Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session AD: Flow Control I |
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Chair: Luciano Castillo, Rensselaer Polytechnic Institute Room: 101D |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AD.00001: Shock Wave/Boundary Layer Interaction Control in a Supersonic Inlet Nathan Webb, Edgar Caraballo, Jesse Little, Jin-Hwa Kim, Mo Samimy A shock wave/boundary layer interaction (SWBLI) occurs in a supersonic mixed compression inlet. The SWBLI could cause boundary layer separation resulting in adverse consequences such as reduced pressure recovery and non-uniform fan loading. Boundary layer bleed is currently used to prevent separation, which incurs a significant performance penalty. We have recently used Localized Arc Filament Plasma Actuators (LAFPAs) with high amplitude and wide bandwidth to control the SWBLI in a Mach 1.9 flow. The preliminary results are promising and show excellent potential for this technique. These actuators may affect the SWBLI by two mechanisms: manipulation of flow instabilities and/or streamwise vorticity generation. Particle image velocimetry measurements have confirmed that instability manipulation is the key to the LAFPAs' ability to significantly energize the boundary layer in the interaction region. The streamwise vorticity effects are currently being investigated. The LAFPAs have been tested at varying frequency, duty cycle, and mode to determine the parameters with the maximum effectiveness. Supported by AFRL and AFOSR. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AD.00002: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AD.00003: Flow Mechanisms Leading to Separation Control over Embedded Hexagonal Cavities Amy Lang, Blake Melnick Digital Particle Image Velocimetry was used to measure the flow over an array of hexagonal cavities, with a focus on discerning the effect of cavity orientation on the adjacent boundary layer flow. Time-averaged velocity profiles above the cavities were measured under transitioning and turbulent boundary layer conditions. Two flow mechanisms leading to separation control, produced by the presence of the embedded cavities, were considered and will be discussed: (1) the presence of a partial slip velocity, produced by the embedded vortices forming within the cavities, on the adjacent boundary layer flow; and (2) turbulence augmentation close to the surface leading to a greater momentum exchange with the higher momentum, outer boundary layer region. The Reynolds stresses over the hexagonal cavities were thus compared to those over the flat plate under turbulent conditions to attempt to discern the effect of cavity orientation on turbulence augmentation. Results will discuss how these flow mechanisms lead to higher momentum in the boundary layer close to the wall as compared to a flat plate. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AD.00004: Flow mechanisms induced by 2D transverse cavities leading to separation control Drew Smith, Amy Lang, Leah Mendelson Cavities embedded in the surface of an object moving through a fluid can help delay flow separation by imposing partial slip velocities and augmenting the turbulence of the boundary layer. Furthermore, recent experiments have shown that embedded cavity surface geometries may have their greatest effect on separation control at the point where the flow initially encounters them. This study investigated whether this effect is observed on a model with two-dimensional, transverse grooves embedded in the surface. The embedded cavities were mounted as a full-span section within a flat plate model in a low-speed water tunnel. Digital Particle Image Velocimetry was used to obtain flow data which were compared to that obtained over a flat surface. An analysis of the boundary layer profiles and Reynolds stresses at multiple locations on the model was conducted. Special attention was paid to the changes in these characteristics with streamwise distance. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AD.00005: Long Lasting Modifications to Karman Vortex by a Single Pulse of DBD Plamsa Kwing-So Choi, Tim Jukes We discovered a unique phenomenon whereby the vortex shedding and force fluctuations on a circular cylinder in cross flow are halted for a considerably long duration by applying a single, short-duration pulse of DBD plasma close to flow separation points. This period of flow modifications is equivalent to over 150 times that of the plasma excitation. We believe this is due to the induced vortex by a short-duration plasma pulse that interacts with the Karman vortex formation. As a result, the drag and lift fluctuations are reduced by 8{\%} and 40{\%}, respectively. This corresponds to the power-saving ratio of nearly 1200, or the energy efficiency of more than 50{\%}. Experiments were conducted in a wind tunnel, where the free-stream velocity was 4.6 m/s and the turbulence intensity was 0.5{\%}. The Reynolds number based on the diameter of circular cylinder was 15000. A single asymmetric DBD plasma actuator was placed on a circular cylinder at 75$^{\circ}$, where a pulse of DBD plasma with a short duration (5{\%} of the vortex shedding period) was applied at a high ac voltage (7.4 kV peak-to-peak, 33 kHz). Simultaneously with the force measurements using a two-component dynamic force balance, the global flow field in the near wake of a circular cylinder was studied using a time-resolved PIV system. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AD.00006: Scalnig of transient lift response to actuation in a 3D separated flow Tim Colonius, David Williams, Gilead Tadmor, Wes Kerstens, Vien Quach, Seth Buntain The transient lift response of a separated flow to short duration (pulsed) blowing is studied on a low Reynolds number, semicircular-planform, flat-plate wing. Actuators were distributed along the leading edge of the wing. The pulse duration, amplitude (supply pressure), and freestream speed were varied in the experiments. We identify two non-dimensional parameters governing the response, and use the data to find functional forms for the lift coefficient increment. We show that the lift coefficient increment is nearly independent of the pulse duration and increases (solely) with the square root of the supply-pressure coefficient up to a saturation. We also find that the shape of the lift response curve is similar to that produced in other experiments with different airfoils and actuators. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AD.00007: Optimized Control of Vortex Shedding from an Inclined Flat Plate Won Tae Joe, Tim Colonius, Doug MacMynowski Optimal control theory is combined with the numerical simulation of an incompressible viscous flow to control vortex shedding in order to maximize lift. A two-dimensional flat plate model is considered at a high angle of attack and a Reynolds number of 300. Actuation is provided by unsteady mass injection near the trailing edge and is modeled by a compact body force. The adjoint of the linearized perturbed equations is solved backwards in time to obtain the gradient of the lift to changes in actuation (the jet velocity), and this information is used to iteratively improve the controls. We investigate how features of the optimized waveform modify the vortex shedding and lead to higher lift, and compare the results with sinusoidal control. In order to obtain a practically implementable control scheme, the optimized waveform is also implemented in a simple closed-loop controller where the control signal is shifted or deformed periodically to adjust to the (instantaneous) frequency of the lift fluctuations. The feedback utilizes a narrowband filter and an Extended Kalman Filter to robustly estimate the phase of vortex shedding and achieve phase-locked, high lift flow states. Finally, the sensitivity of the flow to the phase shift and other features of the optimized waveform are presented. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AD.00008: Separation Control in a 3D Diffuser using Plasma Actuators Sven Grundmann, John K. Eaton Control experiments were conducted for the fully-turbulent flow in a 3D diffuser with an expansion ratio of 4.8. The uncontrolled flow for the same diffuser has a stable, three-dimensional separation zone which begins as a slender bubble in one corner before spreading across the entire width of the diffuser, giving the opportunity to develop and test active separation control devices. Dielectric-barrier discharge actuators were used to actively control the flow separation with the goal of improving the pressure recovery. The most effective control was achieved using spanwise acting plasma actuators in the inlet section of the diffuser which create streamwise vortices. The pressure recovery can be clearly improved or degraded depending on whether the actuators are operated pulsed or continuously. Parameter studies showed the dependence of the pressure recovery along the diffuser wall on the actuator operating parameters, including the modulation frequency and duty cycle. Velocity profile measurements in the inlet and outlet planes of the diffuser show the creation of the streamwise vortices and their influence on the uniformity of the velocity in the end of the diffuser. Frequency spectra taken in the exit plane using a hotwire probe show the influence of the operating parameters on the diffuser flow. A closed-loop control circuit for the automated adaption of the operating parameters is being tested. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AD.00009: Separation Control on a Cascade of Airfoils using Pulsed Vortex Generator Jets R.J. Volino, M.B. Ibrahim, O. Kartuzova Flow through a row of airfoils will separate if the loading (lift) on each airfoil is too high. This can happen if the airfoil turning angle or the spacing between airfoils is too high. If the boundary layers separate, the actual flow turning and lift drop, and aerodynamic losses increase. In applications, such as the flow through turbines, high lift airfoils are desirable, as the same power generation can be achieved using fewer airfoils, thereby saving weight and cost. Advances in understanding of separation and transition have led to high lift airfoils without separation problems, but further increases in loading will likely require flow control. In the present study, flow through a linear cascade of very high lift low pressure turbine airfoils is controlled using pulsed vortex generator jets. Without flow control there is a large unclosed separation bubble at low Reynolds numbers. Separation causes a 20\% drop in lift and increases losses by up to a factor of seven. Transition of the separated shear layer does not guarantee reattachment. Vortex generator jets with very low mass flow successfully control the separation if the jet velocity and pulsing frequency are sufficiently high. Experimental pressure distributions and phase averaged velocity and turbulence results will be presented. [Preview Abstract] |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AD.00010: Flow Separation Control over a High-lift Airfoil using Multiple DBD Plasma Actuators Jesse Little, Munetake Nishihara, Igor Adamovich, Mo Samimy This work continues an ongoing experimental study on the efficacy of plasma actuators for controlling flow separation on a high-lift airfoil. Previous results showed that a single dielectric barrier discharge (DBD) plasma actuator at the shoulder of a simple trailing edge flap can be effective for enhancing lift by increasing momentum transport between the freestream and separated region through amplification of large-scale structures at low frequencies. This work examines the ability of multiple actuators to further generate and amplify flow structures using pulsed actuation with variable phase. Multiple actuators are also operated at high frequency in an effort to reattach the separating boundary layer with quasi-steady plasma induced flow. Results show that low frequency pulsed forcing requires less power input and generates greater lift increases than high frequency actuation, but has a penalty of increased fluctuating pressure loads on the flap. These studies constitute necessary steps in the development and implementation of plasma actuators for control of flow separation at velocities associated with take-off and landing applications in transport aircraft. [Preview Abstract] |
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