Homogenization of linear transport equations. A new approach

Abstract

The paper is devoted to a new approach of the homogenization of linear transport equations induced by a uniformly bounded sequence of vector fields $b_\ep(x)$, the solutions of which $u_\ep(t,x)$ agree at $t=0$ with a bounded sequence of $L^p_{\rm loc}(\RR^N)$ for some $p\in(1,\infty)$. Assuming that the sequence $b_\ep\cdot\nabla w_\ep^1$ is compact in $L^q_{\rm loc}(\RR^N)$ ($q$ conjugate of $p$) for some gradient field $\nabla w_\ep^1$ bounded in $L^N_{\rm loc}(\RR^N)^N$, and that there exists a uniformly bounded sequence $\sigma_\ep>0$ such that $\sigma_\ep\,b_\ep$ is divergence free if $N\!=\!2$ or is a cross product of $(N\!-\!1)$ bounded gradients in $L^N_{\rm loc}(\RR^N)^N$ if $N\!\geq\!3$, we prove that the sequence $\sigma_\ep\,u_\ep$ converges weakly to a solution to a linear transport equation. It turns out that the compactness of $b_\ep\cdot\nabla w_\ep^1$ is a substitute to the ergodic assumption of the classical two-dimensional periodic case, and allows us to deal with non-periodic vector fields in any dimension. The homogenization result is illustrated by various and general examples.