Bodhinanda (Nanda) Chandra

This work presents a unified viscoelastic-viscoplastic continuum framework for modeling rate-dependent granular flows across regimes. The formulation incorporates two distinct rate-dependent mechanisms, namely micro-inertia and viscoelastic dissipation, within a single continuum description. A central contribution is an explicit link between the coefficient of restitution and a continuum viscosity, derived from an analysis of wave attenuation in granular assemblies, thereby establishing a direct connection between particle-scale collision physics and macroscopic damping. This relation is introduced while retaining inertia-dependent plastic flow governed by the classical μ(I) rheology. The constitutive model is constructed by meticulously partitioning elastic and plastic responses within the model and corresponding stress-update routine, such that viscous dissipation governs wave propagation and collisional processes without altering the plastic flow rule. The framework is implemented within the material point method to simulate transient processes involving large deformations, material separation, and subsequent reconsolidation. A range of numerical examples, including steady, transient, vibrational, and impact-driven flows, demonstrates that the model captures wave propagation, diffusion, and rate-dependent granular behavior within a unified continuum setting.

Previous studies have demonstrated that the virtual stress boundary (VSB) method accurately and efficiently imposes nonconforming Neumann boundary conditions in the material point method (MPM). A key advantage of the VSB method is its ability to impose boundary conditions without requiring explicit knowledge of the boundary’s position. This is achieved through a transformation that replaces the original boundary traction with an equivalent virtual stress field. The inclusion of this virtual stress field allows the governing equations to be solved using only volume integrals, which are easily computed in the MPM using a particle‐wise quadrature scheme. In this study, the VSB method is extended to accommodate various high‐order MPM variations. The updates mainly involve addressing the non‐local support domain of high‐order shape functions and enhancing quadrature accuracy in filled cells. The accuracy and versatility of the combined high‐order MPM and VSB method are demonstrated through numerical examples in 1D, 2D, and 3D.

Modeling the incompressible fluid flow in porous media has long been a challenging task in the Material Point Method (MPM). Although widely used, conventional Updated Lagrangian MPM (ULMPM) often suffers from numerical stability and computational efficiency issues in the hydromechanical analysis of saturated porous media. To address these issues, we herein present a novel semi-implicit Total Lagrangian MPM (TLMPM). The proposed TLMPM leverages the fractional step method to decouple pore pressure from kinematic fields and employs the semi-implicit scheme to bypass the small time step constraint imposed by permeability and fluid compressibility. Unlike its UL counterpart, the TLMPM evaluates weighting functions and their gradients only once in the reference configuration, eliminating material point tracking and inherently resolving cell-crossing instabilities. Given the consistent set of active degrees of freedom throughout simulations, the proposed method greatly reduces computational costs associated with system matrix assembly for both kinematics and pore pressure and with free-surface node detection. Furthermore, this feature also facilitates the efficient Cholesky factorization, resulting in a substantial acceleration of the solver performance. The proposed approach has been validated against various benchmark tests, and our results have highlighted the remarkable performance of TLMPM, which can achieve up to 63 times speedup over conventional methods, scaling favorably with problem size, and retaining numerical stability even with low-order basis functions. These advancements position the TLMPM as a transformative tool for poroelastic analysis, with broader applicability to large-deformation problems in geomechanics, energy systems, and environmental engineering.

This study focuses on solving the numerical challenges of imposing absorbing boundary conditions for dynamic simulations in the material point method (MPM). To attenuate elastic waves leaving the computational domain, the current work integrates the Perfectly Matched Layer (PML) theory into the implicit MPM framework. The proposed approach introduces absorbing particles surrounding the computational domain that efficiently absorb outgoing waves and reduce reflections, allowing for accurate modeling of wave propagation and its further impact on geotechnical slope stability analysis. The study also includes several benchmark tests to validate the effectiveness of the proposed method, such as several types of impulse loading and symmetric and asymmetric base shaking. The conducted numerical tests also demonstrate the ability to handle large deformation problems, including the failure of elasto-plastic soils under gravity and dynamic excitations. The findings extend the capability of MPM in simulating continuous analysis of earthquake-induced landslides, from shaking to failure.

This paper presents a novel stabilized mixed material point method (MPM) designed for the unified modeling of free-surface and seepage flow. The unified formulation integrates the Navier-Stokes equation with the Darcy-Brinkman-Forchheimer equation, effectively capturing flows in both non-porous and porous domains. In contrast to the conventional Eulerian computational fluid dynamics (CFD) solver, which solves the velocity and pressure fields as unknown variables, the proposed method employs a monolithic displacement-pressure formulation adopted from the mixed-form updated-Lagrangian finite element method (FEM). To satisfy the discrete inf-sup stability condition, a stabilization strategy based on the variational multiscale method (VMS) is derived and integrated into the proposed formulation. Another distinctive feature is the implementation of blurred interfaces, which facilitate a seamless and stable transition of flows between free and porous domains, as well as across two distinct porous media. The efficacy of the proposed formulation is verified and validated through several benchmark cases in 1D, 2D, and 3D scenarios. Conducted numerical examples demonstrate enhanced accuracy and stability compared to analytical, experimental, and other numerical solutions.