A unified framework for fluid-granular material interaction with elasto-plastic granular continua and immersed rigid bodies

Fluid–granular interactions involving moving rigid bodies pose significant challenges due to complex constitutive behavior and strong interphase coupling. This study develops a unified two-continuum smoothed particle hydrodynamics (SPH) framework coupling a weakly compressible fluid, an elastoplastic granular continuum, and immersed rigid bodies. The granular phase incorporates Drucker–Prager plasticity and μ ( I ) rheology, while fluid–granular interactions are governed by pore-pressure gradients and interphase drag. A key contribution is a dual-state boundary condition enforcement (BCE) method, in which shared boundary particles independently reconstruct fluid and granular states, enabling phase-specific boundary treatment and the transfer of coupled forces and torques to moving rigid bodies. The framework is assessed using five benchmark tests covering fluid-volume conservation, Darcy seepage, single-phase fluid and granular responses, and coupled fluid–granular dam-break dynamics. Two engineering-scale applications further demonstrate its capabilities. Rotating-drum simulations examine dynamic angles of repose under different filling configurations, while a hovercraft beaching simulation illustrates the evolution of fluid–granular support during shore contact. The latter serves as a numerical demonstration rather than an experimentally validated prediction.