SimScale CAE Forum

Multiphase 6DOF simulation set up problem

Hi,

I am having trouble running a simulation on a project of mine which is linked below. This error keeps throwing up " The job execution was aborted, possibly due to a numerical instability. Review the log to identify reasons: nonphysically large field values, extremely small time step size, etc. Modifying numerical settings and time step size could resolve the issue." I played around with the time steps as mentioned but without luck. Cant seem to figure out the issue. Btw i used the project free_floating_boat_in_a_wave_tank as reference and used almost the same simulation conditions. Help would be appreciated.

Project link - https://www.simscale.com/projects/cecil_pabbathi/case1a12/

Thanks and regards,

Cecil

Hi there @cecil_pabbathi!

The first thing I saw was that your bounding box is way too small so I would suggest to increase it in the first step. For the numerical settings as well as for the boundary conditions you can have a look at this project: Floating Boat 6-DOF

Please let me know how things go and make sure to tag Darren @1318980, @Get_Barried or me in case you need help!

Best,

Jousef

Hi @jousefm ,

I did increase the size of my domain but I am still facing the same error. I used the same boundary conditions and solver settings from the referred project. Please do have a look at my case. Looking forward to your reply!

Hi @cecil_pabbathi,

While I’m rather unfamiliar with multi-phase flow simulations I can comment a little on your meshing. You might want to perform an additional surface refinement on your buoy model and propeller to give it better definition since its moderately complex geometry. You also might want to increase the size of both your Cartesian boxes as follows.

  • For “thrsterrefine” increase the box forward to give some time for flow to develop and have better accuracy due to your Cartesian refinement
  • For “waverefine” keep the box within your bounding box parameters and ensure it covers the entire model meaning you need to adjust your Z values to do that while reducing your Y-X values to fit within the bounding box so as to optimize and reduce any unforeseen problems

Your mesh quality check is also not ideal with a couple of illegal triangles so we’ll try to fix that first.

Hope this helps!

Regards,
Barry

Hi Barry.

Thanks for replying! I am pretty sure the error is due to some misstep in the solver settings rather than the mesh. I know my mesh is a bit coarse, however I did make the refinements you proposed and I still get the same error. I am at a loss on how to proceed. Waiting to hear your thoughts.

Regards,

Cecil

Hi @cecil_pabbathi,

Hmm yeah it does look better now. I’m not experienced with multi-phased flow unfortunately and am unsure what is the issue so hopefully @jousefm or @PowerUsers_CFD may be able to help you!

Regards,
Barry

@cecil_pabbathi, just a quick question regarding this.

The bounding box length is just short of 1000m, should you not scale this?

Cheers,
Darren

Hi Darren,

Ofcourse ! Just wanted to get a working simulation first and then definitely scale down the model as I have multiple cases to test in the future! Any ideas as to what the error might be?

Cheers,

Cecil

Hi @cecil_pabbathi

Within the 6DoF setup part, there are parameters for mesh movement, these are in meters. With the size of this geometry, I would imagine the current settings would not be sufficient, increasing them (or decreasing your geometry) might get you unstuck. Certainly worth trying, I did a quick test last night to ensure it was the 6DoF bit that was causing the issue and it was. So the problem lies somewhere in these few parameters.

Kind regards,
Darren

Hi Darren,

Appreciate your reply! I figured out as much as well. When I ran the simulation using linear body motion, the simulation ran without any issues. So coming to the 6DOF part, I used these formulae to calculate the moment of inertia, just want to make sure I am doing them right. Since my mass is only 5 kgs, I get extremely small values of moments as well. I ll take your advice and play around with my mesh movement parameters and see if it makes a difference. Will keep you posted.

Ix=0.5massy^2

Iy=0.5massx^2

Iz=(mass/12)*(x^2+y^2), where x and y are the respective x and y dimensions of the body…

Regards,

Cecil