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-   -   Stability Issue with interDyMFoam / floatingObject tutorial (http://www.cfd-online.com/Forums/openfoam-solving/90404-stability-issue-interdymfoam-floatingobject-tutorial.html)

 ccmccomb July 8, 2011 17:51

Stability Issue with interDyMFoam / floatingObject tutorial

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 December 10, 2012 12:06

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 May 21, 2013 12:22

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

 haze_1986 July 3, 2014 12:50

Quote:
 Originally Posted by neiht (Post 428944) 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.

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