Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session Y12: Vortex Dynamics and Vortex Flows: Simulations (11:30am - 12:15pm CST)Interactive On Demand
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Y12.00001: Effects of convection and diffusion in the viscous Lamb dipole Ling Xu, Robert Krasny We present a numerical study of the viscous Lamb dipole in the Reynolds number range $125 \le Re \le 1000$. The effects of convection and diffusion are examined by comparing solutions of the Navier-Stokes equations (NSE) and diffusion equation (DE) with the inviscid Lamb dipole as the initial condition. The results show that for a given Reynolds number, the vortex core size, shape, and maximum vorticity are diffusion-dominated features. However, the total circulation is affected by convection; at low $Re$, convection in the NSE inhibits circulation decay in comparison to the DE, while it enhances circulation decay at high $Re$. The lateral separation of the vortex cores plays a key role in this transition. [Preview Abstract] |
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Y12.00002: Dynamics of Smooth Elliptical Vortices Computed by a Vortex Method Robert Krasny, Ling Xu A new version of the vortex method is presented for vortex dynamics in 2D ideal fluid flow. The scheme uses Lagrangian particles and panels, Biot-Savart kernel smoothing, remeshing, adaptive mesh refinement, and treecode acceleration. The method is applied to compute the dynamics of elliptical vortices; two cases are considered, one with a smoothed Kirchhoff profile and one with a Gaussian profile. In both cases the rotating core sheds spiral filaments that radiate outward, however the Kirchhoff vortex forms a tripole structure, while the Gaussian vortex becomes axisymmetric on a faster time scale. In addition, the vorticity distribution in the Gaussian vortex has small amplitude bands in the core. [Preview Abstract] |
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Y12.00003: Targeted disruption of vortex rings Robert Handler, David Goldstein, Saikishan Suryanarayanan Vortex rings at low to moderate Reynolds numbers are generated numerically by using an impulsive body force. For a Newtonian fluid, the vortex ring is observed to entrain a passive scalar, introduced at the generation point. This is expected according to standard vortex theorems. The fluid occupied by the scalar is then allowed to take on viscoelastic properties by using a targeting algorithm which employs a FENE-P model for the polymer stress evolution. The effects of such targeting on the evolution of the vortex ring will be discussed. [Preview Abstract] |
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Y12.00004: Unsteady Dynamics of Upswept-Base-Cylinder Afterbody Flows Rajesh Ranjan, Datta Gaitonde The flow behind bluff bodies similar to those behind cargo fuselages, automobiles, and other afterbody configurations results in separation from the upswept base and the ultimate reorganization of the vorticity into, depending on conditions, either a pair of streamwise counter-rotating vortices or a wake. Under certain conditions, hysteretic behavior is also possible. In this work, the unsteady dynamics in each regime (vortex or wake) are elucidated using a well-validated Large-Eddy Simulation (LES) database comprised of numerous upsweep angles. In the vortex regime, an increase in upsweep results in more intense vortex-shear layer interactions near the base and increased wandering of instantaneous vortices downstream of the body. The talk will also discuss the changes in the unsteady coherent structure dynamics with upsweep, using various modal decomposition techniques. Dynamic mode decomposition (DMD) has been employed to extract dominant frequency-segregated modes pertaining to both separation and vortex unsteadiness, while Proper Orthogonal Decomposition (POD) is used to extract energy-rich coherent structures and the rank behavior of the complex flow. These analyses from different upswept configurations are then put into context by examining the unsteadiness due to inclusion of a cavity in the base region. [Preview Abstract] |
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Y12.00005: Fast Potential Flow Computations for Low-order Aerodynamic Modeling Diederik Beckers, Jeff D. Eldredge Potential flow plays an important role in many applications, including flow estimation in aerodynamics. For the models in these applications to work efficiently, it is best to avoid Biot-Savart interactions between the potential flow elements, particularly for 3D models. This work addresses grid-based computations for potential flows in 2D and 3D and their implementation in a low-order vortex model for fast modeling of separated aerodynamic flows and gust interactions. The model uses the immersed boundary projection method to solve for the vector potential field subject to the constraints introduced by the presence of a body, any edge conditions, and Kelvin’s circulation theorem, with each constraint adding a Lagrange multiplier to the overall saddle point system. Sharp edges are treated by decomposing the body forcing Lagrange multiplier into a singular and non-singular part. To enforce the Kutta condition, the non-singular part can then be tuned to remove the singularity introduced by the sharp edge. The equations are discretized on a staggered Cartesian grid and solved using the lattice Green’s function. The accuracy of these computations is demonstrated for a flat plate shedding singular vortex elements in 2D and the extension to 3D flows will be discussed. [Preview Abstract] |
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Y12.00006: Piezoelectric energy harvesting of an inverted flag behind a bluff body Oluwafemi Ojo, Kourosh Shoele Piezoelectric inverted flags can be employed to harness energy from unidirectional flows. The bluff body can be employed to reduce the wind speed at which inverted flags exhibit sustained large amplitude vibration as well as produce vortex streets that oscillate the piezoelectric structure. In this study, we employ large deformation coupled electric-structure-flow interaction model and explore the piezoelectric energy harvesting of an inverted piezoelectric flag behind a bluff body subject to uniform unidirectional flow. Flags with different aspect ratios were simulated with several representative cylinder diameters and their electric energy harvesting efficiency is compared. The roles of flow conditions, structural parameters and electrical setup on the oscillatory behavior of the flag are assessed and combined to predict the optimal parameters that ensure maximum energy harvesting. Also, we employ a nonlinear modal model of the structure and dissect the fluid forces based on their causes to capture the role of different flow features on the flapping dynamics of the inverted flag. [Preview Abstract] |
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Y12.00007: Numerical Investigation of Open and Closed Von Kármán Flow Nidhi Sharma, Joshua R. Brinkerhoff Direct numerical simulations of flow between two counter-rotating coaxial disks with and without cylindrical side walls are carried out to investigate the developed turbulent swirling flow. The developed von K\'{a}rm\'{a}n flow when enclosed within a cylinder is defined as the closed von K\'{a}rm\'{a}n flow and without cylindrical side walls as open von K\'{a}rm\'{a}n flow. The validation study carried out for the smooth flat disks for high radius to height ratio, shows good comparison with the results reported in Gauthier \textit{et al.}\footnote{G. Gauthier et al., \textbf{J.Fluid Mech.} 121, 473, (2002)}. For high value of Reynolds number of the top disk, ($Re_t$), the flow shows the formation of negative spirals extending from the periphery of the disks to the center. On increasing the value of $Re_t$, only positive spirals are observed. From the radial, tangential and axial components of velocity field, high radial and axial bulk flow is noticed for open von K\'{a}rm\'{a}n flow. The vortex extraction method of Q-criteria shows strong and stable vortical structures formed at intermediate times, existing up to later times. The impact of coherent structures on the turbulent kinetic energy budget is ascertained. Grid convergence study is performed using three refined grids. [Preview Abstract] |
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