EXPERIMENTAL INVESTIGATION ON THE EFFECT OF PARTICLES ON THE WAKE OF HEMISPHERIC DISTURBANCE

Particle image velocimetry (PIV) was used to investigate the effect of solid particles on the wake structure of an isolated hemispherical roughness element placed in a laminar boundary layer of a flat plate. The two-dimensional velocity field information for four operating conditions of single phase and addition of 140 \mum, 200 \mum and 350 \mum polystyrene particles was experimentally collected. The Reynolds number based on hemisphere radius was 994 (Re_R=RU/\upsilon). The volume concentration of particles was 3.0\times10^-5. The turbulence statistics such as average velocity profile, comprehensive turbulence intensity and reflux area size under clean water and two-phase conditions are compared to analyze the influence of solid particles on macro characteristics of hemispherical wake flow. The two-dimensional spatial correlation coefficient and power spectral density function of pulsating velocity along the flow direction at different positions along the flow direction are respectively used to analyze the effect of solid particles on the evolution process of wake structure and the shedding frequency of wake structure. The results show that: compared with clear water, the reflux zone gradually increases with the increase of solid particle size; The presence of solid particles increases the comprehensive turbulence intensity and due to the existence of the backflow region, the turbulence intensity in the region before and after the hemispheric backward flow direction position 2R presents different changing trends The presence of solid particles reduces the flow direction dimension of the wake structure and the flow direction dimension firstly decreases and then increases with the increase of particle size The presence of solid particles in the wake structure promotes the periodic acceleration and deceleration of the wake structure, and the promotion effect increases first and then decreases with the increase of solid particle size. The presence of solid particles promotes the shedding of wake structure, and the shedding frequency of wake structure increases first and then decreases with the increase of particle size.