Similar to the Doppler technique the principle of this method is also based on light scattered by small particles as they are normally contained in every fluid. Two light beams (laser beams are not necessary) are focused in two very small light spots in the measuring volume. Particles passing both the spots emit two light pulses which can be detected by a photomultiplier. From the time interval between these two pulses and from the known spots distance the flow velocity can be calculated. A statistical analysis of many signals, taken at the same treasuring point, allows calculation of the mean velocity, the mean flow angle and the turbulence degree of the velocity components in a plane normal to the beam axis. Due to the light intensity of the light spots (102 to 103 times higher than the intensity in a Doppler probe volume under similar conditions) a sufficient signal-to-noise ratio is achieved so that velocity measurements even in unseeded air flows are possible up to 500 m/s in backscattering by using a 5 mW laser. Tests were carried out in a supersonic wind tunnel up to a Mach number of 2.2. For measurements in the rotor blade channels of turbomachines a specific trigger optic has been installed.
A Laser Dual-Beam Method for Flow Measurements in Turbomachines
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Schodl, R. "A Laser Dual-Beam Method for Flow Measurements in Turbomachines." Proceedings of the ASME 1974 International Gas Turbine Conference and Products Show. Volume 1B: General. Zurich, Switzerland. March 30–April 4, 1974. V01BT02A074. ASME. https://doi.org/10.1115/74-GT-157
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