SUBHARMONIC EXCITATION OF THE SHEAR-LAYER BETWEEN 2 RIBS - VORTEX INTERACTION AND PRESSURE FIELD

An experimental study has been undertaken to examine the effect of subharmonic forcing on a curved, reattaching shear layer behind a rib in a ribbed-duct flow. With first subharmonic forcing, the vortex formation in the shear layer becomes regularized, but no pairing occurs. When the flow is forced at the second subharmonic, two vortices pair prior to reattachment. The pairing is accompanied by significant entrainment and shear-layer growth, which, in turn, leads to a dramatic reduction in the reattachment length. Three vortices appear to merge prior to reattachment when the external forcing is a third subharmonic of the most amplified frequency in the flow. The merging process is observed to be strong, and again leads to significant enhancements in entrainment. Phase-averaged pressures are shown to be related to the passage of vortices with the pressure above a pressure tap decreasing as the vortex or vortex pair passes by it and the pressure increasing as the vortices straddle the tap. The pressure spectra indicate that the initial vortex dynamics dictate the pressure field, with the response frequency, for all forcing frequencies and streamwise locations, being the measured natural frequency prior to pairing.