Free Surface

Free surface BCs are frequently adopted at top boundary of a atmospheric flow. They can be implemented from a Neumann BC that considers zero gradient of macroscopic properties.

Neumann

The implemented Neumann BC is used to represent a free surface by forcing a zero normal derivative of macroscopics at the boundary:

\[\frac{\partial A}{\partial x_{n}}=0;\]

Where \(A\) represents any macroscopic variable (\(\rho\), \(u_{\alpha}\), \(S_{\alpha\beta}\)). To implement that, the boundary nodes replicate the macroscopic values from the first node towards normal direction.