Effect of an external excitation on the flow structure in a circular impinging jet

The effect of an external excitation on circular impinging jet flow is studied experimentally. Velocity fields in moderate air flows (Re=10000) are investigated by hot-wire anemometry. The basic flow is excited by a small sinusoidal modulation of the nozzle exit velocity. The phase-averaging technique is used to study the behavior of vortex structures in the jet, specifically rolling-up, pairing, and interaction with the wall. The jet is found to be sensitive to excitation in the frequency range characterized by a Strouhal number, Ste=feD∕U, from 0.3 to 3. Different flow regimes are identified in the excited impinging jet: a periodic flow regime with the same frequency as the excitation, a regime with a frequency corresponding to a subharmonic of the excitation frequency, a regime alternating between these two frequencies, and two border regimes with more complicated behavior. The low-frequency excitation leads to the formation of vortices, which are larger than those occurring in an unexcited jet. Conseq...