Relaxation

Relaxation schemes are used in MOHID Water to relax computed solutions to a reference solution. The standard method follows the equation:

$\frac{\partial \phi}{\partial t} = \frac{ \phi_{ref} - \phi}{\tau}$

where $\phi$ is the relaxed variable, $\phi_{ref}$ is the reference solution and $\tau$ relaxation time scale.

In MOHID Water, the reference fields and the relaxation time scales are read and handled by Module Assimilation. The relaxation algorithm itself is implemented in Module Hydrodynamic and Module WaterProperties to apply the relaxation scheme to properties such as velocity U, velocity V, water level, temperature, salinity, etc. It is normally applied in the boundary condition computation, usually when nesting models or using a climatological reference field, via the definition of a "sponge layer" around the open boundary. However it can also be applied to the entire simulation domain.