Heterodyne laser-Doppler line-sensor for highly resolved velocity measurements of shear flows

We report on the application of a heterodyne laser-Doppler line-sensor for spatially highly resolved shear flow measurements. The line-sensor employs a wavelength-division-multiplexing (WDM) technique, which generates a measurement volume with convergent and divergent fringes for two laser wavelengths, respectively. The evaluation of the ratio of the resulting two Doppler frequencies yields the particle velocity and position inside the measurement volume with a resolution of a few micrometers. A line sensor with directional discrimination was realized by using a heterodyne technique. The measurement signal is generated in the intermediate frequency band, allowing the evaluation of small velocities with high accuracy. We report on the measurement of laminar boundary layers and for the determination of the wall shear stress. Good agreement was obtained between the measured velocity and the Blasius solution.