Approximation of the effective conductivity of ergodic media by periodization

Abstract

This paper is concerned with the approximation of the effective conductivity σ(A, μ) associated to an elliptic operator ∇_xA(x,η)∇_x where for xЄℝ^d,d ≥ 1, A(x,η) is a bounded elliptic random symmetric d×d matrix and η takes value in an ergodic probability space (X, μ). Writing A^N (x, η) the periodization of A(x, η) on the torus T^d_N of dimension d and side N we prove that for μ-almost all η lim ^(N→+∞) σ(A^N, η) = σ(A,μ) We extend this result to non-symmetric operators ∇_x (a+E(x, η))∇_x corresponding to diffusions in ergodic divergence free flows (a is d×d elliptic symmetric matrix and E(x, η) an ergodic skew-symmetric matrix); and to discrete operators corresponding to random walks on ℤ^d with ergodic jump rates. The core of our result is to show that the ergodic Weyl decomposition associated to L^2(X,μ) can almost surely be approximated by periodic Weyl decompositions with increasing periods, implying that semi-continuous variational formulae associated to L^2(X,μ) can almost surely be approximated by variational formulae minimizing on periodic potential and solenoidal functions.