Interaction of external flow with linear cluster plug nozzle jet

Plug nozzle flow in the presence of external flow is experimentally investigated at free-stream Mach numbers of 0.0, 0.7, 0.9, 1.1, and 1.5. The internal nozzle is operated in overexpanded and underexpanded regimes for these external flow Mach numbers. For the subsonic external flow, slight changes in the plug jet flow are observed as compared to the no-external-flow case. The supersonic external flow introduces substantial changes to plug flow jet structure as well as the wave interactions within it, as compared to the no-external-flow case. For the overexpanded internal nozzle operation, the oblique shock waves are asymmetric within the plug core jet. For underexpanded internal nozzle operation, an oblique shock formation is noticed on the plug surface which induces flow separation downstream. The RMS of the pressure fluctuations indicates a distinct peak corresponding to the foot of the overexpansion shock on the plug surface. These peaks are observed at consistent pressure ratios for all the external flow conditions considered. Regarding the cluster plug nozzle flow field, three-dimensional flow features are noticed because of the intermodule interactions. For both the overexpanded and underexpanded internal jet conditions, the interactions between the modules are predominantly compression waves in the spanwise direction. The RMS of the pressure fluctuations measured along the intermodule centreline suggests that the spanwise wave interactions have considerable influence on the flow unsteadiness.