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Converging problem of buoyancy flow

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Old   January 19, 2007, 14:41
Default Converging problem of buoyancy flow
  #1
Cadrian
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Dear all,

I have reduced the under-relaxation factor, used 1st order scheme, follow every steps suggested by buoyancy simulation from the Fluent user guide, yet failed to obtained a converged solution.

The simulated building block size is 20m x 20m x50m, the closing domain of outdoor air is 200m x 200m x 250m.

Boundary conditions: 1 pressure inlet (top face of the domain) and 4 pressure outlets(4 side faces)

For the mesh, I have produced a mesh with boundary layer growth factor (b/a) of 1.3, started at the first row (a)of 0.03 and the number of row of 4.The interval size of the mesh volume is 10. Thus, a fine mesh near the building wall surface and coarse mesh at the surface of the closing air domain is produced. (For Gambit meshing, I used meshing type T-Grid, and the elements option of Tet/Hybrid)

As the solution did not converge, and so the velocity vectors produced looked unexpected. From your experience, can you suggest me about some important points I may ignore in making a good mesh or boundary conditions? And also, how many iteration steps you usually see when the solution starts to converge from your experience?

Thank you very much

Regards,

Cadrian

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Old   January 19, 2007, 15:02
Default Re: Converging problem of buoyancy flow
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Sujith
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which multiphase are you using? are you using any turbulence model? if yes, check your y_plus/ aspect ratio of mesh.. Try with reduced URF..
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Old   January 19, 2007, 15:43
Default Re: Converging problem of buoyancy flow
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Cadrian
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Dear Sujith,

I am using the Boussineq model of the air. And also I am using k-e turbulence model. And I have tried a under-relaxation factor as low as 0.7 (is that enough?)

Anyway, do you have any suggested value of y_plus/aspect ratio of mesh?

Thank you so much =)

Cadrian
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Old   January 19, 2007, 17:08
Default Re: Converging problem of buoyancy flow
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phil
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I had similar issues with buoyancy flows... perhaps there is no stationary solution, so you should use the unsteady solver and see what happens!
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Old   January 19, 2007, 18:09
Default Re: Converging problem of buoyancy flow
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Bak_Flow
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Hi Cadrain,

try running it transient with say 5 iterations per time step. You can calculte a velocity scale given

Bak_Flow
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