# Buoancy

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 May 26, 2008, 10:53 Hi, I am currently developi #1 Member   Flavio Galeazzo Join Date: Mar 2009 Location: Karlsruhe, Germany Posts: 32 Rep Power: 10 Hi, I am currently developing a solver for incompressible (but with variable density), multicomponent, steady state flow based on simpleFoam. It solves a equation for the concentration, which is used to calculate the density of the mixture. I want now to add the effects of buoyancy, due to density gradients created by the mixture, and I found two ways of doing it: 1. Modifying the pressure equation, as in buoyantSimpleFoam; 2. Adding the "weight" rho*g in the equation of velocity, as in reactingFoam. With the first approach I don't get convergence. With the second one it converges, but to a wrong solution. My questions are: What is the difference between these two approaches? There is one that is more suitable to my problem?

 May 27, 2008, 03:58 Hi, buoyantSimpleFoam is ba #2 Member     Stefan Radl Join Date: Mar 2009 Location: Graz, Austria Posts: 82 Rep Power: 11 Hi, buoyantSimpleFoam is based on a compressible fluid solver. Hence, you have to be very careful when using this as you might get problems with stability due to the additional equation for the density! What I can suggest is to throw away the equation for the density and compute it locally from temperature (and your mixture fraction). Then couple this with a standard incompressible solver and underrelax the density calculation - you should have no problems with convergence. Second, your wrong result described under 2. maybe due to the fact that the boussinesq approximation you are using does not hold ( http://en.wikipedia.org/wiki/Boussin...%28buoyancy%29 , i.e., you temperature differences are too big. cheers Stefan

 May 27, 2008, 04:41 Thanks for the reply, Stefan! #3 Member   Flavio Galeazzo Join Date: Mar 2009 Location: Karlsruhe, Germany Posts: 32 Rep Power: 10 Thanks for the reply, Stefan! You are right, my maximum density difference is about 7:1, what is too large for the Boussinesq approximation. I will follow your advice and use the approach from buoyantSimpleFoam. I will post my impressions here, as soon as get good results with it. Flavio

 May 19, 2010, 17:41 #4 Member   Alan Russell Join Date: Aug 2009 Location: Boise, Idaho USA Posts: 61 Rep Power: 10 You could also look at buoyantBoussinesqSimpleFoam, which is an incompressible solver. Alan

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