Molecular Dynamics Simulation Of Microscopic Characteristics Of Wall Slip Of Magnetorheological Fluid

Magnetorheological fluid(MRF)is a controllable intelligent material.Because of its unique magnetorheological effect,it is widely used in automotive,aerospace,medical and other fields.The shear flow of magnetorheological fluid is more common in practical applications,but its microscopic wall slip mechanism is rarely studied.In this paper,the coarse-grained molecular dynamics method is used to simulate the flow of the magnetorheological fluid Couette composed of water and magnetic nanoparticles,and to explore the shear slip of the magnetorheological fluid along the polyethylene wall under the conditions of no magnetic field and an external uniform magnetic field.The evolution of the microstructure,the factors and rules that affect the characteristics of wall slip.The main factors that affect the wall slip characteristics of magnetorheological fluids are:shear rate,magnetic nanoparticle volume fraction,magnetic field strength,and wall potential energy strength.In the absence of a magnetic field,with the increase of the shear velocity(11 dimensionless velocities V*),the fluid near the slip interface will be stratified,a layer of lubrication is formed at the interface,and the wall slip of the magnetorheological fluid is obvious.The slip velocity at the interface increases,and nanoparticle clusters appear in the mainstream area far away from the near-wall area,and the shear stress increases with the increase of the shear velocity.As the volume fraction of magnetic nanoparticles(5%,8%,10%,13%,15%)increases,the slip velocity and shear stress at the interface also increase.The number density keeps decreasing.As the wall potential energy coefficient(0.5,1.0,3.0)increases,the number density at the slip interface increases,and a dense solid-like lubricating layer appears.Therefore,the slip velocity at the interface decreases and the shear stress follows Decrease.Among the above-mentioned factors affecting wall slippage,the increase in wall potential energy has the most obvious effect on the increase in the number density of the fluid at the interface of the magnetorheological fluid;the increase in the shear rate and the volume fraction of magnetic nanoparticles both cause the increase in the interface The fluid number density decreases.Under the condition of applying a uniform magnetic field to the boundary,magnetic nanoparticles form magnetic chains and cluster-like structures.The tightness of the chain-like structures is positively correlated with the applied magnetic field.As the magnetic field strength increases,the magnetic chains and cluster-like structures formed are more stable.And its direction is the same as the direction of the applied magnetic field.After the magnetic field is applied to the slip boundary,the magnetorheological fluid Couette flow is subjected to the magnetic-mechanical coupling simulation calculation.The results show that when the volume fraction of magnetic nanoparticles is 10%and the magnetic field intensity is 140KA/m,the application of shear will destroy the long chain structure formed after the application of the magnetic field,forming many shorter magnetic chains on the wall,and shear stress appears.Partition phenomenon,when 1<V*<3,the shear stress gradually increases;when 3<V*<7,the shear stress gradually decreases;when V*>7,the shear speed increases,and the magnetic link is cut More short chains are formed and attached to the shear wall,which causes the shear stress to continue to increase.The expression of the relationship between the shear speed and the slip length is obtained by fitting.And the shear stress is greater than the shear stress without a magnetic field.When the applied magnetic field intensity is 140KA/m and the shear rate is V*=3,as the volume fraction of magnetic nanoparticles(5%,8%,10%,13%,15%)increases,the magnetorheological fluid approaches the wall The greater the number of zones,the greater the slip velocity and shear stress at the interface.Similar to the non-magnetic field shear,the magnetic field strength and the wall potential energy have the most obvious influence on the wall slip characteristics of the magnetorheological fluid.When the coupled field acts,there is a layer of liquid-carrying particles at the interface,which has a lubricating effect,slows down the friction between the magnetic chain and the wall,and causes apparent wall slip.With the increase of the magnetic field strength(40KA/m,140KA/m,240KA/m),the carrier liquid particles at the sliding interface decrease,the magnetic chain and the shear wall are in direct contact,and the force between the wall and the wall increases,resulting in a wall surface The real slip is similar.As the wall potential energy coefficient(0.5,1.0,3.0)increases,the number density of carrier liquid particles at the interface increases,forming a dense solidlike lubricating layer at the interface,which reduces the friction between the magnetic chain and the wall.The slip between the magnetic chain and the lubricating layer becomes obvious,and the slip speed at the slip interface decreases,and the resulting shear stress decreases.