ALE coupling

Question

Hello, I am trying to simulate a birdstrike using Patran as my preprocessor. At the time of creating an ALE coupling between the surface and the bird. What are the parameters which have to be used (surface, 3d elements, etc) in order to fullfill the coupling? Can somebody please explain step by step what should be done?

Thank you in advance.

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DougMarriott · Accepted Answer

Hi Bhaven,

Regarding the 1.IND error this is a material orientation angle on the CQUAD etc cards in the BDF file that were not translated correctly from Patran. Please verify the element orientations again and use the orientation flag in the PCOMP property set or where you have any inputs. Alternatively remove them if not required as it could be an artifact from a previous file import or something. You can also set the translation option to free format that might help sort it in Analysis | Translation Parameters : Card Format=Free

Regarding the FSI setup. Thanks for the pic, it makes things a bit easier to talk around.

I'm still working on the FSI setup article, so will link it soon(!). But the steps you need to do that I would recommend is:

1. Finalise the structural model. Meaning get the structural model well defined and even run a test model with it just in space to make sure that part of the analysis is initialising correctly (materials/properties/LBC etc).

2. Set up the coupling. Create | Coupling | General. For your structure set the Cover=Inside and select all the external elements (making sure you only select a closed surface). Most times it is easier to make a new group with only the external surface mesh and use the Element tools like Verify | Element Boundaries to make sure that the surface is indeed closed.

3. Now start the FSI side of things. Firstly by creating all the fluid material. In your case then only the bird material. For birdstrike always remember to add shear strength too! Otherwise with only bulk modulus it is like a water balloon! Create | Isotropic | NonLinElas (DMAT) will give you bulk modulus (Coeff A1) and shear modulus (Shear Coeff G0). Because we do birdstrike in void it is important to set the spallation pressure to -1bar. By default in Dytran we spall/cavitate fluid material when the pressure =0, so if you work in void we need to offset this so that we work in a pseudo 'gauge' pressure and spall at -atmospheric pressure.

4. Because we are going to use the abstract fluid domains/regions/mesh we first need to define which fluid solver we'll use. For this create the 3D Euler Property and since our fluid will have to solve the full stress tensor as we have shear, select the MM/Strength.

The PEULER1 option means we will use geometric regions to define which material goes where and not select from a physical mesh/elements.

5. Fluid Mesh or Euler Initial State can be swapped depending on the setup really. In this case it will be easier to do the Initial state setup first. For birdstrike you generally have a hemispherical ended cylinder and you could generate this with the geometry shapes (sphere/cylinder), but it becomes a pain when you want to move things! Easiest is to mesh a surface mesh that you then assign Dummy Shell properties to:

Then in LBC | Create | Init.Cond.Euler | Shape/Surface to set the shape of the bird select these dummy elements. They will therefore only be used to define the region of material and not take part in the structural analysis (dummy element nodes are fixed in space).

For the rest we will define a shape for the void. For this you can generally just create a very very large sphere :)

This means you can then move the bird around in subsequent runs and not need to worry about redefining the void region.

After this we define the initial values that will go with our shapes. In your case then you only need to define the bird values as void is empty by definition:

Lastly you tie it all together with the region definition defining the order that these shape/initial values are applied to the fluid domain space. Order as in level numbers. So Level 1 is void shape, then Level 2 is bird shape/bird values:

Also see that you select the Euler property set to link these things together.

6. Fluid Mesh. In LBC | Create | Mesh Generator you can now set the fluid mesh up. The Option=Box is for external (or internal) fluid domains, while the Option=Adapt is used to fill a fluid mesh inside a coupling surface (and move with it). You can then Preview and see where your fluid mesh will be, so you can move it around to encapsulate the bird shape and structure you think it will mainly interact with.

A key thing to note is that the boundaries of the MESH boxes are by default walls, so if you want to allow flow to go out you need to add the following in the input deck manually as Patran does not support these MESH flow cards yet:

FLOWDEF,999,,MMSTREN,,,,,,+

+,FLOW,OUT

In the Output requests you need to request outputs for the Euler results for the specific Euler property set used, i.e. ALLMULTIEULSTREN.

For the general coupling we also have a feature called Fast coupling. This is setup in the Analysis | Execution Controls | Coupling Parameters. Set this to Active and also failure if required:

Fast coupling is a slight misnomer. We are able to run 50-70% faster by assuming the fluid mesh is aligned with the global orientation (which it is when using MESH boxes). This means we can ignore the step to transform the flow between Euler elements into global X,Y,Z vectors and use them as is. So best to always switch it on :)

Lastly I would strongly recommend to look at the Dytran User Guide Chapter 11 to see how to use Paraview as post processor as this is a great tool to use for fluid results post-processing (isovolumes etc).

All the best!

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