Property and Velocity Measurements in a Supersonic/Hypersonic Flow
Abstract : This work presents a non-intrusive laser-based diagnostics technique to measure instantaneous thermodynamic properties and velocity in a compressible fluid flow. The fluid is irradiated with a pulsed narrow-bandwidth Nd:YAG laser at 532 nm. Light scattering from a point in the flow field is collected over a solid angle of about + or - 20 degrees and focused into a narrow line on an intensified CCD camera using an anamorphic optical system. A molecular iodine filter absorbs the radiation at different degrees with angles, thus producing different intensity levels on each pixel along the line. A portion of the light is split and focused on the same CCD chip unfiltered, thus providing an intensity reference from which the fluid density is found. The scattering spectrum and Doppler frequency shift are dependent on the observation angle and fluid properties, and so the values of pressure, temperature and streamwise velocity at every laser pulse using a well-known scattering model.
On the Kinetic Model Description of Rayleigh–Brillouin Scattering from Molecular Gases
We analyze in detail, in the kinetic regime, the behavior of a model kinetic equation previously used by us for the description of the density fluctuation spectrum in molecular gases. We find that over a small range of wavelengths and frequencies the predicted spectrum contains an unphysical feature, namely a shallow dip in the Rayleigh line. This effect is traced to the modelled elastic collision operator and has gone unnoticed in similar models of monatomic gases. We propose a modified model which corrects this feature and which appears to reproduce the details seen in light-scattering experiments.
A molecular filter based velocimetry technique for high speed flows
A molecular absorption filter-based diagnostic technique has been developed to obtain instantaneous, planar velocity measurements in high speed flows. This paper presents the details of the technique and its application in high Reynolds number compressible mixing layers with convective Mach numbers of 0.51 and 0.86. Pressure broadening was used to “tune” the absorption profile of the filter to a given flow field/optical arrangement. It is shown that two filters can be used when background scattering is significant: one as the velocity-discriminating filter, the other to eliminate unwanted background light from the reference camera. Collected images show instantaneous variations in velocity within both the highly organized roller type structures of the Mc= 0.51 mixing layer and the unorganized structures found in both mixing layers. An uncertainty analysis showed that the measurement uncertainty was ±8.6% for the current measurements, but improvements to reduce the uncertainty to ±3% are suggested.