# Gravity/Buoyancy/Buoyancy reference density in two phase flow

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

November 5, 2013, 10:11
Gravity/Buoyancy/Buoyancy reference density in two phase flow
#1
New Member

Jim KIT
Join Date: Aug 2012
Location: Germany
Posts: 23
Rep Power: 4
Hello,

im trying to modell a gravity driven two phase flow(air - water) in a pipe in CFX. (pic1.)

Boundary:
Velocity Inlet
Pressure outlet up
pressure outlet down

Model:
inhomogene Euler/Euler

Accordint to cfx, flows in which gravity is important can be modeled by CFX by the inclusion of buoyancy source terms.

For buoyancy calculations, a source term is added to the momentum equations as follows:
S= (rho - rhoRef) * g

For multiphase flows, it is important to correctly set the buoyancy reference density(rhoRef). For a flow containing a continuous phase and a dilute dispersed phase, you should set the buoyancy reference density to that of the continuous phase. so the reference density of the continuous phase cancels out buoyancy( eq. 1) and pressure gradients in the momentum equation of continous phase.

For non-dilute cases (which include all free surface cases), all terms can be equally important for each fluid. if there is a significant difference in density you should choose the density of the lighter fluid (that cancels out buoyancy of lighter phase) because this gives an intuitive interpretation of pressure (that is, constant in the light fluid and hydrostatic in the heavier fluid).

I've tried using the two limiting cases. if (rhoRef = rhoAir), no mass flows through the upper outlet. if (rhoRef= rhoWater) no mass through the lower outlet, also the hole tank is full of water and bubles come out.

If (rhoAir <rhoRef < rhoWater) it works, but i think it is not phiscally correct.
In my opinion it shoulde be define as a function. When I try to define rhoRef as a function of Volume fraction,
(rhoRef = Air.Vf * rho.Water + (1- Air.Vf) * rho.Air)
ther is this error message:

The parameter 'Buoyancy Reference Density' is defined to be "Single Valued" but it depends on the following field valued variables: , Air.density, Air.vf, Water.density.

have any one any idea, how can I solve this problem?

thx for ur time
Attached Images
 LangMesh.jpg (24.6 KB, 42 views)

 November 5, 2013, 18:19 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,823 Rep Power: 85 I think you misunderstand the purpose of the reference value. The exact value of this parameter should not matter and it certainly does not vary over the domain. It is a single value used to reduce numerical round-off. You should be able to double or halve it and results are unaffected. If changing the reference value changes your results then you have a very sensitive simulation and you should be using double precision numerics.

 November 6, 2013, 02:26 #3 New Member   Jim KIT Join Date: Aug 2012 Location: Germany Posts: 23 Rep Power: 4 thx for your antwort. Correct me if I wrong. I dont think so. Simply do the tut 9 and change the ref.Density and see the diffrence. However When you see the NS.eq, you find out, that ref.Den has enormous influence. Viele Grüße,

 November 6, 2013, 06:58 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,823 Rep Power: 85 The tutorials are only to show you how to get the models running. They unfortunately do not show you good CFD practise. It the tutorial result changes with different reference densities then it needs double precision numerics as well. You cannot have a result which relies on precise definition of a reference condition. You cannot do accurate CFD unless this is the case.

 November 17, 2013, 06:50 #5 New Member   Jim KIT Join Date: Aug 2012 Location: Germany Posts: 23 Rep Power: 4 hat any one any other idea to simulate such a separator???

 November 20, 2013, 00:14 #6 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 486 Rep Power: 9 What values are you using for your pressure outlets? These two values are determining your flow, and also the height of your liquid head along with the buoyancy forces. Sounds like you are setting them at the same pressure? Which is not true, and would explain why you are getting your results. sharifi likes this.

 November 20, 2013, 10:45 #7 New Member   Jim KIT Join Date: Aug 2012 Location: Germany Posts: 23 Rep Power: 4 thx for the antwort. I use the same pressure for both pressure outlet, p(atm) physically this should be so, since both leave out in the environment. can you explain me plz more, I should put pressure at out differently??!!!

 November 20, 2013, 10:59 #8 New Member   Jim KIT Join Date: Aug 2012 Location: Germany Posts: 23 Rep Power: 4 Also, I do not understand it. the water is at the bottom when Rho(ref)=Rho(air). This should physically lead to lower outlet is clogged and thus the air goes out from the other outlet, but it does not, it even comes out of the lower but not omit from the upper. I mean the transport eq for air hat no extra force-term that it force to go out from upper outlet. I mean, the only reson why air shoud goes out from the upper outlet is that the lower outlet is clogged with water or am I wrong???

 November 20, 2013, 17:15 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,823 Rep Power: 85 A picture showing what you are describing would help.

 November 21, 2013, 14:52 #10 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 486 Rep Power: 9 The botom and top should not be at the same pressure. The bottom would be at a higher pressures due to the liquid head.

 June 9, 2014, 10:23 #11 New Member   Atit Join Date: Jun 2014 Posts: 1 Rep Power: 0 Adding the momentum source should not provide the correct answer. After specifying the Buoyancy reference density and the gravitational acceleration components, CFX will manage the buoyant effect for you. Try to disable this momentum source. It might work. Good luck.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post toolpost OpenFOAM Installation 15 September 21, 2012 09:38 Sas CFX 15 July 13, 2010 08:56 lin123 OpenFOAM 3 April 13, 2010 14:18 allenzhao OpenFOAM Installation 127 January 30, 2009 20:08 youngan CFX 0 June 30, 2003 02:32

All times are GMT -4. The time now is 10:20.

 Contact Us - CFD Online - Top