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-   -   rigid body convergence issue (http://www.cfd-online.com/Forums/cfx/107382-rigid-body-convergence-issue.html)

hamed.majeed September 25, 2012 22:34

rigid body convergence issue
 
Hi,
I have problems with the convergence of my rigid body. Although, the flow parameters are converged. Can anybody specify what the problems might be??

http://pichost.de/images/2012/09/26/GEwkf.png

But look at the flow parameters, they do converge.

http://pichost.de/images/2012/09/26/YlKO0.png

hamed.majeed September 25, 2012 23:23

I applied residuals for torque, force, and mesh motion. Still divergent.
http://pichost.de/images/2012/09/26/qoIX.png
http://pichost.de/images/2012/09/26/p4A1d.png

hamed.majeed September 25, 2012 23:24

Also I tried.
http://pichost.de/images/2012/09/26/Ve9Y.png
http://pichost.de/images/2012/09/26/j7hSB.png

hamed.majeed September 25, 2012 23:25

I also tried.
http://pichost.de/images/2012/09/26/zsOWm.png
http://pichost.de/images/2012/09/26/8op6.png

hamed.majeed September 26, 2012 04:37

For a combination of above settings turned on these were the results.

http://pichost.de/images/2012/09/26/1UK8k.png
http://pichost.de/images/2012/09/26/VNxGW.png

ghorrocks September 26, 2012 08:12

Can you describe what you are modelling?

hamed.majeed September 26, 2012 08:37

I am simulating a rigid body drop in fluid. Using 6 dof Rigid body solver and the rigid body defined as an immersed solid.
The problem is that the fluid parameters u, v, and w show convergence but the rigid body does not. Also in rigid body convergence curves overshoots occur.

hamed.majeed September 26, 2012 11:25

These are the best results that I could obtain, but it has 100 iterations for the stagger/coupling loop.
http://pichost.de/images/2012/09/26/iM0lF.png
http://pichost.de/images/2012/09/26/ypq9.png
http://pichost.de/images/2012/09/26/wOVzp.png
Still unsatisfactory!!

hamed.majeed September 26, 2012 14:37

Hi,
I have explained my case. A rigid body falls under gravity in a column of fluid.

As I have shown that the rigid body convergence diverges. However, in ANSYS help I found this.

Force Convergence

It is defined as ratio of difference of forces for previous and present time step divided by the greater of the two forces.
Now, in my case the rigid body falls under gravity.
At each time step the force acting on the body differs as the rigid body rotates and translates. Is that the explanation of the curve behavior???

Also torque convergence is defined similar to force convergence but for torque.

ghorrocks September 26, 2012 18:36

Are you using immersed solids or moving mesh approach?

hamed.majeed September 26, 2012 22:18

the immersed approach

ghorrocks September 26, 2012 22:31

Have you read section 1.2.10.2 of the documentation listing the limitations of this approach?
  • For transient cases, immersed solids do not interact properly with fluid domains that involve compressibility or multiphase flow.



Would explain what you are seeing. Also have a look at section 11.7.

Also

hamed.majeed September 26, 2012 23:01

Yes, I have. My case is incompressible fluid. However, since the rigid body falls with initially at rest. This limitation applies to it.

"For low Reynolds number turbulence cases, the immersed solids turbulence model cannot accurately predict the pressure fields near the immersed boundaries".

All clear for rigid body limitations

ghorrocks September 27, 2012 07:45

Yes, that restriction may cause problems. What is the Re number of your bodies motion at the time you are having problems?

So your simulation is single phase?

hamed.majeed September 27, 2012 13:07

Yes, it is single phase = water
Rigid body has max. velocity = 2 m/s.
Re = [(rho =1000kg/m^3)*(v=2m/s)*(d=0.02m)]/(mu=1e-3) = 40k.
I am extremely sorry, it isn't low reynolds.

ghorrocks September 27, 2012 18:33

Agreed, that does not sound like low Re. But when the motion first starts it might be. What is the Re at the time of convergence difficulties?

hamed.majeed September 28, 2012 00:02

well if you see the curves above. I would say pretty every where.

ghorrocks September 28, 2012 00:17

Can you try putting the body in a duct with a cross flow in it? Then the body will be starting from a reasonable Re right from the start. I suggest this just to see if the problem is convergence at low Re.

hamed.majeed September 28, 2012 09:37

you mean that I should give some velocity to the fluid i.e. 2 m/s, as max. velocity of rigid body in previous case was 2m/s.
Also, should I make the rigid body stationary??
I will do that, using 6 dof rigid body solver!

hamed.majeed September 28, 2012 11:33

Hi,
Here is a brief explanation of what I am trying to do.
A rigid body defined as an immersed solid and solved by using 6 dof Rigid body solver is dropped from rest into a fluid domain. The fluid domain is single phase. Opening BCs are given to the top and bottom of the fluid domain. Wall BCs the sides.


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