Effective conductivity
= Effective conductivity
{title2=$\langle a\nabla u\rangle=a^*\langle\nabla u\rangle$}
Effective conductivity describes the averaged response of a microscopic conductivity field to a macroscopic temperature or electric-potential gradient. Here the positive conductivity flux is $a\nabla u$; physical heat flux has the opposite sign. For periodic media it is computed by cell correctors, and can be a tensor even for isotropic constituents.