Flow dynamics of an axisymmetric impinging jet under two-frequency external forcing. A study by time-resolved PIV and DMD

The present paper reports on the experimental study of the dynamics of large-scale vortex structures in an axisymmetric impinging jet forced periodically with amplitude modulation. The velocity field measurements are conducted by the particle image velocimetry method. IR imaging is applied to evaluate the effect of external perturbations on local heat transfer coefficient. The Reynolds number of the jet flow is 12,500 and the distance between the nozzle and flat impingement plate is two nozzle diameters. The amplitude modulation corresponds to two-frequency forcing with a main harmonic (for the Strouhal number 0.5) and different sub-harmonics. To analyze quasi-periodic formation and near-wall merging of primary vortex structures and induced secondary vortices in the near-wall boundary layer, the obtained time-resolved velocity data sets are processed by the dynamic mode decomposition method. It is shown that the amplitude modulation can be efficiently used to attenuate the strength of the forced vortices due to the produced blowing events with different momenta and to control vortices pairing process in the near-wall region. It is also shown that two-frequency forcing can provide slightly greater intensification of heat transfer around the stagnation point with considerably smaller energy, consumed during the forcing.