Visualization and Measurement of Velocity Fields by Means Optical Discrimination of the Doppler Frequency Shift of the Scattered Light

Kurzfassung

At Doppler Global Velocimetry (DGV) the frequency of the Doppler shifted light scattered from the fluid has to be determined to obtain the velocity field. Standard DGV is based on the principle of transforming the Doppler shifted frequency into variations of grey levels by means of an Iodine cell, the transmittance of which is frequency dependent. Standard DGV configurations require a second reference camera to monitor the flow field. The major problems of the Standard DGV technique at present are limitations of the velocity resolution at low flow velocities and the influence of multi–particle scattering on the results of the velocity measurements. This paper will discuss different solutions to overcome these problems. First, a method of optical discrimination of the Doppler frequency shift has been realized in an optical Doppler processor (ODP) with coherent feedback. This processor is used as a converter frequency ==> intensity. One implementation of the Optical Doppler Processor consists of a semi–confocal interferometer as wide–band spatial frequency filter with resonant transfer function. The slope of its resonance transfer function is used as frequency discrimination curve. The ODP is included in a feed back loop with the laser. In the German Aerospace Center (DLR) in Göttingen joint experiments on visualization and measurement of velocity fields in gas jets from a test nozzle in the regime of low velocities of a few meters/second have been made utilizing the ODP. A method to eliminate the influence of multi–particle scattering on the result of the measurement of velocity fields based on linear operations with frequency–demodulated images of the flow, formed in various areas of the angular spectrum of the scattered light, is proposed. The registration of the image can be made by only one CCD camera without the need of a second camera for a reference image. Eliminating the influence of multi–particle scattering is subject of an application for an European patent.