[ccmccomb]

I’m trying to model the motion of wave energy converters in openFOAM 1.6-ext using sixDoFRigidBodyDisplacement and the interDyMFoam solver. Starting with the floatingObject tutorial, I modified the mesh to be 40 cells in every direction, and the floating body to be 0.3x0.3x0.1 and the size of the domain is 1.0x1.0x1.0.

Initially, I attempted to model a neutrally buoyant body underwater with no initial motion or external forces. The free surface was defined to be at z = 0 in the coordinate system. The mass of the body was defined so that the density would be equivalent to water. The simulation runs as expected for the first 0.4 seconds (i.e. the body has no motion, the velocity in the domain is approx. 0 everywhere, etc…) The simulation crashed at approximately 0.86 seconds. Shortly prior to this time, the values for pd started growing very large and oscillating, eventually causing the body to become unstable. I’ve attached the case neutralFloatingObject.tar.

Similarly, I attempted to model a floating body half submerged at the water’s surface. The mass of the body was defined so that the density would be half that of water. The simulation crashed at about 0.25 seconds when the body begins to oscillate even though there is no force applied (e.g. waves or a body force) to cause the motion. This case is attached under surfaceFloatingObject.tar. To run cases, just open and run interDyMFoam – mesh is already set up.

Does anyone have any suggestions on how to fix these problems? Any help would be much appreciated. Thanks in advance,

-Chris

[Sagun]

Hello Chris,

I am also facing similar problems with floating objects and interDyMFoam. Were you able to figure out what went wrong?

Thanks,
Sagun

[neiht]

try to change bc movingWallVelocity into fixedValue. manual update the value or restraint object. If it works tell me

[haze_1986]

Originally Posted by neiht

try to change bc movingWallVelocity into fixedValue. manual update the value or restraint object. If it works tell me

changing from movingWallVelocity into fixedValue makes it crash earlier. Just tried for both 2.1.1 and 2.3.0.

Restraining the object helps, if the crash comes after several seconds, when the object floats too far away causing the mesh to be over skewed. It wouldn't fix the issue caused encountered by many in this forum, which is the crash that comes almost instantly, 0.05 - 0.1s when the geometry is changed to be different from the tutorial floatingObject.

[AR91]

Hello,
Thank you for having this discussion on the thread. I have a query about the floatingObject case.

I have changed the floatingObject tutorial from 3D to 2D, and moved the box up to mid tank height such that its weight is equal to the weight of fluid displaced (checked fluid density and made sure the cells are clearly demarcated between the fluids and the box). There are no restraints or constraints on the box and the water level is still and flat so there's so movement in the fluid.

1. Is it impossible to get a hydrostatically stable box in interDyMFoam? Because the box topples over eventually (after 3 seconds of computations) no matter what.
2. Is the use of accelerationRelxation unavoidable? The relaxation factor value in the tutorial is 0.7 and if I take this out (defaulting its value to 1), then the computations crash immediately within the first few time steps.

I have been stuck with trying to get hydrostatic stability for several weeks and progress on my project is severely affected. Any assistance would be truly appreciated.
Regards,
Arun.