Partial optimal transport for a constant-volume Lagrangian mesh with free boundaries

•A new Lagrandian free-surface mesh representation is proposed.•Builds on recent advanced in computational optimal transport.•Volume conservation is accurately enforced.•Changes of topology, interfaces and contacts are accurately tracked.•The representation smoothly depends on the parameters. This article introduces a representation of dynamic meshes, adapted to some numerical simulations that require controlling the volume of objects with free boundaries, such as incompressible fluid simulation, some astrophysical simulations at cosmological scale, and shape/topology optimization. The algorithm decomposes the simulated object into a set of convex cells called a Laguerre diagram, parameterized by the position of N points in 3D and N additional parameters that control the volumes of the cells. These parameters are found as the (unique) solution of a convex optimization problem – semi-discrete Monge-Ampère equation – stemming from optimal transport theory. In this article, this setting is extended to objects with free boundaries and arbitrary topology, evolving in a domain of arbitrary shape, by solving a partial optimal transport problem. The resulting Lagrangian scheme makes it possible to accurately control the volume of the object, while precisely computing the intersections with the domain boundary, the interactions, the collisions, and the changes of topology.