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-   -   Natural convection in water (http://www.cfd-online.com/Forums/star-ccm/132056-natural-convection-water.html)

CneSpaulding March 25, 2014 06:39

Natural convection in water
 
Hi guys,

Here is the case: I want to modelize the water natural convection.
The geometry is quite simple: it's a parallelepiped rectangle.
A surface heatsink represented by a quarter circle in a corner ( the center coordinate is (0,0,0).

MESH: Polyhedral Mesher
Prism Layer Mesher
Surface Remesher

BC: All the surfaces are wall except the 2 vertical walls between the surface are symetrics.

The physics models I used are:

Boussinesq
Constant density
Coupled Flow/energy
Gradients
Gravity
Laminar
3D
Liquid

I tried steady and unstead but the calculation do not converges. The main problem is the Z- momentum (him residuals states at around 1-10)

I saw that replacing the two walls and the top by outlet pressure instead of walls could gives results, but when I tried I had reversed flows..

Anyone can tell me wich are my mistakes and which parameters I have to modifiate?

Thanks for reading

ping March 25, 2014 11:16

natural convection is in real life an unsteady process so i would be surprise if you can get a converged solution in steady state.
so i would be running unsteady and testing a few time steps and checking the courant number and dumping scene images to check for reasonable flows.
also when using the physics you have mentioned be careful to set the coef of expansion of the fluid since star-ccm+ sets this to 0. for some reason it is not in the material database.

CneSpaulding March 25, 2014 12:25

It's working now for some reasons!
Thanks for your answer.

enayath May 5, 2014 15:09

Ping,

Could you please explain what did you do to fix it?
what is you Ra number?

How did you understand that it converged?
just by checking the residuals or you defined some points/lines inside your domain and look for a converged of averaged value for the parameter you defined...

Thank you,
Hooman

ping May 7, 2014 07:07

you had better ask CneSpaulding about how he solved his case.

but you should always have some relevant reports being monitored and plotted to help judge convergence, even for steady state cases.

CneSpaulding May 13, 2014 09:25

Hi,

Sorry for the late answer, I'm quite busy this month.

I used the following parameters:

Implicit Unsteady
Boussinesq
Coupled Flow/Energy
The 2 walls are adiabatic

And I used a better mesh.

I just checked the residuals plot, when I saw that all the parameters converged under 10-5 I decided to stop the calculation.


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