Experimental investigation of flow past a rotationally oscillating tapered cylinder

The flow downstream of a linearly tapered cylinder executing rotational oscillations is experimentally studied at Reynolds number (based on mean diameter) of 250. The tapered cylinder is forced to rotationally oscillate at various oscillation amplitudes and normalized forcing frequencies. The hydrogen bubble technique is used for visualizing the spanwise wake structure, and the laser-induced fluorescence technique is used for visualizing the streamwise wake structure. The cylinder used in the current study has a taper ratio of 70:1 with a mean diameter of 8 mm. Oblique shedding with an oblique angle of 16 degrees is seen for a stationary cylinder which gradually reduces and eventually changes to parallel shedding with an increase of forcing frequency up to a certain forcing frequency which is also depen-dent on the oscillation amplitude. Flow visualization revealed the forcing frequencies and amplitudes for which the flow becomes two-dimensional. Spanwise cellular structures are observed for some specific forcing parameters. For an oscillation amplitude of 3 pi /4 and normalized forcing frequency of 2, a three-dimensional mode is formed at the upper half of the cylinder with a spanwise wavelength of 1.6. Frequency content of the wake measured by hot-film anemometry showed that the flow is governed by multiple frequencies when cellular structures in the wake are observed. It is seen from hot-film measurements that the range of forcing frequencies for which the wake undergoes lock-on increases with increasing oscillation amplitude. A direct drag measurement technique showed that the drag for a stationary tapered cylinder is -10%-20% less than that of a stationary straight cylinder. Particle image velocimetry (PIV) data revealed that the circulation of spanwise vortices loses its strength as we move from the larger diameter end to the smaller diameter end. The drag coefficient for forcing cases is estimated using the momentum deficit formula by finding the mean and fluctuating velocities obtained from PIV. When the forcing frequency enters the lock-on initiation zone, the coefficient of drag becomes maximum, and with further increase in forcing frequency the drag coefficient decreases. The maximum value of the drag coefficient increases as the oscillation amplitude is increased from pi /2 to 3 pi /4.