Rayleigh scattering measurements for obtaining spatiallyresolved absolute gas densities in large scale facilities

Abstract

A systematic approach for the use of Rayleigh scattering to determine spatially resolved (along a line) gas densities is presented. Lessons learned from earlier (2003) measurements on the MARIAH II Radiatively-Driven Hypersonic Wind Tunnel (RDHWT) are incorporated into an approach which seeks to address all of the previous difficulties encountered, providing the requisite spatial resolution, suppression of scattered light, calibration and correction for laser shot-to-shot fluctuations in order to obtain data of sufficient quality to validate the predictive MARIAH II models for a 1 MW electron beam coupled into a supersonic flow. A statistical analysis is used to ascertain the necessary measurement accuracy, based on the final required accuracy for the density for its particular application. Laboratory tests are carried out on a free jet expansion using various test gases - air, helium, methane, ethylene and propane – to simulate the density variations expected in the actual experiments. The results are processed and analyzed quantitatively to verify the approach. Although developed with the above-mentioned wind tunnel in mind, the method is generally and universally applicable to any large scale facility.