Including run time as variable in dynamicMeshDict
 

Hi there,

I'm looking to include a dynamic motion as a function of time in my dynamicMeshDict but now sure how to. I basically need to change the rotation axis depending on the run time. Does anyone have any idea or has done this before?

I saw one post using "scalar t=runTime.value();" but I'm probably using it wrongly (I've been fiddling with a simple case to see if it works). I get an error message saying '$t was not declared in this scope'. I've attached a snippet of my dynamicMeshDict to show how I've used it mainly to see if it can work first.



/*--------------------------------*- 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 "constant";
object dynamicMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dynamicFvMesh solidBodyMotionFvMesh;

motionSolverLibs ( "libfvMotionSolvers.so" );


scalar t=runTime.value();

solidBodyMotionFvMeshCoeffs
{

solidBodyMotionFunction multiMotion;
multiMotionCoeffs

{

Rotation
{
cellZone RotorCylinder;
solidBodyMotionFunction rotatingMotion;
rotatingMotionCoeffs
{

origin (0 0 0);
axis (1 0 0);
omega #calc "3.5*$t"; // rad/s
}
}

.....

Any tips and pointers would be greatly appreciated!


Kind regards,

Ryan

rotation as a function of time
 

cd $HOME/OpenFOAM/OpenFOAM-2.3.1/src/dynamicFvMesh/solidBodyMotionFvMesh/solidBodyMotionFunctions/

cd rotatingMotion

gedit rotatingMotion.C

in the function (line 73)
Foam::septernion
Foam::solidBodyMotionFunctions::rotatingMotion::tr ansformation() const
you can do your stuff

I added ...

const scalar PI = constant::mathematical::pi;

// scalar angle = omega_->integrate(0, t);
scalar angle = Foam::sin(1.0*PI*t)*omega_->integrate(0, t);

Now you have sinus type of rotation. You can add other functions, like atan if you want a steady increase and then steady state rotation.
---------------------------------------------------------------------------

In ./constant/dynamicMeshDict

dynamicFvMesh solidBodyMotionFvMesh;

solidBodyMotionFvMeshCoeffs
{
cellZone none; // none => the whole mesh rotates.
// if you want only part of it to rotate, you have to define
// that region. see examples in openfoam wiki
solidBodyMotionFunction rotatingMotion;

rotatingMotionCoeffs
{
origin ( 0 0 0 );
axis ( 0 0 1 );
omega 1.4451; // rad/s // omega = 2*pi*(0.23 rps)
}
}

Thanks for the reply, JonW! This is helpful, it seems relatively straightforward to tweak the code to do something like this.

However (and this is my fault) I should have given the example code more in line with that I am aiming to do; there I was trying to change omega as a way of playing around with the time value but that's not what my goal is. What I actually need to do is move the origin (centre of rotation) with time. Imagine a pendulum moving, I'm trying to keep the centre of rotation at the tip of it.

I've discovered the tabulated6DoFMotion option, which seems good. It reads the rotation/translation in the form of a table so that there's a rotation/translation in terms of degrees and metres for each time step. If I could do the same with the CofG (origin) that would be ideal! It's like this in the dynamicMeshDict:


Pitching
{
cellZone "Rotor.*";
solidBodyMotionFunction tabulated6DoFMotion;
tabulated6DoFMotionCoeffs
{
CofG (-0.2 -0.2818 0);
timeDataFileName "$FOAM_CASE/constant/6DoF.dat";
}
}


The rotation/translation table listed in the 6DoF.dat file within the /constant folder looks like this, for example:


//(time ((x trans, y trans, z trans) (x rot y rot z rot))) about CofG
5001 //number of entries
(
(0.0 ((0 0 0) (0.144977489506 0 0)))
(0.002 ((0 0 0) (0.289958382958 0 0)))
(0.004 ((0 0 0) (0.434942669873 0 0)))
(0.006 ((0 0 0) (0.579930339763 0 0)))
(0.008 ((0 0 0) (0.724921382128 0 0)))
(0.01 ((0 0 0) (0.869915786463 0 0)))
(0.012 ((0 0 0) (1.01491354225 0 0)))
(0.014 ((0 0 0) (1.15991463898 0 0)))
...

Again, any help in doing the same with the CofG would be great! In the mean time, I'm doing my C++ homework..


Thanks again,

Ryan

Hi Ryan
This is very interesting what you are trying to do. Unfortunately, I don't have an answer. I have been reading (when I have time) about the dynamicMesh thingi for the past weeks, when I have time (which is not much). But I am not where you are now in this. Hopefully, I will be there soon enough. If I come across ideas, I will let you know.
Good luck (to both of us)
J.

Hey RKyle!

Were you able to figure out how to move origin with time? I am working on a similar case where I have to rotate a cylindrical domain in dynamicMeshDict file whose's origin is changing with time.