An experimental investigation of coupled underexpanded supersonic twin-jets

High-resolution particle image velocimetry measurements of coupled underexpanded twin-jets are presented. Two nozzle pressure ratios are examined, which are selected due to a change in coupled plume mode indicated by a discontinuous jump in screech frequency. Estimates of the turbulent flow statistics, shear-layer thickness, merge point, inter-nozzle mixing, and integral length scales are provided. The higher nozzle pressure ratio case shows a strong standing-wave present in the velocity fluctuation amplitude and integral length scale. The ratios of standing, acoustic, and hydrodynamic wavelength are compared and find a close fit to Panda's relation for screech. This indicates that screech in the twin-jet system operates with similar length-scale and frequency characteristics to single jets and provides evidence to suggest screech is an integral part of the twin-jet coupling process. Second-order spatial velocity correlation maps reveal the larger modal structure. A symmetric mode is found for the higher pressure ratio and a weakly symmetric mode for the lower. Comparison is made between where the standing-wave is present and where it is not. It is found that the standing-wave, not the shock structure, is the driver of turbulence coherence modulation near the jet. In regions that are affected only by the standing-wave, it is found that it contributes to both the turbulence intensity and coherence modulation.