Classification of Shear Stress Sensors

Various shear stress measurement techniques have been proposed in literature. Some principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal. 

Stanton tube is a rectangular-shaped pitot tube located very close to the boundary wall, and the mean velocity from this pitot tube is directly related to the shear stress. Preston tube is similar to the concept of Stanton tube using a pitot static tube close to the surface, and the difference between the stagnation pressure at the center of the tube and the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall, and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. The detailed discussion on above three techniques can be found in Hanratty and Campbell (1996). These shear stress measurement techniques are not ideal MEMS-based technique. 

Due to numerous benefits of MEMS-based shear stress sensors, the following shear stress measurement techniques having great promise for future MEMS applications have been discussed in the following sections (1) velocity measurements, (2) thermal sensors, (3) floating 

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