Numerical simulation of viscous flow around a step cylinder

Abstract

Flow around a step cylinder at was investigated numerically with a diameter ratio of $D/d=2$ and Reynolds number $Re_D=600$. The open source software OpenFoam was used for the numerical simulations. The primary focus in this thesis was on vortex shedding and vortex interaction occurring in the wake. The present study showed good agreement with previous numerical and experimental findings.

Three distinct vortex cells was identified in the step cylinder wake. The S-cell forming in the wake of the small cylinder and the N and L-cell forming in the wake of the large cylinder. With that of the S-cell having the highest shedding frequency and the N-cell having the lowest. Due to the difference in shedding frequency vortex splitting and vortex dislocation occurred in two vortex interaction regions located between the adjacent cells. The present result suggest that vortex splitting, loop and half-loop connections and direct cross boundary connection occurs at the cell boundary. Similar vortex connections at the cell boundaries identified by Mortan and Cai \cite{Morton, Cai2} at $Re_D=150$ was also captured in the present study, but severely complicated and weakened by the presence of streamwise vortices forming in the wake.

The N-cell exhibited a cyclic behavior with fluctuations in the duration of the cycle. The N-cell cycle was linked to the spanwise velocity near the step which had a low frequency component corresponding to the average N-cell cycle frequency. The vortex dislocation position was also found to be fluctuated with time and was speculated to fluctuate due to the relative phase difference between the N and L-cell. The fluctuation of the N-cell cycle and vortex dislocation position have never been reported in a numerical study before.

Due to the presence of the step a streamwise vortices pair called the "edge vortex" and oblique vortex shedding in the large cylinder wake was observed.