Rayleigh-Scattering Measurements in Underexpanded Jets

A report describes experiments in which time-averaged and unsteady local variations in the densities of underexpanded supersonic free jets issuing from a choked circular nozzle were measured by laser-induced Rayleigh scattering. This study is part of a continuing effort to understand the generation of screech noise by supersonic jets. The Rayleigh scattering technique used dust-free air for primary and entrained flows, a continuous-wave laser, and photon counting electronics for reliable and accurate measurement. Time-averaged radial density profiles obtained at various axial stations ranging to 10 jet diameters downstream show the development of a jet shear layer and the decay of shock cells. Data on unsteady density variations show the evolution of large turbulent vortices modulated periodically along the flow direction. Comparison of data from these measurements with data from previous measurements revealed the following: The periodic modulation in density and convective velocity of turbulent vortices coincides with the modulation of pressure fluctuations outside the flow boundary. The spatial periodicity of modulation is different from the shock spacing and is associated, instead, with a standing wave. The standing wave is formed between downstream-moving turbulent vortices and the upstream-propagating screech sound waves. It extends from inside the shear layer to the near-field outside the flow. All of these indicate that the sound sources are located a standing wavelength apart.