Learning Forum

[jimbo]

In my analysis, I’m working with FSI where flexible flaps are installed inside an internal flow channel to investigate the heat transfer enhancement on the channel wall.

I have a question regarding the Large Deflection option in the structural domain:

• When this setting is enabled vs. disabled, what are the practical effects in this type of simulation?

• Both cases allow the flaps to deform, but with Large Deflection disabled, the structural solver seems to converge more easily. Could this cause other issues or inaccuracies in the results?

Currently, I am facing difficulties with very low Young’s modulus materials, which often leads to structural divergence. To handle this, I’ve been disabling the Large Deflection option.

So my questions are:

1. Other than disabling Large Deflection, are there additional methods to improve convergence when using low modulus materials?

2. I’ve already reduced the time step to 0.0001 and relaxed the tolerances, but the divergence issue still persists.

Any suggestions or experiences would be greatly appreciated!

[Federico]

Large Deflection is typically needed for slender structures. Use large deflection if the transverse displacements in a slender structure are more than 10% of the thickness.

Small deflection and small strain analyses assume that displacements are small enough that the resulting stiffness changes are insignificant. Setting Large Deflection to On will take into account stiffness changes resulting from changes in element shape and orientation due to large deflection, large rotation, and large strain. Therefore the results will be more accurate. However this effect requires an iterative solution. In addition it may also need the load to be applied in small increments. Therefore, the solution may take longer to solve.

You also need to turn on large deflection if you suspect instability (buckling) in the system. Use of hyperelastic materials also requires large deflection to be turned on.

[jimbo]

So the general criterion is that if the deformation exceeds 10% of the thickness, the Large Deflection option should be enabled. Will this setting have a significant impact on the results?

[Federico]

Are you using System Coupling with a CFD solver (e.g. Fluent, CFX) for this analysis? Or is it purely done in Mechanical?

[jimbo]

I am using the FLUENT module together with the Transient Structural module, exporting them to an external System Coupling R1 setup in order to perform the fluid-structure interaction (FSI) simulation.

[Federico]

What is the working fluid in Fluent?