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-   -   Setting Gravity for Convection (http://www.cfd-online.com/Forums/star-ccm/77350-setting-gravity-convection.html)

 James Lamb June 21, 2010 07:27

Setting Gravity for Convection

I am modelling the passive cooling of a heat sink in an enclosure. I have created a volume of air with a stagnation inlet below and a pressure outlet above. When setting gravity in this model it creates a large flow downwards. I expected to see buoyancy forces at work on the hot components, this does not seem to be the case. Am I not leaving the simulation long enough or could I need more inner iterations? At the moment I am running for a minute with 2 inner iterations per second.

Thanks for the help in advance.

James

 Vinicius June 21, 2010 10:00

Are you using the coupled solver?

 James Lamb June 21, 2010 10:05

Not at the moment no.

 Vinicius June 21, 2010 10:11

Try to use the coupled solver and use more inner iterations, maybe you also can check your time-step, you should have a compromise between your time-step size and your number of inner iterations...

 James Lamb June 21, 2010 10:13

Ok just re-meshing now with a coupled solver. Thank you for your help

 Maddin June 22, 2010 02:30

Look at your courant number.

 WhoAmI June 25, 2010 11:49

To see the gravity effects, your reference density should be really really close to the density of the fluid at that temperature. Make sure the difference between reference density and density less than 1e-7.

 Maddin June 25, 2010 13:20

Jipp or some steady tests before to check the density "effect" - with and withput gravity.
Or what do you think?

 WhoAmI June 25, 2010 14:27

After initializing the solution, write a filed function to calculate the difference between the density in a cell (\$Density) and the reference density value. Create a scene with this field function to see the density difference. Then try to readjust the reference density value (and also the value in field function) to make the difference as small as possible.

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