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

众所周知，壁面和流体流动之间的热质传递以及流体流动的阻力很大程度上取决于靠近壁面或物体表面的流动。为了提高流体机械的效率、强化换热和/或控制蒸发或冷凝速率，了解靠近表面的流体流动的细节是非常必要的。为了了解流体流动的细节，提出了一种与表面流体流动密切相关的壁面剪应力可视化技术。可视化技术是基于马赫-曾德尔干涉仪的。被观察的光被导出到棱镜上，在全内反射过程中在棱镜表面上诱导出逝去波。之后，将该灯光与另一灯光进行干涉以供参考。如果在消逝波区域的流体的反射率由于表面的剪切力而改变，则观测光的相位也改变，并且被观察为干涉图案。在本研究中，我们构建了可视化系统，并对其进行了实证检验，以评估该技术的有效性。阐明了当逝去波直接用作观察光时，观察到的相移较小。我们提出了一种新的切应力观测系统。新系统在棱镜表面引入了一种具有很大光学弹性效应的薄膜。观察光穿过薄膜，而光被薄膜表面全反射，从而使光程变长。在这项研究的第二年建立的新系统使我们能够在应力较大时可视化表面上的剪应力。

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.