Transmittance Correction for Focused Laser Differential Interferometry Measurements in High-Enthalpy Flows

Abstract

Non-intrusive optical measurement techniques provide significant advantages for studying high-speed flows. Focused Laser Differential Interferometry is one such technique that is used to measure the density fluctuations in the flow. The working principle of this technique is that the light intensity measured from the recombination of two orthogonally polarized beams is proportional to the density gradient. In a clean environment, this technique works as expected, however, not all high-speed wind tunnel facilities produce clean flows. In the High-Enthalpy Tunnel Göttingen, the high-enthalpy conditions are contaminated by soot in the flow caused by the extreme temperatures and pressures that the facility operates at. Experiments showed that this soot significantly altered the measurements by reducing the light transmittance, which the data interpreted as a destructive interference in the beams after recombination and polarization. A correction for this effect is presented where one beam from the interferometer is not passed through a second polarizer, allowing for the measured intensity fluctuations to be caused purely by the soot-generated transmittance fluctuations. The intensity range of the system is then scaled according to the time-resolved light transmittance. Results from the application of this correction show that the effect of the soot on the optical measurements is filtered out using this method