Showing papers on "Flow separation published in 2011"

High–Reynolds Number Wall Turbulence

Abstract: We review wall-bounded turbulent flows, particularly high–Reynolds number, zero–pressure gradient boundary layers, and fully developed pipe and channel flows. It is apparent that the approach to an asymptotically high–Reynolds number state is slow, but at a sufficiently high Reynolds number the log law remains a fundamental part of the mean flow description. With regard to the coherent motions, very-large-scale motions or superstructures exist at all Reynolds numbers, but they become increasingly important with Reynolds number in terms of their energy content and their interaction with the smaller scales near the wall. There is accumulating evidence that certain features are flow specific, such as the constants in the log law and the behavior of the very large scales and their interaction with the large scales (consisting of vortex packets). Moreover, the refined attached-eddy hypothesis continues to provide an important theoretical framework for the structure of wall-bounded turbulent flows.

Implicit Large Eddy Simulation of transition to turbulence at low Reynolds numbers using a Discontinuous Galerkin method

Abstract: The present work predicts the formation of laminar separation bubbles at low Reynolds numbers and the related transition to turbulence by means of Implicit Large Eddy Simulations with a high-order Discontinuous Galerkin method. The flow around an SD7003 infinite wing at an angle of attack of 4° is considered at Reynolds numbers of 10 000, 22 000, and 60 000 in order to gain insight into the characteristics of the laminar and turbulent regimes. At the lowest Reynolds number studied, the flow remains laminar and two dimensional over the wing surface, with a periodic vortex shedding. For higher Reynolds numbers, the flow is unsteady over the upper wing surface and exhibits a separation bubble along which the flow transitions to turbulence. Tollmien–Schlichting (TS) waves are observed in the boundary layer, and transition is found to be caused by unstable TS modes as revealed by the growth of the stream-wise amplification factor. Copyright © 2010 John Wiley & Sons, Ltd.

Turbulence in supersonic boundary layers at moderate Reynolds number

Abstract: We study the organization of turbulence in supersonic boundary layers through large-scale direct numerical simulations (DNS) at , and momentum-thickness Reynolds number up to (corresponding to ) which significantly extend the current envelope of DNS in the supersonic regime. The numerical strategy relies on high-order, non-dissipative discretization of the convective terms in the Navier–Stokes equations, and it implements a recycling/rescaling strategy to stimulate the inflow turbulence. Comparison of the velocity statistics up to fourth order shows nearly exact agreement with reference incompressible data, provided the momentum-thickness Reynolds number is matched, and provided the mean velocity and the velocity fluctuations are scaled to incorporate the effects of mean density variation, as postulated by Morkovin’s hypothesis. As also found in the incompressible regime, we observe quite a different behaviour of the second-order flow statistics at sufficiently large Reynolds number, most of which show the onset of a range with logarithmic variation, typical of ‘attached’ variables, whereas the wall-normal velocity exhibits a plateau away from the wall, which is typical of ‘detached’ variables. The modifications of the structure of the flow field that underlie this change of behaviour are highlighted through visualizations of the velocity and temperature fields, which substantiate the formation of large jet-like and wake-like motions in the outer part of the boundary layer. It is found that the typical size of the attached eddies roughly scales with the local mean velocity gradient, rather than being proportional to the wall distance, as happens for the wall-detached variables. The interactions of the large eddies in the outer layer with the near-wall region are quantified through a two-point amplitude modulation covariance, which characterizes the modulating action of energetic outer-layer eddies.

The wake of two side-by-side square cylinders

Abstract: Aerodynamic interference between two cylinders involves most of the generic flow features associated with multiple structures, thus providing an excellent model for gaining physical insight into the wake of multiple cylindrical structures. This work aims to provide an experimental systematic study of the flow behind two side-by-side square cylinders. The square cylinder is a representative model for bluff bodies with sharp corners, characterized by a fixed flow separation point, which are distinct from those of continuous curvature with oscillating separation points, typically represented by the circular cylinder. Experiments were performed at a Reynolds number Re of 4.7 × 10 4 and a cylinder centre-to-centre spacing ratio T/ d (d is the cylinder height) of 1.02–6.00. The flow was measured using different techniques, including hot wires, load cell, particle imaging velocimetry and laser-induced fluorescence flow visualization. Four distinct flow regimes and their corresponding T/ d ranges are identified for the first time on the basis of the flow structure and the Strouhal number. Physical aspects in each regime, such as interference between shear layers, gap flow deflection and changeover, multiple flow modes, entrainment, recirculation bubble, vortex interactions and formation lengths, are investigated in detail and are connected to the characteristics of the time-averaged and fluctuating fluid forces. The flow displays a marked difference in many facets from that behind two side-by-side circular cylinders, which is linked to their distinct flow separation natures. A crucial role played by the gap flow and its passage geometry in contributing to the observed difference is also unveiled.

Experimental Evidence of Rotating Stall in a Pump-Turbine at Off-Design Conditions in Generating Mode

Abstract: An experimental investigation of the rotating stall in reduced scale model of a low specific speed radial pump-turbine at runaway and turbine brake conditions in generating mode is achieved. Measurements of wall pressure in the stator are performed along with high-speed flow visualizations in the vaneless gap with the help of air bubbles injection. When starting from the best efficiency point (BEP) and increasing the impeller speed, a significant increase of the pressure fluctuations is observed mainly in the wicket gates channels. The spectral analysis shows a rise of a low frequency component (about 70% of the impeller rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Analysis of the instantaneous pressure peripheral distribution in the vaneless gap reveals one stall cell rotating with the impeller at sub-synchronous speed. High-speed movies reveal a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas at runaway the flow is highly disturbed by the rotating stall passage. The situation is even more critical at very low positive discharge, where backflow and vortices in the guide vanes channels develop during the stall cell passage. A specific image processing technique is applied to reconstruct the rotating stall evolution in the entire guide vanes circumference for a low positive discharge operating point. The findings of this study suggest that one stall cell rotates with the impeller at sub-synchronous velocity in the vaneless gap between the impeller and the guide vanes. It is the result of rotating flow separations developed in several consecutive impeller channels which lead to their blockage.

Flow around a tall finite cylinder explored by large eddy simulation

Abstract: The flow around a finite circular cylinder with a height-to-diameter ratio of 6 was studied using large eddy simulation (LES) The cylinder was mounted on a ground plane where a thin boundary layer with a thickness of only 7% of the cylinder's diameter was formed The flow resulting from the present LES was used to present a detailed picture of both the instantaneous and the time-averaged flow Among the results of the present study is the explanation for the formation of two time-averaged tip vortices from two pairs of instantaneous vortices The main recirculation region on the cylinder's top was found to be one vortex with both legs attached to the top surface, in agreement with some previous investigations The mean flow in the near wake region was found to contain two horseshoe vortices, one large arch vortex stretching from the cylinder's top and another vortex resulting from the flow moving down below the rear stagnation point on the cylinder The instantaneous horseshoe vortex near the ground was found to be unsteady and changed shape over time The time-averaged flow showed three complete horseshoe vortices, in agreement with previous knowledge The downwash process above the free end into the near wake occurs at a rather constant angle with respect to the plane normal to the streamwise direction The same angle was preserved in the inclination of the arch vortex in the time-averaged flow

Direct numerical simulation of spatially developing turbulent boundary layers with uniform blowing or suction

Abstract: Direct numerical simulation (DNS) of spatially developing turbulent boundary layer with uniform blowing (UB) or uniform suction (US) is performed aiming at skin friction drag reduction The Reynolds number based on the free stream velocity and the 99% boundary layer thickness at the inlet is set to be 3000 A constant wall-normal velocity is applied on the wall in the range, −001U∞ ≤ Vctr ≤ 001U∞ The DNS results show that UB reduces the skin friction drag, while US increases it The turbulent fluctuations exhibit the opposite trend: UB enhances the turbulence, while US suppresses it Dynamical decomposition of the local skin friction coefficient cf using the identity equation (FIK identity) (Fukagata, Iwamoto & Kasagi, Phys Fluids, vol 14, 2002, pp L73–L76) reveals that the mean convection term in UB case works as a strong drag reduction factor, while that in US case works as a strong drag augmentation factor: in both cases, the contribution of mean convection on the friction drag overwhelms the turbulent contribution It is also found that the control efficiency of UB is much higher than that of the advanced active control methods proposed for channel flows

Flow over periodic hills: an experimental study

Abstract: Two-dimensional flow over periodically arranged hills was investigated experimentally in a water channel. Two-dimensional particle image velocimetry (PIV) and one-dimensional laser Doppler anemometry (LDA) measurements were undertaken at four Reynolds numbers ( $$\text{5,600} \le Re \le \text{37,000}$$ ). Two-dimensional PIV field measurements were thoroughly validated by means of point-by-point 1D LDA measurements at certain positions of the flow. A detailed study of the periodicity and the homogeneity was undertaken, which demonstrates that the flow can be regarded as two-dimensional and periodic for $$Re \ge \text{10,000}$$ . We found a decreasing reattachment length with increasing Reynolds number. This is connected to a higher momentum in the near-wall zone close to flow separation which comes from the velocity speed up above the obstacle. This leads to a velocity overshoot directly above the hill crest which increases with Reynolds number as the inner layer depth decreases. The flow speed up above that layer is independent of the Reynolds number which supports the assumption of inviscid flow disturbance in the outer layer usually made in asymptotic theory for flow over small hills.

The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble

Abstract: Short laminar separation bubbles can develop on a flat plate due to an externally imposed pressure gradient. Here, these bubbles are computed by means of direct numerical simulations. Laminar-turbu ...

Laminar superlayer at the turbulence boundary.

Abstract: In this Letter we present results from particle tracking velocimetry and direct numerical simulation that are congruent with the existence of a laminar superlayer, as proposed in the pioneering work of Corrsin and Kistler (NACA, Technical Report No. 1244, 1955). We find that the local superlayer velocity is dominated by a viscous component and its magnitude is comparable to the characteristic velocity of the smallest scales of motion. This slow viscous process involves a large surface area so that the global rate of turbulence spreading is set by the largest scales of motion. These findings are important for a better understanding of mixing of mass and momentum in a variety of flows where thin layers of shear exist. Examples are boundary layers, clouds, planetary atmospheres, and oceans.

Direct numerical simulation of hypersonic turbulent boundary layers. Part 4. Effect of high enthalpy

Abstract: In this paper we present direct numerical simulations (DNS) of hypersonic turbulent boundary layers to study high-enthalpy effects. We study high- and low-enthalpy conditions, which are representative of those in hypersonic flight and ground-based facilities, respectively. We find that high-enthalpy boundary layers closely resemble those at low enthalpy. Many of the scaling relations for low-enthalpy flows, such as van-Driest transformation for the mean velocity, Morkovin’s scaling and the modified strong Reynolds analogy hold or can be generalized for high-enthalpy flows by removing the calorically perfect-gas assumption. We propose a generalized form of the modified Crocco relation, which relates the mean temperature and mean velocity across a wide range of conditions, including non-adiabatic cold walls and real gas effects. The DNS data predict Reynolds analogy factors in the range of those found in experimental data at low-enthalpy conditions. The gradient transport model approximately holds with turbulent Prandtl number and turbulent Schmidt number of order unity. Direct compressibility effects remain small and insignificant for all enthalpy cases. High-enthalpy effects have no sizable influence on turbulent kinetic energy (TKE) budgets or on the turbulence structure.

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

Abstract: [1] Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three-dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193° bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross-stream circulation developing in the deeper part of the bend, several streamwise-oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds-averaged Navier-Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross-stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy-resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.

Flow Separation Control Using Nanosecond Pulse Driven DBD Plasma Actuators

Abstract: This work continues an ongoing effort aimed at development and use of dielectric barrier discharge (DBD) plasma actuators driven by repetitive nanosecond pulses for high Reynolds number aerodynamic flow control. These actuators are believed to influence the flow via a thermal mechanism which is fundamentally different from more commonly studied AC-DBD actuators. Leading edge separation control on an 8-inch chord NACA 0015 airfoil is demonstrated at various post-stall angles of attack for Mach numbers up to 0.26 (free stream velocity up to 93 m/s) and Reynolds numbers up to 1.15 X 106. The nanosecond (NS) pulse driven DBD is shown to extend the stall angle at low Reynolds numbers by functioning as an active trip. At post-stall angles of attack, the device is shown to excite shear layer instabilities and generate coherent spanwise vortices that transfer momentum from the freestream to the separated region, thus reattaching the flow. This is observed for all high Reynolds numbers and Mach numbers spanning th...

Inertial migration of deformable capsules in channel flow

Abstract: Using three-dimensional computer simulations, we study the cross-stream inertial migration of neutrally buoyant deformable particles in a pressure-driven channel flow. The particles are modeled as elastic shells filled with a viscous fluid. We show that the particles equilibrate in a channel flow at off-center positions that depend on particle size, shell compliance, and the viscosity of encapsulated fluid. These equilibrium positions, however, are practically independent of the magnitude of channel Reynolds number in the range between 1 and 100. The results of our studies can be useful for sorting, focusing, and separation of micrometer-sized synthetic particles and biological cells.

A wall-layer model for large-eddy simulations of turbulent flows with/out pressure gradient

Abstract: In this work, modeling of the near-wall region in turbulent flows is addressed A new wall-layer model is proposed with the goal to perform high-Reynolds number large-eddy simulations of wall bounded flows in the presence of a streamwise pressure gradient The model applies both in the viscous sublayer and in the inertial region, without any parameter to switch from one region to the other An analytical expression for the velocity field as a function of the distance from the wall is derived from the simplified thin-boundary equations and by using a turbulent eddy coefficient with a damping function This damping function relies on a modified van Driest formula to define the mixing-length taking into account the presence of a streamwise pressure gradient The model is first validated by a priori comparisons with direct numerical simulation data of various flows with and without streamwise pressure gradient and with eventual flow separation Large-eddy simulations are then performed using the present wall model as wall boundary condition A plane channel flow and the flow over a periodic arrangement of hills are successively considered The present model predictions are compared with those obtained using the wall models previously proposed by Spalding, Trans ASME, J Appl Mech 28, 243 (2008) and Manhart et al, Theor Comput Fluid Dyn 22, 243 (2008) It is shown that the new wall model allows for a good prediction of the mean velocity profile both with and without streamwise pressure gradient It is shown than, conversely to the previous models, the present model is able to predict flow separation even when a very coarse grid is used

The Laminar Generalized Stokes Layer and Turbulent Drag Reduction

Abstract: This paper considers plane channel flow modified by waves of spanwise velocity applied at the wall and travelling along the streamwise direction. Both laminar and turbulent regimes for the streamwise flow are studied. When the streamwise flow is laminar, it is unaffected by the spanwise flow induced by the waves. This flow is a thin, unsteady and streamwise-modulated boundary layer that can be expressed in terms of the Airy function of the first kind. We name it the generalized Stokes layer because it reduces to the classical oscillating Stokes layer in the limit of infinite wave speed. When the streamwise flow is turbulent, the laminar generalized Stokes layer solution describes well the space-averaged turbulent spanwise flow, provided that the phase speed of the waves is sufficiently different from the turbulent convection velocity, and that the time scale of the forcing is smaller than the life time of the near-wall turbulent structures. Under these conditions, the drag reduction is found to scale with the Stokes layer thickness, which renders the laminar solution instrumental for the analysis of the turbulent flow. A classification of the turbulent flow regimes induced by the waves is presented by comparing parameters related to the forcing conditions with the space and time scales of the turbulent flow.

Flow Separation Control over an Airfoil with Nanosecond Pulse Driven DBD Plasma Actuators

Abstract: This work continues an ongoing development and use of dielectric barrier discharge (DBD) plasma actuators driven by repetitive nanosecond pulses for high Reynolds number aerodynamic flow control. These actuators are believed to influence the flow via a thermal mechanism which is fundamentally different from the more commonly studied AC-DBD plasmas. Leading edge separation control on an 8-inch chord NACA 0015 airfoil is demonstrated at various post-stall angles of attack (α) for Reynolds numbers (Re) and Mach numbers (M) up to 1.15x10 6 and 0.26 respectively (free stream velocity, U∞ = 93 m/s). The nanosecond pulse driven DBD can extend the stall angle at low Re by functioning as an active trip. At poststall α, the device generates coherent spanwise vortices that transfer momentum from the freestream to the separated region, thus reattaching the flow. This is observed for all Re and M spanning the speed range of the subsonic tunnel used in this work. The actuator is also integrated into a feedback control system with a stagnation-line-sensing hot film on the airfoil pressure side. A simple on/off type controller that operates based on a threshold of the mean value of the power dissipated by the hot film is developed for this system. A preliminary extremum seeking controller is also investigated for dynamically varying Re. Several challenges typically associated with integration of DBD plasma actuators into a feedback control system have been overcome. The most important of these is the demonstration of control authority at realistic takeoff and landing Re and M.

Unsteady boundary-layer transition in low-pressure turbines

Abstract: This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.

Mixed convection boundary layer flow from a horizontal circular cylinder embedded in a porous medium filled with a nanofluid

Abstract: Steady mixed convection boundary layer flow from an isothermal horizontal circular cylinder embedded in a porous medium filled with a nanofluid has been studied for both cases of a heated and cooled cylinder. The resulting system of nonlinear partial differential equations is solved numerically using an implicit finite-difference scheme. The solutions for the flow and heat transfer characteristics are evaluated numerically for various values of the governing parameters, namely the nanoparticle volume fraction φ and the mixed convection parameter λ. Three different types of nanoparticles are considered, namely Cu, Al2O3 and TiO2. It is found that for each particular nanoparticle, as the nanoparticle volume fraction φ increases, the magnitude of the skin friction coefficient decreases, and this leads to an increase in the value of the mixed convection parameter λ which first produces no separation. On the other hand, it is also found that of all the three types of nanoparticles considered, for any fixed values of φ and λ, the nanoparticle Cu gives the largest values of the skin friction coefficient followed by TiO2 and Al2O3. Finally, it is worth mentioning that heating the cylinder (λ > 0) delays separation of the boundary layer and if the cylinder is hot enough (large values of λ > 0), then it is suppressed completely. On the other hand, cooling the cylinder (λ < 0) brings the boundary layer separation point nearer to the lower stagnation point and for a sufficiently cold cylinder (large values of λ < 0) there will not be a boundary layer on the cylinder.

The influence of boundary layers on supersonic inlet flow unstart induced by mass injection

Abstract: A transverse jet is injected into a supersonic model inlet flow to induce unstart. Planar laser Rayleigh scattering from condensed CO2 particles is used to visualize flow dynamics during the unstart process, while in some cases, wall pressure traces are simultaneously recorded. Studies conducted over a range of inlet configurations reveal that the presence of turbulent wall boundary layers strongly affect the unstart dynamics. It is found that relatively thick turbulent boundary layers in asymmetric wall boundary layer conditions prompt the formation of unstart shocks; in symmetric boundary conditions lead to the propagation of pseudo-shocks; and in both cases facilitate fast inlet unstart, when compared with thin, laminar boundary layers. Incident shockwaves and associated reflections are found to affect the speed of pressure disturbances. These disturbances, which induce boundary layer separation, are found to precede the formation of unstart shocks. The results confirm the importance of and need to better understand shock-boundary layer interactions in inlet unstart dynamics.

Study of subsonic–supersonic gas flow through micro/nanoscale nozzles using unstructured DSMC solver

Abstract: We use an extended direct simulation Monte Carlo (DSMC) method, applicable to unstructured meshes, to numerically simulate a wide range of rarefaction regimes from subsonic to supersonic flows through micro/nanoscale converging–diverging nozzles. Our unstructured DSMC method considers a uniform distribution of particles, employs proper subcell geometry, and follows an appropriate particle tracking algorithm. Using the unstructured DSMC, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number on the flow field in micro/nanoscale nozzles. If we apply the back pressure at the nozzle outlet, a boundary layer separation occurs before the outlet and a region with reverse flow appears inside the boundary layer. Meanwhile, the core region of inviscid flow experiences multiple shock-expansion waves. In order to accurately simulate the outflow, we extend a buffer zone at the nozzle outlet. We show that a high viscous force creation in the wall boundary layer prevents any supersonic flow formation in the divergent part of the nozzle if the Knudsen number exceeds a moderate magnitude. We also show that the wall boundary layer prevents forming any normal shock in the divergent part. In reality, Mach cores would appear at the nozzle center followed by bow shocks and expansion region. We compare the current DSMC results with the solution of the Navier–Stokes equations subject to the velocity slip and temperature jump boundary conditions. We use OpenFOAM as a compressible flow solver to treat the Navier–Stokes equations.

Interaction of viscous and inviscid instability modes in separation–bubble transition

Abstract: This paper describes numerical simulations that are used to examine the interaction of viscous and inviscid instability modes in laminar-to-turbulent transition in a separation bubble. The results of a direct numerical simulation are presented in which separation of a laminar boundary-layer occurs in the presence of an adverse streamwise pressure gradient. The simulation is performed at low freestream-turbulence levels and at a flow Reynolds number and pressure distribution approximating those typically encountered on the suction side of low-pressure turbine blades in a gas-turbine engine. The simulation results reveal the development of a viscous instability upstream of the point of separation which produces streamwise-oriented vortices in the attached laminar boundary layer. These vortices remain embedded in the flow downstream of separation and are carried into the separated shear layer, where they are amplified by the local adverse pressure-gradient and contribute to the formation of coherent hairpin-...

Active Separation Control with Pulsed Jets in a Critically Loaded Compressor Cascade

Abstract: DOI: 10.2514/1.J050931 In this contribution the impact of active flow control by means of pulsed blowing in a critically loaded compressor cascadewillbedescribed.Becauseofthehighloadingoftheblades,afullythree-dimensionalandcomplex flowfieldis developing. Experimental investigations with active flow control were undertaken to increase the turning and the pressure rise of the stator cascade, by suppressing separation phenomena in the passage flowfield. Two different concepts of actuators were used. First, pulsed blowing out of the endwalls was used to reduce the secondary flow structures. Second, the flow was excited with actuators flush-mounted on the blade’s suction surface. The best performance was reached by using both actuator concepts in combination. A multivariable closed-loop control approach was used to control both separation phenomena simultaneously. Heavy disturbances could be compensated and a stabilization of the cascade operation point was achieved.

Visualizing supersonic inlet duct unstart using planar laser Rayleigh scattering

Abstract: Planar laser Rayleigh scattering (PLRS) from condensed CO2 particles is used to visualize flow structure in a Mach 5 wind tunnel undergoing unstart. Detailed flow features such as laminar/turbulent boundary layers and shockwaves are readily illustrated by the technique. A downstream transverse air jet, inducing flow unchoking downstream of the jet, is injected into the free stream flow of the tunnel, resulting in tunnel unstart. Time sequential PLRS images reveal that the boundary layer growth/separation on a surface with a thick turbulent boundary layer, initiated by the jet injection, propagates upstream and produces an oblique unstart shock. The tunnel unstarts upon the arrival of the shock at the inlet. In contrast, earlier flow separation on the opposite surface, initially supporting a thin laminar boundary layer, is observed when a jet induced bow shock strikes that surface. The resulting disturbance to this boundary layer also propagates upstream and precedes the formation of an unstart shock.