CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum

True-VOF vs. False-VOF

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   May 3, 2005, 09:07
Default True-VOF vs. False-VOF
Posts: n/a
Hi to everybody,

I carried out a liquid sloshing CFD study with Fluent 6.2 VOF model, and I have a few questions about the model implemented in the solver.

Well, I know that the VOF model in Fluent is actually a variable-density approximation of the "real" VOF model by Hirt and Nichols and I also know that Flow Science (Hirt is one of the founders, if I'm not wrong) claims Flow 3D to be one of the few codes (the only one?) with all the three essential features needed to properly model free surfaces (-a scheme to describe the shape and location of a surface, -an algorithm is required to evolve the shape and location with time, -free surface boundary conditions at the surface).

I'm actually going into details of the problem these days and trying to understand whenever and however this approximation will affect the results of the simulations. So my questions are:

1) Has anyone ever faced such a situation? I would really appreciate if someone would share his own experience and deal it with mine...

2) I think that Fluent's VOF model lacking feature is the ability to apply a boundary condition at the interface between the phases (velocity gradients, etc...), I'm right?

3) A fine mesh can improve the model and make the lack neglectable?

4) When body forces' effects decrease and surface tensions, wall adhesion become pre-eminent is the variable-density model even worse?

5) What codes (both commercial and freeware) have the best multiphase models implemented (VOF or even other techniques)?

Thank you in advance for any contribution,

  Reply With Quote

Old   May 4, 2005, 03:53
Default Re: True-VOF vs. False-VOF
Junseok Kim
Posts: n/a
For the question number 5), we recently wrote a short review paper about multiphase models, even though it is not evaluations of commerical codes.

take a look at the paper,
  Reply With Quote

Old   May 4, 2005, 04:21
Default Re: True-VOF vs. False-VOF
Posts: n/a
Thank you very much. I just took a look at it. Absolutely interesting.

  Reply With Quote

Old   May 5, 2005, 13:01
Default Re: True-VOF vs. False-VOF
Posts: n/a
Hi Dr. Kim,

It is indeed an informative review paper.

In your classifications, you might consider to include another quite different approach: the lattice Boltzmann method (LBM). Sundaresan¡¯s group are working on the LBM. They made some comparisons with Tryggvason¡¯s front tracking method, see International Journal of Multiphase Flow 29: 109-116 (2003).

Despite the difference in the interface description (either tracking or capturing), there is a direct ideological relation among these methods in the sense that the governing systems are solved on Eulerian grids. The difficulties of multiphase flow simulations are due in part to the singularities (discontinuous pressure and normal derivatives of velocity) at a time-dependent free boundary. At present, the most popular way to deal with the singularities at the interface, particularly in the finite difference context, is the regularization of the delta-function using trigonometric approximation. The interface is no longer sharp but has a finite thickness over a few grid cells even though the interface might be tracked explicitly. However, such an approach results in costly computations because it is at best first-order accurate (in space) and it inherently reduces the accuracy of any formally higher-order scheme when the interfacial force becomes significant.

Therefore, there might be a different way to categorize the interfacial modelling in terms of finite or sharp interface (i.e., low-order or high-order methods). We have also been working on the so-called sharp interface method:

J Comput Phys 187: 255-273 (2003). Int J Numer Meth Fluids 45: 1-19 (2004). Int J Numer Meth Fluids 48: 455-466 (2005).

Hope you may find this information useful.

Congratulations and best regards,

T Chen
  Reply With Quote

Old   May 5, 2005, 15:52
Default Re: True-VOF vs. False-VOF
Junseok Kim
Posts: n/a
Thanks for the comments. The original review paper contains Lattice Gas and Lattice Boltzmann sections, but the requirement of page numbers (Encyclopedia of Math. Phys. by Elsevier) forced us to cut those sections and focus on continumm description of two phase-fluid flows. The longer and detailed version is in preparation now.

  Reply With Quote


Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Compiling OpenFOAM 1.7.1 on Ubuntu 10.10 samiam1000 OpenFOAM Installation 4 November 24, 2010 09:00
VOF Outlet boundary condition in cfd - ace JM Main CFD Forum 0 December 15, 2006 09:07
Moving mesh or VOF? Giovanni Main CFD Forum 16 September 24, 2001 08:25
CFX4.3 -build analysis form Chie Min CFX 5 July 12, 2001 23:19
difference between false and true transient mahesh prakash Main CFD Forum 1 January 21, 1999 14:45

All times are GMT -4. The time now is 14:19.