XANSYS Message: 93027 [Go back to message list]
[bookmark on del.icio.us]
No rating yet
Subject: Re: Ansys FSI nightmare
Author: Y Y Tan
Date: 2008-04-25 22:08:18
Hi guys,
Thanks for the quick reply. Thanks for caring. :)
First of all, i'm an undergrad, and I have to hand up my full dissertation next
week. I havent even started writing it yet. :(
My supervisor,does not know how to do FSI simulations and so far I dont think
there's anyone in my department who does this.
Christopher, I'm not too sure bout the manual part. I dun think i'll have
anytime for that now. I'm in desperate need of a result to put into my
dissertation.
Mason, this is currently the simplest it can be.
Anup, the aim of my exercise is to develop a model for blood flow inside a vein
and the physics of the vein is exactly similar to flow inside a flexible tube.
Thanks for letting me know that sequential is better. I have tried using
Flotran before this but i don't really know how to set up the ALE and there's
not much guide on it. The MFX seems a little more friendly since I can do it
with Workbench.
Let me tell you guys what I did and what happend to my model:
1) Create Solid model in Simulation, which is just a quarter of a pipe since the
pipe would buckle into a 2 lobe shape. Fixed the ends and applied symmetry on
the symmetry planes. Applied an external pressure as well. I modelled my
geometry to be slightly off-circle so that I could get the model to buckle. I'm
using line-search, autotimestepping and non-linear stabilization. The element
used is Solsh190, element formulation was programme controlled. Material
properties nearly incompressible with poison's ratio 0.49.
2) Create CFD model in CFX, applied Inlet velocity, outlet pressure, symmetry,
and the interface. Set coupling time duration 1s and coupling time step 1s and
simulation type steady-state. Did not apply any initialization. Under solver
control, maximum iteration:50, fluid time scale control set to conservative
auto timescale. For the coupling control, max iteration 20, tried playing with
the under-relaxation factors but didn't really help much because I dont really
understand what under-relaxation does.
Problems that occured:
1) Negative volume in CFD, manage to tackle this with a finer mesh at first but
then the problem still came, so I change the mesh displacement stiffness to
1/boundary^10, problem still occured so I did try to change the pseudo CFX time
and also lower the under-relaxation factors which have helped. So as far as
negative volume is concerned, its not happening yet as the solver crashes
before this could happen.
2) Mesh highly distorted for the solid, element turning inside out, element
warping. I've had all this in the solid part, which doesnt happen when I run
the solid part alone. I've already got quite a good mesh. I used to think that
this should not happen because the net pressure which is applied to the solid
is much smaller during the FSI than in the single Solid solution but I have
recently notice one big problem in my model. I use Ansys and keep looking at
the results file as the simulation goes on, one thing is that in one stagger
iteration, the pipe buckles with the vertical part collapsing inwards and the
horizontal part moving outwards. It is suppose to happen this way since I model
the vertical direction to have a slightly smaller radius than the horizontal
part. Anyway, this goes on like that and it seems like it would be converging
but then suddenly, after maybe the 5th stagger iteration, the horizontal part
starts collapsing inwards while the vertical part moves outwards and the
solution starts diverging. So yea, this is the MAIN problem.
>From your experience, do you guys think that it is a good idea if I write my
dissertation focusing on the problems which occured and the steps which I
undertook and have no results? or this would definately not put me in a good
position?
Regards,
Tan Yi Yong
The University of Sheffield
^--------------------------------------------------------
| XANSYS - www.xansys.org |
| The Discussion List for users of ANSYS, Inc. Software |
| Hosted by PADT - www.padtinc.com |
^--------------------------------------------------------