Visualization of Fluid Flow in Wall's Near Field Using Evanescent Wave Mach-Zehnder Interferometer

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It is well known that heat and mass transfer between a wall and fluid flow, as well as drag by fluid flow strongly depend on the flow close to the surface of walls or objects. In order to improve the efficiency of fluid machine, enhance the heat transfer and/or control evaporation or condensation rate, it is very essential to know the detail of fluid flow close to the surface.To understand the detail of the fluid flow, a visualization technique for shear stress on wall surface, which is closely related to the fluid flow on the surface, has been proposed in this study. The visualization technique is based on the Mach-Zehnder Interferometer. Alight for observation is derived to a prism and induces evanescent wave on the prism surface during total internal reflection. After that, the light is interfered with the other light for reference. If the reflective index of fluid at the region of the evanescent wave changes due to shear stress by the surface, the phase of the observation light also changes and is observed as an interference pattern. In this study, the visualization system has been constructed and empirically examined to evaluate the validity of the technique. It was clarified that the phase shift is less to be observed when the evanescent wave is directly used for an observation light. We propose a new system for observing the shear stress. The new system introduces a thin film having a large optical elastic effect on the prism surface. The observation light goes through the thin film while the light is totally reflected by the thin film surface so that the optical pass gets longer. The new system constructed in the second year of this study enable us to visualized the shear stress on the surface when the stress is strong.