Blockage ratio and Reynolds number effects on flows around a rectangular prism

Abstract

The combined effects of blockage ratio (BR) and Reynolds number (Re) on the spatiotemporal characteristics of turbulent flow separation around a rectangular prism with depth-to-thickness ratio of 3 were investigated using a time-resolved particle image velocimetry. Four different blockage ratios (BR = 2.5%, 5%, 10%, and 15%) were examined at Reynolds numbers of 3000, 7500, and 15000. Two regimes (unattached and reattached) were identified; however, the boundary between these regimes shows a complex dependency on BR and Re. The mean flow does not reattach onto the prism at low BR and Re but tends to reattach when BR and Re increase. The wake vortices are relatively larger for the unattached test cases. The separation bubbles over and in the wake of the prism are dynamically coupled for prisms in the unattached regime but independent of each other in the reattached regime. Spectral analyses of the velocity fluctuations and coefficient of the first proper orthogonal decomposition mode pair reveal a single dominant peak at the same fundamental shedding frequency for the reattached test cases, whereas multiple competing frequencies are observed for test cases in the unattached regime. The Kelvin–Helmholtz frequency increases with an increase in BR and Re. The vortical structures are more organized for prisms in the reattached regime, and their convective velocities in the wake are comparatively higher.