add a example of Bretherton-Haidvogel model for 2D turbulence over topography

[miniufo]

This is an example I recently ran into, which adopted a variational calculas to get the steady-state equation between a 2D flow with total kinetic energy $E_0$ and a given topography (small compared with fluid depth).

Once the topography is specified, the steady state of the flow with $E_0$ is also determined.

[miniufo]

This is done here. Closing this.