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

众所周知，壁面和流体之间的传热传质以及流体的阻力在很大程度上取决于壁面或物体表面附近的流动。为了提高流体机械的效率，增强传热和/或控制蒸发或冷凝速率，了解流体靠近表面的流动细节是非常必要的。为了了解流体流动的细节，本研究提出了一种与壁面流体流动密切相关的壁面剪切应力可视化技术。可视化技术是基于马赫-曾德尔干涉仪的。将观测光导出为棱镜，在全内反射过程中在棱镜表面产生倏逝波。之后，该光与另一光进行干涉，以供参考。如果在倏逝波区域流体的反射指数由于表面的剪切应力而发生变化，则观测光的相位也会发生变化，并以干涉图样的形式被观测到。在本研究中，可视化系统已被构建和实证检验，以评估该技术的有效性。结果表明，将倏逝波直接用作观测光时，相移观测值较小。我们提出了一种新的剪切应力观测系统。新系统在棱镜表面引入了具有较大光学弹性效应的薄膜。观测光穿过薄膜时，光被薄膜表面完全反射，使得光路变长。本研究第二年构建的新系统使我们能够在应力较强时可视化地表的剪切应力。

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.