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Old   November 19, 2003, 18:21
Default Natural Convection
  #1
Elyyan
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Hi, I am trying to simulate natural convection within enclosure using Fluent, my Ra is ~10^10 (which means it is fully turbulent), I have been trying to make it work for a while. I am using the K-e model for modeling visc flow, and I am enabling the bouyance effects option, I have tried to use different radiation models, my medium is a gas with absorbtion coeff. 0.14 cm^-1, and my enclosure height is 50 cm. I made my mesh very fine, I used a BL with the first row of 0.01 cm height with growth factor of 1.2, and I have generated 40 rows, I think this should do it. Could you please help me making this simulation work, because I have been trying to make it work for a while now. If possible could you give me advice on what turbulent model to use, and if there are any options that I need to activate. Also, what do you suggest for the radiation model, and what solution schemes should I use, first order, PRESTO ... To tell the truth I will be grateful for any information you give, and I will appreciate all the details you provide. If you advice any source that will give a detailed procedure for the simulation it would be great. Thanks alot in advance

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Old   November 19, 2003, 18:30
Default Re: Natural Convection
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Jim Park
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It would help if you explain what's not working. Is the iteration procedure diverging? Or is it converging to an unrealistic solution? Are you doing a transient or steady-state simulation?

Most of the big CFD codes have pretty powerful iteration controls, and the support staffs have considerable experience in using them. You might want to ask the Fluent folks for their advice as well.
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Old   November 19, 2003, 23:40
Default Re: Natural Convection
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elyyan
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The iterations are not converging, espcially the continuity, I am using the segregated method for solving it. I have tried to start with low Ra (10^5)and increase it gradually to (10^10) but it diverges. Also, I am using the steady state scheme. and I am not using the boussinisque approximation, I have added the density as a polynomial. I hope those additional details will help. Thanks alot for your help and time.
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Old   November 20, 2003, 16:02
Default Re: Natural Convection
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Wei
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What is your boundary condition ?
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Old   November 20, 2003, 16:11
Default Re: Natural Convection
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Elyyan
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I am using isothermal walls, hot and cold, the temperature difference between them will range between 15-30 degrees.
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Old   November 22, 2003, 01:10
Default Re: Natural Convection
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Goicox
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Use the following:

* Simple * Upwind (first order for every solved variable) * An start with the simplest Turbulence model * If possible, turn off the radiation model at the begining.

Try to set the initial values for every variable according the problem, not just 0.

If it does not work, try to use the couple solver, for a small Courant number e.g. < 1.0. Try to iterate for serveral time steps (needed in the coupled solver), and then change to the segregate solver (without re - starting the values of the solved variables).

If it does not work, check your problem setup, boundary conditions, etc.
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