Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EN: Vortex Flows: Vortex Rings |
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Chair: Paul Newton, University of Southern California Room: Long Beach Convention Center 202C |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EN.00001: The stability of a family of vortex rings Clara O'Farrell, John O. Dabiri Jetting swimmers, such as squid or jellyfish, propel themselves by forming axisymmetric vortex rings. In order to evaluate the performance of these swimmers, we must asses the optimality of the vortex wakes they produce, which requires an understanding of their stability. We consider the Norbury family of vortices\footnote{J.~Norbury, J.~Fluid~Mech., {\bf 57}, 417-431, 1973.} as a model for the vortex rings produced by jetting swimmers. Pozrikidis\footnote{C.~Pozrikidis, J.~Fluid~Mech., {\bf 168}, 337-367, 1986.} has studied the stability of Hill's spherical vortex under axisymmetric prolate and oblate shape perturbations. However, the stability of other members of the Norbury family to axisymmetric perturbations of the type that might occur during the vortex formation process in jetting swimmers is unknown. In order to asses the stability of different members of the family, we introduce physically pertinent shape perturbations and simulate their development in a manner akin to Pozrikidis' analysis. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EN.00002: Motion of a vortex ring with swirl Ming Cheng, Jing Lou, Tee Tai Lim Motion of vortex rings has been subject of theoretical and experimental studies since the time of Lord Kelvin simply because of its fundamental significance in flow physics and its practical importance in engineering applications. In this paper, we use a lattice Boltzmann method to simulate the motion of a vortex ring with and without swirl in a viscous incompressible fluid. We study the effect of swirl on the dynamics of an isolated three-dimensional vortex ring at a Reynolds number of 800. Our results show that the evolution of the vortex ring is affected by both the magnitude of swirl and the vortex core size. Increasing swirl for a fixed core size causes vortex ring to slow down or even travel backward initially. Increasing core size not only reduces the propagation speed of the ring, it also increases the duration of backward motion when the swirl is sufficient high. Moreover, it is found that while a weak swirl causes vortex filaments to undergo helical winding, a sufficiently strong swirl transforms these windings into convoluted three-dimensional vortex structure with vortex loops trailing behind it. Each of these vortex loops may reconnect with itself, through the process of vortex reconnection, to form a ringlet. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EN.00003: Effect of a ground plane on a turbulent vortex ring trajectory Maria-Laura Beninati, Michael McErlean, Michael Krane, Arnold Fontaine Experiments were conducted to assess how a turbulent (Re=20000) vortex ring's trajectory is affected by a ground plane parallel to its initial trajectory. This study, part of a larger effort in vortex-particle interaction, aims to characterize the vortex ring flow disturbance that interacts with a particle. Vortex ring motion was characterized for four distances between the initial vortex ring axis and the ground plane. Characterization included vortex centroid motion and diameter from high-speed video, vortex ring circulation from DPIV, and the wall pressure disturbance time traces. It was observed that in all cases the vortex ring trajectory is deflected toward the plane, ending in a collision. As plate height is decreased, the collision occurs closer to the ring generator, the wall pressure signature is also more intense, and the symmetry of the ring is affected more strongly. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EN.00004: Convection of a Vortex Ring Parallel to a Plane Wall Vanora O'Loughlin, Doug Bohl In this work we investigate the motion and structure of a vortex ring convecting in a quiescent fluid parallel to a plane wall. The vortex rings were visualized using Laser Induced Fluorescence and recorded digitally. The plane wall was placed between 0.4-1.7 ring diameters away from the center of the ring. The results show that the vortex ring trajectory diverted towards the wall. The initial trajectory was described by inviscid flow models. As the ring came closer to the wall the interaction became viscous in nature. The portion of the ring closest to the wall interacted with the wall first and lost its coherence. The upper portion of the ring continued to convect towards the wall. This region induced a wall boundary layer that eventually separated and orbited the primary region of vorticity. In some cases the primary vortex ring also rebounded from the wall. The interaction was qualitatively similar to that of a vortex ring/oblique wall interaction once the trajectory was diverted towards the wall. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EN.00005: Vortex Ring Dynamics in Radially Confined Domains Kelley Stewart, Casandra Niebel, Sunghwan Jung, Pavlos Vlachos Vortex ring dynamics have been studied extensively in semi-infinite quiescent volumes. However, very little is known about vortex-ring formation in wall-bounded domains where vortex wall interaction will affect both the vortex ring pinch-off and propagation velocity. This study addresses this limitation and studies vortex formation in radially confined domains to analyze the affect of vortex-ring wall interaction on the formation and propagation of the vortex ring. Vortex rings were produced using a pneumatically driven piston cylinder arrangement and were ejected into a long cylindrical tube which defined the confined downstream domain. A range of confinement domains were studied with varying confinement diameters Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) and were processed using an in-house developed cross-correlation PIV algorithm. The experimental analysis was used to facilitate the development of a theoretical model to predict the variations in vortex ring circulation over time within confined domains. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EN.00006: Interaction of a Vortex Ring with Surfaces of Constant Porosity John Hrynuk, Doug Bohl The interaction of vortices with surfaces is a fundamental process in many natural and technological flow fields. In this work we study the interaction of a vortex ring with porous surfaces using Laser Induced Fluorescence. The surfaces studied were stainless steel screens with a constant open area of 65{\%} but different wire diameters (0.017-0.267 cm). Three distinct interactions were observed: 1) For small wire diameters the vortex rings passed through the screens and maintained their coherence and size but with a much slower convection speed. Secondary rings were formed on the upstream side of the screen and convected back upstream; 2) For medium gage wires the vortex ring broke up as it passed through the screen but reformed into a coherent vortex later downstream; 3) For large gage screens the vortices broke up and did not reform downstream. The transition between the interaction types appeared to be dependent on shedding from the screen wires. Specifically, for the small gage screens no shedding from the screen was observed. The medium gage wire showed the beginning of vortex shedding off of the screen wires while the large gage wires showed clearly formed vortices being shed from the screen wires. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EN.00007: Vortex rings from {\em Sphagnum} moss capsules Dwight Whitaker, Sam Strassman, Jung Cha, Emily Chang, Xinyi Guo, Joan Edwards The capsules of {\em Sphagnum} moss use vortex rings to disperse spores to suitable habitats many kilometers away. Vortex rings are created by the sudden release of pressurized air when the capsule ruptures, and are an efficient way to carry the small spores with low terminal velocities to heights where they can be carried by turbulent wind currents. We will present our computational model of these explosions, which are carried out using a 2-D large eddy simulation (LES) on FLUENT. Our simulations can reproduce the observed motion of the spore clouds observed from moss capsules with high-speed videos, and we will discuss the roles of bursting pressure, cap mass, and capsule morphology on the formation and quality of vortex rings created by this plant. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EN.00008: Formation number of positively and negatively buoyant vortex rings Javier Rodr\'Iguez-Rodr\'Iguez, Carolina Marug\'an-Cruz, Carlos Mart\'Inez-Baz\'an The formation process of both negatively and positively buoyant vortex rings in a piston/cylinder arrangement is investigated numerically with the aim of understanding the effect of buoyancy, characterized by a Richardson number, on the formation number. More specifically, the study focuses on how vorticity is distributed inside the vortex ring and how this vorticity distribution compares with the neutrally buoyant case. It is well known that the kinetic energy of a neutrally buoyant vortex ring, when made dimensionless with its impulse and circulation, has a universal value of $E_{nd} 1/3$. The limits of validity of this value for moderate Richardson numbers, both in the positively and negatively buoyant cases, are examined. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EN.00009: Vortex ring refraction at large Froude numbers Kerry Kuehn, Matthew Moeller, Michael Schulz, Daniel Sanfelippo We have experimentally studied the impact of an initially planar axisymmetric vortex ring, incident at an oblique angle, upon a gravity-induced interface separating two fluids of differing densities. After impact, the vortex ring was found to exhibit a variety of subsequent trajectories, which we organize according to both the incidence angle, $\theta_i$, and the interface strength, defined as the ratio of the Atwood and Froude numbers, $A/F$. For grazing incidence angles ($\theta_i \ga 70$ deg.) vortices either penetrate or reflect from the interface, depending on whether the interface is weak or strong. In some cases, reflected vortices execute damped oscillations before finally disintegrating. For smaller incidence angles ($\theta_i \la 70$ deg.) vortices penetrate the interface. When there is a strong interface, these vortices are observed to curve back up toward the interface. When there is a weak interface, these vortices are observed to refract downward, away from the interface. The critical interface strength below which vortex ring refraction is observed is given by $\log_{10}{(A/F)}= -2.38 \pm 0.05$. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EN.00010: An experimental study on merging of two co-axial co-rotating vortex rings Jagannadha Satti, Jifeng Peng The merging of two co-axial, co-rotating vortex rings is studied experimentally. Two laminar vortex rings were generated consecutively from a piston-cylinder apparatus. The two rings propagate in the same direction and the spatial separation between them decreases until they start merging. Special cases of leapfrogging were also observed. Digital particle image velocimetry was used to measure the flow fields. Core sizes, trajectories and circulations were measured for individual rings before the merging, as well as afterwards for the merged ring. At low Reynolds number, the total circulation in the flow is relatively a constant before and after merging. However, at high Reynolds number, the total circulation decreases quickly upon the contact of two vortex ring cores, indicating the transition to a turbulent vortex ring during merging. The circulation of the merged ring later stabilizes at a less level, indicating the merged ring becomes laminar again after shedding some circulation. Comparison between results from this experimental study and previous theoretical and computational studies in the literature are also discussed. [Preview Abstract] |
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