Optical Measurement of Wall Shear Stress With Emphasis on Flows Near Separation

We describe a new generation of a MEMS based optical microsensor, the Diverging Fringe Doppler sensor, designed to measure the flow velocity gradient within the first 100 µm of a surface. The sensor measures the flow velocity gradient at 75 µm above the sensor surface as well as flow direction. This compact and embeddable sensor uses diffractive optical elements to generate diverging fringes originating at the sensor surface and slanted at 20o from the normal to the sensor face. The slant introduced in the fringes is used to measure flow directionality. The sensor was tested in the boundary layer of a flat plate. Subsequently, the sensor was installed in a cylinder and tested in the boundary layer of that cylinder from the stagnation point to the separation region of the flow, for Reynolds numbers of 104 and 4 x 10 4 . In both cases, the results were compared to that obtained with a miniature laser Doppler velocimeter mounted on a traverse, also embedded into the cylinder. The design of the sensor and instrumentation, and the experimental results are presented in this paper.