Streamwise vortices in an underexpanded axisymmetric jet

This Letter describes an experimental investigation demonstrating the existence of streamwise vortices in an underexpanded jet issuing from a convergent axisymmetric nozzle at a moderate pressure ratio of 5.1, and at a Reynolds number of 1.8×106. These vortices are quite strong and are located in the supersonic region of the jet between the intercepting shock and the jet boundary. They seem to play a significant role by enhancing mixing between the ambient and jet fluids in the supersonic region of the jet. The scale of these vortices increases with downstream distance by a process of vortex merging.