Flow structure in the locked-on wake of a circular cylinder in pulsating flow: Effect of forcing amplitude

The wake structure of a circular cylinder in non-reversing pulsating flow is investigated by means of particle image velocimetry. Measurements are reported for pulsations at twice the natural frequency of vortex shedding in the unforced wake and for forcing amplitudes up to 23% at a constant Reynolds number of 2150. For the parameters employed the vortex shedding locks-on to the sub-harmonic of the pulsation frequency and a resonant wake is formed. The mean velocity field and the distributions of the total and coherent Reynolds stresses show that the averaged wake structure is systematically modified by the flow pulsations. It is shown that this modification is caused by changes in the dynamics of vortex formation and shedding primarily in the near wake. The recirculation bubble and the vortex formation region decrease in size, the global peak values in the distributions of the total and coherent Reynolds stresses increase asymptotically whereas the vortex strength and the mean drag coefficient increase almost linearly with increasing forcing amplitude. The measures of the minimum wake width in the formation region do not exhibit any variation with amplitude even though the wake downstream of the formation region is wider in pulsating flow due to the increased strength of the shed vortices.