Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B10: Vortex Dynamics and Vortex Flow: Simulations |
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Chair: Monika Nitsche, University of New Mexico Room: 3A |
Saturday, November 23, 2019 4:40PM - 4:53PM |
B10.00001: Numerical study of polarized viscous vortex reconnection Jie Yao, fazle hussain Polarized vortical structures (i.e. with axial flow, hence coiled vortex lines) are generic to turbulent flows; hence studying their dynamics and interactions is essential to understanding turbulence phenomena. Vortex reconnection is a frequent event in turbulent shear flows and play an important role in energy cascade, mixing and noise generation. To quantify the polarization effect on vortex reconnection, direct numerical simulation of two anti-parallel helical vortex tubes is performed for vortex Reynolds numbers ($\equiv $\textit{circulation/viscosity)} up to 9 000 and initial swirl numbers $q$ ($\equiv $\textit{peak azimuthal velocity/peak axial velocity}) between 4 and 0.75. For both the same and opposite polarized cases, the reconnection event is delayed as polarization strength increases (i.e., q decreases), but with a higher circulation transfer rate during reconnection. Compared with the unpolarized case, enstrophy and energy dissipation rates are suppressed for weaker polarized (q\textgreater 1.5) cases, but surprisingly enhanced for strong polarized (q\textless 1.5) cases. In addition, increasing the polarization strength alters the energy spectrum in the inertial range with a scaling varying from k$^{\mathrm{-5/3}}$ for the unpolarized case to k$^{\mathrm{-7/3}}$ for the strongly polarized cases. Hence polarization is found to significantly alter vortex reconnection dynamics. [Preview Abstract] |
Saturday, November 23, 2019 4:53PM - 5:06PM |
B10.00002: The Influence of Reynolds Number on the Dynamics of Wing-Tip Vortices Tom Smith, Yiannis Ventikos Tip vortex flows are of considerable interest across a range of technologies and can be significant noise sources, particularly in hydrodynamic applications where tip-vortex cavitation can occur. There are still many open questions regarding the dynamics of this type of flow including Reynolds scale effects and the dynamics of wake-like and jet-like vortex cores. In this study, we use Direct Numerical and Large Eddy Simulations to study the flow over a finite-span elliptical foil at low and moderate Reynolds numbers. Grid convergence is achieved to improve confidence in the validity of the simulations and to develop a better understanding of the mesh requirements for this type of flow. A detailed analysis of the roll-up process is carried out, highlighting significant differences in the flow as the Reynolds number increases. At low Reynolds numbers, a single laminar vortex forms with a very low axial velocity in the core. Higher Reynolds numbers see the emergence of multiple vortices which merge in the near wake. The magnitude and longitudinal position of the minimum pressure is also found to depend on the Reynolds number, which has important consequences for cavitation predictions and scaling. [Preview Abstract] |
Saturday, November 23, 2019 5:06PM - 5:19PM |
B10.00003: Formation and Evaluation of Vortex Rings from Saddle Shape Nozzle Vahid Sadri, Shelly Singh-Gryzbon, Zhenglun Wei, Ajit P Yoganathan The mitral valve (MV) is a complex structure with a saddle-shaped annulus, separating the left atrium and left ventricle (LV). During rapid filling of the LV, a three-dimensional asymmetrical vortex ring forms downstream of the annulus. In this study, we investigated the formation mechanism and evolution of vortex rings from saddle-shaped nozzles using direct numerical simulations. A normal human mitral annulus was used to make the saddle-shaped nozzle geometry. The flow was simulated at a jet Reynolds number of 2,000 (based on hydraulic diameter and jet velocity) with jet pulse length-to- hydraulic diameter ratio (L /D$_{\mathrm{h}})$ ranging from 1 to 6. It was found that the vortex ring from saddle-shaped nozzles has an oscillatory deformation while propagating, which causes the ring to undergo axis-switching. Furthermore, ambient fluid entrainment during vortex ring formation feeds to the main vortex structure, causing circumferential flow that splits the main vortex into smaller portions around the circumference. Additionally, the vortex formation number calculated for low stroke ratio cases. [Preview Abstract] |
Saturday, November 23, 2019 5:19PM - 5:32PM |
B10.00004: Evolution of vortex-surface fields in the flow past a finite plate Wenwen Tong, Yue Yang We investigate the evolution of the vortex-surface field (VSF) in the three-dimensional flow past a finite plate at the Reynolds number of 300, aspect ratio of 2, and angle of attack of 30 degrees. The VSF method is extended to complex flows with an immersed boundary by adding a source term in the VSF evolution equation. The VSF isosurfaces display that near-plate vortex surfaces first roll up from plate edges, and then form hairpin-like structures near the leading edge and semi-ring structures near plate tips and in the wake. We quantitatively distinguish two types of vortical structures by null points of streamwise vorticity on VSF isosurfaces, and refer them to as the leading edge vortex (LEV) and tip vortex (TIV). The VSF characterizes that the development of the LEV near tips is suppressed by the finite growth of TIV. In the wake region, helical vortex lines are generated and their geometry and impulses are quantified based on the VSF isosurfaces for TIV. [Preview Abstract] |
Saturday, November 23, 2019 5:32PM - 5:45PM |
B10.00005: Controlling vortex breakdown through heat addition and extraction Xiao Zhang, Joseph Chung, Carolyn Kaplan, Elaine Oran This work examines how heat addition and extraction can be used to control the modes of vortex breakdown. Three-dimensional, unsteady simulations of gaseous vortex breakdown were carried out by solving the unsteady Navier-Stokes equations. Different modes of vortex breakdown were obtained by matching swirl and Reynolds numbers from the literature. Then, heat was either extracted or added within the vortex core to quantify the influence of non-adiabatic conditions. The results show that a critical value of heat extraction will force a laminar, columnar vortex to transition to a spiral and then to the double helix mode of breakdown. Heat addition with the double helix mode, however, will force the double helix to transition to a columnar vortex, entirely bypassing the spiral mode. We discuss these results and other findings related to the bubble mode of breakdown. [Preview Abstract] |
Saturday, November 23, 2019 5:45PM - 5:58PM |
B10.00006: Role of helicity in vortex breakdown Sameen A, Manjul Sharma We investigate the bubble-type vortex breakdown flow numerically through a model problem of flow inside a cylinder with top rotating lid (also known as Vogel-Escudier flow). The parameters of the flow are $Re=\omega R^{2}/\nu$ and aspect ratio $\Gamma=H/R$, depending on which flow exhibits steady or unsteady breakdown bubble topologies. We find that a negative helicity density is generated by the rotating top lid and is injected in the bulk of the flow at a critical value of the Reynolds number in the event of vortex breakdown. The flow also shows two-dimensional-three-dimensional (2D3C) characteristics for which the helicity density is decomposed into $rz-$component ($h_{r,z}$) and out-of-the-plane component ($h_{\theta}$). We find that the topology of the breakdown bubble correlates directly to the decomposed helicity $h_{r,z}$. Using only the decomposed helicity, $h_{r,z}$, complete breakdown bubble is reconstructed for non-axisymmetric flows. This correlation indicates that the vortex breakdown is the interplay between the axial component of velocity and the axial component of vorticity which are characterized by $h_{r,z}$. [Preview Abstract] |
Saturday, November 23, 2019 5:58PM - 6:11PM |
B10.00007: Accurate evaluation of near-singular integrals in vortex sheet and Stokes flow Monika Nitsche Inviscid vortex sheet or viscous Stokes flow can be efficiently computed using boundary integral formulations, which describe the fluid velocity by integrals over interfaces bounding fluid regions. For points on the interface, the velocity is given by singular integrals, whose computation is well understood. However, for points not on the interface, but near it, the velocity is given by near-singular which are difficult to compute. This talk presents a simple method to accurately compute these integrals. It is based on approximating the integrand by functions that capture the near-singularity and can be integrated exactly. The method is quite general and is applied here to planar vortex sheet and axisymmetric Stokes flow. [Preview Abstract] |
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