Turbulence visualization using reflective flakes

Abstract

The physical mechanism of flow visualizations using reflective flakes is investigated. First, we derive theoretically the governing equations of flake motion based on the assumption that flakes are infinitely thin elliptic disks. Secondly, we verify numerically and experimentally that these equations describe the flake behavior excellently. An important indication of these equations is that the temporal evolution of flake orientations, which determine the intensity in a visualized image, is identical to that of the infinitesimal material surface elements. Since the orientation of material surface elements is governed by velocity gradients, and since the velocity gradient field of turbulent flows is accompanied by coherent vortical structures at the Kolmogorov length, it is expected that such coherent structures in turbulence may be visualized by reflective flakes. It is numerically demonstrated that a flake visualization with appropriate light thickness, indeed, captures the clusters of the coherent structures in isotropic turbulence.