Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session AR: Vortex Dynamics and 3D Vortex Flows I |
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Chair: Paul Krueger, Southern Methodist University Room: Salt Palace Convention Center 251 F |
Sunday, November 18, 2007 8:30AM - 8:43AM |
AR.00001: Nonlinear stability of a swirling jet interacting with a solid wall Joaquin Ortega-Casanova, Ramon Fernandez-Feria We consider in this work the nonlinear stability of a q-vortex interacting with a solid surface perpendicular to its axis at moderately high Reynolds numbers. We use a direct numerical simulation based on a potential vector formulation with a Fourier decomposition in $N$ azimuthal modes. This method is specially suited for the study of the nonlinear stability of axially symmetric flows because one may follow the raising of the different non-axisymmetric modes from just numerical noise, their nonlinear development, and their nonlinear interaction. For a given Reynolds number we find that there exists different transitions as the swirl number is raised, including the development of non-axisymmetric instabilities for different azimuthal modes, their mutual nonlinear interaction, and the formation of a vortex breakdown bubble that turns the flow almost axisymmetric again. We compare these transitions with those appearing in the same flow in absence of the solid wall, and discuss the relevance of the results in the context of seabed excavation processes. [Preview Abstract] |
Sunday, November 18, 2007 8:43AM - 8:56AM |
AR.00002: ABSTRACT WITHDRAWN |
Sunday, November 18, 2007 8:56AM - 9:09AM |
AR.00003: DES of turbulent vortex breakdown in an abrupt axisymmetric expansion at Re = 10$^{5}$. Joongcheol Paik, Fotis Sotiropoulos Turbulent swirling incompressible flow through an abrupt axisymmetric expansion at Re = 100,000 is investigated numerically using detached-eddy simulation. The effects of swirl intensity on the coherent dynamics of the flow are systematically studied by carrying out numerical simulations over a range of swirl numbers from 0.17 to 1.23. Comparisons of computed solutions and the experimental measurements of Dellenback et al. [AIAA Journal. Vol 26, pp. 669-681] show that the numerical simulations resolve both axial and swirling mean velocity profiles and corresponding turbulence statistics with very good accuracy. Coherent structure visualizations with the q-criterion and Lagrangian particle tracking are used to elucidate the rich dynamics of the flow as a function of the swirl number with emphasis on the various modes of vortex breakdown and other large scale instabilities in the shear layers and along the pipe wall. [Preview Abstract] |
Sunday, November 18, 2007 9:09AM - 9:22AM |
AR.00004: Effect of Axial Pressure Gradient on the Bifurcation Structure of Viscous Vortex Breakdown Elena Vyazmina, Joseph Nichols, Jean-Marc Chomaz, Peter Schmid Incompressible open swirling flows are studied by means of direct numerical simulation (DNS) and linear stability analysis. The bifurcation structure is obtained by varying control parameters including: the swirl parameter S, the Reynolds number Re, and the nondimensional external pressure gradient$\beta $. Nonlinear steady states are traced by pseudo-arclength continuation using the Recursive Projection Method (RPM) applied to the fully nonlinear DNS. For zero pressure gradient and large Re, the bifurcation curve shows a characteristic fold representing the existence of multiple solutions associated with vortex breakdown. Large favorable pressure gradients prevent vortex breakdown giving access to new stable or unstable branches corresponding to high swirl number, breakdown-free states. These branches are traced back to the case with zero pressure gradient by applying continuation into the pressure gradient parameter. [Preview Abstract] |
Sunday, November 18, 2007 9:22AM - 9:35AM |
AR.00005: Mechanisms of core perturbation growth in vortex-turbulence interaction Fazle Hussain, Dhoorjaty Pradeep Vortex-turbulence interaction is studied through DNS. Ensemble-averaged statistics show strong core waves induced by organized structures in the core periphery, originating from the ambient turbulence. Three core breakdown mechanisms are studied: (i) growth through instability resulting from a mean circulation overshoot, (ii) resonant forcing of Kelvin waves by ring-like structures wrapping the column's core, and (iii) growth of optimal transient perturbations. Radial transport of mean angular momentum leads to an overshoot of the circulation profile. The overshoot enhances turbulence production, but is found to rapidly diminish through ``turbulent mixing.'' In problem (ii) we find ring-vortex wave resonance even for relatively weak rings. Resonance leads to strong core dynamics, resulting in sheath-like structures in the core, known to be unstable to a Kelvin-Helmholtz-like instability. However, this process requires sustained organized ring-like structures over several vortex turnover times. Amplification of core perturbations in optimal modes also occurs through resonant forcing. Nonlinear evolution of optimal bending modes captures features of vortex interaction with fully developed turbulence: enhanced core diffusion, core perturbation growth, and circulation overshoot. We also discuss possible self-sustaining turbulence and compare DNS results with modeling predictions. [Preview Abstract] |
Sunday, November 18, 2007 9:35AM - 9:48AM |
AR.00006: Experimental Observation of Quantized Vortex Reconnection in Superfluid Helium Matthew Paoletti, Gregory Bewley, Katepalli Sreenivasan, Daniel Lathrop We present experimental studies of the first direct visualization of reconnecting superfluid vortices and the decay of superfluid turbulence in $^{4}$He. Micron-sized solid hydrogen particles allow for particle image velocimetry and particle tracking. As previously shown, the cores of the superfluid vortices can trap the hydrogen, thereby allowing direct visualization of the dynamics of the line-like defects. We generate superfluid turbulence by driving a thermal counterflow. Upon cessation of the counterflow, the system relaxes through a cascade of reconnection events. The velocities and energies of the particles are determined by particle-tracking. Surprisingly, the probability distributions of velocity and energy are shown to have power-law tails, but these may be understood with scaling arguments for reconnecting vortices. [Preview Abstract] |
Sunday, November 18, 2007 9:48AM - 10:01AM |
AR.00007: Swirl ratio effects on tornado-like vortices Pooyan Hashemi-Tari, Roi Gurka, Horia Hangen The effect of swirl ratio on the flow field for a tornado-like vortex simulator (TVS) is investigated. Different swirl ratios are obtained by changing the geometry and tangential velocity which determine the vortex evolution. Flow visualizations, surface pressure and Particle Image Velocimetry (PIV) measurements are performed in a small TVS for swirl ratios S between 0 and 1. The PIV data was acquired for two orthogonal planes: normal and parallel to the solid boundary at several height locations. The ratio between the angular momentum and the radial momentum which characterize the swirl ratio is investigated. Statistical analysis to the turbulent field is performed by mean and rms profiles of the velocity, stresses and vorticity are presented. A Proper Orthogonal Decomposition (POD) is performed on the vorticity field. The results are used to: (i) provide a relation between these 3 sets of qualitative and quantitative measurements and the swirl ratio in an attempt to relate the fluid dynamics parameters to the forensic, Fujita scale, and (ii) understand the spatio-temporal distribution of the most energetic POD modes in a tornado-like vortex. [Preview Abstract] |
Sunday, November 18, 2007 10:01AM - 10:14AM |
AR.00008: Axisymmetric Vortices with Swirl A. Elcrat This talk is concerned with finding solutions of the Euler equations by solving elliptic boundary value problems for the Bragg-Hawthorne equation L u= -urr -(1/r)ur - = r$^2$f (u) + h(u). Theoretical results have been given for previously (Elcrat and Miller, Differential and Integral Equations 16(4) 2003, 949-968) for problems with swirl and general classes of profile functions f, h by iterating Lu(n+1)= rf(u)n)) + h(u(n)), and showing u(n) converges montonically to a solution. The solutions obtained depend on the initial guess, which can be thought of as prescribing level sets of the vortex. When a computational program was attempted these monotone iterations turned out to be numerically unstable, and a stable computation was acheived by fixing the moment of the cross section of a vortex in the merideanal plane. (This generalizes previous computational results in Elcrat, Fornberg and Miller, JFM 433 2001, (315-328) We obtain famillies of vortices related to vortex rings with swirl, Moffatt's generalization of Hill's vortex and tubes of vorticity with swirl wrapped around the symmetry axis. The vortices are embedded in either an irrotational flow or a flow with shear, and we deal with the transition form no swirl in the vortex to flow with only swirl, a Beltrami flow. [Preview Abstract] |
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