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Temperature divergence at high quality mesh

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Old   July 20, 2021, 01:43
Smile Temperature divergence at high quality mesh
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Haeseong. Choi
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Hi. I'm quit new at CFD, but i have essential knowledge about it..

My case file is quit simple. It is the water tank filled with water

and this tank is heated by heating coil in which hot water flow.

I solved the conjugated heat transfer problem with gravity on (the flow in the tank is 100% natural convection)

The problem is, after 1000 iteration. the minimum temperature in the tank diverges with the message 'temperature is limited to 1k'

So. i checked the mesh that has temperature divergence, but the mesh quality and the shape are quit fine. (skewness <0.7, orthogonal quality >0.7)

And it is interesting that, this divergence problem is only occurred when any kind of enhanced wall treatment (kw sst. ke ewt ..)is activated
(Of course, Every wall boundaries have y+value lower than 5~10)

I also tried to control the temperature URF, but it makes interesting problem again.

Infinitesimal decrease of temperature URF (like 1 to 0.999) makes temperature field freeze (temperature is not change at all)

i know the basic principle and equation of URF, but i cant understand this kind of weird problem
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Old   July 20, 2021, 07:40
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Uwe Pilz
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Buoyancy tend to get unstable easily.
- Write the results in close time steps and look what happens.
- use more stable schemes
- use relaxation
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Uwe Pilz
Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)
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cfd, conjugated heat transfer, divergence, fluent, urf

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