Maybe some additional background:
What I found so far is that in OpenFoam (I checked 2006) to obtain the new orientation of the local body fixed coordinate system we first integrate the equation of motion for the angular momentum transformed to the local coordinate system: The subscript C denotes that the angular momentum is computed using the center of gravity as reference point. If we integrate this equation ones we get the angular momentum at the new time step The moment of inertia in the above equation is only diagonal if axis of the local coordinate system are aligned with the axis of inertia. The subcript n+1 denotes that the vectors are expressed in the local coordinate system at the time step n+1 and the superscript n+1 denotes the time step at which the solution is computed. Unfortunately we know only the orientation of the local coordinate system in the last time step n. We get a first order approximation of the above equation if we express the vectors in the local coordinate system at the time step n: is the vector of angle differences between the orientation of the axis of the local coordinate system at the time step n and n+1. So if we rotate the axis of the local coordinate system by the three angles increments we get the orientation of the local coordinate system at the new time step. What i do not understand is why the rotate function is constructed like described in the previous post. |
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