Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Y10: Separated Flows: Wakes (11:30am  12:15pm CST)Interactive On Demand

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Y10.00001: Separated Flow over finite span, cantilevered wings at a moderate Reynolds number Jacob Neal, Michael Amitay Separated flows over airfoils are known to exhibit cellular separation patterns known as stall cells (SCs). A series of wind tunnel experiments explored SC formation over a finite span, square tipped NACA 0015 with aspect ratio 4. For all experiments the Reynolds number was 330,000 and the angle of attack was 22 degrees. Oil flow visualizations (OFV) were performed to qualitatively ascertain the surface topology of the mean flow. The counterrotating foci of a mushroom shaped SC were clearly seen near the midspan. SPIV measurements were taken of the mean flow volume. Threedimensional streamlines were calculated through this volume, showing a focus which originated and terminated at the surface foci identified in the OFV, and extended into a U shape into the wake. To investigate the unsteadiness of the flowfield, timeresolved SPIV measurements were performed at two spanwise locations across the wing. One spanwise location at the center of the SC, and the other bisected the outboard surface focus of the SC. At the SC center location, the dominant frequencies and DMD mode shapes were consistent with von Karman shedding. At the focus center location, a range of frequencies was present and coherent mode shapes were fleeting. [Preview Abstract] 

Y10.00002: Vortical Structures on Low Aspect Ratio Finite Wings at Low Reynolds numbers Shelby Hayostek, Michael Amitay The threedimensional flowfields over finite span wings and in their wake were explored experimentally in a lowspeed, closedreturn, open test section water tunnel using flow visualization and stereoscopic particle image velocimetry. The wings had a NACA 0015 profile and were suspended in the tunnel allowing for the wings to generate tip vortices on either end. Wings with aspect ratios of 1 and 2 were tested at an angle of attack of 22 degrees and chord Reynolds number of 600 and 1000 to understand how the flowfield is affected with alteration of the aspect ratio and/or Reynolds number. At aspect ratio of 1 the tip vortices play a major role in the overall flow field, whereas when the aspect ratio was increased, the flow over the wing is less effected by the tip vortices. In addition, the streamlines close to the surface revealed the presence of two foci close to the wing surface reminiscent of the stall cell phenomenon, supporting previous theoretical finding using stability analysis. [Preview Abstract] 

Y10.00003: Experimental evidence of VortexInduced Vibration in cylinders at subcritical Reynolds numbers Pieter Boersma, Jonathan Rothstein, Yahya ModarresSadeghi Shedding of vortices can be observed in the wake of a cylinder at Reynolds numbers larger than 47. Recent numerical simulations and theoretical work, however, have shown that it is possible to observe Vortex Induced Vibration (VIV) at subcritical Reynolds numbers. i.e., Reynolds numbers smaller than 47. VIV has been observed numerically at Reynolds numbers as low as 22. Here we show the first experimental evidence of VIV at subcritical Reynolds number. We discuss similarities and differences between the VIV response at subcritical Reynolds number and at Reynolds numbers larger than 47. [Preview Abstract] 

Y10.00004: Characteristics of flows over a circular cylinder at critical and supercritical Reynolds numbers Dohyun Jin, Hyunsik Kim, Haecheon Choi We conduct large eddy simulation of flow over a circular cylinder from the critical to supercritical Reynolds numbers (Re $=$ 250,000, 380,000 and 850,000, respectively) with a dynamic global subgridscale model (Lee et al., 2010) and an immersed boundary method (Kim et al., 2001). The drag coefficients and Strouhal numbers agree well with those of previous studies. As the Reynolds number increases, fluctuations of laminar separation and stagnation positions decrease, and the nondimensional pressure at the Karman vortex core increases. At a critical Reynolds number, asymmetric mean pressure distribution on the cylinder surface is observed. At the supercritical Reynolds number, the separated shear layer vortices breakdown into threedimensional turbulent structures near the mean reattachment line at both sides of the cylinder. Although the turbulent reattachment is not observed at the critical Reynolds numbers, the shear layer vortex frequencies normalized with the external azimuthal velocity and momentum thickness at the separation point are almost identical to that at the supercritical Reynolds number. [Preview Abstract] 

Y10.00005: Numerical simulation of flow over a normal flat plate at low Reynolds numbers Daeun Song, Haecheon Choi A normal flat plate is one of the representative twodimensional bluff bodies. Nevertheless, there have been quite a few studies on the flow past a normal flat plate. Those studies are either restricted to very high Reynolds number by experiments, or to low Reynolds number under Re $=$ 1,000 using numerical simulation. In the present study we perform numerical simulation on this flow at low Reynolds numbers (Re $=$ 5  3,000). We show that the flow becomes unsteady at Re $\approx $ 35, and threedimensional at Re $\approx $ 170. The size of the separation bubble behind the normal plate, mean and rms drag coefficients, and vortical evolution behind the plate are investigated and their results will be shown at the presentation. [Preview Abstract] 

Y10.00006: The effect of seams on the aerodynamics of baseballs: A computational study. John Scheffey, Rajat Mittal The aerodynamic force on a ball due to its rotation, known as the Magnus effect, has long been observed and studied in baseball and other ball sports. Recently, there has been interest in the potential existence of ``nonMagnus'' forces, which are thought to be caused by certain seam orientations in ball flight. We present numerical simulations of flows past rotating spheres at varying orientation angles of rotation and investigate the effects of baseball seams on the aerodynamics of such bodies. We examine the role that baseball seams play in modifying the wake and producing asymmetry, leading to transverse forces that generate deviations in the trajectory of pitched and batted balls. [Preview Abstract] 

Y10.00007: Dynamics of a hydrofoil free to oscillate in the wake of a cylinder Adrian Carleton, Todd Currier, Yahya ModarresSadeghi We examine the behavior of a hydrofoil free to oscillate in the wake of a cylinder. We conducted three series of experiments: in the first series of experiments the cylinder was fixed, in the second series, the cylinder was forced to rotate in one direction, and in the third set of experiments, the cylinder was forced to rotate periodically. For all three series of experiments, we measured the displacements of the hydrofoil that was placed in the cylinder's wake. Simultaneously, we conducted hydrogen bubble flow visualization. In the first case, the hydrofoil oscillated with a frequency equal to the shedding frequency initially, and then switched to oscillations with half of the shedding frequency. In the second series of experiments, when the cylinder was forced to rotate in one direction, the oscillations of the hydrofoil had very small amplitudes after the shedding of vortices was suppressed for higher rotation rates. For the third case, where the cylinder was forced to rotate periodically, a 2:1 ratio between the inline and crossflow oscillations was observed initially, resulting in a figureeight trajectory in the response of the hydrofoil. At higher forcing frequencies a 1:1 ratio was observed, and the trajectories did not follow any clear pattern. [Preview Abstract] 
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