Oschillations of an impinging turbulent jet: Coherence characterization via conditional sampling

By using zero crossing statistics, in conjunction with recursive digital filtering, to determine self- and cross-probability densities of velocity and pressure, the degree of phase fluctuation of organized oscillations of turbulent jet flow through a cavity is characterized as a function of mean phase deviation from the "lock-on" condition; "lock-on", producing maximum pressure amplitude, corresponds to a time-averaged phase difference between organized velocities at the exit and entrance of the cavity of 2 nπ. As a frequency jump is approached, there is mean phase deviation from this 2 nπ condition, and the degree of phase (or period) fluctuation increases; at the jump, cross-probability densities show no discernible coherence. Concerning the evolution of the jet in the streamwise direction, growth of the time-averaged organized wave amplitude, obtained by using a wave-education method, is shown to be associated with a decrease in phase fluctuation of the velocity. However, self-probabilities of pressure, and crossprobabilities between pressure and exit velocity, are essentially invariant between cavity exit and inlet, reflecting dominance of the acoustic contribution to the unsteady pressure field at separation.