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-   -   VOF vs LSM (http://www.cfd-online.com/Forums/flow-3d/64824-vof-vs-lsm.html)

ck5285 May 26, 2009 03:21

VOF vs LSM
 
Hi users
Would like to know if anyone did compared or studied on the VOF (volume of fluids) vs LSM (level set method) in free surface tracking? which method provided better accuracy and faster simulation time? :)

isabel July 7, 2009 06:25

I am studying nucleate boiling. Growing of a bubble in a liquid.
For my case, the level set method is more accurate than the VOF method

ck5285 July 8, 2009 04:52

Hi Isabel
thanks for reply, so what CFD code that you are using and do you have any paper on this for sharing? thanks

isabel July 10, 2009 12:13

I am using OpenFOAM. If you are using it too, we can discuss doubts. My mail is lamasgaldo@yahoo.es

ck5285 July 13, 2009 21:31

Isabel
I know OpenFOAM is a open source CFD code but i didn't use it. do you have any paper/doc to compare the results between these two approaches? thanks in advance.

isabel July 15, 2009 03:54

Here you can download some information:

http://www.csi.tu-darmstadt.de/aktue...ls_3520.de.jsp

ck5285 July 16, 2009 20:26

thanks and would like to know how OpenFOAM solve multiphase problem that involved free surface tracking? does it solve as one fluid problem employed by Flow-3D or solve momentum equation for both air and fluid like others typical CFD code?

isabel July 17, 2009 03:09

It is an open code, so you can program the equations you prefer.

ck5285 July 17, 2009 11:30

well that might be interest of academic research but may not for industrial application. anyway thanks for your info.

matteoL July 20, 2009 13:52

Hello Isabel,
I am trying to implement the levelset method for boat-like free
surface problem but unfortunately quite unsuccesfully so far..

Could you please shedm me some light on the topic:

what I ahve done is:
-I have started from rasInterfoam and mainly just substitued the gamma
equation parts with the level set one and solve something like:

solve
(
fvm::ddt(gamma) + fvm::div(phi, gamma)
)

-I initialise gamma with funkysetFields and it is ok.
-So now gamma is the distance form the surface and i solve a simple
transport equation for it (not the MULES...).
-I then reconstruct rho using:
rho == (sign(gamma) -scalar(1))/scalar(-2.0)*rho1 + (scalar(1) +sign(gamma))/scalar(2.0)*rho2;

and rhoPhi:

surfaceScalarField phiGamma=fvc::interpolate(rho);

rhoPhi = ((sign(phiGamma) -scalar(1))/scalar(-2.0)*rho1 + (scalar(1) +sign(phiGamma))/scalar(2.0)*rho2)*phi;

-Checking rho it seems ok.

For some reason, although rho and gamma (the level set variable) seem ok , i have some pressure issues and almost form the start the simulation tends to diverge,mainly driven by pressure issues.


I haven't implemented any reinitialization or similar since I don't think it is that necessary in this case, and anyway the problem doesn't come from that since the simulation divrges from the beginning.

I know I have been very "brutal" in my formulation andh haven't implemented any stabilization or smoothing technique,but it is my first try and wanted to keep it simple.

Do you have any suggestion to make it work? have you implemented something particularly different?any major error i have done?

Thank you very much,
ciao
Matteo

isabel July 21, 2009 04:07

Hello Matteo.

I don't know what can be your problem.

I am simulating nucleate boiling (growing of a bubble). What I have done is the following:

- I started from solver interFoam, but instead of solving the gammaEqn, I solved the level set equation.
- As you said, the initial condition of the level set function is the distance from the interface.
- I have done reinitilialization and smoothing to improve convergence and accurate.

matteoL July 21, 2009 04:15

Hey Isabel, thank you for the fast reply.

Have you done something particularly different than what i wrote? do you see any major error? Do you agree that in principle it shlould be working?
How do you recontruct rho and rhoPhi exactly?

I will add a bit smoothing and see if it there is any improvements..

Do you base your level set formulation on the paper by Shu?

thanks,
ciao
matteo

matteoL July 21, 2009 04:32

One more thing I have just noticed:
I haven't updated the value of the viscosity! Where is it done in interfoam? where do you do it in your level set formulation?
Thanks,
Matteo

isabel July 21, 2009 05:04

I have updated the viscosity at the same way as the density:

rho == H*rho1 + (scalar(1) - H)*rho2;
mu == H*mu1 + (scalar(1) - H)*mu2;

where H is the Heaviside function, which have been smoothed previously.

matteoL July 21, 2009 11:03

Do you have any pressure/velocity peak issues with the level set formulation?

In my formulation, everytime a cell swap from air to water I have a big peak of pressure/velocity and that phenomena become unstable and slowly diverge.. I have smoothed the interface and thinks get better but still far worse than with the vof approach..

I imagine the problem is somewhere in the rhoPhi evaluation that is not smoot enough or something similar..

Is there any smoothing in the VOF approach of that term? or of Rho, or mu?

thanks,
Matteo

isabel July 22, 2009 02:33

I don't understand very well what is your problem. If you want send me your results to lamasgaldo@yahoo.es and I will try to help you if I can.

MuxaB August 13, 2009 22:59

Quote:

Originally Posted by isabel (Post 222698)
Here you can download some information:

http://www.csi.tu-darmstadt.de/aktue...ls_3520.de.jsp

Interesting work, especially on the combined VOF and LSM method for bubble boiling.

To be fair to the VOF method at large - has any one compared LSM in Openfoam with FLOW-3D's VOF? Or have the comparisons been done with the VOF method in Openfoam only?

otd August 14, 2009 09:11

Quote:

Originally Posted by ck5285 (Post 222978)
thanks and would like to know how OpenFOAM solve multiphase problem that involved free surface tracking? does it solve as one fluid problem employed by Flow-3D or solve momentum equation for both air and fluid like others typical CFD code?

Unless flow3d has gone backward over the last few years, you have the options of solving a two-fluid problem (gas over liquid for instance - or two liquids of different density with a sharp interface). Two momentum equations (one for each fluid/phase) are solved.

Perhaps you should check your documentation - or the Flow3d help desk - for help on this one.

ak2009 December 7, 2009 06:54

Hi all,

I have recently started OpenFOAM. I needed some papers, info about LSM in OpenFOAM.

ak

otd December 24, 2009 14:18

Your question is really pretty general, not specific to flow-3d.

Have you tried posting it in the main forum? You'll probably get a lot more comment - and thus exposure to a wide variety of experience.


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