An experimental study of flow past an oscillating cylinder

We present preliminary experimental results on flow past an oscillating cylinder at frequency ratios varying from 0 to 5 and oscillation amplitudes varying from $\pi /8$ to $\pi $. The experiments are conducted at Reynolds number of 185. The frequency ratio, f$_{R}$, is defined as the ratio of cylinder oscillation frequency to vortex shedding frequency from a non-oscillating cylinder. The diagnostic is done using hydrogen bubble technique for flow visualization in a plane. It is found that at one diameter downstream from the cylinder, vortex shedding frequency matches the forcing frequency (lock-on) at all f$_{R} \quad >$1 and all amplitudes; however, for f$_{R} \quad <$1 there is a window adjacent to f$_{R}$ = 1 where lock-on occurs and this depends on oscillation amplitude. In the far wake at nine diameters from the cylinder, the lock-on region is centered around f$_{R}$ = 1 and depends strongly on amplitude. The visualization of the vortical structures showed that near f$_{R}$ =1.0 the vortices became very compact and well formed in the visualization plane. Their size decreases considerably at much higher f$_{R}$'s in the near-wake. The motion pictures reveal interesting phenomenon of merger of vortices at certain f$_{R}$'s.