We develop and fully characterize a meshfree Lagrangian (particle) model for continuum-based numerical modeling of dry and submerged granular flows. The multiphase system of the granular material and the ambient fluid is treated as a multi-density multi-viscosity system in which the viscous behavior of the granular phase is predicted using a regularized viscoplastic rheological model with a pressure-dependent yield criterion. The numerical technique is based on the Weakly–Compressible Moving Particle Semi-implicit (WC–MPS) method. The required algorithms for approximation of the effective viscosity, effective pressure, and shear stress divergence are introduced. The capability of the model in dealing with the viscoplasticity is validated for the viscoplastic Poiseuille flow between parallel plates. The model is then applied and fully characterized (in respect to the various rheological and numerical parameters) for dry and submerged granular collapses with different aspect ratios. The numerical results are evaluated in comparison with the available experimental measurement from the literature as well as some complementary experimental measurements, performed in this study. The results show the capabilities of the presented model and its potential to deal with a broad range of dry and submerged granular flows. They also reveal the impotent role of the regularization, effective pressure, and shear stress divergence calculation methods on the accuracy of the results. For the case of the granular collapse the results characterize the shape, evolution, and flow regimes of granular deposit, as well as the important effect of the ambient fluid.
