Filtered Rayleigh scattering for multi-parameter fluid flow analysis

In aerospace test facilities, for example wind tunnels, gas turbine compressors and combustion experiments, it is necessary to obtain data about the fluid flows in these experiments to improve performance thus reducing harmful emissions and noise and to gain a better understanding of the fundamental fluid processes. The data required includes the density, pressure, temperature and velocity of the flow. Presently available techniques allow the measurement of these flow properties but have limitations: multiple techniques may need to be used to measure the different properties. The techniques may be limited to making single point measurements, requiring an expensive and time-consuming scanning mechanism to 'map' the flow. Additionally some of the techniques for measuring flow velocity require small 'seed' particles to be added to the flow which may result in inaccurate velocity measurements if these seed particles do not follow the flow accurately. Also, the addition of 'seed' particles to the flow is limiting in some applications, causing optical windows to become fouled limiting the measurement time and increasing the expense of the technique.Filtered Rayleigh scattering (FRS) is a promising optical technique that can potentially measure multiple properties (temperature, pressure, density and velocity) of the flow simultaneously. The measurements are made non-intrusively, so the flow is not changed by the measurement. Measurements can be made at a single point, or more significantly at multiple points over a plane defined by the laser light sheet in the flow. In FRS the properties of the flow are determined by measuring the Rayleigh scattered light. This is light scattered from the molecules of the gas itself, and as such no 'seed' particles are required. When the light is scattered from the gas molecules it will have its spectrum altered by the properties of the gas. The density of the gas can be found from the scattered intensity. The temperature can be found from the width of the scattered spectrum and the velocity from the shift in frequency compared to the optical frequency of the laser. Both of these effects are due to the Doppler shift, with the flow velocity causing a shift from the illumination frequency and the temperature provide a widening of the spectrum due to the Doppler shifts from the thermal motion of the molecules. Finally the pressure can be found by looking at the shape of the spectrum.The research proposed is the development of FRS instrumentation as a multi-parameter measurement technique for application in wind tunnels, gas turbine compressor and combustion facilities. Trials of the instruments constructed will be made, but not sustained measurement campaigns, rather the capabilities and limitations of the technique will be disseminated to end-users and industry for subsequent collaborative programmes.