GGU-CONTAM-FE: The Peclet criterion

As you know, FEM allows almost unlimited subdivision of the system into triangular elements. Concentrations are assumed to be linearly distributed across each element. Where the contaminant front is discrete, such an approach is no longer capable of precisely modelling contaminant distribution. The contaminant fronts then become smeared. This effect is known as numerical dispersion. It occurs predominantly when the proportion of convection is large compared to other mechanisms (dispersion and diffusion). This can be avoided by selecting a denser triangular mesh.

The "Peclet criterion" serves to check whether a system displays a tendency towards numerical dispersion and thus to compute the necessary mesh density. This criterion is checked by the program after modelling has begun. If it is exceeded, mesh refinement will be required. You will then need to reprocess the system using GGU-2D-SSFLOW. Under certain conditions, however, a very dense mesh is required. If necessary, you can obtain a solution using the RANDOM-WALK method, as this procedure does not recognise the numerical dispersion effect (GGU-CONTAM-RW program).