A dynamic capillarity equation with stochastic forcing on manifolds: a singular limit problem

Abstract: We consider a dynamic capillarity equation with stochastic forcing on a compact Riemannian manifold $(M,g)$. \begin{equation*}\tag{P} d \left(u_{\varepsilon,\delta}-\delta \Delta u_{\varepsilon,\delta}\right) +\operatorname{div} f_{\varepsilon}(x, u_{\varepsilon,\delta})\, dt =\varepsilon \Delta u_{\varepsilon,\delta}\, dt \Phi(x, u_{\varepsilon,\delta})\, dW_t, \end{equation*} where $f_{\varepsilon}$ is a sequence of smooth vector fields converging in $L^p(M\times \Bbb{R})$ ($p>2$) as $\varepsilon\downarrow 0$ towards a vector field $f\in L^p(M;C^1(\Bbb{R}))$, and $W_t$ is a Wiener process defined on a filtered probability space. First, for fixed values of $\varepsilon$ and $\delta$, we establish the existence and uniqueness of weak solutions to the Cauchy problem for (P). Assuming that $f$ is non-degenerate and that $\varepsilon$ and $\delta$ tend to zero with $\delta/\varepsilon^2$ bounded, we show that there exists a subsequence of solutions that strongly converges in $L^1_{\omega,t,x}$ to a martingale solution of the following stochastic conservation law with discontinuous flux: $$ d u +\operatorname{div} f(x, u)\,dt=\Phi(u)\, dW_t. $$ The proofs make use of Galerkin approximations, kinetic formulations as well as $H$-measures and new velocity averaging results for stochastic continuity equations. The analysis relies in an essential way on the use of a.s.~representations of random variables in some particular quasi-Polish spaces. The convergence framework developed here can be applied to other singular limit problems for stochastic conservation laws.