Planar Doppler Velocimetry for Aircraft Exhausts

A diagnostic is being developed for aircraft engine exhausts that measures three velocity components in a slice through the plume. The method measures the Doppler shift of laser light scattered from particles naturally present in the flow, such as soot or dust. Two key areas of uncertainty were investigated in the Phase I study: 1) scattered light intensity from soot particles of sizes and concentrations that are representative of jet exhaust flows, and 2) image registration limitations that determine how accurately Doppler images can be normalized by reference images. Predictions were performed using a mathematical model that showed signal levels would be high enough in a typical aircraft engine exhaust to enable single shot measurements with uncertainties as low as 2 m/s. Experiments were conducted using a soot generator that showed uncertainties in Doppler shifts corresponding to about 6 m/s. Planned improvements in image registration should lead to even lower uncertainties. Photon fluxes were measured in a small turbojet engine, and were found to be on the same order of those measured in the soot generator. Scattering from dust was observed in regions outside the plume that may provide an opportunity for velocity measurements in these regions as well.