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Showing papers on "Flow separation published in 2003"


Journal ArticleDOI
TL;DR: In this paper, a general formula for skin friction, including heat transfer to a flat plate, was developed for a thin turbulent boundary layer in compressible fluids with zero pressure gradient, and curves were presented giving skin-friction coefficients and heat-transfer coefficients for air for various wall-to-free-stream temperature ratios and free-stream Mach Numbers.
Abstract: The continuity, momentum, and energy differential equations for turbulent flow of a compressible fluid are derived, and the apparent turbulent stresses and dissipation function are identified. A general formula for skin friction, including heat transfer to a flat plate, is developed for a thin turbulent boundary layer in compressible fluids with zero pressure gradient. Curves are presented giving skin-friction coefficients and heat-transfer coefficients for air for various wall-to-free-stream temperature ratios and free-stream Mach Numbers. In the special case when the boundary layer is insulated, this general formula yields skin-friction coefficients higher than those given by the von Karman wall-property compressible-fluid formula but lower than those given by the von Karman incompressible-fluid formula. Heat transfer from the boundary layer to the plate generally increases the friction and heat-transfer coefficients.

616 citations


Journal ArticleDOI
TL;DR: In this article, the aerodynamic problems that must be addressed in order to design a successful small aerial vehicle are described, including the effects of Reynolds number and aspect ratio (AR) on the design and performance of fixed-wing vehicles.
Abstract: ▪ Abstract In this review we describe the aerodynamic problems that must be addressed in order to design a successful small aerial vehicle. The effects of Reynolds number and aspect ratio (AR) on the design and performance of fixed-wing vehicles are described. The boundary-layer behavior on airfoils is especially important in the design of vehicles in this flight regime. The results of a number of experimental boundary-layer studies, including the influence of laminar separation bubbles, are discussed. Several examples of small unmanned aerial vehicles (UAVs) in this regime are described. Also, a brief survey of analytical models for oscillating and flapping-wing propulsion is presented. These range from the earliest examples where quasi-steady, attached flow is assumed, to those that account for the unsteady shed vortex wake as well as flow separation and aeroelastic behavior of a flapping wing. Experiments that complemented the analysis and led to the design of a successful ornithopter are also described.

537 citations


Journal ArticleDOI
TL;DR: In this article, the viability and accuracy of large-eddy simulation with wall modeling for high Reynolds number complex turbulent flows is investigated by considering the flow around a circular cylinder in the supercritical regime.

315 citations


Journal ArticleDOI
TL;DR: In this article, the accuracy of Reynolds averaged Navier-Stokes (RANS) turbulence models in predicting complex flows with separation is examined, and the unsteady flow around a square cylinder and over a wall-mounted cube are simulated and compared with experimental data.

263 citations


Journal ArticleDOI
TL;DR: In this paper, a similarity formulation is proposed to describe the streamwise turbulence intensity across the entire smooth-wall zero-pressure-gradient turbulent boundary layer, which is an extension of the Marusic, Uddin and Perry [Phys. Fluids 9, 3718 (1997)] formulation that was restricted to the outer region of the boundary layer.
Abstract: A similarity formulation is proposed to describe the streamwise turbulence intensity across the entire smooth-wall zero-pressure-gradient turbulent boundary layer. The formulation is an extension of the Marusic, Uddin, and Perry [Phys. Fluids 9, 3718 (1997)] formulation that was restricted to the outer region of the boundary layer, including the logarithmic region. The new formulation is found to agree very well with experimental data over a large range of Reynolds numbers varying from laboratory to atmospheric flows. The formulation is founded on physical arguments based on the attached eddy hypothesis, and suggests that the boundary layer changes significantly with Reynolds number, with an outer flow influence felt all the way down to the viscous sublayer. The formulation may also be used to explain why the empirical mixed scaling of DeGraaff and Eaton [J. Fluid Mech. 422, 319 (2000)] appears to work.

244 citations


Journal ArticleDOI
TL;DR: In this paper, large eddy simulations are presented for the flow in a periodic channel segment, which is laterally constricted by hill-shaped obstructions on one wall, having a height of 33% of the unconstricted channel.

230 citations


Journal ArticleDOI
TL;DR: Three‐dimensional time‐averaged computational fluid dynamics models of two natural bends with inner‐bank separation with substantial differences in detail appear to be due to differences in upstream planform, manifested through the lateral distribution of inflow velocity.
Abstract: In highly curved river bends, flow may separate at the inner bank to create a recirculation eddy with weak upstream flow. Very little is known about how recirculation eddies connect with the downstream flow or how the latter is affected by their presence. We investigate these questions using three-dimensional time-averaged computational fluid dynamics models of two natural bends with inner-bank separation. Test measurements of velocity in one bend agree well with the simulation. Common points in the two simulations are that (1) an outer-bank helix is only present in the upstream part of the bend, (2) maximum near-bank velocities are highest here rather than beyond the apex as in most bends, (3) reverse flow extends farther across the channel at the surface than the bed, and (4) flow within the recirculation eddy has a three-dimensional structure, linked with that in the outer-bank free stream. Substantial differences in detail between the two bends appear to be due to differences in upstream planform, manifested through the lateral distribution of inflow velocity.

225 citations


Journal ArticleDOI
TL;DR: In this article, the authors highlight the importance of a certain linear mechanism and its contribution to skin-friction drag in turbulent boundary layers and the implication that significant drag reduction can be achieved by altering this linear mechanism.
Abstract: The objective of this paper is to give an overview of recent progress on boundary layer control made by the author’s research group at University of California, Los Angeles. A primary theme is to highlight the importance of a certain linear mechanism and its contribution to skin-friction drag in turbulent boundary layers—and the implication that significant drag reduction can be achieved by altering this linear mechanism. Examples that first led to this realization are presented, followed by applications of linear optimal control theory to boundary-layer control. Results from these applications, in which the linear mechanism in turbulent channel flow was targeted, indirectly confirm the importance of linear mechanisms in turbulent—and hence, nonlinear—flows. Although this new approach has thus far been based solely on numerical experiments which are yet to be verified in the laboratory, they show great promise and represent a fundamentally new approach for flow control. The success and limitations of various controllers and their implications are also discussed.

217 citations


Journal ArticleDOI
TL;DR: In this article, the aerodynamics of membrane and corresponding rigid wings under the MAV flight conditions are reviewed. And the proper orthogonal decomposition method is also discussed as an economic tool to describe the flow structure around a wing and to facilitate flow and vehicle control.

213 citations


Journal ArticleDOI
TL;DR: The second AIAA Drag Prediction Workshop as discussed by the authors focused on absolute and configuration delta drag prediction of the DLR, German Aerospace Research Center F6 geometry, which is representative of transport aircraft designed for transonic flight.
Abstract: Results from the Second AIAA Drag PredictionWorkshop are summarized. The workshop focused on absolute and configuration delta drag prediction of the DLR, German Aerospace Research Center F6 geometry, which is representative of transport aircraft designed for transonic flight. Both wing–body and wing–body–nacelle–pylon configurations are considered. Comparisons are made using industry relevant test cases that include single-point conditions, drag polars, and drag-rise curves. Drag, lift, and pitching moment predictions from several different Reynolds averagedNavier–Stokes computational fluid dynamics codes are presented and compared to experimental data. Solutions on multiblock structured, unstructured, and overset structured grids using a variety of turbulence models are considered. Results of a grid-refinement study and a comparison of tripped transition vs fully turbulent boundary-layer computations are reported.

202 citations


Journal ArticleDOI
TL;DR: In this paper, four different turbulence models were employed to study their influence on the results of pulsatile turbulent flow in axisymmetric stenoses and it was found that the low Reynolds number k-omega turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-epsilon turbulence model and the standard k-EPsilon model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow
Abstract: Pulsatile turbulent flow in stenotic vessels has been numerically modeled using the Reynolds-averaged Navier-Stokes equation approach. The commercially available computational fluid dynamics code (CFD), FLUENT, has been used for these studies. Two different experiments were modeled involving pulsatile flow through axisymmetric stenoses. Four different turbulence models were employed to study their influence on the results. It was found that the low Reynolds number k-omega turbulence model was in much better agreement with previous experimental measurements than both the low and high Reynolds number versions of the RNG (renormalization-group theory) k-epsilon turbulence model and the standard k-epsilon model, with regard to predicting the mean flow distal to the stenosis including aspects of the vortex shedding process and the turbulent flow field. All models predicted a wall shear stress peak at the throat of the stenosis with minimum values observed distal to the stenosis where flow separation occurred.

Journal ArticleDOI
TL;DR: In this paper, the aerodynamic and the vortical flow structure over simple delta wings undergoing either a pitching or rolling motion are presented, and the importance of unsteady aerodynamics on the flight dynamics of aircraft maneuvering at large angles of attack is examined.

01 Jan 2003
TL;DR: In this article, the aerodynamic and the vortical flow structure over simple delta wings undergoing either a pitching or rolling motion is presented, and several aircraft configurations are examined to show the importance of unsteady aerodynamics on the flight dynamics ofaircraf t maneuvering at large angles of attack.
Abstract: Aircraft that maneuver through large angles ofattack will experience large regions offlow separation over the wing and fuselage. The separated flow field is characterized by unsteadiness and strong vortical flow structures that can interact with various components ofthe aircraf t. These complicated flow interactions are the primary cause ofmost flight dynamic instabilities, airload nonlinearities and flow field time lags. The aerodynamic and the vortical flow structure over simple delta wings undergoing either a pitching or rolling motion is presented. This article reviews experimental information on the flow structure over delta wings and complete aircraft configurations. First, the flow structure of leading-edge vortices and their influence on delta wing aerodynamics for stationary models is presented. This is followed by a discussion of the effect of large amplitude motion on the vortex structure and aerodynamic characteristic ofpitching and rolling delta wings. The relationship between the flow structure and the unsteady airloads is reviewed. The unsteady motion ofthe delta wing results in a modification ofthe flow field. Delays in flow separation, vortex formation, vortex position and the onset of vortex breakdown are all affected by the model motion. These flow changes cause a corresponding modification in the aerodynamic loads. Data is presented which shows the importance offlow field hysteresis in either vortex position or breakdown and the influence on the aerodynamic characteristics ofa maneuvering delta wing. The free-to-roll motion of a double-delta wing is also presented. The complicated flow structure over a double-delta wing gives rise to damped, chaotic and wing rock motions as the angle ofattack is increased. The concept ofa critical state is discussed and it is shown that crossing a critical state produces large transients in the dynamic airloads. Next, several aircraft configurations are examined to show the importance of unsteady aerodynamics on the flight dynamics ofaircraf t maneuvering at large angles ofattack. The rolling characteristics ofthe F-18 and X-31 configurations are examined. The influence ofthe vortical flow structure on the rolling motion is established. Finally, a briefdiscussion ofnonlinear aerodynamic modeling is presented. The importance ofcritical states and the transient aerodynamics associated with crossing a critical state are examined. r 2003 Elsevier Science Ltd. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, a high-speed schlieren technique was used to characterize the phase-averaged properties of the flow over a cavity at a Mach number 0.8, and it was shown that the formation of coherent vortices in the region close to the boundary layer separation has some resemblance to the collective interaction mechanism introduced by Ho & Huang (1982) to describe mixing layers subjected to strong sub-harmonic forcing.
Abstract: The flow over a cavity at a Mach number 0.8 is considered. The cavity is deep with an aspect ratio (length over depth) L/D = 0.42. This deep cavity flow exhibits several features that makes it different from shallower cavities. It is subjected to very regular self-sustained oscillations with a highly two-dimensional and periodic organization of the mixing layer over the cavity. This is revealed by means of a high-speed schlieren technique. Analysis of pressure signals shows that the first tone mode is the strongest, the others being close to harmonics. This departs from shallower cavity flows where the tones are usually predicted well by the standard Rossiter’s model. A two-component laser-Doppler velocimetry system is also used to characterize the phase-averaged properties of the flow. It is shown that the formation of coherent vortices in the region close to the boundary layer separation has some resemblance to the ‘collective interaction mechanism’ introduced by Ho & Huang (1982) to describe mixing layers subjected to strong sub-harmonic forcing. Otherwise, the conditional statistics show close similarities with those found in classical forced mixing layers except for the production of random perturbations, which reaches a maximum in the structure centres, not in the hyperbolic regions with which turbulence production is usually associated. An attempt is made to relate this difference to the elliptic instability that may be observed here thanks to the particularly well-organized nature of the flow.

Journal ArticleDOI
TL;DR: In this article, the authors present a derivation of a benchmark problem for control of high-speed supercavitating vehicles (HSSVs) which focuses exclusively on the pitch-plane dynamics of the body.
Abstract: At very high speeds, underwater bodies develop cavitation bubbles at the trailing edges of sharp corners or from contours where adverse pressure gradients are sufficient to induce flow separation. Coupled with a properly designed cavitator at the nose of a vehicle, this natural cavitation can be augmented with gas to induce a cavity to cover nearly the entire body of the vehicle. The formation of the cavity results in a significant reduction in drag on the vehicle and these so-called high-speed supercavitating vehicles (HSSVs) naturally operate at speeds in excess of 75 m s-1. The first part of this paper presents a derivation of a benchmark problem for control of HSSVs. The benchmark problem focuses exclusively on the pitch-plane dynamics of the body which currently appear to present the most severe challenges. A vehicle model is parametrized in terms of generic parameters of body radius, body length, and body density relative to the surrounding fluid. The forebody shape is assumed to be a right cylindri...

Journal ArticleDOI
TL;DR: In this paper, Germano et al. used a dynamic Smagorinsky type subgridstress (SGS) model and Detached Eddy Simulation (DES) to simulate the flow around a sphere at a Reynoldsnumber of 104 in the subcritical regime.
Abstract: Large Eddy Simulation (LES) using a dynamic Smagorinsky type subgridstress (SGS) model and Detached Eddy Simulation (DES) are applied toprediction and investigation of the flow around a sphere at a Reynoldsnumber of 104 in the subcritical regime. In this regime the boundarylayers at separation are laminar, and transition to turbulence occursfarther downstream in the separated shear layers via Kelvin–Helmholtz(K–H) instabilities. The dynamic eddy viscosity model of Germano et al.(Physics of Fluids3 (1991) 1760–1765) is used in the LES, while the current implementation of the DESemploys a formulation based on the Spalart–Allmaras (S–A) model. DES isa hybrid approach in which the closure is a modification to theproduction/destruction term of the original Reynolds-AveragedNavier–Stokes (RANS) model, reducing to RANS in the attached regions,and to LES away from the wall. In the present work where we simulate theflow over a sphere in the subcritical regime in which the boundarylayers at separation are laminar, DES can be viewed as LES with adifferent SGS model. Effects of the discretization scheme used toapproximate the convective terms are considered, along with sensitivityof predictions to changes in the additional model coefficient, CDES, in the DES formulation. DES and LES yield similar predictions of the wakestructure, large-scale vortex shedding and the Strouhal numberassociated with the low frequency mode in the wake. Predictions ofquantities such as the drag coefficient, wake frequencies, position oflaminar separation on the sphere, and the mean pressure andskin-friction distributions along the sphere are in good agreement withthe measurements of Achenbach (Journal of Fluid Mechanics54 (1972) 565–575). Predictions of the primaryReynolds shear stress, turbulent kinetic energy, eddy viscosity, andturbulent dissipation for the two models are also similar. In addition,both models successfully resolve the formation of the vortex tubes inthe detached shear layers along with the value of the Strouhal numberassociated with the high frequency instability mode, provided that thelevel of numerical dissipation introduced by the discretization schemeremains sufficiently low. Flow physics investigations are focused onunderstanding the wake structure in the subcritical regime.

Journal ArticleDOI
TL;DR: In this article, the Strouhal-Reynolds-number profiles are determined for a range of ring aspect ratios, as well as critical Reynolds numbers for the onset of flow separation, unsteady flow and asymmetry.
Abstract: The low-Reynolds-number wake dynamics and stability of the flow past toroids placed normal to the flow direction are studied numerically. This bluff body has the attractive feature of behaving like the sphere at small aspect ratios, and locally like the straight circular cylinder at large aspect ratios. Importantly, the geometry of the ring is described by a single parameter, the aspect ratio (Ar), defined as a ratio of the torus diameter to the cross-sectional diameter of the ring. A rich diversity of wake topologies and flow transitions can therefore be investigated by varying the aspect ratio. Studying this geometry allows our understanding to be developed as to why the wake transitions leading to turbulence for the sphere and circular cylinder differ so greatly. Strouhal–Reynolds-number profiles are determined for a range of ring aspect ratios, as are critical Reynolds numbers for the onset of flow separation, unsteady flow and asymmetry. Results are compared with experimental findings from the literature. Calculated Strouhal–Reynolds-number profiles show that ring wakes shed at frequencies progressively closer to that of the straight circular cylinder wake as aspect ratio is increased from Ar =3 . For Ar > 8, the initial asymmetric transition is structurally analogous to the mode A transition for the circular cylinder, with a discontinuity present in the Strouhal–Reynolds-number profile. The present numerical study reveals a shedding-frequency decrease with decreasing aspect ratio for ring wakes, and an increase in the critical Reynolds numbers for flow separation and the unsteady flow transition. A Floquet stability analysis has revealed the existence of three modes of asymmetric vortex shedding in the wake of larger rings. Two of these modes are analogous to mode A and mode B of the circular cylinder wake, and the third mode, mode C, is analogous to the intermediate wavelength mode found in the wake of square section cylinders and circular cylinder wakes perturbed by a tripwire. Furthermore, three distinct asymmetric transition modes have been identified in the wake of small aspect ratio bluff rings. Fully developed asymmetric simulations have verified the unsteady transition for rings that exhibit a steady asymmetric wake.

Journal ArticleDOI
TL;DR: In this paper, the stability of a separating boundary-layer flow at the rear of a two-dimensional bump mounted on a flat plate is numerically investigated, and the flow field is shown to undergo self-sustained low-frequency fluctuations in the upstream region of the separation bubble, evolving into aperiodic vortex shedding further downstream.
Abstract: The stability of a separating boundary-layer flow at the rear of a two-dimensional bump mounted on a flat plate is numerically investigated. Above a critical Reynolds number, the flow field is shown to undergo self-sustained two-dimensional low-frequency fluctuations in the upstream region of the separation bubble, evolving into aperiodic vortex shedding further downstream. The computed steady flow states below the critical Reynolds number are shown to be convectively unstable. On extrapolating the flow field to Reynolds numbers above criticality, some evidence is found that the onset of the oscillatory behaviour coincides with topological flow changes near the reattachment point leading to the rupture of the (elongated) recirculation bubble. The structural changes near reattachment are shown to trigger an abrupt local transition from convective to absolute instability, at low frequencies. On preventing the separation bubble from bursting by reaccelerating the flow by means of a second bump further downstream, the separated flow remains steady for increasing Reynolds numbers, until a local region of absolute instability in the upper part of the geometrically controlled recirculation bubble is produced. The resulting global instability consists of self-sustained nonlinear saturated perturbations oscillating at a well-defined frequency, very distinct from the the low-frequency motion of the elongated recirculation bubble in the single-bump geometry. A frequency selection criterion based on local absolute frequencies, which has been successfully applied to wake flows, is shown to accurately predict the global frequency.

Proceedings ArticleDOI
06 Jan 2003
TL;DR: In this paper, a spanwise-oriented phased-plasma-array actuator is used for boundary-layer separation in a closed-circuit wind tunnel with a shaped insert on the opposite wall.
Abstract: Active flow control of boundary-layer separation using glow-discharge plasma actuators is studied experimentally. Separation is induced on a flat plate installed in a closed-circuit wind tunnel by a shaped insert on the opposite wall. The flow conditions represent flow over the suction surface of a modern low-pressure-turbine airfoil. The Reynolds number, based on wetted plate length and nominal exit velocity, is varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low (0.2 percent) and high (2.5 percent) free-stream turbulence intensities are set using passive grids. A spanwise-oriented phased-plasma-array actuator, fabricated on a printed circuit board, is surface-flush-mounted upstream of the separation point and can provide forcing in a wide frequency range. Static surface pressure measurements and hot-wire anemometry of the base and controlled flows are performed and indicate that the glow-discharge plasma actuator is an effective device for separation control.

Journal ArticleDOI
TL;DR: In this article, the authors consider the separated flow of an inviscid fluid around a moving flat plate and use a boundary integral representation for the complex-conjugate velocity field to determine the force and torque on the plate.
Abstract: In this study we consider the separated flow of an inviscid fluid around a moving flat plate. The motion of the plate, which is initially started from rest, is prescribed and unconstrained and we set ourselves the task of fully characterizing the resulting motion in the surrounding fluid. To do this we use a boundary integral representation for the complex-conjugate velocity field $\Phi(z,t)$ and require that the force and torque on the plate be determined as part of the solution. The flow solution is assumed to consist of a bound vortex sheet coincident with the plate and two free vortex sheets that emanate from each of the plate's two sharp edges. The time evolution of these vortex sheets is then considered in general. For physical reasons, the flow solution is required to satisfy the unsteady Kutta condition, which states that $\Phi(z,t)$ must be bounded everywhere, and the rigorous imposition of this condition then yields two types of additional constraint. The first governs the rate at which circulation is shed from the plate's edges and the second ensures that the free vortex sheets are shed tangentially. In fact, all the familiar flow characteristics associated with the imposition of the steady Kutta condition are rigorously shown to have exact parallels in the unsteady case. In addition, explicit expressions for the normal force and torque on the plate are derived. An asymptotic solution to the full system of evolution equations is developed for small times $t > 0$ and a simplified version of this solution is used as an initial condition for a desingularized numerical treatment of the full problem. A fast numerical algorithm is proposed and implemented and the results of several example calculations are presented. The featured examples are limited to high effective angles of attack due to the occurrence of a specific type of event that prevents further time-integration of the evolution equations using the current numerical method. The event corresponds physically to a situation in which a Lagrangian point placed at one of the plate's edges moves onto instead of away from the edge.

Journal ArticleDOI
TL;DR: In this paper, Stolz et al. computed supersonic turbulent boundary layers at a Mach number of 2.5 and momentum-thickness Reynolds numbers at inflow of 4530 and 10'049, respectively, with large-eddy simulation using the approximate deconvolution model.
Abstract: Spatially developing supersonic turbulent boundary layers at a Mach number of 2.5 and momentum-thickness Reynolds numbers at inflow of 4530 and 10 049, respectively, are computed with large-eddy simulation using the approximate deconvolution model [Stolz et al., Phys. Fluids 13, 2985 (2001)]. Turbulent inflow conditions are generated by rescaling the turbulent boundary layer at some distance downstream of inflow and reintroducing the rescaled mean profiles and fluctuation fields at inflow. This technique follows essentially that of Lund et al. [J. Comput. Phys. 140, 233 (1998)], albeit simplified and adapted for compressible flow. The simulations feature rather short spatial transient behavior and the results agree well with experimental data and theoretical correlations. The validity of assumptions of the strong Reynolds analogy is addressed.

Journal ArticleDOI
TL;DR: In this article, a database of wall-pressurearray measurements was compiled for studying the space-time character of the surface-pressure field within a separating/reattaching flow region, and two distinctive regions, defined based on their location relative to the position of the mean reattachment point (xr) of the shear layer, emerged from this investigation.
Abstract: A database of wall-pressure-array measurements was compiled for studying the space–time character of the surface-pressure field within a separating/reattaching flow region. The experimental setup consisted of a long splitter plate located within the wake of a fence and instrumented with an array of flush-mounted microphones. Data were acquired for a Reynolds number of 7900, based on the fence height above the splitter plate. Two distinctive regions, defined based on their location relative to the position of the mean reattachment point (xr) of the shear layer, emerged from this investigation. Upstream, from the fence to 0.25xr, the surface-pressure signature was dominated by large time scale disturbances and an upstream convection velocity of 0.21U∞. Beyond 0.25xr, turbulent structures with smaller time scales and a downstream convection velocity of 0.57U∞ generated most of the pressure fluctuations. Interestingly, the low-frequency wall-pressure signature typically associated with the flapping of the sep...

Journal ArticleDOI
TL;DR: In this article, micro-flow control actuation embedded in a stator vane was used to successfully control separation and improve near stall performance in a multistage compressor rig at NASA Glenn.
Abstract: Micro-flow control actuation embedded in a stator vane was used to successfully control separation and improve near stall performance in a multistage compressor rig at NASA Glenn. Using specially designed stator vanes configured with internal actuation to deliver pulsating air through slots along the suction surface, a research study was performed to identify performance benefits using this microflow control approach. Pressure profiles and unsteady pressure measurements along the blade surface and at the shroud provided a dynamic look at the compressor during microflow air injection. These pressure measurements lead to a tracking algorithm to identify the onset of separation. The testing included steady air injection at various slot locations along the vane. The research also examined the benefit of pulsed injection and actively controlled air injection along the stator vane. Two types of actuation schemes were studied, including an embedded actuator for on-blade control. Successful application of an online detection and flow control scheme will be discussed. Testing showed dramatic performance benefit for flow reattachment and subsequent improvement in diffusion through the use of pulsed controlled injection. The paper will discuss the experimental setup, the blade configurations, and preliminary CFD results which guided the slot location along the blade. The paper will also show the pressure profiles and unsteady pressure measurements used to track flow control enhancement, and will conclude with the tracking algorithm for adjusting the control.

Journal ArticleDOI
TL;DR: In this article, the authors show that laminar-turbulence coexistence dynamics (turbulent spots, spiral turbulence, etc.) can be seen as the ultimate stage of a modulation of the turbulent flows present at higher Reynolds number leading to regular, long-wavelength, inclined stripes.

Journal ArticleDOI
TL;DR: In this article, numerical simulations of the subcritical flow over a sphere are presented, where the main aim is to compare prediction of some of the main physics and flow parameters from solutions of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, large-eddy simulation (LES), and DES.
Abstract: Numerical simulations of the subcritical flow over a sphere are presented. The primary aim is to compare prediction of some of the main physics and flow parameters from solutions of the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, large-eddy simulation (LES), and detached-eddy simulation (DES). URANS predictions are obtained using two-layer κ-e, κ-ω, ν 2 -f, and the Spalart-Allmaras model. The dynamic eddy viscosity model is used in the LES. DES is a hybrid technique in which the closure is a modification to the Spalart-Allmaras model, reducing to RANS near solid boundaries and LES in the wake. The techniques are assessed by evaluating simulation results against experimental measurements, as well as through their ability to resolve time-dependent features of the flow related to vortex shedding. Simulation are performed at a Reynolds number of 10 4 , where laminar boundary-layer separation occurs at approximately 83 deg

Journal ArticleDOI
TL;DR: Oscillating vortex generator jets have been used to control boundary layer separation from the suction side of a low-pressure turbine airfoil as mentioned in this paper, and the results showed that losses will be substantially lower with the jets than in the baseline or passively controlled cases.
Abstract: Oscillating vortex generator jets have been used to control boundary layer separation from the suction side of a low-pressure turbine airfoil. A low Reynolds number (Re = 25,000) case with low free-stream turbulence has been investigated with detailed measurements including profiles of mean and fluctuating velocity and turbulent shear stress. Ensemble averaged profiles are computed for times within the jet pulsing cycle, and integral parameters and local skin friction coefficients are computed from these profiles. The jets are injected into the mainflow at a compound angle through a spanwise row of holes in the suction surface. Preliminary tests showed that the jets were effective over a wide range of frequencies and amplitudes. Detailed tests were conducted with a maximum blowing ratio of 4.7 and a dimensionless oscillation frequency of 0.65. The outward pulse from the jets in each oscillation cycle causes a disturbance to move down the airfoil surface. The leading and trailing edge celerities for the disturbance match those expected for a turbulent spot. The disturbance is followed by a calmed region. Following the calmed region, the boundary layer does separate, but the separation bubble remains very thin. Results are compared to an uncontrolled baseline case in which the boundary layer separated and did not reattach, and a case controlled passively with a rectangular bar on the suction surface. The comparison indicates that losses will be substantially lower with the jets than in the baseline or passively controlled cases.Copyright © 2003 by ASME

Book ChapterDOI
06 Jan 2003
TL;DR: In this paper, a Spalart-Allmaras-based Detached-Eddy Simulation (DES) of the Ahmed reference car model with 25° and 35° slant angles using unstructured grids and the solver Cobalt is presented.
Abstract: This paper presents a Spalart-Allmaras based Detached-Eddy Simulation (DES) of the Ahmed reference car model with 25° and 35° slant angles using unstructured grids and the solverCobalt. Comparisons are made to experimental laser doppler velocity measurements as well as total and surface pressure integrated drag. The Reynolds number based on body length was 2.78 ×106, making the boundary layers approaching the slant fully turbulent. The flow over the base slant in the experiments is attached at 25° and separated at 35°. This causes a large drop in the drag with the increased slant angle as the vortices on the side of the slant are weakened due to the separation. These cases stress turbulence models due to the need to accurately predict the boundary layer separation over the slant as well as predict the pressures in the massively separated base region accurately. The DES results are compared to the experiments as well as the Spalart-Allmaras RANS model. DES is seen to predict separation at 25◦ slant angle, in contrast to the experiments. Drag is relatively close to the experiments, but the distribution of drag is more on the rear than on the slant due to the separation. At the 35 ° slant angle, DES is in good agreement to the experimental drag, with the correct distribution, while RANS over-predicts the drag.

Journal ArticleDOI
TL;DR: In this article, the effects of the flow elasticity and inertia in polymer-induced drag reduction through (pseudo)spectral simulations of a turbulent channel flow of a dilute polymer solution are investigated.
Abstract: In this work we systematically investigate the effects of the flow elasticity and inertia in polymer-induced drag reduction through (pseudo)spectral simulations of a turbulent channel flow of a dilute polymer solution. Viscoelastic effects are modeled by the finite-extensibility nonlinear elastic dumbbell model with the Peterlin approximation. The present work updates the low Weissenberg results (Weτ0⩽50) reported in earlier works by Sureshkumar et al. [Phys. Fluids 9, 743 (1997)] and Dimitropoulos et al. [J. Non-Newtonian Fluid Mech. 79, 433 (1998)] for a zero shear rate friction Reynolds number, Reτ0=125, by allowing for a lower value for the numerical diffusivity. In addition, we examine two effects on drag reduction: (A) high elasticity, by varying Weτ0 from 62.5 to 125 for a constant Reτ0=125, (B) friction Reynolds number, Reτ0=180, 395, and 590, for a constant Weτ0=50. In the high elasticity region, the mean Reynolds, Remean, continues to increase with increasing Weτ0, albeit at a smaller rate. Thus...

Journal ArticleDOI
TL;DR: In this article, the authors measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number, and found that Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied.
Abstract: Reynolds-number dependence of flow fields within a modelled urban area was studied in a wind tunnel. We measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number. Our results led to the following conclusions. Firstly, the flow around the models in the wind tunnel was roughly divided into three parts according to the intensities of viscous stress and Reynolds stress as follows: (1) the flow in the vicinity of the ground or the surfaces of the model, where viscous stress became dominant under certain conditions; (2) the flow detached from the surfaces of the model, where Reynolds stress was always dominant; and (3) the flow around the separation bubble at the leading edge of the building model, where the influences of both viscous stress near the wall and the Reynolds stress in the separated boundary layer were mixed.Secondly, the critical Reynolds number of the flow in the modelled urban area could be defined by using both the roughness Reynolds number Rez0 (= z0u*/ν) and the dimensionless height z+ (= zu*/ν). Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied.

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TL;DR: In this paper, data from 2435 gas-liquid flow experiments in horizontal pipelines, taken from different sources, including new data for heavy oil, are compiled and processed for power law and composite power law friction factor correlations.