Study On The Slip Characteristics Between Free Flow And Porous Media When Turbulent Over And Within Porous Media

The solid material with pores inside is called porous medium.Generally,the porous medium in nature has a complicated pore structure,which is difficult to be characterized by models or parameters.Porous media are indispensable for nature and human daily life.The soil layer for filtering and purifying fresh water,the temperature insulation materials in daily life and the clothes we wear are all porous media.The problem of material transport in the coupled flow of porous media and fluid is widely used in many subjects,from biology to geophysics.For example,the pore size of the river bed will affect the transport and dispersion of solute in the river;the permeability of cell membrane plays an important role in the exchange of materials in the cell;The pore structure of geological layer will affect the oil recovery rate.A better understanding of the slip law and flow characteristics of fluid in porous media will be of great help to many fields and industries,so it is of great significance to study the coupled flow of fluid and porous media.In particular,the boundary conditions at the interface of turbulent flow across porous media have not been studied theoretically,and still need futher study and exploration.Based on the previous research foundation,this paper uses a combination of numerical simulation and experimental research to study the velocity distribution,slip law and mass transfer mechanism of turbulent fluid sweeping through porous media.In this paper,a three-dimensional numerical model of the flow area of pure fluid and porous media is established,and a three-dimensional model of porous media area is constructed by excavating solid phase to form pore structure channel.The single domain method is used for simulation,the same control equations are used in the pure fluid and porous media regions,so it is no longer necessary to deal with the boundary conditions at the fluid solid interface,Import the three-dimensional model into Fluent software and use the standard k-?model to close the system of equations.The effects of porosity,relative water depth and Reynolds number on the results are studied by changing the parameters of simulation.The results of numerical calculation are in good agreement with the experimental data of two-dimensional velocity,the feasibility and accuracy of the numerical model are verified.In order to carry out the experimental research,a visualized flume experiment platform is designed and built in this paper.The circulation system and the velocity measurement system of the experiment platform are used to measure the velocity distribution when the fluid passes through the porous medium.In this paper,the ultrasonic Doppler velocimeter is used to measure the velocity of the fluid,and the velocity is measured by the frequency of transmitting and receiving ultrasonic waves,so the flow velocity inside the porous medium can be measured without intruding into the porous medium.The method of stacking spherical beads is used to form a rectangular cuboid of porous media,and a variety of stacking methods are used to obtain porous media with different porosities.In order to better explore the fluid flow characteristics in porous media and its interface,this paper has carried out experimental research from several research directions,mainly including one-dimensional flow rate experiment,two-dimensional flow rate experiment and bottom bed experiment of longitudinal change porosity.The main flow velocity in one-dimensional horizontal direction is obtained by ultrasonic Doppler velocimeter.The effects of Reynolds number,relative water depth and porosity on the velocity distribution are studied by using a combination of various experimental schemes.The slip law,the distribution trend of velocity and the cause of its formation are analyzed,and the sensitivity of the penetration depth of the fluid in the porous medium to each parameter is discussed.The results show that the increase of Reynolds number leads to the increase of slip velocity,and the increase of porosity and relative water depth leads to the decrease of slip velocity;The change of Reynolds number and relative water depth will not affect the transition zone.By using the two-dimensional measurement mode of ultrasonic Doppler velocimeter,the horizontal and vertical velocity can be measured at the same time.By using the two-dimensional measurement method,the velocity distribution of turbulent fluid across porous media is studied in depth,and the mass transfer phenomenon of fluid solid coupling flow is discussed.The distribution trend of diffusion velocity above and inside the porous media is studied.It is concluded that the vertical diffusion of fluid is determined by gravity,resistance and capillary force of porous media,static pressure difference and arrangement of spherical bodies.And different positions in the vertical direction will have different factors to play a leading role,while the change of Reynolds number and relative water depth has no significant impact on the diffusion velocity.The experiment of turbulent flow across porous media was carried out in a porous media bed with longitudinal porosity change.The influence of various parameters on the experimental results was also analyzed.How the change of porosity in porous media affects the distribution trend of velocity is studied.The results show that the stratification of porosity makes the barrier effect of porous media on fluid different,which leads to the flow velocity in the large porosity layer is obviously larger,so the velocity stratification phenomenon also occurs in the bottom bed of porous media.Velocity slip occurs at the interface of fluid solid interface and the interface of different porosity layers in porous media,and the trend of flow velocity changing with each parameter is more obvious in the large porosity layer.