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VOF simulation of a liquid surrounded by vacuum?

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Old   May 10, 2012, 10:20
Default VOF simulation of a liquid surrounded by vacuum?
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Pharg Mandadapu
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Hi! I want to simulate a water surface with the finite volume method and the volume of fluid method, but instead of having to simulate air around the water I want to remove these cells from the simulation and just assume there is vacuum there instead to save computational power (I'm using a kind of adaptive grid). Is there any special way to use the VOF method in when your simulating a liquid surrounded by vacuum instead of two fluids like you normally do? Thanks in advance.
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Old   May 10, 2012, 13:24
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can you do that? What would be the BC at the intersection between the fluid and the vacuum? And, wouldn't the thermodynamics at the intersection would be a problem?
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Old   May 10, 2012, 14:05
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The surfaceTracking solver in OpenFOAM can do something similar. You could impose a p=0 BC directly on the surface. There would be capillary forces applied but no external pressure forces.
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Old   May 10, 2012, 14:13
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just curious, but what are the units of pressure for p=0 in openfoam, and would that be a vacuum?

thanks
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Old   May 10, 2012, 16:52
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You could check out Flow3d and their TrueVoF.

http://www.flow3d.com/cfd-101/cfd-101-VOF.html
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Old   September 6, 2012, 14:03
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Sorry that I haven't answered to your replies yet, but I've been busy trying to figure out how to tackle this!

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Originally Posted by mettler View Post
can you do that? What would be the BC at the intersection between the fluid and the vacuum? And, wouldn't the thermodynamics at the intersection would be a problem?
If capillary forces are neglected (which they can be in my case since I don't have any need for simulating capillary effects), the boundary condition would be p=0, where p is the pressure. If on the other hand capillary effects are not neglected, the boundary condition would be p=0 just outside of the interface, and the pressure difference across the interface would be -\gamma \nabla \cdot \hat n, according to Young–Laplace equation.

Besides, then the sheer stress along the surface is 0 since there is no friction between the liquid and the vacuum, which can be simulated by considering the sheer velocity to be zero.

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The surfaceTracking solver in OpenFOAM can do something similar. You could impose a p=0 BC directly on the surface. There would be capillary forces applied but no external pressure forces.
Okay, how does the surfaceTracking solver work? Does it use a moving mesh to track the surface or does it use some surface capturing method like the level set method or volume of fluid method?

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Originally Posted by mettler View Post
just curious, but what are the units of pressure for p=0 in openfoam, and would that be a vacuum?

thanks
I haven't used OpenFOAM myself, but p=0 doesn't necessarily mean that you have a vacuum. In some cases, you can even have negative pressures, although this mostly happens for solids. For a gas this is impossible, and for a liquid you would need to have an extremely high surface tension or the liquid will be prone to break up and internally form small bubbles of vacuum before you can measure any negative pressure. If you want a vacuum, you have to set \rho=0, where \rho is the density.

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You could check out Flow3d and their TrueVoF.

http://www.flow3d.com/cfd-101/cfd-101-VOF.html
That is interesting. Do you have any reference to any paper in which this "true VOF" method they are talking about is described to more detail? It sounds like they are defining separate velocities for the different phases in cells that contain a little of both phases, which is something I have never seen anyone do before (except for in flux corrected transport, where the separation in velocity exists for a completely other reason).

They say: "Pseudo-VOF methods produce a growth at the tip of the jet (Fig. 2). This growth is numerical, not physical, because it is independent of the density of air". Well, that depends in the implementation. My implementation for example (I turned out to make a normal VOF implementation, or acording to them, a "pseudo-VOF"), transports momentum between cells, rather than velocity, which means that the motion of the water is almost unchanged by the motion of the air since the air contributes with very little momentum.

It is also interesting that they are trying to discredit other VOF implementations by saying that they are "pseudo VOF" implementation, while basically all physics simulation implementation suffer from various forms of numerical errors, but that's another thing
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Old   August 26, 2013, 08:34
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Hi Pharg,

I wanted to know how did your simulation turn out.. did the VOF method work with vacuum surrounding the interface? I have to simulate a similar problem and hence wanted some advise. Thanks in advance.
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Old   August 26, 2013, 17:55
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Originally Posted by MANSOOMM View Post
Hi Pharg,

I wanted to know how did your simulation turn out.. did the VOF method work with vacuum surrounding the interface? I have to simulate a similar problem and hence wanted some advise. Thanks in advance.
I didn't implement the method, since I never found any detailed explanation about how it worked. I did an implementation containing air around the water instead, and it didn't work very well. I can imagine if you succeed with actually simulating water surrounded by a vacuum it would work better and you would get rid of some of the problems I had.

Tell me, what do you want to simulate? Why do you want to simulate a vacuum next to a liquid (assuming that is what you want to do)? If you want to simulate the same thing as I wanted to simulate, which was an ocean surface, you should probably go for a two-dimensional method instead, like the Fourier Synthesis method or one using Laplacian Pyramid Decomposition.
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Old   August 27, 2013, 10:31
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Hi..thanks for the reply...i will be performing the simulation of drop impacts in the presence of vacuum ..I would definitely do a 2D simulation as this problem is symmetric... the main concern is whether the VOF model would work well in this situation or not... the simulation using a VOF model code written by my fellow on Gerris crashes if the atmospheric pressure drops below 1/3 its original value...he says perhaps Gerris cannot handle very large pressure gradients at the boundary...if its a program issue then i might redo the problem using FLUENT. What do you suggest?
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Old   August 28, 2013, 17:57
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Quote:
Originally Posted by MANSOOMM View Post
Hi..thanks for the reply...i will be performing the simulation of drop impacts in the presence of vacuum ..I would definitely do a 2D simulation as this problem is symmetric... the main concern is whether the VOF model would work well in this situation or not... the simulation using a VOF model code written by my fellow on Gerris crashes if the atmospheric pressure drops below 1/3 its original value...he says perhaps Gerris cannot handle very large pressure gradients at the boundary...if its a program issue then i might redo the problem using FLUENT. What do you suggest?
Have you looked at the report Drop impact onto a liquid layer of finite thickness: Dynamics of the cavity evolution? I don't know how it models any of the phases as a vacuum, but it sounds like you want to do something similar as what is done in this work.

If you want to do a two-dimensional simulation because you have a cylindrical symmetric system, you should not use any of the two-dimensional methods I mentioned; they are for simulating gravity waves on sea and ocean surfaces.
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