Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A9: Flow Instability: Wakes |
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Chair: Patricia Ern, IMFT Room: 109 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A9.00001: Vortex pairing in the wake of an oscillating bubble rising in a thin-gap cell Patricia Ern, Audrey Filella, V\'eronique Roig We investigate experimentally the oscillatory motion and wake of a bubble rising in a counter flow in a thin gap cell (3 mm) by shadowgraphy and PIV. The equivalent diameter $d$ of the bubble in the plane of the cell is used to define the Archimedes number $Ar = \frac{\sqrt{g d^3}}{\nu}$ ($\nu$ is the kinematic viscosity and $g$ the gravitational acceleration). The counter flow is characterized by the Reynolds number $Re_{cf}$ based on the mean liquid velocity and the gap thickness. For $500 \leq Ar \leq 5500$ and $0 \leq Re_{cf} \leq 200$, the mean vertical velocity of the bubble relative to the counter flow, $V_{br}$, corresponds to the mean rising velocity in liquid at rest; and the frequency and the amplitude of the oscillatory motion superpose for all $Re_{cf}$ when normalized with $V_{br}$ and the timescale $d/V_{br}$. For a given size of the bubble ($d \approx 9.5$ mm and $Ar \approx 2800$) corresponding to a Reynolds number based on $V_{br}$ and $d$ of about $1900$, we then investigate in detail the wake associated to the bubble in several counter flows. As $Re_{cf}$ increases, the number of vortices released increases. Furthermore, the wake of the bubble undergoes vortex pairing for $0 \leq Re_{cf} \leq 110$), whereas no vortex pairing is observed for $Re_{cf} \geq 140$. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A9.00002: Experimental investigation of freely falling thin disks: Transition of three-dimensional motion from zigzag to spiral Zhuang Su, Cunbiao Lee The motion of a freely falling thin disk was investigated experimentally. Different motion modes, including plannar zigzag and three dimensional spiral, were identifed based on the measurements of the whole six degrees of freedom of the disk. The final motion modes in the fall were found to change with the dimensionless moments of inertial ($I^*$), which is determined by the aspect ratio of the disk and the density ratio between the disk and water. The motion mode is zigzag in the range of $2.95\times10^{-3}$ to $1.17\times10^{-2}$ and spiral in the range of $7.36\times10^{-4}$ to $1.47\times10^{-3}$ in our experiments. A zigzag to spiral transition process was found in the lower $I^*$ range. Two differet types of transition were identified, which are zigzag-spiral monotonous transition in the lower and higher Reynolds number range (600 to 1000 and above 2900 in our experiments) and zigzag-spiral-zigzag intermittence transition in the middle range. The forces acted on the disk were also investigated. Different force behaviors corresponding to different types of wake structures were identified and analyzed. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A9.00003: Experimental investigation of the elastic flag spontaneous flapping in water flow YongXia Jia, LiChao Jia, Zhuang Su, YiDing Zhu, HuiJing Yuan, CunBiao Lee The flapping stability and the response of a thin two-dimensional flag of low bending rigidity to the Reynolds number was investigated. The three relevant non-dimensional parameters governing fluid-structure problems that concern the interaction of elastic flags with high-speed fluid flows are the structure-to-fluid mass ratio, the non-dimensional bending rigidity and the Reynolds number. To study the mechanisms of the transition from the periodic flapping to chaotic flapping, we use PIV and flow visualization techniques to obtain the whole flow field around the midspan of the immersed elastic flag interacting with fluid in both periodic and chaotic states. A moving interface detection technique is used to determine the flag position and velocity. Virtual particle images are imposed in the flag region in the PIV algorithm, of which the displacements are evaluated by the flag movement. We find that the value of St is constrained in the narrow range of 0.2 $<$ St $<$ 0.31 based on the flapping amplitude.We find that the transition to chaos occurs at a critical Reynolds number Re = 60800. For the larger Reynolds number, the high-strength vortices are distributed in a detached region away from the free end of the flag during the intermittent snapping events in the chaotic regime. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A9.00004: The effect of Reynolds number on the drag of a rectangular cylinder Robert Breidenthal, Jonathan Wai Direct numerical simulations of the flow past a rectangular cylinder at low Reynolds number reveal that the aspect ratio for maximum drag is much less than that measured at high Reynolds number. Nakaguchi et al. (1967) discovered a remarkably sharp peak in the drag coefficient at a cylinder aspect ratio of 0.62 for Re $=$ 20,000. In contrast, our numerical simulations at Re $=$ 500 indicate a maximum-drag aspect ratio of 0.2. This dramatic difference is attributed to the rollup station of the laminar vortex sheet from the separating boundary layer. Essentially inviscid, the rollup process scales with the thickness of the vortex sheet at the separation point, which in turn varies inversely with the square root of the Reynolds number. Consequently, at low Reynolds number, the sheet remains thin and laminar, curving tightly toward the cylinder. On the other hand, at high Reynolds number, the vortex sheet promptly rolls up into a rapidly growing, turbulent shear layer. The thick, turbulent layer has a large displacement thickness, deflecting the outer streamlines and altering its own trajectory so that it curves relatively gradually toward the cylinder. Bearman and Trueman (1972) showed that the peak drag corresponds to the shear layer nearly reattaching to the bluff body and rolling up into vortices very close to the base of the cylinder. The low pressure of the vortex cores is reflected in a low base pressure and thus high drag. The critical aspect ratio is much smaller for the laminar vortex sheet because of its more tightly curved trajectory. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A9.00005: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A9.00006: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A9.00007: Stability Analysis of Flow Past a Wingtip Adam Edstrand, Peter Schmid, Kunihiko Taira, Louis Cattafesta Trailing vortices are commonly associated with diminished aircraft performance by increasing induced drag and producing a wake hazard on following aircraft. Previously, stability analyses have been performed on the Batchelor vortex (Heaton et al., 2009), which models a far field axisymmetric vortex, and airfoil wakes (Woodley \& Peake, 1997). Both analyses have shown various instabilities present in these far field vortex-wake flows. This complicates the design of control devices by excluding consideration of near field interactions between the wake and vortex shed from the wing. In this study, we perform temporal and spatial bi-global stability analyses on the near field wake of the flow field behind a NACA0012 wing computed from direct numerical simulation at a chord Reynolds number of 1000. The results identify multiple instabilities including a vortex instability, wake instability, and mixed instability that includes interaction between the wake and vortex. As these modes exhibit wave packets, we perform a wave packet analysis (Obrist \& Schmid, 2010), which enables the prediction of spatial mode structures at low computational cost. Furthermore, a bi-global parabolized stability analysis is performed, highlighting disparities between the parallel and parabolized analysis. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A9.00008: Bifurcations beneath the bluff body instability modes Amalendu Sau A new family of Hopf bifurcations is detected in a cylinder wake. Besides widely known streamwise bifurcations, our study reveals a new route to wake transition via cross-stream flow undulations and a class of previously unknown spanwise bifurcations along von Karman vortex cores; leading to an improved understanding of wake transformation and the transitional flow physics. It shows, alternate vortex shedding generates significant cross-stream momentum transfer, which facilitates self-sustained spanwise wake oscillation and growth of sequence of bifurcations along Karman corelines. The study shows how spanwise oscillation of pressure/velocity/KE keep growing with \textit{Re}, and influence onset of ``Mode A'' and ``Mode B'' instabilities. It reports two distinct stages of wake undulation for 125$\le $\textit{Re}$\le $240. While weakly subcritical periodic-oscillation of pressure/velocity/vorticity along Karman corelines and the uniform/wider length-scale bifurcations dominate during ``Mode A'' instability, the transition to ``Mode B' is prompted following eruption/swapping of significantly smaller variable length-scale bifurcations, and the spanwise flow irregularity. Onset of a secondary frequency in the flow appeared crucial for transition to ``Mode B.'' [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A9.00009: Global stability analysis of turbulent 3D wakes Georgios Rigas, Denis Sipp, Matthew Juniper At low Reynolds numbers, corresponding to laminar and transitional regimes, hydrodynamic stability theory has aided the understanding of the dynamics of bluff body wake-flows and the application of effective control strategies. However, flows of fundamental importance to many industries, in particular the transport industry, involve high Reynolds numbers and turbulent wakes. Despite their turbulence, such wake flows exhibit organisation which is manifested as coherent structures. Recent work has shown that the turbulent coherent structures retain the shape of the symmetry-breaking laminar instabilities and only those manifest as large-scale structures in the near wake (Rigas \emph{et al.}, JFM vol. 750:R5 2014, JFM vol. 778:R2 2015). Based on the findings of the persistence of the laminar instabilities at high Reynolds numbers, we investigate the global stability characteristics of a turbulent wake generated behind a bluff three-dimensional axisymmetric body. We perform a linear global stability analysis on the experimentally obtained mean flow and we recover the dynamic characteristics and spatial structure of the coherent structures, which are linked to the transitional instabilities. A detailed comparison of the predictions with the experimental measurements will be provided. [Preview Abstract] |
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