An experimental study of rectangular under-expanded supersonic jets collision

Abstract

Rectangular under-expanded supersonic jet collision experiment is carried out under different nozzle distances and jet pressures and compared with that in the case of free jet. Experiments indicate that there are four screech tone modes of supersonic jet collision, switched from one mode to another depending on the nozzle distance and jet pressure. Two normal shock waves are present between nozzles as jet pressure is more than 0.5 MPa and nozzle distance is less than 50 mm, radiating a stable screech tone with a frequency of about 3 kHz. With nozzle distance increasing or jet pressure decreasing, a bow shock is present at one nozzle exit and a normal shock wave appears at the other exit with the collision surface oscillating between them. Collision surface might be kept balanced in the centre of two nozzles with a 9 kHz frequency screech tone, however, it is vulnerable to disturbance and would return to the equilibrium position near nozzle exit or oscillate between nozzles with large amplitude. When jet pressure is less than 0.36 MPa and nozzle distance greater than 70 mm, the collision surface substantially oscillates between the nozzles, radiating a screech tone with a frequency of about 1 kHz which decreases with jet pressure decreasing and nozzle distance increasing.