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Racing_34 December 20, 2010 09:02

Unexpected capillary action missing
hi everyone, it's my first post here.. Firt of all i want to say sorry for my bad english :(

I'm writing my thesis, i have to simulate the flow in a mesoscale cell of material for a rtm process.
I'm trying to replicate the results of this article, expecially the example on page 1342 " FBP mesolevel results for flow across fiber tows"

focusing on the flow across the fibre. the ellipses are the fiber tows
On page 1345 you can see cell's model.

[what i want to model and what i expect]

A flow of resin (of known data) enters the cell from the left and have to fill the whole space, both free space and fiber tows, modeled as porous. Two simulation are proposed with different inlet velocity, leaving the same values for the other parameters.
The filling is shown for both cases on page 1345

the equation used for the flow are
stoke's for free space
brinkmann's for porous

[how i modeled it in cfx]
i recreated the cell respecting all geometrical given data,
created two doimani, one fluid, one porous,
apply symmetry boundary on the bottom, an inlet with fixed velocity, an opening with pressure equal to atmosferic pressure and a free slip wall for the other boundaries, and the interface fluid-porous.

I used two fluids, air and resin, for the second alla data are given in the article

I tryed both omogeneous an inomogeneus model, free surfece or not but i can't reproduce the article's results

[my results]
when i don't use a free surface model, with low velocity inlet condition, i can't fill the porous domain.. It happens only if i use a higher permeability, but if I use the other inlet condition, i have more or less the same filling, and that's not expected..
If i use the free surface model, the flow moves really slowly, even in the free space.. If i have an inlet velocity of 1 mm/s i expet to fill a 6mm long cell in 6s (in free spaces)... But with free surface option turned on i need more than 300s o.O

I think there's something wrong with capillary action, but i don't know how to make it work
could someone please help me?

ghorrocks December 20, 2010 18:13

This is a tricky field of work. The physics are quite complex. The only way to approach a difficult problem like this is to get it working one piece at a time. I assume you already know the basics of single phase CFD (if not you have a lot of learning to do). First, get yourself confident on general free surface flow modelling. Then add surface tension and wall contact angles.

Also I cannot see how porous materials are going to help you here. You won't get the correct resin motion with a porous material as it is linked to the surface tension and wall contact angle properties. I suspect you will have to model the free surface in the fibre bundle.

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