This work aims at understanding the physics governing the effect of mechanical tabs on the vortical structures in the near field of jet mixing region. Jets from a sonic nozzle with and without tabs operated at nozzle pressure ratios from 2 to 7 were studied in the present investigation. Tabs with various combinations of length to width ratios were investigated by keeping the blockage area constant. The tabs offered a blockage of 10.18 percent of nozzle exit area. It was found that when the tabs are introduced, two pairs of counter rotating streamwise vortices are shed all along the edges of the tabs and these vortices act as effective mixing promoters. It is evident from centerline pitot pressure decay that, for the tabbed jet a maximum core reduction of 75% was achieved for NPR 7 compared to uncontrolled jet. Direct shadowgraph technique was employed to capture the waves in the controlled and uncontrolled jets. It showed that the tabs are effective in weakening the shock structure in the jet core. To gain an insight into the jets spread rate and the distortion of tabbed jets a surface flow visualization method was developed and employed. Presence of two pairs of streamwise vortices in the vicinity of nozzle exit and the bifurcation of the jet field at the downstream for the tabbed jets were also captured by the surface coating technique.
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