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Subject: Re: Ansys FSI nightmare
Author: Dan Bohlen
Date: 2008-04-26 14:22:57
Y Y Tan said:
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.
I had this for my grad school thesis and my first advisor. I quickly
switched advisors when mine admitted he had no clue on the topic I
wanted to do (FEA related). But no one at that school can help? Why
would they expect you to be able to do it then?
Sounds like a recipe for failure or BS'ing. Why launch into some study
in which one on campus can help you? Pick another topic. On the more
cynical side - if no one knows how to do what you are doing - totally
fake it and no one will be able to prove you wrong! :-)
The problem you are tackling doesn't sound like something someone should
be attempting to get out of undergrad...
Dan Bohlen
GE Aviation
-----Original Message-----
From: xansys-bounces_at_xansys.org [mailto:xansys-bounces_at_xansys.org] On
Behalf Of Y Y Tan
Sent: Friday, April 25, 2008 6:08 PM
To: ANSYS User Discussion List
Subject: Re: [Xansys] Ansys FSI nightmare
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
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