Roeland Nieboer

Hi Peter and Matteo,

I have about the same (2D vertical, trans flow/trans transport) problem:
It's the [b]Horton-Rogers-Lapwood [/b]problem in subcritical rayleigh domain (no convection), but with [u]h=0 upper boundary and q=0.6885 W/m2 at lower bound[/u] (corresponds to 0.03K/m geothermal gradient with Feflow default material parameters).

For [b]initial conditions[/b] I first run the problem with a lower bound constant temperature (20.5 deg C) at steady state. This gives a nice linear geothermal gradient. I then switch to transient/transient with constant q.
I use 'streamline upwinding' and a constant(T), thermal expansion coefficient = 1,5 e-4 /K

[b]Meshing[/b]
Domain: W=2500 m, H=350 m (1 polygon)
Mesh: Triangle, refinement around all polygon borders, gradation 6, target 1m (rest default triangle settings)
The maximum element size in the middle of the model is no more than ~4 m and the model consists of 280k elements.

Even when increasing the BICGSTABP iterations to 2500 I still get residuals of 10e+8, which seems similar to Matteo findings.

The expectation is that I will get the same temperature distribution as the initial conditions and I do, but instability might very possibly lead to artefacts upon complexifying the model.This seems like a pretty straight forward problem, so [b]what do you reckon could cause the instability? [/b]

Thank you very much, keep up the good work!
[u][/u]
Kind regards,
Roeland