Dual vortex structure shedding from low aspect ratio, surface-mounted pyramids

The flow around surface-mounted, square-based pyramids, of apex angles ranging from 5 to 90, placed in a thin boundary layer with one face normal to the on-coming flow was investigated for Reynolds numbers of 10 000-150 000 using surface pressure measurements and particle image velocimetry as well as oil-film surface-flow patterns. Based on the periodicity of the surface pressure on the pyramid side faces, four distinct shedding regimes were identified: constant local Strouhal number, discrete cellular, low aspect ratio and suppressed. Closer inspection of the low aspect ratio regime indicates that two distinct vortical structures are shed at the same frequency. Vortices are shed alternately from opposing faces in the obstacle base region, while hairpin vortices are shed in-phase and are most easily observed near the tip region of the pyramid. It is further shown that the existence of these structures is consistent with the mean surface-flow topology observed in the wake and side faces of the obstacles.