Conditional sampling study of acoustically active turbulent structures in cold subsonic jets

An instantaneous distribution of acoustic sources in the sense of Lighthill can be defined through the causality method in nonaveraged form. This method is applied to a cold subsonic jet on the basis of velocity fluctuations provided by a hot wire probe and the far-field radiated acoustic pressure. The intermittent character of the instantaneous distribution of each point in the jet is revealed. As an example, the method is applied to a point downstream of the potential cone. It is observed that 70 percent of the acoustic energy is radiated during less than 20 percent of the time.