Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A15: Wakes and Separation Bubbles |
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Chair: Michael Plesniak, George Washington University Room: Georgia World Congress Center B302 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A15.00001: The wake of a bluff body in highly pulsatile flow: effects of freestream inflow frequency Ian A Carr, Nikolaos Beratlis, Elias Balaras, Michael W Plesniak Bluff body wake dynamics in highly pulsatile flows are of interest due to their potential for complexity and sensitivity to flow parameters. Large fluctuations in freestream velocity occur in biological, geophysical, and engineering flows in a multitude of contexts, e.g. blood flow, speech production, coastal flows, gusting winds. Large amplitude freestream pulsatility has, in our prior studies, provided insight into bluff body wakes in general. In this study we focus on the effect of the frequency of the large-scale freestream fluctuations, spanning from vortex lock-in frequencies at the upper bound down to quasi-steady inflow. The bluff body studied is a surface-mounted hemisphere that protrudes out of the boundary layer. A series of direct numerical simulations (DNS) supplement experimental studies performed using planar particle image velocimetry and hot-wire anemometry in a low-speed, pulsatile wind tunnel. Characterizations of freestream penetration, wake turbulence, and qualitative wake regimes through a sweep of inflow frequencies will be presented and compared. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A15.00002: Amplitude modulation and suppressed shedding in the wake of a streamwise-oscillating cylinder Maysam Shamai, Igor Mezic, Beverley J McKeon In this study we investigate the development of amplitude modulation in the wake of a streamwise-oscillating cylinder. Our forcing regime consists of oscillation frequencies, ff, much lower than corresponding shedding frequencies, fs. In contrast to cases where these frequencies are of the same order of magnitude, our regime of interest has been relatively less studied. We combine experimental and theoretical approaches using particle image velocimetry and Koopman mode decomposition, respectively, to study frequency ratios in the range ff / fs ∼ 0.02-0.2 with mean Reynolds number, Re = 800. Although the amplitude of oscillation is such that the instantaneous Reynolds number is far from the critical value, a range of interesting phenomena appear in the wake, including amplitude modulation and periods of suppressed vortex shedding. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A15.00003: Experimental Investigations of Flow Past Axially-Aligned Cylinders Pasquale Carlucci, Liam Buckley, Igbal Mehmedagic, Donald Carlucci, Siva Thangam Turbulent flow past spinning cylinders whose axis is aligned with the mean flow is investigated experimentally. Such flows are of common occurrence in projectiles, missiles and munitions and the modeling and the prediction of flow field is crucial for effective design. The objective of this study is to examine the flow over the cylinders and in their wakes to characterize the flow field and the flow separation to provide a better understanding of the force field over the cylinders. The experiments were conducted at Reynolds numbers of 60,000 and 100,000 with rotation numbers of up to 2 (based on cylinder diameter). Time-averaged mean flow and turbulence profiles in the wake flow are presented with and without spin along with comparison to published experimental data. The results are compared with the available data in the literature. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A15.00004: Effects of Fineness Ratio on Flow Field of Axial Circular Cylinder Taku Ochiai, Takumi Ambo, Yuta Ozawa, Taku Nonomura, Keisuke Asai In this study, flow fields of the circular cylinder aligned to free stream were measured and dependencies of them on the fineness ratio (length to diameter, L/D) were investigated. All the tests were conducted under interference-free condition using a magnetic suspension and balance system. The velocity fields in the plane including the center axis of the cylinder were obtained by particle image velocimetry (PIV) under the condition of Re = 33,000 and 50,000 and L/D = 1.5 and 2.0, respectively, whereas the previous study shows that the flow separated at the leading edge reattaches at the position downstream 1.5 to 1.7 times the diameter and the drag becomes minimum in the case of L/D ∼ 1.7. As a result, fluctuation in the frequency close to that in the circumferential fluctuation in the previous study was observed. Furthermore, this phenomenon is observed in the case of L/D = 2.0 despite the occurrence of the reattachment. In order to investigate the relationship between aerodynamic force and flow field, synchronous measurement of flow field and aerodynamic force will be performed. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A15.00005: Flow adjustment in the vicinity of a cylindrical canopy suspended in deep water Jian Zhou, Subhas Karan Venayagamoorthy The time-averaged flow dynamics of a suspended cylindrical canopy patch with a bulk diameter of D is investigated using large-eddy simulations (LES). The patch consists of Nc constituent solid circular cylinders of height h and diameter d, mimicking patchy vegetation suspended in deep water (i.e. total water depth much larger than the canopy height). After validation against published data, LES of a uniform incident flow impinging on the canopy patch was conducted to study the effects of canopy density (0.16 ≤ ∅ = Nc(d/D)2 ≤ 1, by varying Nc) and the bulk aspect ratio (0.25 ≤ AR = h/D ≤ 1, by varying h) on the near-wake structure and adjustment of flow pathways. The relationships between patch geometry, local flow bleeding (three-dimensional re-distribution of flow entering the patch) and global flow diversion (streamwise re-distribution of upstream undisturbed flow) are identified. The spatial extents of the wake, the flow diversion dynamics, and the bulk drag coefficients of the canopy patch jointly reveal the fundamental differences of flow responses between porous patches and their solid counterparts. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A15.00006: Large Eddy Simulation of flow past a circular cylinder in the drag crisis regime: effect of aspect ratio and the existence of multiple states Gaurav Chopra, Sanjay Mittal In this work, we present Large Eddy Simulation of flow past a three-dimensional circular cylinder in the drag crisis regime, 1×104≤ Reynolds number≤ 4×105. A stabilized Finite Element Method is used to solve the incompressible Navier Stokes equations. The Sigma model is implemented to model the effects of the sub-grid scales in the flow. First, we investigate the effect of aspect ratio of the cylinder on drag crisis. It is found that critical Reynolds number for the onset of drag crisis is sensitive to the aspect ratio of the cylinder. The drag crisis occurs at a lower Reynolds number for a cylinder with a small aspect ratio compared to one with large aspect ratio. The existence of multiple solutions in the drag crisis regime is explored by carrying out computations with different initial conditions. It is found that multiple states exist near the onset and also towards the end of drag crisis. Multiple states are identified in the variation of time-averaged coefficient of drag and lift, rms of coefficient of lift and vortex shedding frequency with Reynolds number.
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Sunday, November 18, 2018 9:18AM - 9:31AM |
A15.00007: Response of a Turbulent Separation Bubble to Variations in Freestream Pressure Gradients Wen Wu, Charles Meneveau, Rajat Mittal Separating turbulent boundary layers over a flat plate are investigated by DNS at $Re_\theta$ = 600. A suction-blowing velocity distribution is imposed at the top boundary to produce an adverse-to-favorable pressure gradient (PG), leading to a closed turbulent separation bubble. The separation bubble is found to naturally oscillate at a Strouhal number $St \equiv fL_{sep}/U_\infty \approx 0.37$. In order to gain insights into the natural time-scales of these bubbles we impose different magnitudes of the imposed PG and also varying the PG in time. Analysis of the simulation data shows that the change in the bubble size is mainly due to shear layer ``flapping" at the downstream end of the bubble. The analysis of the separating shear layer confirms its similarity to a plane mixing layer, albeit one that is formed by counter-flowing, and not co-flowing streams. Variations in the magnitude of the PG enable us to modulate the velocity ratio $R = (U_1-U_2)/(U_1+U_2)$ (subscripts denote the two sides of the mixing layer) in the separating shear layer. For some cases, the value of this critical parameter is found to exceed the convective to a global instability transition threshold in the separating shear layer. |
(Author Not Attending)
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A15.00008: Abstract Withdrawn |
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