Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session JT: Separated Flows I |
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Chair: Jamey Jacob, University of Oklahoma Room: Salt Palace Convention Center Ballroom FH |
Monday, November 19, 2007 3:35PM - 3:48PM |
JT.00001: Effects of flow control over a 3D turret -- Part I M. Andino, R. Wallace, R. Schmit, R. Camphouse, J. Myatt, M. Glauser Turbulence causes losses in the performance and effectiveness of optical systems and separated turbulent flow in particular is a phenomena present in several aero-optics applications. In an attempt to reduce the adverse effects of separated turbulent flow on such systems, we are exploring the use of both open and closed-loop flow control. In this presentation results from an experiment performed in Syracuse University's 2' x 2' wind tunnel of the flow over a 3D turret at a Reynolds number of 300,000 and Mach number of 0.1 will be presented. The 3D turret used has a diameter of 6in with a flat aperture of 2.8in diameter. Our actuation system consists of 11 synthetic jets created by 22 piezoelectric disks. The actuation system was placed 0.4in upstream from the leading edge of the aperture and two different actuation cases were tested to then evaluate the effects of the flow control over the aperture area. Simultaneous surface pressure and PIV velocity measurements in the separated region were performed, both with and without flow control. The PIV results suggest that with the open-loop control the separation zone above the aperture is reduced spatially and the mean square velocity fluctuations reduced in amplitude by 15 -- 20 percent. We are currently applying the successful POD/LSE based NACA 4412 closed-loop separation control work of Pinier et al (2007) to the 3D turret. Pinier, J.T., Ausseur, J.M., Glauser, M.N. and Higuchi, H., (2007), ``Proportional Closed-loop Feedback Control of Flow Separation'', AIAA Journal, Volume 45, Issue 1, pp 181 - 190. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JT.00002: Effects of flow control over a 3D turret -- Part II Ryan Wallace, Marlyn Andino, Ryan Schmit, Chris Camphouse, James Myatt, Mark Glauser Building upon the 3D turret work done at Syracuse University an extended study was conducted in the Air Force Research Laboratory's Subsonic Aerodynamic Research Laboratory (SARL) wind tunnel at Wright-Patterson Air Force Base. The SARL experiments were performed at higher Reynolds and Mach numbers and therefore present a more complex, more challenging flow. Synthetic jets mounted upstream of the aperture were used to generate multiple actuation cases in order to provide a rich ensemble for plant model development based on the split POD method of Camphouse (2007). PIV velocity data was acquired along with simultaneous surface pressure data at various planes across the turret with and without open-loop control. In addition, a simple proportional closed-loop control was performed using the bandpass filtered first POD mode coefficient of the surface pressure as the feedback signal. The amplitude of the feedback signal was calibrated using the open-loop results which were the most effective in reducing the separation zone of the turret. [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JT.00003: Effects of flow control over a 3D turret -- Part III Ryan Schmit, Marlyn Andino, Ryan Wallace, Russel Camphouse, James Myatt, Mark Glauser Additional 3D turret work from Wright-Patterson Air Force Base's Subsonic Aerodynamic Research Laboratory (SARL) will be presented in the paper. The research involved simultaneously sampling aero-optic measurement with the dynamic surface pressure measurements. Two aero-optics devices the Malley Probe and the PhaseCam, a digital electronic holography system, where used to measure the aero-optic distortion created by the turbulent separated region of the turret. Open and closed loops flow control was used to reduce the aero-optical distortion and improve the flow quality. POD and LSM analysis techniques will be used to correlate the aero-optic distortion movement over the turret to the dynamic surface pressure measurements. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JT.00004: Closed-loop control of vortex shedding on two-dimensional flat-plate airfoil at low Re. Won Tae Joe, Tim Colonius, Doug MacMynowski, Kunihiko Taira We investigate a two-dimensional model problem related to using closed-loop control to enhance lift on a separated flat-plate airfoil at low Reynolds number. Navier-Stokes simulations are performed using an immersed boundary method. Pulsed mass injection is modeled by treating the actuation slot as a boundary with a pre-defined velocity. Open-loop actuation is examined for different frequencies, angles of attack (AOA), actuator locations (leading or trailing edge), and injection angles. The natural flow undergoes a Hopf bifurcation around 15 degrees after which vortex shedding occurs. When forced, the flow becomes phase-locked up to a certain AOA, but displays a pulling-out phenomena, in which subharmonic resonance is excited, at higher AOA. In this region, the highest maximum lift occurs when actuation is in phase with the lift, and the highest average lift (over the forcing period) occurs with an associated optimal phase shift between lift and actuation. We design a simple controller that feeds back the lift to the actuator in order to phase-lock the flow with either the highest average or maximum lift. [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JT.00005: Flow Control via a Single Spanwise Wire on the Surface of a Stationary Cylinder Alis Ekmekci, Donald Rockwell The flow structure arising from a single spanwise wire attached along the surface of a circular stationary cylinder is investigated experimentally via a cinema technique of digital particle image velocimetry (DPIV). Consideration is given to wires that have smaller and larger scales than the thickness of the unperturbed boundary layer that develops around the cylinder prior to flow separation. The wires have diameters that are 1{\%} and 3{\%} of the cylinder diameter. Over a certain range of angular positions with respect to the approach flow, both small- and large-scale wires show important global effects on the entire near-wake. Two critical angles are identified on the basis of the near-wake structure. These critical angles are associated with extension and contraction of the near-wake, relative to the wake in absence of the effect of a surface disturbance. The critical angle of the wire that yields near-wake extension is associated with bistable oscillations of the separating shear layer, at irregular time intervals, much longer that the time scale associated with classical Karman vortex shedding. Moreover, for the large scale wire, in specific cases, either attenuation or enhancement of the Karman mode of vortex formation is observed. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JT.00006: Fluidic Control of Aerodynamic Forces on an Axisymmetric Body Philip Abramson, Bojan Vukasinovic, Ari Glezer The aerodynamic forces and moments on a wind tunnel model of an axisymmetric bluff body are modified by induced local vectoring of the separated base flow. Control is effected by an array of four integrated aft-facing synthetic jets that emanate from narrow, azimuthally-segmented slots, equally distributed around the perimeter of the circular tail end within a small backward facing step that extends into a Coanda surface. The model is suspended in the wind tunnel by eight thin wires for minimal support interference with the wake. Fluidic actuation results in a localized, segmented vectoring of the separated base flow along the rear Coanda surface and induces asymmetric aerodynamic forces and moments to effect maneuvering during flight. The aerodynamic effects associated with quasi-steady and transitory differential, asymmetric activation of the Coanda effect are characterized using direct force and PIV measurements. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JT.00007: Control of a Sphere Wake by Sting Interference and Localized Disturbances Adam Norman, Joshua Feingold, Beverley McKeon Vortex shedding in the wake of a sphere that is simply supported using a streamwise-aligned cylindrical sting is investigated at sub-critical Reynolds numbers of order $10^4$. The effect of the sting size on the K\'{a}rm\'{a}n vortex shedding and Kelvin-Helmholtz shear layer instability is examined. The blockage of the sting will be compared with the two-dimensional analog of the splitter plate introduced into a cylinder wake. The controlling mechanism of a small stud placed upstream of the average azimuthal separation angle is also explored. High speed stereo particle image velocimetry is used to understand the average and temporal aspects of the sting and stud controlling mechanisms, and Lagrangian Coherent Structure (LCS) analysis is implemented to probe the wake structure. This research is a first step towards active control of a sphere wake using surface actuation. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JT.00008: Separation Control on a Hydrofoil Using Leading Edge Protuberances Derrick Custodio, Charles Henoch, Hamid Johari The humpback whale's maneuvarability has been attributed to their use of pectoral flippers, on which protuberances are present along the leading edge. To examine the effects of protuberances on hydrofoil performance, the lift, drag, and pitching moments of two-dimensional hydrofoils with leading edge sinusoidal protuberances were measured in a water tunnel and compared to those of a baseline NACA 63(4)-021 hydrofoil. The amplitude and spanwise wavelengths of the protuberances ranged from 2.5{\%} to 12{\%} and 25{\%} to 50{\%} of the mean chord length respectively. Flow visualizations using tufts and dye, as well as Laser Doppler Velocimetry (LDV) measurements were performed to examine the flow patterns surrounding the hydrofoils. At angles of attack lower than the stall angle of the baseline, the modified foils revealed lower lift and increased drag. However, above this angle the lift generated by the modified foils was up to 50{\%} greater than the baseline foil with little or no drag penalty. The amplitude of the protuberances has a large effect on the performance of the hydrofoils whereas the wavelength has little. Flow topology on the protuberances will be discussed by means of the visualization and measured velocities. [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JT.00009: Simulation and Control of Separated Flow over Low-Aspect-Ratio Airfoils Kunihiko Taira, Tim Colonius Direct numerical simulation is used to study the three-dimensional formation and evolution of wake vortices behind low-aspect-ratio, flat-plate airfoils at a Reynolds number of 300. For impulsively-started translation at high angles of attack, flow separates and forms leading-edge and tip vortices that interact as they detach from the plate. Since added spanwise circulation enhances lift exerted on the plate, it is useful to prevent the leading-edge vortex from shedding. Stabilizing mechanisms for the leading-edge vortex are identified to be the downward induced velocity from the tip vortices and the release of vorticity through diffusion and spanwise convection. Aspect ratio, planform geometry, and angle of attack are varied to examine differences in the convection of vorticity and influence of tip vortices. Flow control by momentum injection (blowing) is also considered at various locations and is found to alter the vortical wake structure by strengthening the tip vortices inducing stronger downward velocity. Such controlled flows enhanced lift and, in some instances, achieved a stable steady state. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JT.00010: Sub-Optimal Control of Unsteady Separation in a Channel Kevin Cassel, Chetan Sardesai Incompressible flow through a two-dimensional channel with localized suction from the upper surface is considered as a framework within which to consider the control of unsteady separation within the context of boundary-layer theory. Control is implemented through a body force throughout the boundary layer at the lower surface where the unsteady separation takes place. The control objective is to suppress the onset of unsteady separation by minimizing a cost functional that retards the separation process while minimizing the energy input required to accomplish the control. In the present investigation, sub-optimal control is considered in which the control, i.e. adjoint, equation is solved in a quasi-steady manner as the unsteady boundary-layer equations evolve in time. Without control the unsteady boundary layer forms a recirculation region and terminates in a separation singularity for suction values above a critical value. Although cases in which recirculation forms for the no- control case always result in unsteady separation, it is possible to eliminate unsteady separation without eliminating recirculation using the domain-based control employed here. The controlling body force is found to concentrate primarily in the reattachment region for cases in which unsteady separation is suppressed. [Preview Abstract] |
Monday, November 19, 2007 5:45PM - 5:58PM |
JT.00011: Mechanism of drag reduction on a three-dimensional model vehicle using a passive control device Wook Yi, Woong Sagong, Haecheon Choi It has been well known that the boat-tail device reduces drag on a three-dimensional vehicle. However, its detailed mechanism is not clearly known yet. To understand this mechanism, we conduct an experiment for flow over a three-dimensional model vehicle in ground proximity. We consider various lengths ($l/H = 0.1 \sim 0.5$) and slant angles ($\theta=0^{\circ} \sim 40^{\circ}$) of the boat tail, and conduct velocity measurements near the boat tail and oil visualizations on the boat-tail surface. We find that the slant angle is an important parameter for drag reduction. The maximum drag reduction occurs at $\theta=12.5^{\circ}, 15^{\circ}$ and $15^{\circ}$ for $l/H = 0.1, 0.3$ and $0.5$, respectively, and the amounts of maximum drag reduction are 20, 41 and 45\%. For the case of $l/H = 0.3$, separation starts to occur from $\theta= 6^{\circ}$ at the leading edge of the boat tail. This separated flow reattaches on the boat-tail surface and forms a small secondary separation bubble, which provides strong near-wall momentum and delays main separation down to the trailing edge of boat tail. The size of secondary separation bubble increases with increasing $\theta$. At $\theta>16^{\circ}$, main separation occurs at the leading edge of boat tail, and drag increases from the minimum value and reaches that of no control at large $\theta$'s. [Preview Abstract] |
Monday, November 19, 2007 5:58PM - 6:11PM |
JT.00012: Linear proportional control of flow over a sphere Seung Jeon, Haecheon Choi In the present study, we apply a linear proportional control to flow over a sphere for reduction of drag and lift fluctuations. For this purpose, the radial velocity at the centerline in the wake region is measured for the feedback and the control input (blowing/suction) varying in the azimuthal direction is provided from a slot located near the separation point. The azimuthal angle of maximum blowing is in phase or out of phase to the measured velocity according to the sign of feedback gain and the amplitude of blowing/suction is proportional to the measured velocity. Zero-net mass flow rate is satisfied during the control. We consider two different Reynolds numbers of $Re=300$ and $425$. We vary the sensing location from $x_s = 0.8d$ to $1.3d$ ($d$ is the sphere diameter). The present control is found to be very sensitive to the sensing location, as was also observed for the control of flow over a cylinder by Park et al. (PoF, 1994). Using the present linear proportional control, the drag and lift fluctuations are significantly reduced for both $Re=300$ and 425. It is found that the best sensing location for the present control is well correlated with the radial velocity induced by the vortex shedding. [Preview Abstract] |
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