Visualisation of underexpanded jet flowfields in the presence of an intrusive tab

This paper investigates the influence of a single intrusive tab has on the flowfield of a supersonic jet. When a tab is added to the jet, an increase in the fundamental screech frequency is observed in acoustic spectra. High resolution schlieren imaging reveals alteration of both the shock structure and preferred instability mode in the jet. Two-point spatial correlation of schlieren images shows the transition from a lateral flapping mode to a punctuated toroid screech mode. This change in dominant instability mode produces a change in coherent structure wavelength that correlates well to the observed screech frequency shift in the acoustic data. The high quality of the schlieren images also allows the counter-rotating vortex pair produced by an individual tab to be identified. Images of the radial density gradient are used to estimate vortex pair location, and a statistical estimate for the trajectory of the vortex pair is produced. The vortex pair path identification technique shows that the path of the vortex pair is constrained close to the exit of the nozzle and increasingly variant in space with greater downstream distance. The vortex trajectory is shown to alter the sonic line and thus the spatial extent of the embedded shock structures.