Implementation of non-newtonian rheology for granular flow simulation

Abstract

Landslides of the debris flow type pose a serious natural hazard. These land-slides are often triggered by hydro-meteorological processes during extreme precipitationevents. Debris flows usually form a dense flow composed of water and poorly graded soilparticles. The propagation of these landslides greatly influences the consequences theyhave. The run-out of debris flows is usually simulated with depth-averaged models. Theseare fast to simulate due to the integration over the flow height, which reduces the problemfrom three to two dimensions. For the design of countermeasures resisting the pressurefrom the flow, it can be advantageous to use more advanced 3D numerical methods, suchas computational fluid dynamics (CFD). The particle phase of debris flows has here beenconsidered as a granular flow, and implemented as a non-Newtonian viscoplastic rheologyin the open-source CFD code REEF3D. In the numerical model, the Reynolds-Averaged Navier-Stokes (RANS) equations are discretized with the fifth-order accurate Weighted Essentially Non-Oscillatory (WENO) scheme in space and with a third-order Runge-Kuttabased fractional step scheme in time. The level set method used for representing the free surface handles the complex air-granular flow interface topology. The pressure gradient is modelled with Chorin’s projection method for incompressible flow. The granular flow rheology includes a Coulomb frictional yield stress, increasing with the normal stress, anda viscous term that is non-linear dependent on the shear rate. The implementation has been validated using results from laboratory dam break experiments with dry sands.