Coupling modes of an underexpanded twin axisymmetric jet

An experimental investigation into the coupling behavior of screeching axisymmetric twin supersonic jets is presented. Acoustic measurements and schlieren photography are used to identify four distinct coupling modes over a wide range of pressure ratios and nozzle spacings. Nozzle spacing is found to have a significant effect on the selection of these modes as well as the transition between them. A discrete jump in the peak screech frequency is seen to accompany most but not all mode switches. Statistical analysis of schlieren photography provides evidence that the modes consist of two flapping (symmetric and antisymmetric) and two different helical oscillations. These coupling modes are shown to have a limited effect on the convection velocity of the large-scale structures within the jet, and as such, the characteristic length scales are investigated as a source of mode switching.