Spatial-temporal patterns of three-dimensional subsonic turbulent cavity flow

Three-dimensional, high Reynolds number cavity flow is investigated. The cavity was exposed to a free-stream Mach number of 0.40 and a Reynolds number (based on cavity depth) of 1.6 × 106, with a thick incoming turbulent boundary-layer. Dominant modes and structures were studied by conducting unsteady numerical flow simulations (CFD), and applying Proper-Orthogonal-Decomposition (POD) in both two and three-dimensions. Fully three-dimensional POD analysis of cavity flow has been rarely conducted in other studies, and it is demonstrated that two-dimensional POD analysis is valid in order to emphasize patterns which are limited in space, but could be misleading without the comparison with the three-dimensional POD analysis. The analysis sheds new light on three-dimensional cavity flow by disclosing novel non-intuitive dominant structures and modes. Stretched streamwise vortices were found to be dominant, at least as the spanwise structures that originate from the classical two-dimensional shear-layer instability. Furthermore, the effect of symmetry plane assumption, which is usually disregarded, was studied rigorously for the first time. POD decomposition of symmetric and anti-symmetric flow modes revealed that anti-symmetric modes contain a significant portion of the total energy; however, they only interact weakly with the symmetric modes.