Showing papers on "Flow separation published in 2000"

Detached-Eddy Simulations past a circular cylinder

Abstract: The flow is calculated with laminar separation (LS) at Reynolds numbers 50,000 and 140,000, and with turbulent separation (TS) at140,000 and 3 × 106. The TS cases are effectively tripped, but compared with untripped experiments at very high Reynolds numbers. The finest grid has about 18,000 points in each of 56 grid planes spanwise; the resolution is far removed from Direct Numerical Simulations, and the turbulence model controls the separation if turbulent. The agreement is quite good for drag, shedding frequency, pressure, and skin friction. However the comparison is obscured by large modulations of the vortex shedding and drag which are very similar to those seen in experiments but also, curiously, durably different between cases especially of the LS type. The longest simulations reach only about 50 shedding cycles. Disagreement with experimental Reynolds stresses reaches about 30%, and the length of the recirculation bubble is about double that measured. The discrepancies are discussed, as are the effects of grid refinement, Reynolds number, and a turbulence-model curvature correction. The finest grid does not give the very best agreement with experiment. The results add to the validation base of the Detached-Eddy Simulation (DES) technique for smooth-surface separation. Unsteady Reynolds-averaged simulations are much less accurate than DES for LS cases, but very close for TS cases. Cases with a more intricate relationship between transition and separation are left for future study.

Direct numerical simulation of'short' laminar separation bubbles with turbulent reattachment

Abstract: Direct numerical simulation of the incompressible Navier–Stokes equations is used to study flows where laminar boundary-layer separation is followed by turbulent reattachment forming a closed region known as a laminar separation bubble. In the simulations a laminar boundary layer is forced to separate by the action of a suction profile applied as the upper boundary condition. The separated shear layer undergoes transition via oblique modes and [Lambda]-vortex-induced breakdown and reattaches as turbulent flow, slowly recovering to an equilibrium turbulent boundary layer. Compared with classical experiments the computed bubbles may be classified as ‘short’, as the external potential flow is only affected in the immediate vicinity of the bubble. Near reattachment budgets of turbulence kinetic energy are dominated by turbulence events away from the wall. Characteristics of near-wall turbulence only develop several bubble lengths downstream of reattachment. Comparisons are made with two-dimensional simulations which fail to capture many of the detailed features of the full three-dimensional simulations. Stability characteristics of mean flow profiles are computed in the separated flow region for a family of velocity profiles generated using simulation data. Absolute instability is shown to require reverse flows of the order of 15–20%. The three-dimensional bubbles with turbulent reattachment have maximum reverse flows of less than 8% and it is concluded that for these bubbles the basic instability is convective in nature.

Approximate Wall Boundary Conditions in the Large-Eddy Simulation of High Reynolds Number Flow

Abstract: The near-wall regions of high Reynolds numbers turbulent flows must be modelled to treat many practical engineering and aeronautical applications. In this review we examine results from simulations of both attached and separated flows on coarse grids in which the near-wall regions are not resolved and are instead represented by approximate wall boundary conditions. The simulations use the dynamic Smagorinsky subgrid-scale model and a second-order finite-difference method. Typical results are found to be mixed, with acceptable results found in many cases in the core of the flow far from the walls, provided there is adequate numerical resolution, but with poorer results generally found near the wall. Deficiencies in this approach are caused in part by both inaccuracies in subgrid-scale modelling and numerical errors in the low-order finite-difference method on coarse near-wall grids, which should be taken into account when constructing models and performing large-eddy simulation on coarse grids. A promising new method for developing wall models from optimal control theory is also discussed.

Computation of Trailing-Edge Flow and Noise Using Large-Eddy Simulation

Abstract: Turbulent boundary layers near the trailing edge of a lifting surface are known to generate intense, broadband scattering noise as well as surface pressure e uctuations. Numerically predicting the trailing-edge noise requires that the noise-generating eddies over a wide range of length scales be adequately represented. The large-eddy simulation (LES) technique provides a promising tool for obtaining the unsteady wall-pressure e elds and the acousticsourcefunctions. An LES iscarried out forturbulent boundary-layere ow pastan asymmetrically beveled trailing edge ofa e at strut at a chord Reynolds number of 2 :15 £ 10 6 . The computed velocity and surface pressure statistics compare reasonably well with previous experimental measurements. The far-e eld acoustic calculation is facilitated by the integral solution to the Lighthill equation derived by Ffowcs-Williams and Hall. Computations havebeen carried outto determine thefar-e eld noisespectra,thesource-term characteristics, and therequirement for the integration domain size. It is found that the present LES domain is adequate for predicting noise radiation over a range of frequencies. At the low-frequency end, however, the spanwise source coherenceestimated based on surface pressure e uctuations does not decay sufe ciently, suggesting the need for a wider computational domain.

A note on the overlap region in turbulent boundary layers

Abstract: Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters desc...

Turbulent structures in accelerating boundary layers

Abstract: The vortical structures in spatially developing turbulent boundary layers subjected to streamwise acceleration are studied. Two cases are examined: one in which the acceleration is insufficient to cause reversion to the laminar state of the initially turbulent flow, and another in which the acceleration is stronger and relaminarization begins to take place; the pressure gradient, however, is not maintained long enough for full reversion to occur. The turbulent statistics show the expected trends: the mean velocity profile deviates significantly from the logarithmic law-of-the-wall and shows, in the strongly accelerated case, a tendency to approach the laminar profile; the turbulent kinetic energy increases less rapidly than the energy of the mean flow. The structure of the inner layer is also significantly altered. In the near-wall region, the streaks become more elongated and show fewer undulations, owing to a significant decrease of the spanwise fluctuations relative to the streamwise ones. The coherent...

Direct simulation of the turbulent boundary layer along a compression ramp at M = 3 and Reθ = 1685

Abstract: The turbulent boundary layer along a compression ramp with a deflection angle of 18° at a free-stream Mach number of M = 3 and a Reynolds number of Reθ = 1685 with respect to free-stream quantities and mean momentum thickness at inflow is studied by direct numerical simulation. The conservation equations for mass, momentum, and energy are solved in generalized coordinates using a 5th-order hybrid compact- finite-difference-ENO scheme for the spatial discretization of the convective fluxes and 6th-order central compact finite differences for the diffusive fluxes. For time advancement a 3rd-order Runge–Kutta scheme is used. The computational domain is discretized with about 15 × 106 grid points. Turbulent inflow data are provided by a separate zero-pressure-gradient boundary-layer simulation. For statistical analysis, the flow is sampled 600 times over about 385 characteristic timescales δ0/U∞, defined by the mean boundary-layer thickness at inflow and the free-stream velocity. Diagnostics show that the numerical representation of the flow field is sufficiently well resolved.Near the corner, a small area of separated flow develops. The shock motion is limited to less than about 10% of the mean boundary-layer thickness. The shock oscillates slightly around its mean location with a frequency of similar magnitude to the bursting frequency of the incoming boundary layer. Turbulent fluctuations are significantly amplified owing to the shock–boundary-layer interaction. Reynolds-stress maxima are amplified by a factor of about 4. Turbulent normal and shear stresses are amplified differently, resulting in a change of the structure parameter. Compressibility affects the turbulence structure in the interaction area around the corner and during the relaxation after reattachment downstream of the corner. Correlations involving pressure fluctuations are significantly enhanced in these regions. The strong Reynolds analogy which suggests a perfect correlation between velocity and temperature fluctuations is found to be invalid in the interaction area.

Eddy structure in turbulent boundary layers

Abstract: In this paper a new analysis is proposed for the driving mechanisms and the statistics for turbulent boundary layers at very high Reynolds numbers. It differs from theories for moderate to low Reynolds numbers and is based on the results of (linear) rapid distortion theory, and both laboratory and field experimental data. The large-scale eddy structure near the wall in boundary layers is distorted in several ways: by the strong mean shear, by the blocking of the normal velocity component and by the moving internal shear layers produced by large eddies as they impinge and scrape along the wall. Elongated streamwise vortices are formed with length scales that are several times the boundary layer height. An approximate stability argument suggests that if the Reynolds number for the turbulence, Re τ ≫10 4 , these internal layers are fully turbulent and that the large eddies can burst upward where the vortical eddies interact. The forms of the main statistical quantities, such as variances, spectra, length scales, are derived in terms of outer layer quantities using surface similarity and inhomogeneous linear theory. These `top-down' eddy-impingement, inner-layer/eddy-interaction/ejection mechanisms at very high Reynolds number are sensitive to changes in surface conditions and to variations in pressure gradients. They may therefore require different techniques for their control from those used at lower Reynolds number when boundary layers are driven by `bottom-up' instability/surface-interaction mechanisms. Furthermore, accurate numerical modelling of boundary layers at high Reynolds number requires resolving surface processes at very fine resolution. By inference, it is likely that there is some residual `top-down' influence, even at low Re τ .

Boundary layer separation control with directed synthetic jets

Abstract: A new concept for boundary layer separation control has been developed that is a derivative of the synthetic jet concept (being used primarily for virtual shape control) which converts acoustic oscillations into mean fluid motions. The new concept, the so-called “directed synthetic jet”, has an acoustically excited neck like the synthetic jet, however the neck is curved in the downstream tangential direction. In this manner, the boundary layer flowing over the neck or slot is energized via suction removal of the approaching low momentum fluid on the in-stroke and tangential blowing of high momentum on the out-stroke, thereby making it in the time average more resistant to separation. The concept has been demonstrated to be energy efficient comparing input power to system benefit, yielding a net power gain and also potent enough to completely suppress boundary layer separation. An electroacoustic model is presented which describes the actuator characteristics and enables realistic application analysis.

Numerical and experimental studies of flat-walled diffuser elements for valve-less micropumps

Abstract: An investigation of flat-walled diffuser elements for valve-less micropumps is presented. The diffuser element is a small angle flow channel with a rounded inlet and a preferably sharp outlet. The diverging-wall direction is the positive flow direction. The flow-directing capability under steady flow conditions was determined experimentally for several different diffuser elements. The flow-pressure characteristic was studied in detail for one of them. The result is compared with previously published results on pump performance. Numerical simulations were done using the Computational Fluid Dynamics program ANSYS/Flotran. The simulations show the flow-directing capability of the diffuser elements and predict the flow-pressure characteristics well for Reynolds numbers below 300-400. For higher Reynolds numbers, the simulations show the flow-directing capability, but there is a larger discrepancy between simulations and measurements. Simulations were also done for a nozzle element, a wide-angle flow channel with sharp inlet and outlets used in the micropump with dynamic passive-valves. A nozzle element has the converging-wall direction as positive flow direction. The simulations show differences in the flow patterns for diffuser elements and nozzle elements that explain the opposite positive flow directions. The diffuser element has an ordered flow and takes advantage of the pressure recovery in the diverging-wall direction. The nozzle element has gross flow separation in the diverging-wall direction and there is a vena-contracta effect instead of pressure recovery. The effective cross-sectional area is smaller in the diverging-wall direction than in the converging-wall direction.

Numerical simulation of three-dimensional, time-averaged flow structure at river channel confluences

Abstract: Current confluence research emphasizes three broad controls on flow structure generation: (1) planform curvature; (2) topographic steering; and (3) anisotropic turbulence associated with flow separation and shear layer dynamics. The relative importance of these processes in explaining observed flow structures is controversial, a situation that may be related to the fact that different investigators have examined different confluence configurations. This paper uses a three-dimensional numerical model, with a fully elliptic solution, a free surface treatment, and a turbulence model based on a renormalized group (RNG) to help to provide a physically based explanation of the controls upon flow structure generation for both a laboratory (rectangular) and a field confluence (the confluence of the Kaskaskia River and Copper Slough) and to identify the particular conditions under which particular flow structures are observed. Results suggest that an analogy with back-to-back meanders is possible for symmetrical configurations but that there will be progressive divergence from this state as confluence asymmetry increases. In asymmetric situations a dual-cell structure may be limited to the immediate vicinity of the junction because of the effects of streamline curvature and topographic steering. These differences can be explained by consideration of the dynamic pressure field, which may be specific to each confluence configuration. As such, this study partially reconciles differing views over what controls time-averaged flow structures in river channel confluences, although further research into the interaction of these processes with instantaneous velocity fluctuations is required.

Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

Abstract: Laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two-dimensional dunes in a sediment-starved flow. Contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures. Spectral analysis reveals that peak spectral energies generally occur at 1–2 Hz for the stream wise velocity component and 2–4 Hz for the cross-stream and vertical velocity components. Spectra show larger and better defined energy peaks near the shear layer. Peak spectral energies for suspended sediment concentration occur near 1 Hz throughout the flow. Squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant. Integral timescales for velocity range from 0.20 s for the streamwise component to 0.06 s for the cross-stream and vertical components. Integral length scales for velocity range from 0.065 to 0.135 m for the streamwise component, which is comparable to flow depth, and from 0.020 to 0.030 m for the crossstream and vertical components, which is comparable to dune height. For suspended sediment concentration, integral timescales and length scales are similar to the streamwise velocity component.

Direct numerical simulation of turbulent thermal boundary layers

Abstract: In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature...

Restricted Shock Separation in Rocket Nozzles

Abstract: In overexpanded rocket nozzles the e ow separates from the nozzle wall at a certain pressure ratio of wall pressure to ambient pressure. Flow separation and its theoretical prediction have been the subject of several experimental and theoretical studies in the past decades. Two distinctive e ow separation phenomena, the freeshock and restricted-shock separation, were observed in experiments with nozzles. Both phenomena are discussed in detail, and the system of recompression shocks and expansion waves is described. For the free-shock case three different shock structures in theplume can occur, namely the regular shock ree ection, the Mach disk, or a cap-like shock pattern. Theappearanceofthesedifferentplumepatternsis discussed. Theseshock structuresareconserved for the full-e owing, but overexpanded, nozzle. Numerical results obtained for existing rocket nozzles, e.g., Space ShuttleMain EngineorVulcain, show a qualitativegood agreement with experimental photographs.Furthermore, an explanation for the appearance of restricted shock separation, which has been widely unknown up to now, is given, analyzing why and under what conditions it occurs. The type of nozzle contour strongly ine uences this form of e ow separation, and restricted shock separation also occursin full-scale, thrust-optimized rocket nozzles. Based on the results established for e ow separation, an outlook on the generation of side loads is given.

LES and DES investigations of turbulent flow over a sphere

Abstract: Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) are applied to prediction and investigation of the ow around a sphere. DES is a hybrid approach in which the closure is a simple modi cation to the Spalart-Allmaras model, reducing to Reynolds-Averaged Navier-Stokes (RANS) in attached regions, and LES away from the wall . Calculations are performed at a Reynolds number, Re = 10, in the sub-critical regime where there is a laminar boundary layer separation from the sphere. A fth-order upwind-biased treatment of convection is su cient for capturing the development of the separated shear layers and for the velocity spectra to display an inertial range without excessive decay at the smallest resolved scales. The closest agreement with LES predictions is obtained in DES with the model constant CDES = 0:65. Both techniques yield predictions of the drag, position of laminar separation, and the mean pressure and skin-friction distributions along the sphere are in good agreement with measurements .

Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Abstract: Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2% and 7%) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains non-turbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re = 50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence and intermittency profiles are presented.

Steady flow separation patterns in a 45 degree junction

Abstract: Numerical and experimental techniques were used to study the physics of flow separation for steady internal flow in a 45° junction geometry, such as that observed between two pipes or between the downstream end of a bypass graft and an artery. The three-dimensional Navier–Stokes equations were solved using a validated finite element code, and complementary experiments were performed using the photochromic dye tracer technique. Inlet Reynolds numbers in the range 250 to 1650 were considered. An adaptive mesh refinement approach was adopted to ensure grid-independent solutions. Good agreement was observed between the numerical results and the experimentally measured velocity fields; however, the wall shear stress agreement was less satisfactory. Just distal to the ‘toe’ of the junction, axial flow separation was observed for all Reynolds numbers greater than 250. Further downstream (approximately 1.3 diameters from the toe), the axial flow again separated for Re [ges ] 450. The location and structure of axial flow separation in this geometry is controlled by secondary flows, which at sufficiently high Re create free stagnation points on the model symmetry plane. In fact, separation in this flow is best explained by a secondary flow boundary layer collision model, analogous to that proposed for flow in the entry region of a curved tube. Novel features of this flow include axial flow separation at modest Re (as compared to flow in a curved tube, where separation occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.

Test Of Turbulence Models For Wind Flow Over Terrain With Separation And Recirculation

Abstract: Several two-equation turbulence models using isotropic eddy viscosity and wall functions are assessed by solution of the neutral atmospheric boundary layer over a flat surface and wind flow over two- and three-dimensional models and real terrain. Calculations are presented for wind flow over the Sirhowy Valley in Wales, an embankment along the Rhine in Germany and the Askervein Hill in Scotland. Comparisons of predictions with previous work, and laboratory and field data, show that the RNG-based k–∈ model gives the best agreement with respect to the flow profiles and length of the separated flow region. The results of this model are analyzed with a non-linear stress-strain relation to gauge the potential effect of turbulence anisotropy.

Three-dimensional oscillatory flow over steep ripples

Abstract: The process which leads to the appearance of three-dimensional vortex structures in the oscillatory flow over two-dimensional ripples is investigated by means of direct numerical simulations of Navier–Stokes and continuity equations. The results by Hara & Mei (1990a), who considered ripples of small amplitude or weak fluid oscillations, are extended by considering ripples of larger amplitude and stronger flows respectively. Nonlinear effects, which were ignored in the analysis carried out by Hara & Mei (1990a), are found either to have a destabilizing effect or to delay the appearance of three-dimensional flow patterns, depending on the values of the parameters. An attempt to simulate the flow over actual ripples is made for moderate values of the Reynolds number. In this case the instability of the basic two-dimensional flow with respect to transverse perturbations makes the free shear layer generated by boundary layer separation become wavy as it leaves the ripple crest. Then the amplitude of the waviness increases and eventually complex three-dimensional vortex structures appear which are ejected in the irrotational region. Sometimes the formation of mushroom vortices is observed.

Analysis of Transient Thermal Choking Processes in a Model Scramjet Engine

Abstract: Shadowgraph flow visualisation and floor static pressure measurements have been used to examine the transient behaviour of a thermally choked combusting flow. Experiments were performed to examine the effect of varying inlet Mach number and fuel-air equivalence ratio on the nature and extent of the interaction. In all cases a sudden increase in static pressure was measured, followed by a highly turbulent region of sonic flow which was seen to propagate upstream along the duct. The nature of the dominant processes causing this pressure discontinuity are still not certain. Some mechanisms which may contribute to this phenomenon are presented. These include separation of the boundary layer in the duct, formation of a detonation and formation of a near-normal shock wave by the region of thermally choked flow.

Theoretical modelling of cavitation in piston ring lubrication

Abstract: Mathematical models of piston ring dynamics and lubrication are sensitive to the boundary conditions adopted to describe the cavitation occurring in the diverging outlet region of the lubricant film between the piston ring and cylinder wall In this paper, such sensitivity is investigated by applying different models of gaseous cavitation, flow separation and fluid film reformation to the analysis of a single compression ring from a diesel engine Significant differences are predicted in hydrodynamic pressure profiles, lubricant film boundaries, lubricant film thickness, oil flow and frictionSuch indications of substantial differences in piston ring operating characteristics associated with the distinct cavitation boundary conditions considered highlights the need for further research in this field However, the lack of detailed experimental data to validate the predictive models suggests that future progress must be based upon combined theoretical and experimental approaches to the problem It i

Spontaneous growth of fluctuations in the viscous flow of a fluid past a soft interface

Abstract: The flow induced instability in the flow past a soft material is studied in the limit of low Reynolds number where inertial effects are insignificant. A transition from laminar flow to a more complicated flow profile is observed when the strain rate of the base flow increases beyond a critical value; the transition is found to be reproducible. The experimental results are compared with theoretical predictions and quantitative agreement is found with no adjustable parameters.

Numerical and experimental investigation of double-cone shock interactions

Abstract: A series of experiments was conducted in the Princeton University Mach 8 Wind Tunnel to study shock interactions on axisymmetric double-cone geometries. Schlieren images and surface-pressure data were taken. Two models were tested, which were expected to produce steady Type VI and Type V shock interactions. The experiments are compared to computational fluid dynamics calculations, and the features of these complicated flowfields are discussed. The comparison is excellent for the laminar Type VI shock interaction. The computations accurately reproduce the size of the separation zone and the surface pressure. However, for the Type V interaction the laminar computation overpredicts the size of the separation region. In addition, the experimental results for the Type V interaction show that the size of the separation region decreases with increasing Reynolds number, whereas the laminar computations predict the opposite trend. Turbulent computations show much better agreement with experimental data and reproduce the experimentally observed relationship between the size of the separation region and the Reynolds number, indicating that the reattachment shocks cause transition to turbulence in these flows

Hypersonic Flow Predictions Using Linear and Nonlinear Turbulence Closures

Abstract: Two- and three-dimensional hypersonic flow cases are computed using linear one-equation closures and a nonlinear two-equation model, where the anisotropy tensor is modeled as a cubic function of mean strain and vorticity tensors. The latter is found to excel in predicting bypass transition, whereas the one-equation R t model is very good at heat-transfer prediction. Both closures excel in predicting pressure distributions; however, the nonlinear model is found to overpredict heat-transfer. This suggests that in separated flow regions with simultaneously low mean-flow kinetic energy (and therefore low strain magnitude) and high temperature gradients, overpredicted levels of turbulence length scale can lead to rather small errors in the turbulent shear stress, while at the same time leading to a large overprediction of the turbulent heat fluxes

Some effects of localized injection on the turbulence structure in a boundary layer

Abstract: The effects of localized injection through a porous strip on a turbulent boundary layer at zero pressure gradient conditions were examined experimentally. The magnitude of the injection velocity was kept very small (less than 1% of the free-stream velocity) to prevent separation near the injection strip and to keep the perturbations small. It was found that the injection increases all the Reynolds stresses and that this perturbation dies out very slowly as the affected layer is sandwiched between the outer edge of the incoming boundary layer and a new layer that develops at the wall. A study of the anisotropy tensor indicated no effects of the blowing rate on the flow anisotropy downstream of the injection region, although a quadrant analysis showed that the contributions to the shear stresses from the various quadrants were affected.

Passively driven acoustic jet controlling boundary layers

Abstract: Existing pressure oscillations created by axial or centrifugal fans in a diverging shroud are utilized to power a passive, acoustic jet, the nozzle of which directs high momentum flux gas particles essentially tangentially into the boundary layer of the flow in a diffuser, or a duct, the fluid particles in the resonant chamber of the passive acoustic jet being replenished with low momentum flux particles drawn from the fluid flow in a direction normal to the surface, thereby to provide a net time averaged flow of increased momentum flux particles to defer, even eliminate, the onset of boundary layer separation in the diffuser or duct. The passive acoustic jet is used in the vicinity of fan blade tips to alleviate undesirable flow effects in the tip region, such as leakage.