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

In CFX10 or 11?

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

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