Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G25: Flow Control: Wakes 
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Chair: Philippe Lavoie, University of Toronto Room: 31A 
Monday, November 19, 2012 8:00AM  8:13AM 
G25.00001: Optimal openloop control of the recirculation length in the wake of a cylinder Edouard Boujo, Francois Gallaire We consider steady equilibrium solution of the 2D NavierStokes equations describing the flow around a circular cylinder. It is well known that, for moderate Reynolds numbers, the recirculation length is an increasing function of the Reynolds number. We first express the recirculation length as a costfunctional and then use a Lagrangianbased optimization procedure to compute the linear sensitivity of the recirculation length with respect to base flow modifications, localized bulk point forces and blowing and suction at the cylinder wall. The results are compared to nonlinear steadystate computations. The influence of a shortened or increased recirculation on the stability of the flow above threshold (Re=47) is finally discussed. [Preview Abstract] 
Monday, November 19, 2012 8:13AM  8:26AM 
G25.00002: Dynamic Mode Decomposition of PIV measurements for the cylinder wake flow in turbulent regime Laurent Cordier, Gilles Tissot, Nicolas Benard, Bernd R. Noack For historical reasons, Proper Orthogonal Decomposition (POD) is the most wellknown and used reduction approach in the turbulence community. POD is widely used since it extracts from a sequence of data an orthonormal basis which captures optimally the flow energy. Unfortunately, energy level is not necessarily the correct criterion in terms of dynamical modelling and deriving a dynamical system based on POD modes leads sometimes to irrelevant models. In this communication, the Dynamic Mode Decomposition (DMD) as recently proposed by Schmid (JFM 2010) will be used to determine the eigenvalues and eigenvectors of the Koopman operator (Rowley \textit{et al.}, JFM 2009), an infinitedimensional linear operator associated with the full nonlinear system. Without explicit knowledge of the dynamical operator, frequencies and growth rates associated to each DMD modes can be easily determined based on the eigenvalues. The DMD will be demonstrated on experimental data corresponding to a PIV dataset of a cylinder wake flow at Reynolds number 40000. Moreover, the link between DMD and temporal discrete Fourier transform will be analysed and discussed. [Preview Abstract] 
Monday, November 19, 2012 8:26AM  8:39AM 
G25.00003: Characterization of an Actively Controlled ThreeDimensional Turret Wake Patrick Shea, Mark Glauser Threedimensional turrets are commonly used for housing optical systems on airborne platforms. As bluff bodies, these geometries generate highly turbulent wakes that decrease the performance of the optical systems and the aircraft. The current experimental study looked to use dynamic suction in both open and closedloop control configurations to actively control the turret wake. The flow field was characterized using dynamic pressure and stereoscopic PIV measurements in the wake of the turret. Results showed that the suction system was able to manipulate the wake region of the turret and could alter not only the spatial structure of the wake, but also the temporal behavior of the wake flow field. Closedloop, feedback control techniques were used to determine a more optimal control input for the flow control. Similar control effects were seen for both the steady openloop control case and the closedloop feedback control configuration with a 45\% reduction in the suction levels when comparing the closedloop to the openloop case. These results provide unique information regarding the development of the baseline threedimensional wake and the wake with three different active flow control configurations. [Preview Abstract] 
Monday, November 19, 2012 8:39AM  8:52AM 
G25.00004: Flow Visualization of a von K\'{a}rm\'{a}n Ogive Forebody with Plasma Actuation John Farnsworth, Zachary Francis, Reid Witt, Chris Porter, Thomas McLaughlin The flow field around an axisymmetric forebody at a moderate angle of attack can produce a significant side force, and thus a yawing moment, on the body. The side force results from an asymmetric vortex state and therefore pressure distribution that forms on the body. This asymmetric vortex state originates from a convective instability in the flow field, meaning that minor geometric or flow disturbances near the apex of the model can perturb the flow into an asymmetric state. In the current experiments two single dielectric barrier discharge plasma actuators are used to perturb the flow and control the vortex state. Smoke flow visualization techniques were utilized to better understand the behavior of the vortices under plasma actuation. It was found that the vortex state responds proportionally to the voltage of the plasma actuation. Additionally, the response of the vortex state to control changed drastically with changes in Reynolds number, suggesting a relation between the blowing ratio and the behavior of the vortex state. [Preview Abstract] 
Monday, November 19, 2012 8:52AM  9:05AM 
G25.00005: Feedback control of vortex shedding: An explanation of the gain window Simon Illingworth, Hiroshi Naito, Koji Fukagata This presentation explains the gain window phenomenon seen in early experimental and computational studies on active, closedloop control of vortex shedding, whereby shedding is completely suppressed only if the feedback gain lies within some narrow window of stabilizing gains. Using twodimensional direct numerical simulations and reducedorder modeling techniques, a loworder, linear model of the cylinder wake is formed at a Reynolds number of 60. This model is used to reproduce and to explain the gain window seen in previous studies. It is shown that the gain window is not caused by the destabilization of a higher mode, but rather is determined entirely by the behaviour of the openloop unstable mode under the action of the closedloop controller. The time taken for actuated fluid to convect to the sensor location plays an important part in explaining this gain window. A similar analysis at a higher Reynolds number of 80 reveals that the wake remains unstable for all choices of the feedback gain. The study illustrates the limitations of closedloop suppression of vortex shedding when a very simple control strategy is used. [Preview Abstract] 
Monday, November 19, 2012 9:05AM  9:18AM 
G25.00006: A Comparative Study of Spatially Modulated Forcing of Cylinder Wake with Segmented Plasma Actuators of Different Wavelengths Samik Bhattacharya, James W. Gregory The wake of a 1inch diameter (D) circular cylinder was forced three dimensionally by mounting dielectric barrier discharge plasma actuators at specific spanwise locations. Actuators with spanwise wavelengths of 1, 2, 4 and 6D were used. Considerable drag reduction was achieved with 4D and 6D actuators compared to 1D and 2D actuators which showed no effect on the drag. In the case of the 6D actuator, prominent spanwise differences were observed in the wake mean velocity profiles, with a much wider wake behind the region of no plasma formation and differential development persisting well downstream. The different wavelength actuators were also compared in terms of their ability to induce streamwise vorticity in the wake. Segmented forcing with the 4D actuator augmented the generation of streamwise vorticity in the wake which was revealed in the crossplane timeaveraged data. A detailed study of vortex dislocation was carried out with a rake of eight hot wires. Segmented forcing with 4 and 6 D actuator introduced vortex dislocation in the Karman vortex street. The phenomenon of vortex dislocation due to phase mismatch is inferred as the primary reason for the effectiveness of 4D actuator in reducing drag. [Preview Abstract] 
Monday, November 19, 2012 9:18AM  9:31AM 
G25.00007: Open loop control of an axisymmetric turbulent wake using periodic jet blowing Jonathan Morrison, Anthony Oxlade In this study we investigate the effects of periodic jet blowing on the turbulent wake of an axisymmetric bulletshaped body with a sharp trailing edge. The jet is formed from an annular orifice situated immediately below the trailing edge and oriented in the direction of the freestream. By varying the frequency and amplitude of the perturbation, we achieve a mean pressure increase on the base of the body of up to 35\%. This pressure rise is obtained only when the forcing frequency is greater than four times the frequency of the shear layer mode. The mechanism of pressure recovery is characterised via measurements of fluctuating pressure on the base of the body, and high resolution 2C PIV in the wake. Modal decomposition of the pressure fluctuations reveals a nonlinear coupling between the symmetric ($m=0$) perturbation and higher order azimuthal modes that results in an asymmetric mean pressure distribution. [Preview Abstract] 
Monday, November 19, 2012 9:31AM  9:44AM 
G25.00008: Single frequency lockon of wake behind a circular cylinder using oscillatory actuation Phillip Munday, Kunihiko Taira We numerically investigate the influence of oscillatory flow control on the twodimensional wake behind a circular cylinder at Re = 100. Understanding under what conditions the shedding frequency locks solely onto the actuation frequency and drag reduction is of particular interest. We investigate the influence of steady and oscillatory components of actuation with the actuator position, direction, frequency, and amplitude varied. For oscillatory forcing a Vshaped lockon region can be seen in the frequencyamplitude plot about the natural shedding frequency, which resemble stability horns in oscillator dynamics. Steady actuation is observed to reduce mean and fluctuating components of drag. With oscillatory forcing reducing drag fluctuation allows the wake to lockon to a wider range of actuation frequencies. We find that there exists a set of forcing amplitudes and frequencies that achieves both lockon and drag reduction. Low frequency oscillation due to the competition between low and high drag states are observed in the vicinity of the lockon boundary. [Preview Abstract] 
Monday, November 19, 2012 9:44AM  9:57AM 
G25.00009: Wake Control of a Blunt Trailing Edge Profiled Body Using Dielectric Barrier Discharge Plasma Actuators Arash NaghibLahouti, Philippe Lavoie The periodic shedding of von Karman vortices is the source of cyclic aerodynamic forces on nominally 2D bluff bodies. Beyond a threshold Reynolds number, which can be as high as 700 depending on profile geometry, secondary instabilities, appearing as undulations in the von Karman vortices and pairs of counterrotating streamwise vortices, emerge in the wake. The secondary instabilities are found to persist at Reynolds numbers in the order of 10$^{4}$. It has been shown that amplification of the secondary instabilities can lead to disorganization of the von Karman vortices, and attenuation of the cyclic forces. In the present study, this relationship is used as the basis of a wake flow control approach for a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge. An array of dielectric barrier discharge plasma actuators placed at the trailing edge is used for control actuation, with a spanwise spacing based on the wavelength of the secondary instabilities, to achieve maximum amplification of the instabilities. PIV and hotwire measurements have been conducted at Reynolds numbers between 2,000 and 24,000 to determine the effect of flow control on the wake characteristics, and the total drag. [Preview Abstract] 

G25.00010: ABSTRACT WITHDRAWN 
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