Anisotropic heat conduction

Summary

Anisotropic heat conduction is conduction in a material that conducts heat better in some directions than in others. Crystals, wood, fibre composites and layered rocks are examples. The thermal conductivity becomes a symmetric positive definite matrix $K$, and Fourier's law becomes $\mathbf{q} = -K \nabla T$. The heat flux then need not point straight down the temperature gradient.