Velocity and Temperature Measurements in High-speed Flows with Naturally Present Dust Particles Using Rayleigh and Mie Scattering

Dust particles and occasional moisture condensations are an unavoidable reality of all wind tunnels. On the path to pursue a goal of velocity and temperature measurements in high-speed wind tunnels we have created a tabletop, spectrally resolved, Rayleigh-Mie scattering setup around a small jet fed by ambient air and air with different particle concentration to determine the accuracy of the technique. The other reality of a wind tunnel setup is the flare light from spurious scattering of the incident laser beam by solid surfaces. Such lights further contaminate the Rayleigh-Mie scattered light. This is simulated by backgrounds with different reflectivity towards the collection optics. Light from a CW laser is delivered via an optical fiber and the scattered light from a point on the laser path is spectrally resolved using a stabilized Fabry-Perot interferometer, followed by imaging on an EMCCD camera. A model of the combined spectra from laser flare, Mie scattered and Rayleigh scattered light was fitted to the camera images using maximum likelihood estimation. It was observed that the present modeling approach can extract velocity and temperature with reasonable accuracy when the intensity of the Mie scattered light is less than or comparable to that of the Rayleigh scattered light; beyond that level of Mie scattering error in velocity measurement remains reasonable ±12m/s, but the temperature measurement becomes progressively more inaccurate. Increasingly, the flare light is found to cause a bias error in velocity. Similar trend is observed in the presence of both the flare light and the Mie scattered light. The setup has provided a set of data for the future improvement of the modeling procedure and demonstrated that even in the case of large amount of dust particles and large flare light, the present technique can provide measurement of velocity with reasonable accuracy.