Exponential-algebraic transition in the near-field of low supersonic jets

Abstract

The paper is concerned with the transition from exponential to algebraic cross-stream decay of instability waves in supersonic axisymmetric cold jets. The cross-stream structure of these waves is analysed for phase velocities close to the sound speed when the streamwise inhomogeneities of the mean flow are characterized by a small parameter. It is shown how algebraic decays are completely compatible with the features of the near-field, and a selection criterion for the occurence of such a behaviour is given. Near-field pressure fluctuations are then determined as functions of azimuthal properties of instability waves and control parameters. In some cases, where the dominant contribution to the sound generation is due to the axisymmetric Kelvin–Helmholtz instability mode, algebraic decays are not confined in the core region of the jet and the acoustic wavelength becomes larger than the envelope scale.