Shock Motion and Flow Structure of an Underexpanded Jet in the Helical Mode

Abstract

A large-eddy simulation is performed to study the shock motions and flow structures of an underexpanded free jet. The jet is characterized by a nozzle pressure ratio of 3.4 and a fully expanded Mach number of 1.45. At this condition, the dominant instability mode of the jet is the helical (C) mode. The numerical results of mean velocity fields, shock structures, and the screech frequency are in good agreement with the experimental results. A single helical vortex appears in the jet shear layer, which coincides with the helical mode. At the fourth shock cell, the dominant acoustic source location for the screech tone, the shock interacts with vortices and oscillates periodically. The flow structures associated with the generations of the screech tone and the second harmonic are analyzed by the proper orthogonal decomposition. The coherent structures with the azimuthal wave number and 2 are related to the screech tone and its second harmonic, respectively.