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FEM Theory Question: Creating a local model from a global model

Crashphys

Hi,

I realise this site is mostly for software help, however I am wondering if anyone can recommend any documentation on how to take characteristic sections of your model and do FEM on those? For example, imagine you are designing a plane wing iteratively. It is really hard to get convergence if you optimize each beam and panel all at once, so you take one characteristic panel and optimize that in a separate model.

Now you need to generate an FEM of this characteristic panel, however this panel was part of a much more complex system of moving parts. My question is how you would constrain this panel. In the global model, the edges of this panel might have a displacement by a certain amount. Is there a specific and easy way to tell the BCs to allow for a certain amount of displacement before loads begin deforming the panel?

Thanks

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Mr.Alb

Hello Crashphys,

FEM is pretty general as field. In RADIOSS solver, you can performe a cut to consider only a part of your whole system for explicit analysis.

(https://altairuniversity.com/28761-radioss-sub-modeling-cut-approach/)

If you work with linear problems, maybe the field of most typical optimization problems, with OPTISTRUCT you have to consider how real components contraint the item you want to optimize. Unfortunately, I don't know how you can make a binding bond...

In my opinion, when you performe an optimization the real limit is the submodel you're considering rather than the constraints you've imposed. Usually accounting all part as possible is a good choice to do a meaningful optimum. For instance, are you really sure that optimizing one panel at once, globally it results in an optimum structure? Loads pattern could be very different in my opinion...

Let me know

Crashphys

Hello Mr. Alb,

I had to take some time to consider what you're saying. The problem with optimizing my whole model all at once is that it is infeasible. I would end up with hundreds of design variables which are highly non-linear and convergence would be unlikely. The only way I can think of is to create a submodel. In another post I made:

https://community.altair.com/community?id=community_question&sys_id=1666003a1b2bd0908017dc61ec4bcbd8

I found out how to create a break-out model in linear static FEM. What I want to do is take those equivalent forces and apply them on my submodel. The problem is I don't see how I can constrain it. For example, see my two models attached. One is my local and one is my global model. The local model has the resultant forces on each node from the global model applied through linear interpolation. The thing is, the edges of the local panel model I have are not stationary, they should be able to move a certain amount, but unconstraining them too much also results in disagreement in results. What I am wondering is the best methodology to constrain the panel. How should the edges of my local panel be constrained such that the model is at least close to reality as possible?

To my understanding, there must be some kind of way, otherwise how could such huge and complex systems such as aircraft be optimized?

Edit: As an example, see this outside link: https://structures.aero/blog/submodels-breakout-models-fea/

In this article, they mention one use of submodelling/breakout models is to iteratively design local component so that you don't need to run your full model. Quoting:

'When should I use submodels?
In what circumstances might you use a submodel?

When a small design change requires analysis.

When a small portion of a large design needs to be iteratively designed
You don’t want to have to run your large analysis model over and over again. Instead, you can iterate the submodel and save computation time on the solution.

When a feature is added to an existing part.
In our first example, we will look at a scenario where avionics wants to add a pass-through to a wing-rib. It will be computationally expensive to run the entire model just to analyze the new mesh on the rib, so a breakout model would be useful.'

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