A phase-field formulation of frictional sliding contact for 3D fully Eulerian fluid-structure interactions

• Phase-field Eulerian framework for 3D frictional contact in multiphase FSI • Overlap-based contact detection with phase-averaged slip velocity formulation • Robust enforcement of Coulomb friction in fully Eulerian simulations • Verified against Hertzian contact, sliding block, and ironing benchmarks • Demonstrated for ship–ice interaction with free-surface and contact dynamics Frictional sliding contact in hydrodynamic environments can be found in a range of engineering applications. Accurate modeling requires an integrated numerical framework capable of resolving large relative motions, multiphase interactions, and nonlinear contact responses. Building on our previously developed fully Eulerian fluid-structure formulation, we introduce a phase-field based formulation for dynamic frictional contact in three dimensions. While the structural deformations are represented through the evolution of individual left Cauchy-Green strain tensors, contact detection is achieved via the overlap of diffuse interfaces of colliding solids. Specifically, we utilize phase fraction functions to estimate the amount of overlap between the two interfaces, thereby circumventing explicit distance computations in 3D. The normal contact response is defined as a volumetric body force proportional to the overlap parameter, while the tangential response is computed using the Coulomb friction model. The direction of the friction forces are derived by projecting phase-averaged relative velocities onto the local tangent plane of colliding bodies. This proposed unified treatment enables the computation of both normal and frictional forces within a single momentum balance equation, avoiding separate velocity fields for individual solids. We present several test cases with increasing complexity to verify and demonstrate our proposed frictional contact model. Verification against the Hertzian contact problem shows excellent agreement with the analytical solution, with errors below 3 % in the traction profile. In the sliding block benchmark, the computed displacement profiles closely follow the analytical solution for point-mass systems across multiple friction coefficients. The ironing problem demonstrates stable force predictions under finite deformation, with normal and tangential forces matching kinetic friction laws. The robustness and scalability of the proposed formulation are further demonstrated through a representative ship-ice interaction scenario with free-surface and frictional sliding effects.