Planar Doppler Velocimetry for Aircraft Exhausts

A diagnostic is proposed for aircraft engine exhausts that provides three velocity components at each point in an image of a slice through the plume. The method measures the Doppler shift of laser light scattered from soot particles naturally present in the exhaust, and may also be useful for measuring non-combustion flows outside the exhaust using naturally occurring environmental aerosols. An image processing filter that employs iodine vapor absorption transforms frequency variations in the scattered light to variations in image intensity. The technique can provide two-dimensional instantaneous velocity profiles over an area large enough to cover the cross-section of a typical aircraft engine exhaust. A laser system that is insensitive to vibration effects makes it suitable for field measurements on a full-scale aircraft. Velocity uncertainties are dependent on signal level, and thus on the level of soot particles present in the flow. However, the method should be capable of operating over a large range of particle loading levels and is capable of measuring supersonic as well as subsonic velocities.