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-   -   Could someone give me a hand about floating exception in floating object case? (https://www.cfd-online.com/Forums/openfoam-solving/114754-could-someone-give-me-hand-about-floating-exception-floating-object-case.html)

 shchao March 17, 2013 05:11

Could someone give me a hand about floating exception in floating object case?

Dear Foamers:
i just change the box weight and flume size in the floatingobject case in tutorial but the floating exception problem happens.all changes i've made is the box weight changed from 9.6 to 5.4 and the moment of inertia changed as follow to ensure the rho of box is half of water and it could floating steadily in the flume:
floatingobject
{
type sixDoFRigidBodyDisplacement;
centreOfMass (10.15 0.5 0.5);
momentOfInertia (0.04698 0.04698 0.081);
mass 5.4;
rhoInf 1; // needed only for solvers solving for kinematic pressure
report on;
value uniform (0 0 0);
}
and the size of box is set default(0.3m*0.3m*0.12m,represents length,width and heigth) ,the initial postion of box is from (10,0.35,0.44) to (10.3,0.65,0.56) in toposetdict and the flume size is 20m*1m*1m with free surface setted at height = 0.5m,it is a still water case without flow or wave.While calculating, one step of outputted information is as follows:

Interface Courant Number mean: 0.000721590695662 max: 0.301937993368
Courant Number mean: 0.00818754720977 max: 0.94912951812
deltaT = 1.14527594706e-05
Time = 0.104541
Centre of mass: (10.1499926389 0.499990272469 0.309453775244)
Linear velocity: (-0.0111517260846 -0.00058010598683 -70.4234893345)
Angular velocity: (-0.018433659261 0.0332520957249 0.000499827186231)
GAMG: Solving for cellDisplacementx, Initial residual = 0.0130422365202, Final residual = 6.61657788476e-06, No Iterations 4
GAMG: Solving for cellDisplacementy, Initial residual = 0.00508763930987, Final residual = 9.43010228328e-06, No Iterations 3
GAMG: Solving for cellDisplacementz, Initial residual = 0.00470125419757, Final residual = 6.90773821862e-06, No Iterations 3
Execution time for mesh.update() = 6 s
time step continuity errors : sum local = 0.000284692001246, global = 6.98727910686e-12, cumulative = -4.25377217883e-06
GAMGPCG: Solving for pcorr, Initial residual = 1, Final residual = 3.54688252264e-06, No Iterations 5
time step continuity errors : sum local = 0.000168330725798, global = 3.06371892438e-10, cumulative = -4.25346580693e-06
MULES: Solving for alpha1
Phase-1 volume fraction = 0.625041930789 Min(alpha1) = -1.82735702121e-20 Max(alpha1) = 1.0000000594
MULES: Solving for alpha1
Phase-1 volume fraction = 0.625041851886 Min(alpha1) = -1.13702270128e-20 Max(alpha1) = 1.0000000591
MULES: Solving for alpha1
Phase-1 volume fraction = 0.625041851886 Min(alpha1) = -2.17004667899e-19 Max(alpha1) = 1.0000000588
GAMG: Solving for p_rgh, Initial residual = 0.000242043912232, Final residual = 1.24916860273e-06, No Iterations 6
time step continuity errors : sum local = 2.2856519015e-07, global = 1.22579261119e-07, cumulative = -4.13088654581e-06
GAMGPCG: Solving for p_rgh, Initial residual = 0.000193291568891, Final residual = 1.34761526591e-09, No Iterations 5
time step continuity errors : sum local = 3.59992746348e-10, global = 1.52468105825e-11, cumulative = -4.130871299e-06
smoothSolver: Solving for epsilon, Initial residual = 0.036117382078, Final residual = 4.06277782323e-07, No Iterations 10
smoothSolver: Solving for k, Initial residual = 0.0562500613244, Final residual = 5.00328374544e-07, No Iterations 12
ExecutionTime = 689.38 s ClockTime = 695 s

I am confused about why the value of linear velocity of my box is such high?The rho of it is 500kg/m3 clearly.:confused:

 wyldckat March 18, 2013 18:47

Greetings shchao,

I'm sorry to say that the "floatingObject" tutorial is one of the hardest tutorials I know of. If you search this forum for the name of this tutorial, you'll find a few threads discussing some solutions, but there is no clear solution for all test cases.

One such attempt to figure out what was wrong is reported here: http://www.openfoam.org/mantisbt/view.php?id=417
In essence: any small change in any direction, can lead to a crash. Some possible solutions are given on the first (and only comment) on that bug report.

Good luck!
Bruno

 shchao March 19, 2013 00:21