Numerical study of dense particle flows under planar shear

Sammanfattning: In this thesis I conducted numerical simulations to study the flow behavior of dense particle flows composed of hard particles under planar shear using the discrete element method. The simulations were carried out in two dimensional systems, where the particles are modelled as circular discs. The discs are non-Brownian and neutrally buoyant. The granular flows can be either dry or immersed in a Newtonian fluid, where the fluid is treated in a mean field manner and represented by a velocity profile. The works that are included in this thesis can be divided into two parts. The first (Paper I+II) focus on the rheology of discontinuous shear thickening (DST) granular flows under steady planar shear (i.e.~with a constant shear-rate). The DST behavior is reproduced using the critical load model (CLM), where a threshold force is introduced for determining whether there is friction between the discs at contact. A contact is frictional if the normal force between the discs is larger than the threshold. It is found that a key parameter that controls the rheology of such flow is the fraction of frictional contacts, defined as the ratio of the number of frictional contacts to the total number of contacts. By performing simulations under controlled imposed pressure, we are able to investigate behaviors of suspensions close to shear jamming points as well as suspensions with intermediate fraction of frictional contacts. The constitutive laws are then presented, which are used to predict rheology of discontinuous shear thickening particle flows under various shear protocols. The types of particle flows range from viscous suspensions where the particles are strictly overdamped so that the particle inertia are negligible to dry granular flows where the particle inertia are dominant, as well as suspensions where both particle inertia and viscous drag is important. The second part (Paper III and IV) focuses on the behaviors of dense viscous suspensions under oscillatory planar shear. The simulations wereconducted both with constant packing fraction and constant imposed pressure. The oscillatory shear is either a pure oscillation or with an extra oscillatory shear parallel to a primary shear. It is found that by having an oscillatory shear parallel to the primary shear, the viscosity of the suspensions decreased. Furthermore, the shear jamming packing fractions for the suspensions composed of frictional particles are found to be increased under oscillation conditions, possibly due to the microstructure of the suspensions.