A measuring system based on a differential laser-Doppler velocimeter has been extended to determine one-dimensional velocity profiles with a spatial resolution inside the measurement volume. The principle of the velocity profile sensor is based on the generation of two fringe systems with different gradients of the fringe spacings. The determination of the corresponding two Doppler frequencies yields the position as well as the velocity component of individual tracer particles, which results in the velocity profile for detecting several particles. The sensor was used to determine velocity profiles of flat-plate laminar boundary layers for varying Reynolds numbers. A precise determination of the wall shear stress was accomplished. All results are in good agreement with the theory. The main application of the velocity profile sensor is the spatial high-resolved investigation of turbulent boundary layers.

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