Studyon The Surface Flow Field Structure And Particle Starting Mechanism Of Bulk Yard

The dispersion of fine particles caused by dust particles in the bulk yard is one of the main sources of urban particle pollution.In this paper,numerical simulation and PIV(Particle Image Velocimetry)experiment are used to study the influence of different arrangement of rough elements on the flow field structure on the surface of the pile.Through the stress analysis,the starting wind speed equation of particles at different positions on the surface of the pile is established.The research work is of great significance to accurately understand the flow field structure on the surface of the pile and reveal the starting mechanism of particles in the bulk pile.The main research contents and conclusions are as follows:(1)The large eddy simulation is used to study the flow characteristics and pressure distribution around the feature structure under different arrangement of rough elements.The simulation results show that a pair of carmen vortex streets appear at the tail of the feature structure in staggered arrangement,while a pair of carmen vortex streets appear after two adjacent rough elements in parallel arrangement;the average flow velocity and flow pattern in the feature structure are relatively small in staggered arrangement,the flow pattern is regular and the average velocity is large in parallel arrangement.The arrangement of the characteristic structure has little effect on the surface pressure distribution of the upstream rough elements.The front of the upstream rough elements is high pressure area(5.5pa),and the top is negative pressure area(-2.2pa).The surface pressure distribution of the downstream rough elements is the same as that of the upstream rough elements when they are staggered.However,theparallel arrangement of the upstream rough elements has a better diversion effect on the fluid,so the surface pressure of the downstream rough elements is smaller(-0.5pa)and the pressure gradient is smaller when they are parallel arranged.(2)The flow field structure near the wall of different surface material piles is analyzed.The results show that near the wall with a height of 0.25 mm from the wall,the flow field of smooth material piles is relatively regular,but the average flow velocity is relatively high.The average flow velocity is relatively low when staggered,but there are many vortices,and the flow field is relatively chaotic.Due to the blocking effect of rough elements,although the flow velocity at the wall is reduced,a large number of vortices are also generated,the stress of particles on the upper surface will change,which will lead to the unbalanced starting of particles;in the area less than 4 times the height of rough element from the wall,the rough element has a greater impact on the flow field in the windward area of the pile top;in the area more than 4times the height of rough element from the wall,the flow field structure is less affected by rough element.(3)By analyzing the flow characteristics of particles near the wall,it can be concluded that the velocity gradient and pressure gradient of the fluid around the particles change little under the shielding effect of the rough element,which reduces the critical velocity of the unbalanced starting of particles;However,due to the high velocity of the fluid between the particles and the rough element,the impact and friction effect of the local fluid on particles is increased.(4)The position of particles will directly affect the angle between the direction of force and the direction of fluid flow.At the same time,the distribution of flow field in different areas of the pile is also different.Using a single equation to describe the starting model of particles will produce large errors.In order to be able to more accurately describe the starting state of particles,the starting wind speed equations of particles at different positions on the surface of the pile are established respectively.(5)The flow field structure near the wall of the pile was obtained by PIV experiment in wind tunnel.The experimental results are in good agreement with the simulation results.The results of flow field analysis show that although the vertical velocity of the fluid is much lower than the horizontal velocity,there is a maximum value at the front of the windward side,which is about half of the horizontal velocity atthe same location,which will produce a large drag force on the vertical direction of the particles at that location,leading to the imbalance and start of the particles;the standard deviation of the velocity and vorticity of the fluid in the region of the top wind surface is relatively large,while the standard deviation of the velocity and vorticity of the fluid in the region of the back wind surface is relatively small due to the existence of the flow structure.