Computation of three-dimensional flows past circular cylinder of low aspect ratio

Abstract

Results are presented for finite element simulation of three-dimensional unsteady flows past cylinders of low aspect ratio. The end-conditions are specified to model the effect of a wall that may correspond to the flow in a wind tunnel, water channel or a tow-tank experiment with a cylinder having large end-plates. Results are computed for Reynolds number 100, 300, and 1000 for a cylinder of aspect ratio 16. The computations confirm that it is the end conditions for the finite cylinder that determine the mode of vortex shedding (parallel or oblique). Preliminary results for Re=106 flow past a cylinder of aspect ratio 8 are also presented. The Re=100 and 1000 flows exhibit oblique mode of vortex shedding. The flow for Re=100 is very organized, devoid of any vortex dislocations and is associated with only one cell along the cylinder span. The flow at Re=1000 is interspersed with vortex dislocations and the vortex shedding angle varies, both, temporally and spatially. The presence of vortex dislocations is responsible for the breakdown of spanwise coherence of vortex structures. Mode B pattern of vortex shedding is observed. Flow at Re=300 results in flow patterns that correspond to the wake transition regime. Mode A and Mode B patterns of vortex shedding in addition to vortex dislocations are observed at different time instants. The present results indicate that the wake transition regime, that is known to occur in the Re range 190–250 for large aspect-ratio cylinders, is either extended and/or delayed for a cylinder of small aspect ratio with “no-slip” walls.