Experimental Study On The Effect Of Four Pyramid Rough Elements On Liquid-Solid Turbulent Boundary Layer

To study the wake shedding behavior of the pyramid rough element in the liquid-solid two-phase boundary layer,the instantaneous velocity fields of the single-phase and two-phase flow fields are obtained using particle image velocimetry(PIV)for the experimental study of its wake flow field.The variation of macroscopic statistics of the pyramid wake is analyzed by the variation of the mean velocity field and turbulence intensity.The flow characteristics and periodicity pattern of the shedding structure are analyzed using the power spectral density function combined with the proper orthogonal decomposition.The variation trend of the shedding structure position is analyzed by the dynamic mode decomposition.To study the effect of the rough elements on the liquid-solid two-phase flow field,the interaction pattern between the pyramid wake and the particles are investigated,and the motion of the particles and the concentration distribution characteristics of the particles are discussed.The basic flow structure of the flow field around the pyramid reveals two regions of high turbulence intensity in the streamwise normal plane,which are influenced by different shedding structures.The POD and DMD results of the streamwise normal plane and streamwise spanwise plane demonstrate the existence of periodic structure shedding at both the top and bottom of the pyramid,and the PSD results show that the periods of the two different shedding structures are consistent.The recirculation region of different normal heights shows different patterns,and the energy occupied by the first mode of POD decreases with the increasing of normal height.The Reynolds number directly affects the turbulence intensity,and the two groups of data show a positive correlation,but the Reynolds number do not change the size of the recirculation zone in the mean flow structure.This indicates that the Reynolds number does not change the flow pattern of the pyramid wake within the turbulent boundary layer,and the effect is limited to the scalar parameters that respond to the mean flow velocity,such as turbulence intensity and shedding frequency.The particulate phase affects the macroscopic statistics of the pyramid wake,with a reduction in the size of the reflux zone and the magnitude of turbulence intensity.The POD analysis reveals that the particles break up some small-scale structures,reduce the velocity pulsation,and the first few orders of POD results have a reduced contribution to the total energy of the flow field.The particles reduce the period and energy of the pyramid wake vortex shedding structure,but the flow pattern around the pyramid is not affected.In the two phase turbulent boundary layer,the change of yaw angle α causes the wake evolution.As the α angle increases,the cross-sectional area obstructing the flow increases,resulting in the increase of the size of the recirculation zone and the turbulence intensity.The results of PSD and POD show that the larger α angle makes the rough element gradually change from blunt body to streamlined,which makes the size and energy of the wake vortex structure increase.However,the shedding pattern and shedding period are almost unaffected by the variations of yaw angle.With the spatial and temporal evolution of the flow structure,the particle phase affected by the pyramid extends all the way to the outer boundary layer.A strong shear layer formed at the location of the pyramid wake shedding structure,and only particles caught in the recirculation zone have a greater chance of being carried to the top shedding location.The velocity of particles moving close to the leeward side of the pyramid is significantly lower than the velocity of movement with the top shedding.