Vortex shedding resonance from a rotationally oscillating cylinder

Vortex shedding resonance of a circular cylinder wake to a forced rotationa l oscillation has been investigated experimentally by measuring the velocit y fluctuations in the wake, pressure distributions over the cylinder surfac e, and visualizing the flow field with respect to cylinder oscillations. Th e vortex shedding resonance occurs near the natural shedding frequency at s mall amplitude of cylinder oscillations, while the peak resonance frequency shifts to a lower value with an increase in oscillation amplitude. The dra g and lift forces acting on the cylinder at fixed forcing Strouhal number i ndicate that the phase lag of fluid forces to the cylinder oscillations inc reases with an increase in oscillation amplitude, supporting the variation of resonance frequency with oscillation amplitude. The comparative study of the measured pressure distributions and the simultaneous flow visualizatio ns with respect to cylinder rotation shows the mechanisms of phase lag, whi ch is due to the strengthened vortex formation and the modification of the surface pressure distributions. (C) 1998 Academic Press.