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 L17: Vortex Evolution and Interaction |
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Chair: Robert Kerr, University of Warwick Room: Georgia World Congress Center B304 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L17.00001: Straining effect in the process of vortex reconnection Yoshifumi Kimura, Keith Moffatt During the process of vortex reconnection, the vortices are subject to the local velocity gradient, and the finite vortex cores are inevitably deformed by the straining action. The nature of the deformation is determined by the eigenvalues of the rate-of-strain tensor. Using a tent model for vortex reconnection which consists of vortex filaments in the form of two anti-parallel tilted hyperbolae, we investigated the structure of the eigenvalues and eigenvectors of the rate-of-strain tensor near the points of the closest approach (tipping points). We observed the following things: (1) The second largest eigenvalue λ_{2 }is positive and its eigenvector e_{2} is in the axial direction; each vortex is therefore persistently stretched at the tip. (2) The eigenvectors e_{1} and e_{3} are in the plane perpendicular to the axial direction and are mutually orthogonal as expected for a real symmetric matrix. (3) At the tipping point on one vortex, the rate-of-strain produced by the other vortex alone is dominant, and the magnitudes of the components in the perpendicular plane of e_{1} and e_{3} are nearly equal; these vectors are therefore close to the directions along a line inclined at an angle of ±π/4. These observations can be verified asymptotically using a model of two tilted vortex rings. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L17.00002: Visualization of Lagrangian Coherent Structures in Vortex Formation and Advection Braxton N. Harter, Matthew H. McCrink, James W. Gregory In recent years, the formation of vortex rings generated through impulsively started jets using a piston-cylinder mechanism has been a popular topic of discussion, as these fluid structures are commonly found in nature. It is well known that there is a limiting stroke to diameter (L/D) ratio that causes two distinct modes of formation. For short stroke ratios, a single advecting vortex ring is created; however, above a ubiquitous L/D (~ 4) threshold, the flowfield generated consists of a leading vortex ring followed by a trailing jet. The transition between these two modes of formation is studied using particle image velocimetry (PIV) of a submerged piston-cylinder vortex ring generator. Finite Time Lyapunov Exponent analysis of the PIV flowfield identifies Lagrangian Coherent Structures that provide insight to the existence of this inherent threshold between the two ring generation modes. Results validate the findings of other researchers, in that when the stroke ratio is too large an adverse pressure gradient behind the leading vortex ring inhibits vorticity transport into the ring. Further analysis on the vortex wake and shedding of vorticity is presented, as well as the mechanisms by which a vortex ring reaches optimum strength. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L17.00003: Interactions of periodically generated vortex rings Hossein Asadi, Hafez Asgharzadeh, Iman Borazjani Periodically generated vortex rings are observed in nature such as cardiovascular flows. We set up a series of simulations to study the dynamics of vortex rings in both transient and quasi-steady state. Our set-up consists of a circular cylinder through which a periodic pulsed jet is pushed into a tank. Our visualizations showed that for test cases with high non-dimensional period (T^{*}, i.e., equivalent to formation time) or low Re, the stopping vortices surrounding the leading ones remain circular and intact; however, by decreasing T^{*} and increasing Re the stopping vortices become unstable and form hairpin vortices. Our observations in quasi-steady state showed that two distinct interaction patterns exist. For cases with short T^{*}, vortex pairing occurs after about one period downstream, which makes reaching the quasi-steady take longer. The stability of pairing process is affected by the Re. For cases with higher T^{*}, a train of vortex rings is generated in the quasi-steady state. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L17.00004: The influence of collision angle on vortex reconnection during the symmetrical collision of vortex rings JiaCheng Hu, Sean D. Peterson Limitations on the common vortex tube arrangements, namely the anti-parallel and the orthogonal configurations, constrain the parameter space of vortex reconnection studies to Reynolds number only in the literature. In a recent article by Hernandez and Reyes [Phys. Fluids 29, 103604 (2017)], the symmetrical collisions of three and six vortex rings in the incompressible laminar regime were numerically investigated. This particular configuration of vortex collision exhibits a similar vortex reconnection process to the common setups, but offers the ability to adjust the collision angle, which enables the investigation of the geometrical influence on the vortex reconnection phenomenon. We employ pseudo-spectral simulations to perform a parametric study by varying both the collision angle and Reynolds number for the symmetrical collision of vortex rings. We find that the collision angle does play a role in the reconnection process, where the reduction in the impact angle leads to increases in both the circulation transfer rate and the final circulation transferred to the collision plane. Furthermore, the circulation transfer mechanism is reported, which provides additional insights into the vortex reconnection phenomenon. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L17.00005: Interactions of nested, coiled vortex rings Robert M Kerr Pairs of nested vortex rings, one with $m$-coils, as well as single rings, are evolved numerically to determine their global helicities $H$, enstrophies $Z$ and their individual topological properties: twist, writhe, self-linking numbers and centreline helicities ${\cal H}_c$. The questions are: Can the experimental values of Scheeler et al. {\it Science} {\bf 357}, 487 (2017) can be validated and do the numbers have roles in the dynamics? Only the $t=0$ writhe measurements are validated, with twist $gg$ writhe and $\leq$ self-linking, along with surprisingly small global helicities compared to the predicted $m\Gamma^2$: ${\cal H}\sim{\cal H}_c\ll m\Gamma^2$, $\Gamma$ the circulation. Nonetheless, the writhe and twist retain important complementary roles in the dynamics of ${\cal H}$ and enstrophy $Z$, with the evolution of the torsion $\tau(s)$ profile showing the beginnings of a cascade to small scales. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L17.00006: Helicity Decay in Vortex Rings Robert Morton, William T M Irvine The helicity of vortex loops is captured in the loops' linking, writhe, and twist. In viscous fluids, helicity loses its conserved status and is dissipated through only one of these modes -- twist. By generating helical vortices using hydrofoils, we are able to measure their helicity and its decay over a range of viscosities. We report the dissipation and exchange dynamics and interpret them in terms of the evolution of the vortex core geometry and size, which we resolve using a novel method. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L17.00007: An analysis of significant flow scale of key flow for vortex generation in terms of local flow geometry in isotropic homogeneous turbulence Katsuyuki Nakayama The key flow for vortex generation in terms of the invariant local topology derived from the velocity gradient tensor is analysed with decomposing the flow scale in an isotropic homogeneous turbulence in a low Reynolds number. Swirlity that specifies the unidirectionality and intensity of the azimuthal flow in the local topology monitors the flow transition into a vortex. Then the specific coordinate system is defined associated with the predicted swirl plane, and the all velocity gradient tensor components are expressed as invariant quantities relating to the flow topology. The statistical analysis of the time derivatives of the swirlity and tensor components shows that the key flow for the vortex generation is an invariant shear flow in the predicted swirl plane which direction is specified by the eigenvectors of the velocity gradient tensor. In the vortex generation, both vortical and non-vortical flows exist in the several decomposed flow scales. However, the intermediate flow scale exists to produce a predominant key flow component. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L17.00008: Effect of temperature gradients on axial vortex breakdown Vishnu Ravindran, Manjul Sharma, A Sameen In this study, we do a direct numerical simulation to understand the effect of temperture gradient on an axial vortex breakdown. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L17.00009: Comparison of vortex formation by linearly or angularly accelerated objects in a quiescent flow Guillaume De Guyon-Crozier, Diego Francescangeli, Karen Mulleners The formation of vortices in a quiescent flow is studied for different types of vortex generators. Three types of vortex generators are considered: a classical piston-cylinder configuration, a sharp edge object translated along a straight trajectory from rest, and a two dimensional rectangular flat plate that is rotated 180° about its span-wise axis. The velocity field induced by the motion of these objects is measured by means of time-resolved particle image velocimetry and Lagrangian methods are used to track the evolution of the shear layer, the generation of coherent vortical structures, and the characteristics of these vortices. Special attention is directed toward identifying non-dimensional values describing the vortex formation time and vortex circulation and energy. The geometry of the translated objects and the linear or angular acceleration profiles for the different vortex generators are varied to explore their influence on the vortex formation process and the characteristics of the shed vortices. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L17.00010: Dynamics of electron plasma vortices subject to time-dependent external strain flows Noah Hurst, James Danielson, Daniel Dubin, Clifford Surko The behavior of two-dimensional (2D) vortices in response to time-dependent strain flows is studied experimentally using pure electron plasmas. These plasmas obey guiding-center drift dynamics in the plane perpendicular to the magnetic field which are isomorphic to the dynamics of a 2D inviscid, incompressible fluid, where electron density plays the role of fluid vorticity [1]. External strain flows are applied by imposing a time-dependent, quadrupolar boundary condition on the circular container [2]. The experiments focus on dynamical orbits of the vortices [3] and shear instabilities which occur when the vortices are highly distorted [4]. Vortex-in-cell simulations are carried out to complement the experimental data. These studies are potentially relevant to a variety of quasi-2D fluid systems, including geophysical fluids, astrophysical disks, and magnetically confined plasmas. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L17.00011: DNS of the interaction between counter rotating vortices Aditya Madabhushi, Krishnan Mahesh Understanding the nature of interaction between a pair of vortex filaments is essential to predict cavitation inception in the filaments. DNS is used to study the effect of core radius on the dynamics of the interaction between a pair of unequal strength counter-rotating vortices. The simulations examine conditions that lead to lowest pressure in either of the vortices. The extent of stretching and deformation of the filament cores is analysed and the variation in collapse time, minimum pressure and maximum vorticity with radii ratio are discussed. |
Monday, November 19, 2018 6:28PM - 6:41PM |
L17.00012: On the formation of vortices under transient and pulsatile inflow conditions in a curved pipe Mohammad Reza Najjari, Michael W Plesniak Formation of vortical structures in a 180° curved pipe under transient flows was investigated using particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations. Several vortex identification methods were employed to identify various vortices in the flow. Vorticity transport equation analysis revealed that the origin of the Lyne-Type vortex was around the 30° location from the inlet and close to the curved pipe inner wall. Our study showed that the axis of the Lyne-type vortex (unlike that of the Dean vortices) is primarily in the direction normal to the pipe’s central plane in the vicinity of its origin until around the 60° cross-section, where it starts to gain a weak swirling component normal to the pipe's cross-section. In order to identify the effect of inner wall vorticity on the Lyne-type vortex, additional CFD simulations were performed with a slip wall boundary condition, which revealed the Lyne-type vortex originates from vorticity introduced by the incoming velocity profile close to the inner wall. During the formation of the Lyne-type vortex, a Dean vortex pair distorts into the two deformed-Dean vortices to provide space to accommodate the Lyne-type vortex. |
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