An experimental investigation of a rotationally oscillating cylinder

The flow patterns of the wake region of a rotationally oscillating cylinder were investigated experimentally by using the Particle Image Velocimetry (PIV) technique and dye visualization. The experiments were conducted at Reynolds number R-e = 1000 and different values of oscillation amplitude (theta(A) = 60 degrees, 120 degrees, 180 degrees) and frequency ratio (F-R = 0.8, 1.7, 3.3, 6.7). Dye visualizations, instantaneous vorticity contours, time-averaged streamlines, vorticity, streamwise and crossflow velocity contours, Reynolds stress correlations, turbulent kinetic energy (< TKE >) and autospectral density contours were analysed. For F-R = 0.8, the vortices formation behind the cylinder wake disappeared and the only saddle point formation was observed at theta(A) = 120. and theta(A) = 180 degrees. The maximum and minimum shear stress < u'v'> were obtained at F-R = 0.8, theta(A) = 180 degrees and F-R = 3.3, theta(A) = 120 degrees, respectively. The turbulent kinetic energy decreased with the increase in the frequency ratio at theta(A) = 180 degrees, and it has been observed that the vortex shedding weakens the interaction between the upper and lower vortices of the cylinder. The maximum decreasing in the drag coefficient of the stationary cylinder was by around 46% at F-R = 6.7 theta(A) = 120 degrees. It is concluded that the rotationally oscillating cylinder has a significant effect on the cylinder wake region and changes the wake flow structures and vortex shedding patterns.