Oscillation modes of underexpanded jets issuing from square and equilateral triangular nozzles

The oscillation modes of underexpanded jets exhausted from square and equilateral triangular nozzles were investigated experimentally. Two stages were observed for both jets. The oscillation modes corresponding to these stages were identified by using optical and acoustical methods. The experimental results show that oscillation modes for both jets are first axisymmetric and then sinuous with increasing jet pressure ratio. The helical oscillation mode, which appeared for the circular jet, was not observed in these jets. The hysteresis phenomenon found for the circular jet cannot be seen in these jets.