Learning Forum

[Pegah Mehrabian]

Hi everyone. For a two-way FSI simulation of flow over a cylinder, I defined four springs (body-to-ground) with similar stiffness in transient structural ANSYS. With the defined inlet velocity of flow in CFX, I expect to get the resonance amplitude in transient structural. But after 20 seconds, the simulation got diverged. In the result, it seems springs start twisting because of the nonlinearity of the problem. How could I solve this problem?

[Dave Looman]

Perhaps add stiff torsional springs at the same four locations.  You may need to turn on beta features.

[Pegah Mehrabian]

Would you please explain more? like how could I add stiff torsional springs? or what do you mean by using beta features? 
Thanks

[Dave Looman]

Just like you created the translational springs you could create torsional springs in the same locations to prevent rotation.   You can turn on beta features on the project page with Tools>  Options...>  Appearance> Scroll to the bottom of the Appearance options.

[Pegah Mehrabian]

Thank you, I found it. How could I define torsional stiffness based on longitudinal stiffness, now? Could you please provide me with some formulas? 

[peteroznewman]

A rigid body has 6 degrees of freedom. Do you want the cylinder to move in all 3 translations and all 3 rotations?

The way you drew the springs, perhaps you only want the cylinder to translate in 2 directions: X and Y.  If that is the case, add constraints to the structural model to fix the other 4 degrees of freedom. A simple way to add a General Joint. That has integral springs so you can delete the 4 springs you added. 

[Pegah Mehrabian]

 

It works so well, thank you so much!

 

[Dave Looman]

If you are trying to include a realistic representation of the spring's torsional stiffness that would be something a spring manufacturer might provide.  I say "might" because springs typically aren't used to provide torsional stiffness.

[Pegah Mehrabian]