feedback information on airflows over the bat wing for enhanced stability or maneuverability during flight. Here, we study the geometric specialization of hair-like structures for the detection of changes in boundary layer velocity profiles (shapes). A quasi-steady model that relates the flow velocity profile incident on the longitudinal axis of a hair to the resultant moment and shear force at the hair base is developed. The hair length relative to the boundary layer momentum thickness that maximizes the resultant moment and shear-force sensitivity to changes in boundary layer shape is determined. The sensitivity of the resultant moment and shear force is shown to be highly dependent on hair length. Hairs that linearly taper to a point are shown to provide greater output sensitivity than hairs of uniform cross-section. On an order of magnitude basis, the computed optimal hair lengths are in agreement with the range of hair receptor lengths measured on individual bat species. These results support the hypothesis that bats use hair receptors for detecting changes in boundary layer shape and provide geometric guidelines for artificial hair sensor design and application.
Three dimensional boundary layers on submarine conning towers and rudders
Gleyzes, C.
1988-01-01
Solutions for the definition of grids adapted to the calculation of three-dimensional boundary layers on submarine conning towers and on submarine rudders and fins are described. The particular geometry of such bodies (oblique shaped hull, curved fins) required special adaptations. The grids were verified on examples from a test basin.
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.
Modeling of particulate plumes transportation in boundary layers with obstacles
Karelsky, K. V.; Petrosyan, A. S.
2012-04-01
This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high
Transportation of particulate plumes in boundary layer with obstacles
Petrosyan, A.; Karelsky, K.; Smirnov, I.
2010-05-01
This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high
Transport of Particulates in Boundary Layer with Obstacles
Karelsky, Kirill; Petrosyan, Arakel
2014-05-01
This presentation is aimed at creating and realization of new physical model of impurity transfer (solid particles and heavy gases) in areas with non-flat and/or nonstationary boundaries. The main idea of suggested method is to use non-viscous equations for solid particles transport modeling in the vicinity of complex boundary. In viscous atmosphere with as small as one likes coefficient of molecular viscosity, the non-slip boundary condition on solid surface must be observed. This postulates the reduction of velocity to zero at a solid surface. It is unconditionally in this case Prandtle hypothesis must be observed: for rather wide range of conditions in the surface neighboring layers energy dissipation of atmosphere flows is comparable by magnitude with manifestation of inertia forces. That is why according to Prandtle hypothesis in atmosphere movement characterizing by a high Reynolds number the boundary layer is forming near a planet surface, within which the required transition from zero velocities at the surface to magnitudes at the external boundary of the layer that are quite close to ones in ideal atmosphere flow. In that layer fast velocity gradients cause viscous effects to be comparable in magnitude with inertia forces influence. For conditions considered essential changes of hydrodynamic fields near solid boundary caused not only by nonslip condition but also by a various relief of surface: mountains, street canyons, individual buildings. Transport of solid particles, their ascent and precipitation also result in dramatic changes of meteorological fields. As dynamic processes of solid particles transfer accompanying the flow past of complex relief surface by wind flows is of our main interest we are to use equations of non-viscous hydrodynamic. We should put up with on the one hand idea of high wind gradients in the boundary layer and on the other hand disregard of molecular viscosity in two-phase atmosphere equations. We deal with describing high
Directory of Open Access Journals (Sweden)
Ravinder Kumar
2014-01-01
Full Text Available The present investigation is concerned with the study of propagation of shear waves in an anisotropic fluid saturated porous layer over a semi-infinite homogeneous elastic half-space lying under an elastic homogeneous layer with irregularity present at the interface with rigid boundary. The rectangular irregularity has been taken in the half-space. The dispersion equation for shear waves is derived by using the perturbation technique followed by Fourier transformation. Numerically, the effect of irregularity present is analysed. It is seen that the phase velocity is significantly influenced by the wave number and the depth of the irregularity. The variations of dimensionless phase velocity against dimensionless wave number are shown graphically for the different size of rectangular irregularities with the help of MATLAB.
Cumulative second-harmonic generation of Lamb waves propagating in a two-layered solid plate
Institute of Scientific and Technical Information of China (English)
Xiang Yan-Xun; Deng Ming-Xi
2008-01-01
The physical process of cumulative second-harmonic generation of Lamb waves propagating in a two-layered solid plate is presented by using the second-order perturbation and the technique of nonlinear reflection of acoustic waves at an interface.In general,the cumulative second-harmonic generation of a dispersive guided wave propagation does not occur.However,the present paper shows that the second-harmonic of Lamb wave propagation arising from the nonlinear interaction of the partial bulk acoustic waves and the restriction of the three boundaries of the solid plates does have a cumulative growth effect if some conditions are satisfied.Through boundary condition and initial condition of excitation,the analytical expression of cumulative second-harmonic of Lamb waves propagation is determined.Numerical results show the cumulative effect of Lamb waves on second-harmonic field patterns.
Magnetic Domination of Recollimation Boundary Layers in Relativistic Jets
Kohler, Susanna
2012-01-01
We study the collimation of relativistic magnetohydrodynamic jets by the pressure of an ambient medium, in the limit where the jet interior loses causal contact with its surroundings. This follows up a hydrodynamic study in a previous paper, adding the effects of a toroidal magnetic field threading the jet. As the ultrarelativistic jet encounters an ambient medium with a pressure profile with a radial scaling of p ~ r^-eta where 2
New Theories on Boundary Layer Transition and Turbulence Formation
Directory of Open Access Journals (Sweden)
Chaoqun Liu
2012-01-01
Full Text Available This paper is a short review of our recent DNS work on physics of late boundary layer transition and turbulence. Based on our DNS observation, we propose a new theory on boundary layer transition, which has five steps, that is, receptivity, linear instability, large vortex structure formation, small length scale generation, loss of symmetry and randomization to turbulence. For turbulence generation and sustenance, the classical theory, described with Richardson's energy cascade and Kolmogorov length scale, is not observed by our DNS. We proposed a new theory on turbulence generation that all small length scales are generated by “shear layer instability” through multiple level ejections and sweeps and consequent multiple level positive and negative spikes, but not by “vortex breakdown.” We believe “shear layer instability” is the “mother of turbulence.” The energy transferring from large vortices to small vortices is carried out by multiple level sweeps, but does not follow Kolmogorov's theory that large vortices pass energy to small ones through vortex stretch and breakdown. The loss of symmetry starts from the second level ring cycle in the middle of the flow field and spreads to the bottom of the boundary layer and then the whole flow field.
Direct spatial resonance in the laminar boundary layer due to a rotating-disk
Indian Academy of Sciences (India)
M Turkyilmazoglu; J S B Gajjar
2000-12-01
Numerical treatment of the linear stability equations is undertaken to investigate the occurrence of direct spatial resonance events in the boundary layer flow due to a rotating-disk. A spectral solution of the eigenvalue problem indicates that algebraic growth of the perturbations shows up, prior to the amplification of exponentially growing instability waves. This phenomenon takes place while the flow is still in the laminar state and it also tends to persist further even if the non-parallelism is taken into account. As a result, there exists the high possibility of this instability mechanism giving rise to nonlinearity and transition, long before the unboundedly growing time-amplified waves.
Transient thermal response of turbulent compressible boundary layers
DEFF Research Database (Denmark)
Li, Hongwei; Nalim, M. Razi; Merkle, Charles L.
2011-01-01
and Smith, and the turbulent Prandtl number formulation originally developed by Kays and Crawford. The governing differential equations are discretized with the Keller-box method. The numerical accuracy is validated through grid-independence studies and comparison with the steady state solution......-dimensional semi-infinite flat plate. The compressible Reynolds-averaged boundary layer equations are transformed into incompressible form through the Dorodnitsyn-Howarth transformation and then solved with similarity transformations. Turbulence is modeled using a two-layer eddy viscosity model developed by Cebeci....... In turbulent flow as in laminar, the transient heat transfer rates are very different from that obtained from quasi-steady analysis. It is found that the time scale for response of the turbulent boundary layer to far-field temperature changes is 40% less than for laminar flow, and the turbulent local Nusselt...
Measuring Plume Meander in the Nighttime Stable Boundary Layer with Lidar
Hiscox, A.; Miller, D. R.; Nappo, C. J.
2009-12-01
Complex dynamics of the stable planetary boundary layer (PBL), such as the effects of density currents, intermittent turbulence, surface-layer decoupling, internal gravity waves, cold air pooling, and katabatic flows affect plume transport and diffusion. A better understanding of these effects is needed for nighttime transport model development. The JORNADA (Joint Observational Research on Nocturnal Atmospheric Dispersion of Aerosols) field campaign, conducted in the New Mexico desert during April 2005, sought to address some of these issues The JORNADA data set includes simultaneous micrometeorological measurements of the boundary layer structure, turbulence, and wave activity along with continuous lidar measurement of aerosol plume releases. What makes JORNADA unique is the real-time monitoring of an elevated plume with a lidar. The quantification of plume meander will be presented in this paper. The application of these techniques to the JORNADA data allows for a more complete understanding of the nocturnal boundary layer (NBL). We will present an in-depth analysis of lidar measurements of plume meander and dispersion and their relationship to the complexities of NBL structure.
A Diagnostic Diagram to Understand the Marine Atmospheric Boundary Layer at High Wind Speeds
Kettle, Anthony
2014-05-01
Long time series of offshore meteorological measurements in the lower marine atmospheric boundary layer show dynamical regimes and variability that are forced partly by interaction with the underlying sea surface and partly by the passage of cloud systems overhead. At low wind speeds, the dynamics and stability structure of the surface layer depend mainly on the air-sea temperature difference and the measured wind speed at a standard height. The physical processes are mostly understood and the quantified through Monin-Obukhov (MO) similarity theory. At high wind speeds different dynamical regimes become dominant. Breaking waves contribute to the atmospheric loading of sea spray and water vapor and modify the character of air-sea interaction. Downdrafts and boundary layer rolls associated with clouds at the top of the boundary layer impact vertical heat and momentum fluxes. Data from offshore meteorological monitoring sites will typically show different behavior and the regime shifts depending on the local winds and synoptic conditions. However, the regular methods to interpret time series through spectral analysis give only a partial view of dynamics in the atmospheric boundary layer. Also, the spectral methods have limited use for boundary layer and mesoscale modellers whose geophysical diagnostics are mostly anchored in directly measurable quantities: wind speed, temperature, precipitation, pressure, and radiation. Of these, wind speed and the air-sea temperature difference are the most important factors that characterize the dynamics of the lower atmospheric boundary layer and they provide a dynamical and thermodynamic constraint to frame observed processes, especially at high wind speeds. This was recognized in the early interpretation of the Froya database of gale force coastal winds from mid-Norway (Andersen, O.J. and J. Lovseth, Gale force maritime wind. The Froya data base. Part 1: Sites and instrumentation. Review of the data base, Journal of Wind
Body surface adaptations to boundary-layer dynamics.
Videler, J J
1995-01-01
Evolutionary processes have adapted nektonic animals to interact efficiently with the water that surrounds them. Not all these adaptations serve the same purpose. This paper concentrates on reduction of drag due to friction in the boundary layer close to the body surface. Mucus, compliant skins, scales, riblets and roughness may influence the flow velocity gradient, the type of flow and the thickness of the boundary layer around animals, and may seriously affect their drag in a positive or negative way. The long-chain polymers found in mucus decrease the pressure gradient and considerably reduced drag due to friction. The effect is probably due to channelling of the flow particles in the direction of the main flow, resulting in a reduction of turbulence. Compliant surfaces could probably reduce drag by equalising and distributing pressure pulses. However, the existing evidence that drag reduction actually occurs is not convincing. There is no indication that instantaneous heating, reducing the viscosity in the boundary layer, is used by animals as a drag-reducing technique. Small longitudinal ridges on rows of scales on fish can reduce shear stress in the boundary by a maximum of 10% compared with the shear stress of a smooth surface. The mechanism is based on the impedance of cross flow under well-defined conditions. The effect has been visualized with the use of particle image velocimetry techniques. The function of the swords and spears of several fast, pelagic, predatory fish species is still enigmatic. The surface structure of the sword of a swordfish is shown to be both rough and porous. The height of the roughness elements on the tip of the sword is close to the critical value for the induction of a laminar-to-turbulent flow transition at moderate cruising speeds. A flow tank is described that is designed to visualize the effects of surface imperfections on flow in the boundary layer in direct comparison with a smooth flat wall. The flow in a 1 m long, 10 cm
On buffer layers as non-reflecting computational boundaries
Hayder, M. Ehtesham; Turkel, Eli L.
1996-01-01
We examine an absorbing buffer layer technique for use as a non-reflecting boundary condition in the numerical simulation of flows. One such formulation was by Ta'asan and Nark for the linearized Euler equations. They modified the flow inside the buffer zone to artificially make it supersonic in the layer. We examine how this approach can be extended to the nonlinear Euler equations. We consider both a conservative and a non-conservative form modifying the governing equations in the buffer layer. We compare this with the case that the governing equations in the layer are the same as in the interior domain. We test the effectiveness of these buffer layers by a simulation of an excited axisymmetric jet based on a nonlinear compressible Navier-Stokes equations.
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
Boundary control of long waves in nonlinear dispersive systems
DEFF Research Database (Denmark)
Hasan, Agus; Foss, Bjarne; Aamo, Ole Morten
2011-01-01
Unidirectional propagation of long waves in nonlinear dispersive systems may be modeled by the Benjamin-Bona-Mahony-Burgers equation, a third order partial differential equation incorporating linear dissipative and dispersive terms, as well as a term covering nonlinear wave phenomena. For higher...... orders of the nonlinearity, the equation may have unstable solitary wave solutions. Although it is a one dimensional problem, achieving a global result for this equation is not trivial due to the nonlinearity and the mixed partial derivative. In this paper, two sets of nonlinear boundary control laws...... that achieve global exponential stability and semi-global exponential stability are derived for both linear and nonlinear cases....
A boundary integral approach to analyze the viscous scattering of a pressure wave by a rigid body
Homentcovschi, Dorel; Miles, Ronald N.
2008-01-01
The paper provides boundary integral equations for solving the problem of viscous scattering of a pressure wave by a rigid body. By using this mathematical tool uniqueness and existence theorems are proved. Since the boundary conditions are written in terms of velocities, vector boundary integral equations are obtained for solving the problem. The paper introduces single-layer viscous potentials and also a stress tensor. Correspondingly, a viscous double-layer potential is defined. The properties of all these potentials are investigated. By representing the scattered field as a combination of a single-layer viscous potential and a double-layer viscous potential the problem is reduced to the solution of a singular vectorial integral equation of Fredholm type of the second kind. In the case where the stress vector on the boundary is the main quantity of interest the corresponding boundary singular integral equation is proved to have a unique solution. PMID:18709178
Lamb waves propagation in layered piezoelectric/piezomagnetic plates.
Ezzin, Hamdi; Ben Amor, Morched; Ben Ghozlen, Mohamed Hédi
2017-04-01
A dynamic solution is presented for the propagation of harmonic waves in magneto-electro-elastic plates composed of piezoelectric BaTiO3(B) and magnetostrictive CoFe2O4(F) material. The state-vector approach is employed to derive the propagator matrix which connects the field variables at the upper interface to those at the lower interface of each layer. The ordinary differential approach is employed to determine the wave propagating characteristics in the plate by imposing the traction-free boundary condition on the top and bottom surfaces of the layered plate. The dispersion curves of the piezoelectric-piezomagnetic plate are shown for different thickness ratios. The numerical results show clearly the influence of different stacking sequences as well as thickness ratio on dispersion curves and on magneto-electromechanical coupling factor. These findings could be relevant to the analysis and design of high-performance surface acoustic wave (SAW) devices constructed from piezoelectric and piezomagnetic materials.
Boundary Layer Effect on Behavior of Discrete Models
Directory of Open Access Journals (Sweden)
Jan Eliáš
2017-02-01
Full Text Available The paper studies systems of rigid bodies with randomly generated geometry interconnected by normal and tangential bonds. The stiffness of these bonds determines the macroscopic elastic modulus while the macroscopic Poisson’s ratio of the system is determined solely by the normal/tangential stiffness ratio. Discrete models with no directional bias have the same probability of element orientation for any direction and therefore the same mechanical properties in a statistical sense at any point and direction. However, the layers of elements in the vicinity of the boundary exhibit biased orientation, preferring elements parallel with the boundary. As a consequence, when strain occurs in this direction, the boundary layer becomes stiffer than the interior for the normal/tangential stiffness ratio larger than one, and vice versa. Nonlinear constitutive laws are typically such that the straining of an element in shear results in higher strength and ductility than straining in tension. Since the boundary layer tends, due to the bias in the elemental orientation, to involve more tension than shear at the contacts, it also becomes weaker and less ductile. The paper documents these observations and compares them to the results of theoretical analysis.
NONLINEAR BOUNDARY STABILIZATION OF WAVE EQUATIONS WITH VARIABLE C OEFFICIENTS
Institute of Scientific and Technical Information of China (English)
冯绍继; 冯德兴
2003-01-01
The wave equation with variable coefficients with a nonlinear dissipative boundary feedbackis studied. By the Riemannian geometry method and the multiplier technique, it is shown thatthe closed loop system decays exponentially or asymptotically, and hence the relation betweenthe decay rate of the system energy and the nonlinearity behavior of the feedback function isestablished.
The quasi-monochromatic ULF wave foreshock boundary at Venus
Shan, Lican; Mazelle, Christian; Meziane, Karim; Romanelli, Norberto; Ge, Yasong; Du, Aimin; Lu, Quanming; Zhang, Tielong
2017-04-01
The location of ULF quasi-monochromatic wave onsets upstream of Venus bow shock is explored using VEX magnetic field data. We report the existence of a spatial foreshock boundary from which ULF waves are present. It is found that the ULF boundary is sensitive to the interplanetary magnetic field (IMF) direction and appears well defined for a cone-angle larger than 30 degrees. In the Venusian foreshock, the slope of the boundary increases with the cone-angle and for a nominal direction of the IMF, it makes an inclination of 70 degrees with the Sun-Venus direction. Moreover, we have found that the velocity of an ion traveling along the ULF boundary presents a qualitative agreement with the hypothesis of a quasi-adiabatic reflection of a portion of the solar wind at the bow shock. For a nominal IMF direction, the ions associated with the boundary have enough momentum to overcome the solar wind convection. These elements strongly suggest that backstreaming ions upstream of Venus bow shock provide the main energy source of the ULF foreshock waves.
Three-dimensional instability analysis of boundary layers perturbed by streamwise vortices
Martín, Juan A.; Paredes, Pedro
2016-08-01
A parametric study is presented for the incompressible, zero-pressure-gradient flat-plate boundary layer perturbed by streamwise vortices. The vortices are placed near the leading edge and model the vortices induced by miniature vortex generators (MVGs), which consist in a spanwise-periodic array of small winglet pairs. The introduction of MVGs has been experimentally proved to be a successful passive flow control strategy for delaying laminar-turbulent transition caused by Tollmien-Schlichting (TS) waves. The counter-rotating vortex pairs induce non-modal, transient growth that leads to a streaky boundary layer flow. The initial intensity of the vortices and their wall-normal distances to the plate wall are varied with the aim of finding the most effective location for streak generation and the effect on the instability characteristics of the perturbed flow. The study includes the solution of the three-dimensional, stationary, streaky boundary layer flows by using the boundary region equations, and the three-dimensional instability analysis of the resulting basic flows by using the plane-marching parabolized stability equations. Depending on the initial circulation and positioning of the vortices, planar TS waves are stabilized by the presence of the streaks, resulting in a reduction in the region of instability and shrink of the neutral stability curve. For a fixed maximum streak amplitude below the threshold for secondary instability (SI), the most effective wall-normal distance for the formation of the streaks is found to also offer the most stabilization of TS waves. By setting a maximum streak amplitude above the threshold for SI, sinuous shear layer modes become unstable, as well as another instability mode that is amplified in a narrow region near the vortex inlet position.
Boundary-layer model of pattern formation in solidification
Ben-Jacob, E.; Goldenfeld, N.; Langer, J. S.; Schon, G.
1984-01-01
A model of pattern formation in crystal growth is proposed, and its analytic properties are investigated. The principal dynamical variables in this model are the curvature of the solidification front and the thickness (or heat content) of a thermal boundary layer, both taken to be functions of position along the interface. This model is mathematically much more tractable than the realistic, fully nonlocal version of the free-boundary problem, and still recaptures many of the features that seem essential for studying dendritic behavior, for example. Preliminary numerical solutions produce snowflakelike patterns similar to those seen in nature.
Direct Numerical Simulation of Hypersonic Turbulent Boundary Layer inside an Axisymmetric Nozzle
Huang, Junji; Zhang, Chao; Duan, Lian; Choudhari, Meelan M.
2017-01-01
As a first step toward a study of acoustic disturbance field within a conventional, hypersonic wind tunnel, direct numerical simulations (DNS) of a Mach 6 turbulent boundary layer on the inner wall of a straight axisymmetric nozzle are conducted and the results are compared with those for a flat plate. The DNS results for a nozzle radius to boundary-layer thickness ratio of 5:5 show that the turbulence statistics of the nozzle-wall boundary layer are nearly unaffected by the transverse curvature of the nozzle wall. Before the acoustic waves emanating from different parts of the nozzle surface can interfere with each other and undergo reflections from adjacent portions of the nozzle surface, the rms pressure fluctuation beyond the boundary layer edge increases toward the nozzle axis, apparently due to a focusing effect inside the axisymmetric configuration. Spectral analysis of pressure fluctuations at both the wall and the freestream indicates a similar distribution of energy content for both the nozzle and the flat plate, with the peak of the premultiplied frequency spectrum at a frequency of [(omega)(delta)]/U(sub infinity) approximately 6.0 inside the free stream and at [(omega)(delta)]/U(sub infinity) approximately 2.0 along the wall. The present results provide the basis for follow-on simulations involving reverberation effects inside the nozzle.
Lidar investigation of tropical nocturnal boundary layer aerosols and cloud macrophysics
Manoj, M. G.; Devara, P. C. S.; Taraphdar, S.
2013-10-01
Observational evidence of two-way association between nocturnal boundary layer aerosols and cloud macrophysical properties over a tropical urban site is reported in this paper. The study has been conducted during 2008-09 employing a high space-time resolution polarimetric micro-pulse lidar over a tropical urban station in India. Firstly, the study highlights the crucial role of boundary layer aerosols and background meteorology on the formation and structure of low-level stratiform clouds in the backdrop of different atmospheric stability conditions. Turbulent mixing induced by the wind shear at the station, which is associated with a complex terrain, is found to play a pivotal role in the formation and structural evolution of nocturnal boundary layer clouds. Secondly, it is shown that the trapping of energy in the form of outgoing terrestrial radiation by the overlying low-level clouds can enhance the aerosol mixing height associated with the nocturnal boundary layer. To substantiate this, the long-wave heating associated with cloud capping has been quantitatively estimated in an indirect way by employing an Advanced Research Weather Research and Forecasting (WRF-ARW) model developed by National Center for Atmospheric Research (NCAR), Colorado, USA, and supplementary data sets; and differentiated against other heating mechanisms. The present investigation as well establishes the potential of lidar remote-sensing technique in exploring some of the intriguing aspects of the cloud-environment relationship.
On the coupling between a supersonic boundary layer and a flexible surface
Frendi, Abdelkader; Maestrello, Lucio; Bayliss, Alvin
1992-01-01
The coupling between a two-dimensional, supersonic, laminar boundary layer and a flexible surface is studied using direct numerical computations of the Navier-Stokes equations coupled with the plate equation. The flexible surface is forced to vibrate by plane acoustic waves at normal incidence emanated by a sound source located on the side of the flexible surface opposite to the boundary layer. The effect of the source excitation frequency on the surface vibration and boundary layer stability is analyzed. We find that, for frequencies near the fifth natural frequency of the surface or lower, large disturbances are introduced in the boundary layer which may alter its stability characteristics. The interaction between a stable two-dimensional disturbance of Tollmien-Schlichting (TS) type with the vibrating surface is also studied. We find that the disturbance level is higher over the vibrating flexible surface than that obtained when the surface is rigid, which indicates a strong coupling between flow and structure. However, in the absence of the sound source the disturbance level over the rigid and flexible surfaces are identical. This result is due to the high frequency of the TS disturbance which does not couple with the flexible surface.
On the wave equation with semilinear porous acoustic boundary conditions
Graber, Philip Jameson
2012-05-01
The goal of this work is to study a model of the wave equation with semilinear porous acoustic boundary conditions with nonlinear boundary/interior sources and a nonlinear boundary/interior damping. First, applying the nonlinear semigroup theory, we show the existence and uniqueness of local in time solutions. The main difficulty in proving the local existence result is that the Neumann boundary conditions experience loss of regularity due to boundary sources. Using an approximation method involving truncated sources and adapting the ideas in Lasiecka and Tataru (1993) [28], we show that the existence of solutions can still be obtained. Second, we prove that under some restrictions on the source terms, then the local solution can be extended to be global in time. In addition, it has been shown that the decay rates of the solution are given implicitly as solutions to a first order ODE and depends on the behavior of the damping terms. In several situations, the obtained ODE can be easily solved and the decay rates can be given explicitly. Third, we show that under some restrictions on the initial data and if the interior source dominates the interior damping term and if the boundary source dominates the boundary damping, then the solution ceases to exists and blows up in finite time. Moreover, in either the absence of the interior source or the boundary source, then we prove that the solution is unbounded and grows as an exponential function. © 2012 Elsevier Inc.
Transitional boundary layers in low-Prandtl-number convection
Schumacher, Jörg; Bandaru, Vinodh; Pandey, Ambrish; Scheel, Janet D.
2016-12-01
The boundary layer structure of the velocity and temperature fields in turbulent Rayleigh-Bénard flows in closed cylindrical cells of unit aspect ratio is revisited from a transitional and turbulent viscous boundary layer perspective. When the Rayleigh number is large enough, the dynamics at the bottom and top plates can be separated into an impact region of downwelling plumes, an ejection region of upwelling plumes, and an interior region away from the side walls. The latter is dominated by the shear of the large-scale circulation (LSC) roll, which fills the whole cell and continuously varies its orientation. The working fluid is liquid mercury or gallium at a Prandtl number Pr=0.021 for Rayleigh numbers 3 ×105≤Ra≤4 ×108 . The generated turbulent momentum transfer corresponds to macroscopic flow Reynolds numbers with 1.8 ×103≤Re≤4.6 ×104 . In highly resolved spectral element direct numerical simulations, we present the mean profiles of velocity, Reynolds stress, and temperature in inner viscous units and compare our findings with convection experiments and channel flow data. The complex three-dimensional and time-dependent structure of the LSC in the cell is compensated by a plane-by-plane symmetry transformation which aligns the horizontal velocity components and all its derivatives with the instantaneous orientation of the LSC. As a consequence, the torsion of the LSC is removed, and a streamwise direction in the shear flow can be defined. It is shown that the viscous boundary layers for the largest Rayleigh numbers are highly transitional and obey properties that are directly comparable to transitional channel flows at friction Reynolds numbers Reτ≲102 . The transitional character of the viscous boundary layer is also underlined by the strong enhancement of the fluctuations of the wall stress components with increasing Rayleigh number. An extrapolation of our analysis data suggests that the friction Reynolds number Reτ in the velocity boundary
Indian Academy of Sciences (India)
N V Sam; U C Mohanty; A N V Satyanarayana
2000-06-01
The present study is based on the observed features of the MBL (Marine Boundary Layer) during the Bay of Bengal and Monsoon Experiment (BOBMEX) - Pilot phase. Conserved Variable Analysis (CVA) of the conserved variables such as potential temperature, virtual potential temperature, equivalent potential temperature, saturation equivalent potential temperature and specific humidity were carried out at every point of upper air observation obtained on board ORV Sagar Kanya. The values are estimated up to a maximum of 4 km to cover the boundary layer. The Marine Boundary Layer Height is estimated from the conserved thermodynamic profiles. During the disturbed period when the convective activity is observed, the deeper boundary layers show double mixing line structures. An attempt is also made to study the oceanic heat budget using empirical models. The estimated short-wave radiation flux compared well with the observations.
Boundary-Value Problem for Two-Dimensional Fluctuations in Boundary Layers
1985-07-01
inviscid analysis by P. Durbin "Distortion of turbulence by a constant-shear layer adjacent to a wall," private communication (1977). (l.2e) 2-D...vortices near a boundary," ~ of the Americ~ p ~ ~ , Volume 20, Number 9 (November 1975). 21. Hultgren, Lennart S. and Gustavsson, L. Hakan, " Algebraic
Experimental investigation of absolute instability of a rotating-disk boundary layer
Othman, H.; Corke, T. C.
2006-10-01
A series of experiments were performed to study the absolute instability of Type I travelling crossflow modes in the boundary layer on a smooth disk rotating at constant speed. The basic flow agreed with analytic theory, and the growth of natural disturbances matched linear theory predictions. Controlled temporal disturbances were introduced by a short-duration air pulse from a hypodermic tube located above the disk and outside the boundary layer. The air pulse was positioned just outboard of the linear-theory critical radius for Type I crossflow modes. A hot-wire sensor primarily sensitive to the azimuthal velocity component, was positioned at different spatial (r,theta) locations on the disk to document the growth of disturbances produced by the air pulses. Ensemble averages conditioned on the air pulses revealed wave packets that evolved in time and space. Two amplitudes of air pulses were used. The lower amplitude was verified to produced wave packets with linear amplitude characteristics. The space time evolution of the leading and trailing edges of the wave packets were followed past the critical radius for the absolute instability, r_{c_{A}}. With the lower amplitudes, the spreading of the disturbance wave packets did not continue to grow in time as r_{c_{A}} was approached. Rather, the spreading of the trailing edge of the wave packet decelerated and asymptotically approached a constant. This result supports previous linear DNS simulations where it was concluded that the absolute instability does not produce a global mode and that linear disturbance wave packets are dominated by the convective instability. The larger-amplitude disturbances were found to produce larger temporal spreading of the wave packets. This was accompanied by a sharp growth in the wave packet amplitude past r_{c_{A}}. Explanations for this are discussed.
On the global existence and uniqueness of solutions to the nonstationary boundary layer system
Institute of Scientific and Technical Information of China (English)
ZHANG; Jianwen; ZHAO; Junning
2006-01-01
In this paper, we study the problem of boundary layer for nonstationary flows of viscous incompressible fluids. There are some open problems in the field of boundary layer. The method used here is mainly based on a transformation which reduces the boundary layer system to an initial-boundary value problem for a single quasilinear parabolic equation. We prove the existence of weak solutions to the modified nonstationary boundary layer system. Moreover, the stability and uniqueness of weak solutions are discussed.
Boundary layer ozone - An airborne survey above the Amazon Basin
Gregory, Gerald L.; Browell, Edward V.; Warren, Linda S.
1988-01-01
Ozone data obtained over the forest canopy of the Amazon Basin during July and August 1985 in the course of NASA's Amazon Boundary Layer Experiment 2A are discussed, and ozone profiles obtained during flights from Belem to Tabatinga, Brazil, are analyzed to determine any cross-basin effects. The analyses of ozone data indicate that the mixed layer of the Amazon Basin, for the conditions of undisturbed meteorology and in the absence of biomass burning, is a significant sink for tropospheric ozone. As the coast is approached, marine influences are noted at about 300 km inland, and a transition from a forest-controlled mixed layer to a marine-controlled mixed layer is noted.
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 ...
Lumley decomposition of turbulent boundary layer at high Reynolds numbers
Tutkun, Murat; George, William K.
2017-02-01
The decomposition proposed by Lumley in 1966 is applied to a high Reynolds number turbulent boundary layer. The experimental database was created by a hot-wire rake of 143 probes in the Laboratoire de Mécanique de Lille wind tunnel. The Reynolds numbers based on momentum thickness (Reθ) are 9800 and 19 100. Three-dimensional decomposition is performed, namely, proper orthogonal decomposition (POD) in the inhomogeneous and bounded wall-normal direction, Fourier decomposition in the homogeneous spanwise direction, and Fourier decomposition in time. The first POD modes in both cases carry nearly 50% of turbulence kinetic energy when the energy is integrated over Fourier dimensions. The eigenspectra always peak near zero frequency and most of the large scale, energy carrying features are found at the low end of the spectra. The spanwise Fourier mode which has the largest amount of energy is the first spanwise mode and its symmetrical pair. Pre-multiplied eigenspectra have only one distinct peak and it matches the secondary peak observed in the log-layer of pre-multiplied velocity spectra. Energy carrying modes obtained from the POD scale with outer scaling parameters. Full or partial reconstruction of turbulent velocity signal based only on energetic modes or non-energetic modes revealed the behaviour of urms in distinct regions across the boundary layer. When urms is based on energetic reconstruction, there exists (a) an exponential decay from near wall to log-layer, (b) a constant layer through the log-layer, and (c) another exponential decay in the outer region. The non-energetic reconstruction reveals that urms has (a) an exponential decay from the near-wall to the end of log-layer and (b) a constant layer in the outer region. Scaling of urms using the outer parameters is best when both energetic and non-energetic profiles are combined.
Bandgap tunability at single-layer molybdenum disulphide grain boundaries
Huang, Yu Li
2015-02-17
Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40±0.05 eV for single-layer, 2.10±0.05 eV for bilayer and 1.75±0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85±0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.
CFD Modeling of Non-Neutral Atmospheric Boundary Layer Conditions
DEFF Research Database (Denmark)
Koblitz, Tilman
to the atmospheric boundary-layer, are mostly ignored so far. In order to decrease the uncertainty of wind resource assessment, the present work focuses on atmospheric flows that include atmospheric stability and the Coriolis effect. Within the present work a RANS model framework is developed and implemented......For wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics models that focus on modeling the airflow in a neutrally stratified surface-layer. Physical processes like the Coriolis force, buoyancy forces and heat transport, that are important...
Goertler instability. [for boundary layer flow over curved walls
Ragab, S. A.; Nayfeh, A. H.
1981-01-01
Goertler instability for boundary-layer flows over generally curved walls is considered. The full-linearized disturbance equations are obtained in an orthogonal curvilinear coordinate system. A perturbation procedure to account for second-order effects is used to determine the effects of the displacement thickness and the variation of the streamline curvature on the neutral stability of the Blasius flow. The streamwise pressure gradient in the mean flow is accounted for by solving the nonsimilar boundary-layer equations. Growth rates are obtained for the actual mean flow and compared with those for the Blasius flow and the Falkner-Skan flows. The results demonstrate the strong influence of the streamwise pressure gradient and the nonsimilarity of the basic flow on the stability characteristics.
Simulation of Wind turbines in the atmospheric boundary layer
DEFF Research Database (Denmark)
Chivaee, Hamid Sarlak; Sørensen, Jens Nørkær; Mikkelsen, Robert Flemming
and higher moments. As an example, figure (1) shows a 2D snapshot of stream-wise velocity contours (in SI units) in an infinite row of wind turbines simulated in stably stratified flow. Simulations are performed usind the in-house CFD code Ellipsys3D, which is a multi-block general purpose, parallelized...... of the specific turbine, however the method reduces the computational costs significantly while giving accurate prediction of wakes and statistical quantities behind the turbine. The simulations start with a neutral prescribed boundary layer that follows a logarithmic profile with the velocity of 8 m/s at the hub......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...
Influence of localised double suction on a turbulent boundary layer
Oyewola, O.; Djenidi, L.; Antonia, R. A.
2007-07-01
The effects of localised suction applied through a pair of porous wall strips on a turbulent boundary layer have been quantified through the measurements of mean velocity and Reynolds stresses. The results indicate that the use of second strip extends the pseudo-relaminarisation zone but also reduces the overshoot in the longitudinal and normal r.m.s. velocities. While the minimum r.m.s. occurs at x/δo=3.0 (one strip) and x/δo=12 (two strips), the reduction observed for the latter case is larger. Relative to no suction, the turbulence level is modified by suction and the effect is enhanced with double suction. This increased effectiveness reflects the fact that the second strip acts on a boundary layer whose near-wall active motion has been seriously weakened by the first strip.
Optimal control of wind turbines in a turbulent boundary layer
Yilmaz, Ali Emre; Meyers, Johan
2016-11-01
In recent years, optimal control theory was combined with large-eddy simulations to study the optimal control of wind farms and their interaction with the atmospheric boundary layer. The individual turbine's induction factors were dynamically controlled in time with the aim of increasing overall power extraction. In these studies, wind turbines were represented using an actuator disk method. In the current work, we focus on optimal control on a much finer mesh (and a smaller computational domain), representing turbines with an actuator line method. Similar to Refs., optimization is performed using a gradient-based method, and gradients are obtained employing an adjoint formulation. Different cases are investigated, that include a single and a double turbine case both with uniform inflow, and with turbulent-boundary-layer inflow. The authors acknowledge support from the European Research Council (FP7-Ideas, Grant No. 306471).
Stereoscopic PIV measurement of boundary layer affected by DBD actuator
Directory of Open Access Journals (Sweden)
Procházka Pavel
2016-01-01
Full Text Available The effect of ionic wind generated by plasma actuator on developed boundary layer inside a narrow channel was investigated recently. Since the main investigated plane was parallel to the channel axis, the description of flow field was not evaluated credibly. This paper is dealing with cross-section planes downstream the actuator measured via 3D time-resolved PIV. The actuator position is in spanwise or in streamwise orientation so that ionic wind is blown in the same direction as the main flow or in opposite direction or perpendicularly. The interaction between boundary layer and ionic wind is evaluated for three different velocities of main flow and several parameters of plasma actuation (steady and unsteady regime, frequency etc.. Statistical properties of the flow are shown as well as dynamical behaviour of arising longitudinal vortices are discussed via phase-locked measurement and decomposition method.
Temporally optimized spanwise vorticity sensor measurements in turbulent boundary layers
Morrill-Winter, C.; Klewicki, J.; Baidya, R.; Marusic, I.
2015-12-01
Multi-element hot-wire anemometry was used to measure spanwise vorticity fluctuations in turbulent boundary layers. Smooth wall boundary layer profiles, with very good spatial and temporal resolution, were acquired over a Kármán number range of 2000-12,700 at the Melbourne Wind Tunnel at the University of Melbourne and the University of New Hampshire's Flow Physics Facility. A custom hot-wire probe was necessary to simultaneously obtain velocity and spanwise vorticity measurements centered at a fixed point in space. A custom calibration/processing scheme was developed to utilize single-wall-parallel wires to optimize the accuracy of the measured wall-normal velocity fluctuations derived from the sensor's ×-array.
Full-Scale Spectrum of Boundary-Layer Winds
DEFF Research Database (Denmark)
Larsén, Xiaoli Guo; Larsen, Søren Ejling; Lundtang Petersen, Erik
2016-01-01
Extensive mean meteorological data and high frequency sonic anemometer data from two sites in Denmark, one coastal onshore and one offshore, have been used to study the full-scale spectrum of boundary-layer winds, over frequencies f from about 1 yr−1 to10 Hz. 10-min cup anemometer data are used...... to estimate the spectrum from about 1 yr−1 to 0.05 min−1; in addition, using 20-Hz sonic anemometer data, an ensemble of 1-day spectra covering the range 1 day−1 to 10 Hz has been calculated. The overlapping region in these two measured spectra is in good agreement. Classical topics regarding the various...... of the three velocity components over a wide range from 1 day−1 to 10 Hz, which is useful in determining the necessary sample duration when measuring turbulence statistics in the boundary layer....
Small Scale Forcing in a Turbulent Boundary Layer
Lorkowski, Thomas; Rathnasingham, Ruben; Breuer, Kenneth S.
1996-11-01
In order to understand the effect of small scale forcing on turbulent flows and its implications on control, an experimental investigation is made into the forcing of the inertial scales in the wall region of a turbulent boundary layer. A wall-mounted resonant actuator is used to produce a local vortical structure in the streamwise direction which is convected downstream by the boundary layer flow. The frequency associated with this structure is governed by the resonant frequency of the device and falls in the range of the inertial scales at the Reynolds number of the experiment (Re_θ = 2000). Hot-wire anemometry is used to map the velocity field at several stations downstream of the actuator. The signals are also conditioned to identify the effect of the actuator on different scales in the flow. Amplitude and modulation effects are also discussed. ^*Supported by ONR Grant N00014-92-J-1910.
Turbulence measurements in high Reynolds number boundary layers
Vallikivi, Margit; Smits, Alexander
2013-11-01
Measurements are conducted in zero pressure gradient turbulent boundary layers for Reynolds numbers from Reθ = 9,000 to 225,000. The experiments were performed in the High Reynolds number Test Facility (HRTF) at Princeton University, which uses compressed air as the working fluid. Nano-Scale Thermal Anemometry Probes (NSTAPs) are used to acquire data with very high spatial and temporal precision. These new data are used to study the scaling behavior of the streamwise velocity fluctuations in the boundary layer and make comparisons with the scaling of other wall-bounded turbulent flows. Supported under ONR Grant N00014-09-1-0263 (program manager Ron Joslin) and NSF Grant CBET-1064257 (program manager Henning Winter).
Anisotropic Boundary Layer Adaptivity of Multi-Element Wings
Chitale, Kedar C; Sahni, Onkar; Shephard, Mark S; Jansen, Kenneth E
2014-01-01
Multi-element wings are popular in the aerospace community due to their high lift performance. Turbulent flow simulations of these configurations require very fine mesh spacings especially near the walls, thereby making use of a boundary layer mesh necessary. However, it is difficult to accurately determine the required mesh resolution a priori to the simulations. In this paper we use an anisotropic adaptive meshing approach including adaptive control of elements in the boundary layers and study its effectiveness for two multi-element wing configurations. The results are compared with experimental data as well as nested refinements to show the efficiency of adaptivity driven by error indicators, where superior resolution in wakes and near the tip region through adaptivity are highlighted.
Flight Experiment Verification of Shuttle Boundary Layer Transition Prediction Tool
Berry, Scott A.; Berger, Karen T.; Horvath, Thomas J.; Wood, William A.
2016-01-01
Boundary layer transition at hypersonic conditions is critical to the design of future high-speed aircraft and spacecraft. Accurate methods to predict transition would directly impact the aerothermodynamic environments used to size a hypersonic vehicle's thermal protection system. A transition prediction tool, based on wind tunnel derived discrete roughness correlations, was developed and implemented for the Space Shuttle return-to-flight program. This tool was also used to design a boundary layer transition flight experiment in order to assess correlation uncertainties, particularly with regard to high Mach-number transition and tunnel-to-flight scaling. A review is provided of the results obtained from the flight experiment in order to evaluate the transition prediction tool implemented for the Shuttle program.
Wind-US Code Contributions to the First AIAA Shock Boundary Layer Interaction Prediction Workshop
Georgiadis, Nicholas J.; Vyas, Manan A.; Yoder, Dennis A.
2013-01-01
This report discusses the computations of a set of shock wave/turbulent boundary layer interaction (SWTBLI) test cases using the Wind-US code, as part of the 2010 American Institute of Aeronautics and Astronautics (AIAA) shock/boundary layer interaction workshop. The experiments involve supersonic flows in wind tunnels with a shock generator that directs an oblique shock wave toward the boundary layer along one of the walls of the wind tunnel. The Wind-US calculations utilized structured grid computations performed in Reynolds-averaged Navier-Stokes mode. Four turbulence models were investigated: the Spalart-Allmaras one-equation model, the Menter Baseline and Shear Stress Transport k-omega two-equation models, and an explicit algebraic stress k-omega formulation. Effects of grid resolution and upwinding scheme were also considered. The results from the CFD calculations are compared to particle image velocimetry (PIV) data from the experiments. As expected, turbulence model effects dominated the accuracy of the solutions with upwinding scheme selection indicating minimal effects.
Turbulent boundary layer control through spanwise wall oscillation using Kagome lattice structures
Bird, James; Santer, Matthew; Morrison, Jonathan
2015-11-01
It is well established that a reduction in skin-friction and turbulence intensity can be achieved by applying in-plane spanwise forcing to a surface beneath a turbulent boundary layer. It has also been shown in DNS (M. Quadrio, P. Ricco, & C. Viotti; J. Fluid Mech; 627, 161, 2009), that this phenomenon is significantly enhanced when the forcing takes the form of a streamwise travelling wave of spanwise perturbation. In the present work, this type of forcing is generated by an active surface comprising a compliant structure, based on a Kagome lattice geometry, supporting a membrane skin. The structural design ensures negligible wall normal displacement while facilitating large in-plane velocities. The surface is driven pneumatically, achieving displacements of 3 mm approximately, at frequencies in excess of 70 Hz for a turbulent boundary layer at Reτ ~ 1000 . As the influence of this forcing on boundary layer is highly dependent on the wavenumber and frequency of the travelling wave, a flat surface was designed and optimised to allow these forcing parameters to be varied, without reconfiguration of the experiment. Simultaneous measurements of the fluid and surface motion are presented, and notable skin-friction drag reduction is demonstrated. Airbus support agreement IW202838 is gratefully acknowledged.
Effect of leading-edge geometry on boundary-layer receptivity to freestream sound
Lin, Nay; Reed, Helen L.; Saric, W. S.
1991-01-01
The receptivity to freestream sound of the laminar boundary layer over a semi-infinite flat plate with an elliptic leading edge is simulated numerically. The incompressible flow past the flat plate is computed by solving the full Navier-Stokes equations in general curvilinear coordinates. A finite-difference method which is second-order accurate in space and time is used. Spatial and temporal developments of the Tollmien-Schlichting wave in the boundary layer, due to small-amplitude time-harmonic oscillations of the freestream velocity that closely simulate a sound wave travelling parallel to the plate, are observed. The effect of leading-edge curvature is studied by varying the aspect ratio of the ellipse. The boundary layer over the flat plate with a sharper leading edge is found to be less receptive. The relative contribution of the discontinuity in curvature at the ellipse-flat-plate juncture to receptivity is investigated by smoothing the juncture with a polynomial. Continuous curvature leads to less receptivity. A new geometry of the leading edge, a modified super ellipse, which provides continuous curvature at the juncture with the flat plate, is used to study the effect of continuous curvature and inherent pressure gradient on receptivity.
Evolution and formation of shear layers in a developing turbulent boundary layer
Lee, Junghoon; Monty, Jason; Hutchins, Nicholas
2016-11-01
The evolution and formation mechanism of shear layers in the outer region of a turbulent boundary layer are investigated using time-resolved PIV datasets of a developing turbulent boundary layer from inception at the trip up to Reτ = 3000 . An analysis of a sequence of instantaneous streamwise velocity fluctuation fields reveals that strong streamwise velocity gradients are prevalent along interfaces where low- and high-speed regions interact. To provide an insight on how such regions are associated with the formation of shear layers in the outer regions, we compute conditional averages of streamwise velocity fluctuations based on a strong shear layer. Our results reveal that one possible mechanism for the generation of shear layers in the outer region is due to the mismatch in the convection velocities between low- and high-speed regions. The results also indicate that the angle of the inclined shear layer is developing in time. In addition, the conditionally averaged velocity fluctuations exhibit a local instability along these shear layers, leading to a shear layer roll-up event as the layers evolve in time. Based on these findings, we propose a conceptual model which describes dynamic interactions of shear layers and their associated large-scale coherent motions. The authors wish to acknowledge the financial support of the Australian Research Council.
Modeling Turbulence Generation in the Atmospheric Surface and Boundary Layers
2015-10-01
index. In the boundary layer, atmospheric temperature fluctuations are primarily responsible for the variations in refractive index at ultraviolet...parameterization of the atmospheric emissivity, in the early 1980s a parallel study of the SEB was conducted by the US Army Waterways Experiment Station...period of rotation of the atmosphere can be defined as TI = 2π/fc. At most mid- latitude locations this period is approximately 17 h. This quantity is
Neutral stability calculations for boundary-layer flows
Nayfeh, A. H.; Padhye, A.
1980-01-01
An analysis is presented of the parallel neutral stability of three-dimensional incompressible, isothermal boundary-layer flows. A Taylor-series expansion of the dispersion relation is used to derive the general eigenvalues. These equations are functions of the complex group velocity. These relations are verified by numerical results obtained for two- and three-dimensional disturbances in two- and three-dimensional flows.
Transport of particles in an atmospheric turbulent boundary layer
Institute of Scientific and Technical Information of China (English)
Xiongping Luo; Shiyi Chen
2005-01-01
A program incorporating the parallel code of large eddy simulation (LES) and particle transportation model is developed to simulate the motion of particles in an atmospheric turbulent boundary layer (ATBL). A model of particles of 100-micrometer order coupling with large scale ATBL is proposed. Two typical cases are studied, one focuses on the evolution of particle profile in the ATBL and the landing displacement of particles, whereas the other on the motion of particle stream.
Extreme Vertical Gusts in the Atmospheric Boundary Layer
2015-07-01
with tornadogenesis [Mueller and Carbone (1987), Wilson (1986) and McCaul and Bluestein (1986)], although tornadoes are part of the hazard of...Burns, C. Nappo, R. Banta, R. Newsom and J. Cuxart (2002). CASES-99: A comprehensive investigation of the stable nocturnal boundary layer. Bulletin of...Meteorology 64(1-2): 55-74. Wilson , J. W. (1986). Tornadogenesis by nonprecipitation induced wind shear lines. Monthly Weather Review 114(2): 270-284
Alpine boundary layer development observed with airborne lidar
Energy Technology Data Exchange (ETDEWEB)
Furger, M.; Nyeki, S. [Paul Scherrer Inst. (PSI), Villigen (Switzerland); Whiteman, C.D. [Pacific Northwest National Laboratory, Richland, WA (United States)
2000-07-01
Airborne lidar cross-sections of the atmospheric boundary layer (ABL) over the Bernese Alps were obtained during the STAAARTE campaign in 1997, providing information on the distribution of aerosols over this complex terrain area. Lidar offers a much better spatial and temporal data coverage than conventional instrumentation, and the good spatial resolution allows many small-scale features and phenomena of the mountain atmosphere to be recognized and quantitatively or qualitatively described. (authors)
Imaging the transient boundary layer on a free rotating disc.
Matijasević, Branimir; Guzović, Zvonimir; Martinis, Vinko
2002-10-01
This report presents a visual study of the transition process of the laminar boundary layer (BL) in a turbulent BL on a free rotating disc. The imaging is based on an experimental investigation that aimed to analyze the structure of the BL by relating it to the ratio between turbulent energy and vortex energy, the critical and the transient Reynolds numbers (Re), the vortex numbers and their dependence on Re, and on the distance from the rotating disc.
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.
Coupled wake boundary layer model of wind-farms
Stevens, Richard J A M; Meneveau, Charles
2014-01-01
We present and test a coupled wake boundary layer (CWBL) model that describes the distribution of the power output in a wind-farm. The model couples the traditional, industry-standard wake expansion/superposition approach with a top-down model for the overall wind-farm boundary layer structure. The wake expansion/superposition model captures the effect of turbine positioning, while the top-down portion adds the interaction between the wind-turbine wakes and the atmospheric boundary layer. Each portion of the model requires specification of a parameter that is not known a-priori. For the wake model the wake expansion coefficient is required, while the top-down model requires an effective span-wise turbine spacing within which the model's momentum balance is relevant. The wake expansion coefficient is obtained by matching the predicted mean velocity at the turbine from both approaches, while the effective span-wise turbine spacing depends on turbine positioning and thus can be determined from the wake expansion...
Minnowbrook II 1997 Workshop on Boundary Layer Transition in Turbomachines
LaGraff John E. (Editor); Ashpis, David E. (Editor)
1998-01-01
The volume contains materials presented at the Minnowbrook II - 1997 Workshop on Boundary Layer Transition in Turbomachines, held at Syracuse University Minnowbrook Conference Center, New York, on September 7-10, 1997. The workshop followed the informal format at the 1993 Minnowbrook I workshop, focusing on improving the understanding of late stage (final breakdown) boundary layer transition, with the engineering application of improving design codes for turbomachinery in mind. Among the physical mechanisms discussed were hydrodynamic instabilities, laminar to turbulent transition, bypass transition, turbulent spots, wake interaction with boundary layers, calmed regions, and separation, all in the context of flow in turbomachinery, particularly in compressors and high and low pressure turbines. Results from experiments, DNS, computation, modeling and theoretical analysis were presented. Abstracts and copies of viewgraphs, a specifically commissioned summation paper prepared after the workshop, and a transcript of the extensive working group reports and discussions are included in this volume. They provide recommendations for future research and clearly highlight the need for continued vigorous research in the technologically important area of transition in turbomachines.
Using UAV's to Measure the Urban Boundary Layer
Jacob, R. L.; Sankaran, R.; Beckman, P. H.
2015-12-01
The urban boundary layer is one of the most poorly studied regions of the atmospheric boundary layer. Since a majority of the world's population now lives in urban areas, it is becoming a more important region to measure and model. The combination of relatively low-cost unmanned aerial vehicles and low-cost sensors can together provide a new instrument for measuring urban and other boundary layers. We have mounted a new sensor and compute platform called Waggle on an off-the-shelf XR8 octo-copter from 3DRobotics. Waggle consists of multiple sensors for measuring pressure, temperature and humidity as well as trace gases such as carbon monoxide, nitrogen dioxide, sulfur dioxide and ozone. A single board computer running Linux included in Waggle on the UAV allows in-situ processing and data storage. Communication of the data is through WiFi or 3G and the Waggle software can save the data in case communication is lost during flight. The flight pattern is a deliberately simple vertical ascent and descent over a fixed location to provide vertical profiles and so flights can be confined to urban parks, industrial areas or the footprint of a single rooftop. We will present results from test flights in urban and rural areas in and around Chicago.
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.
Numerical Investigation of a Fuselage Boundary Layer Ingestion Propulsion Concept
Elmiligui, Alaa A.; Fredericks, William J.; Guynn, Mark D.; Campbell, Richard L.
2013-01-01
In the present study, a numerical assessment of the performance of fuselage boundary layer ingestion (BLI) propulsion techniques was conducted. This study is an initial investigation into coupling the aerodynamics of the fuselage with a BLI propulsion system to determine if there is sufficient potential to warrant further investigation of this concept. Numerical simulations of flow around baseline, Boundary Layer Controlled (BLC), and propelled boundary layer controlled airships were performed. Computed results showed good agreement with wind tunnel data and previous numerical studies. Numerical simulations and sensitivity analysis were then conducted on four BLI configurations. The two design variables selected for the parametric study of the new configurations were the inlet area and the inlet to exit area ratio. Current results show that BLI propulsors may offer power savings of up to 85% over the baseline configuration. These interim results include the simplifying assumption that inlet ram drag is negligible and therefore likely overstate the reduction in power. It has been found that inlet ram drag is not negligible and should be included in future analysis.
Thermocapillary Bubble Migration: Thermal Boundary Layers for Large Marangoni Numbers
Balasubramaniam, R.; Subramanian, R. S.
1996-01-01
The migration of an isolated gas bubble in an immiscible liquid possessing a temperature gradient is analyzed in the absence of gravity. The driving force for the bubble motion is the shear stress at the interface which is a consequence of the temperature dependence of the surface tension. The analysis is performed under conditions for which the Marangoni number is large, i.e. energy is transferred predominantly by convection. Velocity fields in the limit of both small and large Reynolds numbers are used. The thermal problem is treated by standard boundary layer theory. The outer temperature field is obtained in the vicinity of the bubble. A similarity solution is obtained for the inner temperature field. For both small and large Reynolds numbers, the asymptotic values of the scaled migration velocity of the bubble in the limit of large Marangoni numbers are calculated. The results show that the migration velocity has the same scaling for both low and large Reynolds numbers, but with a different coefficient. Higher order thermal boundary layers are analyzed for the large Reynolds number flow field and the higher order corrections to the migration velocity are obtained. Results are also presented for the momentum boundary layer and the thermal wake behind the bubble, for large Reynolds number conditions.
A planetary boundary layer observational capability in Kansas
Energy Technology Data Exchange (ETDEWEB)
Wesely, M.L.; Coulter, R.L.; Klazura, G.E. [and others
1997-03-01
An initiative is underway to establish the Argonne Boundary Layer Experiments (ABLE) facility to provide continuous, long-term observations of the planetary boundary layer (PBL) with state-of-the-art instruments. Planning for ABLE began during 1995, and implementation is expected to be mostly complete by 1998. ABLE will be located within the area now occupied by the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site of DOE`s Atmospheric Radiation Measurement (ARM) program. The Argonne facility will concentrate on measuring at spatial scales considerably smaller than addressed with CART. When it is fully functional, ABLE will offer atmospheric scientists the opportunity to remotely {open_quote}collect{close_quote} data in real time without necessarily leaving their home offices. Specialized computer analysis and visualization software will be developed and provided by ABLE to facilitate analysis by remote users. ABLE will host specialized field campaigns for which it can provide supplementary measurements and the required facilities for shorter-term instrument deployments. In addition, ABLE will function as the proving ground for new technologies for atmospheric boundary layer research. 1 ref., 1 fig.
Some characteristics of bypass transition in a heated boundary layer
Sohn, K. H.; Reshotko, E.; O'Brien, J. E.
Experimental measurements of both mean and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent boundary layers on a heated flat plate are presented. Measurements were obtained in air over a range of freestream turbulence intensities from 0.3 percent to 6 percent with a freestream velocity of 30.5 m/s and zero pressure gradient. Conditional sampling performed in the transitional boundary layers indicate the existence of a near-wall drop in intermittency, especially pronounced at low intermittencies. Nonturbulent intervals were observed to possess large levels of low-frequency unsteadiness, and turbulent intervals had peak intensities as much as 50 percent higher than were measured at fully turbulent stations. Heat transfer results were consistent with results of previous researches and Reynolds analogy factors were found to be well predicted by laminar and turbulent correlations which accounted for unheated starting length. A small dependence of the turbulent Reynolds analogy factors on freestream turbulence level was observed. Laminar boundary layer spectra indicated selective amplification of unstable frequencies. These instabilities appear to play a dominant role in the transition process only for the lowest freestream turbulence level studied, however.
Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability
Belyaev, Mikhail
2016-01-01
We present an instability for exciting incompressible modes (e.g. gravity or Rossby modes) at the surface of a star accreting through a boundary layer. The instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. We call this instability the acoustic CFS instability, because of the direct analogy to the Chandrasekhar-Friedman-Schutz instability for exciting modes on a rotating star by emission of energy in the form of gravitational waves. However, the acoustic CFS instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (i.e.\\ the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. For this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. We discuss applications of o...
Universal High Order Subroutine with New Shock Detector for Shock Boundary Layer Interaction
Oliveria, M; Liu, X; Liu, C
2014-01-01
The goal of this work is to develop a new universal high order subroutine for shock boundary layer interaction. First, an effective shock/discontinuity detector has been developed.The detector has two steps.The first step is to check the ratio of the truncation errors on the coarse and fine grids and the second step is to check the local ratio of the left and right slopes. The currently popular shock/discontinuity detectors can detect shock, but mistake high frequency waves and critical points as shock and then damp the physically important high frequency waves.Preliminary results show the new shock/discontinuity detector is very delicate and can detect all shocks including strong, weak and oblique shocks or discontinuity in function and the first, second, and third order derivatives without artificial constants, but never mistake high frequency waves and critical points, expansion waves as shock. This will overcome the bottle neck problem with numerical simulation for the shock-boundary layer interaction, sh...
Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability
Belyaev, Mikhail A.
2017-02-01
We present an instability for exciting incompressible modes (e.g., gravity or Rossby modes) at the surface of a star accreting through a boundary layer. The instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. We call this instability the acoustic Chandrasekhar–Friedman–Schutz (CFS) instability, because of the direct analogy to the CFS instability for exciting modes on a rotating star by emission of energy in the form of gravitational waves. However, the acoustic CFS instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (i.e., the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. For this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. We discuss applications of our results to high-frequency quasi-periodic oscillations in accreting black hole and neutron star systems and dwarf nova oscillations in cataclysmic variables.
Boundary-value problems for wave equations with data on the whole boundary
Directory of Open Access Journals (Sweden)
Makhmud A. Sadybekov
2016-10-01
Full Text Available In this article we propose a new formulation of boundary-value problem for a one-dimensional wave equation in a rectangular domain in which boundary conditions are given on the whole boundary. We prove the well-posedness of boundary-value problem in the classical and generalized senses. To substantiate the well-posedness of this problem it is necessary to have an effective representation of the general solution of the problem. In this direction we obtain a convenient representation of the general solution for the wave equation in a rectangular domain based on d'Alembert classical formula. The constructed general solution automatically satisfies the boundary conditions by a spatial variable. Further, by setting different boundary conditions according to temporary variable, we get some functional or functional-differential equations. Thus, the proof of the well-posedness of the formulated problem is reduced to question of the existence and uniqueness of solutions of the corresponding functional equations.
The response of the Ocean Surface Boundary Layer and Langmuir turbulence to tropical cyclones
Wang, Dong; Kukulka, Tobias; Reichl, Brandon; Hara, Tetsu; Ginis, Isaac
2016-11-01
The interaction of turbulent ocean surface boundary layer (OSBL) currents and the surface waves' Stokes drift generates Langmuir turbulence (LT), which enhances OSBL mixing. This study investigates the response of LT to extreme wind and complex wave forcing under tropical cyclones (TCs), using a large eddy simulation (LES) approach based on the wave-averaged Navier-Stokes equations. We simulate the OSBL response to TC systems by imposing the wind forcing of an idealized TC storm model, covering the entire horizontal extent of the storm systems. The Stokes drift vector that drives the wave forcing in the LES is determined from realistic spectral wave simulations forced by the same wind fields. We find that the orientations of Langmuir cells are vertically uniform and aligned with the wind in most regions despite substantial wind-wave misalignment in TC conditions. LT's penetration depth is related to Stokes drift depth and limited by OSBL depth. A wind-projected surface layer Langmuir number is proposed and successfully applied to scale turbulent vertical velocity variance in extreme TC conditions. Current affiliation: Princeton University/NOAA GFDL.
Inorganic bromine in the marine boundary layer: a critical review
Directory of Open Access Journals (Sweden)
R. Sander
2003-06-01
Full Text Available The cycling of inorganic bromine in the marine boundary layer (mbl has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is depleted in bromine by about 50% relative to conservative tracers, whereas marine submicrometer aerosol is often enriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that these depletions reflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and other important constituents of marine air. However, currently available techniques cannot reliably quantify many chem{Br}-containing compounds at ambient concentrations and, consequently, our understanding of inorganic Br cycling over the oceans and its global significance are uncertain. To provide a more coherent framework for future research, we have reviewed measurements in marine aerosol, the gas phase, and in rain. We also summarize sources and sinks, as well as model and laboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans, excluding the polar regions. The generation of sea-salt aerosol at the ocean surface is the major tropospheric source producing about 6.2 Tg/a of bromide. The transport of Br from continents (as mineral aerosol, and as products from biomass-burning and fossil-fuel combustion can be of local importance. Transport of degradation products of long-lived Br-containing compounds from the stratosphere and other sources contribute lesser amounts. Available evidence suggests that, following aerosol acidification, sea-salt bromide reacts to form Br_{2 } and BrCl that volatilize to the gas phase and photolyze in daylight to produce atomic Br and Cl. Subsequent transformations can destroy
Inorganic bromine in the marine boundary layer: a critical review
Directory of Open Access Journals (Sweden)
R. Sander
2003-01-01
Full Text Available The cycling of inorganic bromine in the marine boundary layer (mbl has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is substantially depleted in bromine (often exceeding 50% relative to conservative tracers, whereas marine submicrometer aerosol is often enriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that the supermicrometer depletions reflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and other important constituents of marine air. Mechanisms for the submicrometer enrichments are not well understood. Currently available techniques cannot reliably quantify many Br containing compounds at ambient concentrations and, consequently, our understanding of inorganic Br cycling over the oceans and its global significance are uncertain. To provide a more coherent framework for future research, we have reviewed measurements in marine aerosol, the gas phase, and in rain. We also summarize sources and sinks, as well as model and laboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans outside the polar regions. The generation of sea-salt aerosol at the ocean surface is the major tropospheric source producing about 6.2 Tg/a of bromide. The transport of Br from continents (as mineral aerosol, and as products from biomass-burning and fossil-fuel combustion can be of local importance. Transport of degradation products of long-lived Br containing compounds from the stratosphere and other sources contribute lesser amounts. Available evidence suggests that, following aerosol acidification, sea-salt bromide reacts to form Br2 and BrCl that volatilize to the gas phase and photolyze in daylight
Long-lived force patterns and deformation waves at repulsive epithelial boundaries
Rodríguez-Franco, Pilar; Brugués, Agustí; Marín-Llauradó, Ariadna; Conte, Vito; Solanas, Guiomar; Batlle, Eduard; Fredberg, Jeffrey J.; Roca-Cusachs, Pere; Sunyer, Raimon; Trepat, Xavier
2017-10-01
For an organism to develop and maintain homeostasis, cell types with distinct functions must often be separated by physical boundaries. The formation and maintenance of such boundaries are commonly attributed to mechanisms restricted to the cells lining the boundary. Here we show that, besides these local subcellular mechanisms, the formation and maintenance of tissue boundaries involves long-lived, long-ranged mechanical events. Following contact between two epithelial monolayers expressing, respectively, EphB2 and its ligand ephrinB1, both monolayers exhibit oscillatory patterns of traction forces and intercellular stresses that tend to pull cell-matrix adhesions away from the boundary. With time, monolayers jam, accompanied by the emergence of deformation waves that propagate away from the boundary. This phenomenon is not specific to EphB2/ephrinB1 repulsion but is also present during the formation of boundaries with an inert interface and during fusion of homotypic epithelial layers. Our findings thus unveil a global physical mechanism that sustains tissue separation independently of the biochemical and mechanical features of the local tissue boundary.
Swept shock/boundary-layer interactions: Scaling laws, flowfield structure, and experimental methods
Settles, Gary S.
1993-01-01
A general review is given of several decades of research on the scaling laws and flowfield structures of swept shock wave/turbulent boundary layer interactions. Attention is further restricted to the experimental study and physical understanding of the steady-state aspects of these flows. The interaction produced by a sharp, upright fin mounted on a flat plate is taken as an archetype. An overall framework of quasiconical symmetry describing such interactions is first developed. Boundary-layer separation, the interaction footprint, Mach number scaling, and Reynolds number scaling are then considered, followed by a discussion of the quasiconical similarity of interactions produced by geometrically-dissimilar shock generators. The detailed structure of these interaction flowfields is next reviewed, and is illustrated by both qualitative visualizations and quantitative flow images in the quasiconical framework. Finally, the experimental techniques used to investigate such flows are reviewed, with emphasis on modern non-intrusive optical flow diagnostics.
Efficiency of eddy mixing in a stable stratified atmospheric boundary layer
Kurbatskiy, A. F.; Kurbatskaya, L. I.
2011-12-01
Based on a mesoscale RANS model of turbulence, the behavior of turbulent eddy mixing parameters is found to agree with the latest data of laboratory and atmospheric measurements. Some problems of the description of turbulent eddy mixing in the atmospheric boundary layer are studied. When the flow transforms to an extremely stable state, in particular, it is found the flux Richardson number Ri f can change nonmonotonically: it increases with increasing gradient Richardson number Rig until the state of saturation is reached at Ri g ≃ 1 and then decreases. The behavior of the coefficients of eddy diffusion of momentum and heat agrees with the concept of momentum (but not heat) transfer by internal waves propagating in an extremely stable atmospheric boundary layer.
Semenov, A. N.; Gaponov, S. A.
2016-10-01
The boundary layer receptivity process due to the interaction of three-dimensional slow acoustic disturbances is numerically investigated at a free stream Mach number of 2.0. Problem is solved in the linear approximation relatively excited disturbances by an acoustic wave. Numerical simulations were conducted with using the program complex Ansys. In general, matching the results of the approximate method (based on stability equations for low-frequency fluctuations) with direct numerical simulation data is satisfactory. Normalized solutions on the corresponding maxima of the velocity perturbations amplitudes are coincided well enough about a wall. The greatest discrepancy occurs in the area of the boundary layer edge where the approximation theory is inapplicable.
Stability and coherent structures of the asymptotic suction boundary layer over a heated plate
Zammert, Stefan; Eckhardt, Bruno
2016-01-01
The asymptotic suction boundary layer (ASBL) is a parallel shear flow that becomes turbulent in a bypass transition in parameter regions where the laminar profile is stable. We here add a temperature gradient perpendicular to the plate and explore the interaction between convection and shear in determining the transition. We find that the laminar state becomes unstable in a subcritical bifurcation and that the critical Rayleigh number and wave number depend strongly on the Prandtl number. We also track several secondary bifurcations and identify states that are localized in two directions, showing different symmetries. In the subcritical regime, transient turbulent states which are connected to exact coherent states and follow the same transition scenario as found in linearly stable shear flows are identified and analyzed. The study extends the bypass transition scenario from shear flows to thermal boundary layers and shows the intricate interactions between thermal and shear forces in determining critical po...
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 of MHD waves...
Investigation of Materials for Boundary Layer Control in a Supersonic Wind Tunnel
Braafladt, Alexander; Lucero, John M.; Hirt, Stefanie M.
2013-01-01
During operation of the NASA Glenn Research Center 15- by 15-Centimeter Supersonic Wind Tunnel (SWT), a significant, undesirable corner flow separation is created by the three-dimensional interaction of the wall and floor boundary layers in the tunnel corners following an oblique-shock/ boundary-layer interaction. A method to minimize this effect was conceived by connecting the wall and floor boundary layers with a radius of curvature in the corners. The results and observations of a trade study to determine the effectiveness of candidate materials for creating the radius of curvature in the SWT are presented. The experiments in the study focus on the formation of corner fillets of four different radii of curvature, 6.35 mm (0.25 in.), 9.525 mm (0.375 in.), 12.7 mm (0.5 in.), and 15.875 mm (0.625 in.), based on the observed boundary layer thickness of 11.43 mm (0.45 in.). Tests were performed on ten candidate materials to determine shrinkage, surface roughness, cure time, ease of application and removal, adhesion, eccentricity, formability, and repeatability. Of the ten materials, the four materials which exhibited characteristics most promising for effective use were the heavy body and regular type dental impression materials, the basic sculpting epoxy, and the polyurethane sealant. Of these, the particular material which was most effective, the heavy body dental impression material, was tested in the SWT in Mach 2 flow, and was observed to satisfy all requirements for use in creating the corner fillets in the upcoming experiments on shock-wave/boundary-layer interaction.
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...
Evidence of tropospheric layering: interleaved stratospheric and planetary boundary layer intrusions
2007-01-01
International audience; We present a case study of interleaving in the free troposphere of 4 layers of non-tropospheric origin, with emphasis on their residence time in the troposphere. Two layers are stratospheric intrusions at 4.7 and 2.2 km altitude with residence times of about 2 and 6.5 days, respectively. The two other layers at 7 and 3 km altitude were extracted from the maritime planetary boundary layer by warm conveyor belts associated with two extratropical lows and have residence t...
Spina, Eric F.
1995-01-01
The primary objective in the two research investigations performed under NASA Langley sponsorship (Turbulence measurements in hypersonic boundary layers using constant temperature anemometry and Reynolds stress measurements in hypersonic boundary layers) has been to increase the understanding of the physics of hypersonic turbulent boundary layers. The study began with an extension of constant-temperature thermal anemometry techniques to a Mach 11 helium flow, including careful examinations of hot-wire construction techniques, system response, and system calibration. This was followed by the application of these techniques to the exploration of a Mach 11 helium turbulent boundary layer (To approximately 290 K). The data that was acquired over the course of more than two years consists of instantaneous streamwise mass flux measurements at a frequency response of about 500 kHz. The data are of exceptional quality in both the time and frequency domain and possess a high degree of repeatability. The data analysis that has been performed to date has added significantly to the body of knowledge on hypersonic turbulence, and the data reduction is continuing. An attempt was then made to extend these thermal anemometry techniques to higher enthalpy flows, starting with a Mach 6 air flow with a stagnation temperature just above that needed to prevent liquefaction (To approximately 475 F). Conventional hot-wire anemometry proved to be inadequate for the selected high-temperature, high dynamic pressure flow, with frequent wire breakage and poor system frequency response. The use of hot-film anemometry has since been investigated for these higher-enthalpy, severe environment flows. The difficulty with using hot-film probes for dynamic (turbulence) measurements is associated with construction limitations and conduction of heat into the film substrate. Work continues under a NASA GSRP grant on the development of a hot film probe that overcomes these shortcomings for hypersonic
Kaufman, L. G., II; Johnson, C. B.
1981-01-01
Empirical anaytic methods are presented for calculating thermal and pressure distributions in three-dimensional, shock-wave turbulent-boundary-layer, interaction-flow regions on the surface of controllable hypersonic aircraft and missiles. The methods, based on several experimental investigations, are useful and reliable for estimating both the extent and magnitude of the increased thermal and pressure loads on the vehicle surfaces.
PIV-based pressure fluctuations in the turbulent boundary layer
Ghaemi, Sina; Ragni, Daniele; Scarano, Fulvio
2012-12-01
The unsteady pressure field is obtained from time-resolved tomographic particle image velocimetry (Tomo-PIV) measurement within a fully developed turbulent boundary layer at free stream velocity of U ∞ = 9.3 m/s and Reθ = 2,400. The pressure field is evaluated from the velocity fields measured by Tomo-PIV at 10 kHz invoking the momentum equation for unsteady incompressible flows. The spatial integration of the pressure gradient is conducted by solving the Poisson pressure equation with fixed boundary conditions at the outer edge of the boundary layer. The PIV-based evaluation of the pressure field is validated against simultaneous surface pressure measurement using calibrated condenser microphones mounted behind a pinhole orifice. The comparison shows agreement between the two pressure signals obtained from the Tomo-PIV and the microphones with a cross-correlation coefficient of 0.6 while their power spectral densities (PSD) overlap up to 3 kHz. The impact of several parameters governing the pressure evaluation from the PIV data is evaluated. The use of the Tomo-PIV system with the application of three-dimensional momentum equation shows higher accuracy compared to the planar version of the technique. The results show that the evaluation of the wall pressure can be conducted using a domain as small as half the boundary layer thickness (0.5δ99) in both the streamwise and the wall normal directions. The combination of a correlation sliding-average technique, the Lagrangian approach to the evaluation of the material derivative and the planar integration of the Poisson pressure equation results in the best agreement with the pressure measurement of the surface microphones.
Spontaneous generation of inertial waves from boundary turbulence in a librating sphere
Sauret, Alban; Bars, Michael Le
2013-01-01
In this work, we report the excitation of inertial waves in a librating sphere even for libration frequencies where these waves are not directly forced. This spontaneous generation comes from the localized turbulence induced by the centrifugal instabilities in the Ekman boundary layer near the equator and does not depend on the libration frequency. We characterize the key features of these inertial waves in analogy with previous studies of the generation of internal waves in stratified flows from localized turbulent patterns. In particular, the temporal spectrum exhibits preferred values of excited frequency. This first-order phenomenon is generic to any rotating flow in the presence of localized turbulence and is fully relevant for planetary applications.
Grain-boundary layering transitions and phonon engineering
Rickman, J. M.; Harmer, M. P.; Chan, H. M.
2016-09-01
We employ semi-grand canonical Monte Carlo simulation to investigate layering transitions at grain boundaries in a prototypical binary alloy. We demonstrate the existence of such transitions among various interfacial states and examine the role of elastic fields in dictating state equilibria. The results of these studies are summarized in the form of diagrams that highlight interfacial state coexistence in this system. Finally, we examine the impact of layering transitions on the phononic properties of the system, as given by the specific heat and, by extension, the thermal conductivity. Thus, it is suggested that by inducing interfacial layering transitions via changes in temperature or pressure, one can thereby engineer thermodynamic and transport properties in materials.
Investigation of the outer and inner low-latitude boundary layers
Directory of Open Access Journals (Sweden)
T. M. Bauer
Full Text Available We analyze 22 AMPTE/IRM crossings of the day-side low-latitude boundary layer for which a dense outer part can be distinguished from a dilute inner part. Whereas the plasma in the outer boundary layer (OBL is dominated by solar wind particles, the partial densities of solar wind and magnetospheric particles are comparable in the inner boundary layer (IBL. For 11 events we find a reasonable agreement between observed plasma flows and those predicted by the tangential stress balance of an open magnetopause. Thus, we conclude that, at least in these cases, the OBL is formed by a local magnetic reconnection. The disagreement with the tangential stress balance in the other 11 cases might be due to reconnection being time-dependent and patchy. The north-south component of the proton bulk velocity in the boundary layer is, on average, directed toward high latitudes for both low and high magnetic shear across the magnetopause. This argues clearly against the possibility that the dayside low-latitude boundary layer is populated with solar wind plasma primarily from the cusps. "Warm", counterstreaming electrons that originate primarily from the magnetosheath and have a field-aligned temperature that is higher than the electron temperature in the magnetosheath by a factor of 1–5, are a characteristic feature of the IBL. Profiles of the proton bulk velocity and the density of hot ring current electrons provide evidence that the IBL is on closed field lines. Part of the IBL may be on newly opened field lines. Using the average spectra of electric and magnetic fluctuations in the boundary layer, we estimate the diffusion caused by lower hybrid drift instability, gyroresonant pitch angle scattering, or kinetic Alfvén wave turbulence. We find that cross-field diffusion cannot transport solar wind plasma into the OBL or IBL at a rate that would account for the thickness ( ~ 1000 km of these sublayers. On the duskside, the dawn-dusk component of the proton
Perfectly Matched Layer (PML) for Transient Wave Propagation in a Moving Frame of Reference
Madsen, Stine Skov; Krenk, Steen; Hededal, Ole
2013-01-01
In relation to the development of a Rolling Wheel Deflectometer (RWD), which is a non-destructive testing device for measuring pavement deflections, a finite element model for obtaining the soil/pavement response is developed. Absorbing boundary conditions are necessary in order to prevent reflections of the waves propagating through the soil due to the dynamic loading. The Perfectly Matched Layer (PML) has proven to be highly efficient when solving transient wave propagation problems in a fi...
A numerical-physical planetary boundary layer model
Padro, Jacob
1983-07-01
A numerical-physical model for the planetary boundary layer has been formulated for the purpose of predicting the winds, temperatures and humidities in the lowest 1600 m of the atmosphere. An application of the model to the synoptic situation of 30 August, 1972, demonstrates its ability to produce useful forecasts for a period of 24 h. Results are illustrated in terms of horizontal maps and time-height sections of winds and temperatures. The model is divided in the vertical direction into three layers that are governed, respectively, by different physical formulations. At the lowest level, which is the surface of the earth, forecasts of temperature and humidity are computed from empirical relations. In the first layer, the surface layer, application is made of the similarity theories of Monin-Obukhov, Monin-Kazanski and Businger’s form of the universal functions. The second layer, the Ekman layer, is 1550 m deep and is governed by diagnostic momentum and time-dependent thermodynamic and humidity equations. External input to the model are large-scale pressure gradients and middle-level cloudiness. Cressman’s objective analysis procedure is applied to conventional surface and upper air data over a horizontal region of about 2500 km by 2500 km, centered about Lake Ontario. With a grid distance of 127 km and a time interval of 30 min, the computer time required on Control Data Cyber 76 for a 24 h forecast for the case study is less than two minutes.
Visualization of phase conjugate ultrasound waves passed through inhomogeneous layer.
Yamamoto, K; Pernod, P; Preobrazhensky, V
2004-04-01
Compensation of phase distortions of ultrasound beams by means of parametric phase conjugation is visualized. Quasi-plane and focused primary beams were distorted by a polymer aberration layer introduced between the primary wave source and the wave phase conjugator. It is demonstrated acousto-optically that, while the acoustic field is strongly irregular in the area between aberration layer and conjugator, the phase conjugate wave visibly reproduces the primary beams in the area between the layer and the primary wave source. The phenomenon is observed in supercritical mode of parametric amplification when intensity of phase conjugate wave is high enough for manifestations of acoustic nonlinearities in water.
Identification of Lagrangian coherent structures in the turbulent boundary layer
Institute of Scientific and Technical Information of China (English)
无
2009-01-01
Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vor- tices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is im- plemented to get the ensemble-averaged inclination angle θ R of typical LCS. θ R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y+=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y+=100. The ensem- ble-averaged convection velocity Uc of typical LCS is finally calculated from temporal-spatial correla- tion analysis of FTLE field. It is found that the wall-normal profile of the convection velocity Uc(y) ac- cords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the down- stream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond.
Identification of Lagrangian coherent structures in the turbulent boundary layer
Institute of Scientific and Technical Information of China (English)
PAN Chong; WANG JinJun; ZHANG Cao
2009-01-01
Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vor-tices extending deep into the near wall region with an inclination angle θto the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of # accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is im-plemented to get the ensemble-averaged inclination angle θR of typical LCS. θR first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of 8 saturates at Y+=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around Y+=100. The ensem-ble-averaged convection velocity Uc of typical LCS is finally calculated from temporal-spatial correla-tion analysis of FTLE field. It is found that the wall-normal profile of the convection velocity Uc(Y) ac-cords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the down-stream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond.
Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer II: MHD Simulations
Belyaev, Mikhail A; Stone, James M
2013-01-01
We perform global unstratified 3D magnetohydrodynamic simulations of an astrophysical boundary layer (BL) -- an interface region between an accretion disk and a weakly magnetized accreting object such as a white dwarf -- with the goal of understanding the effects of magnetic field on the BL. We use cylindrical coordinates with an isothermal equation of state and investigate a number of initial field geometries including toroidal, vertical, and vertical with zero net flux. Our initial setup consists of a Keplerian disk attached to a non-rotating star. In a previous work, we found that in hydrodynamical simulations, sound waves excited by shear in the BL were able to efficiently transport angular momentum and drive mass accretion onto the star. Here we confirm that in MHD simulations, waves serve as an efficient means of angular momentum transport in the vicinity of the BL, despite the magnetorotational instability (MRI) operating in the disk. In particular, the angular momentum current due to waves is at times...
Vagov, A; Zalipaev, V V
2009-01-01
We extend the asymptotic boundary layer (ABL) method, originally developed for stable resonator modes, to the description of individual wavefunctions localized around unstable periodic orbits. The formalism applies to the description of scar states in fully or partially chaotic quantum systems, and also allows for the presence of smooth and sharp potentials, as well as magnetic fields. We argue that the separatrix wave function provides the largest contribution to the scars on a single wave function. This agrees with earlier results on the wave-function asymptotics and on the quantization condition of the scar states. Predictions of the ABL formalism are compared with the exact numerical solution for a strip resonator with a parabolic confinement potential and a magnetic field.
An LDA investigation of three-dimensional normal shock-boundary layer interactions in a corner
Chriss, R. M.; Keith, T. G., Jr.; Hingst, W. R.; Strazisar, A. J.; Porro, A. R.
1987-01-01
Nonintrusive, three-dimensional, measurements have been made of a normal shock wave-turbulent boundary layer interaction. The measurements were made in the corner of the test section of a continuous supersonic wind tunnel in which a normal shock wave had been stabilized. LDA, surface pressure measurement and flow visualization techniques were employed for two freestream Mach number test cases: 1.6 and 1.3. The former contained separated flow regions and a system of shock waves. The latter was found to be far less complicated. The reported results are believed to accurately define the flow physics of each case and may be used as benchmark data to verify three-dimensional computer codes.
A boundary value problem for the wave equation
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Nezam Iraniparast
1999-01-01
Full Text Available Traditionally, boundary value problems have been studied for elliptic differential equations. The mathematical systems described in these cases turn out to be “well posed”. However, it is also important, both mathematically and physically, to investigate the question of boundary value problems for hyperbolic partial differential equations. In this regard, prescribing data along characteristics as formulated by Kalmenov [5] is of special interest. The most recent works in this area have resulted in a number of interesting discoveries [3, 4, 5, 7, 8]. Our aim here is to extend some of these results to a more general domain which includes the characteristics of the underlying wave equation as a part of its boundary.
Forced wave motion with internal and boundary damping.
Louw, Tobias; Whitney, Scott; Subramanian, Anu; Viljoen, Hendrik
2012-01-01
A d'Alembert-based solution of forced wave motion with internal and boundary damping is presented with the specific intention of investigating the transient response. The dynamic boundary condition is a convenient method to model the absorption and reflection effects of an interface without considering coupled PDE's. Problems with boundary condition of the form [Formula: see text] are not self-adjoint which greatly complicates solution by spectral analysis. However, exact solutions are found with d'Alembert's method. Solutions are also derived for a time-harmonically forced problem with internal damping and are used to investigate the effect of ultrasound in a bioreactor, particularly the amount of energy delivered to cultured cells. The concise form of the solution simplifies the analysis of acoustic field problems.
Nagata, Kouji; Sakai, Yasuhiko; Komori, Satoru
2011-01-01
Effects of weak, small-scale freestream turbulence on turbulent boundary layers with and without thermal convection are experimentally investigated using a wind tunnel. Two experiments are carried out: the first is isothermal boundary layers with and without grid turbulence, and the second is non-isothermal boundary layers with and without grid turbulence. Both boundary layers develop under a small favorable pressure gradient. For the latter case, the bottom wall of the test section is heated...
The effects of external conditions in turbulent boundary layers
Brzek, Brian G.
The effects of multiple external conditions on turbulent boundary layers were studied in detail. These external conditions include: surface roughness, upstream turbulence intensity, and pressure gradient. Furthermore, the combined effects of these conditions show the complicated nature of many realistic flow conditions. It was found that the effects of surface roughness are difficult to generalize, given the importance of so many parameters. These parameters include: roughness geometry, roughness regime, roughness height to boundary layer thickness, (k/delta), roughness parameter, ( k+), Reynolds number, and roughness function (Delta B+). A further complication, is the difficulty in computing the wall shear stress, tauw/rho. For the sand grain type roughness, the mean velocity and Reynolds stresses were studied in inner and outer variables, as well as, boundary layer parameters, anisotropy tensor, production term, and viscous stress and form drag contributions. To explore the effects of roughness and Reynolds number dependence in the boundary layer, a new experiment was carefully designed to properly capture the x-dependence of the single-point statistics. It was found that roughness destroys the viscous layer near the wall, thus, reducing the contribution of the viscous stress in the wall region. As a result, the contribution in the skin friction due to form drag increases, while the viscous stress decreases. This yields Reynolds number invariance in the skin friction, near-wall roughness parameters, and inner velocity profiles as k + increases into the fully rough regime. However, in the transitionally rough regime, (i.e., 5 component shows the largest influence of roughness, where the high peak near the wall was decreased and became nearly flat for the fully rough regime profiles. In addition, the Reynolds stresses in outer variables show self-similarity for fixed experimental conditions. However, as the roughness parameter, k +, increases, all Reynolds stress