Effect of In Phase and Out of Phase Forcing on Circular Cylinder Wake

Abstract

Using periodic forcing, the control of instabilities in the wake of a circular cylinder was investigated. The cylinder model had two straight slits, one on each side, that were connected to separate low frequency and high frequency acoustic drivers. In order to provide equal periodic forcing, the top and bottom slits were separated by a partition located in the middle of the cylinder. This feature also allowed the implementation of in-phase (IP) and out-of-phase (OP) forcing. The experiments were conducted at six different forcing frequencies and Reynolds numbers of 12,000 and 24,000. Excitation frequencies (fe) ranging from 1/2fs0 to 4fs0 were utilized in an attempt to decrease and eliminate the amplitude of von Karman vortex shedding. A unique random noise frequency excitation was also investigated and was found to similarly suppress the oscillatory wake. Forcing from the two dimensional slit was able to disrupt the formation of the von Karman instability, therefore reducing the oscillatory wake and the pressure drag. A maximum reduction of 28% was observed for the in-phase driver mode and 26% for out-of-phase. At the lower Reynolds number the wake was more responsive to the periodic forcing and required a lower blowing coefficient. It was found that the out-of-phase excitation caused the shedding frequency to lock on to the corresponding excitation frequency until approximately 4fs0 . In-phase forcing was more efficient at suppressing the cylinder wake, while the out-of-phase driver mode had more control authority.