CIRF Seminar on "Rheology and Segregation of Granular Mixtures"
Speaker: Anurag Tripathi, Fluid Mechanics Lab., Dept. of Chemical Engineering, Indian Institute of Technology, Bombay
Title: Rheology and Segregation of Granular Mixtures
Abstract: Flow and segregation of dry particulate materials is observed widely in nature (debris flow, avalanche, river bed transport) and finds enormous application in industry (cement, pharmaceutical, mining, agro-products). We study the rheology and segregation of dry granular materials by means of DEM simulations. Kinetic theory of dissipative gases is found to describe the behaviour of frictionless, inelastic particles. For realistic case of mono-disperse frictional particles, a visco-plastic fluid model, with apparent viscosity being dependent not only on shear rate but also on the pressure, is found to be appropriate. The shear stress to pressure ratio, termed as effective friction coefficient, is found to depend on a single non-dimensional parameter called 'Inertial number' relating the shear rate and pressure. We extend this visco-plastic model to the generic case of mixtures of different size and/or density particles by proposing a generalised inertial number and show that the model describes the rheology of binary granular mixtures of same size different density particles, different size same density particles and different size different density particles very well.
To understand the segregation of granular materials due to difference in particle density/size, we perform detailed study of trace particles of different density/size particles in a sheared granular medium. We propose the concept of granular buoyancy by accounting for the presence of voids and show that Archimedes principle holds for this discontinuous medium as well. We perform granular Stokes' experiment at low Reynolds numbers and show that the drag force on a sphere of same size can be given as $F=4.5\pi\eta rv$. However, there are significant qualitative differences between the granular flow and the hydrodynamic flow which reflect the different rheology for the two cases. Ratio of the form to friction drag for granular fluids is approximately 6:1 as opposed to 1:2 in the case of Newtonian fluids. We use this grain level information to predict the segregation of same size different density particles and show that the segregation is determined by an "effective temperature" relating the diffusivity and mobility of the particles. Effect of micro-mechanical parameters of the grain like inelasticity and friction coefficient on segregation would also be discussed.
Host: Prof. Eckart Meiburg, Mechanical Engineering