Crosswind Diffusion
Transport of diluted species applications can often result in models with a very high cell Péclèt number—that is, systems where convection or migration dominates over diffusion. Streamline diffusion and crosswind diffusion are of paramount importance to obtain physically reasonable results. The Transport of Diluted Species interface provides two crosswind diffusion options using different formulations. Observe that crosswind diffusion makes the equation system nonlinear even if the transport equation is linear.
Do Carmo and Galeão
This is the formulation described in Numerical Stabilization in the COMSOL Multiphysics Reference Manual. The method reduces over- and undershoots to a minimum, even for anisotropic meshes.
In some cases, the resulting nonlinear equation system can be difficult to converge. This can happen when the cell Péclèt number is very high and the model contains many thin layers, such as contact discontinuities. You then have three options:
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Refine the mesh, especially in regions with thin layers.
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Use a nonlinear solver with a constant damping factor less than one.
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Switch to the Codina crosswind formulation.
Codina
The Codina formulation is described in Ref. 1. It adds diffusion strictly in the direction orthogonal to the streamline direction. Compared to the do Carmo and Galeão formulation, the Codina formulation adds less diffusion but is not as efficient at reducing over- and undershoots. It also does not work as well for anisotropic meshes. The advantage is that the resulting nonlinear system is easier to converge and that under-resolved gradients are less smeared out.