Boiko V.M.   Poplavski S.V.   Nesterov A.  

Laser Doppler anemometer based on Fizeau interferometer

Reporter: Boiko V.M.

When measuring flow velocities, methods of laser Doppler anemometry (LDA) are of interest, which have a number of advantages in comparison with Pitot tube, thermoanemometers, etc., and in multiphase media they are usually the only ones that are operable. The so-called "differential" scheme of heterodyne LDA is the most developed and brought to commercial product, but when investigating high-speed (> 300 m/s) flows, the use of heterodyne LDA causes difficulties. In these conditions, the LDA is more promising, measuring the Doppler frequency shift with the help of spectral instruments of high resolving power. Unlike heterodyne methods, direct spectral analysis has no limitations on the maximum measured velocity, and the lower limit is determined by the resolution of the spectral device. Thus, in studies of high-velocity multiphase flows, LDA methods with direct spectral analysis are preferable. Return to these methods, developed in the 70 - 90 years of the twentieth century, seems relevant, and the progress in laser technology, digital recording of spectra and computer processing of data allows them to be implemented at a new technical level.
In this paper, we present the results of the implementation and testing of two types of LDA with direct spectral analysis: based on the Fabry-Perot interferometer (IFP) and on the basis of the Fizeau interferometer. A comparative analysis of the spectral characteristics of interferometers is given, it is shown that under the same conditions the IFP instrument function is narrower, hence, the resolving power is higher. Therefore, with a single-channel recording of the reference signal and a frequency-shifted scattered signal, the Fabry-Perot interferometer is preferable. In a two-channel scheme with spatial separation of the reference and scattered signals, the scheme with the Fizeau interferometer has the advantage. Its interference pattern significantly simplifies data processing and improves the accuracy of Doppler frequency measurement.
Examples of LDA implementation on the modern element base are presented, the results of development and testing of LDA with the Fizeau standard are given. The optic scheme is based on the wavelength meter “Angstrom WS-7” (High Precision Wavelength Meters: WS HighFinesse Angstrom Series) manufactured by “Angstrom” (Russia).

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