Bladed users are sometimes surprised to discover that ‘Nacelle Yaw Displacement’ and ‘Yaw Bearing Angular Position’ are defined with opposite signs. In other words, a clockwise (from above) rotation of the nacelle results in positive values of Yaw Bearing Angular Displacement and negative values of Nacelle Yaw Displacement.
What is the reason for this choice of sign convention?The reason that these outputs are defined this way comes from the fact that Bladed needs to incorporate two well established and conflicting angular definition systems. These systems are:
1. The standard convention for wind direction. This has incoming wind direction given as a positive angle moving clockwise from north, as shown below. The variable ‘Yaw Bearing Angular Position’ follows convention 1 to match the wind direction definition.
2. The coordinate system for the yaw bearing is shown below. For a wind turbine facing towards the north, this has the x axis pointing south, and the z axis pointing vertically upwards. The “sense” of Z rotation is anti-clockwise when viewed from above. The variable 'Nacelle Yaw Displacement' comes directly from the structural dynamics code that uses the right-hand screw rule when defining the sense of structural rotations. Therefore, this variable follows convention 2.
