Showing papers on "Boundary layer published in 1989"

XFOIL: An Analysis and Design System for Low Reynolds Number Airfoils

Abstract: Calculation procedures for viscous/inviscid analysis and mixed-inverse design of subcritical airfoils are presented. An inviscid linear-vorticity panel method with a Karman-Tsien compressiblity correction is developed for direct and mixed-inverse modes. Source distributions superimposed on the airfoil and wake permit modeling of viscous layer influence on the potential flow. A two-equation lagged dissipation integral method is used to represent the viscous layers. Both laminar and turbulent layers are treated, with an e 9-type amplification formulation determinining the transition point. The boundary layer and transition equations are solved simultaneously with the inviscid flowfield by a global Newton method. The procedure is especially suitable for rapid analysis of low Reynolds number airfoil flows with transitional separation bubbles. Surface pressure distributions and entire polars are calculated and compared with experimental data. Design procedure examples are also presented.

Parameterization of Orography-Induced Turbulence in a Mesobeta--Scale Model

Abstract: The possibility of extending existing techniques for turbulence parameterization in the planetary boundary layer to attitude, orography-induced turbulence events is examined. Starting from a well-tested scheme, we show that it is possible to generalize the specification method of the length scales, with no deterioration of the scheme performance in the boundary layer. The new scheme is implemented in a two-dimensional version of a limited-area, numerical model used for the simulation of mesobeta-scale atmospheric flows. Three well-known cases of orographically induced turbulence are studied. The comparison with observations and former studies shows a satisfactory behavior of the new scheme.

Scaling of hard thermal turbulence in Rayleigh-Bénard convection

Abstract: An experimental study of Rayleigh-Benard convection in helium gas at roughly 5 K is performed in a cell with aspect ratio 1. Data are analysed in a ‘hard turbulence’ region (4 × 107 < Ra < 6 × 1012) in which the Prandtl number remains between 0.65 and 1.5. The main observation is a simple scaling behaviour over this entire range of Ra. However the results are not the same as in previous theories. For example, a classical result gives the dimensionless heat flux, Nu, proportional to . A new scaling theory is described. This new approach suggests scaling indices very close to the observed ones. The new approach is based upon the assumption that the boundary layer remains in existence even though its Rayleigh number is considerably greater than unity and is, in fact, diverging. A stability analysis of the boundary layer is performed which indicates that the boundary layer may be stabilized by the interaction of buoyancy driven effects and a fluctuating wind.

Coherent structure of the convective boundary layer derived from large-eddy simulations

Abstract: Turbulence in the convective boundary layer (CBL) uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for zero mean horizontal flow using large-eddy simulations (LES). The Rayleigh number is effectively infinite, the Froude number of the stable layer is 0.09 and the surface roughness height relative to the height of the convective layer is varied between 10−6 and 10−2. The LES uses a finite-difference method to integrate the three-dimensional grid-volume-averaged Navier–Stokes equations for a Boussinesq fluid. Subgrid-scale (SGS) fluxes are determined from algebraically approximated second-order closure (SOC) transport equations for which all essential coefficients are determined from the inertial-range theory. The surface boundary condition uses the Monin–Obukhov relationships. A radiation boundary condition at the top of the computational domain prevents spurious reflections of gravity waves. The simulation uses 160 × 160 × 48 grid cells. In the asymptotic state, the results in terms of vertical mean profiles of turbulence statistics generally agree very well with results available from laboratory and atmospheric field experiments. We found less agreement with respect to horizontal velocity fluctuations, pressure fluctuations and dissipation rates, which previous investigations tend to overestimate. Horizontal spectra exhibit an inertial subrange. The entrainment heat flux at the top of the CBL is carried by cold updraughts and warm downdraughts in the form of wisps at scales comparable with the height of the boundary layer. Plots of instantaneous flow fields show a spoke pattern in the lower quarter of the CBL which feeds large-scale updraughts penetrating into the stable layer aloft. The spoke pattern has also been found in a few previous investigations. Small-scale plumes near the surface and remote from strong updraughts do not merge together but decay while rising through large-scale downdraughts. The structure of updraughts and downdraughts is identified by three-dimensional correlation functions and conditionally averaged fields. The mean circulation extends vertically over the whole boundary layer. We find that updraughts are composed of quasi-steady large-scale plumes together with transient rising thermals which grow in size by lateral entrainment. The skewness of the vertical velocity fluctuations is generally positive but becomes negative in the lowest mesh cells when the dissipation rate exceeds the production rate due to buoyancy near the surface, as is the case for very rough surfaces. The LES results are used to determine the root-mean-square value of the surface friction velocity and the mean temperature difference between the surface and the mixed layer as a function of the roughness height. The results corroborate a simple model of the heat transfer in the surface layer.

Turbulent oscillatory boundary layers at high Reynolds numbers

Abstract: This study deals with turbulent oscillatory boundary-layer flows over both smooth and rough beds. The free-stream flow is a purely oscillating flow with sinusoidal velocity variation. Mean and turbulence properties were measured mainly in two directions, namely in the streamwise direction and in the direction perpendicular to the bed. Some measurements were made also in the transverse direction. The measurements were carried out up to Re = 6 × 106 over a mirror-shine smooth bed and over rough beds with various values of the parameter a/ks covering the range from approximately 400 to 3700, a being the amplitude of the oscillatory free-stream flow and ks the Nikuradse's equivalent sand roughness. For smooth-bed boundary-layer flows, the effect of Re is discussed in greater detail. It is demonstrated that the boundary-layer properties change markedly with Re. For rough-bed boundary-layer flows, the effect of the parameter a/ks is examined, at large values (O(103)) in combination with large Re.

Turbulent Boundary-Layer Separation

Abstract: This article summarizes our present understanding of the physical behavior of two-dimensional turbulent separated flows, which occur due to adverse pressure gradients around streamlined and bluff bodies. The physical behavior of turbulence is flow dependent, so detailed experimental infor­ mation is needed for understanding such flows and modeling their physics for calculation methods. An earlier review (Simpson 1 985) discussed in much detail prior experimental and computational work, and this was followed by an updated review of calculation methods only (Simpson 1 987). Here additional recent references are added to those cited in the two other works. By separation, we mean the entire process of departure or breakaway, or the breakdown of boundary-layer flow. An abrupt thickening of the rotational-flow region next to a wall and significant values of the normal­ to-wall velocity component must accompany breakaway, or otherwise this region would not have any significant interaction with the free-stream flow. This unwanted interaction causes a reduction in the performance of the flow device of interest (e.g. a loss of lift on an airfoil or a loss of pressure rise in a diffuser). It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two­ dimensional flow the surface shear stress can change sign with flow reversal without the occurrence of breakaway_ Conversely, the breakdown of the boundary-layer concept can occur before any flow reversal is encountered. In three-dimensional flow the rotational layer can depart without the

Ozone concentration in leaf intercellular air spaces is close to zero.

Abstract: Transpiration and ozone uptake rates were measured simultaneously in sunflower leaves at different stomatal openings and various ozone concentrations. Ozone uptake rates were proportional to the ozone concentration up to 1500 nanoliters per liter. The leaf gas phase diffusion resistance (stomatal plus boundary layer) to water vapor was calculated and converted to the resistance to ozone multiplying it by the theoretical ratio of diffusion coefficients for water vapor and ozone in air (1.67). The ozone concentration in intercellular air spaces calculated from the ozone uptake rate and diffusion resistance to ozone scattered around zero. The ozone concentration in intercellular air spaces was measured directly by supplying ozone to the leaf from one side and measuring the equilibrium concentration above the other side, and it was found to be zero. The total leaf resistance to ozone was proportional to the gas phase resistance to water vapor with a coefficient of 1.68. It is concluded that ozone enters the leaf by diffusion through the stomata, and is rapidly decomposed in cell walls and plasmalemma.

Low Froude Number Flow Past Three-Dimensional Obstacles. Part I: Baroclinically Generated Lee Vortices

Abstract: We study the flow of a density-stratified fluid past a three-dimensional obstacle, using a numerical model. Our special concern is the response of the fluid when the Froude number is near or less than unity. Linear theory is inapplicable in this range of Froude number, and the present numerical solutions show the rich variety of phenomena that emerge in this essentially nonlinear flow regime. Two such phenomena, which occupy Parts I and II of this study, are the formation of a pair of vertically oriented vortices on the lee side and a zone of flow reversal on the windward side of the obstacle. The Ice vortices have been explained as a consequence of the separation of the viscous boundary layer from the obstacle however, this boundary layer is absent (by design) in the present experiments and lee vortices still occur. We argue that a vertical component of vorticity develops on the lee side owing to the tilting of horizontally oriented vorticity produced baroclinically as the isentropes deform in r...

The viscous flow on surfaces with longitudinal ribs

Abstract: The viscous sublayer of a turbulent boundary layer on a surface with fine longitudinal ribs (riblets) is investigated theoretically. The mean flow constituent of this viscous flow is considered. Using conformal mapping, the velocity distributions on various surface configurations are calculated. The geometries that were investigated include sawtooth profiles with triangular and trapezoidal grooves as well as profiles with thin blade-shaped ribs, ribs with rounded edges and ribs having sharp ridges and U-shaped grooves. (This latter riblet configuration is also found on the tiny scales of fast sharks.) Our calculations enable us to determine the location of the origin of the velocity profile that lies somewhat below the tips of the ridges. The distance between this origin and the tip of the ridge we call ‘protrusion height’. The upper limit for the protrusion height is found to be 22% of the lateral rib spacing; the coefficient 0.22 being the value of the expression π−1 In 2. This limit is valid for two-dimensional riblet geometries. Analogous experiments with an electrolytic tank are carried out as an additional check on the theoretical calculations. This is also an easy way to determine experimentally the location of the origin of the velocity profile for arbitrary new riblet geometries. A possible connection between protrusion height and drag reduction in a turbulent boundary layer flow is discussed. Finally, the present theory also produces an orthogonal grid pattern above riblet surfaces which may be utilized in future numerical calculations of the whole turbulent boundary layer.

Second-moment closure for the near-wall sublayer - Development and application

Abstract: This paper extends the widely used second-moment closure of Gibson and Launder so as to be applicable within the near-wall sublayer where viscous effects are substantial. This extension, which allows the usual log-law/local-equilibrium matching to be discarded, enables boundary-layer problems to be tackled where the flow structure in the inner region departs from what is usually termed the "universal" wall law. Applications are reported of the flat-plate boundary layer and three cases in which strong streamwise pressure gradients are imposed. Agreement with experiment is better than when the simpler A-e model is adopted.

Large-Eddy Simulation of the Convective Atmospheric Boundary Layer

Abstract: Large-eddy simulations of a free convective atmospheric boundary layer with an overlying capping inversion are considered. Attention is given to the dependence of the results upon the various factors influencing the simulation: the subgrid model, the domain size, and the mesh resolution. By providing artificial constraints upon the convection the results also provide extra insight into the underlying dynamics. The gross features of the boundary layer, such as the overall energy budget, are not sensitive to the details of the simulations but a number of important factors are revealed. It has been found that near the surface the subgrid diffusivity must be larger than is usually supposed, in order for the vertical velocity skewness to have the correct sign. This region of the flow has a significant subgrid-scale heat flux and it seems that the subgrid model requires improvement in such cases. A revised model which under statically unstable conditions allows the mixing-length of the subgrid-scale tu...

Near-wall structure of a turbulent boundary layer with riblets

Abstract: A detailed wind tunnel study has been carried out on the near-wall turbulence structure over smooth and riblet wall surfaces under zero pressure gradient. Time-average quantities as ‘well as conditionally sampled profiles were obtained using hotwire/film anemometry, along with a simultaneous flow visualization using the smoke-wire technique and a sheet of laser light. The experimental results indicated a significant change of the structure in the turbulent boundary layer near the riblet surface. The change was confined within a small volume of the flow close to the wall surface. A conceptual model for the sequence of the bursts was then proposed based on an extensive study of the flow visualization, and was supported by the results of conditionally sampled velocity fields. A possible mechanism of turbulent drag reduction by riblets is discussed.

Is the Tropical Atmosphere Conditionally Unstable

Abstract: We examine the mean and standard deviation of the bouyancy of lifted boundary layer parcels from a large sample of soundings from the western equatorial Pacific. The inclusion of condensate loading in the definition of buoyancy is emphasized as is the precise originating level of parcels in the subcloud layer. We confirm the observation of Betts that those parts of the tropical atmosphere experiencing deep convection are nearly neutral to adiabatic parcel ascent from the subcloud layer when adiabatic condensate loading is included in the definition of buoyancy. Parcels lifted from the top of the subcloud layer are more nearly neutral than those originating near the surface, and atmospheres with higher levels of free convection (LFCs) are closer to neutral than those with low LFCs. We explore possible reasons for these observations and discuss their implications for tropical meteorology.

Turbulence and the diffusive layers around small organisms

Abstract: We consider whether the diffusion of substances to and from a small spherical organism that is motionless relative to the surrounding water is significantly affected by the turbulent motions in the water. Viscosity, by smoothing out turbulent eddies less than a few millimeters across, dictates that the flow of nutrients and wastes to and from a very small organism must occur by molecular diffusion through a thin surrounding boundary layer. Motion of the organism relative to the water through sinking or swimming distorts this boundary layer. This alters the gradients and causes a fairly well understood increase in diffusive flux. The effect of turbulent motion on the flux, however, is less well understood. We first clarify the relationship between the size of the smallest turbulent eddies and the Kolmogorov or viscous length and go on to contend that turbulent motion over the small distances near small organisms is manifested as a linear velocity gradient whose magnitude is determined by the rate of turbulent energy dissipation. Knowing the shear enables us to calculate, from the experimental results of Purcel (1978, Journal of Fluid Mechanics, 84, 551–559), the quantitative effect of the turbulence on the diffusive flux to and from an idealized spherical organism. For motionless cells 100 μm in diameter, high levels of turbulence (dissipation rate 10−6 W kg−1_ produce a ⋍2% increase in flux. This escalates to a 100% increase for cells ⋍1 mm in diameter. Our results also lead to the conclusion that the microzones of increased nutrient levels surrounding small organisms, proposed by Mitchell et al. (1985, Nature, 316, 58–59), are much more robust than they suggested.

Climatic Equilibrium of the Atmospheric Convective Boundary Layer over a Tropical Ocean

Abstract: A radiative-convective boundary layer model was developed by coupling a thermodynamic model of a partially mixed convective boundary layer (CBL) with a radiation model, and energy balance constraints were used to study coupled boundary layer (CBL) equilibrium over three timescales (about 1 day, about 10 days, and more than 100 days). It is shown that the variation in cloud top decreases with greater coupling to the atmosphere and the ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverses sign with a coupled ocean.

Vertical mixing due to the breaking of critical internal waves on sloping boundaries

Abstract: A laboratory experiment is used to examine the vertical mixing resulting from the breaking of internal waves on a sloping boundary in a continuously stratified fluid. Attention is confined to the case of critical waves when the slope of the group velocity vector of the incident waves is equal to the bottom slope. Along the sloping boundary a turbulent bottom boundary layer forms with a thickness dependent on the incident wave amplitude. The mixing efficiency, defined as the ratio of the increase in potential energy due to mixing to the loss of kinetic energy by the incident waves, is dependent upon the stability of the flow and has an upper bound of approximately 0.20.By examining the increase in potential energy of the fluid as a result of sustained mixing, we are able to compute the transition value of the dissipation etr below which no mixing occures. For mixing due to the breaking of critical internal waves on sloping boundaries we find that etr = (8±2)νN2. From comparisons with experiments with grid-generated turbulence, this suggests that while etr/νN2 = 0(10) in the available data sets, the specific value of etr may be mechanism dependent.

Flow and heat transfer in a viscoelastic fluid over a stretching sheet

Abstract: The paper discusses the flow of an incompressible second-order fluid due to stretching of a plane elastic surface in the approximation of boundary layer theory. An exact analytical solution of the non-linear equation governing the self-similar flow is given. The skin friction decreases with increase in the elastic parameter k1. The analysis of heat transfer in this flow reveals that when the wall and the ambient temperature are held constant, temperature at a point increases with increase in k1, for fixed Prandtl number σ.

Boundary-Layer Receptivity to Long-Wave Free-Stream Disturbances

Abstract: The present treatment of the early stages of boundary layer-transition phenomena, where the unsteady motion is of small amplitude and can be accordingly treated as a small perturbation of an appropriate mean flow, elaborates the Heinrich et al. (1988) discussion of the role played by this 'receptivity' stage: in which the unsteady flow exhibits the same harmonic time-dependence as the externally-imposed forcing. Freestream disturbance wavelengths are noted to often be much longer than the Tollmien-Schlichting wavelength. Attention is given to the variety of wavelength-reduction mechanisms able to couple the long-wavelength, freestream disturbances to the comparatively short Tollmien-Schlichting waves.

Boundary Conditions at the Cartilage-Synovial Fluid Interface for Joint Lubrication and Theoretical Verifications

Abstract: The objective of this study is to establish and verify the set of boundary conditions at the interface between a biphasic mixture (articular cartilage) and a Newtonian or non-Newtonian fluid (synovial fluid) such that a set of well-posed mathematical problems may be formulated to investigate joint lubrication problems. A "pseudo-no-slip" kinematic boundary condition is proposed based upon the principle that the conditions at the interface between mixtures or mixtures and fluids must reduce to those boundary conditions in single phase continuum mechanics. From this proposed kinematic boundary condition, and balances of mass, momentum and energy, the boundary conditions at the interface between a biphasic mixture and a Newtonian or non-Newtonian fluid are mathematically derived. Based upon these general results, the appropriate boundary conditions needed in modeling the cartilage-synovial fluid-cartilage lubrication problem are deduced. For two simple cases where a Newtonian viscous fluid is forced to flow (with imposed Couette or Poiseuille flow conditions) over a porous-permeable biphasic material of relatively low permeability, the well known empirical Taylor slip condition may be derived using matched asymptotic analysis of the boundary layer at the interface.

Inclusion of a TKE Boundary Layer Parameterization in the Canadian Regional Finite-Element Model

Abstract: The formulation of the regional model recently implemented by the Atmospheric Environment Service of Canada for its operational 48 h NWP forecasts is presented. The emphasis is put on the parameterization of the physical processes, especially those affecting the atmospheric boundary layer. The originality of this model, in addition to the use of 3-D finite elements, of variable meshes in both the horizontal and vertical, and of being non-nested (as previously described by Staniforth and Daley), consists in the treatment of the time-dependent turbulent Kinetic energy (TKE) and the inclusion of the full diurnal cycle. The overall organization of the model calculations it also presented in order to convey a more accurate description of this integrated system. Sample results from the well-known case of the Presidents' Day Cyclone of 1979 and general performance are covered in the last section.

Mechanics of flow over ripples and dunes

Abstract: A simple predictive model for velocity and boundary shear stress fields over two-dimensional bedforms of finite amplitude is presented herein, and the results of that model are compared with flume data. The production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of McLean and Smith (1986). Their model is modified in order to take account of the flow response to short-wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior. The theory is also extended to include the region between the bedform crest and the reattachment point, thereby providing a spatially complete field of velocity and shear stress. Results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser-Doppler velocimeter. Our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well-developed natural bedforms.

An experimental study of global instabilities due to the tidal (elliptical) distortion of a rotating elastic cylinder

Abstract: Broad band secondary instability of elliptical vortex motion has been proposed as a principal source of shear-flow turbulence. Here experiments on such instability in an elliptical flow with no shear boundary layer are described. This is made possible by the mechanical distortion in the laboratory frame of a rotating fluid-filled elastic cylinder. One percent ellipticity of a 10 cm diameter cylinder rotating once each second can give rise to an exponentially-growing mode stationary in the laboratory frame. In first order this mode is a sub-harmonic parametric Faraday instability. The finite-amplitude equations represent angular momentum transfer on an inertial time scale due to Reynolds stresses. The growth of this mode is not limited by boundary friction but by detuning and centrifugal stabilization. On average, a generalized Richardson number achieves a marginal value through much of the evolved flow. However, the characteristic flow is intermittent with the cycle: rapid growth, stabilizing mom...

Oxygen uptake kinetics in the benthic boundary layer

Abstract: A simple, one-dimensional, transport-reaction model of oxygen uptake by sediments during incubation experiments assumes zero-order reaction kinetics for oxygen removal in the pore water of sediments and takes the apparent diffusive limitation posed by the benthic boundary layer into account. Model calculations are in excellent agreement with experimental data obtained during in situ benthic flux chamber experiments in Gullmarsfjorden, Sweden, during fall (water temp, 10°C) and winter (- 1’C). The boundary layer “resistance,” represented by a mean diffusive sublayer thickness, was estimated with a series of radioactive tracers. The oxygen uptake curve was initially approximately linear but became exponential below - 100 PM oxygen. The half-removal time of oxygen below 100 PM corresponds to a boundary layer resistance similar to that obtained with the radiotracers. The nearly constant rate of decrease of oxygen during the initial phase of the incubations suggests that incubation techniques can be used to obtain an approximate measure of the oxygen uptake rate of sediments as long as the concentration remains above the transition where the curve changes from approximately linear to exponential. This procedure, however, underestimates the oxygen uptake rate by 28-34% compared to model results. Model calculations also suggest that oxygen uptake rates can differ substantially between chamber and outside sediments, especially when hydrodynamic conditions influencing the apparent boundary layer resistance are different within and outside chambers.

Numerical Calculation of Bubble Growth in Nucleate Boiling From Inception Through Departure

Abstract: The relative contributions of the fundamental mechanisms accounting for theenhanced heat transfer in nucleate boiling are difficult to quantifyanalytically or experimentally. A comprehansive model was developed thatpermits some accurate insights into this problem. An essential feature involvedthe numerical mapping of the complicated geometry to a plane where the bubbleand wall boundaries lie along constant coordinate lines. The results show thatmicrolayer evaporation accounts for 87 percent of the enhanced wall heattransfer during saturated boiling of water at 1 atm and 8.5 K wall superheat.In contrast, enhanced convective effects were essentially nonexistent duringgrowth and minimal following depature. The analysis predicts an extremelynonuniform thermal boundary layer around the bubble, and shows that the wallthermal boundary layer regenerates almost immediately following departure.

Numerical simulation of boundary-layer transition and transition control

Abstract: The laminar-turbulent transition process in a parallel boundary-layer with Blasius profile is simulated by numerical integration of the three-dimensional incompressible Navier-Stokes equations using a spectral method. The model of spatially periodic disturbances developing in time is used. Both the classical Klebanoff-type and the subharmonic type of transition are simulated. Maps of the three-dimensional velocity and vorticity fields and visualizations by integrated fluid markers are obtained. The numerical results are compared with experimental measurements and flow visualizations by other authors. Good qualitative and quantitative agreement is found at corresponding stages of development up to the one-spike stage. After the appearance of two-dimensional Tollmien-Schlichting waves of sufficiently large amplitude an increasing three-dimensionality is observed. In particular, a peak-valley structure of the velocity fluctuations, mean longitudinal vortices and sharp spike-like instantaneous velocity signals are formed. The flow field is dominated by a three-dimensional horseshoe vortex system connected with free high-shear layers. Visualizations by time-lines show the formation of A-structures. Our numerical results connect various observations obtained with different experimental techniques. The initial three-dimensional steps of the transition process are consistent with the linear theory of secondary instability. In the later stages nonlinear interactions of the disturbance modes and the production of higher harmonics are essential.We also study the control of transition by local two-dimensional suction and blowing at the wall. It is shown that transition can be delayed or accelerated by superposing disturbances which are out of phase or in phase with oncoming Tollmien-Schlichting instability waves, respectively. Control is only effective if applied at an early, two-dimensional stage of transition. Mean longitudinal vortices remain even after successful control of the fluctuations.

Resonant growth of three-dimensional modes in transitioning blasius boundary layers

Abstract: By carefully controlled phase-coupled input of simultaneous two- and three-dimensional disturbances, the nonlinear evolution and breakdown of the laminar flow in a boundary layer was examined. This involved the generation of plane Tollmien–Schlichting waves and pairs of oblique waves so as to promote nearresonance conditions which have been theoretically shown to lead to the rapid development of three-dimensionality in unstble boundary layers. Special emphasis is placed on the two prominent mechanisms, namely resonant-triads of Orr–Sommerfeld modes and the secondary instability of the streamwise periodic flow to spanwise periodic three-dimensional disturbances. The sensitivity of these mechanisms on the amplitudes and wavenumbers of the input disturbances was of special focus.The simultaneous two- and three-dimensional wave generation was accomplished using a spanwise array of line heaters suspended just above the wall at the approximate height of the critical layer in the laminar boundary layer. These were operated to produce, through local heating, time-periodic spanwise-phase-varying velocity perturbations. Of primary emphasis in this paper are conditions obtained by the combined forcing of fundamental plane waves with wavenumbers (α, 0) and pairs of subharmonic oblique waves (½α, ± β). The reslults document resonant growth of energy in the subharmonic modes, the formation of staggered lambda vortex patterns with a cross-stream scale commensurate with the seeded ± β condition, and their subsequet transition to turbulence. Complete documentation of the flow field at these various stages is presented using smoke-wire flow visualization and through phase-conditioned hot-wire surveys measuring all three velocity components in three space dimensions.

Turbulence structure of a boundary layer beneath a turbulent free stream

Abstract: Measurements have been made in the turbulent boundary layer on a flat plate in the presence of grid-generated free-stream turbulence with a wide range of lengthscales Complete balances of turbulent kinetic energy and shear stress have been evaluated, dissipation and pressure strain redistribution having been deduced by difference

Direct numerical study of leading-edge contamination

Abstract: Instability, turbulence, and relaminarization in the attachment-line region of swept and unswept cylindrical bodies are studied by numerical solution of the full Navier-Stokes equations. The flow is simulated over a strip containing the attachment-line and treated as homogeneous in the spanwise direction; the disturbances decay exponentially upstream. Transpiration through the wall may be prescribed. The new method, which admits completely general disturbance, agrees with published linear-stability results, which were limited to an apparently restrictive form of disturbance. Fully developed turbulent solutions with sweep are generated and compare well with the experiment. The turbulence is subcritical (except for blowing), resulting in large hysteresis loops. By lowering the sweep Reynolds number, or increasing the suction, the turbulent flow is made to relaminarize. The relaminarization Reynolds number is much less sensitive to suction than the linear-stability Reynolds number. Extensive attempts to detect the postulated nonlinear instability of the unswept flow failed, suggesting that this flow is linearly and nonlinearly stable.