Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EM: Separated Flows III |
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Chair: Hamid Johari, Worcester Polytechnic Institute Room: Hilton Chicago PDR 2 |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EM.00001: Three-dimensional instabilities in detached boundary layers Fran\c{c}ois Gallaire, Matthieu Marquillie, Uwe Ehrenstein The three-dimensional direct numerical simulation of the incompressible Navier-Stokes equations of the flow above a bump at a Reynolds number of Re=400 shows stationary streamwise elongated structures in the wake of the bump. A three-dimensional global mode linear analysis is used to interpret these results and shows that the most unstable eigenmode is steady and localized in the recirculation bubble, with spanwise wavelength approximatively ten bump heights. What is the physical mechanism at the origin of this instability? Is this a G\"ortler instability, a centrifugal instability? These questions motivate an inviscid geometrical optics analysis along closed streamlines, which identifies the most unstable streamline in the flow. We then show that this method, though designed for short wave inviscid instabilities, provides a quantitatively correct estimate of the growth-rate, once suitably modified to take into account viscous effects. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EM.00002: Effect of Flow Control on the Mean Pressure Field over a NACA 4412 Airfoil Marlyn Andino, Julie Ausseur, Jeremy Pinier, Mark Glauser, Hiroshi Higuchi We are applying real time closed loop separation control to the flow over a NACA 4412 airfoil using zero net-mass flow actuators. The feedback study involves analyzing the dynamics of the separating shear flow using low-dimensional methods and suppressing wing stall via zero net-mass flow actuators utilizing pressure sensors on the surface alone for input in feedback control. In this talk we will explore the effects of the interaction between the actuators and flow on the mean pressure distribution over the NACA 4412 airfoil. Our approach involves an application of the method developed by Honohan et al. (2003) in which they use PIV data coupled with the RANS equations to obtain the effect of the flow control actuation on the averaged pressure field. We have already demonstrated (Andino et al 2005) the utility of this approach on a NACA 4412 hydrofoil to help understand the global effects of flow control. We clearly see a major effect of the actuation on the mean pressure field backed out from the combined PIV/RANS approach. In this study we will present a similar application to higher resolution PIV data from a NACA 4412 airfoil, obtained at higher Reynolds number compared to the hydrofoil studies. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EM.00003: Nonlinear Theory of Three-Dimensional Separation George Haller In this talk, I describe a recent theory of steady and unsteady three-dimensional separation. The theory provides analytic criteria for the location and shape of separation surfaces emanating from a no-slip boundary of a three-dimensional flow. We construct these surfaces as nonhyperbolic unstable manifolds using nonlinear dynamical systems methods. I show applications of the theory to direct numerical simulations of a backward-facing step flow and of a lid-driven cavity flow. I also mention related two-dimensional experiments and applications to separation control. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EM.00004: Characterization of Zero Mass Flux Flow Control of Airfoil Separation at Low Re Nan Jou Pern, Ray Lebeau, Jamey Jacob Zero mass flux flow control using an oscillating upper wing profile is investigated at low Re over a NACA 4415 airfoil. Time-averaged particle image velocimetry (PIV) measurements over the suction surface shows that reduced frequencies of $F^+\sim\cal{O}$(1) using sinusoidal oscillations significantly reduces flow separation at $Re$ ranging from 25,000 to 100,000. Phase-locked PIV at $Re=$25,000 is used to characterize the physical flow control mechanism. Preliminary visualization indicates that a vortex structure is formed at the point of separation and it travels downstream as the phase of the actuator increases. Detailed phase-locked PIV measurements of different phases of the actuation at $Re=$25,000 and $F^+$ ranging from 4 to 12 will be shown. It is expected that the vortex grows in size as it travels downstream and then dissipates. Comparison to and predictions from numerical simulations using an unsteady code with moving grid will also be presented. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EM.00005: Flow Control Using Zero-Mass Flux (ZMF) with Superposition of Steady Suction. B. Demanett, T. Reynolds, J. Kiedaisch, H. Nagib Most studies of Active Flow Control (AFC) applied to separated flows have centered upon blowing. More recently, focus has been on oscillatory blowing and Zero-Mass Flux (ZMF). However, ZMF with superposition of steady suction, or oscillatory suction, has been largely neglected. This investigation was initiated in an effort to reopen avenues of using more efficient suction for flow control, particularly through use of unsteady means other than ZMF. Oscillatory suction was made possible because of improvements made in the ZMF actuators designed at IIT. The stability of the new design allowed steady suction to be used in conjunction with the ZMF actuators in a generic separated flow field over a hump model. The study demonstrated that ZMF and steady suction, both alone and in combination, have a dramatic effect in altering pressure distributions over the aerodynamic surface. Steady suction was found to have a marginal advantage over the combination, and it provided a slightly lower minimum pressure before saturation of the flow occurred. The oscillatory suction control, with smaller input amplitudes, was able to perform nearly as well as steady suction and better than ZMF alone. Contrary to general belief, the results of this study indicate that for separated flows similar to the one examined here, ZMF may not be the most effective, and traditional parameters used to characterize the AFC performance, such as blowing momentum coefficient, are not the proper choice. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EM.00006: AISCOS a New Approach to Active Flow Control of Separation. P. Reinhard, B. Demanett, J. Kiedaisch, H. Nagib Most studies of Active Flow Control (AFC) of separated flows have centered upon steady or periodic blowing, and Zero-Mass Flux (ZMF). However, ZMF with superposition of steady suction, or oscillatory suction, has recently demonstrated improved effectiveness in controlling large separated zones. Coupled with demonstrated hysteresis in the reattachment of such zones, we were led to the development of the concept of Adaptive Intermittent Suction Control of Separation (AISCOS). The concept and its effectiveness were demonstrated using a generic separated flow field over a hump model. While we have not tested AISCOS within a feedback arrangement to demonstrate its full potential, parametric testing of variables such as frequency and duty cycle have been carried out. When comparing the improvement as a result of AISCOS to other approaches such as ZMF, and to best potentially available improvement in peak C$_{p}$ value over the hump for a fully attached flow, we find that nearly double the change in peak C$_{p}$ could be hypothetically achieved in the case of ZMF. The best steady suction performance resulted in 22{\%} of additional improvement over ZMF, and comparable improvements are achieved with AISCOS using various duty cycles. Most probably, the less than optimum location of the AFC slot on this model limited the potential of the improvement from the application of suction-based methods; the model and its AFC slot were originally designed to work most effectively with ZMF only. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EM.00007: Active Flow Control of Large Separation: A New Look at Scaling Parameters. J. Kiedaisch, B. Demanett, H. Nagib Recently, there have been significant advances in the application of various methods of Active Flow Control (AFC) for controlling separated flows. The most popular method of AFC used recently is Zero-Mass Flux (ZMF). Traditionally, the scaling parameters used to evaluate effectiveness of ZMF control have been the momentum coefficient C$_{\mu }$ and the non-dimensional frequency F$^{+}$. While this scaling has worked well for certain types of flows, i.e. flows with small separated regions, exhibiting separated shear layers susceptible to instabilities and development of coherent structures, it is not the proper choice for flows with large separated regions, especially with three dimensionality and high Reynolds numbers. This became evident in a couple of recent tests where ZMF was used with large separated regions, including the first ever full-scale flight demonstration of AFC; the DARPA/Boeing/Bell Helicopter XV-15 download reduction program. The key element in this program and others, where AFC technology is transitioned from small-scale laboratory tests to large-scale tests and eventually to full-scale flight, was the realization that traditional scaling parameters did not apply, and a new way of characterizing the performance of AFC was needed. This new way of thinking, where the dominant parameter is the ratio of the peak AFC jet velocity to the local flow velocity, has lead to advances in development of new AFC actuators and concepts, including Adaptive Intermittent Suction Control of Separation (AISCOS). [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EM.00008: Characteristics of a Plasma Synthetic Jet Arvind Santhanakrishnan, Karthik Ramakumar, Jamey Jacob The term plasma (aka, 1 atm glow discharge or dielectric barrier discharge) actuator is used to describe an arrangement of two electrodes separated by dielectric material. Under input of a high voltage, high frequency AC, this leads to dielectric breakdown, and the plasma region produced at the interfacial air gap can be used for flow control applications. One such novel actuator design consisting of an annular electrode array embedded on a flat plate is examined here. This particular actuator produces a jet that can be applied continuously or pulsated to resemble a synthetic (zero-net mass flux) jet. The purpose of this work is to examine the evolution of the plasma synthetic jet, and compare the jet characteristics with a conventional synthetic jet. Flow visualization is employed for qualitative observation of the flowfield with and without actuation. Phase-locked PIV measurements are used to examine the development of regular vortical structures and jet profiles. Two sets of counter-rotating vortex rings are generated upon plasma actuation, one of which advects downstream ahead of the jet while the other is ``trapped'' peripherally to the annulus. The effects of changing the forcing frequency and actuator dimensions in relation to the resultant jet momentum are investigated. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EM.00009: Unsteady Properties of a Separated and Reattaching Flow Sarah Blackmar, Richard Hillier Separated flows are commonly responsible for buffet on buildings, cars, and aircraft. The process of separation and reattachment that occurs in these instances is highly unsteady and turbulent, inducing large pressure fluctuations on the surface below. These unsteady effects are poorly understood and cannot be accurately predicted for many common practical problems. Low speed wind tunnel experiments (Re = 1x10$^{5}$ based on model diameter) have studied the unsteady properties of the separated and reattaching flow around a blunt-faced circular cylinder axially aligned with the freestream. The model was configurable with three different nose pieces: one flat front face, and two circular domed front faces, altogether providing three different flow separation angles. Extensive coverage of surface pressure fluctuation data provides detailed streamwise and spanwise distributions for mean and RMS pressure distributions, autospectra, streamwise and spanwise correlations, and cross-spectra. The separated shear layer is largely characterized by the low-frequency `flapping' of the shear layer as a whole and the development of large-scale vortices within the shear layer. Correlations between surface pressure fluctuations and flow field velocity fluctuation are used to relate these shear layer vortex structures to the mechanism of pressure generation. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EM.00010: Global linear stability analysis of separated flow over a rounded backward facing step Olivier Marquet, Matteo Lombardi, Denis Sipp The global linear stability of a two-dimensional flow over a rounded backward facing step to three-dimensional transverse perturbations is numerically investigated. The steady two- dimensional base flow is obtained from time-dependent simulations based on a finite-element spatial discretization and a Lagrange-Galerkin temporal discretization. The generalized eigenvalue problem is solved using the Implicity Restarted Arnoldi Method implemented in the ARPACK library. The most unstable linear mode is three-dimensional, non-oscillating and appears at a critical Reynolds number of Re=675. The associated eigenmode is localized within the separation bubble, and the reconstruction of the total flow shows a three- dimensional deformation of this recirculation region. The adjoint stability problem is solved in order to locate the core of the instability and to determine the influence of numerical boundary conditions on the direct stability problem. Our results suggest that the characteristics of the global mode are mainly dictated by the recirculation zone of the base flow. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
EM.00011: Scaling Laws for Flow Control in a Hump Diffuser. Ciro Cerretelli A study has been performed to characterize the effects of steady boundary layer injection on the separated flow of a hump diffuser. Results of parametrically varying discrete hole blowing characteristics have identified two separate regimes where the injection momentum coefficient and velocity ratio, respectively, are the primary scaling parameters for pressure recovery. Both discrete hole and slot injection have been investigated under varying degrees of adverse pressure gradient in the stream-wise direction, indicating optimal discrete hole injection to be more efficient than slot injection in terms of necessary injection momentum coefficient to achieve maximum levels of increased pressure recovery. The effects of discrete injection parameters such as hole diameter, hole spacing and stream-wise injection (yaw) angle have been studied. Angled injection (45\r{ } yaw) has been shown to be most effective in removing separated flow and increasing the pressure recovery. The angled injection enhances shear layer mixing through large-scale co-rotating vortical motion induced by the yawed jet -- main flow interaction. A CFD/data comparison study has been performed on the results of the discrete injection tests, capturing overall data trends and representing the net impact of stream-wise and angled injection on pressure recovery in the hump diffuser. [Preview Abstract] |
Sunday, November 20, 2005 6:33PM - 6:46PM |
EM.00012: Evaluation of Turbulence Models Through Predictions of a Separated Flow Over a Hump. D. Madugundi, J. Kiedaisch, H. Nagib Although a number of popular turbulence models are now commonly used to predict complex 3D separated flows, in particular for industrial applications, very limited full evaluation of their performance has been carried out using thoroughly documented experiments. Activities in this area have been recently intensified in view of the growing interest in Active Flow Control (AFC) of separation, as can be seen from the papers by Greenblatt et al. (AIAA-2005-0485) and Rumsey et al. (AIAA-2004-2217). Unlike the hump model used in these recent publications, where the point of separation is highly localized by the surface geometry, many applications develop separation over surfaces with more gradual changes in local surface curvature. A hump model generating such a separated flow has been used as a test bed for comparing results from various models including $k-\varepsilon $, Spalart-Alamaras, $k-\omega $, Menter's SST, and RSM. Our goal is a better understanding of the strengths and limitations of the various models by comparing them to each other and to experimental data recently documented on this model. The results include cases without and with AFC, applied by suction and blowing from a nearly tangential narrow slot in the vicinity of the start of separation. While very high suction and blowing rates lead to near elimination of the separation zone, the surface pressure contours reveal interesting behavior that can be used to arrive at optimum AFC parameter selection. [Preview Abstract] |
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