Experimental study of the near wake of a circular cylinder and its detached shear layers

The near wake of a cylinder (x < 1.6D) and its detached shear layers are studied at Re = 3.5 x 1(0)4 through phase averaging of 2D-PIV snapshots using as trigger the pressure at phi = 90 degrees. The shedding cycle is examined at 8 time instants as well as at 16 time instants based on POD analysis. In the shear layer region, the flow becomes fully turbulent at x similar to 0.45D according to the momentum thickness spatial variation, whereas based on the velocity profile shapes there is a transition from convective to absolute type of instabilities at the same x station. The Karman vortices once formed at (0.8D, +/- 0.5D),accelerate streamwise towards the wake centre line following curved trajectories with speeds much smaller than those at the distant field. The Reynolds stresses and the turbulent kinetic energy production of each phase exhibit peaks downstream of the vortex centres in contrast to the spanwise vorticity, swirling strength and turbulent kinetic energy which maximize at the vortex centres, seen from a moving frame of reference. In the wake, normally <(v)over bar>'(2) > (u) over bar'(2) in contrast to the shear layer region in which (u) over bar'(2) > (v) over bar'(2). The Reynolds shear (u) over bar 'v being the smallest of the Reynolds stresses in the wake, shows two peaks of opposite sign, one being close to the vortex. The pressure at phi = 90 degrees (which might be related to the lift applied on the cylinder) takes its peak values with a time delay after the Karman vortex formation and particularly when the latter passes through x similar to 1.05D, its fluctuations being high or low according to the size of the vortex.