[EmaManu]

Hi guys!
I need your help because I’m trying to simulate a planing motorboat (using DTC Hull tutorial as usual) but I’m addressing the same problem some of you already faced: interDyMFoam becomes instable and terminates.
I have done many trials attempting to follow the hints I found on the forum but I think I have done a passage not in the correct way. I’m referring particularly to the thread:

interDyMFoam - planing hulls at high speed

Specifically in the dynamicMeshDict (sixDoF motion solver) file I have simply copied the rows relative to the “force ramp” from the dynamicMeshDict (RigidBody motion solver) file suggested by Jan @JNSN.
So I would like to ask you which is the correct way to insert the force ramp in the dynamicMeshDict (motion solver sixDofRigidBodyMotion)
Thanks in advance for the help!!
Best regards,
Emanuele

PS: Some data of my simulation
-motorboat: 7 meters runabout, beam 2.5 meters, draft 0.43 meters, displacement 2700 kg, speed 35 knots
-mass and inertia moments of half hull
-acceleration relax 0.4
-translational damper 85960
-rotational damper 115860 (I increased them by an order of magnitude wrt DTCHull values aiming at increasing solver stability but it doesn’t work)
-Values I set for turbulence parameters (hope they are reasonable!):
k 0.01215
omega 18
nut 6.75 x 10^(-7)
-Mesh: 1.8 milion cells, built similar to the DTChull mesh through topoSet and refineMesh
-OpenFOAM v17.12

[Fauster]

Dear EmaManu,

I thought the "ramp coefficients" were only available with openfoam-dev and you are using OF1712+. Are you sure they are working ? EDIT : Yes it seems they are available with of1712+ (getting mad with all these OpenFOAM versions and different functionalities ...)

Can you show your checkMesh, shemes and BC ?

Thanks

[EmaManu]

Dear Fauster,

thank you for answering!! Of course: I have just uploaded on dropbox my entire case folder... the link is the following:

https://www.dropbox.com/s/g9ukleb75z...Barca.zip?dl=0

[Fauster]

Originally Posted by EmaManu

Dear Fauster,

thank you for answering!! Of course: I have just uploaded on dropbox my entire case folder... the link is the following:

https://www.dropbox.com/s/g9ukleb75z...Barca.zip?dl=0

You will find here your case modified. The problem is due to excessive mesh deformation. I change your mesh (coarsened and add layer), schemes and dynamicMeshDict. I run just 1 sec of your case on 10 cores.

I am not sure the quadraticRamp is working. If you have still some problem you will need to damp the strong acceleration at the beginning. Play also with inner and outer distance in your dynamicMeshDict.

https://www.dropbox.com/s/oj35smm6xm...arca2.zip?dl=0

cheers

[EmaManu]

Dear Fauster
thank you so much for the help!!! I have downloaded the file and I'm going to launch the simulation! I will tell you how it will go
However thanks again!

[EmaManu]

Dear Fauster,
the simulation with your settings works well!
I uploaded the case folder with 8 seconds of simulation so that maybe can be helpful for other people, the following is the link:

https://www.dropbox.com/sh/s0cwrcpbt...ThLOFdE2a?dl=0

Have a great day and weekend!
Best regards
Emanuele

[Fauster]

Dear EmaManu,

Have you been able to stabilized your simulation ? I mean do you get stabilized trim and sinkage time evolution ?

I am doing similar simulation and I am not able to get a stabilized solution. My pressure coefficient oscillates randomly wich affect the hull position and. Or maybe it's the opposite !

F

[EmaManu]

Dear Fauster,
for the time being I can't say because until now I have done several short simulations but now I am doing a new calculation which I hope wuold simulate 20-25 seconds by next monday/tuesday and I will tell you how it will go! Hoping the boat reaches a stable condition
Best regards
Emanuele

[EmaManu]

Dear Fauster,
sorry for the delay!
I'm still searching the useful parameter for my simulation because with 5x10^6 for both the translational and rotational dampers the bow rises too fastly and thus trim increases too mach while the aft remains immersed... so at 12-13 sec the simulations fail
I'm looking for the useful values to fix this problem: now I'm trying with 3x10^6 for translational damper and 7x10^6 for rotational one.
And you? How are going your simulations?
Best regards,
Emanuele

[Fauster]

Originally Posted by EmaManu

Dear Fauster,
sorry for the delay!
I'm still searching the useful parameter for my simulation because with 5x10^6 for both the translational and rotational dampers the bow rises too fastly and thus trim increases too mach while the aft remains immersed... so at 12-13 sec the simulations fail
I'm looking for the useful values to fix this problem: now I'm trying with 3x10^6 for translational damper and 7x10^6 for rotational one.
And you? How are going your simulations?
Best regards,
Emanuele

My simulations are not fine. I am not able to stabilize trim and sinkage and by consequence pressure coefficient is not stabilized.

Some keys points to help you:

• I realized since my last message about the quadraticRamp that the key word is quadraticRamp :

ramp
{
type quadraticRamp;
start 0;
duration 10;
}

• After investigating it appears that the ramp is not working !! The ramp works only with rigidBodyMotion !! Change your dynamicMeshDict and rerun your case.
• If it's still not ok try to use interFoam with LTS option (in fvschemes file) to obtain a stabilized flow.
• Then switch to interDyMFoam with a quadraticRamp (maybe 5 sec of duration) without damping and with accelerationRelaxation of 0.4 (or maybe 0.3).

Can I ask you where the hull comes from ? The stl triangulation is nice. This improves the meshing step.

I hope this will help you.
F

[EmaManu]

Dear Fauster,
first of all thanks for your suggestions!! ...I haven't put them in practice yet because I was attempting to find the suitable values for damping force and moment, and these trials took some time: now I have managed to find two values (7e6 for the rotational damper and 9e5 for the translational one) that work (quite) well beacause now the boat rises and rotates more normally, but I think sinkage and trim will be characterised by an oscillatory trend which doesn't end like it has happened to your simulations. I'm at 39 seconds of motion and the bow of the boat is coming down.
I have the curiosity to see how it will go, then I will applly your hints. About this: once I would have completed the simulation with InterFoam how should I practically use its results for the next simulation with interdymFoam? That is which data should I take from the output of the InterFoam simulation to put as input of the interDymFoam one?
Thanks in advance, again!!
EmaManu
PS: I uploaded the graphs of trim, sinkage and damping forces and moments If you want to see and compare to your results

And about the STL of course I can tell you! I realized it with Vacanti at first and then modified and exported with Rhino5

[Fauster]

Originally Posted by EmaManu

About this: once I would have completed the simulation with InterFoam how should I practically use its results for the next simulation with interdymFoam? That is which data should I take from the output of the InterFoam simulation to put as input of the interDymFoam one?

Before answering to your whole message I can give you how I proceed for doing that (I guess there are plenty of possibilities).

• Once interFoam (LTS) is finished you have to reconstruct the last results (reconstructPar -latestTime).
• Then rename the created folder to 0lts.orig (or any other name).
• Create a new clean case suitable for interDyMFoam (good fvsolution, controlDict, dynamicMesh etc...)
• Use the 0lts.orig as your 0 folder for the interDyMFoam run.
• Decompose the case and run interDymFoam in parallel

I hope it will help you !

[EmaManu]

Dear Fauster of course it helps!! Thanks a lot!

[Fauster]

Originally Posted by EmaManu

Dear Fauster,
first of all thanks for your suggestions!! ...I haven't put them in practice yet because I was attempting to find the suitable values for damping force and moment, and these trials took some time: now I have managed to find two values (7e6 for the rotational damper and 9e5 for the translational one) that work (quite) well beacause now the boat rises and rotates more normally, but I think sinkage and trim will be characterised by an oscillatory trend which doesn't end like it has happened to your simulations. I'm at 39 seconds of motion and the bow of the boat is coming down.
I have the curiosity to see how it will go, then I will applly your hints. About this: once I would have completed the simulation with InterFoam how should I practically use its results for the next simulation with interdymFoam? That is which data should I take from the output of the InterFoam simulation to put as input of the interDymFoam one?
Thanks in advance, again!!
EmaManu
PS: I uploaded the graphs of trim, sinkage and damping forces and moments If you want to see and compare to your results

And about the STL of course I can tell you! I realized it with Vacanti at first and then modified and exported with Rhino5

The figures of trim and sinkage evolution show that your simulation is not stabilized. I guess your damping coefficients are too high and the hull can't reach a stabilized trim and sinkage position. The inertia of the movement are way too big from my point of view. Your hull is small with reasonable inertia coefficient. You should reach a stabilized position in 10 sec of 20 sec maybe.

If you still want to use interDyMFoam at the beginning I would lower the value of damping coefficients and increase the ramp time (15 sec or 20 sec for example) to see if it helps.

Try also with a lower speed and then increase to 20 m/s. I manage to stabilized the hull with a speed of 8 m/s.

Can I ask you how you monitor trim and sinkage evolution ? Can you share your program ?

Paul

[EmaManu]

Dear Paul
I agree with you about the eccessive slowness and inertia of my actual simulation, next days I will apply both your stategies: at first interdymfoam with dinamicmeshdict.rigidbodymotion so that the 15-20 sec ramp can work, and dampers lowered: I am thinking to mantain the same ratio but with lower vaues, for istance 6e5 the linear and 4.67e6 the rotational one
...then if it doesn't work i will apply your second way: interfoam LTS and then interdymfoam without dampers
About the graphs of course i can share my scripts! Ihave uploaded the ones for the sinkage and trim (cosine(trim))
But I don't think that is very smart my path: they are java scripts that extract data from the file log.interdymfoam wrtiting in a new text file two columns: one for the time and one for the quantity of interest (you can download eclipse java and run them)
After that I import these files in excel and do the graphs
..it's really not the smartest way I think...

[Fauster]

Originally Posted by EmaManu

Dear Paul
I agree with you about the eccessive slowness and inertia of my actual simulation, next days I will apply both your stategies: at first interdymfoam with dinamicmeshdict.rigidbodymotion so that the 15-20 sec ramp can work, and dampers lowered: I am thinking to mantain the same ratio but with lower vaues, for istance 6e5 the linear and 4.67e6 the rotational one
...then if it doesn't work i will apply your second way: interfoam LTS and then interdymfoam without dampers
About the graphs of course i can share my scripts! Ihave uploaded the ones for the sinkage and trim (cosine(trim))
But I don't think that is very smart my path: they are java scripts that extract data from the file log.interdymfoam wrtiting in a new text file two columns: one for the time and one for the quantity of interest (you can download eclipse java and run them)
After that I import these files in excel and do the graphs
..it's really not the smartest way I think...

Awesome ! Thanks a lot . Let me know if your simulation run fine.

[EmaManu]

Dear Paul I manage to get a stabilized simulation with the 0.LTS initial conditions, 5 sec quadratic ramp and zero dampers and 15 m/s speed of the boat
Thanks again, have a nice day!
Emanuele

[shogan50]

EmaManu,
Would you mind sharing your case files where you initialize with LTS interfoam? I have a similar problem with a different hull and only 6.5 m/s.

[Fauster]

See the tutorial dtcHull in interfoam. You Will find the setup. It s realy basic. Set localEuler as scheme for d/dt. Copy also fvsheme and fvsolution

[EmaManu]

Dear shogan50,

here the files, related to the first simulation with interFoam, with the boat at 24 knots

Best regards,

Emanuele