Visualization of near-wall three-dimensional structures in a natural convection boundary layer via mirror-assisted three-dimensional background-oriented schlieren

Three-dimensional (3D) background-oriented schlieren (BOS) is a promising technique for volumetric measurements in variable-density flows; however, its application to wall-bounded flows is severely limited because the wall obstructs the optical paths required for multiview reconstruction near the surface. In this study, we demonstrate a mirror-assisted 3D BOS method that overcomes this limitation using a mirror-finished heated vertical plate to secure reflected viewing paths between synchronized cameras and background monitors. A dedicated calibration procedure is employed to estimate the relative positions of the cameras, mirror surface, and background monitors, and a regularized least-squares reconstruction is used to obtain a 3D density difference between consecutive frames from multiview displacement data. This method is demonstrated for a natural convection boundary layer in air along the vertical heated plate. The reconstructed fields successfully capture near-wall 3D structures and clearly distinguish different flow patterns over a wide range of local Rayleigh numbers. Qualitative analyses based on the fluctuation intensity of frame-to-frame signals and correlation-length anisotropy further support the ability of this method to resolve structural changes in the flow. Although such structures have previously been discussed primarily based on direct numerical simulations, the present results provide direct experimental access to them in wall-bounded buoyancy-driven flows. The results indicate that the mirror-assisted 3D BOS provides a practical approach for volumetric near-wall measurements in wall-bounded variable-density flows.