Volume fraction calculation

eric weddle · September 30, 2011, 08:48

Hello there.

I work with multiphasic simulations and I'm currently evaluating OpenFOAM to check the possibility of using it for the simulations I work with.

My doubt rests over how OpenFOAM ensures that the sum of all components' volume fractions are equal to one.

In the TwoPhaseEulerFoam solver, the component a volume fraction (alpha) is calculated by a transport equation and the volume fraction of the second phase, b, is simply (1-alpha).

But I'm more interested in simulations with three and more components.

In multiphaseInterFoam, I couldn't understand how they ensure that the sum of all the fractions are equal to one.
Could any of you explain it to me?

Actually, my goal is to have a code in which I would calculate the volume fractions through transport equations (with source terms) for all the phases and apply a constraint so that the summation of their values are always equal to 1. Any thoughts on this?

Thank you guys for your colaboration

kwardle · October 5, 2011, 10:55

Eric,
Good question. multiphaseInterFoam in fact does not quite ensure that the volume fractions sum to 1. If you take a look at the min, max values for any phase it is typically as much as a few percent over 1 (~1.05 -- can be worse on tet meshes). That said, I have been working with Henry Weller on a different multiphase solver which required him to write a multiphase implementation of his MULES solver which indeed can do just this--the sums are several orders of magnitude better and the overall volume fraction is well conserved. My guess is that you will see this out in new versions of multiphaseInterFoam soon. It may already be in 2.0.x, I am not sure, but I don't think so.
Regards,
Kent

September 30, 2011, 08:48	Volume fraction calculation	#1
eric weddle New Member Join Date: Sep 2011 Posts: 12 Rep Power: 15	Hello there. I work with multiphasic simulations and I'm currently evaluating OpenFOAM to check the possibility of using it for the simulations I work with. My doubt rests over how OpenFOAM ensures that the sum of all components' volume fractions are equal to one. In the TwoPhaseEulerFoam solver, the component a volume fraction (alpha) is calculated by a transport equation and the volume fraction of the second phase, b, is simply (1-alpha). But I'm more interested in simulations with three and more components. In multiphaseInterFoam, I couldn't understand how they ensure that the sum of all the fractions are equal to one. Could any of you explain it to me? Actually, my goal is to have a code in which I would calculate the volume fractions through transport equations (with source terms) for all the phases and apply a constraint so that the summation of their values are always equal to 1. Any thoughts on this? Thank you guys for your colaboration

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October 5, 2011, 10:55		#2
kwardle Senior Member Kent Wardle Join Date: Mar 2009 Location: Illinois, USA Posts: 219 Rep Power: 22	Eric, Good question. multiphaseInterFoam in fact does not quite ensure that the volume fractions sum to 1. If you take a look at the min, max values for any phase it is typically as much as a few percent over 1 (~1.05 -- can be worse on tet meshes). That said, I have been working with Henry Weller on a different multiphase solver which required him to write a multiphase implementation of his MULES solver which indeed can do just this--the sums are several orders of magnitude better and the overall volume fraction is well conserved. My guess is that you will see this out in new versions of multiphaseInterFoam soon. It may already be in 2.0.x, I am not sure, but I don't think so. Regards, Kent