# Density boussinesq Problem

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 July 27, 2007, 07:31 Density boussinesq Problem #1 matrix-1.0 Guest   Posts: n/a Hi, I am simulation a big entry hall and try to activate the variable density option. Under Material Air I activate "boussinesq" and set the default Density to 1.19 Kg/m³ and the thermal expansion coeff. 1/k to 0.003 ( 1:300K ). the other values are not changed. Then, under Operation Conditions, I leave the 15 °C under boussinesq and mark the variable density parameter and set the density there to 1.225. The iteration becomes convergent, even unsteady, but under display/contors/density it ranges between 1.18999 and 1.190001 So is my setting wrong, or is there a different view panel ? My temperatures in this hall range fom 14 °C to 80 °c so there should be more... Can anybody help me out, please ???

 July 27, 2007, 11:01 Re: Density boussinesq Problem #2 Ken Adams Guest   Posts: n/a Hello, Just one quick thought. Boussinesq approximation is only valid for small temperature variations (Fluent manual 12.5.3). So according to your temperature range, that approximation may not work. There're several posts on natural convection problems on this forum. Lots of people suggest using "Ideal Gas" for density. Hope it helps, Ken

 July 29, 2007, 17:33 Re: Density boussinesq Problem #3 matrix-1.0 Guest   Posts: n/a Hi, thanks for your advice ! It really helped. The density now behaves as it should ! But..., now I do not get it convergent. The continuity is constant at 1e-01 and the other parameters at 1e-02. I am using a Do radiation with solar calculator. Can this be the problem in conjunction with a non constant density ? ( with constant density it becomes convergent after 800 Iterations ) I tried changing the under-rellaxation factors pressure and momentunm, sadly to no avail. Then I changed the density rell. to 0.3, that also didn't help. Does any body has an idea ??

 July 30, 2007, 04:08 Re: Density boussinesq Problem #4 ritmat Guest   Posts: n/a the problem could be if you are using gravity. If it is the case in operating conditions decrease the gravity , and increase step by step waiting for convergence. Sometimes it can be the problem. let us know

 July 30, 2007, 10:14 Re: Density boussinesq Problem #5 matrix-1.0 Guest   Posts: n/a Hi, thanks for this advice, but I am not allowed to reduce the gravity. Could you please tell me where I can increase the sep by step waiting ?? Many thanks !

 July 31, 2007, 02:02 Re: Density boussinesq Problem #6 ritmat Guest   Posts: n/a i mean run your model with 0.001 m/s2 ( or less) and wait convergence. Then increase little by little and do the same until 9.81.

 August 1, 2007, 07:05 Re: Density boussinesq Problem #7 matrix-1.0 Guest   Posts: n/a Ah, understood !! I will do it and let you now ! Many thanks !!! Matrix

 August 4, 2007, 07:08 Re: Density boussinesq Problem #8 matrix-1.0 Guest   Posts: n/a Hi, thanks al lot ! It really worked !!! I started wit 0.001 went to 1 then to 5 and at last to 9.81. Many thanks !!! Matrix

January 17, 2014, 22:05
#9
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Ke Wu
Join Date: Jan 2012
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Quote:
 Originally Posted by matrix-1.0 ;144146 Hi, thanks al lot ! It really worked !!! I started wit 0.001 went to 1 then to 5 and at last to 9.81. Many thanks !!! Matrix
Hi

I wonder why we need to try different value of gravity acceleration

If we do so, I have to run several times, what's the point of doing this

Shouldn't the gravity acceleration be a constant value in 9.81

January 17, 2014, 22:17
#10
Senior Member

Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 6
Quote:
 Originally Posted by ritmat ;144006 i mean run your model with 0.001 m/s2 ( or less) and wait convergence. Then increase little by little and do the same until 9.81.
Hi I wonder why we need to change the value of gravity acceleration

Shouldn't it be a constant value in 9.81