An experimental study on the control of plug nozzle jets

Experimental studies are performed for the near- and far-field characteristics of a plug nozzle jet with the objective to see the effect of truncation of the plug on mean flow characteristics of the jet and its control using vortex generators (VGs). To this end, the plug is truncated at 50%, 40%, and 30% of the axial length from the throat, and mean flow characteristics are studied by the schlieren technique and a pitot pressure survey. Nozzle pressure ratio (NPR) is varied from 2 to 8 for a Mach 1.8 nozzle designed to give correctly expanded flow at NPR 5.74. During over-expansion mode, all the truncation levels are observed to increase core length and strengthen shock cells of the jet, whereas, during under-expansion, truncation up to 50% of the plug is not found to alter the core length. To promote mixing, two rectangular VGs of different aspect ratios but same blockage area of 2% are attached to the base of the truncated plugs. The influence of pressure ratio on the flow structure of controlled and uncontrolled jets is discussed in detail. Results have shown VGs to be effective in mixing enhancement and weakening of shock cells over the nozzle pressure ratios studied.