Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session GU: Vortex Rings |
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Chair: Jim Liburdy, Oregon State University Room: 204B |
Monday, November 24, 2008 8:00AM - 8:13AM |
GU.00001: A Study of Vortex Rings from Cylinders with Inclined Exits Using V3V Daniel Troolin, Ellen Longmire Volumetric 3-component velocimetry (V3V) was used to examine the vortical structures generated by impulsively driven pistons within surrounding cylinders. The piston stroke length L was equal to the cylinder inner diameter of 72.8 mm. The Reynolds number based on piston velocity and L was 2500. The measurement volume was 140mm x 140mm x 100mm, and the spatial resolution was 4mm. While an axisymmetric cylindrical exit yields a primary ring that is largely decoupled from a trailing ring of opposite circulation, inclined exits yield a much more complicated topology. The primary ring exits the cylinder with an initial inclination of approximately half the cylinder exit angle. Then, the primary ring interacts directly with two trailing rings associated with the piston stopping. The downstream portions of the trailing vortices are drawn in through the center of the primary vortex and subsequently stretched as they wind around the core of the primary ring. The upstream portions of the trailing rings intertwine. These interactions lead to earlier breakdown of the primary ring as well as decreases in propagation speed. Movies showing the detailed evolution of these structures will be shown. [Preview Abstract] |
Monday, November 24, 2008 8:13AM - 8:26AM |
GU.00002: Ambient fluid entrainment by pinched off laminar vortex rings Ali Olcay, Paul Krueger A starting jet produced with a jet slug length-to-diameter (L/D) ratio greater than a limiting value called the formation number leads to a leading vortex ring pinched off from its generating jet. It is our interest to understand the significance of formed trailing jet on ambient fluid entrainment by the leading vortex ring. In this study, starting jets are produced from a piston -- cylinder mechanism for jet Reynolds number of 1400 at L/D of 3.0, 6.0, and 9.0 for a trapezoidal velocity program. Using Lagrangian Coherent Structures (LCS) along with experimental velocity field data, a gap between the vortex spiral and trailing jet is identified where entrainment into the ring takes place. It is noticed that entrainment occurs through much smaller area for a pinched off vortex ring than an isolated vortex ring due to the existence of a trailing jet. This indicates that entrainment into the leading vortex ring is limited by the trailing jet and changes in L/D will not significantly affect entrainment into the ring once pinch off has occurred. [Preview Abstract] |
Monday, November 24, 2008 8:26AM - 8:39AM |
GU.00003: Dynamics of non newtonian vortex rings C. Palacios, I. Monsivais, R. Zenit The dynamics of formation and evolution of vortex rings created in a piston-cylinder arrangement is studied. The purpose of this investigation is to compare the formation process of Newtonian and non Newtonian vortex rings. These kind of vortices are present in many engineering applications and natural phenomena. The ratio of the piston displacement L and the nozzle diameter D determine the vortex size and evolution. Experiments with different conditions are presented: translation velocity of the piston, stroke ratio L/D, a Newtonian fluid and a shear thinning fluid. Measurements of the 2D velocity field were obtained with a PIV technique. The vortex circulation was computed considering a vortex identification scheme (Q criterion). In its evolution the vortex ``feeds'' of vorticity and increases its size; after some time its strength begins to dissipate. In the case of Newtonian fluids the results show that there is a critical number of L/D above which the circulation inside the vortex can not increase and remains constant (Gharib et al., 1998); in other words the vortex has a finite size. The critical number, called formation number, is between 3.5 and 4.5 for Newtonian fluids. We have observed that for a shear thinning fluid the critical number can increase considerably (up to twice the Newtonian value). Also, we observed that the shape of the rings is modified by the non-Newtonian properties of the fluid. [Preview Abstract] |
Monday, November 24, 2008 8:39AM - 8:52AM |
GU.00004: Dynamics of collision of a vortex ring and a planar surface Michael McErlean, Michael Krane, Arnold Fontaine The dynamics of the impact between a vortex ring and a planar surface is presented. The vortex rings, generated by piston injection of a slug of water into a quiescent water tank, collide with a surface oriented normally to the ring's direction of travel. The time evolution of both the force imparted to a planar surface and the wall pressure are presented. These are supplemented by DPIV measurements of the evolution of ring strength and structure, before and during impact. The relation between changes in ring structure during collision and the waveforms of impact force and wall pressure will be discussed. [Preview Abstract] |
Monday, November 24, 2008 8:52AM - 9:05AM |
GU.00005: Vortex Ring Interaction with Multiple Permeable Screens Mustafa N. Musta, Paul S. Krueger Previous experiments on the interaction of a vortex ring impinging on single thin permeable screen demonstrated the formation of secondary vortices and a transmitted vortex ring. The present work concerns experimental investigation of the interaction of a vortex ring with multiple permeable screens. Vortex rings are formed by piston-cylinder type vortex ring generator and impinge on an array of parallel, transparent screens. The screens have an open ratio of 84{\%} and the spacing between screens is variable. The vortex rings were formed with an approximate jet Reynolds number of 1300 and a piston stroke-to-jet diameter ratio (L/D) of approximately 4. Dye visualization of the vortex rings shows that they break into multiple vortices after impinging on first screen The vortices subsequently disintegrate, but the total distance required for disintegration is relatively unaffected by the number of screens through with the vortices pass due to the regular structure of the screens. It is also observed that the location of the initial vortex ring axis relative to the screen rods has a significant effect on the vortex breakup and disintegration process. [Preview Abstract] |
Monday, November 24, 2008 9:05AM - 9:18AM |
GU.00006: Vortex Ring State and Asymmetric Thrust Oscillations Gregory McCauley, Omer Savas, Francis Caradonna When the helical vortices of a rotor are not convected away, the vortices may form a ring-like structure about the rotor disk. This vortex ring state (VRS) is most common during rapid descent and leads to thrust oscillations coupled to the formation and subsequent breakdown of the ring. Experimental observations at and near VRS were made using strobed particle image velocimetry on a three-blade rotor in a towing tank. Simultaneous strain gage readings allowed direct measurement of the rotor's thrust history in this state. Operating conditions near the cusp of VRS were investigated to offer insight into the initial evolution of this undesirable state. In addition, asymmetries in the periodic thrust histories during non-axial descent are analyzed in conjunction with corresponding vorticity evolutions. Salient features of the vortex wake structure during highly asymmetric thrust oscillations are discussed in contrast to VRS cases with nearly symmetric thrust oscillations. [Preview Abstract] |
Monday, November 24, 2008 9:18AM - 9:31AM |
GU.00007: Vortex Ring Interaction with a Heated Screen Jason Smith, Paul S. Krueger Previous examinations of vortex rings impinging on porous screens has shown the reformation of the vortex ring with a lower velocity after passing through the screen, the creation of secondary vortices, and mixing. A heated screen could, in principle, alter the vortex-screen interaction by changing the local liquid viscosity and density. In the present investigation, a mechanical piston-cylinder vortex ring generator was used to create vortex rings in an aqueous sucrose solution. The rings impinged on a screen of horizontal wires that were heated using electrical current. The flow was visualized with food color and video imaging. Tests with and without heat were conducted at a piston stroke-to-jet diameter ratio of 4 and a jet Reynolds number (Re) of 1000. The vortex rings slowed after passing through the screen, but in tests with heat, they maintained a higher fraction of their before-screen velocity due to reduction in fluid viscosity near the wires. In addition, small ``fingers'' that developed on the front of the vortex rings as they passed through the screen exhibited positive buoyancy effects in the heated case. [Preview Abstract] |
Monday, November 24, 2008 9:31AM - 9:44AM |
GU.00008: Vortex rings with swirl Stefan Llewellyn Smith, Vincent Morin Vortex rings with azimuthal vorticity $\zeta \propto r$, where $r$ is the distance from the axis of symmetry, have an unsteady contour dynamical formulation on the one hand and a family of steady solutions due to Norbury on the other hand. We consider the effect of adding swirl to such rings. Taking the swirl $w \propto r^{-1}$ maintains the contour dynamics formulation, but it becomes necessary to add a vortex sheet at the boundary of the rings. Steady and unsteady solutions are presented, and the relation of these results to previous work is discussed. [Preview Abstract] |
Monday, November 24, 2008 9:44AM - 9:57AM |
GU.00009: Vortex Rings in Superfluid Helium Sultan Z. Alamri, Carlo F. Barenghi We present results of numerical simulations of large-scale vortex rings in superfluid helium. These large-scale vortex rings consists of many discrete (quantized) vortex filaments which interact with each other moving according to the Biot-Savart law. Lifetime, structural stability and speed of large-scale vortex rings will be discussed and compared to experimental results. [Preview Abstract] |
Monday, November 24, 2008 9:57AM - 10:10AM |
GU.00010: Buoyancy Effects on the Development of the Leading Vortex Ring in a Starting Jet Carolina Marugan-Cruz, Javier Rodriguez-Rodriguez, Carlos Martinez-Bazan The initial development of negatively buoyant jets has been investigated experimental and numerically, more specifically the role played by gravity in the development of the leading vortex ring. A classical piston-cylinder arrangement has been used to produce the negatively buoyant jets. Under the experimental conditions considered in this work, the Froude number, Fr, which compares the jet momentum and the buoyancy flux is the most important parameter characterizing the dynamics of the flow. When the value of this parameter is sufficiently small the initial vortex ring generated at the start of the motion is pushed upwards by the gravity force before it can entrain enough vorticity to acquire a self induced velocity. However when the Froude number exceeds a critical value, Frc~$\sim$~1, the vortex ring can travel downwards and entrainins vorticity from the trailing jet during a longer time. Total and vortex circulation, as well as the trajectory of the leading vortex have been measured to clarify the effect of gravity on the distribution of vorticity during the initial development of negatively buoyant jets. [Preview Abstract] |
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