FSI. simple pipe case crashes

  • Last Post 07 December 2018
pwkt posted this 20 November 2018


I've tried to simulate an FSI case - flow through an elastic pipe.
Fluent and Transient Structural were used.

I've set up both participants and joined them using a system coupling block.
Then I've defined forth and back data transfers between the components.

Unfortunately, during a simulation I've got an error, saying:

(DP 0) Element 720 located in Body "FFF\vessel" (and maybe other elements) has become highly distorted. You may select the offending object and/or geometry via RMB on this warning in the Messages window. Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere. Try incrementing the load more slowly (increase the number of substeps or decrease the time step size). You may need to improve your mesh to obtain elements with better aspect ratios. Also consider the behavior of materials, contact pairs, and/or constraint equations. If this message appears in the first iteration of first substep, be sure to perform element shape checking. Named Selections for the offending element can be created via the Identify Element Violations property on the Solution Information Object.

I've tried to decrease the time step, but with no success. I'm wondering why so simple case crashes... 

Could anyone help with this case?
Thank you in advance!

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rwoolhou posted this 20 November 2018

As staff I can't download the files, but can you run through the mesh quality and settings to make sure they're sensible. It could be the mesh is poor in a region, or simply too coarse: also check the model scale incase you've got mm / m / km  muddled up due to CAD import or dimension scaling. 

kkanade posted this 21 November 2018

Please check following. 

1. Please check orthogonal quality of the mesh. It should be more than 0.1.

2. Please run fluid and structure separately and make sure that they run successfully

3. Once they run separately then you can start setting up fsi problem. Please make sure that fsi settings are correct. 

pwkt posted this 22 November 2018

Dear rwoolhou,


I've carefully checked both meshes and improved orthogonality for structural mesh.

Then I've run both fluid and structural problems separately. They all simulated fine.

Then I've connected them again to system coupling element, assigned data transfers and time steps.

The coupling simulation crashes again just after few initial steps, giving me the same error.

During the coupling ANSYS said that vessel wall element became distorted; however, when I run a single structural case deformations are good and no errors occured:

I've attached the updated project file here: https://yadi.sk/d/3w_w1YA3b6Du_w

pwkt posted this 22 November 2018

Dear kkanade,


I've double checked my settings using the videos sample you've provided; my settings were similar, however, the simulation still crashes.

Could you suggest what could be wrong?

kkanade posted this 23 November 2018

Did you define all dynamic mesh zones i.e. stationary, deforming, system coupling?

Make sure that the deformation in mech is not too much which will give negative cells in fluid.

pwkt posted this 24 November 2018

 Dear kkanade,

I've defined all zones in the Dynamic mesh section.

Moreover, I've applied zero inlet velocity and zero outlet pressure for test.

Nevertheless, the simulation still crashes, with no reason.

Very strange........


The settings are the following:




pwkt posted this 26 November 2018

Dear rwoolhou,


could you suggest what could be a reason for this ?

pwkt posted this 06 December 2018

Dear kkanade,

I've just found that when I use a stiff material like steal a simulation goes with no problem.

Also I've observed that initial perturbations of structure are really high, even for steal.

Maybe these perturbations are really the cause, since I want to use a vessel wall, which is really elastic.

Is it any way to omit this high amplitude initial deformations ??

With best regards,


kkanade posted this 07 December 2018

can you please make changes so that force on wall reduces.