Learning Forum

[Ben_Ben]

I posted the error previously but I have been waiting for a reply since Decemeber 9th.

Please see post here: https://forum.ansys.com/forums/topic/runaway-velocity-error-using-dynain-file-for-sequential-simulations-with-beams/

I will repost the main issue here:

I simplified my sim down to one beam but the second simulation does not behave as expected. I have outlined my test scenario below, all analysis settings are the same etc. 

In my first sim I applied a displacement inwards towards centre at each end

Here are the results 

In the second simulation I applied the same displacements but towards the outside. 

But I get ridiculous results, here are the results on non-deformed geometry and deformed geometry

Please could advise on if I can run this scenario with a restart analysis. Where I can change my boundary conditions. 

Thanks, 

Ben

[Ram Gopisetti]

Hi Ben, what version of ANSYS are you using, have you tried to open dynain file in prepost?, how does it look, did you tried to load via external data method?

cheers, Ram 

[Ben_Ben]

Hi Ram,

I'm using Ansys2022R2. Trying dynain in LS-PrePost now. 

Thanks for reply, 

Ben

[Reno Genest]

Hello Ben,

Could you do this in one simulation without the dynain? Why do you need to do this in 2 steps an 2 simulations?

If you do it in one simulation, you need to specify different times for the displacement BCs moving in and out.

 

Let me know how it goes.

 

Reno.

[Ben_Ben]

Hi Reno, 

The example problem I'm sharing here is a simplification of my actual modelling scenario. I get the exact same issues with my full model. I will describe it below so you can better understand. 

My modelling problem involves two steps. It consists of several helical beams (like the one in my picture). 

1. I displace the top node of beams in the x-direction. The bottom nodes are free.
2. Then I displace the top node and bottom nodes toward the centre. Effectively compressing the helical beams. 

I can't do in one simulation because I need bottom nodes free in first step and then a displacement BC in the second step. 

Many thanks for your reply,

Ben

[Ben_Ben]

Here is my project schematic:

I load in the dynain file via an external model tile. 

[Reno Genest]

Hello Ben,

Also, what process do you use with the dynain file in WB? Can you show a screenshot of  your WB project schematic?

 

Reno.

[Reno Genest]

Hello Ben,

You could do that in one simulation with *BOUNDARY_SPC and *BOUNDARY_PRESCRIBED_MOTION using the BIRTH/DEATH options to activate  and deactivate the BCs at specific times. Have look at the user manual for more information on these keywords:

https://lsdyna.ansys.com/manuals/

 

If you are a commercial customer, please submit a service request on the Ansys Customer Portal:

support.ansys.com

We will be able to get your model and troubleshoot it. Ansys  employees are not allowed to upload and download models in this forum.

 

Let me know how it goes.

 

Reno.

[Ben_Ben]

Hi Reno, 

Would I need to do this in LS-PrePost? Rather than LS-Dyna Workbench?

Thanks, 

Ben

[Ben_Ben]

Hi Reno, 

I used 3 *BOUNDARY_PRESCRIBED_MOTION

First one - Displacement on Top Node (Bottom Node Free)

Second one - Compression on top node 

 

Third one - Compression on bottom node


Is this what you hd in mind with birth and death times. I use 0-0.005 for the displacement of Top and then 0.005-0.01 for compression of bottom and top nodes. 

I haven't used *BOUNDARY_SPC. Do I need to if my bottom node is free in all DOF. 

Thanks for your suggestion Reno,

Ben

 

[Reno Genest]

Hello Ben,

You can use both LSPP or WB depending on what you prefer. In WB, you can use the birth/death tool:

 

 

Reno.

[Reno Genest]

Hello Ben,

Yes, this is what I had in mind with birth and death. *BOUNDARY_SPC is used to fix degrees of freedom (DOFs) for some nodes in your model. You can have fixed or free DOFs. You can achieve the same with *BOUNDARY_PRESCRIBED_MOTION and settting the displacement to zero (fixed). 

Let me know how it goes.

 

Reno.

[Ben_Ben]

Hey Reno, 

It worked for my test model. Am running my full model now. Thanks for your input!

Ben