Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R23: Flow Control VI |
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Chair: Jens Fransson, Linne Flow Center, KTH Room: 326 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R23.00001: Advanced Fluid Research On Drag reduction In Turbulence Experiments -- AFRODITE Jens H.M. Fransson A hot topic in today's debate on global warming is drag reduction in aeronautics. The most beneficial concept for drag reduction is to maintain the major portion of the airfoil laminar. Estimations show that the potential drag reduction can be as much as 15\%, which would give a significant reduction of NOx and CO emissions in the atmosphere considering that the number of aircraft take offs, only in the EU, is over 19 million per year. In previous tuned wind tunnel measurements it has been shown that roughness elements can be used to sensibly delay transition to turbulence$\footnote{Fransson et al. 2006 {\emph{Phys. Rev. Lett.}} {\bf{96}}, 064501.}$. The result is revolutionary, since the common belief has been that surface roughness causes earlier transition and in turn increases the drag, and is a proof of concept of the passive control method per se. The beauty with a passive control technique is that no external energy has to be added to the flow system in order to perform the control, instead one uses the existing energy in the flow. Within the research programme AFRODITE, funded by ERC, we will take this passive control method to the next level by making it twofold, more persistent and more robust. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R23.00002: Closed-Loop Aerodynamic Flow Control of a Maneuvering Airfoil Daniel P. Brzozowski, John R. Culp, Ari Glezer The unsteady interaction between trailing edge aerodynamic flow control and airfoil motion in pitch and plunge is investigated in wind tunnel experiments using a 2-DOF traverse which enables application of time-dependent external torque and forces by servo motors. The global aerodynamic forces and moments are regulated by controlling vorticity generation and accumulation near the surface using hybrid synthetic jet actuators. The dynamic coupling between the actuation and the time-dependent flow field is characterized using simultaneous force and velocity measurements that are taken phase-locked to the commanded actuation waveform. The effect of the unsteady motion on the model-embedded flow control is assessed in unsteady several maneuvers. Circulation time history that is estimated from a PIV wake survey shows that the entire flow over the airfoil readjusts within about 1.5 TCONV, which is about two orders of magnitude shorter than the characteristic time associated with the controlled maneuver of the wind tunnel model. This illustrates that flow-control actuation can be typically effected on time scales that are commensurate with the flow's convective time scale, and that the maneuver response is primarily limited by the inertia of the platform. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R23.00003: A Method to Control Compliance of Blades and Flaps Julia Cosse, Morteza Gharib Compliant plates experience lower drag forces than rigid plates primarily due to reconfiguration. From this concept it follows that through modifying the compliance of a plate, the aerodynamic forces can be controlled. To achieve this control, the concept of hydroskeletons -- which are fluid filled cavities that resist deformation through the pressure of an internal fluid -- were used. Using this notion a compliant structure with an internal chamber was developed. Shape change was detected when filling the chamber with fluid and controlling the pressure. Preliminary testing involved simple internal geometries filled with water and pressurized up to 20 psi. Using this method a plate was built with several internal chambers, each with individual pressure control. The plate was attached to a force balance perpendicularly in a wind tunnel. Drag and lift forces were modified through changing the internal pressure both globally and locally. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R23.00004: Manipulation of Laminar Separation Utilizing Dynamic Roughness at the Leading Edge of an Idealized Airfoil Ryan Wallace, Beverley McKeon Low Reynolds number flow over a symmetric, idealized airfoil with a reasonably constant laminar separation point was manipulated using a leading edge roughness element with small, time-dependent amplitude. At a fixed height and low Reynolds number the roughness element was able to reduce the extent of laminar separation over the airfoil as compared to a smooth airfoil. Further reduction of the separation was achieved by dynamically oscillating the roughness element in an appropriate range of actuation frequencies. Proper orthogonal decomposition performed upon the flow over the airfoil for both the baseline and active open loop case shows the introduction of persistent structures within the flow due to the oscillating roughness element. The coupling of this small input perturbation with the flow and the resultant manipulation of the separation bubble will be discussed for a range of flow and roughness conditions. The support of NSF CAREER award {\#}0747672 is gratefully acknowledged. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R23.00005: Vortex shedding response of streamwise driven cylinders Daniel Tudball Smith, Justin Leontini, John Sheridan, David Lo Jacono Bluff body cylinders exhibit a range of responses when externally forced. This study experimentally investigated the modes of response and trends in shedding frequency of square and circular cylinders undergoing inline forced oscillations in a steady flow. Experiments were conducted in a free surface water channel over a range of Reynolds numbers from 1500 to 6300. With the driving frequency held constant at the natural unperturbed shedding frequency, the forcing amplitude was varied and the response examined. Frequency analysis of velocity, lift and drag forces showed that for low amplitude oscillation the shedding frequency decreases as amplitude increases, displaying a quadratic relationship. As amplitude increases further a region of possible mode competition exists until a critical amplitude is reached where the shedding locks to a period-doubled subharmonic of the forcing frequency. The region of mode competition is brief with an abrupt lock for the circular cylinder, while the square cylinder exhibits a gradual transition to the subharmonic over a larger range of amplitudes. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R23.00006: Transition delay by means of base flow modulations S.S. Sattarzadeh, S. Shahinfar, B.E. Fallenius, J.H.M. Fransson, A. Talamelli Recent experimental investigations have shown that spanwise modulations of the base flow may delay transition to turbulence.\footnote{Fransson et al. 2006 {\emph{Phys. Rev. Lett.}} {\bf{96}}, 064501.} In this study we explore the possibility to generate streaks of much larger amplitude than previously reported by using a row of miniature vortex generators (MVGs). Here, we present the first boundary layer experiment where streak amplitudes exceeding 30\% have been produced without having any secondary instability acting on them. Furthermore, the induced skin-friction drag due to the streaky base flow is quantified and it is demonstrated that the streaks can be reinforced by placing a second array of MVGs downstream of the first one. In this way it is possible to make the control more persistent in the downstream direction. We conclude that the specially designed set of MVGs, as a boundary layer modulator, is a promising candidate for successfully setting up robust and persistent streamwise streaks, which is a prerequisite for a successful flow control. This work is carried out within the AFRODITE programme funded by ERC. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R23.00007: Electroactive Polymer based flow control at Low Reynolds Numbers Sarah Zaremski, Michael Amitay Electroactive polymers (EAPs) are used to achieve distributed, conformal actuation on aerodynamic surfaces, promote transition in places where laminar separation occurs, and maintain laminar flow where there is no separation. In this work, the feasibility of the EAPs in mitigating a laminar separation bubble on a flat plate was examined. First, the performance of the EAPs was evaluated using a Laser vibrometer and a high-speed camera to quantify the response of the EAP to input voltage and driving frequency and to better understand the physics of the actuator itself. Then, measurements using Stereoscopic particle image velocimetry were conducted in the vicinity of the EAP. Several parameters, such as the driving frequency, the input voltage, and the dimple size, were tested. Preliminary data show promising results, where the size of the separation bubble was significantly reduced when the EAPs were driven at the appropriate voltages and frequencies. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R23.00008: Experimental Evaluation of Control Algorithms for a Supercavitating Vehicle David Escobar Sanabria, Roger Arndt, Gary Balas High speed supercavitating vehicles offer significant challenges regarding control. Vehicles with actuated control surfaces, such as cavitators and fins, are of considerable interest for maneuverability and control. To study the interaction of control surfaces and the body dynamics of the vehicle, a new hardware and software infrastructure has been developed at Saint Anthony Falls Laboratory (SAFL-U of Minn). In addition, a new vehicle prototype that utilizes a cavitator disk and fins for control, a 6 degree-of-freedom force balance to measure forces and moments, and a ventilation system to insure a fully developed supercavity was designed and tested in the high-speed water tunnel at SAFL. Based on experiments in presence of a supercavity surrounding the vehicle, mathematical models that map cavitator and fins angles to pitch moment, drag force and lift force are obtained. These mathematical models and the new platform enable the use of closed-loop control to significantly reduce pitch moment oscillations induced by a gust flow. This achievement shows a promising path towards the experimental validation of control algorithms for high-speed supercavitating vehicles. The platform architecture, experimental design, mathematical models, and validation process are presented here. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R23.00009: Bubble Gate for In-Plane Flow Control Ali Kazemi Oskooei, Axel Guenther The ability to control fluid flow is of key importance for microfluidic devices. While a large number of sophisticated solutions have been demonstrated, there is still a great amount of interest in developing simple strategies that do not require complex fabrication steps and electrical connections. A small footprint, compatibility with different substrate materials, working fluids and temperatures are amongst other desirable characteristics. We demonstrate a bubble gate strategy that meets all the above. In this strategy, flow control is achieved using a controlled gas stream that intercepts a liquid stream at a T-junction, forming a gas-liquid interface (i.e. bubble). Closely positioned micropillars are employed to limit the bubble motion to a single degree of freedom. The bubble breaks into the liquid stream and occupies the entire liquid cross-section, when the gas pressure is continued. Hence, the bubble movement is able to stop or manipulate the liquid flow. Several control operations are discussed herein, including, but not limited to, valves, liquid metering and peristaltic pumping. PIV measurements are employed to investigate the transient flow structure. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R23.00010: Vertical Fence Wake Manipulation Using Periodic Variation of Upstream Flow Seung-Hwan Lee, Xin Cheng Tu, Hyoung-Woo Kim, Hyoung-Bum Kim The effect of periodic variation of upstream flow on the separated shear flows behind the vertical fence was experimentally investigated. Upstream flow was modified using small obstacles and this device made the periodic change of streamwise velocities in front of the fence. The experiments were performed in a circulating water channel. The Reynolds number based on the height of fence and freestream velocity was varied from 2000 to 6000. The vertical fence was submerged in the turbulent boundary layer. Stereoscopic-PIV method was used to measure the instantaneous velocity fields around a vertical fence. 800 instantaneous velocity fields were acquired in each experimental condition and the mean properties were calculated using the ensemble average method. The obtained results were compared with those of uncontrolled fence flow. The results showed the vertical fence under the upstream flow change has the local downwash flow behind the fence and this flow suppressed the separation bubble and made the smaller recirculation region compared with uncontrolled fence wake. [Preview Abstract] |
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