[alg]

Hi,

Just wondering if anyone could have a look at a problem I'm having getting a CHT simulation to converge/reach steady output values?

The residuals and values of interest (a number of different heat fluxes) behave well to begin with and almost reach a level/stability that I would be happy with. Then at 20-50 iterations (depending on the mesh/timescales) the T-energy residual begins to increase again and heat flux through the top boundary (fixed heat transfer coefficient and external temperature on a solid domain) and at a conductive (solid-solid) interface near there begin to fluctuate. The fluctuation grows and spreads until the whole simulation looks like a heart-rate monitor.

I've stopped the run at the beginning of this instability and seen a number of hot spots on what should be cold boundaries. Results from later on in the unstable simulations involve hot spots many times hotter than anything in the model should be. These spots are nowhere near any fluid, but are near a fairly complex geometry involving 4-5 different solids with different properties. The top domain is a thin (0.6mm) steel sheet (in the context of a 10x0.8x0.45m geometry) too, which is meshed with thin plate-like elements.

I'm not sure if the problem could be my mesh in this top domain? I thought the heat transfer equations should behave fairly well given a semi-decent mesh - maybe mine isn't semi-decent? I've tried a range of solid timescales but this just shifts the onset of instability. I've also tried refining the mesh in the region of the hotspots but that brought on instability after only 10 iterations (instead of ~30). I've attached some images that show an initial hotspot, and the coarser and finer mesh in this area.

Any help would be great - I'm at a bit of a loss with this one...

Cheers,
Alan