[BenMUC]

Hey all,

I'm trying to model the microflow inside a carbon fiber roving using ANSYS CFX 14.5.7. The geometry I'm using is one third of hexagonal unit cell ( radius of the filament 3.5µm ), please see the attached picture of the geometry and the mesh:



The resin infusion is driven by a pressure gradient, therefore I've calculated the pressure gradients for the length of the geometry of 200µm based on experiments that were done with 2 and 3 bar pressure on a 400mm tool. So basically it's laminar pipe flow in a weird-shaped pipe. As Darcy's law has been deducted for laminar ( creep ) and stationary flow, I'm using the Static Pressure ( Zero Gradient ) boundary on the Inlet. On the Outlet I'm also using the Static Pressure option ( 0.0 Pa. ) The manual states that this combination isn't recommended, however it also states that the Zero Gradient boundary is the best way to get a fully developed velocity profile.

These are the options I've chosen in the Solver Control and the Solver Manager:

(can be found in the second post since I'm limited to 10 pictures in one post)

The values that I'm interested in a both the velocity profile and the shear strain rate. Now, when choosing a static pressure of 2000 Pa the solver converges and I get these results which seem correct to my expectations:





When choosing smaller/bigger pressure gradients than say 1500 Pa and 3500 Pa, the solver still converges but I get this warning:

 +--------------------------------------------------------------------+
 |                     ****** Notice ******                           |
 |  A wall has been placed at portion(s) of an INLET                  |
 |  boundary condition (at  14.3% of the faces,   3.5% of the area)   |
 |  to prevent fluid from flowing out of the domain.                  |
 |  The boundary condition name is: inlet.                            |
 |  The fluid name is: Fluid 1.                                       |
 |  If this situation persists, consider switching                    |
 |  to an Opening type boundary condition instead.                    |
 +--------------------------------------------------------------------+
 +--------------------------------------------------------------------+
 |                     ****** Notice ******                           |
 |  A wall has been placed at portion(s) of an OUTLET                 |
 |  boundary condition (at   7.1% of the faces,   0.5% of the area)   |
 |  to prevent fluid from flowing into the domain.                    |
 |  The boundary condition name is: outlet.                           |
 |  The fluid name is: Fluid 1.                                       |
 |  If this situation persists, consider switching                    |
 |  to an Opening type boundary condition instead.                    |
 +--------------------------------------------------------------------+

Sometimes up to 100% of the faces are covered by a wall and the resulting velocity profile and shear strain rate look like this ( e.g. this is for 1000 Pa, 100% coverage ):





It gets really interesting when switching to formerly stable pressure gradients, for example the following pictures were taken for 2000 Pa but the calculation was run after I got the "A wall has been placed warning" and then I'm getting the warning at 2000 Pa as well. The velocity profile and the shear strain rate look like this:





It looks as if the solver is using the formerly produced results to calculate further, and it's doing this although I changed the static pressure so it shouldn't continue from the results done prior to that. Changing to Initial Conditions at Initializations Option in the Solver Manager didn't change anything.

The manual states that this warning can be ignored if it doesn't persist until the solver converges ( in my case it persists ). If it does, one should change to an opening type boundary, which doesn't really seem adequate in my case. The three options given here don't seem to be applicable in my case either.
I found I can get better results if I change Timescale Control from Auto Timescale to Physical Timescale ( 100 - 500s ) in the Solver Control. I'll still get the warning but only for ~0.2% of the faces, which is a great improvement, and the results for 2000 Pa look like this again:





Changing to Physical Timescale improves the results for 1500 Pa and 3000 Pa a lot as well, I've already tested it, but it's still not entirely perfect.

In the end, my questions are the following:
What can I do to get rid of the warning and get good results? Are my boundary conditions wrong? Do you have any ideas?
What do I have to do that the solver starts to calculate from the start again, so I can avoid getting the warning on formerly "stable" pressure gradients?

I also want to get a shear strain rate distribution ( shear strain rate values and their percentage of occurence ), do you have an idea how I can do that? Can I specify, say 200 or 1000 points where ANSYS probes the shear strain rate? And can ANSYS calculate an averaged shear strain rate?

I hope you can help me out as this problem is driving me nuts.