Study of decay, spread, and shock structure of a supersonic jet issuing from a C-D nozzle with semi-circular grooves

Two diametrically opposite internal grooves were introduced in the diverging section of a supersonic nozzle. Experiments were conducted to find the effect of grooves at three different nozzle pressure ratios ( NPR s), i.e., 3.6, 5.5, and 7.2 and comparisons were made with circular jet without grooves. Data reported here are for a design exit Mach number of 1.8 with an area ratio 1.44. Results are presented as pitot pressure decay along the extended geometric centerline of the nozzle and the radial distribution of pitot pressure at various axial ( x ′ = 1, 3, 5, 8, 12) locations. Empirical relations have been developed to identify the lengths of various regions involved in the pitot pressure decay (along the X -direction). Groove effectiveness was used as a parameter to find the effect of grooves on the jet decay characteristics. The jet from grooved nozzle exhibited totally different results compared to that from plain circular nozzle. Results show that grooves played a more significant role at overexpansion condition of the nozzle than at under-expansion case. Schlieren flow visualization technique was used to capture the shock cell structure of the jets for the same NPRs and it was found that the structure of the jet emanating from the grooved nozzle was highly distorted. To understand decay characteristics, supersonic core length and the pitot pressure decay lengths for 50 % decay and 90 % decay were determined from the pitot pressure data along the extended centerline of the nozzle. A significant reduction in spatial pressure fluctuations and supersonic core length were observed at overexpansion condition of the nozzle when grooves were introduced.