Visualization of supersonic free jet flow structures subjected to various temperature and pressure ratio conditions

The standard Z-type schlieren visualization technique was used to experimentally investigate the effects of inlet boundary conditions (temperature and pressure) and nozzle geometry (convergent and convergent-divergent nozzles (C-D)) on a supersonic flow structure. The focus was on the Mach disk's length (Lm) and width (Wm). The inlet temperature (Tin) was 123 to 288 K, and the nozzle pressure ratio (NPR = P-in/P-atm) was 3 to 20. When varying the NPR, over and under-expanded flow conditions were observed in the studied nozzle configurations. The results illustrate that when increasing NPR at Tin = 288 K, Lm increases for both over and under-expanded flow conditions. However, when increasing NPR in over-expanded conditions, Wm increases to reach the saturation point and then decreases for a C-D nozzle with a larger exit diameter, while it becomes constant for a small exit diameter. In contrast, Wm increased when increasing NPR for all nozzles in the under-expanded regime. At 123 K < Ti-n < 173 K, the shock cells were shrunken for C-D nozzles at NPR = 5, and subsequently, Lm decreased. Nevertheless, Wm increased until Tin = 153 K and then decreased until Tin = 123 K, which disturbed the Mach disks and shock cells that governed the decrease in Wm.