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Old   June 12, 2007, 10:27
Default Pressure fluctuations, free surface
  #1
Terje
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Hi!

I am performing a 2d - filling simulation where a high viscous fluid is replacing air in a cavity. During the simulation there is a heavy fluctuation in the pressure over the interface, which eventually crashes the simulation. The fluctuation has a checkerboard shape with large positive and negative values and changes in time. Anyone have an idea on how to fix such a problem?

The domain has three boundaries with prescribed velocity (opening) and an outlet with prescribed pressure (outlet).

Thanks!

- Terje
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Old   June 12, 2007, 10:41
Default Re: Pressure fluctuations, free surface
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Joe
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In CFX10 or 11?
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Old   June 12, 2007, 11:32
Default Re: Pressure fluctuations, free surface
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Rui
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Hi,

What's your time step and Courant number?

I've done some filling, including 2D, simulations (Reaction Injection Moulding filling simulations). Can you specify more details, like: what does your 2D geometry look like, what mesh you use. Does the simulation crash after a few timesteps, or after many? What's your Re number? Can you post a link to some pictures showing the volume fractions distribution, and those presure flutuations?

And by the way: what's the third boundary? A wall? And why do you set the inlet boundary as an "Opening" and not an "Inlet"?

Rui
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Old   June 12, 2007, 12:31
Default Re: Pressure fluctuations, free surface
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Terje
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Hi!

Thanks for your answers. I will try to provide some more details. I do not have acces to the simulation from here so pictures will have to wait.

I am using CFX 11 and am modellng injection molding of a plate which is 1 mm thick. On top of this plate there are located microstructures with dimension approximately 1 um and what is really interesting is the filling of these. However, I do not regard it possible to perform one simulation that includes both the main structure and the microstructures because of the size differences. Therefore, I have used an analytical solution to the free surface flow between parallel plates as a boundary condition to a small scale (10 um x 10 um) simulation around the microstructure.

The boundary can be divided in three:

- One wall, no slip

- One interaction with the main flow field. Here fluid is both entering and leaving at different places at different times and I thus found that neither inlet nor outlet could describe it. So I used opening and prescribe the velocity field and volume fraction from the analytical solution.

- A venting channel on the top of the microstructure where I just use a pressure outlet.

The simulation runs for quite some time before crashing, but I think this is due to a very low Courant number Co_max = 1.5. which stops the fluctuations from growing too large. Re < 1. I am using a hexagonal mapped mesh.

- Terje

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