[jiadongw]

Hello Foamers,

Recently, I’m trying to conduct the tutorial wingMotion with the new motion library rigidBodyMeshMotion. This tutorial originally employs the sixDoFRigidBodyMotion.

Unfortunately, the new case with rigidBodyMeshMotion has some problems. One of quite weird things is the moment of linearAxialAngularSpring has a same direction with angle, which means the axis spring accelerate the rotation instead of restraining it.
Also, I’m quite confused about the parameter ‘transform’ in dynamicMeshDict of rigidBodyMeshMotion.

I have attached the modified dynamicMeshDict (rigidBodyMeshMotion) and original dynamicMeshDict (sixDoFRigidBodyMotion) to this post.
Does anyone have the experience with this new motion library rigidBodyMeshMotion?

Any suggestions will be appreciated.

Thank you.

dynamicMeshDict of rigidBodyMeshMotion:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  5                                     |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      dynamicMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dynamicFvMesh      dynamicMotionSolverFvMesh;

motionSolverLibs ("libRY05rigidBodyMeshMotion.so");

motionSolver    rigidBodyMotion;

report          on;

solver
{
    type symplectic;
}

g               (0 -9.81 0);
rho             rhoInf;
rhoInf          1;

bodies
{
   wing
  {

    type            rigidBody;
    parent          root;
    centreOfMass    (0.4974612746 -0.01671895744 0.125);
    mass            22.9;
    inertia         (1.958864357 0 0 3.920839234 0 2.057121362);
    transform       (1 0 0 0 1 0 0 0 1) (0.25 0.007 0.125);
   
    joint
    {
       type   composite;
       joints
       (
            {
                 type     Py;
            }                      
            {
                 type     Rz;
            }
       );
    }

        patches         (wing);
        innerDistance   0.3;
        outerDistance   1;

  }
}

restraints
{
    verticalSpring
    {
        type            linearSpring;
        body            wing;
        anchor          (0.25 0.007 0.125);
        refAttachmentPt (0.25 0.007 0.125);
        stiffness       4000;
        damping         2;
        restLength      0;
    }

    axialSpring
    {
        type            linearAxialAngularSpring;
        body            wing;
        axis            (0 0 1);
        referenceOrientation (1 0 0 0 1 0 0 0 1);
        stiffness       700;
        damping         0.5;
    }
}


// ************************************************************************* //

dynamicMeshDict of sixDoFRigidBodyMotion:

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  5                                     |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      dynamicMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dynamicFvMesh      dynamicMotionSolverFvMesh;

motionSolverLibs ("libtest00.so");

motionSolver    sixDoFRigidBodyMotion;

patches         (wing);
innerDistance   0.3;
outerDistance   1;

mass            22.9;
centreOfMass    (0.4974612746 -0.01671895744 0.125);
momentOfInertia (1.958864357 3.920839234 2.057121362);
orientation
(
    0.9953705935 0.09611129781 0
    -0.09611129781 0.9953705935 0
    0 0 1
);
angularMomentum (0 0 -2);
g               (0 -9.81 0);
rho             rhoInf;
rhoInf          1;
report          on;

solver
{
    type symplectic;
}

constraints
{
    yLine
    {
        sixDoFRigidBodyMotionConstraint line;
        centreOfRotation    (0.25 0.007 0.125);
        direction           (0 1 0);
    }

    zAxis
    {
        sixDoFRigidBodyMotionConstraint axis;
        axis                (0 0 1);
    }
}

restraints
{
    verticalSpring
    {
        sixDoFRigidBodyMotionRestraint linearSpring;

        anchor          (0.25 0.007 0.125);
        refAttachmentPt (0.25 0.007 0.125);
        stiffness       4000;
        damping         2;
        restLength      0;
    }

    axialSpring
    {
        sixDoFRigidBodyMotionRestraint linearAxialAngularSpring;

        axis            (0 0 1);
        stiffness       700;
        damping         0.5;
        referenceOrientation $orientation;
    }
}


// ************************************************************************* //

[jiadongw]

Just ignore the name of library in both dynamicMeshDict. That’s the recompiled version without any changes except I add some output orders using Info.

[jiadongw]

Any ideas?

[tecmul]

I'm struggling with the same questions as OP.
In interFoam's floating object tutorial (OpenFOAM v6), there's two dynamic mesh dictionaries, one for the sixDoF library and one for the rigidBodyMotion library.

sixDoF:

centreOfMass        (0.5 0.45 0.35);

rigidBodyMotion:

centreOfMass    (0 0 0.25);
transform       (1 0 0 0 1 0 0 0 1) (0.5 0.45 0.1);

Why is it defined like this?

[tecmul]

Anyone? Any ideas?

[Sachin m]

Any ideas??

[Himanshu_Shrivastava]

any ideas?..
all confused here..

[tecmul]

So I've been working with and testing the rigidBodyDynamics library for some time. I'm not 100% certain so take everything I say with a grain of salt, especially regarding the coordinate transform.


The tensor and vector specified under the transform keyword represent a coordinate transform from the parent frame to the local frame of the body's joint. The vector is the displacement of the origin of the local frame relative to the parent frame and the tensor is its rotation (google 3D rotation matrix). The frame of the root body is identical to the global frame. The centre of mass of a body is specified relative to its local frame. So the following dictionary from the floating object tutorial

centreOfMass    (0 0 0.25);
transform       (1 0 0 0 1 0 0 0 1) (0.5 0.45 0.1);

is telling us that the origin of the local frame (the joint) of the floating body is displaced by the vector (0.5 0.45 0.1) with respect to the origin of the parent body, here the global frame. The axes of the local frame are aligned with the global frame because of the identity rotation tensor. The centre of mass is located at (0 0 0.25) relative to the local frame, or (0.5 0.45 0.35) in the global coordinate system.



About the problem with the axial spring restraint mentioned in the OP, I think the problem is with the direction of the vector "a" that the class uses to determine the sense of the applied moment. This was a while ago but if I remember correctly, rectifying the problem required only a very simple modification to the class.

[Sachin m]

Originally Posted by tecmul

So I've been working with and testing the rigidBodyDynamics library for some time. I'm not 100% certain so take everything I say with a grain of salt, especially regarding the coordinate transform.


The tensor and vector specified under the transform keyword represent a coordinate transform from the parent frame to the local frame of the body's joint. The vector is the displacement of the origin of the local frame relative to the parent frame and the tensor is its rotation (google 3D rotation matrix). The frame of the root body is identical to the global frame. The centre of mass of a body is specified relative to its local frame. So the following dictionary from the floating object tutorial

centreOfMass    (0 0 0.25);
transform       (1 0 0 0 1 0 0 0 1) (0.5 0.45 0.1);

is telling us that the origin of the local frame (the joint) of the floating body is displaced by the vector (0.5 0.45 0.1) with respect to the origin of the parent body, here the global frame. The axes of the local frame are aligned with the global frame because of the identity rotation tensor. The centre of mass is located at (0 0 0.25) relative to the local frame, or (0.5 0.45 0.35) in the global coordinate system.



About the problem with the axial spring restraint mentioned in the OP, I think the problem is with the direction of the vector "a" that the class uses to determine the sense of the applied moment. This was a while ago but if I remember correctly, rectifying the problem required only a very simple modification to the class.

Originally Posted by Bloerb

Yes, those can change, depending on how you constrain your part.



transform can be used to define your inertia inside a different coordinate system.

transform       (1 0 0 0 1 0 0 0 1) (2.929541 0 0.2);

The first part is the roation matrix. the second a vector by which to move it.


Hence:
1 0 0

0 1 0

0 0 1



is the unit matrix. Meaning no rotation. And (2.929541 0 0.2) is a translation by that vector. You can hence always use

transform       (1 0 0 0 1 0 0 0 1) (0 0 0);

but you need to make sure to calculate your moment of inertia correctly

https://ibb.co/tqfHZ7v
This is the image of my case set up.
It consists of a domain and two floating objects.
I have to specify center of mass and transform for both the objects.
Dimension of the bodies are Lx=0.3 , Ly=0.2 , Lz=0.5.
Iam confused on how to specify the Center of mass and transform.
As per the above quote, the center of mass has to be specified based on the local co ordinate of the body. So COM will be (0,0,0.25) for both the body, assuming the local co ordinate system to be at the bottom of either bodies.(Correct me if iam wrong)
I presume the center of rotaion to be at 0.1 m above the center of mass, Which has to be defined in the global co ordinate system (Correct me if iam wrong).
Both bodies are identical.
Domain size is (1 1 1) (-1 -1 -1).
COM for first body with respect to Global Co ordinate system is (-0.5,0.45,0.35)
COM for second body with respect to Global Co ordinate system is (0.5,0.45,0.35)
Please help me on how to specify the center of mass and transform. Iam using Openfoam V 2006.
How can we find the Local co ordinate system values using paraview?



https://ibb.co/m4Q9z90
Sketch of Domain and body.

[tecmul]

Originally Posted by Sachin m

I presume the center of rotaion to be at 0.1 m above the center of mass,
COM for first body with respect to Global Co ordinate system is (-0.5,0.45,0.35)
Please help me on how to specify the center of mass and transform. Iam using Openfoam V 2006.
How can we find the Local co ordinate system values using paraview?

If the centre of rotation of the body is to be 0.1 m above the centre of mass, then for the left body you would specify

centreOfMass    (0 0 -0.1);
transform       (1 0 0 0 1 0 0 0 1) (-0.5 0.45 0.45);

The centre of rotation is the origin of the local coordinate system. I hope this helps.

[Sachin m]

Originally Posted by tecmul

If the centre of rotation of the body is to be 0.1 m above the centre of mass, then for the left body you would specify

centreOfMass    (0 0 -0.1);
transform       (1 0 0 0 1 0 0 0 1) (-0.5 0.45 0.45);

The centre of rotation is the origin of the local coordinate system. I hope this helps.

How can we know the origin of local co ordinate system?

[tecmul]

Originally Posted by Sachin m

How can we know the origin of local co ordinate system?

The user sets the origin of the local coordinate system. Or are you asking how to monitor the origin as it moves over time?

[FloB]

Hi tecmul,

Thanks for your explanations, now I understand a bit more the function "transform" in the file dynamicMeshDict.


Do you (or any other foamer) know what is the point of having a local frame different from the global frame ?

In the tutorial of interDyMFoam, I think they have to do this to precise to which point is applied the joint Py. Indeed, I think the joint Py (or Pxy, Pxyz, Pz etc...) is always applied to the origin of the local frame, which is here the point (0.5 0.45 0.1) in global frame. Therefore, it is in agreement with, the constraint fixedLine in the file "dynamicMeshDict.sixDoF" : the point (0.5 0.45 0.1) can only translate along y axis.

Am I right ? Are there others points to set a different local frame ?


Moreover, why do we have to work with these two files (dynamicMeshDict and dynamicMeshDict.sixDoF) whereas most information are redundant between them ?

I think that dynamicMeshDict is dedicated to the mesh of the cuboid whereas the other file is dedicated to rest of the mesh but I'm not sure...

[tecmul]

Hello Florian, glad I could help.
I don't know the answer to your first question, but if I had to guess I'd say it's something to do with Roy Featherstone's algorithm. The rigid body dynamics library is based on his work and mirrors its logic.
The two dynamic mesh dictionary files are for use with the two rigid body dynamics libraries currently available in OpenFoam. The former file is for use with the rigidBodyDynamics library, the latter for sixDoF.
See 212949-flow-induced-3d-motion-openfoam.html#post719121 for a post on the differences between the two libraries.

[FloB]

Originally Posted by tecmul

Hello Florian, glad I could help.
I don't know the answer to your first question, but if I had to guess I'd say it's something to do with Roy Featherstone's algorithm. The rigid body dynamics library is based on his work and mirrors its logic.
The two dynamic mesh dictionary files are for use with the two rigid body dynamics libraries currently available in OpenFoam. The former file is for use with the rigidBodyDynamics library, the latter for sixDoF.
See 212949-flow-induced-3d-motion-openfoam.html#post719121 for a post on the differences between the two libraries.

Hi tecmul, thank you very much for your quick answer.

I have a question about this sentence :

These are nearly identical, however the rigidBodyDynamics library can solve for more than one rigid body. The sixDoFRigidBodyMotion on the other hand can be used as a boundary condition for every mesh motion solver

When he says that the sixDoFRigidBodyMotion can be used as a boundary condition, is it not also the case for the rigidBodyDynamics library ?
Moreover what is mesh motion solver ? Is it the solver mentionned like this :

solver
{
type Newmark;
}

so it would mean that the sixDoFRigidBodyMotion can be solved with Newmark, Crank-Nicolson and Symplectic (which is confirmed here https://openfoamwiki.net/index.php/P...onSolverFvMesh) but for rigidBodyDynamics, less solvers can be used ? (there is at least Newmark according to the tutorial)


Another question :

Do you know why no inertia is mentionned in the tutorial using rigidBodyMeshMotion library ? I tried to add diagonal coefficients, but it does not change the results..


Thanks in advance,

Florian

[tecmul]

Originally Posted by FloB

When he says that the sixDoFRigidBodyMotion can be used as a boundary condition, is it not also the case for the rigidBodyDynamics library ?

No, it isn't; and besides, the ability to use the sixDoFRigidBodyMotion as a boundary condition was removed from OpenFoam 5 onwards. See this for a discussion that might be helpful.

Originally Posted by FloB

Moreover what is mesh motion solver ? Is it the solver mentionned like this :

solver
{
type Newmark;
}

The Newmark solver solves for the position of the rigid body from its acceleration. Each solver uses a particular integration scheme. Once the rigid body has moved, then the fluid mesh around it must also be displaced. How this is done depends on the mesh motion solver.

Originally Posted by FloB

Do you know why no inertia is mentionned in the tutorial using rigidBodyMeshMotion library ? I tried to add diagonal coefficients, but it does not change the results..

If you mean the floating object tutorial, then that's because the library is calculating the moment of inertia for you. It can do this because the shape (type->cuboid), dimensions and density of the floating body have been specified.

[Himanshu_Shrivastava]

Originally Posted by tecmul

So I've been working with and testing the rigidBodyDynamics library for some time. I'm not 100% certain so take everything I say with a grain of salt, especially regarding the coordinate transform.


The tensor and vector specified under the transform keyword represent a coordinate transform from the parent frame to the local frame of the body's joint. The vector is the displacement of the origin of the local frame relative to the parent frame and the tensor is its rotation (google 3D rotation matrix). The frame of the root body is identical to the global frame. The centre of mass of a body is specified relative to its local frame. So the following dictionary from the floating object tutorial

centreOfMass    (0 0 0.25);
transform       (1 0 0 0 1 0 0 0 1) (0.5 0.45 0.1);

is telling us that the origin of the local frame (the joint) of the floating body is displaced by the vector (0.5 0.45 0.1) with respect to the origin of the parent body, here the global frame. The axes of the local frame are aligned with the global frame because of the identity rotation tensor. The centre of mass is located at (0 0 0.25) relative to the local frame, or (0.5 0.45 0.35) in the global coordinate system.



About the problem with the axial spring restraint mentioned in the OP, I think the problem is with the direction of the vector "a" that the class uses to determine the sense of the applied moment. This was a while ago but if I remember correctly, rectifying the problem required only a very simple modification to the class.

Hello tecMul,

Thank you for sharing your knowledge. what ever you said , i was able to make sense from tutorial "multiphase\interFoam\RAS\floatingObject".
when i check thoroughly the result, what ever you said the relation of COM and vector (in transform)....
but, when I saw tutorial "multiphase\overInterDyMFoam\boatAndPropeller" , then in dynamicMeshDict, its written as:

centreOfMass (-0.2847 0.03 0); // Relative to parent CoM
mass 0.0288;
inertia (7.6e-6 0 0 4.2e-6 0 4.2e-6);
// transform and CoR - relative to parent CoR
transform (1 0 0 0 1 0 0 0 1) (-0.2757 0.03 0);

i.e. COM w.r.t parent COM and COR w.r.t parent COR..

is this because in "multiphase\overInterDyMFoam\boatAndPropeller" , we have multi rigid connected bodies?

Thank You

[tecmul]

Yes the boat and propeller tutorial confused me as well. I think the comment is wrong here because I've run tests of double pendulums and the centre of mass does indeed seem to be specified relative to the body's own joint, not the parent's centre of mass. But again, I'm not 100% sure.