Observations on the development of a two-optical-channel laser Doppler anemometer



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A justification is made, based on a discussion of the disadvantages of hot wire anemometers, for the need of developing a laser anemometer which can measure instantaneous axial (u) and normal (v) turbulence velocities. Theoretical equations are presented and experimental equipment were developed for a laser velocimeter capable of measuring instantaneous turbulent velocities and turbulence shear stress. Photographs of Doppler frequency spectral broadening from a spectrum analyzer were made at various positions in the flow channel. Axial turbulence intensities were measured with the optics in a horizontal plane. These data compare approximately with that in the literature. It is suspected that excessive noise in the Doppler signals is due to effects of polarization, thermal electronic noise, time broadening the Doppler signal, additional Doppler heterodyning along the light path, and the scattering particles used. It is concluded that at this date of development, turbulence quantities cannot be reliably extracted from a two- component laser anemometer. Suggestions are given for redesign of the optical and electronic systems.