Finite volume approximation of optimal transport and Wasserstein gradient flows

This thesis is devoted to the design of locally conservative and structure preserving schemes for Wasserstein gradient flows, i.e. steepest descent curves in the Wasserstein space. The time discretization is based on variational approaches that mimic at the discrete in time level the behavior of steepest descent curves. These discretizations involve the computation of the Wasserstein distance, an instance of optimal transport problem. The space discretization is based on Two-Point Flux Approximation (TPFA) finite volumes, a well-known methodology particularly suited for the discretization of partial differential equations that present a conservative structure. In order to preserve the variational structure at the discrete level, we follow a first discretize then optimize approach. We start by presenting TPFA discretizations for the Wasserstein distance based on the Benamou-Brenier dynamical formulation. We expose some stability issues related to these discetizations, propose a possible solution to overcome them and derive quantitative estimate on the convergence of the discrete model. To solve the discrete optimization problem, we introduce an interior point strategy. Then, we propose first and second order accurate schemes for Wasserstein gradient flows. At this level, to reduce the computational complexity, we use an implicit linearization of the Wasserstein distance. By taking adavantage of the monotonicity of the upwind reconstruction, we propose a first order scheme which can be efficiently solved with a Newton method and show its convergence towards distributional solutions of the Fokker-Planck equation. In order to higher the accuracy in space, we use a centered reconstruction, which requires a different optimization technique. We use again the interior point strategy for this purpose. Finally, we propose a modified variational BDF2 time discretization and prove its convergence towards Wasserstein gradient flows. Thanks to these new discretizations, we design a second order accurate scheme in both time and space. All our approaches are validated with several numerical results.