Spatial resolving laser Doppler velocity profile sensor using slightly tilted fringe systems and phase evaluation

In this paper we report on a differential laser Doppler velocimeter which offers spatially resolved measurements in the probe volume. For the first time, two laser wavelengths are employed to generate two interference fringe systems with identical fringe spacing which are slightly tilted towards each other. The relative phase shift varies linearly with the position along the optical axis, thus allowing spatially resolved measurements. In the centre of the measurement volume a spatial resolution in the nanometre range is achieved. The sensor was applied in a wind tunnel for the determination of the wall shear stress of a laminar flat-plate flow without mechanical traversing. Spatially resolved measurements of the Blasius velocity profile are presented. All results agree well with the theoretical prediction.