Experimental investigation on the effect of particles on large scale vortices of an isolated hemispherical roughness element

Abstract

Particle image velocimetry (PIV) was used to investigate the effect of 155 μm polystyrene particles on the wake structure of an isolated hemispherical roughness element placed in a laminar boundary layer of a flat plate. The main purpose of this study was to investigate the influence of particles on the large-scale vortices induced by the hemisphere. The turbulence statistics and the main modes of proper orthogonal decomposition (POD) in whole field of view (FOV) were comparatively analyzed. The shedding frequency was analyzed by the power spectral density (PSD) function, and the hairpin vortex head structure was conditionally detected by the spatial multi-scale local average function. The results showed that by introducing particles, the statistics in the near-wall region downstream of the hemisphere were significantly changed. PSD results showed that the existence of particles led to the shedding of structures with multiple frequencies and inhibited the shedding of primary structures. Further conclusions could be drawn through POD and hairpin vortex head extraction: the existence of particles changed the type of vortex shedding behind the hemisphere, thereby reducing the number and the vorticity intensity of hairpin vortex heads in the range near the peak, and promoting the scale of the prograde vortex structure.