Secondary wake mode instabilities induced by vortex-structure interaction

Experimental tests were conducted in a small wind tunnel to investigate the effect of mechanical cylinder oscillation in the 'inflow' direction on the wake vortex structure. Depending on the excitation to Karman shedding frequency ratio, mode locked states, in the form of spatio-temporally repeatable patterns, could be observed. The primary pattern involves symmetry breaking of the initially symmetry shed pair of vortices. Analysis based on symmetry (group) theory is presented to support the foregoing experimental observations. By considering two oscillators having inline shedding symmetry and the Karman wake having the spatio-temporal symmetry, the possible symmetries of subsequent flow perturbations resulting from the modal interaction are determined.