Showing papers on "Flow separation published in 2008"

Turbulent Shear Layers in Supersonic Flow

Abstract: Contents: 1. Introduction. 2. The Equations of Motion. 3. The Equations for Turbulent Flow. 4. Fundamental Concepts. 5. Boundary Layer Mean Flow Behavior. 6. Boundary Layer Turbulence Behavior. 7. Mixing Layers. 8. Perturbed Boundary Layers. 9. Two-Dimensional Interactions. 10. Three-Dimensional Interactions.

High-order large-eddy simulation of flow over the “Ahmed body” car model

Abstract: The structure of the turbulent flow over a simplified automotive model, the Ahmed body (S. R. Ahmed and G. Ramm, SAE Paper No. 8403001, 1984) with a 25° slanted back face, is investigated using high-order large-eddy simulations (LESs) at Reynolds number Re=768000. The numerical approach is carried out with a multidomain spectral Chebyshev–Fourier solver and the bluff body is modeled with a pseudopenalization method. The LES capability is implemented thanks to a spectral vanishing viscosity (SVV) technique, with particular attention to the near wall region. Such a SVV-LES approach is extended for the first time to an industrial three-dimensional turbulent flow over a complex geometry. In order to better understand the interactions between flow separations and the dynamic behavior of the released vortex wake, a detailed analysis of the flow structures is provided. The topology of the flow is well captured showing a partial separation of the boundary layer over the slanted face and the occurrence of two stro...

Abridged Summary of the Third AIAA Computational Fluid Dynamics Drag Prediction Workshop

Abstract: Results from the Third AIAA Drag Prediction Workshop (DPW-III) are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configurations that are representative of transonic transport aircraft The baseline DLR-F6 wing-body geometry, previously used in DPW-II, is also augmented with a side-of-body fairing to help reduce the complexity of the flow physics in the wing-body juncture region. In addition, two new wing-alone geometries have been developed for DPW-III. Numerical calculations are performed using industry-relevant test cases that include lift-specific and fixed-alpha flight conditions, as well as full drag polars. Drag, lift, and pitching-moment predictions from numerous Reynolds-averaged Navier-Stokes computational fluid dynamics methods are presented, focused on fully turbulent flows. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets are composed of tetrahedral, pyramid, and prismatic elements. Effort was made to provide a high-quality and parametrically consistent family of grids for each grid type about each configuration under study. The wing-body families are composed of a coarse, medium, and fine grid, whereas the wing-alone families also include an extra-fine mesh. These mesh sequences are used to help determine how the provided flow solutions fare with respect to asymptotic grid convergence, and are used to estimate an absolute drag for each configuration.

Colloquium : Theory of drag reduction by polymers in wall-bounded turbulence

Abstract: The flow of fluids in channels, pipes, or ducts, as in any other wall-bounded flow (like water along the hulls of ships or air on airplanes) is hindered by a drag, which increases manyfold when the fluid flow turns from laminar to turbulent. A major technological problem is how to reduce this drag in order to minimize the expense of transporting fluids like oil in pipelines, or to move ships in the ocean. It was discovered that minute concentrations of polymers can reduce the drag in turbulent flows by up to 80%. While experimental knowledge had accumulated over the years, the fundamental theory of drag reduction by polymers remained elusive for a long time, with arguments raging whether this is a ``skin'' or a ``bulk'' effect. In this Colloquium the phenomenology of drag reduction by polymers is summarized, stressing both its universal and nonuniversal aspects, and a recent theory is reviewed that provides a quantitative explanation of all the known phenomenology. Both flexible and rodlike polymers are treated, explaining the existence of universal properties like the maximum drag reduction asymptote, as well as nonuniversal crossover phenomena that depend on the Reynolds number, on the nature of the polymer and on its concentration. Finally other agents for drag reduction are discussed with a stress on the important example of bubbles.

Near-Surface Electrical Discharge in Supersonic Airflow : Properties and Flow Control

Abstract: The results of experimental and numerical investigations of the interaction between the near-wall electrical discharge and supersonic airflow in an aerodynamic channel with constant and variable cross sections are presented. Peculiar properties of the surface quasi-direct-current discharge generation in the flow are described. The mode with flow separation developing outside the discharge region is revealed as a specific feature of such a configuration. An interaction model is proposed on the basis of measurements and observations. A regime of gas-dynamic screening of a mechanical obstacle installed on the channel wall is studied. Variation of the main flow parameters caused by the surface discharge is quantified. The ability of the discharge to shift an oblique shock in an inlet is demonstrated experimentally. The influence of relaxation processes in nonequilibrium excited gas on the flow structure is analyzed. Comparison of the experimental data with the results of calculations based on the analytical model and on numerical simulations is presented.

Scaling Effects of an Aerodynamic Plasma Actuator

Abstract: We present experimental results to yield insight into the scalability and control effectiveness of single-dielectricbarrier-discharge plasma actuators for leading-edge separation control on airfoils. The parameters investigated are chord Reynolds number, Mach number, leading-edge radius, actuator amplitude, and unsteady frequency. This includes chord Reynolds numbers up to 1:0 � 106 and a maximum freestream speed of 60 m=s corresponding to a Mach number of 0.176. The main objective of this work is to examine the voltage requirements for the plasma actuators to reattach the flow at the leading edge of airfoils at poststall angles of attack for a range of flow parameters in order to establish scaling between laboratory and full-flight conditions. For the full range of conditions, an optimum unsteady actuator frequency f is found to minimize the actuator voltage needed to reattach the flow, such that F� � fLsep=U1 � 1. At the optimum frequencies, the minimum voltage required to reattach the flow is weakly dependent on chord Reynolds number and strongly dependent on the poststall angle of attack and leading-edge radius. The results indicate that the voltage required to reattach the flow scales as the square of the leading-edge radius.

Laminar Flow Control - Back to the Future?

Abstract: In the 21 st Century, reducing the environmental impact of aviation will become an increasingly important priority for the aircraft designer. Among the various environmental impacts, emission of CO2 can be expected to emerge as the most important in the long term and reducing fuel burn to become the overriding environmental priority. Increasing fuel costs and the world’s limited oil reserves will add to the pressure to reduce fuel burn. Starting from the limitations imposed on the aircraft designer by the laws of physics – the Breguet Range Equation, the Second Law of Thermodynamics, the behaviour of real, viscous fluids – the paper discusses the technological and design options available to the designer. Improvements in propulsion and structural efficiency have valuable contributions to make but it is in drag reduction through laminar flow control that the greatest opportunity lies. The physics underlying laminar flow control is discussed and the key features and limitations of natural, hybrid and full laminar flow control are explained. Experience to date in this field is briefly reviewed, with particular attention drawn to the substantial body of work in the 1950s and 1960s that demonstrated the potential of full laminar flow control by boundary-layer suction. The case is argued for revisiting the design of an aircraft with full laminar flow control, taking into account the advances over the past half century in all aspects of aircraft engineering, notably in propulsion and materials. With approximately half the thrust provided by the boundary layer suction system, this aircraft presents a completely new challenge in airframe-propulsion integration. We understand the physics of boundary layer control, we know that an aircraft with full laminar flow is potentially much more fuel efficient than the alternatives, what is needed now is a wholehearted attack on the engineering obstacles in its path.

An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil

Abstract: An experimental study was conducted to characterize the transient behavior of laminar flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds number of 70,000. In addition to measuring the surface pressure distribution around the airfoil, a high-resolution particle image velocimetry (PIV) system was used to make detailed flow field measurements to quantify the evolution of unsteady flow structures around the airfoil at various angles of attack (AOAs). The surface pressure and PIV measurements clearly revealed that the laminar boundary layer would separate from the airfoil surface, as the adverse pressure gradient over the airfoil upper surface became severe at AOA ≥8.0 deg. The separated laminar boundary layer was found to rapidly transit to turbulence by generating unsteady Kelvin-Helmholtz vortex structures. After turbulence transition, the separated boundary layer was found to reattach to the airfoil surface as a turbulent boundary layer when the adverse pressure gradient was adequate at AOA 12.0 deg.

Experimental studies of the viscous boundary layer properties in turbulent Rayleigh–Bénard convection

Abstract: We report high-resolution measurements of the properties of the velocity boundary layer in turbulent thermal convection using the particle image velocimetry (PIV) technique and measurements of the temperature profiles and the thermal boundary layer. Both velocity and temperature measurements were made near the lower conducting plate of a rectangular convection cell using water as the convecting fluid, with the Rayleigh number Ra varying from 10 9 to 10 10 and the Prandtl number Pr fixed at 4.3. From the measured profiles of the horizontal velocity we obtain the viscous boundary layer thickness δυ. It is found that δυ follows the classical Blasius-like laminar boundary layer in the present range of Ra, and it scales with the Reynolds number Re as δυ/H =0 .64Re −0.50±0.03 (where H is the cell height). While the measured viscous shear stress and Reynolds shear stress show that the boundary layer is laminar for Ra < 2.0 × 10 10 , two independent extrapolations, one based on velocity measurements and the other on velocity and temperature measurements, both indicate that the boundary layer will become turbulent at Ra ∼ 10 13 . Just above the thermal boundary layer but within the mixing zone, the measured temperature r.m.s. profiles σT (z) are found to follow either a power law or a logarithmic behaviour. The power-law fitting may be slightly favoured and its exponent is found to depend on Ra and varies from −0. 6t o−0.77, which is much larger than the classical value of −1/3. In the same region, the measured profiles of the r.m.s. vertical velocity σw(z) exhibit a much smaller scaling range and are also consistent with either a power-law or a logarithmic behaviour. The Reynolds number dependence of several wall quantities is also measured directly. These are the wall shear stress τw ∼ Re 1.55 , the viscous sublayer δw ∼ Re −0.91 , the friction velocity uτ ∼ Re 0.80 , and the skinfriction coefficient cf ∼ Re −0.34 . All of these scaling properties are very close to those predicted for a classical Blasius-type laminar boundary layer, except that of cf . Similar to classical shear flows, a viscous sublayer is also found to exist in the present system despite the presence of a nested thermal boundary layer. However, velocity profiles normalized by wall units exhibit no obvious logarithmic region, which is likely to be a result of the very limited distance between the edge of the viscous sublayer and the position of the maximum velocity. Compared to traditional shear flows, the peak position of the wall-unit-normalized r.m.s. profiles is found to be closer to the plate (at z + = z/δw � 5). Our overall conclusion is that a Blasius-type laminar boundary condition is a good approximation for the velocity boundary layer in turbulent thermal convection for the present range of Rayleigh number and Prandtl number.

Time-resolved and volumetric PIV measurements of a transitional separation bubble on an SD7003 airfoil

Abstract: To comprehensively understand the effects of Kelvin–Helmholtz instabilities on a transitional separation bubble on the suction side of an airfoil regarding as to flapping of the bubble and its impact on the airfoil performance, the temporal and spatial structure of the vortices occurring at the downstream end of the separation bubble is investigated. Since the bubble variation leads to a change of the pressure distribution, the investigation of the instantaneous velocity field is essential to understand the details of the overall airfoil performance. This vortex formation in the reattachment region on the upper surface of an SD7003 airfoil is analyzed in detail at different angles of attack. At a Reynolds number Re c 4°. Due to transition processes, turbulent reattachment of the separated shear layer occurs enclosing a locally confined recirculation region. To identify the location of the separation bubble and to describe the dynamics of the reattachment, a time-resolved PIV measurement in a single light-sheet is performed. To elucidate the spatial structure of the flow patterns in the reattachment region in time and space, a stereo scanning PIV set-up is applied. The flow field is recorded in at least ten successive light-sheet planes with two high-speed cameras enclosing a viewing angle of 65° to detect all three velocity components within a light-sheet leading to a time-resolved volumetric measurement due to a high scanning speed. The measurements evidence the development of quasi-periodic vortex structures. The temporal dynamics of the vortex roll-up, initialized by the Kelvin–Helmholtz (KH) instability, is shown as well as the spatial development of the vortex roll-up process. Based on these measurements a model for the evolving vortex structure consisting of the formation of c-shape vortices and their transformation into screwdriver vortices is introduced.

Unsteady shock wave dynamics

Abstract: An experimental study of an oscillating normal shock wave subject to unsteady periodic forcing in a parallel-walled duct has been conducted. Measurements of the pressure rise across the shock have been taken and the dynamics of unsteady shock motion have been analysed from high-speed schlieren video (available with the online version of the paper). A simple analytical and computational study has also been completed. It was found that the shock motion caused by variations in back pressure can be predicted with a simple theoretical model. A non-dimensional relationship between the amplitude and frequency of shock motion in a diverging duct is outlined, based on the concept of a critical frequency relating the relative importance of geometry and disturbance frequency for shock dynamics. The effects of viscosity on the dynamics of unsteady shock motion were found to be small in the present study, but it is anticipated that the model will be less applicable in geometries where boundary layer separation is more severe. A movie is available with the online version of the paper.

Statistical properties of wall pressure fluctuations over a forward-facing step

Abstract: This work describes an experimental study of the flow field and wall pressure fluctuations induced by quasi-two-dimensional incompressible turbulent boundary layers overflowing a forward-facing step (FFS). Pressure fluctuations are measured upstream and downstream of an instrumented FFS step model installed inside a large scale recirculation water tunnel, while two-dimensional (2D) velocity fields are measured close to the step via 2D particle image velocimetry (PIV). The overall flow physics is studied in terms of averaged velocity and vorticity fields for different Reynolds numbers based on the step’s height. The wall pressure statistics are analyzed in terms of several indicators, including the root mean squares and probability density functions of the pressure fluctuations, demonstrating that the most relevant flow structure is the unsteady recirculation bubble formed at the reattachment region downstream of the step. Pressure spectra and cross correlations are computed as well, and the convection vel...

Dynamics of Airfoil Separation Control using Zero-Net Mass-Flux Forcing

Abstract: Zero-net mass-flux jet based control of flow separation over a stalled airfoil is examined using numerical simulations. Two-dimensional simulations are carried out for a NACA 4418 airfoil at a chord Reynolds number of 40,000 and angle of attack of 18 deg. Results for the uncontrolled flow indicate the presence of three distinct natural time scales in the flow corresponding to the shear layer, separation bubble, and wake regions. The natural frequencies are used to select appropriate forcing frequencies, and it is found that forcing frequencies closer to the separation bubble frequency elicit the best response in terms of reduction of separation extent and an improvement in aerodynamic performance. In contrast, higher forcing frequencies closer to the natural shear layer frequency tend to enhance separation. The vortex dynamics and frequency response of flow are examined in detail to gain insight into mechanisms underlying the observed behavior.

Two-dimensional global low-frequency oscillations in a separating boundary-layer flow

Abstract: A separated boundary-layer flow at the rear of a bump is considered. Two-dimensional equilibrium stationary states of the Navier–Stokes equations are determined using a nonlinear continuation procedure varying the bump height as well as the Reynolds number. A global instability analysis of the steady states is performed by computing two-dimensional temporal modes. The onset of instability is shown to be characterized by a family of modes with localized structures around the reattachment point becoming almost simultaneously unstable. The optimal perturbation analysis, by projecting the initial disturbance on the set of temporal eigenmodes, reveals that the non-normal modes are able to describe localized initial perturbations associated with the large transient energy growth. At larger time a global low-frequency oscillation is found, accompanied by a periodic regeneration of the flow perturbation inside the bubble, as the consequence of non-normal cancellation of modes. The initial condition provided by the optimal perturbation analysis is applied to Navier–Stokes time integration and is shown to trigger the nonlinear ‘flapping’ typical of separation bubbles. It is possible to follow the stationary equilibrium state on increasing the Reynolds number far beyond instability, ruling out for the present flow case the hypothesis of some authors that topological flow changes are responsible for the ‘flapping’.

Engineering fluid mechanics

Abstract: Introduction Preface 1. Fundamentals in Continuum Mechanics 1.1 Dynamics of fluid motion 1.2 Dynamics in rotating reference frame 1.3 Material objectivity and convective derivatives 1.4 Displacement gradient and relative strain 1.5 Reynold's transport theorem 1.6 Forces on volume element Exercise Problems Bibliography Nomenclature for chapter 1 2. Conservation equations in continuum mechanics 2.1 Mass conservation 2.2 Linear momentum conservation 2.3 Angular momentum conservation 2.4 Energy conservation 2.5 Thermodynamic relations Exercise Problems Bibliography Nomenclature for chapter 2 3. Fundamental Treatment for Fluid Engineering 3.1 Fluid static 3.1 Fluid-fluid interfaces Exercise Problems Bibliography Nomenclature for chapter 3 4. Perfect flow 4.1 Potential and inviscid flows Exercise Problems 4.2 General theories of turbomachinery 4.2.1 Moment of momentum theory 4.2.2 Airfoil theory 4.2.3 Efficiency and similarity rules of turbomachinery 4.2.4 Cavitation Exercise Problems Bibliography Nomenclature for chapter 4 5. Compressible flow 5.1 Speed of sound and Mach number 5.2 Isoentropic flow 5.3 Fanno and Reyleigh lines 5.4 Normal shock waves 5.5 Oblique shock wave Exercise Problems Bibliography Nomenclature for chapter 5 6. Newtonian flow 6.1 Navier-Stokes Equation Problems 6.2 Similitude and Nondimensionalization Exercise Problems 6.3 Basic flows derived from Navier-Stokes equation 6.3.1 Unidirectional flow in a gap space 6.3.2 Lubrication theory 6.3.3 Flow around sphere Problems 6.4 Flow through pipe 6.4.1 Entrance flow 6.4.2 Fully developed flow pipe 6.4.3 Transient Hagen-Poiseuille flow in pipe Exercise Problems 6.5 Laminar boundary layer theory 6.5.1 Flow over a flat plate 6.5.2 Integral Analysis of Boundary Layer equation 6.5.3 Boundary layer separation 6.5.4 Integral relation for thermal energy Exercise Problems 6.6Turbulent flow 6.6.1 Turbulence models 6.6.2 Turbulence heat transfer Exercise Problems Bibliography Nomenclature for chapter 6 7. Non-Newtonian fluid and flow 7.1 Non-Newtonian fluid and generalized Newtonian fluid flow 7.1.1 Rheological classifications 7.1.2 Generalized Newtonian fluid flow Exercise Problems 7.2 Standard flow and material functions 7.2.1 Simple shear flow 7.2.2 Shearfree flow 7.2.3 Oscillatory rheometric flow 7.2.4 Viscometric flow in rheomery Exercise Problems 7.3 Viscoelastic fluid and flow 7.3.1 Linear viscoelastic rheological equation 7.3.2 Linear and nonlinear viscoelastic models 7.3.3 Viscoelastic models to standard flow and application to some engineering flow problems 7.3.3.1 UCM, CRM and Giesekus equation 7.3.3.2 Unidirectional basic flow problems Exercise Problems Bibliography Nomenclature for chapter 7 8. Magnetic fluid and flow 8.1 Thermophysical properties Exercise Problems 8.2 Ferrohydrodynamics equation Exercise Problems 8.3 Basic flows and applications 8.3.1 Generalized Bernoulli equation 8.3.2 Hydrostatics 8.3.3 Thermoconvective phenomena Exercise Problems Bibliography Nomenclature for chapter 8

Flexible-Membrane Airfoils at Low Reynolds Numbers

Abstract: An experimental study was conducted to assess the benefits of using flexible-membrane airfoils/wings at low Reynolds numbers for micro air vehicle applications compared with using a conventional rigid airfoil/wing. In addition to measuring aerodynamic forces acting on flexible-membrane airfoils/wings, a high-resolution particle image velocimetry system was used to conduct flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the flexible-membrane airfoils/wings to elucidate the associated underlying fundamental physics. The aerodynamic force measurements revealed that flexible-membrane airfoils could provide better aerodynamic performance compared with their rigid counterpart at low Reynolds numbers. The flexibility (or rigidity) of the membrane skins of the airfoils was found to greatly affect their aerodynamic performance. Particle image velocimetry measurements elucidated that flexible-membrane airfoils could change their camber (i.e., crosssectional shape) automatically to adapt incoming flows to balance the pressure differences on the upper and lower surfaces of the airfoils, therefore suppressing flow separation on the airfoil upper surfaces. Meanwhile, deformation of the flexible-membrane skins was found to cause significant airfoil trailing-edge deflection (i.e., lift the airfoil trailing edge up from its original designed position), which resulted in a reduction of the effective angles of attack of the flexible-membrane airfoils, thereby delaying airfoil stall at high angles of attack. The nonuniform spanwise deformation of the flexible-membrane skins of the flexible-membrane airfoils was found to significantly affect the characteristics of vortex and turbulent flow structures around the flexible-membrane airfoils.

Investigation of the backflow phenomenon in falling liquid films

Abstract: The phenomenon of backflow in the capillary wave region of laminar falling liquid films is studied in detail. For the first time, the mechanism leading to the origination of the phenomenon is identified and explained. It is shown that backflow forms as the result of a separation eddy developing at the bounding wall similar to the case of classical flow separation. Results show that the adverse pressure distribution causing the separation of the flow in the capillary wave region is induced by the strong third-order deformation (i.e. change in curvature) of the liquid–gas free surface there. This deformation acts on the interfacial pressure jump, and thereby the wall pressure distribution, as a result of surface tension forces. It is shown that only the capillary waves, owing to their short wavelength and large curvature, impose a pressure distribution satisfying the conditions for flow separation. The effect of this capillary separation eddy on momentum and heat transfer is investigated from the perspective of modelling approaches for falling liquid films. The study is centred on a single case of inclined liquid film flow in the visco-capillary regime with surface waves externally excited at a single forcing frequency. Investigations are based on temporally and spatially highly resolved numerical data obtained by solving the Navier–Stokes equations for both phases. Computation of phase distribution is performed with the volume of fluid method and the effect of surface tension is modelled using the continuum surface force approach. Numerical data are compared with experimental data measured in the developed region of the flow. Laser-Doppler velocimetry is used to measure the temporal distribution of the local streamwise velocity component, and confocal chromatic imaging is employed to measure the temporal distribution of film thickness. Excellent agreement is obtained with respect to film thickness and reasonable agreement with respect to velocity.

Scanning PIV investigation of the laminar separation bubble on a SD7003 airfoil

Abstract: A laminar separation bubble occurs on the suction side of the SD7003 airfoil at an angle of attack α = 4–8° and a low Reynolds number less than 100,000, which brings about a significant adverse aerodynamic effect. The spatial and temporal structure of the laminar separation bubble was studied using the scanning PIV method at α = 4° and Re = 60,000 and 20,000. Of particular interest are the dynamic vortex behavior in transition process and the subsequent vortex evolution in the turbulent boundary layer. The flow was continuously sampled in a stack of parallel illuminated planes from two orthogonal views with a frequency of hundreds Hz, and PIV cross-correlation was performed to obtain the 2D velocity field in each plane. Results of both the single-sliced and the volumetric presentations of the laminar separation bubble reveal vortex shedding in transition near the reattachment region at Re = 60,000. In a relatively long distance vortices characterized by paired wall-normal vorticity packets retain their identities in the reattached turbulent boundary layer, though vortices interact through tearing, stretching and tilting. Compared with the restricted LSB at Re = 60,000, the flow at Re = 20,000 presents an earlier separation and a significantly increased reversed flow region followed by “huge” vortical structures.

Large Eddy/Reynolds-Averaged Navier-Stokes Simulation of a Mach 5 Compression-Corner Interaction

Abstract: Simulations of Mach 5 turbulent flow over a 28-deg compression corner are performed using a hybrid large-eddy/ Reynolds-averaged Navier-Stokes method. The model captures the mean-flow structure of the interaction reasonably well, with observed deficiencies relating to an underprediction of the displacement effects of the shock-induced separation region. The computational results provide some support for a recent theory concerning the underlying causes of low-frequency shock-wave oscillation. In the simulations, the sustained presence of a collection of streaks of fluid with lower/higher momentum than the average induces a low-frequency undulation of the separation front. Power spectra obtained at different streamwise stations are in good agreement with experimental results. Downstream of reattachment, the simulations capture a three-dimensional mean-flow structure, dominated by counter-rotating vortices that produce wide variations in the surface skin friction. Predictions of the structure of the reattaching boundary layer agree well with experimental pitot pressure measurements. In comparison with Reynolds-averaged model predictions, the hybrid large-eddy/Reynolds-averaged Navier-Stokes model predicts more amplification of the Reynolds stresses and a broadening of the Reynolds stress distribution within the boundary layer that is probably due to reattachment-shock motion.

Numerical investigation of unsteady flow past a circular cylinder using 2-D finite volume method

Abstract: The dynamic characteristics of the pressure and velocity fields of unsteady incompressible laminar and turbulent wakes behind a circular cylinder are investigated numerically and analyzed physically. The governing equations, written in the velocity pressure formulation are solved using 2-D finite volume method. The initial mechanism for vortex shedding is demonstrated and unsteady body forces are evaluated. The turbulent flow for Re = 1000 & 3900 are simulated using k-e standard, k-e Realizable and k-ω SST turbulence models. The capabilities of these turbulence models to compute lift and drag coefficients are also verified. The frequencies of the drag and lift oscillations obtained theoretically agree well with the experimental results. The pressure and drag coefficients for different Reynolds numbers were also computed and compared with experimental and other numerical results. Due to faster convergence, 2-D finite volume method is found very much prospective for turbulent flow as well as laminar flow. Keywords: Viscous unsteady flow, laminar & turbulent flow, finite volume method, circular cylinder. DOI: 10.3329/jname.v4i1.914 Journal of Naval Architecture and Marine Engineering 4(2007) 27-42

Numerical simulations of a cylinder wake under a strong axial magnetic field

Abstract: We study the flow of a liquid metal in a square duct past a circular cylinder in a strong externally imposed magnetic field. In these conditions, the flow is quasi-two-dimensional, which allows us to model it using a two-dimensional (2D) model. We perform a parametric study by varying the two control parameters Re and Ha (Ha2 is the ratio of Lorentz to viscous forces) in the ranges [0…6000] and [0…2160], respectively. The flow is found to exhibit a sequence of four regimes. The first three regimes are similar to those of the non-magnetohydrodynamic (non-MHD) 2D circular wake, with transitions controlled by the friction parameter Re∕Ha. The fourth one is characterized by vortices raising from boundary layer separations at the duct side walls, which strongly disturbs the Karman vortex street. This provides the first explanation for the breakup of the 2D Karman vortex street first observed experimentally by Frank, Barleon, and Muller [Phys. Fluids 13, 2287 (2001)]. We also show that, for high values of Ha (H...

Discrete conservation principles in large-eddy simulation with application to separation control over an airfoil

Abstract: An unstructured-grid large-eddy simulation (LES) technique is used to investigate the turbulent flow separation over an airfoil with and without synthetic-jet control. Numerical accuracy and stability on arbitrary shaped mesh elements at high Reynolds numbers are achieved using a finite-volume discretization of the incompressible Navier–Stokes equations based on higher-order conservation principles—i.e., in addition to mass and momentum conservation, kinetic energy conservation in the inviscid limit is used to guide the selection of the discrete operators and solution algorithm. Two different stall configurations, which consist of flow over a NACA 0015 airfoil at 16.6° and 20° angles of attack, are simulated at Reynolds number of 896 000 based on the airfoil chord length and freestream velocity. In the case of 16.6° angle of attack where flow separates around a midchord location, LES results show excellent agreement with the experimental data for both uncontrolled and controlled cases. LES confirms the ex...

Large Trucks Drag Reduction using Active Flow Control

Abstract: Aerodynamic drag is the cause for more than two-thirds of the fuel consumption of large trucks at highway speeds. Due to functionality considerations, the aerodynamic efficiency of the aft-regions of large trucks was traditionally sacrificed. This leads to massively separated flow at the lee-side of truck-trailers, with an associated drag penalty of at least a third of the total aerodynamic drag. Active Flow Control (AFC), the capability to alter the flow behavior using unsteady, localized energy injection, can very effectively delay boundary layer separation. By attaching a compact and relatively inexpensive “add-on” AFC device to the back side of truck-trailers (or by modifying it when possible) the flow separating from it could be redirected to turn into the lee-side of the truck, increasing the back pressure, thus significantly reducing drag. A comprehensive and aggressive research plan that combines actuator development, computational fluid dynamics and bench-top as well as wind tunnel experiments was performed. The research uses an array of 15 newly developed Suction and Oscillatory Blowing actuators housed inside a circular cylinder attached to the aft edges of a generic 2D truck model. Preliminary results indicate a net drag reduction of 10% or more.