Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GB: Turbulence Control I |
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Chair: Mo Samimy, The Ohio State University Room: 001B |
Monday, November 24, 2008 8:00AM - 8:13AM |
GB.00001: Periodic forcing of a Turbulent Axisymmetric Wake Jonathan Morrison, Ala Qubain The near wake of a blunt, axisymmetric body subject to periodic forcing is investigated. A high-fidelity speaker located inside the cylinder is used to generate a pulsed jet from a small circumferential gap located on the underside of the separating boundary layer, with its axis aligned in the streamwise direction. A detailed investigation of the growth of the disturbances is performed using hot wires, PIV and base-pressure transducers. It is shown that, with azimuthal symmetric forcing ({\it m} =0), the base pressure may be reduced by 30\% at ``low'' frequencies or increased by 10\%, at ``high'' frequencies with consistent changes to the velocity field. As in previous, similar investigations, it is shown that the important scaling parameter is the boundary-layer momentum thickness at separation - in contrast to other geometries such as a 2D bluff body for example, where the von K\'{a}rm\'{a}n vortex shedding is universal, or control of separating-reattaching flows, where a range of actuation frequencies is often effective. Moreover, caution is required when comparing to other axisymmetric bodies because the wake is quite sensitive to boundary conditions and the nature of separation from the body. Many previous studies have demonstrated successful alterations of the wake of a 3D bluff body, all using passive geometric modifications. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GB.00002: Dynamics and control of hydrofoil wakes Morten Kjeldsen, Martin Wosnik, Roger Arndt The problem of rotor-stator interaction (RSI) is an issue within the field of turbomachinery. The flow field entering the rotor cascade will depend on the stator blade to blade velocity distributions, and the viscous wake trailing cascade blades. This flow field is also dependent on the mode of operation, e.g by changing the angle of each blade in hydroturbines. Manipulating the stator viscous wakes is one method to minimize the problems associated RSI; i.e. noise and vibration. In order to explore this concept, a comprehensive experimental program was carried out in a high-speed water tunnel utilizing a series of NACA 0015 hydrofoils. Baseline wake data were collected with a hydraulically smooth foil and compared with two foils modified with two sizes of vortex generators (VG) positioned close to the leading edge of the foil. Not only was the effect of the modifications on wake spreading investigated but also the effect on wake dynamics such as vortex shedding was studied. A high frame-rate PIV system was used at recording rates of 1 and 10 kHz to map the near wake region, extending roughly 1 chord-length downstream the trailing edge, over a range of angles of attack and velocities. The results show that wake dynamics and wake characteristics, i.e. velocity deficit and width, scale with average drag. It was demonstrated that the use of VGs can improve both the dynamics and spreading characteristics of the wake. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GB.00003: Bluff Body Flow Control Using Dielectric Barrier Discharge Plasma Actuators Flint Thomas, Alexey Kozlov The results of an experimental investigation involving the use of dielectric barrier discharge plasma actuators to control bluff body flow is presented. The motivation for the work is plasma landing gear noise control for commercial transport aircraft. For these flow control experiments, the cylinder in cross-flow is chosen for study since it represents a generic flow geometry that is similar in all essential aspects to a landing gear strut. The current work is aimed both at extending the plasma flow control concept to Reynolds numbers typical of landing approach and take-off and on the development of optimum plasma actuation strategies. The cylinder wake flow with and without actuation are documented in detail using particle image velocimetry (PIV) and constant temperature hot-wire anemometry. The experiments are performed over a Reynolds number range extending to $Re_D =10^5$. Using either steady or unsteady plasma actuation, it is demonstrated that even at the highest Reynolds number Karman shedding is totally eliminated and turbulence levels in the wake decrease by more than 50{\%}. By minimizing the unsteady flow separation from the cylinder and associated large-scale wake vorticity, the radiated aerodynamic noise is also reduced. [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GB.00004: Optimal Control of Thermo--Fluid Phenomena in Variable Domains Oleg Volkov, Bartosz Protas This presentation concerns our continued research on adjoint--based optimization of viscous incompressible flows (the Navier--Stokes problem) coupled with heat conduction involving change of phase (the Stefan problem), and occurring in domains with variable boundaries. This problem is motivated by optimization of advanced welding techniques used in automotive manufacturing, where the goal is to determine an optimal heat input, so as to obtain a desired shape of the weld pool surface upon solidification. We argue that computation of sensitivities (gradients) in such free--boundary problems requires the use of the shape--differential calculus as a key ingredient. We also show that, with such tools available, the computational solution of the direct and inverse (optimization) problems can in fact be achieved in a similar manner and in a comparable computational time. Our presentation will address certain mathematical and computational aspects of the method. As an illustration we will consider the two--phase Stefan problem with contact point singularities where our approach allows us to obtain a thermodynamically consistent solution. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GB.00005: Vortex-model based control of turbulent wakes in an experiment Mark Pastoor, Lars Henning, Bernd R. Noack, Rudibert King, Gilead Tadmor High energy costs, dwindling resources and the threat of global warming necessitates the development of efficient engines and transport vehicles. As aerodynamic design is technically mature in many cases flow control has the potential to dramatically increase the performance. Vortex-based reduced-order models explain, develop and improve control strategies for wake flows. By synchronizing the vortex shedding in a bluff body wake the pressure-induced drag decreases by 15\% at Reynolds number 70000. This rate has been achieved with less than 50\% of the actuation power compared to other strategies. Furthermore, the strategy is robust under changing operating conditions. A model of the flow around a second body in the wake of a body ahead is derived from vortex methods and utilized to design a dynamic observer for the reconstruction of the flow field. This model enables a real-time prediction of vortex positions and sensor readings in parallel to an experiment. [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GB.00006: Turbulent skin-friction reduction by electroactive polymer surfaces with in-plane and out-of-plane motion Kevin Gouder, Jonathan Morrison An electroactive polymer (EAP) surface capable of in-plane and out-of-plane deformations is used to apply periodic near-wall forcing to a fully-developed turbulent boundary layer to reduce friction drag. The in-plane wall deformation consists of a flat surface with streamwise-oriented narrow strips of electrode which cause the EAP to expand and contract in the spanwise direction in response to an applied voltage. The out-of-plane wall deformation takes the form of a spanwise travelling harmonic surface wave with amplitude of the order of the viscous sublayer height. Direct measurements of friction drag by means of a drag balance (with accuracy $\pm2\%$) and hot-wire measurements of mean and higher-order moments are presented for a range of frequencies, amplitudes and wavelengths. It is expected that the imposition of a spatially and temporally varying Stokes-like layer will enable the direct manipulation of the quasi-streamwise near-wall structures and the reduction of the frequency and intensity of sweep events. While the effectiveness of this form of near-wall forcing has been demonstrated in simulations, the development of EAP surfaces allows experimental assessment of the technique at laboratory Reynolds numbers. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GB.00007: Separation Control from the Flap of a High-Lift Airfoil Using DBD Plasma Actuators Jesse Little, Munetake Nishihara, Igor Adamovich, Mo Samimy Control of separation from the flap of a high-lift airfoil using a single dielectric barrier discharge (DBD) plasma actuator has been investigated experimentally. This project is motivated by the desire to replace existing multi-element flap configurations with a single simple flap to allow more efficient high-lift generation. The results show that a single DBD plasma actuator located at the flap shoulder can increase or reduce the size of the time-averaged separation bubble over the flap depending on the frequency of actuation. In the latter case, the lift on the airfoil is increased due to improved circulation around the model, but it does not result in full reattachment on the deflected flap. These findings are consistent with previous research on high-lift airfoil configurations. The work will be expanded by exploring the effect of multiple actuators as well as their geometry and location on the size and structure of the separated region over the flap. This portion of the work will be done with an emphasis on optimizing the relative phase of each actuator and its effect on the separated flow region. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GB.00008: Control of a heated high Reynolds number jet using plasma actuators Martin Kearney-Fischer, Jin-Hwa Kim, Mo Samimy Recent developments in the heating capabilities of the free jet facility at the Ohio State University's Gas Dynamics and Turbulence Laboratory using a storage-based off-line electric heater have made it possible to test the effectiveness of the localized arc filament plasma actuators (LAFPAs) over a wide range of temperatures. These actuators have been used successfully to control high Reynolds number, high-speed unheated jets. Characteristics of this jet with heating capabilities show consistency with previously published results in both far-field acoustics as well as jet centerline velocity and turbulent kinetic energy. Initial experiments show that LAFPAs can consistently produce significant mixing enhancement. Across the range of temperatures collected, the control authority of LAFPAs improves as temperature increases. Further experimental results exploring the behavior of LAFPAs will be presented, attempting to focus on the potential of the system for mixing enhancement and noise mitigation. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GB.00009: Development of Feedback Control for High-Speed and High-Reynolds Number Jets Aniruddha Sinha, Kihwan Kim, Jin-Hwa Kim, Andrea Serrani, Mo Samimy Currently an effort is underway at GDTL to develop feedback control for high-speed and high Reynolds number jets. In this abstract we present preliminary experimental results on the feedback control of a Mach 0.9 axisymmetric jet with a Reynolds number based on jet diameter of 7.8 $\times $ 10$^{5}$. An azimuthal array of 8 plasma actuators is employed at the nozzle exit; they are driven by pulse-based switching signals. Open-loop forcing is shown to have two distinct effects on the irrotational near field pressure. At low forcing Strouhal numbers (\textit{St}$_{DF}$'s) (near the jet column mode instability), a sharp peak in the pressure fluctuations is observed. At higher \textit{St}$_{DF}$'s (close to the initial-shear-layer instability), a broad minimum is observed in the pressure fluctuations. An online gradient-based extremum-seeking feedback control scheme is implemented. The cost function can be selected as the RMS of various individual azimuthal pressure modes, or a combination thereof. The controller can be setup to optimize the \textit{St}$_{DF}$ to seek either a maximum or a minimum of the cost function with negligible reconfiguration. [Preview Abstract] |
Monday, November 24, 2008 9:57AM - 10:10AM |
GB.00010: Drag Reduction by Laminarization of Pulsating Turbulent Channel Flow Kaoru Iwamoto, Hiroshi Kawamura Direct numerical simulations of a pulsating turbulent channel flow have been carried out at a friction Reynolds number of $Re_{\tau}=110$ with high pressure gradient for laminarization. In the case of laminarization, the bulk velocity is increased since the Reynolds shear stress is decreased to zero. Finally, the mean velocity profile averaged over one cycle coincides with that of a laminar flow. It is found that laminarization occurs when the Reynolds shear stress of pulsating turbulent flow is lower than that of a steady turbulent flow at the critical Reynolds number of $Re_{\tau}=56$. [Preview Abstract] |
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