Measurement of shock structure and shock-vortex interaction in underexpanded jets using Rayleigh scattering

The density field of underexpanded supersonic free jets issuing from a chok ed circular nozzle was measured using a Rayleigh scattering-based technique . This reliable and nonintrusive technique is particularly suitable for hig h-speed flows and is fundamentally superior to the intrusive probes and par ticle-based techniques such as laser Doppler velocimetry. A continuous wave laser and photon counting electronics were employed for time and phase-ave raged density measurements. The use of dust-free air for the entrained flow allowed measurements in the shear layer region. The free jets were produce d in the plenum to ambient pressure ratio range of 1.88-5.75, which corresp onded to a fully expanded Mach number range of 0.99 less than or equal to M (j)less than or equal to 1.8. A comparative study of schlieren photographs and time-averaged density data provided insight into the shock-cell structu res. The radial profiles obtained at various axial stations covering a down stream distance of 10 jet diameters show the development of the jet shear l ayer and the decay of the shock-cells. The supersonic free jets produced sc reech sound. A phase-averaged photon counting technique, using the screech tone as the trigger source, was used to measure the unsteady density variat ion. The phase-averaged density data show the evolution of the large-scale turbulent vortices that are found to be modulated periodically along the fl ow direction. A comparison with previously obtained data showing near-field pressure fluctuation and convective speed of the organized vortices reveal s many interesting dynamics. All quantities show regular spatial modulation . The locations of local maxima in density fluctuations are found to coinci de with the high convective speed and the antinode points in the near-field pressure fluctuation. Interestingly, the periodicity of modulation is foun d to be somewhat different from the shock spacing. Instead it shows that th e standing wave system, known to exist in the near-field pressure fluctuati on, extends into the jet shear layer. The standing wave is formed between t he downstream moving Kelvin-Helmholtz instability waves and the upstream pr opagating part of sound waves. A detailed field measurement of the unsteady density fluctuation was conducted for the M-j = 1.19 and 1.42 jets for whi ch the near-field pressure fluctuation data were obtained previously. The p hase-matched, combined plots of the density fluctuation present inside the jet flow, and the pressure fluctuation present just outside the jet boundar y provide a charming insight into the shock-vortex interaction leading to t he sound wave generation. (C) 1999 American Institute of Physics. [S1070-66 31(99)01712-2].