The visualization of the acoustic feedback loop in impinging underexpanded supersonic jet flows using ultra-high frame rate schlieren

The use of modern ultra-high speed cameras to acquire time-resolved schlieren image sequences of supersonic jet impingement is presented. The use of these cameras, with framerates of up to 1 million frames per second, allows for the first time-resolved visualizations of the impinging jet acoustic feedback loop. The role of upstream travelling acoustic waves in generating perturbations in the jet shear layer at the nozzle exit is also directly observed for the first time. The arrival of the acoustic wave at the nozzle lip generates a sinusoidal variation in density gradient that persists until a distance of $$\frac{x}{d}=0.3$$. A structure that rapidly evolves into a large-scale vortex ring forms at the trailing edge of this initial instability, first observed at approximately $$\frac{x}{d}=0.25$$.Graphical abstract