An openfoam numerical investigation of leapfrogging of vortex rings
Chua, Darren Yong Sheng
Date of Issue2017
School of Mechanical and Aerospace Engineering
In this report, coaxial circular and elliptical vortex rings at AR=3 were simulated with OpenFOAM 2.4.0 by solving the incompressible Navier-Stokes equations. A modified pimpleFoam solver and Large Eddy Simulation (LES) were used in order to reduce the effects of numerical dissipation and capture the large-scale eddy structures. The study aims to investigate if leapfrogging is possible for elliptical vortex rings and whether bifurcation and axis-switching will affect the occurrence. Leapfrogging of two coaxial circular vortex rings at Re=3000, axis-switching of single elliptical vortex ring at Re=2200 were first simulated and validated with existing literature. The results obtained satisfactory comparability at the specified mesh size of R0=30Δx. Two coaxial elliptical vortex rings were then simulated next at Re=2200 and Re=4400. One through pass was observed for both cases even though the dynamics were very convoluted. Bifurcation of Ring 2 (trailing) into 2 side rings and 1 centre ring, and leapfrogging occurred in tandem at the early stages of the simulation. The side rings impinged on Ring 1 (leading) and vortex reconnections were observed. Consequently, only the bifurcated centre ring went through a complete pass. It was observed that Ring 2 axis switched with a higher frequency than Ring 1 for both Re cases after the first full pass. Based on the current initial core size, separation distance and aspect ratio, it was concluded that a second pass is not plausible due to the increasing stream-wise distance between the two rings after the first pass.
Final Year Project (FYP)
Nanyang Technological University