Coherent structure and sound production in the helical mode of a screeching axisymmetric jet

The structure of a screeching axisymmetric jet in the helical C mode at a nozzle pressure ratio of 3.4 issuing from a convergent nozzle is studied using high-resolution particle image velocimetry. Proper orthogonal decomposition (POD) is used to extract the dominant coherent structures within the jet. The first two modes produced by the POD are used to reconstruct a phase-averaged data sequence. A triple decomposition into mean, coherent and random velocity components is performed. The embedded shock structures within the jet are shown to strongly modulate the coherent axial stresses within the shear layer and to weakly modulate the random axial stresses. Analysis of the third and fourth moments of the velocity probability density function is used as an indicator of possible regions of shock–vortex interaction and thus screech tone generation. Peaks of kurtosis (flatness) occur at the second, third and fourth shock–boundary intersection points, with the radial position shifting towards the centreline with increasing downstream distance. Analysis of the coherent component of vorticity shows that the largest fluctuations in coherent vorticity occur at the high-speed side of the shear layer in an area extending from the second to the fourth shock cell. With reference to prior literature, the argument is made that it is this increased magnitude of coherent vorticity fluctuation that is the primary factor in the determination of which shock cells act as dominant screech sources.