Fluid effects in model granular flows

Abstract

Pore fluid plays a crucial role in many granular flows, especially those in geophysical settings. However, the transition in behaviour between dry flows and fully saturated flows and the underlying physics that relate to this are poorly understood. In this paper, we report the results of small-scale flume experiments using monodisperse granular particles with varying water content and volume in which the basal pore pressure, total pressure, flow height and velocity profile were measured at a section. We compare the results with theoretical profiles for granular flow and with flow regimes based on dimensional analysis. The runout and the centre of mass were also calculated from the deposit surface profiles. As the initial water content by mass was increased from zero to around 10%, we first observed a drop in mobility by approximately 50%, as surface tension caused cohesive behaviour due to matric suction. As the water content was further increased up to 45%, the mobility also increased dramatically, with increased flow velocity up to 50%, increased runout distance up to 240% and reduced travel angle by up to 10° compared to the dry case. These effects can be directly related to the basal pore pressure, with both negative pressures and positive pore pressures being measured relative to atmospheric during the unsteady flow. We find that the initial flow volume plays a role in the development of relative pore pressure, such that, at a fixed relative water content, larger flows exhibit greater positive pore pressures, greater velocities and greater relative runout distances. This aligns with many other granular experiments and field observations. Our findings suggest that the fundamental role of the pore fluid is to reduce frictional contact forces between grains thus increasing flow velocity and bulk mobility. While this can occur by the development of excess pore pressure, it can also occur where the positive pore pressure is not in excess of hydrostatic, as shown here, since buoyancy and lubrication alone will reduce frictional forces. Graphical abstract

Numerical simulation of potential impulse waves generated by Mogu toppling rock slope at varying water levels in Lianghekou Reservoir, China

“…Du et al Du et al (2021) took full advantage of MPM in simulating large deformation of soil and gravity-driven flow of water to reproduce the entire process of landslide-induced surge and proposed the two-phase double-point material point method (TPDP-MPM) to handle the boundary without setting boundary particles. The most used commercial software for impulse wave simulation is FLOW3D based on CFD theory (Franco et al 2020, Franco et al 2021, Khoolosi and Kabdaşlı 2016, Kim et al 2020, Zhao et al 2023. FLOW3D software models commonly employed to simulate impulse waves generated by landslides include rigid body models, elasto-visco-plasticity models, drift-flux, and granular flow models (Flow Science Inc 2020).…”