Velocity and Acceleration Outputs from BeamDyn

[jjonkman]

According to the BeamDyn documentation (https://openfast.readthedocs.io/en/master/_images/bd_output_channel.pdf), the nodal and tip velocities and accelerations output by BeamDyn are absolute and expressed in the global inertial-frame coordinate system (g). However, the source-code implementation has the absolute velocities expressed in g, but while the accelerations are output absolutely, they are expressed in the floating coordinate system local to the deflected blade (l). This is a bug in the documentation. That said, a typical motion sensor installed on a blade would measure the absolute velocity and accelerations expressed in l, not g, so, it would make more sense to express the velocity and acceleration outputs both in l instead of g. It may be preferred to simply add both sets of outputs so that the absolute velocities and accelerations can be output in both l and g.

[andrew-platt]

Does it make sense to also add the option to express the outputs in the blade root coordinate frame (r)? I find it easier to think in those terms than in a local frame when there are large deflections.

[jjonkman]

The blade deflections are easiest to interpret when output in r. A typical motion sensor would measure the absolute velocity and acceleration expressed in l, but outputs could be added in all three coordinates (r, l, g) if there is a desire for that.