[ste_ge]

Hi everyone,

I am a bit confused having an issue with the sixDoFRigidBodyMotion solver.

First of all, what is exactly, from a physical point of view, the object called "centreOfRotation"?
I presume it represents the point for which the equation that rules the rotation (i.e. Newton's second law for angular momentum) is written. In fact, all the moment arms are calculated as it was.
If I'm right, the theory says that one can choose any arbitrary point to be the center of rotation, having the same result (the moments and the moment of inertia will vary accordingly).
Of course, this point moves with the body, i.e. the distance between centreOfRotation and centreOfMass is a constant vector in the body local reference, while in the fixed reference only the magnitude of the vector is constant.

In the problem that I am studying, I need to choose "smartly" this point, so that the contribution of certain forces is zero, and the point does not coincide with the center of mass.

Now, I'm not being able to set the case successfully. I tried with setting in the constant/dynamicMeshDict an additional centreOfRotation entry, for example:

...
centreOfRotation (-5 0 0);
centreOfMass (0 0 0);
...

that I think is somehow read by the code, but I see in the output that "centreOfMass" is overwritten by "centreOfRotation".

Looking at the source code at src/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion/sixDoFRigidBodyMotion.C (lines 103-111), I found strange the following:

...
    initialCentreOfMass_
    (
        dict.lookupOrDefault
        (
            "initialCentreOfMass",
            vector(dict.lookup("centreOfMass"))
        )
    ),
    initialCentreOfRotation_(initialCentreOfMass_),
...

Why is the centreOfRotation assumed to be coincident with the centreOfMass?

Did anyone have a similar problem before?

Any help is appreciated!
Stefano

P.S.: the solver application I use is pimpleDyMFoam.

[iy-a]

Hi,

if you check dynamicMeshDict from pimpleDyMFoam's wingMotion case,
you will notice that centreOfRotation is set in the constraints.
If you built your case on a specific example, it would have been nice to know which one.

iy-a.

[vava10]

Hey,

I am trying to simulate a ship flow. I have few questions regarding dynamic mesh. Hope someone can help. Its urgent

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. In another thread I asked if I am using the half geometry should the mass properties be of the half geometry and I was told yes. Then if so doesn't it affect the centre of mass and moment of inertia?

3. 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.

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

[random_ran]

When the model is defined to move along the y-axis only, the user
needs to explicitly specify it. It means the model is not allowed to
move along the x-axis or z-axis. In terms of the rotation, the user
needs to specify how the rotation is defined. For example, the
following code only allows the object to rotate along the z-axis, but
it is unclear how the rotation exactly is. `centreOfRotation' bridges
this gap. Both conditions meet the requirement of rotating along the
z-axis, but a rotating centre at Pt-1 results in a different rigid
body motion compared to the centre at Pt-2.

,----
| constraints
| {
| yLine
| {
| sixDoFRigidBodyMotionConstraint line;
| centreOfRotation (0.25 0.007 0.125);
| direction (0 1 0);
| }
|
| zAxis
| {
| sixDoFRigidBodyMotionConstraint axis;
| axis (0 0 1);
| }
| }
`----

screen_2021-04-25_12-28-39.png