Experimental Study Of The Effect Of Solid Particles On The Wake Structure Induced By Hemispheric Rough Element By PIV

The research on the influence of solid particles on the turbulent flow structure of semi-spherical rough elements lays a foundation for the study of liquid-solid two-phase on rough walls,and has reference significance for the study of liquid-solid two-phase on smooth walls,and also provides reference for theoretical research and engineering application.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 140μm,200μm and 350μm polystyrene particles was experimentally collected.The Reynolds number based on hemisphere radius was 994（Re_R（28）RU/v）.The volume concentration of particles was3×10^-5.The effects of particles on macro characteristics such as average velocity profile,turbulence intensity,Reynolds stress and average spanwise vorticity are analyzed.The power spectral density function is used to analyze the effect of particles on the shedding frequency of wake structure.The spatial local structure function and the new quadrant splitting method are used to extract burst events and carry out topology.The effect of particles on the spatial scale of wake structure is analyzed by using the correlation of flow pulsation.Vortex detection criteria and conditional sampling are used to extract spanwise vortex structure and analyze the influence of particles on it.Through comparative analysis,it is found that the existence of particles has great influence on the average flow in the posterior 3R hemisphere.The recirculation zone gradually increases with the increase of particle size.The existence of particles promotes the shedding of large-scale vortex structures,and the promoting effect increases first and then decreases with the increase of particle size.The presence of particles increases the shedding frequency,and with the increase of particle size,the shedding frequency increases first and then decreases.The existence of particles increases the turbulence intensity,Reynolds stress and average spanwise vorticity.Due to the existence of recirculation zone,the turbulence intensity,Reynolds stress and average spanwise vorticity show two different trends with the change of particle size before and after the flow direction 2R.At the normal position y=0.5R near the wall surface,the existence of particles increases the pulsating velocity and spanwise vorticity in the injection and sweep phases,and increases first and then decreases with the increase of particle size.At the normal position y=1R in the wake structure evolution process,the existence of particles reduces the flow direction pulsation in the injection stage and sweeping event,and decreases first and then increases with the increase of particle size,while the normal pulsation and spanwise vorticity increase,and increases first and then decreases with the increase of particle size.The existence of particles increases the number of ejection and sweep events,and the number of burst events decreases with the increase of normal position.Comparing the number of ejection and sweep events,it is found that particles have a stronger promoting effect on sweep events.Compared with the clean water condition,the existence of particles reduces the two-dimensional spatial scale of the wake structure,and with the increase of particle size,the spatial scale of the wake structure first decreases and then increases.The increase of wake structure dimension is inhibited,and the inhibition firstly increases and then decreases with the increase of particle size.The particles accelerate and decelerate the periodic velocity of the vortex structure,and the accelerating effect first increases and then decreases with the increase of particle size.The existence of particles promotes the generation,evolution and development of vortex structure,and the promotion effect increases first and then decreases with the increase of particle size.Through experimental research,it is found that the existence of solid particles makes macro statistics such as turbulence intensity and Reynolds stress in the boundary layer induced by rough elements promote turbulent transport.It promotes the formation and evolution of rough element induced structure and changes nonlinearly with particle size.