Aerodynamic Characterisation of a Supersonic Jet Using Two-point Laser Doppler Velocimetry

Abstract

Statistical characterisation of the turbulent velocity field in the shear layer of a cold Mach 1.2 axisymetric jet is investigated. Multi-point velocity measurements have been performed using a laser Doppler velocimetry in order to estimate the spatiotemporal correlation functions and the features of supersonic jet aerodynamics most relevant to the acoustic analogies. Of first interest was the aerodynamic structure of the jet. Thus, mean velocity, turbulence kinetic energy and shear stress have been examined. In addition, turbulence spectra have been estimated in both the potential core and the shear layer. From the two-point spatiotemporal correlation function estimates, a cartography of turbulent length scales and convection velocities was eected in the mixing layer of the jet, showing how the dimensions of characteristic turbulent structures vary as a function of position. In addition, the turbulent time scales were calculated in both fixed and mobile reference frames. Using these quantities, estimates of acoustic compactness are obtained as a function of position in the jet and imply that supersonic jet noise sources cannot be considered compact. Finally, a new statistical source model for the spatiotemporal correlation function of velocity fluctuations is proposed which shows good agreement with experimental data.