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# Documentation

## Diffusion

Since heat transfer is a diffusion process, the generic diffusion equation has the same structure as the heat equation:

$\frac{\partial c}{\partial t}-\nabla \text{\hspace{0.17em}}·\text{\hspace{0.17em}}\left(D\nabla c\right)=Q,$

where c is the concentration, D is the diffusion coefficient and Q is a volume source. If diffusion process is anisotropic, in which case D is a 2-by-2 matrix, you must solve the diffusion equation using the generic system application mode of the PDE app. For more information, see PDE Menu.

The boundary conditions can be of Dirichlet type, where the concentration on the boundary is specified, or of Neumann type, where the flux, $n\cdot \left(D\nabla c\right)$, is specified. It is also possible to specify a generalized Neumann condition. It is defined by $n\cdot \left(D\nabla c\right)+qc=g$, where q is a transfer coefficient.

Visualization of the concentration, its gradient, and the flux is available from the Plot Selection dialog box.