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 F17: Vortex Turbulence and Superfluids |
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Chair: Stefan Llewellyn Smith, University of California, San Diego Room: Georgia World Congress Center B304 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F17.00001: Vortex Interactions in Two-Dimensional Turbulence Scott Carlson, Patrick Folz, Keiko K Nomura Interactions of coherent vortices in decaying two-dimensional turbulence are quantitatively assessed. Direct numerical simulations are performed, and a tracking algorithm is implemented to autonomously identify vortex structures and track their evolution in time. Vortex properties such as circulation, area, vorticity, strain rate, and enstrophy are used to assess Individual interactions and their outcomes. Inelastic interactions such as merging and straining out, observed here in a field of vortices, are compared with those characterized by isolated asymmetric vortex pairs (Folz and Nomura, JFM 2017). The statistics of these interactions are interpreted in terms of their cumulative effects on the overall flow. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F17.00002: What can two vortex tubes tell us about the turbulent cascade? Rodolfo Ostilla Monico, Ryan McKeown, Shmuel M Rubinstein, Alain Jack Pumir, Michael Phillip Brenner We numerically explore the violent interaction of two parallel, counter-rotating vortex tubes which causes the transfer of energy from the initial large, smooth tubes to the late-stage small ``worm''-like structures similar to those in Homogeneous Isotropic Turbulence simulations. Reynolds numbers of up to Re=Γ/ν=6000 are reached, where Γ is the is the vortex ring circulation, and ν the kinematic viscosity of the fluid. When the tubes are initially very close, their deformation is due to the elliptical instability. The tubes generate perpendicular filaments which undergo transformations of filaments into sheets into filaments, according to an iterative cascade of the type suggested by Brenner et al. (Phys. Rev. Flu., 1, 084503 (2016)). By numerically introducing different perturbations, we compare the dynamics resulting from the elliptical instability, to that induced by the Crow-like instabilities, which lead to reconnection. We show that the iterative mechanisms, triggered by the elliptical instability, transfers energy across scales much more efficiently as a whole than reconnection. We propose that this iterative mechanism is crucial for understanding the turbulent cascade. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F17.00003: Identifying the tangle of twisted vortex tubes in homogeneous isotropic turbulence Shiying Xiong, Yue Yang We apply the vortex-surface field (VSF) to the direct numerical simulation of homogeneous isotropic turbulence. The VSF is constructed from a given vorticity field and its isosurface is a vortex surface consisting of vortex lines. The VSF construction involves pseudo-transport in frozen vorticity and local optimization for compromising satisfactory VSF deviation and smoothness of VSF solutions. In the visualization of VSF isosurfaces with the fixed zero isocontour level, stretched spiral vortex tubes constitute a complex network in isotropic turbulence at moderate Reynolds numbers, and the tangle of vortex tubes appears to be an attractor in the chaotic vorticity system. The VSF isosurface reveals physically reasonable vortex dynamics, such as twisting of vortex tubes and rolling-up of vortex sheets, instead of the visual vortex ‘breakdown’ identified by the isosurface of vorticity magnitude. The successful construction of VSFs in isotropic turbulence implies that the VSF can be constructed in arbitrary flow fields. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F17.00004: Vortex axis tracking by iterative propagation (VATIP): a method for analyzing three-dimensional turbulent structures Lu Zhu, Li Xi Study of turbulent vortices in DNS data relies heavily on direct visual inspection, anecdotal observations, and intuitive arguments. Quantitative analysis is limited by the lack of computational tools for the objective detection and extraction of vortex structures. Much progress has been made in the identification criteria of vortices, which, however, only label spatial regions belonging to vortices, without any information on the identity, topology, and shape of individual vortices. This latter information requires vortex tracking techniques and existing methods so far only focused on quasi-linear streamwise vortices. In this study, a new tracking algorithm is proposed which propagates along the vortex axis-lines and iteratively search for new directions for growth. It is the first tracking method designed for general three-dimensional vortices. The method is tested in transient flow fields with specific vortex types as well as DNS. A new procedure is also proposed that classifies vortices into commonly-observed shapes, including quasi-streamwise vortices, hairpins, hooks, and branches, based on their axis-line topology. The study offers a new tool-set for the fundamental study of the dynamics of turbulent coherent structures. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F17.00005: Breakup and Reorganization of a Turbulent Batchelor Vortex Eric Stout, Fazle Hussain The breakup and reorganization, i.e. recombination of the vorticity filaments from the disrupted core into a columnar vortex due to pairing, merging and viscous diffusion, of a Batchelor vortex is studied using Smagorinsky model Large Eddy Simulation (LES). At Reynolds numbers (vortex circulation/viscosity) up to 500,000, an unstable Batchelor vortex embedded in random turbulence breaks up in to vorticity filaments. These filaments tend to form dipoles and advect away from the original axis due to the swirling jet flow. Regardless of the strength of the jet flow causing the instability, filaments near the core are nearly axial, and thus eventually undergo mutual rotation and eventually pair, merge and reorganize into a columnar structure. The effect of the core reorganization on vortex decay – decrease in the peak azimuthal velocity and increase in the radius of the peak azimuthal velocity – is discussed for the potential to model vortex evolution and predict the time to “safe” vortex states. Evolutions of the Oseen vortex embedded in turbulence as well as vortices excited with linear transient growth optimal perturbation eigenfunctions are compared to the Batchelor vortex cases to illustrate the changes to vortex evolution, breakup and reorganization with increasing axial flow. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F17.00006: From rings to smoke: visualizing the breakdown of colliding vortex rings Ryan McKeown, Rodolfo Ostilla Monico, Alain Pumir, Michael Brenner, Shmuel Rubinstein We experimentally probe the head-on collision of two vortex rings at high Reynolds numbers and visualize in real-time how the initially coherent cores rapidly break down into a turbulent cloud. The colliding vortex rings are seeded with fluorescent dye and illuminated with a scanning laser sheet that is synchronized with a high-speed camera in order to visualize the breakdown dynamics of the flow in full 3D. We observe that for collisions at sufficiently high Reynolds numbers, the vortex cores develop perturbations consistent with the elliptical instability and form an array of slender vortex filaments perpendicular to the collision plane that traverse the narrow gap between the colliding vortex rings. The close-range interactions of these perpendicular filaments with both each other and the cores of the vortex rings trigger the rapid breakdown of the vortices, resulting in the generation of fine-scale vortex filaments. This breakdown is mediated by the iterative flattening and splitting of these perpendicular vortices into successively smaller filaments. We find that the breakdown of these colliding vortex rings at high Reynolds numbers could thus provide new insights into the mechanistic underpinnings of the turbulent cascade. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F17.00007: Critical formation parameters of orifice generated turbulent vortex rings Raphael Limbourg, Jovan Nedic Vortex rings are generated experimentally by a brief discharge of fluid through a sharp-edge nozzle or orifice. Based on initial parameters, the ring can be either laminar or turbulent and if the amount of fluid released exceeds a threshold value, energy is shed in a trailing wake in the form of hairpin vortices. In this study we reproduce the transition map of Glezer (1988) for an orifice outlet and assess the universality of the formation number (Gharib et al. (1998)). An actuator pushes fluid through a tube following a specific velocity program. In the case of nozzle geometries, the rolling-up of the vortex sheet occurs in the extension of the tube. Because an orifice apparatus consists of a flat plate covering the end of the tube, the ratio of the tube diameter Dp to the orifice diameter D0 is expected to modify the formation process and the transition to turbulence. More precisely, the transition line is found to be shifted toward lower values of circulation-based Reynolds numbers ReΓ = U0L0/2ν while the formation number is observed to be closer to unity. As the ratio Dp/D0 tends to 1, the formation number tends towards those of Gharib. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F17.00008: Evolution of Turbulent Vortex Rings in Uniformly Stratified Environments Daniel Curtis Saunders, Alan Brandt Vortex-induced stratified mixing is a topic that has received significant attention. Past studies have examined vortex rings impacting a density interface, traveling at an oblique angle through a stratified region, and laminar rings traveling through either a sharp pycnocline or a continuously stratified region. In this study laboratory experiments were performed to investigate the long-time behavior of turbulent vortex rings in both constant density and uniformly stratified environments. Existing empirical models of ring behavior in constant density environments were compared to the extant data and were extended to turbulent rings traveling through a uniformly stratified region. The vortex rings were found to exhibit three distinct behaviors, primarily dependent upon the local Froude number of the ring as it entered the stratified region: (1) expansion and collapse while transitioning into the uniformly stratified region, (2) rapid decrease in velocity and core radius and stopping within the uniformly stratified region, or (3) traveling totally through the stratified layer. In addition, the degree of transfer of energy from the vortex ring to the internal wavefield was quantified. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F17.00009: Abstract Withdrawn
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Monday, November 19, 2018 9:57AM - 10:10AM |
F17.00010: The rotating CryoLEM (Cryogenic Lagrangian Exploration Module), spin-up, spin-down Emeric Durozoy, Mathieu Gibert The seminal work of Packard’s group in the 80s [1] showed us pictures of quantum vortices in HeII. Twelve years ago, G. Bewley’s PhD work[2], convinced the community that frozen gas micron-sized particles are a good tool to study the dynamics of these angstrom-sized vortex lines. Since then, Lagrangian Particle Tracking has proven to be an insightful tool to study quantum turbulence[3]. Building on these progresses, we have developed a cryostat with 8 optical accesses allowing performing all possible visualization techniques (from 2D-PIV to 3D-LPT). Its temperature can be adjusted between 4.2 and 1.12K, and its uniqueness relies on the fact that it can spin (in order to polarize the vorticity field) up to 2Hz. We will present this unique infrastructure (including particle generation) and report on our first results focusing on the transient and steady states reached during and after spin-up and spin-down of the cryostat. [1]E. J. Yarmchuk , et al., Phys. Rev. Lett. 43, 214 (1979) |
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