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October 13, 2019, 20:39 
Inertia in rigidBodyMotion

#1 
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Morteza
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Hello Foamers,
I have a question about the inertia entry in dynamicMeshDict for a rigidBodyMotion solver: type rigidBody; parent root; // Cuboid mass  very simple setup. For more complicated shapes // see utility surfaceInertia mass 0.0005; rho 0.1; inertia (0.0002 0 0 0.01 0 0.01); centreOfMass (0.11 0.1 0.008); How can I calculate inertia for a complex geometry? The surfaceInertial tool does not give such an information. Thanks. 

October 14, 2019, 16:30 

#2 
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Morteza
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The answer to my question:
The surfaceInertia tool gives the following matrix as "Inertia tensor around centre of mass": (Ixx Ixy Ixz Iyx Iyy Iyz Izx Izy Izz) The entry "inertia" in rigidBodyMotion dynamicMeshDict requires six elements of the aforementioned matrix: (Ixx Ixy Ixz Iyy Iyz Izz) The surfaceInertia tool works like: surfaceInertia surface.stl In order to extract a STL file out of a patch existing in your grid you may use: surfaceMeshTriangulate patches '(patchName)' output.stl 

March 4, 2021, 18:02 

#3 
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Deutschland
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Hey,
I am simulating a ship motion using dynamicMEsh and I tried surfaceInertia and following result mass 0.208208201605; centreOfMass (2.63037231059 1.17234355678e05 0.0244737162072); momentOfInertia (0.00199990142845 0.225304731178 0.2257076848); But I am given a mass of 77 kg I another thread posted in cfd forum it was said the moment of inertia is mass moment of inertia. Id that so? If yes, is it possible to find it from the momentOfInertia (0.00199990142845 0.225304731178 0.2257076848); or using surfaceInertia thanks in advance 

March 4, 2021, 18:11 

#4 
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Morteza
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Hello,
You should give the density value when you run surface inertia. For example: surfaceInertia density 5 file.stl This way, you will get the correct mass, if your density*volume (of the STL file) results in 77. 

March 13, 2021, 12:28 
Mass property

#5 
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Deutschland
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Hey,
THANK YOU FOR THE REPLY I am trying to simulate a ship flow. I have few questions regarding dynamic mesh. Hope someone can help. 1. in DTCHull case entire ship is considered in the snappyHexDict even though one half is simulated. Does this not affect the parameters in the entire case setup? So is it fine if I use my entire geometry rather than cutting the geometry into half and then using it? If I use the entire geometry wall all think should I pay attention to? 2. why is the rhoInf in the dynamic mesh 1. should it not be the density of the ship (mass/volume). I have the weight for my geometry (77 kg) and I got the volume using the Solidwork. and used to find density for my case. 4. The moment of inerta. there are 3 moment of inertia a. Principal axes of inertia and principal moments of inertia: ( kilograms * square meters ) Taken at the center of mass. b.Moments of inertia: ( kilograms * square meters )Taken at the center of mass and aligned with the output coordinate system c. Moments of inertia: ( kilograms * square meters ) Taken at the output coordinate system. My guess for the moment of inertia was about the center of mass since that is what asked in the dynamic mesh. Bot I try to clarify it using Solidwork, I am not getting the same moment of inertia nor mass using the rhoInf as 1 so I tried to find the density using the mass in the dynamicMeah and the volume I got from the software. But I still don't have answers Please please help. I am lost and out of ideas Kind regards vava10 

March 16, 2021, 04:30 

#6 
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1) snappyHexMesh uses the stl file of the ship and cuts this out of the mesh block. Since the mesh block covers only half the rest isn't used. You could use half, but it is easier to use the full ship, since this makes it easier to detect edges in the meshing run. E.g if your half ship stl file has one point at x=1e15 and your mesh goes to x=0 you might get artifacts during meshing.
2) rhoInf is there because you are using basically two solvers. One for the rigid body and one for the flow simulation. Coupled together on a patch. Incompressible openfoam solvers do not use a density. They instead solve for p/rho. Hence you need to add rhoInf so that the rigid body solver can calculate the fluid forces correctly. rhoInf is hence the density of the fluid. I haven't looked in the code, but this is probably not read for solvers which use a density field. Like interFoam etc. 3) no 3 question asked 4) There is one moment of inertia. You can however transform it into a different coordinate system. Which basically means rotating your part until x y z align with principle axis. You obviously need to calculate it here using the coordinate system you are calculating in. And around the centre of mass. surfaceInertia density 1 file.stl calculates the moment of inertia of the stl file around it's centre of mass with density 1. surfaceInertia density 1 referencePoint '(0 0 1)' file.stl calculates the moment of inertia of the stl file around this point 

March 16, 2021, 04:52 

#7 
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I should also add, that the inertia of the stl file is not necessarily the one used in the calculation. This would mean that the ship is of uniform density. But in reality the engine/cargo etc are heavier. Hence you might not get the exact value just from the stl file since this is only used for meshing.


March 16, 2021, 05:32 
inertia

#8 
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Thank you so much for the reply I have been stuck soooooo hard
I was finding the moment of inertia of the ship from the stl (MeshLab, Solidworks and surfaceInertia) and using it for the calculation. My kayak was sinking what I have is a kayak and it does not have any additions cargo or engines. and I have 77 kg as the weight of the kayak. I obtained the volume of the mesh using MeshLAb and used this volume and given mass to find the density. This density was used for the calculation of the moment of inertia If the inertia from the STL file is not used for the calculation, that probably can be the reason for sinking? How exactly should I proceed for finding the moment of inertia for the calculation purpose? Should I just use the moment of inertia for the bounding box? if so I need mass for that. should I use 77kg for the mass?? Again thank you 

March 16, 2021, 08:56 

#9 
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Just use the inertia of your cad program or the openfoam surfaceInertia as demonstrated above.
And no the inertia is just a measure for how difficult it is to change a systems velocity. It has however no influence whatsover on a boat sinking. (well it makes it easier for the boat to roll over and fill up) It will sink if the integral of the pressure around the boat (it's lift force) is lower than it's weight force (mass*gravity). The mass of your rigidBody is specified in the dynamicmeshdict and the lift force can be calculated using the forces function object. Or you can simply calculate how much water it displaces. Since this is what it boils down to and should equal. 

March 16, 2021, 09:51 
Mass property

#10 
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Thank you for the help
1.So Since the mesh block covers only half the rest isn't used, does that mean the mass properties of the complete stl obtained using surfaceInertia density <scalar> should be divided by 2 to use it in the dynamicMeshDict? P:S I am obtaining density by dividing given mass/volume obtained using MeshLab 2.Incompressible openfoam solvers do not use a density. They instead solve for p/rho. Hence you need to add rhoInf so that the rigid body solver can calculate the fluid forces correctly. rhoInf is hence the density of the fluid.. In the controlDict for finding the forces i am using rhoInf of 1000. But the rhoInf in dynamicMEsh seems different because I checked the DTCHull case setup provided by openFoam. In dynamicMeshDict rhoInf is 1 Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: v2012   \\ / A nd  Website: www.openfoam.com   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; object dynamicMeshDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dynamicFvMesh dynamicMotionSolverFvMesh; motionSolverLibs (sixDoFRigidBodyMotion); motionSolver sixDoFRigidBodyMotion; patches (hull); innerDistance 0.3; outerDistance 1; centreOfMass (2.929541 0 0.2); mass 412.73; momentOfInertia (40 921 921); rhoInf 1; report on; value uniform (0 0 0); accelerationRelaxation 0.4; solver { type Newmark; } constraints { zAxis { sixDoFRigidBodyMotionConstraint line; direction (0 0 1); } yPlane { sixDoFRigidBodyMotionConstraint axis; axis (0 1 0); } } restraints { translationDamper { sixDoFRigidBodyMotionRestraint linearDamper; coeff 8596; } rotationDamper { sixDoFRigidBodyMotionRestraint sphericalAngularDamper; coeff 11586; } } // ************************************************************************* // 

March 16, 2021, 10:07 

#11 
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The rigid motion solver basically solves F=m*a. With m the mass inside your dynamicMeshDict. a the acceleration you want to solve for. The same applies for it's velocity v and the distance s. Forces can be either from constraints/restraints in your dynamicMeshDict or fluid forces from the flow calculation. In other words it does nothing else than a forces function object. Or even more precise it even uses the forces function object library internally to calculate the force. rhoInf is hence the same one used in the actual function object. And this is only read if you are using an incompressible solver without a density field (e.g simpleFoam pisoFoam pimpleFoam icoFoam ). If you are running interFoam this entry is not used but rather the solvers density field. It calculates the resulting force vector and point of attack (the moment) and does so on the specified patch.
This is the implementation Code:
dictionary forcesDict; forcesDict.add("type", functionObjects::forces::typeName); forcesDict.add("patches", patches_); forcesDict.add("rhoInf", rhoInf_); forcesDict.add("rho", rhoName_); forcesDict.add("CofR", motion_.centreOfRotation()); functionObjects::forces f("forces", db(), forcesDict); f.calcForcesMoment(); motion_.update ( firstIter, ramp*(f.forceEff() + motion_.mass()*g.value()), ramp *( f.momentEff() + motion_.mass()*(motion_.momentArm() ^ g.value()) ), t.deltaTValue(), t.deltaT0Value() ); Hence symmetry is not baked into the rigid motion solvers. You need to half the weight. 

March 16, 2021, 10:23 

#12 
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I am using interFoam solver with sixDoFRigidBodyMotion so rhoInf 1000?
(I am sorry if the questions are stupid but I am completely new to CFD. I appreciate your help GREATLY) And I also have a point mass at the centre of gravity representing the passenger weight. How will this include into dynamic mesh? 77 is the combined weight of boat and passenger. I was using this value for the calculation and this could be the reason why it was sinking 

March 16, 2021, 10:36 

#13 
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Start reading my posts above. InterFOAM does not care about rhoInf. InterFoam uses an internal density field. Hence pressure is in Pa not divided by density. You can enter any value you want. 1, 1000 100000. It does not matter.
And your second question isn't even worth answering since you clearly do not even try. You are using a single rigid body. It hence has ONE mass. Hence mass in your dynamicMeshDict is 77. But you need to divide it by half if you are using a symmetryPlane. And if you have a passenger you clearly need to add that to the moment of inertia. 

March 16, 2021, 10:57 

#14  
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Quote:


March 12, 2022, 15:47 

#15  
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Tony Zhang
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Quote:
I just came across the same problem. I am simulating a ship in calm water condition using OpenFOAMv7. I have successfully simulated the hull in fixed condition and now want to make hull free to heave and pitch. I have changed the dynamicMeshDict and also the centre of rotation located in controlDict file. When I enter interFoam, it seems ok from the beginning but then the time step drops to a very small value, and linear & angular velocity became very big, then the error is reported. Code:
/**\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ Build : 7 Exec : interFoam Date : Mar 12 2022 Time : 20:22:11 Host : "Tony" PID : 29051 I/O : uncollated Case : /home/OpenFOAM/tony7/run/sotonsnappDone nProcs : 1 sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE). fileModificationChecking : Monitoring runtime modified files using timeStampMaster (fileModificationSkew 10) allowSystemOperations : Allowing usersupplied system call operations // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Create time Create mesh for time = 0 Selecting dynamicFvMesh dynamicMotionSolverFvMesh Selecting motion solver: rigidBodyMotion Selecting rigidBodySolver Newmark PIMPLE: No convergence criteria found PIMPLE: No corrector convergence criteria found Calculations will do 3 corrections PIMPLE: Operating solver in transient mode with 3 outer correctors Reading field p_rgh Reading field U Reading/calculating face flux field phi Reading transportProperties Selecting incompressible transport model Newtonian Selecting incompressible transport model Newtonian Selecting turbulence model type RAS Selecting RAS turbulence model kOmegaSST Selecting patchDistMethod meshWave RAS { RASModel kOmegaSST; turbulence on; printCoeffs on; alphaK1 0.85; alphaK2 1; alphaOmega1 0.5; alphaOmega2 0.856; gamma1 0.555556; gamma2 0.44; beta1 0.075; beta2 0.0828; betaStar 0.09; a1 0.31; b1 1; c1 10; F3 false; } Reading g Reading hRef Calculating field g.h No MRF models present No finite volume options present GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000693716, No Iterations 14 time step continuity errors : sum local = 6.13717e11, global = 1.60616e11, cumulative = 1.60616e11 Constructing face velocity Uf Courant Number mean: 0.000801996 max: 0.151765 forces totForce: Not including porosity effects Starting time loop Courant Number mean: 0.000801996 max: 0.151765 Interface Courant Number mean: 0 max: 0 deltaT = 0.000119999 Time = 0.000119999 PIMPLE: Iteration 1 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.84309e07 0.170527) Orientation: (1 0 1.50333e06 0 1 0 1.50333e06 0 1) Linear velocity: (0 0 0.0468714) Angular velocity: (0 0.0250556 0) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000752343, No Iterations 12 time step continuity errors : sum local = 8.96779e11, global = 2.75192e11, cumulative = 4.35808e11 smoothSolver: Solving for alpha.water, Initial residual = 9.59931e08, Final residual = 7.56999e12, No Iterations 1 Phase1 volume fraction = 0.536621 Min(alpha.water) = 0 Max(alpha.water) = 1 MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.536621 Min(alpha.water) = 2.60186e10 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 1, Final residual = 4.30904e08, No Iterations 16 time step continuity errors : sum local = 9.00489e12, global = 5.89621e12, cumulative = 3.76846e11 PIMPLE: Iteration 2 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.84309e07 0.170258) Orientation: (1 0 0.000142199 0 1 0 0.000142199 0 1) Linear velocity: (0 0 4.43959) Angular velocity: (0 2.37 0) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000771057, No Iterations 12 time step continuity errors : sum local = 1.17222e09, global = 8.5936e10, cumulative = 8.97045e10 smoothSolver: Solving for alpha.water, Initial residual = 3.61728e06, Final residual = 1.1771e11, No Iterations 4 Phase1 volume fraction = 0.536621 Min(alpha.water) = 1.78302e34 Max(alpha.water) = 1.00001 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.536621 Min(alpha.water) = 6.09734e08 Max(alpha.water) = 1.00001 GAMG: Solving for p_rgh, Initial residual = 0.00425161, Final residual = 4.07793e08, No Iterations 18 time step continuity errors : sum local = 2.38151e11, global = 1.71943e11, cumulative = 9.14239e10 PIMPLE: Iteration 3 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.84309e07 0.177328) Orientation: (0.999993 0 0.0036235 0 1 0 0.0036235 0 0.999993) Linear velocity: (5.55112e17 0 113.406) Angular velocity: (0 60.3923 0) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000625751, No Iterations 12 time step continuity errors : sum local = 2.49812e08, global = 1.83191e08, cumulative = 1.74049e08 smoothSolver: Solving for alpha.water, Initial residual = 9.46965e05, Final residual = 9.63334e11, No Iterations 8 Phase1 volume fraction = 0.536621 Min(alpha.water) = 3.11261e43 Max(alpha.water) = 1.00002 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.536621 Min(alpha.water) = 0.000972877 Max(alpha.water) = 1.00002 GAMG: Solving for p_rgh, Initial residual = 0.00824394, Final residual = 4.5727e08, No Iterations 19 time step continuity errors : sum local = 3.4876e10, global = 2.68154e10, cumulative = 1.71367e08 smoothSolver: Solving for omega, Initial residual = 0.000320324, Final residual = 4.87026e08, No Iterations 7 smoothSolver: Solving for k, Initial residual = 1, Final residual = 4.00958e08, No Iterations 12 ExecutionTime = 97.03 s ClockTime = 97 s forces totForce write: sum of forces: pressure : (2.24726e+07 64526.1 2.77865e+08) viscous : (4.05731 0.459928 83.9502) porous : (0 0 0) sum of moments: pressure : (27959.1 1.82698e+08 184772) viscous : (0.210328 23.3017 0.395045) porous : (0 0 0) Courant Number mean: 0.00589213 max: 163.91 Interface Courant Number mean: 0.000496804 max: 96.5416 deltaT = 1.83026e05 Time = 0.000138302 PIMPLE: Iteration 1 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.84309e07 0.171931) Orientation: (1 0 0.000746945 0 1 0 0.000746945 0 1) Linear velocity: (0 0 703.147) Angular velocity: (0 374.727 0) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000949863, No Iterations 12 time step continuity errors : sum local = 3.45495e08, global = 2.62545e08, cumulative = 9.11781e09 smoothSolver: Solving for alpha.water, Initial residual = 7.18828e05, Final residual = 2.83216e11, No Iterations 13 Phase1 volume fraction = 0.536621 Min(alpha.water) = 1.27194e18 Max(alpha.water) = 1.00002 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.536621 Min(alpha.water) = 7.54874e14 Max(alpha.water) = 1.00002 GAMG: Solving for p_rgh, Initial residual = 0.00601612, Final residual = 2.78072e08, No Iterations 20 time step continuity errors : sum local = 2.05845e10, global = 1.51615e10, cumulative = 9.26942e09 PIMPLE: Iteration 2 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.84309e07 0.472629) Orientation: (0.98707 0 0.16029 0 1 0 0.16029 0 0.98707) Linear velocity: (0 0 32155.3) Angular velocity: (0 17135.1 0) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000999593, No Iterations 11 time step continuity errors : sum local = 1.67722e06, global = 1.34222e06, cumulative = 1.35149e06 smoothSolver: Solving for alpha.water, Initial residual = 0.00402176, Final residual = 6.77955e11, No Iterations 35 Phase1 volume fraction = 0.536621 Min(alpha.water) = 3.62896e06 Max(alpha.water) = 1.00009 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.536621 Min(alpha.water) = 3.79914 Max(alpha.water) = 1.08871 #0 Foam::error::printStack(Foam::Ostream&) at ??:? #1 Foam::sigFpe::sigHandler(int) at ??:? #2 ? in "/lib/x86_64linuxgnu/libc.so.6" #3 Foam::GAMGSolver::scale(Foam::Field<double>&, Foam::Field<double>&, Foam::lduMatrix const&, Foam::FieldField<Foam::Field, double> const&, Foam::UPtrList<Foam::lduInterfaceField const> const&, Foam::Field<double> const&, unsigned char) const at ??:? #4 Foam::GAMGSolver::Vcycle(Foam::PtrList<Foam::lduMatrix::smoother> const&, Foam::Field<double>&, Foam::Field<double> const&, Foam::Field<double>&, Foam::Field<double>&, Foam::Field<double>&, Foam::Field<double>&, Foam::Field<double>&, Foam::PtrList<Foam::Field<double> >&, Foam::PtrList<Foam::Field<double> >&, unsigned char) const at ??:? #5 Foam::GAMGSolver::solve(Foam::Field<double>&, Foam::Field<double> const&, unsigned char) const at ??:? #6 Foam::fvMatrix<double>::solveSegregated(Foam::dictionary const&) at ??:? #7 Foam::fvMatrix<double>::solve(Foam::dictionary const&) in "/home/OpenFOAM/OpenFOAM7/platforms/linux64GccDPInt32Opt/bin/interFoam" #8 Foam::fvMatrix<double>::solve() in "/home/OpenFOAM/OpenFOAM7/platforms/linux64GccDPInt32Opt/bin/interFoam" #9 ? in "/home/OpenFOAM/OpenFOAM7/platforms/linux64GccDPInt32Opt/bin/interFoam" #10 __libc_start_main in "/lib/x86_64linuxgnu/libc.so.6" #11 ? in "/home/OpenFOAM/OpenFOAM7/platforms/linux64GccDPInt32Opt/bin/interFoam" Floating point exception (core dumped) Code:
/** C++ **\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ FoamFile { version 2.0; format ascii; class dictionary; object dynamicMeshDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dynamicFvMesh dynamicMotionSolverFvMesh; motionSolverLibs ("librigidBodyMeshMotion.so"); motionSolver rigidBodyMotion; report on; solver { type Newmark; } accelerationRelaxation 0.4; bodies { hull { type rigidBody; parent root; centreOfMass (1.86617058305 4.84309040657e07 0.170524016331); mass 229.5; inertia (0.0122617483413 3.05290279692e07 0.00566631344423 0.430050980166 3.19195024184e08 0.435116910148); transform (1 0 0 0 1 0 0 0 1) (1.86617058305 4.84309040657e07 0.170524016331); joint { type composite; joints ( { type Pz; } { type Ry; } ); } patches (hull); innerDistance 0.3; outerDistance 1; } } restraints { translationDamper { type linearDamper; body hull; coeff 8596; } rotationDamper { type sphericalAngularDamper; body hull; coeff 11586; } } // ************************************************************************* // Code:
/**\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ Build : 7 Exec : surfaceInertia /home/OpenFOAM/Tony7/run/sotonsnappDone/constant/triSurface/KCS_SOTON_ModelScale.stl Date : Mar 12 2022 Time : 20:03:08 Host : "Tony" PID : 27323 I/O : uncollated Case : /home/ nProcs : 1 sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE). fileModificationChecking : Monitoring runtime modified files using timeStampMaster (fileModificationSkew 10) allowSystemOperations : Allowing usersupplied system call operations // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // > FOAM Warning : From function int main(int, char**) in file surfaceInertia.C at line 107 Negative mass detected, the surface may be insideout. > FOAM Warning : From function int main(int, char**) in file surfaceInertia.C at line 324 Nonunique eigenvectors, cannot compute transformation from Cartesian axes Density: 1 Mass: 0.4534574004 Centre of mass: (1.86617058305 4.84309040657e07 0.170524016331) Surface area: 5.76446631858 Inertia tensor around centre of mass: (0.0122617483413 3.05290279692e07 0.00566631344423 3.05290279692e07 0.430050980166 3.19195024184e08 0.00566631344423 3.19195024184e08 0.435116910148) eigenValues (principal moments): (0.435192825846 0.430050980166 0.0121858326428) eigenVectors (principal axes): (0.0133965207656 5.41183306631e06 0.999910262575) (8.03987066635e07 0.999999999985 5.40154715773e06) (0.999910262589 7.31552980241e07 0.0133965207697) Writing scaled principal axes at centre of mass of "/home/OpenFOAM/Tony7/run/sotonsnappDone/constant/triSurface/KCS_SOTON_ModelScale.stl" to "axes.obj" End Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.3.0   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; location "system"; object controlDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // application interFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 50; deltaT 0.0001; writeControl adjustableRunTime; writeInterval 5; purgeWrite 0; writeFormat binary; writePrecision 6; writeCompression off; timeFormat general; timePrecision 6; runTimeModifiable yes; adjustTimeStep yes; maxCo 25; maxAlphaCo 15; maxDeltaT 0.01; functions { ///ALLFORCE_SCOMM totForce { type forces; functionObjectLibs ( "libforces.so" ); patches (hull); rhoInf 999.10260000000000000000; rho rho; UName U; log on; writeControl timeStep; writeInterval 1; CofR (1.86617058305 4.84309040657e07 0.170524016331); } } Many thanks, Tony 

March 14, 2022, 11:17 

#16 
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Your surfaceInertia results in a negative value, because your stl is inside out. I.e you need to flip the normals of the stl file. For example:
Code:
surfaceOrient input.stl '(10 10 10)' output.stl From the limited info I have another real problem is this Code:
maxCo 25; maxAlphaCo 15; Code:
....... Courant Number mean: 0.000801996 max: 0.151765 Courant Number mean: 0.000801996 max: 0.151765 Interface Courant Number mean: 0 max: 0 deltaT = 0.000119999 Time = 0.000119999 PIMPLE: Iteration 1 Centre of rotation: (1.86617 4.84309e07 0.170527) Orientation: (1 0 1.50333e06 0 1 0 1.50333e06 0 1) Linear velocity: (0 0 0.0468714) Angular velocity: (0 0.0250556 0) Centre of rotation: (1.86617 4.84309e07 0.170258) Orientation: (1 0 0.000142199 0 1 0 0.000142199 0 1) Linear velocity: (0 0 4.43959) Angular velocity: (0 2.37 0) Centre of rotation: (1.86617 4.84309e07 0.177328) Orientation: (0.999993 0 0.0036235 0 1 0 0.0036235 0 0.999993) Linear velocity: (5.55112e17 0 113.406) Angular velocity: (0 60.3923 0) forces totForce write: sum of forces: pressure : (2.24726e+07 64526.1 2.77865e+08) viscous : (4.05731 0.459928 83.9502) porous : (0 0 0) sum of moments: pressure : (27959.1 1.82698e+08 184772) viscous : (0.210328 23.3017 0.395045) porous : (0 0 0) Courant Number mean: 0.00589213 max: 163.91 Interface Courant Number mean: 0.000496804 max: 96.5416 deltaT = 1.83026e05 Time = 0.000138302 1 more outer iterations needed: 3 outer loops are not even close to enough to converge the solution...as you can see your velocity is drastically increasing from outer loop to outer loop...hence increase those drastically and add a convergence criterion 2 Your initial rigid body definition is wrong: check the definition of your dampers, center of rotation and gravity/ the mass etc. Keep in mind the translation you are adding to those with the transform. Your ships weight is really 200 kg? 3 your flow solution is wrong: it could also be that your initial force on the hull is extremely inaccurate because your problem is ill initialized. I.e your first pressure and flow velocity solution is immensely inaccurate which results in a force on the boat which is wrong. Write out the first time step and look at the pressure and drag forces on the hull Also use the rigid body state function object instead of forces Code:
{ type rigidBodyState; libs ("librigidBodyState.so"); angleFormat degrees; } 

March 15, 2022, 09:57 

#17 
Member
Tony Zhang
Join Date: Nov 2019
Location: soton
Posts: 45
Rep Power: 5 
[QUOTE=Bloerb;824081]
Dear Bloerb, Many thanks for your detailed reply! 1) I used the meshLab to invert my STL file and now surfaceInertia gives me a positive value. Since this is a modelscale ship hull (Lpp=3.773m), the mass is 229.5kg. 2) In terms of the controlDict file, I use the tutorial of DTCHullMoving (https://develop.openfoam.com/Develop.../DTCHullMoving) as a base and then change the corresponding parameters. In this tutorial, the maxCo is 25 and maxAlphaCo is 15. But I have changed to 1 and 1 respectively. But I am a little bit confused about this tutorial, it has restraints coefficients in the dynamicDict file, is the DTCHull free to heave and pitch? 3)I have modified the dynamicMeshDict file and controlDict file according to your suggestions and also, add one more out iterations (from 3 to 4). The details of dynamicMeshDict file and controlDict are attached: Code:
/** C++ **\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ FoamFile { version 2.0; format ascii; class dictionary; object dynamicMeshDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dynamicFvMesh dynamicMotionSolverFvMesh; motionSolverLibs ("librigidBodyMeshMotion.so"); motionSolver rigidBodyMotion; report on; solver { type Newmark; } accelerationRelaxation 0.4; bodies { hull { type rigidBody; parent root; centreOfMass (1.86617058305 4.84309040657e07 0.170524016331); mass 229.5; inertia (6.20581170707 0.00015451091794 2.8677863327 217.653741789 1.61548268875e05 220.217667153); transform (0.999910262589 7.31552980241e07 0.0133965207697 8.03987066635e07 0.999999999985 5.40154715773e06 0.0133965207656 5.41183306631e06 0.999910262575) (1.86617058305 4.84309040657e07 0.170524016331); joint { type composite; joints ( { type Pz; } { type Ry; } ); } patches (hull); innerDistance 0.3; outerDistance 1; } } restraints { translationDamper { type linearDamper; body hull; coeff 7217; } rotationDamper { type sphericalAngularDamper; body hull; coeff 9727; } } // ************************************************************************* // Code:
/** C++ **\  =========    \\ / F ield  OpenFOAM: The Open Source CFD Toolbox   \\ / O peration  Version: 2.3.0   \\ / A nd  Web: www.OpenFOAM.org   \\/ M anipulation   \**/ FoamFile { version 2.0; format ascii; class dictionary; location "system"; object controlDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // application interFoam; startTime 0; startFrom startTime; stopAt endTime; endTime 30; deltaT 0.01; writeControl adjustableRunTime; writeInterval 1; purgeWrite 0; writeFormat binary; writePrecision 6; writeCompression uncompressed; timeFormat general; timePrecision 6; runTimeModifiable yes; adjustTimeStep yes; maxCo 1; maxAlphaCo 1; maxDeltaT 0.1; functions { totForce { type forces; functionObjectLibs ( "libforces.so" ); patches (hull); rhoInf 999.1; rho rho; UName U; log on; writeControl timeStep; writeInterval 1; CofR (1.86617058305 4.84309040657e07 0.170524016331); } } // ************************************************************************* // Code:
PIMPLE: Iteration 1 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (3.03881e+07 12276 2.26815e+09) Angular velocity: (799.724 9.94697e+08 5372.9) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.00099521, No Iterations 11 time step continuity errors : sum local = 4.88642e15, global = 8.62449e16, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 1.97572e12, Final residual = 3.06837e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.984401, Final residual = 4.9944e08, No Iterations 184 time step continuity errors : sum local = 4.04897e16, global = 4.39492e18, cumulative = 1.00854e08 PIMPLE: Iteration 2 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (9.72907e+07 39302.8 7.26173e+09) Angular velocity: (2791.19 3.47168e+09 18752.5) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000900381, No Iterations 11 time step continuity errors : sum local = 2.35324e15, global = 4.49763e16, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 1.42279e12, Final residual = 3.00484e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.604041, Final residual = 4.93915e08, No Iterations 108 time step continuity errors : sum local = 2.10258e15, global = 2.55039e17, cumulative = 1.00854e08 PIMPLE: Iteration 3 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (4.93233e+08 199253 3.68147e+10) Angular velocity: (13820.6 1.71901e+10 92853) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000842764, No Iterations 13 time step continuity errors : sum local = 2.97691e15, global = 2.53722e16, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 4.0255e12, Final residual = 2.98851e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.815495, Final residual = 4.87706e08, No Iterations 115 time step continuity errors : sum local = 3.47524e15, global = 4.23773e17, cumulative = 1.00854e08 PIMPLE: Iteration 4 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (4.16159e+08 168117 3.10619e+10) Angular velocity: (10656.3 1.32544e+10 71594.1) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000775915, No Iterations 11 time step continuity errors : sum local = 4.08494e15, global = 1.85092e15, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 5.98422e12, Final residual = 4.25305e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.854974, Final residual = 4.8514e08, No Iterations 107 time step continuity errors : sum local = 6.66147e15, global = 8.05259e17, cumulative = 1.00854e08 smoothSolver: Solving for omega, Initial residual = 1.57746e08, Final residual = 1.60001e11, No Iterations 1 smoothSolver: Solving for k, Initial residual = 4.55715e09, Final residual = 7.22209e13, No Iterations 1 ExecutionTime = 5819.41 s ClockTime = 5838 s forces totForce write: sum of forces: pressure : (2.77316e+33 1.35636e+33 1.28499e+34) viscous : (6.50696e+11 1.01531e+11 1.85448e+11) porous : (0 0 0) sum of moments: pressure : (1.94071e+32 1.41346e+34 1.34365e+33) viscous : (2.87414e+10 3.03959e+11 1.41163e+11) porous : (0 0 0) Courant Number mean: 7.31397e08 max: 2.0195 Interface Courant Number mean: 8.31808e11 max: 2.34747e05 deltaT = 7.22564e22 Time = 0.861924 PIMPLE: Iteration 1 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (7.71943e+07 31184.4 5.76175e+09) Angular velocity: (827.559 1.02932e+09 5559.92) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000902617, No Iterations 12 time step continuity errors : sum local = 2.92597e15, global = 8.46416e17, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 4.48082e12, Final residual = 3.09593e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50709e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.816907, Final residual = 4.85215e08, No Iterations 115 time step continuity errors : sum local = 4.28464e15, global = 5.24605e17, cumulative = 1.00854e08 PIMPLE: Iteration 2 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (1.69724e+08 68563.8 1.26681e+10) Angular velocity: (5025.58 6.25083e+09 33764.1) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000996818, No Iterations 7 time step continuity errors : sum local = 7.95507e15, global = 2.225e15, cumulative = 1.00854e08 smoothSolver: Solving for alpha.water, Initial residual = 1.49025e11, Final residual = 5.13629e17, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50708e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.50708e13 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.811872, Final residual = 4.90291e08, No Iterations 119 time step continuity errors : sum local = 1.41397e14, global = 1.73317e16, cumulative = 1.00854e08 PIMPLE: Iteration 3 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86048e07 0.170203) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (8.29019e+09 3.34901e+06 6.18776e+11) Angular velocity: (227874 2.8343e+11 1.53096e+06) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.00099994, No Iterations 69 time step continuity errors : sum local = 5.62137e07, global = 5.51393e09, cumulative = 4.57149e09 smoothSolver: Solving for alpha.water, Initial residual = 5.64671e07, Final residual = 2.0355e11, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.29505e13 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 6.84271e11 Max(alpha.water) = 1 GAMG: Solving for p_rgh, Initial residual = 0.999722, Final residual = 4.9867e08, No Iterations 339 time step continuity errors : sum local = 2.80599e11, global = 4.54197e15, cumulative = 4.57148e09 PIMPLE: Iteration 4 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86051e07 0.170202) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (1.93416e+13 7.8135e+09 1.44365e+15) Angular velocity: (4.69789e+08 5.84324e+14 3.15625e+09) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.000965759, No Iterations 24 time step continuity errors : sum local = 5.71193e07, global = 6.43151e09, cumulative = 1.86003e09 smoothSolver: Solving for alpha.water, Initial residual = 5.37648e07, Final residual = 2.25343e13, No Iterations 1 Phase1 volume fraction = 0.53662 Min(alpha.water) = 7.49442e13 Max(alpha.water) = 1.00001 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 0.53662 Min(alpha.water) = 2.57565e09 Max(alpha.water) = 1.00001 GAMG: Solving for p_rgh, Initial residual = 0.98905, Final residual = 4.95097e08, No Iterations 252 time step continuity errors : sum local = 9.00697e09, global = 1.07553e10, cumulative = 1.75248e09 smoothSolver: Solving for omega, Initial residual = 5.76433e06, Final residual = 2.7095e08, No Iterations 1 smoothSolver: Solving for k, Initial residual = 2.84242e06, Final residual = 4.47391e09, No Iterations 1 ExecutionTime = 5833.65 s ClockTime = 5852 s forces totForce write: sum of forces: pressure : (2.52119e+37 4.06381e+35 7.84299e+37) viscous : (6.91233e+14 1.18334e+13 4.0348e+15) porous : (0 0 0) sum of moments: pressure : (4.5691e+34 2.45591e+37 3.64627e+35) viscous : (8.40372e+12 2.20753e+15 1.81932e+13) porous : (0 0 0) Courant Number mean: 0.000253879 max: 210.617 Interface Courant Number mean: 3.62046e07 max: 0.0692958 deltaT = 3.43069e24 Time = 0.861924 PIMPLE: Iteration 1 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (1.86617 4.86051e07 0.170202) Orientation: (0.976646 1.94577e06 0.214856 8.03987e07 1 5.40155e06 0.214856 5.10266e06 0.976646) Linear velocity: (1.94766e+13 7.86804e+09 1.45373e+15) Angular velocity: (4.73072e+08 5.88408e+14 3.17831e+09) GAMG: Solving for pcorr, Initial residual = 1, Final residual = 0.00099803, No Iterations 165 time step continuity errors : sum local = 30.0278, global = 0.333752, cumulative = 0.333752 smoothSolver: Solving for alpha.water, Initial residual = 0.000638797, Final residual = 9.17205e11, No Iterations 190 Phase1 volume fraction = 0.231448 Min(alpha.water) = 7.66302e64 Max(alpha.water) = 1 Applying the previous iteration compression flux MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water MULES: Correcting alpha.water Phase1 volume fraction = 673.428 Min(alpha.water) = 2.7388e+10 Max(alpha.water) = 6.97531e+10 GAMG: Solving for p_rgh, Initial residual = 1, Final residual = 4.73618e08, No Iterations 150 time step continuity errors : sum local = 0.00172731, global = 0.000342047, cumulative = 0.334094 PIMPLE: Iteration 2 forces forces: Not including porosity effects Rigidbody motion of the hull Centre of rotation: (114902 46.4165 8.57608e+06) Orientation: (0.542239 4.97447e06 0.840224 8.03987e07 1 5.40155e06 0.840224 2.2534e06 0.542239) Linear velocity: (6.69835e+28 2.70595e+25 4.99962e+30) Angular velocity: (1.78355e+24 2.21838e+30 1.19827e+25) [0] #[57] #00 Foam::error::printStack(Foam::Ostream&)Foam::error::printStack(Foam::Ostream&) A process has executed an operation involving a call to the "fork()" system call to create a child process. Open MPI is currently operating in a condition that could result in memory corruption or other system errors; your job may hang, crash, or produce silent data corruption. The use of fork() (or system() or other calls that create child processes) is strongly discouraged. The process that invoked fork was: Local host: [[59254,1],0] (PID 211497) If you are *absolutely sure* that your application will successfully and correctly survive a call to fork(), you may disable this warning by setting the mpi_warn_on_fork MCA parameter to 0. The case is simulated at calm water condition Fn=0.26 (U=1.59m/s) with free surface effect without any appendage. Please let me know if you need more information from me.Also, I am not sure about the difference between rigidBodyMotion and sixDoFRigidBodyMotion. Should I try sixDoFRigidBodyMotion for motionsolver? Finally, I need to compute the force on the hull, so I have to use the force function in controlDict. Please let me know if you have any idea to resolve this error and many thanks for your generous help! Best regards, Tony 

March 15, 2022, 12:29 

#18 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
You probably do not understand the basics, is my feeling. Sooo a rigid body is a point with inertia...Your centre of Mass is at roughly (1.86 0.00 0.17) in the coordinate system of your rigidBody...you now transform those into the coordinate system of your mesh. With a matrix (you rotate it):
1 0 0 0 1 0 0 0 1 I.e you do nothing to it. No rotation of that point. Now you shift everything once again by (1.86 0.00 0.17). Why? My feeling is that your centre of mass or transform is wrong. Why aren't you using this: Code:
transform (1 0 0 0 1 0 0 0 1) (0 0 0); Code:
centreOfMass (1.86617 4.84309e07 0.170); mass 229.5; inertia (0.012 3.05e07 0.0056 0.4300 3.191e08 0.4351); transform (1 0 0 0 1 0 0 0 1) (1.8661 4.8457e07 0.17331); F_fluid is the force on the patch hull...which you now move by the solution of your rigid body....depending on your fvSolution setting once or several times per time step....3 or 4 outer correctors are still not enough...add atleast another 50. You need to converge both solutions. Before you enter the next time step. Once you can solve it you can lower them and increase the maxCo etc. But you need to have a good solution first. In other words look into your logfile...and make sure that within a time step...so within the outerCorrector loops...pimple 1 2 3 4 etc your linear velocity, angular velocity etc is not changing anymore...you need atleast so many outerCorrectors that those values become steady. Simply use grep or awk to search the logfile for those lines, and plot them. Make sure they are fully converged each time step. Simple example.... Code:
awk '{if ($0 ~ "Linear velocity") print}' logfile You also need to keep in mind that you are using accelerationRelaxation 0.4; which means you are using a relaxation of the acceleration of your rigid body. Which is not realistic for transient runs. Hence you need to iterate this out using outer loops to get a realistic solution. A rigid body has 6 degrees of freedom. You restrict those with a Pz and Ry joint. So your body can only move in z and rotate around y. Hence 2 degrees of freedom are left. rigidBody or sixDoF libaries are both possible. The rigidBody one you are using allows for multiple bodies. But you can't set initial velocites etc or use it as a boundary condition. Hence It does not really matter which one you use. Since you are solving a rigid body with no internal forces...like a spring and are only adding dampers You ship shouldn't move at all if there is no force from the fluid. You are also not adding gravity. Since there is no line like g (0 0 9.81) in your dynamicMeshDict file...why are you not using gravity in your rigid body solution?....It is therefore highly likely that your fluid force on the patch hull is either wrong, or due to your center of mass being not correctly placed you are spinning your rigid body up..... So please create a plot of position, velocity and angular velocity from your logfile (by using the awk line as shown) and post those here. 

March 17, 2022, 09:45 

#19  
Member
Tony Zhang
Join Date: Nov 2019
Location: soton
Posts: 45
Rep Power: 5 
Quote:
Many thanks for your detailed explanation and patience with me... I think now it works since I choose to use 6dof rigid body motion solver instead, as follows: Code:
/** C++ **\ =========  \\ / F ield  OpenFOAM: The Open Source CFD Toolbox \\ / O peration  Website: https://openfoam.org \\ / A nd  Version: 7 \\/ M anipulation  \**/ FoamFile { version 2.0; format ascii; class dictionary; object dynamicMeshDict; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dynamicFvMesh dynamicMotionSolverFvMesh; motionSolverLibs ("libsixDoFRigidBodyMotion.so"); motionSolver sixDoFRigidBodyMotion; patches (hull); innerDistance 0.19; outerDistance 0.63; centreOfMass (1.86617058305 4.84309040657e07 0.170524016331); mass 229.5; momentOfInertia (6.20581170707 217.653741789 220.217667153); rhoInf 1; report on; value uniform (0 0 0); accelerationRelaxation 1; solver { type Newmark; } constraints { zAxis { sixDoFRigidBodyMotionConstraint line; direction (0 0 1); } yPlane { sixDoFRigidBodyMotionConstraint axis; axis (0 1 0); } } restraints { translationDamper { sixDoFRigidBodyMotionRestraint linearDamper; coeff 7217; } rotationDamper { sixDoFRigidBodyMotionRestraint sphericalAngularDamper; coeff 9727; } } // ************************************************************************* // Since the hull is free to heave and pitch, I want to plot the heave and pitch RAO vs simulation time, do you have any idea how to do this? Should I set this in controlDict file or can by postprocessing after the simulation is done? Many thanks for your help as always and best regards, Tony 

March 17, 2022, 10:37 

#20 
Senior Member
Join Date: Sep 2013
Posts: 353
Rep Power: 20 
You are using rhoInf=1. This is the density of air. It's been a long time since i used these solvers. It is likely that it isn't read and the density field of interFoam is used....RhoInf used to calculate the forces on the patch. Since many OpenFOAM solvers use a density normalized pressure p/rho. Hence to calculate the Force on the patch F=p*n you'd need to multiply it by a density. F=rhoInf*p*n. Please make sure this isn't the case. Otherwise there is basically 1000 times less force on the patch as realisitc (density of water=1000). From looking into the code it seems that it isn't used. It is basically a forces function object that calculates the forces on the patch which are given to the rigidbody solver. Hence removing that line altogether should work.
For plotting you have to keep in mind that you are only saving your result every so often. Hence if you need something for plotting over time make sure to use functionObjects to output the needed values. I am not familiar with RAOs what quantity do you need to calculate it? Try this for starters in your controlDict: Code:
functions { sixDoFRigidBodyState { type sixDoFRigidBodyState; libs ("libsixDoFRigidBodyState.so"); angleFormat degrees; } } 

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