Transition of oscillation modes of rectangular supersonic jet in screech

An experimental investigation has been carried out on supersonic jets issuing from a convergent-divergent nozzle. In order t o treat supersonic jets as close to 2-dimensional as possible, we used a high aspect ratio rectangular nozzle and set side walls from the nozzle exit to about 300mm downstream. We can see a phase locked images of the jet oscillation and the screech sound field by a newly developed visualization technique. Then two modes of jet oscillation can he observed. One is the flapping mode and the other is the symmetrical mode. This is the first case that the symmetrical mode is reported for the rectangular free jet. The shock cell length and the convection velocity are independent of the oscillation mode. However, the jet oscillation frequency of the symmetrical mode is about twice as high as that of the flapping mode, and the jet behavior is quite different each other. In the flapping mode the third shock cell behaves quite dynamically, and the traveling wave ( shock) is observed inside the cell. This wave is actually produced by the interaction between the shear layer vortexes and the shock cell structure, and plays the primary role in screech sound generation.