Influence of Mie and Geometric Scattering Contributions on Temperature and Density Measurements in Filtered Rayleigh Scattering

Unwanted signals from particle and surface scattering cause bias errors and substantial uncertainties when using conventional filtered Rayleigh scattering approaches. A means for accounting and removing these effects are investigated by developing a comprehensive scattering model including Rayleigh scattering (via Tenti’s S6 Rayleigh lineshape model), Mie scattering, and geometric scattering contributions. The insights gained from this model are then applied for the first time using several cases that vary in flow conditions and spectral bandwidth to reduce the uncertainties of the in situ multi-property measurements of vector velocity, temperature, and density. Both Mie and geometric scattering are individually found to cause significant distortions in the shape and intensity of a measured Rayleigh scattering/iodine transmission convolution spectrum. However, these contributions may be filtered out using the molecular iodine vapor cell when optimal bands in the iodine absorption spectrum are applied. For cases considering only Mie scattering, temperature and density uncertainties are found to be below 3.5% and 4.5%, respectively, while the uncertainties for cases considering only geometric scattering are found to be below 3% and 3.5% for temperature and density estimation, respectively, over a range of subsonic flow Mach numbers, 0.4≤M≤0.7. Owing to a large Doppler shift in the Rayleigh spectrum, a supersonic case at M=1.9 is found to yield a greater uncertainty in temperature of 8%. When Mie and geometric scattering contributions are combined, the intersection of the individual rejection regions must be used, resulting in temperature and density uncertainties of 2.0% and 2.6% for the M = 0.4 case and 2.6% and 2.8% for the M = 0.7 case, respectively. An alternative method is proposed and demonstrated qualitatively for applications where the Doppler shift magnitude is too large to rely solely upon the iodine filter rejection region method. In comparison to experimental measurements, the developed composite scattering model captures the majority of the observed scattering physics.