Learning Forum

[Abhemanyu Palaniswamy Chandrasekaran]

I am recreating the floating ship model found in https://ftp.lstc.com/anonymous/outgoing/hao/sale/ by replacing all the keywords with the ones for Structured FSI. (Structured mesh volume filling, Structured_FSI, Structured multi-material group) etc. I am sucessfully able to replicate this example and also include ambient elements of AETYPE =4 to simulate an infinite fluid region.

But I do not understand the nature of Fluid structure interaction and the behaviour of the lagrangian part. 

The example has the lagrangian container defined with a shell element of thickness 0.2 m and steel as the material. But the deformations observed from the simuations are very unrealistic. I have also attempted changing the material properties and shell element thickness, but they seem to show no difference in the results.

Similarly i have also tried to change the structure to a rigid by chaning the material keyword. But again the entire lagrangian structure remains fixed in space (similar scenario happens also with the example file).

Could someone please guide me to a proper 3D setup of hydrostatic equillibrium for a floating body.

Thanks & Regards

Abhemanyu

[Ian Do]

Hi Abhemanyu,

Can you list out exactly which KW you modified and how were they modified?

During the conversion, please make sure you check:  (IC = initial condition, BC = boundary condition)

• unit system used
• mat data - check EOS (IC) input & the resulting P0(t=0), rho0(t=0) in all ALE fluids
• IC's BC's - vel(t=0) 

It looks like there may be something wrong in the input that causes the strange ship hull deformation(?). You can try:

• go back to run the original model (no changes)
• then convert 1 KW at a time and rerun
• This will pin down exactly which change may cause the strange result.

Hope this helps.

 

Ian Do, PhD

[Abhemanyu Palaniswamy Chandrasekaran]

Dear Ian, 

Thank you for the reply. I think I did not explain myself very well in the first post.

The hull deformation is unrealistic in the example file too.

Unfortunately, I am now unable to access the example files

 

 

I have changed the following keywords. They make no difference to the setup of the original file.

*CONSTRAINED_LAGRANGE_IN_SOLID ----> *STRUCTURED_FSI

*INITIAL_VOLUME_FRACTION_GEOMETRY ---------> *STRUCTUED_MESH_VOLUME_FILLING

*ALE_MULTI-MATERIAL_GROUP-------->*STRUCTURED_MULTI-MATERIAL_GROUP

To change the structure to a rigid body, Only the material definition was changed from ELASTIC to RIGID.

 

It would be very helpful if there are some other simple examples of objects floating in hydrostatic equilibrium, just to verify if my FSI setup is correctly modelled.

 

Regards

Abhemanyu

[Ian Do]

which exe version are you using?

 

[Abhemanyu Palaniswamy Chandrasekaran]

I am using the smp version

ls-dyna_smp_d_R13.1.1_27-g8731a0d8c5_winx64_ifort190

 

[Ian Do]

can  you please try the released R14 version? MPP (faster), double precision?  

Which organization are you writing from? 

[Abhemanyu Palaniswamy Chandrasekaran]

I am working on my master's thesis at ICAM, Nantes.

I have tried running the same with version 14, but it makes no difference. The hull deformation is still the same.

ls-dyna_smp_d_R14.0_winx64_ifort190

The following image is from Version 14 SMP double precesion with the original input file

 

The part definition of the ship from the keyword is below

 

 

[Ian Do]

i ran with:

[1]   ls-dyna_smp_d_R14.1_205-geb5348f751_winx64_ifort190.exe

     |  Date    : 10/16/2023    Time: 19:29:09           |
     |  Version : smp d R14                              |
     |  Revision: R14.1-205-geb5348f751                  |
     |  AnLicVer: 2024 R1 (20231025+39429f6)             |

and

[2]    ls-dyna_mpp_d_R14.1_205-geb5348f751_winx64_ifort190_msmpi.exe

     |  Date    : 10/16/2023    Time: 19:29:09           |
     |  Version : mpp d R14                              |
     |  Revision: R14.1-205-geb5348f751                  |
     |  AnLicVer: 2024 R1 (20231025+39429f6)             |

Both give good result as expected.

Can you try to test with these exe?

 

[Abhemanyu Palaniswamy Chandrasekaran]

Hi Ian,

I have tried running the original file with MPP version 14. I get the following

Could you please share an image of the hull deformation results that you obtain.

The material is modelled as steel with a thickness of 0.2m (20cm). The fluid is water and the fluid inside is an arbitrary fluid with half the density of water. In any case, such a thick hull cannot deform due to the fluid.

Similarly by only changing the material behaviour in the original file, changing the material for PART=5 (lagrange shell)  from ELASTIC to RIGID, the results are completely unrealistic.

In the original model, the node set of the lagrange part 5 (ship) is only constrained from moving out of plane. So Ideally the wave must be able to move the structure in the X and Y directions. Instead, the part remains fixed in space. Could you please explain if I am missing any details in this model.

Thanks & Regards

Abhemanyu

[Ian Do]

MPP run:

 

  C P U   T i m i n g   i n f o r m a t i o n

  Processor   Hostname                              CPU/Avg_CPU  CPU(seconds)
  ---------------------------------------------------------------------------
  #       0   AAPG0ILAE0h14Og                           1.00825   8.3797E+01
  #       1   AAPG0ILAE0h14Og                           0.99773   8.2922E+01
  #       2   AAPG0ILAE0h14Og                           0.99942   8.3062E+01
  #       3   AAPG0ILAE0h14Og                           0.99566   8.2750E+01
  #       4   AAPG0ILAE0h14Og                           1.00581   8.3594E+01
  #       5   AAPG0ILAE0h14Og                           1.00149   8.3234E+01
  #       6   AAPG0ILAE0h14Og                           1.00280   8.3344E+01
  #       7   AAPG0ILAE0h14Og                           1.00487   8.3516E+01
  #       8   AAPG0ILAE0h14Og                           1.00243   8.3312E+01
  #       9   AAPG0ILAE0h14Og                           0.99415   8.2625E+01
  #      10   AAPG0ILAE0h14Og                           0.99547   8.2734E+01
  #      11   AAPG0ILAE0h14Og                           1.00261   8.3328E+01
  #      12   AAPG0ILAE0h14Og                           1.00562   8.3578E+01
  #      13   AAPG0ILAE0h14Og                           1.00243   8.3312E+01
  #      14   AAPG0ILAE0h14Og                           1.01690   8.4516E+01
  #      15   AAPG0ILAE0h14Og                           0.96708   8.0375E+01
  #      16   AAPG0ILAE0h14Og                           1.00224   8.3297E+01
  #      17   AAPG0ILAE0h14Og                           1.00468   8.3500E+01
  #      18   AAPG0ILAE0h14Og                           0.99716   8.2875E+01
  #      19   AAPG0ILAE0h14Og                           1.00205   8.3281E+01
  #      20   AAPG0ILAE0h14Og                           1.00111   8.3203E+01
  #      21   AAPG0ILAE0h14Og                           1.00167   8.3250E+01
  #      22   AAPG0ILAE0h14Og                           0.99754   8.2906E+01
  #      23   AAPG0ILAE0h14Og                           0.99603   8.2781E+01
  #      24   AAPG0ILAE0h14Og                           1.00543   8.3562E+01
  #      25   AAPG0ILAE0h14Og                           1.00149   8.3234E+01
  #      26   AAPG0ILAE0h14Og                           0.99754   8.2906E+01
  #      27   AAPG0ILAE0h14Og                           0.99848   8.2984E+01
  #      28   AAPG0ILAE0h14Og                           1.00431   8.3469E+01
  #      29   AAPG0ILAE0h14Og                           0.99867   8.3000E+01
  #      30   AAPG0ILAE0h14Og                           0.99904   8.3031E+01
  #      31   AAPG0ILAE0h14Og                           0.98983   8.2266E+01
  ---------------------------------------------------------------------------
  T o t a l s                                                     2.6595E+03

 Start time   05/23/2024 13:24:42  
 End time     05/23/2024 13:26:18  
 Elapsed time      96 seconds for   50315 cycles using    32 MPP procs
             (      0 hour   1 minute  36 seconds)

 N o r m a l    t e r m i n a t i o n                          05/23/24 13:26:18

[Ian Do]

SMP run:

 

 Problem time       =    2.0000E+01
 Problem cycle      =     50315
 Total CPU time     =       276 seconds (   0 hours  4 minutes 36 seconds)
 CPU time per zone cycle  =       1338.943 nanoseconds
 Clock time per zone cycle=       1344.606 nanoseconds

 Number of CPU's   32
 NLQ used/max     136/  136
 Start time   05/23/2024 13:34:22  
 End time     05/23/2024 13:38:57  
 Elapsed time     275 seconds for   50315 cycles using 32 SMP threads
             (      0 hour   4 minutes 35 seconds)

 N o r m a l    t e r m i n a t i o n                          05/23/24 13:38:57

 

 

MPP is always better 

[Ian Do]

i ran the original model.

 

[Ian Do]

I have modified the mat properties of the steel to make it soft to amplify the deformation for visualization. This is completely conceptual and unrealistic. Please never rely on my mat properties because they can be scaled for such purpose or to run faster with larger dt.