Visualization of stagnation point inside the closed wake of a 20%-truncated plug nozzle at starting process

Flow visualization using the background-oriented schlieren (BOS) technique was used to investigate the characteristics of the stagnation point shift near the nozzle base of a 20%-truncated planar plug nozzle designed for a Mach number of 2.87 at the nozzle exit. The nozzle pressure ratios ranged from 7 to 120. A shock tube with a cross-section of 3×4 cm3×4 cm is used as a high-pressure gas reservoir. The obtained images were compared with the Navier–Stokes numerical solution obtained with the weighted averaged flux method. First, to confirm the establishment of steady flow, the starting process inside the nozzle was visualized using shadowgraph. Next, BOS images were used to determine the location of the stagnation point inside the closed wake. The images revealed that the distance of the stagnation point from the base surface was 1.3 times the throat width after the closed wake mode was established.