[arjun]

Originally Posted by Miguel Hernandez

Flow3D allows simulations with only one fluid (water for example) and uses TruVOF (I'm not an expert but I think it is a modified version of VOF). In this type of simulations the user is not required to provide boundary conditions for the free-surface (nothing prevents the software to consider a boundary condition without showing it to the end user).
I have little experience with openFoam, but from the few projects I've done, I've noticed that interFoam (2 fluids) tends to be more easily unstable than what you get with Flow3D, as well as being much slower computationally. I thought this was due to the fact that interFoam needs to solve the equations for two fluids, hence my question about the existence of an openFoam solver that allow the simulation of open channel considering only one fluid, as Flow3D allows to do.

TruVOF is two phase system. In past they used to have documents on their site about it (if i remember correctly).

[Santiago]

Originally Posted by arjun

You mean like better descretization ie better reconstruction and solving continuity equation is a trick??

No we are not adding or substracting any fluid to maintain the mass conservation if this is what you mean by trick.

If solving equations by descretisation is a problem to you, then there is nothing that could be done because without it there is no solution to the problem.

Some points: (1) last time I checked the projection operation in interFoam is also performed at every step, so I dont undertand your "solving continuity equation is a trick"; (2) if by better reconstruction schemes you mean CICSAM, mHRIC, PLIC, then again, these can be either implemented or found in the wild, nothing special; by (3) tricks, I mean many tricks, including the one you just mentioned, and yes, explicitely bounding scalars on the spatial operators are also tricks. Everything that enforces mass conservation OUTSIDE from the projection operation in PISO IS A TRICK; as I said, the condition is to keep velocities solenoidal, not the face fluxes equal to zero.

Originally Posted by arjun

The scheme that we use is none of what you mentioned. The scheme that i use is something i have designed and the whole thinking about it is very different than the interface tracking that you think of. The method is at its base implicit in nature. This is why it can run on large courants and keep interface sharp.

Care to share a link to your published work?

Originally Posted by arjun

Edited to add: It occured to me that it is openfoam that actually uses the 'trick'. They have extra term added to VOF equation to keep the interface sharp. Wildkatze does not do even that and there is no such option for user to chose from too.

OpenFOAM doesn't use any trick, INTERFOAM does. Besides, I never said INTERFOAM doesn't do tricks of its own, or is better than other solvers. What I like of OF is that at least you can easily see the tricks. Plus, you can perfectly use isoAdvector, or PLIC, or implement a CLSVOF, or just turn off the 'extra' term and use CICSAM. As I said, OpenFOAM is not a SOLVER, is a library for FVM, and one needs to make efforts to implement things, even tricks, whenever necessary.

It seems to me that you are taking this thing with a bit too much passion, Do you work at Flow3D?

BTW, What is wildkatze?

[Andrea1984]

It would be very interesting indeed to have a link to something (a peer-reviewed paper ideally) describing your scheme.

Originally Posted by arjun

You mean like better descretization ie better reconstruction and solving continuity equation is a trick??

No we are not adding or substracting any fluid to maintain the mass conservation if this is what you mean by trick.

If solving equations by descretisation is a problem to you, then there is nothing that could be done because without it there is no solution to the problem.


The scheme that we use is none of what you mentioned. The scheme that i use is something i have designed and the whole thinking about it is very different than the interface tracking that you think of. The method is at its base implicit in nature. This is why it can run on large courants and keep interface sharp.


Edited to add: It occured to me that it is openfoam that actually uses the 'trick'. They have extra term added to VOF equation to keep the interface sharp. Wildkatze does not do even that and there is no such option for user to chose from too.

[arjun]

If you need extra terms to keep interface sharp then it is a trick whichever version of openfoam does it. You can switch it off but then the interface is smeared.

Originally Posted by Santiago

Some points: (1) last time I checked the projection operation in interFoam is also performed at every step, so I dont undertand your "solving continuity equation is a trick"; (2) if by better reconstruction schemes you mean CICSAM, mHRIC, PLIC, then again, these can be either implemented or found in the wild, nothing special; by (3) tricks, I mean many tricks, including the one you just mentioned, and yes, explicitely bounding scalars on the spatial operators are also tricks. Everything that enforces mass conservation OUTSIDE from the projection operation in PISO IS A TRICK; as I said, the condition is to keep velocities solenoidal, not the face fluxes equal to zero.



Care to share a link to your published work?



OpenFOAM doesn't use any trick, INTERFOAM does. Besides, I never said INTERFOAM doesn't do tricks of its own, or is better than other solvers. What I like of OF is that at least you can easily see the tricks. Plus, you can perfectly use isoAdvector, or PLIC, or implement a CLSVOF, or just turn off the 'extra' term and use CICSAM. As I said, OpenFOAM is not a SOLVER, is a library for FVM, and one needs to make efforts to implement things, even tricks, whenever necessary.

It seems to me that you are taking this thing with a bit too much passion, Do you work at Flow3D?

BTW, What is wildkatze?

Is another multiphysics solver. We have been developing for almost 5 years now.

[arjun]

Originally Posted by Andrea1984

It would be very interesting indeed to have a link to something (a peer-reviewed paper ideally) describing your scheme.

Wish to convince company to make it available to others and let the ideas about it more public. I might just make it an library that others can call it to calculate the volume fraction at the face.

The paper shall be published about it but i am not sure how much i am allowed to write in detail. Though to Holger I tried to tell as much detail as I could to help him get this in openfoam. He has a student working on this idea. So at least I could provide him the global hint and ideas (not the fine details). There is certainly now someone working on similar ideas for openfoam.

Hopefully it takes shape of library that others can directly use. This has very high probability. This week I am actually working on the library part and demonstrate the cases with higher courant (benchmark problems).

[arjun]

This was actually tested again after i wrote in this thread just to see how the solver stands at the moment (current version).

The dt is based on maximum courant number (happens at the bottom of the bubble).

[Bloerb]

As far as i know there are no TrueVOF like solvers in OpenFOAM readily available. A VOF like interFoam has problems with high viscosity or density differences between the two media. Which often results in artificial velocities at the free surface. Using VOF for simulating a blob of highly viscous plastic/resin for example is not a good idea. And And another big "problem" are channel flows where the air is vented through a small outlet . And your water is moving hence much slower. This is for example typical in simulations used for injection molding. The "problem" here is that the air is moving near the speed of sound while the other fluid is an order of magnitude slower. And the air itself isn't really changing the fluid flow. Hence all commercial solvers for injection molding or casting are typically neglecting the air phase since it slows down the time step by an order of magnitude without much benefit to the user. They simply use particles to track pressure in these void regions.


Everything else said here still applies though. It is however beneficial for some applications to use such a VOF formulation.


It should however be possible to implement this in OpenFOAM with simple methods. You need to check which cells belong to the interface or are near the interface and change the velocities there. And zero out everything in the air phase. You are still solving both phases and you need to be careful about mass conservation etc. Still a bit of work though