Vortex dynamics in the near wake of a surface-mounted hemisphere

Abstract

The dynamics of vortical structures for the flow over an isolated surface-mounted hemisphere are investigated with time-resolved particle image velocimetry and hydrogen bubble visualization techniques. The 10 mm height hemisphere is completely submerged in the laminar boundary layer, and the Reynolds number based on the hemisphere height is 780. The interactions for a quasi-streamwise vortex, hairpin vortex, and counter-rotating hairpin vortex are explored for the first time in the hemispherical wake, and the three-dimensional hemisphere wake is reconstructed based on the time-resolved stereoscopic particle image velocimetry measurement. It is found that these three structures shed or oscillate at the same frequency, and the periodic oscillations of the quasi-streamwise vortex are caused by the periodic shedding of the hairpin vortex. Furthermore, the mechanisms for the formation and evolution of the counter-rotating vortex structures upstream of hairpin vortices are revealed. Finally, a refined instantaneous three-dimensional structure of the vortex system in the near wake of the hemisphere is presented.