RADIOSS AMS For Time Step Increase

Crashphys
Crashphys Altair Community Member
edited October 2020 in Community Q&A

Hi,

 

I am using AMS for the attached rad files. I have uploaded my results and models for reference. I had help in this thread previously to reduce time step:

https://community.altair.com/community?id=community_question&sys_id=125648f61b2bd0908017dc61ec4bcb7a

 

Here has been my process so far:

In Test3, I ran the solver without any AMS, and the mesh was poorly optimized. My total run time was about 30 min, and my maximum displacement was 0.15 m. This is a ridiculously high result, as it can't be possible to have a deformation of 15 cm. I tried to add more accuracy by increasing the number of SPH nodes, however the run time was over 60,000 seconds.

 

I was advised in the attached thread to get rid of any elements with bad aspect ratios etc. I did so and created Test4. Note, I had to used RBODYs for certain parts of the mesh as it was the only way to get rid of the bad aspect ratios entirely. At that point, I was able to almost triple the amount of SPH node, however it was even less accurate than before, with a max deformation of 18 cm. It also takes about an hour to run - double the time. 

 

I don't understand, seeing as the time step was reduced by at least one order of magnitude between the two iterations. I have more nodes, it takes longer to run, yet is somehow less accurate. When I try to use AMS, the simulation run time is either not decreased significantly, or results in the simulation being killed for exceeding the mass and energy error.

 

Can anyone please advise what is wrong with my model? I suspect it may be the RBODYs, but I'm not sure.

 

Thanks in advance,

Ayman

 

EDIT: I know it is not accurate because I ran it with very few SPH nodes, and got a deformation of 0.25 m (25 cm), which is almost double what I got with what I have in Test3.

 

EDIT2: Attached Test6. That is my solver with AMS enabled.

Unable to find an attachment - read this blog

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Answers

  • Andy_20955
    Andy_20955 New Altair Community Member
    edited April 2019

    Hi,

    Please check that the physics of your model is correct as defined.  If you take the mass of the particles in, /PROP/SPH, mp*#particles does that equal the expected weight of the material in the car?   Also check material properties and units used.  I assume the units are kg, m, s.  If yes then, the wall of your car is 0.00635 m which is 6.35 mm or 0.25 inches which I am sure is to thin and most likely the issue. 

     

    What is the loading condition your are simulating using 5 m/s initial velocity.  What about gravity?

    Please set /INTER/TYPE7, GapMin= the 1/2 the pitch you used to generate the SPH particles + 1/2 the thickness of the walls.  The pitch is the distance between the particles so 1/2 the pitch is the radius.  Then set Inacti=5 to remove any initial penetrations due the meshing. 

     

    We normally recommend face centered cubic for SPH meshes if you redo the SPH mesh you can use that option in HyperMesh by the pitch: setting.

     

    Last for improved speedup, please check the number of core your have in your CPU and use the run option, -np #cores to improve run times.

     

    Thanks,

    Andy

  • Crashphys
    Crashphys Altair Community Member
    edited April 2019

    Hi Andy,

     

    CPU option drastically improved results time. Thank you for the suggestion, it was incredibly helpful. 

     

    I have readjusted the mass. Generally, a hopper car carries somewhere around 100 tons. I changed my analysis to have it at 90,000 kg now, which makes sense. I believe I did use FCC for the SPH Mesh. I also set the interface settings as recommended.

     

    As for the physics, it is difficult to say. From parts I have acquired, the measured thickness of the walls in the car were 6.35 mm. I accompanying those walls is likely some sort of frame structure. I suppose I will have to find some way to include that in the stiffness of the walls somehow. As for the loading, I am currently just trying to make it work with load. My objective is to see what will happen if the car turns and experiences a lateral inertial load of 0.3 Gs. Unfortunately, from what I can see, RADIOSS does not do inertial loads, so I used an /inivel on the particles instead. I intend to apply gravity eventually for sure.

     

    If you have any advice on the above, it would be much appreciated.

     

    Thanks,

    Ayman

     

    EDIT: I just noticed that there is in fact a /GRAV load, and it can be applied in any direction. Sorry for the confusion there. I will try to apply that now.

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