Particle image velocimetry analysis of the twin supersonic jet structure and standing-wave

High resolution particle image velocimetry measurements of under-expanded twin-jets are presented. Two nozzle pressure ratios are examined, which are selected due to a change in coupling family indicated by a discontinuous jump in screech frequency. Estimates of the turbulent flow statistics, shear-layer thickness, and shear-layer merge point are provided. The effect of the standing-wave on the velocity field is quantified for the first time. The higher nozzle pressure ratio case shows a strong standing-wave with a mismatch between the shock-cell spacing and standing-wavelength. A very weak standing-wave is observed in the lower pressure case, which more closely matches the shock-cell spacing. A snapshot based proper orthogonal decomposition of the instantaneous velocity fields reveal a symmetric coupling mode for the higher pressure case, which could either be flapping or helical in nature. The lower pressure case provided little evidence for plume coupling belonging to an in-plane oscillation.