Experimental study of interaction of wall boundary layer and vortices in the wake of a rotationally oscillating cylinder

The wake of a rotationally oscillating cylinder in presence of a plane wall with various gap to diameter ratio is studied experimentally for Re=190 and Re= 250. Effects of forcing frequency and oscillation amplitude on the wake structure is studied. It is found that flow transitions from nominally two dimensional to three dimensional at certain forcing parameters. The planar boundary affects the flow physics of the wake behind the oscillating cylinder which in turn affects the Strouhal number, transition regime and drag behind the cylinder. Qualitative analysis using laser induced fluorescence and hydrogen bubble flow visualization were performed for various forcing parameters. Quantitative analysis providing information on wake lock-on characteristics and Reynolds number-Strouhal number relationship are performed using hot wire anemometer measurements. Study of three dimensional effects behing the oscillating cylinder showed presence of two new modes and these modes didnot change with the presence of wall at any gap to diameter ratio.