| Mojtaba.a |
February 13, 2016 07:43 |
Massless 6DOF Simulation
Hello,
My question is as simple as it appears. Whenever I use zero mass in my 6DOF simulations, the solver (namely pimpleDyMFoam) blows up at the start of the process with Floating point exception. However zero mass in a sixDOF problem isn't impossible. Here is my DynamicMeshDict:
Code:
dynamicFvMesh dynamicMotionSolverFvMesh;
motionSolverLibs ("libsixDoFRigidBodyMotion.so");
solver sixDoFRigidBodyMotion;
sixDoFRigidBodyMotionCoeffs
{
patches (cylinder);
innerDistance 1;
outerDistance 20;
mass 0;
momentOfInertia (0.01 0.01 0.01);
centreOfMass (0 0 0);
orientation
(
1 0 0
0 1 0
0 0 1
);
velocity (0 0 0);
acceleration (0 0 0);
angularMomentum (0 0 0);
torque (0 0 0);
g (0 0 0);
rhoName rhoInf;
rhoInf 1;
report on;
accelerationRelaxation 0.3;
value uniform (0 0 0);
solver
{
type symplectic;
}
constraints
{
transverse
{
sixDoFRigidBodyMotionConstraint line;
tolerance 1e-6;
relaxationFactor 0.7;
refPoint (0 0 0);
direction (0 1 0);
}
noRotation
{
sixDoFRigidBodyMotionConstraint orientation;
orientation (1 0 0 0 1 0 0 0 1);
}
zAxis
{
sixDoFRigidBodyMotionConstraint axis;
axis (0 0 1);
}
}
restraints
{
translationSpring&Damper
{
sixDoFRigidBodyMotionRestraint linearSpring;
anchor (0 -10 0);
refAttachmentPt (0 0 0);
stiffness 0;
damping 0;
restLength 10;
}
}
}
It doesn't have any problems with a non-zero mass. Well This means that mass parameter is in a denominator somewhere in the main code. So I checked the code and found this:
Code:
void Foam::::updateAcceleration
(
const vector& fGlobal,
const vector& tauGlobal
)
{
static bool first = false;
// Save the previous iteration accelerations for relaxation
vector aPrevIter = a();
vector tauPrevIter = tau();
// Calculate new accelerations
a() = fGlobal/mass_;
tau() = (Q().T() & tauGlobal);
applyRestraints();
// Relax accelerations on all but first iteration
if (!first)
{
a() = aRelax_*a() + (1 - aRelax_)*aPrevIter;
tau() = aRelax_*tau() + (1 - aRelax_)*tauPrevIter;
}
first = false;
}
This is the only place where mass in in the denominator. From Newton's Second law a = F/m.
Any Ideas how I can solve a 6DOF problem with zero mass? |
