A volume-adaptive mixture-theory MPS model for transitional fluid–granular dynamics and impulse waves

Highly dynamic fluid–granular flows transitioning from subaerial to submerged conditions exhibit strongly nonlinear behaviour involving large deformations, evolving pore pressure, seepage, and granular dilatancy or compaction, posing significant challenges for computational modelling. This study presents a novel mixture-theory-based two-phase Moving Particle Semi-implicit (MPS) model for transitional fluid–granular dynamics. We develop the first volume-adaptive MPS formulation that explicitly accounts for particle volume variations induced by fluid seepage and granular deformation, enabling consistent modelling of dilatancy, compaction, and dry-to-saturated transitions. The granular phase is represented as a deformable porous medium using a generalized constitutive law that captures solid- and fluid-like behaviour under varying saturation conditions. A unified coupling strategy ensures conservative interphase interactions between overlapping continua with evolving volumes. Stabilization techniques are reformulated to accommodate variable-volume particle behaviour while maintaining numerical stability. The model is validated against benchmark problems involving subaerial granular column collapse, dam-break flows into porous media, and hydrostatic stress development in initially dry granular columns, and is further applied to two- and three-dimensional granular collapses interacting with free-surface flows. The results demonstrate accurate predictions of granular dynamics, pore-pressure evolution, and impulse-wave generation and propagation, establishing a volume-consistent particle framework for transitional fluid–granular flows.