Blockage effect on the wake of a rotationally oscillating linearly tapered cylinder*

The wake of a linearly tapered cylinder executing rotational oscillations constrained by two stationary walls is studied experimentally at Reynolds number (based on mean diameter D_m) of 250. The tapered cylinder is forced to perform rotational oscillations at various oscillation amplitudes and normalized forcing frequencies. Tapered cylinders are characterized by taper ratio defined as the ratio of the length to the difference in diameters at the two ends and the cylinder used in the present study has a taper ratio of 70:1 and a mean diameter D_m of the cylinder is 8mm. Laser Induced Fluroscence technique is used for flow visualization of the wake structure. The normalized gap distance between the walls were T/D_m=2,4,6 for the present study. For T/D_m=2, it was found that the vortex shedding is supressed till FR<=0.4 and with further increase in FR, vortex shedding starts. At high forcing frequencies (FR>=2.5) vortex shedding again ceases. At T/D_m=4, the range of forcing frequencies for which the vortex shedding occured widens and at T/D_m=6, the wake behaves like an isolated and unbounded rotationally oscillating tapered cylinder. PIV experiments were performed to validate the flow visualization results.