[frossi]

Hi everybody,

I am a college student learning ansys and CFD. I am running a two-ways FSI simulation of a wing with Transient Structural and CFX (the wing is hollow inside with a skin thickness of 0.5 inches). I used one side of the wing as the fixed support.
The goal is to find the equivalent stress and the deformation of the wing (is it correct to assume the "Total Mesh Displacement" variable to be the wing deformation?).

First, I modeled the geometry using the thin surface option. I used 3 different surfaces: the wing skin, the support (where it attaches to the fuselage), and the wingtip surface. I set the surfaces thickness offset type to "middle" (This causes the mesh of the skin to merge with the mesh of the support and wingtip. Is this a problem?)
When I look at the FSI results, I get a deformation of about 2 cm, and an equivalent stress of 37 MPa.
Then, to check my results, I ran the same FSI simulation with a 3D hollow body of the same wing imported from Creo. The equivalent stress is very similar (35 MPa) but the deformation is much larger (23 cm!!!).

Could someone please explain me why I get such a big difference in the deformation, while the stress is almost the same?
Also, when is it appropriate to use thin surfaces, and when is it better to use 3D solid bodies?

The first two pics show the thin surface wing, while the last two pics show the 3D solid body wing.



I am stuck and would really appreciate some help. Thank you very much!

[ghorrocks]

I cannot explain the stress/deflection issue without looking at your simulation in detail - and the forum is not suitable for that sort of thing anyway.

But I can say that your 3D mesh looks very coarse and I would be very suspicious of stresses predicted by it. You need to do mesh sensitivity studies on the FEA side of a simulation as well as the CFD side.

Also don't forget about the different element formulations - high/low order, reduced integration, tets vs hex etc etc. You need to know these basics before you do FEA.

[frossi]

Ok thank you, I will look at the element formulations in depth.

Some questions I have:
1. When you say sensitivity study, do you refer to convergence study? If yes, what do you suggest as a good convergence? is 5% or less acceptable?

2. In general, how does FEM of a solid 3D body differs from FEM of a thin surface body? When should I use thin surfaces, and when should I use solid body? Someone on the forum was mentioning that it is important to have a minimum number of layers when working with solid bodies. Could you please clarify on this one?

Thank you in advance.

[ghorrocks]

Convergence study: There is more to an accurate simulation than just getting the convergence tight enough. You need to check mesh density as well.

Is 5% good enough: That depends on what you are trying to achieve. For high precision, no; for a general idea of what is going on, a coarser tolerance is OK. So it is up to you to determine.

FEM questions - best ask these questions on a FEA forum.