Sorry that I haven't answered to your replies yet, but I've been busy trying to figure out how to tackle this! :D
Originally Posted by mettler
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
is the pressure. If on the other hand capillary effects are not neglected, the boundary condition would be
of the interface, and the pressure difference across the interface would be
, 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.
Originally Posted by kmooney
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?
Originally Posted by mettler
just curious, but what are the units of pressure for p=0 in openfoam, and would that be a vacuum?
I haven't used OpenFOAM myself, but
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
is the density.
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 ;)