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ashtonJ May 30, 2012 20:07

Negative ELEMENT Volume Problem-FSI
Dear All,

I am doing FSI modelling in arteries. I encountered with following error at the beginning of second stagger iteration.

A negative ELEMENT volume has been detected. This is a fatal |
error and execution will be terminated. The location of the first |
negative volume is reported below. |

Volume: -0.9954E-12

I got this error when I specified frictionless support at one end of the model. When I fix both ends, the solution converges; however, when I specify frictionless support at one end, the above error is given.

Could anyone help me to fix it pls.
Kind regards,

p.galimutti May 30, 2012 21:28

'Friction-less support' means you are restricting the face to displace in its normal direction. Hence, you have displacement in other two directions. When the solid displaces in other two directions say 'y' and 'x', the fluid mesh also needs to be displaced accordingly. The mesh motion should be set to 'unspecified' for that boundary.

This is my initial view of you model, but we can not go further with out more input from you! Which means please post as much information as you can regarding your model, setup, bc's, a ccl file etc..

ashtonJ May 30, 2012 21:46

Thanks for your helpful comments.

For my next questions regarding the FSI modelling, I will provide more information.

Thank you.

ashtonJ May 31, 2012 19:21

Thanks for your help. I could converge my model with fixed support at inlet and frictionless support at outlet. I tried to use frictionless supports at both inlet and outlet; however, it diverged.

Here is the settings of my model;

Structural Model:
Young module: 1MPa
Poisson ratio: 0.3
Density: 1050 kg/m^3

Structural setting
Tetrahedral-Patch Conforming Mesh

Analysis Settings:
Autotime stepping: Off
Define by: substeps
Number of substeps: 1
Time Integration: Off

Boundary Conditions:
Inlet and outlet: Frictionless support
Inner surface of the wall: Fluid-solid interface

Fluid setting

Tetrahedral-Patch Conforming Mesh

Boundary Conditions:
Inlet: Static Pressure 100 mmHg, Mesh Motion: Unspecified
Outlet: Static Pressure 50 mmHg, Mesh Motion: Unspecified
Wall: fluid-solid interface

Initial conditions:

Solver Control:
Fluid part:
Advection scheme: High resolution
transient scheme :Second order backward
max loops: 10
convergence criteria:0.0001
Structural part:
max iteration: 45
Under relaxation factor: 0.25
Convergence target: 0.01

In CFX-Pre, the following warning is given:
In Analysis 'Flow Analysis 1': Export results are unavailable as no suitable boundaries exist for use as export surfaces. Mesh motion on a boundary invalidates its use as an export surface.
I would be too please if you could help me to fix this problem.

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