VORTEX PAIRING IN A CIRCULAR JET UNDER CONTROLLED EXCITATION - 1. GENERAL JET RESPONSE.

Hot-wire and flow-visualization studies have been carried out in three air jets subjected to pure-tone acoustic excitation, and the instability, vortex roll-up and transition as well as jet response to the controlled excitation have been investigated. The conditions most favorable to vortex pairing were determined as a function of the excitation Strouhal number, the Reynolds number (Re//D), and the initial shear-layer state. It is shown that the rolled-up vortex rings undergo pairing under two distinct conditions of excitation: ″the shear layer mode″ when the Strouhal number based on the intitial shear-layer momentum thickness is about 0 multiplied by (times) 012, and ″the jet column mode″ when the Strouhal number based on the jet diameter is about 0 multiplied by (times) 85. The trajectories of the vortex centers and their convection velocities during a pairing event have been recorded through phase-locked measurements. Refs.