Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F16: Flow Control: Active IIControl
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Chair: Ebenezer Gnanamanickam, Embry-Riddle Aeronautical University Room: 603 |
Monday, November 20, 2017 8:00AM - 8:13AM |
F16.00001: Skin Friction Reduction Through Large-Scale Forcing Shibani Bhatt, Sravan Artham, Ebenezer Gnanamanickam Flow structures in a turbulent boundary layer larger than an integral length scale ($\delta$), referred to as large-scales, interact with the finer scales in a non-linear manner. By targeting these large-scales and exploiting this non-linear interaction wall shear stress (WSS) reduction of over 10\% has been achieved. The plane wall jet (PWJ), a boundary layer which has highly energetic large-scales that become turbulent independent of the near-wall finer scales, is the chosen model flow field. It’s unique configuration allows for the independent control of the large-scales through acoustic forcing. Perturbation wavelengths from about 1 $\delta$ to 14 $\delta$ were considered with a reduction in WSS for all wavelengths considered. This reduction, over a large subset of the wavelengths, scales with both inner and outer variables indicating a mixed scaling to the underlying physics, while also showing dependence on the PWJ global properties. A triple decomposition of the velocity fields shows an increase in coherence due to forcing with a clear organization of the small scale turbulence with respect to the introduced large-scale. The maximum reduction in WSS occurs when the introduced large-scale acts in a manner so as to reduce the turbulent activity in the very near wall region. [Preview Abstract] |
Monday, November 20, 2017 8:13AM - 8:26AM |
F16.00002: Direct numerical simulation of turbulent Taylor-Couette flow controlled by a traveling wave-like blowing and suction for drag reduction. Kohei Ogino, Hiroya Mamori, Naoya Fukushima, Makoto Yamamoto Flow control to decrease the skin-friction drag in wall turbulence is important in engineering viewpoints. Recently, a traveling wave-like control is known to not only decrease the skin-friction drag, but also induce relaminarization phenomenon in the turbulence channel flow. Therefore, it is worth to investigate the control effect in other canonical flows. In this study, we investigated the drag reduction of a traveling wave-like control in fully developed turbulent Taylor-Couette flow using direct numerical simulations. The Reynolds number based on the rotating velocity of the inner wall and the radius of the centerline was set to be 84000. We imposed a traveling wave-like control only in the inner wall and investigated the control effect parametrically. We found that this control resulted in 30{\%} drag reduction when the wave traveled in the same direction as the rotation of the inner wall and faster than the rotation. In this presentation, we will discuss the drag reduction mechanisms comparing with the other control techniques such as an opposition control. [Preview Abstract] |
Monday, November 20, 2017 8:26AM - 8:39AM |
F16.00003: Skin friction drag reduction on a flat plate turbulent boundary layer using synthetic jets Randy Belanger, Pieter D. Boom, Ronald E. Hanson, Philippe Lavoie, David W. Zingg In these studies, we investigate the effect of mild synthetic jet actuation on a flat plate turbulent boundary layer with the goal of interacting with the large scales in the log region of the boundary layer and manipulating the overall skin friction. Results will be presented from both large eddy simulations (LES) and wind tunnel experiments. In the experiments, a large parameter space of synthetic jet frequency and amplitude was studied with hot film sensors at select locations behind a pair of synthetic jets to identify the parameters that produce the greatest changes in the skin friction. The LES simulations were performed for a selected set of parameters and provide a more complete evaluation of the interaction between the boundary layer and synthetic jets. Five boundary layer thicknesses downstream, the skin friction between the actuators is generally found to increase, while regions of reduced skin friction persist downstream of the actuators. This pattern is reversed for forcing at low frequency. Overall, the spanwise-averaged skin friction is increased by the forcing, except when forcing at high frequency and low amplitude, for which a net skin friction reduction persists downstream. The physical interpretation of these results will be discussed. [Preview Abstract] |
Monday, November 20, 2017 8:39AM - 8:52AM |
F16.00004: On The Persistence Of Turbulent Boundary Layer Drag Reduction Under Pulsed DC Plasma Actuation Samaresh Midya, Flint Thomas, Thomas Corke Experiments are reported which use a novel method of active flow control explicitly designed to intervene in Streak Transient Growth which was first postulated by Schoppa and Hussain (POF 1998, JFM 2002) as the dominant mechanism for the production of streamwise vortices in wall-bounded turbulent flows. The flow control method utilizes pulsed-DC plasma actuator arrays that are mounted flush with the wall in a ZPG turbulent boundary layer. A key finding of Schoppa and Hussain (1998) was the persistence of drag reduction for a finite time interval $T_M^+=T_Mu_\tau^2/\nu=O(10^3)$ after termination of near-wall actuation. This time scale ultimately governs the required streamwise stagger between successive actuators for a given drag reduction application. In the reported experiment the time scale $T_M^+$ is established in a ZPG TBL. Oil film interferometry is used to directly measure the local wall shear stress at several consecutive locations downstream of a pulsed-DC actuator array. In this manner, the spatial evolution of the local wall shear stress is obtained and contrasted for the controlled flow and non-actuated flows. Subsequently, the characteristic streamwise distance $\Delta x \propto U_e T_M$, over which local skin friction relaxes back to natural values is determined. [Preview Abstract] |
Monday, November 20, 2017 8:52AM - 9:05AM |
F16.00005: Feedforward and Feedback Control of Boundary Layer Streaks Induced by Freestream Turbulence Kevin A. Gouder, Ahmed M. Naguib, Philippe L. Lavoie, Jonathan F. Morrison Sensing and cancellation of streaks early within their growth extent could enable the delay of bypass transition and eventual turbulence. Previously, we had evaluated the capability of plasma-actuator-based feedforward-feedback (FF-FB) control systems to weaken streaks induced ``synthetically'' in a Blasius boundary layer via dynamic roughness elements. In contrast, the current work aims to delay bypass boundary layer transition, where in the presence of freestream turbulence intensity exceeding about 1\%, streaks form naturally and stochastically in the underlying boundary layer. A wall-shear-stress sensor $S_U$, a twin-plasma actuator, and a second sensor $S_D$, are installed along the streamwise direction of a flat plate, in the streaks' linear transient growth region, upstream of any turbulent spot formation. A FF control system uses the $S_U$ output and single-point Linear Stochastic Estimation (LSE), to produce a counter-disturbance aimed to cancel the convected original streak at the $S_D$ location. A FB loop uses any $S_D$ output in a PI controller to correct for uncancelled disturbances resulting from, say, inaccuracies in the LSE model of the streak growth dynamics. Results demonstrate the viability of the control scheme to weaken streaks and delay bypass transition. [Preview Abstract] |
Monday, November 20, 2017 9:05AM - 9:18AM |
F16.00006: Airfoil Drag Reduction using Controlled Trapped Vorticity Concentrations Michael DeSalvo, Ari Glezer The aerodynamic performance of a lifting surface at low angles of attack (when the base flow is fully attached) is improved through fluidic modification of its ``apparent'' shape by superposition of near-surface trapped vorticity concentrations. In the present wind tunnel investigations, a controlled trapped vorticity concentration is formed on the pressure surface of an airfoil (NACA 4415) using a hybrid actuator comprising a passive obstruction of scale O(0.01c) and an integral synthetic jet actuator. The jet actuation frequency [\textit{St}$_{\mathrm{act}}\sim $ O(10)] is selected to be at least an order of magnitude higher than the characteristic unstable frequency of the airfoil wake, thereby decoupling the actuation from the global instabilities of the base flow. Regulation of vorticity accumulation in the vicinity of the actuator by the jet effects changes in the local pressure, leading in turn to changes in the airfoil's drag and lift. Trapped vorticity can lead to a significant reduction in drag and reduced lift (owing to the sense of the vorticity), e.g. at $\alpha =$ 4$^{\mathrm{o}}$ and \textit{Re} $=$ 6.7$\cdot $10$^{\mathrm{5}}$ the drag and lift reductions are 14{\%} and 2{\%}, respectively. PIV measurements show the spatial variation in the distribution of vorticity concentrations and yield estimates of the corresponding changes in circulation. [Preview Abstract] |
Monday, November 20, 2017 9:18AM - 9:31AM |
F16.00007: Optimal Control of Gortler Vortices by Means of Local Wall Deformations Adrian Sescu, Mohammed Afsar We explore an optimal control strategy in the framework of high Reynolds number asymptotics in which the growth of Gortler vortices is reduced by local wall deformations. The Gortler vortices are excited by a row of roughness elements that enter the analysis through upstream conditions derived previously using an asymptotic analysis (Goldstein et al., J. Fluid Mech., 613, pp. 95-124, 2011). Since the leading order Navier-Stokes equations reduce to the boundary region equations (BRE) in a transverse region that scales on the local boundary layer thickness, they are parabolic in the streamwise direction, and may be solved by marching downstream. Wall deformations are introduced into the BREs via a Prandtl transformation for an arbitrary streamwise/spanwise wall surface shape. The vortex energy is then controlled using an optimal control algorithm formulated in the framework of the Lagrange multipliers method, wherein the solution to the adjoint equations are determined by an arbitrary variation in the Lagrangian, in which the cost functional is associated with the local energy of the Gortler vortices. Our numerical results indicate that the optimal control algorithm is very effective in reducing the amplitude of the Gortler vortices. [Preview Abstract] |
Monday, November 20, 2017 9:31AM - 9:44AM |
F16.00008: Flow Characteristics of Ground Vehicle Wake and Its Response to Flow Control Prabu Sellappan, Jonathan McNally, Farrukh Alvi Air pollution, fuel shortages, and cost savings are some of the many incentives for improving the aerodynamics of vehicles. Reducing wake-induced aerodynamic drag, which is dependent on flow topology, on modern passenger vehicles is important for improving fuel consumption rates which directly affect the environment. In this research, an active flow control technique is applied on a generic ground vehicle, a 25\textdegree Ahmed model, to investigate its effect on the flow topology in the near-wake. The flow field of this canonical bluff body is extremely rich, with complex and unsteady flow features such as trailing wake vortices and c-pillar vortices. The spatio-temporal response of these flow features to the application of steady microjet actuators is investigated. The responses are characterized independently through time-resolved and volumetric velocity field measurements. The accuracy and cost of volumetric measurements in this complex flow field through Stereoscopic- and Tomographic- Particle Image Velocimetry (PIV) will also be commented upon. [Preview Abstract] |
Monday, November 20, 2017 9:44AM - 9:57AM |
F16.00009: Feedback Control of Unsteady Flow and Vortex-Induced Vibration Rajeev Jaiman, Weigang Yao We present an active feedback blowing and suction (AFBS) procedure via model reduction for unsteady wake flow and the vortex-induced vibration (VIV) of circular cylinders. The reduced-order model (ROM) for the AFBS procedure is developed by the eigensystem realization (ERA) algorithm, which provides a low-order representation of the unsteady flow dynamics in the neighbourhood of the equilibrium steady state. The actuation is considered via vertical suction and blowing jet at the porous surface of a circular cylinder with a body mounted force sensor. The resulting controller designed by linear low-order approximation is able to suppress the nonlinear saturated state. A systematic linear ROM-based stability analysis is performed to understand the eigenvalue distributions of elastically mounted circular cylinders. The results from the ROM analysis are consistent with those obtained from full nonlinear fluid-structure interaction simulations. A sensitivity study on the number of suction/blowing actuators, the angular arrangement of actuators, and the combined versus independent control architectures has been performed. Overall, the proposed control is found to be effective in suppressing the vortex street and the VIV for a range of reduced velocities and mass ratios. [Preview Abstract] |
Monday, November 20, 2017 9:57AM - 10:10AM |
F16.00010: Extension of suboptimal control theory for flow around a square cylinder Yosuke Fujita, Koji Fukagata We extend the suboptimal control theory to control of flow around a square cylinder, which has no point symmetry on the impulse response from the wall in contrast to circular cylinders and spheres previously studied. The cost functions examined are the pressure drag ($J_1$), the friction drag ($J_2$), the squared difference between target pressure and wall pressure ($J_3$) and the time-averaged dissipation ($J_4$). The control input is assumed to be continuous blowing and suction on the cylinder wall and the feedback sensors are assumued on the entire wall surface. The control law is derived so as to minimize the cost function under the constraint of linearized Navier-Stokes equation, and the impulse response field to be convolved with the instantaneous flow quanties are numerically obtained. The amplitide of control input is fixed so that the maximum blowing/suction velocity is $40\%$ of the freestream velocity. When $J_2$ is used as the cost function, the friction drag is reduced as expected but the mean drag is found to increase. In constast, when $J_1$, $J_3$, and $J_4$ were used, the mean drag was found to decrease by $21\%$, $12\%$, and $22\%$, respectively; in addition, vortex shedding is suppressed, which leads to reduction of lift fluctuations. [Preview Abstract] |
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