Secondary vortex street in the intermediate wake of a circular cylinder

Abstract

Particle image velocimetry measurements are carried out in the wake of a circular cylinder at two values of the Reynolds number (3460 and 5800), based on the free stream velocity and the cylinder diameter, to investigate the spatial organization of vortical motions in the intermediate wake. The proper orthogonal decomposition method (POD) is used to extract information from the vorticity data. While the coherent motion associated with the von Karman vortex street is well reflected in the first two POD modes which account for about 8% of the total enstrophy, the motion associated with the secondary vortex street is captured in the third and fourth POD modes which account for only about 2.4% of the total enstrophy. The measurements show that the secondary vortex street only alternates in space. This is significantly different from the far wake where there is continuous switching between symmetric and antisymmetric arrangements about the centreline (e.g. Bisset et al., J Fluid Mech 218:439–461, 1990), the alternative regime being nearly twice as frequent as the opposite regime.