# natural convection at high Rayleigh

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 February 23, 2005, 10:05 natural convection at high Rayleigh #1 mauricio Guest   Posts: n/a Hi I would like to ask about natural convection problems at high Rayleigh number in closed cavities using Fluent. I have been testing the natural convection with Fluent with the article Xu W. and Chen Q.1998. "Numerical Simulation of Airflow in a Room with Differentially Heated Vertical Walls", ASHRAE Transaction 104(1):168-175. The author of the article simulates a real dimension room with a Rayleigh number around 10e10. I repeated the case and after some tests I discovered that Fluent have difficulties in natural convection in closed cavities running with a high Rayleigh number (even using a fine mesh, presto!, and hybrid). I have been using the real physical properties of the air except for the gravity because I use it to adjust the Rayleigh number. The convergence for all residual is okay until Ra=10e6. The convergence is very hard with Ra=10e7 and it is impossible with Ra=10e8, even when it is supposed to be laminar. I will really appreciate some comments about this situation and/or suggestions to solve the problem. Thank you so much

 February 23, 2005, 14:57 Re: natural convection at high Rayleigh #2 Michael Guest   Posts: n/a I was working on a high Ra natural convection. However the fluid was water, there were some tips and trics. I don't renember everything but i'll try. -segregated solver, sec. order except the pressure: body force weighted, -important thing, operating density strongly coupled with corresponding temperature, I'm not shure, but try lowest value of density for your conditions -I didnt use a bousinesq app. It is good practise to write own properties UDF with exact dependence of density in the term of temperature (only) -try to manipulate under-relaxation factors. Renember about strong coupling between energy and momentum eq. Take a special care about this. I hope it will help. Good luck. Michael

 February 23, 2005, 19:43 Re: natural convection at high Rayleigh #3 mauricio Guest   Posts: n/a Michel I am very glad with your feedback. I will tray your suggestion and then I will comment what happened.