This article explains the axis-angle representation used in Bladed to represent node orientations. Users will interact with this orientation through a variety of Bladed inputs & outputs such as:
The support structure deflection and support structure global motions output groups.
The orientation of flexible body nodes (blades and support structure) and multibody nodes accessed via the external loads dll.
Inputting floating platform initial orientiation in the "initial conditions" window.
This article explains some counter intuitive results that may occur in particular for floating turbines with yaw rotations.
Bladed describes the orientation of support structure nodes using a representation known as axis-angle which is a vector of length 3. It is a method of representing rotation where the rotation is characterized by an axis of rotation and an angle. The axis is defined by the vector and the angle of rotation about the axis is given by the magnitude of the vector.
It is important to emphasise that the axis-angle is not equivalent to Euler angles. To explain this we consider two examples where the initial mooring position of a floating vertical cylinder is pitched.
-------Example 1 ------------------------------
The platform is pitched by 10 deg about the global y-axis. This would be represented in Bryant angles (yaw, pitch, roll) as:
X: 0, Y: 10 deg, Z: 0 deg
An axis-angle would represent this rotation by
[0, 10, 0] deg
-------Example 2 ------------------------------
The platform is yawed by 180 deg about global z-axis and then pitched by 10 deg about the local y-axis. This would be represented in Bryant angles as:
X: 0, Y: 10 deg, Z: 180 deg
An axis-angle would represent this rotation by
[15.6880337, 0, 179.3150457] deg
Counter intuitively the axis-angle has no component in the y-direction even though the platformed is pitched about the y-axis.