Vorticity Generation and Transport Characteristics in Flow around a Wall-Mounted Hemisphere and the Influence on Flow Structures

In this study, direct numerical simulation (DNS) is carried out to investigate flow around a wall-mounted hemisphere at a Reynolds number Re = 1000. The generation and transport characteristics of vorticity are analyzed based on the simulation results, deepening the understanding of the evolution mechanisms of vortex structures. The main flow features include near-wall recirculation vortices wrapping around the hemisphere, a large recirculation zone formed by flow separation at the apex, and hairpin vortices shedding downstream of the recirculation region along with several secondary vortical structures. From a vortex dynamics perspective, spanwise vorticity consistently dominates, contributing more than 60% to the total enstrophy. In the upstream recirculation vortices, spanwise vorticity is mainly amplified by stretching, while streamwise vorticity is generated through transfer from spanwise vorticity, manifested as spanwise stretching, spreading, and streamwise twisting of the recirculation structures. Vorticity generation occurs primarily on the windward face and upstream of the separation points due to fluid–surface interaction, followed by redistribution under the influence of surface curvature. In the near-wake evolution of arch vortices, strong transfer from wall-normal and spanwise vorticity to streamwise vorticity is identified as the key mechanism for the formation of hairpin vortices.