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Diverging solutions for natural convection flow at differentially heated vertical wal

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Old   May 26, 2021, 13:20
Default Diverging solutions for natural convection flow at differentially heated vertical wal
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Hi guys,

this is my very first thread in this forum.

Currently I am working on natural convection flow in a closed cavity with two differentially heated, opposing, vertical side walls using Boussinesq approach using OF v2006.
I use the chtMultiRegionFoam, because for one of the walls, the temperature profile is a function of hight. The solid wall represents a thin copper plate.
So far so good, my simulation run quite good for wall hights until 7.5m... but when I furtherly increase my wall hight to 10m, the solution diverges (exploding Courant-Number/shrinking time steps). I already limited my temperature fields in fvOptions, so no non-physical temperature values anymore.

I run those simulations in 2D, so front and back patches are set as empty (2D, because number of cells would fastly exceed my HPCs computation capacities, while maintaining y+~1)

I guess my problem lies with the pressure. But I am not able to find my mistake.

All walls got "fixedFluxPressure"-BC for p_rgh and "calculated"-BC for p of course. InternalField values I sat to 0, because my equation of state is Boussinesq (I tried 1e5, but it didn't change anything of course) Thus I needed to define a pRefPoint and a pRefValue in fvSolutions. I tried setting pRefPoint to the very top of my geometry und to the very bottom - still getting same error. pRefValue is still 0 (also tried 1e5 here).
I use kOmegaSSTSAS, but also tried laminar for checking if it's a matter of the turbulence model, but it seems not to be the problem.

My error message is basically saying, that my time steps became much too small for getting a solution. Also number of iterations for p_rgh increase significantly together with rapidly decreasing timeStep size.

All of my walls have noSlip conditions in U and wallfunctions for turbulence:
alphat --> compressible::alphatWallFunction
nut --> nutkWallFunction (for low- and highRe)
k --> kLowReWallFunction (for low- and highRe)
omega --> omegaWallfunction (for low- and highRe)

For Temperature I have "fxedValue" on the right vertical wall and the typical "compressible::turbulentTemperatureCoupledBaffleMi xed"-BC for fluid-solid-interface on the right vertical wall. Top and bottom walls are set to "zeroGradient".

As I said before, my simulatios ran pretty good and fast with lower vertical wall hights. But as I increase wall hight and cavity width (proportionally with an aspect ratio of hight/width=10), keeping mesh resolution nearly constnat (still maintaining y+~1), the simulation won't run anymore.

As an additional information: the physical behavior is a natural convection loop, bacause the left solid wall has a higher Temperature than the fixed temperature right wall.

Two attached pictures show the Temperature field of the complete geometry (in this case hight: 0.384m, cavity width: 0.038m, copper plate thickness: 9.5mm) and the velocity field with mesh resolution of the top part of the geometry.



I'd be really happy, if anyone could help.

SonnyD is offline   Reply With Quote


boussin, heat transfer, openfoam

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