### Session BT: Turbulent Flow Control II

Chair: Mo Samimy, Ohio State University
Room: Hilton Chicago Stevens 5

 Sunday, November 20, 2005 10:56AM - 11:09AM BT.00001: Alternatives to Kelvin-Helmholtz instabilities to control separation bubbles Mark P. Simens , Javier Jim\'{e}nez We study the control of two-dimensional laminar separation bubbles on a flat plate at low Reynolds numbers, using two-dimensional DNS. A range of steady separation bubbles is obtained varying the pressure gradient. They are forced by a zero-mass flow, oscillatory wall blowing with different perturbation amplitudes and frequencies. The reduction in bubble length as a function of frequency has two minima for sufficient high amplitudes. One of them is related to the Kelvin-Helmholtz instability of the separated boundary layer, while the other, most effective one, is here denoted as the low-frequency regime. In this regime large vortices are created which are not a consequence of an instability of the original bubble. On the contrary the forcing creates an unsteady separation bubble which evolves into a large vortex. These vortices have large radii and attach to the wall due to their self-induced pressure field while convecting across the adverse pressure gradient zone. Scaling relations for the effect of the forcing are proposed and tested. Sunday, November 20, 2005 11:09AM - 11:22AM BT.00002: Prediction and Control of Turbulent Separation Over a Wall-Mounted Hump Donghyun You , Meng Wang , Parviz Moin In recent years control methods employing synthetic jets, or zero-net-mass-flux oscillatory jets, have shown good promise for controlling flow separation in industrial applications. The flow fields under control are typically complex and involve a wide range of spatial and temporal scales. In this study, we use LES to predict the flow over a wall-mounted hump at Reynolds number of $9.75 \times 10^{5}$ based on the hump chord length (Test Case 3 in 2004 NASA Langley Workshop on CFD Validation of Synthetic Jets and Separation Control). A small slot across the entire span is used to produce either steady suction or sinusoidal suction/blowing. The model synthetic jet actuator is shown to be effective in suppressing flow separation. The flow solutions for both controlled and uncontrolled cases show excellent agreement with experimental data and are much more accurate than those obtained using RANS methods. Sunday, November 20, 2005 11:22AM - 11:35AM BT.00003: Turbulent Boundary Layer Separation Control on a Convex Ramp using Plasma Actuators David M. Schatzman , Flint O. Thomas , Thomas C. Corke This work is focused toward the development of active feedback control of turbulent boundary layer separation from a convex ramp surface. The work reported here is performed in a subsonic wind tunnel facility and utilizes single dielectric barrier discharge plasma actuators for separation control. Smoke and oil surface flow visualization are used to characterize the separation in the absence of actuation. The surface mounted plasma actuators are positioned upstream of the flow separation locations. Plasma-induced blowing transfers additional momentum to the boundary layer along the ramp surface and has a beneficial effect on flow reattachment. Experimental results are presented which demonstrate the effects of both steady and unsteady actuation. The effectiveness of the active flow control is documented through surface pressure measurements, LDV measurements, and downstream wake surveys. Sunday, November 20, 2005 11:35AM - 11:48AM BT.00004: Control of flow over a backward-facing step using vertical tabs Hyungmin Park , Woo-Pyung Jeon , Haecheon Choi , Jung Yul Yoo In this study, mixing enhancement behind a backward-facing step is experimentally investigated using thin rectangular tabs attached on the trailing edge. The parameters considered are the height ($l_y$) and width ($l_z$) of the tab and the spanwise spacing between the adjacent tabs at the Reynolds number of 24,000 based on the free-stream velocity and step-height $(h)$. The reattachment length is about 5.8$h$ without the tab. For each tab configuration, we measure the distributions of wall pressure and reattachment length along the spanwise direction. With the tab, the reattachment length and wall pressure show significant variations in the spanwise direction. For example, with single tab of $l_y=l_z=0.5h$, the reattachment length slightly increases at the spanwise location of tab ($-0.25 < z/h < 0.25$), but significantly decreases at other spanwise locations; it becomes about $2h$ at $z = 1.5 \sim 2h$. This indicates that the tab attached on the trailing edge drastically increases mixing behind the backward-facing step. The pressure on the backward-facing wall also decreases at most spanwise positions, showing the increase of vortical strength in the recirculating zone. Sunday, November 20, 2005 11:48AM - 12:01PM BT.00005: Control of Small- and Large- Scales in a Shear layer Zvi Rusak , Inness Eisele The response of a backward facing step flow to upstream flow perturbations at both low and high actuation frequency is investigated. This investigation entails the use of local linear temporal and spatial stability analyses, a second-order model equation, a global stability study, and direct numerical simulations. The model problems attempt to mathematically mimic the setup and flow conditions in the experiments of Vukasinovic et al. (2005). The computed results shed light on the perturbed flow structure and the measured flow characteristics along the shear layer of both free and directly manipulated states. The flow evolution may be described by a linear combination of the shear layer instability modes and the global mode of the base flow. It is shown that at low forcing frequencies, the periodic vortical perturbations grow over a distance in the near field of the step. This distance is directly related to position where the imposed perturbations' frequency matches twice the local natural frequency of the flow. Beyond this distance, the flow perturbations in the mid field decay exponentially, yet feeding energy to the far-field global mode which dominates the flow at large distances from the step. At high forcing frequencies, above twice the maximum local natural frequency in the domain, the near-field behavior is suppressed and flow perturbations decay with distance from the step (the mid field), thereby feeding much less energy to the far-field global mode. Sunday, November 20, 2005 12:01PM - 12:14PM BT.00006: Flow Structure in Baseline and Controlled Subsonic Cavity Flows$^{\ast }$ Marco Debiasi , Jesse Little , Mo Samimy The self-excited resonance of flow over an open shallow cavity occurs in several practical applications and understanding its physics plays an important role in the development of efficient techniques for its suppression. To this aim, we use advanced diagnostics to examine the flow over such a cavity recessed in a small continuously operating subsonic wind tunnel. A compression driver actuator is utilized to control the resonance by forcing the cavity shear layer at its receptivity region. Arrays of dynamic pressure transducers are used to measure and correlate the pressure fluctuations in the test section in conjunction with qualitative flow visualizations and 2D PIV images phase-locked to either the forcing frequency or the shedding frequency of shear layer vortices. Both instantaneous and phase-averaged images are obtained in an effort to identify variations between controlled and baseline cases. The results obtained show that the behavior of the coherent structures spanning the cavity matches the empirical predictions given in classical literature. Reduction of the acoustic resonance with control is accompanied by subtle changes in the flow structure and behavior. These detailed results will be presented, discussed and matched against the most recent models of cavity flow. $^{\ast }$Supported by AFRL/VA {\&} AFOSR Sunday, November 20, 2005 12:14PM - 12:27PM BT.00007: Control of instabilities in a cavity-driven separated boundary-layer flow Jerome Hoepffner , Espen {\AA}kervik , Uwe Ehrenstein , Dan Henningson A two-dimensional incompressible boundary-layer flow along a smooth-edged cavity is considered. Unstable global modes appear above a critical inflow Reynolds number of approximatively 500. We aim at stabilizing the flow using feedback control. Sensors measure shear stress at the downstream lip of the cavity, where the unstable modes are most energetic, and actuators apply blowing and suction upstream, where sensitivity is highest. The optimal control loop, in the form of control and estimation feedback gains, is computed through the solution of two Riccati equations. The high dimentionality of this strongly nonparallel flow, once discretized, challenges the design of an optimal feedback controller. A reduced dynamic model is thus constructed for small perturbations to the basic flow by selecting the least stable eigenmodes of the linearized Navier-Stokes equations for this geometry. The flow system is discretized using Chebyshev collocation in both the streamwise and wall-normal direction and the global eigenmodes are computed by means of the Krylov-Arnoldi method. Flow stabilization is demonstrated using a model reduced to 50 states, provided the actuators are smooth, with slow time scales. Sunday, November 20, 2005 12:27PM - 12:40PM BT.00008: Reduced-order Model Based Feedback Control of Cavity Flows M. Samimy , M. Debiasi , E. Caraballo , J. Little , A. Serrani , X. Yuan The focus of this work is on the development of reduced-order model based feedback control of low- to mid-subsonic cavity flows. A reduced-order model was developed via Proper Orthogonal Decomposition using particle imaging velocimetry (PIV) in conjunction with the Galerkin projection of the governing equations - simplified Navier-Stokes - onto the resulting spatial eigenfunctions. The stochastic estimation method was used for real-time estimation of the model time coefficients from simultaneous PIV and dynamic surface pressure measurements. The reduced-order model was linearized around the equilibrium point and a linear-quadratic optimal controller was designed and implemented in the experiments. The actuator was a compression driver type and its output was channeled through a one millimeter slit spanning the entire width of the cavity leading edge. Promising results show that the controller is capable of reducing the cavity flow resonance at Mach 0.3 flow, for which the model was specifically derived, and also at other flows near the design Mach number.