New optical method for measurement of the local wall shear stress has been developed by using thermo-chromic liquid crystal temperature measurement based on hue ,  of the camera view. The flow field is the fully developed turbulent channel flow. Thin film made of thermo-chromic liquid crystal is placed on the wall. A rectangular shaped obstacle is glued on the film. The obstacle is within a region of buffer layer with height from the wall. Temperature of the film and the obstacle are slightly raised by a heater below the wall. The air flow makes non-uniform temperature distribution and non-uniform color distribution appears on the surface of the film. Relations between hue and local skin friction coefficient were examined in a turbulent air channel flow. It is indicated that a certain hue of a point is varying linearly against the corresponding local skin friction coefficient.
Measurement of the Wall Shear Stress With Sublayer Thin Plate of Thermochromic Liquid Crystal
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Kodama, T, & Mochizuki, S. "Measurement of the Wall Shear Stress With Sublayer Thin Plate of Thermochromic Liquid Crystal." Proceedings of the ASME/JSME/KSME 2015 Joint Fluids Engineering Conference. Volume 1A: Symposia, Part 2. Seoul, South Korea. July 26–31, 2015. V01AT25A006. ASME. https://doi.org/10.1115/AJKFluids2015-25474
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