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.
Application of absorption filter planar Doppler velocimetry to sonic and supersonic jets
An absorption filter planar Doppler velocimeter has been constructed and tested on a pressure-matched sonic jet and an overexpanded supersonic jet (M design = 1.9). The current system can acquire single-velocity-component single-shot planar images (15-ns exposures) at 30 Hz. A unique aspect of the system is the use of a single camera and lens, made possible by the application of an image splitter system. This development reduces the expense and experimental complexity of the technique. The most extensive of the three data sets taken comprises 1100 centerline planes of instantaneous velocity acquired in the overexpanded supersonic jet. Mean and rms velocity fields are presented for this data set ; a mean centerline and a mean transverse profile are extracted and compared with profiles from a computational fluid dynamics solution generated Wing the LARCK computer code employing a k-e turbulence model.