Coordinate systems

Articles on this page:

• Difference between ‘Nacelle Yaw Displacement’ and ‘Yaw Bearing Angular Position’
• Overhang definition: Where is the rotor centre?
• Orientation of the blade cross-section
• Cone angle vs "blade mounting cone" - what's the difference?
• Is it possible to get global positions of hub, and also of tower nodes (stations)?

Problem

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?

Solution

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.
   

Keywords

Nacelle yaw displacement; Yaw bearing angle; Yaw angle

Problem

In the user manual, overhang is defined as the distance between tower centre line and rotor centre; but where is the rotor centre exactly?

Solution

The rotor centre is the point where the 3 blade pitch axes meet. Note that there is a built-in assumption in the rotor model that the pitch axes are co-planar and all intersect at a common point. This is also the point where the hub centre load outputs are defined / measured.

Keywords

Overhang, rotor centre, hub centre, hub, pitch axis

Problem

In the Blade Geometry section of the User Manual, there are diagrams illustrating the blade cross-section, showing such things as aerodynamic twist and neutral axis position relative to the chord. But how is each of these cross-sections oriented in 3D space? Are they all perpendicular to the pitch axis, i.e. parallel to the blade flange? Or is each section perpendicular to the local orientation of the neutral axis?

Solution

These cross-sections are perpendicular to the local orientation of the neutral axis. It follows that they are in general not parallel to each other, as their orientation depends on both pre-bend and dynamic deflection of the blade.

Keywords

Blade cross-section; Neutral axis; Pitch axis; Blade section; Frame; Coordinates

Problem

There are two places in Bladed where cone angle is defined - in the Rotor screen and in the Blade screen. What is the difference, if any - and which direction does the blade tilt for positive / negative cone angle?

Solution

There are two different cone angles that you can set independently of each other. Also (perhaps confusingly) the direction conventions are opposite. The two types are:

1. The cone angle on the rotor screen:
   
   
   
   
2. The blade screen setting "Blade mounting cone angle":
   
   
   
   

For (1), positive values of cone will make the blades point more downwind. For (2), it's the other way round - positive values make the blades point more upwind.

The other difference, which is explained in the User Manual (section 3.4 in current 4.12 edition), is that (2) is applied outboard of the pitch bearing. So if (2) is non-zero and you change the pitch angle on a stationary rotor, a straight blade will trace out part of a cone-shape as it pitches. For (1), this is not the case.

A general tip for answering questions like this yourself is to just experiment with changing values, such as blade mount cone, to a value large enough to be visually noticeable. Then carry out a 1-second parked / idling run and animating the outputs so you can visualise the effects of your changes. (There is also the Turbine Graphic window, but this does not always show effects of change correctly, so we recommend Results Animation).

Keywords

Cone; Blade; Mount; Mounting; Cone angle; Rotor

Problem

For multi-member towers, the instantaneous positions of tower nodes and hub are not available in global coordinate frame. Tower node deflections are outputted, but these are in a local frame.

Solution

There is a workaround for this. One caveat is that you do need to have the Seismic module enabled on your licence. There is a downloadable example project which illustrates the scheme. The model is based on the 5MW Tripod supplied as standard. The modifications which were made are as follows:

1. Set up an earthquake - this is a "trick" to make Bladed produce outputs for tower node positions in global coordinates. The earthquake does not affect the results at all; this is a 5 second simulation and the earthquake start time is at 8 sec:
   
   
   
   
2. Set up the tower outputs so that you have deflections enabled at all the tower stations you are interested in.
   
   
   
   
3. Set up "Specific Node outputs" as described in Section 7.21.11 of the User Manual (this is the section number in the latest edition, 4.16 at time of writing) to give kinematics at the LSS node, which is equivalent to the hub position:
   
   
   
   

After doing all this, run the simulation and look at the outputs "Hub global position" and "Support structure global positions".

Keywords

Offshore support structure; Tower; Coordinate frame