Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B12: Separated Flows: Control |
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Chair: Amy Lang, University of Alabama Room: 303 |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B12.00001: Reversing Flow Formation Induced by an Increasing Adverse Pressure Gradient in a Separating Laminar Boundary Layer Andrew Bonacci, Amy Lang, Leo Santos Reversing flow development in a laminar boundary layer is a determining factor in the development of dynamic stall. It has been demonstrated that shark scales are capable of being bristled by reversing flow leading to a passive mechanism for separation control. An investigation of the formation of the reversing flow over a smooth surface is key to understanding this passive flow-actuated bio-inspired separation control mechanism. A water tunnel facility is utilized to generate a separated region by using a rotating cylinder to induce an increasing unsteady adverse pressure gradient over a smooth flat plate. Growing the laminar boundary layer to a thickness of about 10 mm and Reynolds numbers of 1.67*10$^{\mathrm{5}}$ to 2.98*10$^{\mathrm{5}}$ makes the separation region more measurable with DPIV. Results are analyzed for reversing flow development in size and magnitude to better understand the viscous development of the separation point as a function of the strength and timescale of the increasing adverse pressure gradient. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B12.00002: Effect of perimetric suction at the rear of a square-back bluff body on the bi-stable dynamics of the wake flow Luc Pastur, En-Chi Hsu, Vladimir Parezanovic Wake flows behind 3D bluff bodies are known to exhibit random switches between two mirror reflectional symmetry-breaking wake flows\footnote{M. Grandemange {\it et al}, {\bf PRE} 86, 035302 (2012).}, each associated with an increase of the drag. Consequently, it may be desirable to symmetrize the flow, if the symmetrization would consist in stabilizing the unstable symmetric flow state associated with a lower drag. One attempt in this direction may consist in steadily blowing symmetrically through perimetric slits at the rear of the body. However, it was recently shown that this mode of actuation could not symmetrize the flow\footnote{M. Lorite-Di\'ez {\it et al}, to appear in {\bf PAMM}.}. The present work is a preliminary investigation of the effect of a steady symmetric perimetric suction at the rear of the body, among other modes of actuation of the wake flow. For moderate flow rates, it is found that the transitions between the two states are promoted, indicating that the actuator has authority on the flow bistability. The resulting resymmetrization of the probability distribution function of the pressure gradient is accompanied by a decrease of the base pressure, together with the slight reduction of the bubble length, as revealed by rear pressure and PIV measurements. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B12.00003: Control of the reflectional symmetry breaking mode of a square-back bluff body wake using a sweeping jet actuator Vladimir Parezanovic, Abdul Raouf Tajik, Luc Pastur The wake of a square-back 3D bluff body is dominated by a stochastic switching between two reflectional symmetry breaking wake flows\footnote{M. Grandemange et al., \textbf{Phy. Rev. E} 86, 035302}. The two mirror wakes are associated with an increased drag. We attempt to symmetrize the wake using a Sweeping Jet (SWJ) actuator located at the top of the bluff body base. Arrays of SWJs have been used to reduce the drag of the 3D bluff body by reattaching the flow to an angled back\footnote{M. Metka and J. W. Gregory, \textbf{J. Fluids Eng.} 137 (5), 051108} or to trailing edge flaps\footnote{J. Schmidt et al., \textbf{Exp. Fluids} 56, 151}. In our study, only a single SWJ is used, and its exit nozzle almost spans the entire width of the bluff body base. The SWJ produces a jet which oscillates horizontally, along the same direction of the bistable wake switching. The sweeping motion of the jet is intended to interact with the wake state switches, which may lead to a symmetric wake. Base pressure measurements reveal a wake locked in a symmetric state in the horizontal plane, for a certain range of actuator mass flow rates. This result is accompanied by a change in the vertical orientation of the wake. Force and PIV measurements are used to elucidate on the wake modifications. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B12.00004: Elimination of velocity deficit behind a cylinder using reinforcement learning Feng Ren, Hui Tang In this study, we present an active flow control strategy through deep reinforcement learning (DRL) for eliminating the velocity deficit in the wake of a circular cylinder. A group of windward-suction-leeward-blowing actuators are adopted. Their individual velocities are automatically adjusted by the DRL agent through feedback signals from a downstream sensor array. Simulations are conducted using a GPU-accelerated Lattice Boltzmann solver with multi-block mesh partition. The high-dimensionality and non-linearity features of this problem make it challenging to explicitly determining the control strategy. By adopting the DRL, the agent can learn from the time sequences of the sensors, actuators and a specified reward function through trials and errors, and finally converge and determine the optimal control strategy. Results show that the well-trained control strategy can eliminate 99.7{\%} of the velocity deficit. The policy is further tranferred for fluid-structure interaction situations, and results based on four representative cases show that the transferred control strategy is robust and can effectively eliminate the velocity deficit by around 96{\%}. Overall, the current study offers an innovative view that could potentially help underwater vehicles achieve low detectability. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B12.00005: DSMC Simulation of Flow Past a Circular Cylinder at Re~$=$~100 J.R. Torczynski, M.A. Gallis The Direct Simulation Monte Carlo (DSMC) method is used to simulate vortex shedding in the flow of a gas past a circular cylinder at Re~$=$~100, Ma~$=$~0.1 (incompressible), and Kn~$=$~0.00162 (continuum). Sandia's DSMC code SPARTA is used. The domain is 5~diameters upstream, 10~diameters downstream, and 5~diameters on both sides and is meshed with 0.72~billion square cells with sides of 1/4 mean free path. Each cell contains \textasciitilde 100~particles, for 72~billion particles total. The time step is 1/3 of the mean collision time. Simulations are run on Sequoia, an IBM Blue~Gene/Q petascale supercomputer at LLNL. The K\'{a}rm\'{a}n vortex street arises without using any perturbation, and the lateral force on the cylinder is periodic with a Strouhal number of 0.175. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B12.00006: On The Flows Of Low-Aspect Ratio Rotating Circular Cylinders Hisham Shehata, Albert Medina, Matthew Rockwood A low-aspect ratio circular cylinder is placed on a laminar boundary layer plate in a wall-normal orientation, and rotated about its respective axial coordinate. It is found that the rotary motion of the cylinder may disrupt the symmetry of mainstay vortices associated with a static low-aspect ratio pin, resulting in a dominant streamwise vortex formation. Such formation stems from asymmetry of the near body wake, induced by cylinder motion and downwash effects at the free-end of the cylinder. The influence of cylinder aspect ratio, cylinder height to boundary layer thickness, and rotational rate are explored in detail through volumetric reconstruction of the flow field via stereoscopic particle image velocimetry (SPIV). [Preview Abstract] |
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