[ghorrocks]

You have mentioned the pressure gradient many times, but never once mentioned where the pressure gradient comes from. Rather than trying to artifically create the pressure gradient you think is there, how about you model the conditions which make the pressure gradient come about - then the pressure gradient will form naturally.

I presume this fibre mat is in a pressure vessel into which resin is injected. What do they do in the pressure vessel to handle the air?

[marcoymarc]

Ghorrocks, i'll try to do my best to make you understand the process with my poor English. Please, forget everything about this simulation before starting to read this.

Imagine you have a resin container which is at atmospheric pressure, let's call it 1. Then you have a fiber vessel, let's call it 2. At start, the valve connecting 1 to 2 is closed, so you have two separate environments.

A pump empties B of air and puts it under negative pressure. You have no more air, but vacuum instead. Then, you open the valve and let resin flow due to pressure gradient. Substantially, what i am trying to model using fluid = air is vacuum in truth. But Cfx has no way to model vacuum, and i think this is the best choice. We have small viscosity effects due to air, so the guess is to introduce very little error caused by this substitution. Again, in cfx i am setting up pressures different from the physics, in which we have like 0Pa at inlet and -10kpa at outlet; infact i am using 10kpa at inlet and 0kpa at outlet. Basically i don't think to introduce any issue with this, since what really matters is pressure gradient and not pressure itself.

Moreover, we start with Resin volume fraction = 1 in the fluid domain because we can assume fiber impregnation starts only after the elementary cell (fluid domain) is already filled; this is because resin flow in fluid domain should be way faster than fiber impregnation, and so our assumption should introduce a very little error, again.

Of course, pressure gradient along flow direction will not be fixed for every elementary cell and in time, so i'll need to make different simulations with different outlet pressures/inlet mass flows.

I hope to have been clear.
Sorry again for my English.

[ghorrocks]

Your English is very clear, I understand what you are saying. Some important points:
* You do not have a vacuum. You have air under a low pressure. There is still air in the fibre reservoir, just not much - but the air which is there is important.
* CFX cannot model a vacuum because they don't exist At least not in the Navier Stokes world anyway.
* So this should be modelled as air at whatever pressure it is at.
* I disagree that the absolute pressure is not important. The absolute pressure sets the air density and therefore the trapped air mass. This is critical, so you must set the absolute pressure level correctly.

I think the convergence problems youare having is because you are not modelling the resin flow into the fibre chamber correctly. You seem to be missing some important points. Another key point you have not discussed is whether the vacuum chamber keeps appying a vacuum as the resin enters, or whether it is sealed. And do you keep the vacuum on until you pull resin out the other side to ensure the fibres are fully wet? These types of details affect the way you should model it.

[marcoymarc]

Sorry for this absence.
Finally i think i solved my problem. Since i had never used ICEM before, i had to give a look into tutorials et cetera.
When my simulations diverged, i noticed everything started in a little region in which pressure values growed like crazy. That was due to low quality elements. Now Everything looks fine thanks to some working in ICEM.
Thank you really for your help. Again.