ABSTRACT

The vortex shedding mode from an impulsively started rotating and translating circular cylinder is investigated by compact scheme FDM (finite-difference method) for Reynolds number between 200 and 3000. The open boundary condition is utilized as the remote boundary condition with the 4th order accurate wall vortex generation formula proposed by us. The initial time step is increased from 10−8 to 10−2 till the time reaching the normal time step. The FDM scheme is 2nd accurate in space step and 1st accurate in time step. The streamline pattern at Re=1000, α=0.5 is a little different from previous calculated results, the present pattern indicates the double transposition phenomenon just as Coutanceau & Menard (1985b) pointed out. And at Re=3000, α=0.5, the lower vortex is found to be shedding prior to the upper primary vortex. The present vortex mode transition Re number is shifted from 20000 to at least 3000 compared with results calculated by Chang(1991) etc. Compared with the M. Coutanceau's flow visual work in France for Re less than 1000, the present results are good.

1. INTRODUCTION

Because the flow around rotating and translating circular cylinder has two characteristics, limiting the vortex shedding and raising the lift force, so many scholars have been attracted to this problem investigation since the well known Magnus experiment in 1853. Asymptotic analysis, finite difference and flow visualization have been utilized for the problem study. For inviscid fluids, Bachelor(1967)gave a clear explanation about the lift force existence. Due to the asymmetry of the two flows on each side of the cylinder, the speed on the lower side which moves downstream is higher than that on the upper side which moves upstream, leading to the asymmetrical pressure distributions produced by the Bernoulli effect and consequently to a transverse force towards the high velocity side.

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