Abstract
Wake structures behind a circular cylinder at Reynolds number of 3.7 × 104 are investigated using a computer aided flow visualization technique, which includes our newly developed conditional sampling procedure. Phase-averaged velocity distributions in the boundary layer on a cylinder are measured and the process of generation and shedding of vorticity are presented. Total circulation discharged from the cylinder surface in a shedding cycle is directly evaluated. Phase-averaged stream-lines and vorticity fields in the wake are obtained, and using these results the coherent structures of the wake flow, i.e., geometrical arrangement, convection speed, a total circulation of a shedding vortex and inner structures of vortices are described. Less than 40% of the discharged vorticity appears in a shedding vortex, and a vorticity distribution in the vortex in the near wake is quite similar to that in an isolated theoretical viscous vortex. But the spacing ratio, convection speed and the drag coefficient obtained do not agree with those calculated from a classical theory.
