Imaging Laser Doppler Velocimetry using a High-Speed Camera

Imaging Laser Doppler Velocimetry (ILDV) is a flow measurement technique which facilitates the measurement of one velocity component of a flow field in an imaging plane. It is an evolution of Heterodyne Doppler Global Velocimetry and may be regarded as the planar extension of the classical dual beam LDV by crossing two light sheets in the flow instead of focused laser beams. With different light sheet configurations different velocity components of the flow field can be measured. Seeding particles within the flow are illuminated with two light sheets from two different directions. The light scattered from the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of the illumination and the velocity of the particle. With two different illumination directions the scattered light exhibits two different frequency shifts. The superposition of the two different frequency shifted signals creates interference on the detector and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity of the particle. This signal is detected using a conventional high speed camera and the acquired data is analyzed using an autocorrelation based data analysis technique. To demonstrate the feasibility of the technique the velocity distribution of a fully turbulent jet in water is measured with two different setups. In the first setup the light sheets are arranged such that the in-plane velocity distribution can be measured. This setup facilitates a simultaneous PIV measurement using the same high speed camera and the same image data set, therefore allowing a validation of the technique. The second setup measures the out-of-plane velocity distribution of the jet. Using this out-of-plane velocity measurement in combination with PIV a so called 2D3C measurement could be performed, that is a measurement of all three velocity components of the flow field in a plane.