# Coordinate System in FAST

Dear Jason,

This does help and make sense to me now. Thanks a lot for your active responses and considerations. I really appreciate it!

Best Regards,
Neelabh

Hello everyone,

I have some questions regarding the coordinate system. Right now I am trying to evaluate the semisubmersible platform displacements and tension in the lines varying both wave and wind direction.

My problem is that I don’t know how the coordinate system works for any of that, so if I get a fairlead with much higher tension than the others I cant check if that is because of the wave or wind direction as I don’t know how it works.

Can anybody explain it to me?

Thanks,

Carmen

Dear Carmen,

Which coordinate system do you have a question about? Are you asking about the definition of wind and wave direction? Or are you asking about the mooring line definition? Or something else entirely?

Best regards,

Dear Jason,

I am pretty lost overall. I would like to know about which axis both wind and wave direction are defined. Also, what is the coordinate system for surge/sway/heave and the mooring lines?
I basically want to be sure of all of them, since I want to see if my lines tension and movements make sense for the different wave/wind direction scenarios I am simulating.

Best regards,

Carmen

Dear Carmen,

In FAST’s HydroDyn module, the wave direction input (WaveDir) is defined counter-clockwise when looking from above (that is, a positive rotation about the Z axis of the inertia frame, which points verticically upwards). Zero degrees corresponds to waves propogating along the X axis of the inertia frame, and 90 degrees corresponds to waves propogating along the Y axis of the inertia frame.

For definition of the wind direction and nacelle-yaw error, please see the following forum topic: http://forums.nrel.gov/t/is-positive-yaw-error-clockwise/582/1.

In FAST, the platform translational displacements surge, sway, and heave are positive along the X, Y, and Z axes of the inertia frame, respectively. The platform rotational displacements roll, pitch, and yaw are positive about the X, Y, and Z axes of the inertia frame, respectively. See Figure 20 from the FAST User’s Guide for more information.

The mooring line angles in FAST v7 follow the same convention as WaveDir.

Best regards,

Hello Jason,

From the user guide and your responses I understand that WaveDir, displacements and mooring lines all refer to the inertial fram coordinates.
From the guide I get that the xi axis of those coordinates is defined by the 0º Wind Direction, that is the part that confuses me. Is the wind the parameter from which we set our inertial coordinates and all of the rest depend on them?

For example, I have a scenario where WaveDir is -50º and the wind direction is 293º, how would it look like? I don’t know where to place the xi (inertial frame x axis) according to the wind direction. I attach example of what I think but I guess its wrong, I basically placed the xi axis from behind the platform… is that what it would look like?
imgur.com/IZntRTN

Best regards,

Carmen

Dear Carmen,

I see a few problems with your picture:

1. The wave direction should be defined counter-clockwise when viewed from above. So, a 51-deg angle would be defined up and to the left. If you mean -51 degrees, then the figure is correct.
2. The 0-degree wind direction should be directed along the positive Xi axis (there is no arrow shown on the figure, so, I don’t know if that is what you had in mind).
3. The wind direction input in AeroDyn is opposite the wind direction output from FAST (the input is clockwise, but the output is counter-clockwise). Your figure shows the wind direction input convention, not the output convention. (I’m not sure whether that is what you had in mind.)

Best regards,

Hi Jason,

This is the result from what I understood
imgur.com/VTIrC8Z

WinDir= -51º and wind direction is 293º in the Aerodyn file, is that correct? Is the 0º wind direction correct there?

Carmen

Dear Carmen,

You’ve fixed the wave direction and 0-deg wind direction, but the 293-deg wind direction is now incorrect. Because you are referring to a wind direction input to AeroDyn, and the wind direction input to AeroDyn is defined clockwise, 293-deg wind should come from the upper-left of the image–like you had previously.

Best regards,

Hi Jason,

I understand now what you mean, thank you very much for your help.

Best regards,

Carmen

Dear all,

I have made a figure of the convention of wind and wave. The wind direction is for AeroDyn (especially, in the *.wnd file) and the wave direction is for FAST. Could anyone confirm my figure? Do I have right understanding with this convention?

Many thanks!
Sungwon

Dear Sungwon,

I agree with the wind directions in your figure (in terms of wind input to AeroDyn), but the arrows on the wave directions are reversed. A wave direction of 0 degrees means that waves propogate along the positive X axis (identical to 0-degree wind). A wave direction of 90 degrees means that the waves propogate along the positive Y axis (opposite 90-degree wind as input to AeroDyn).

I’m also not sure why you drew four rotors. The rotor on the left represents an upwind rotor with 0-degrees yaw angle.

Best regards,

Dear Jason,

I checked wave direction with some cases; 0 and 180 degrees of waves and mooring line below

```LRadAnch LAngAnch LDpthAnch LRadFair LAngFair LDrftFair LUnstrLen LDiam LMassDen LEAStff LSeabedCD LTenTol (m) (deg) (m) (m) (deg) (m) (m) (m) (kg/m) (N) (-) (-) 853.87 0.0 320.0 5.2 0.0 70.0 902.2 0.09 77.7066 384.243E6 0.0 0.0000001 853.87 120.0 320.0 5.2 120.0 70.0 902.2 0.09 77.7066 384.243E6 0.0 0.0000001 853.87 240.0 320.0 5.2 240.0 70.0 902.2 0.09 77.7066 384.243E6 0.0 0.0000001```
0 “WaveDir” and 180 “WaveDir”

I could find that results of PtfmSurge start negative value with 0 degree waves, and I got opposite values with 180 degree (See attached). Both cases are without wind. That is why I thought waves propagate along the negative X axis. Could you confirm the directions of mooring line and the result?

In addition, I have another question about FAST input. If I try to simulate upwind case with 180 degree of wind situation, do I have to put all direction inputs with opposite sign or just put 180 degree of angle? (eg. (+)overhang with 180 angle OR (-)overhang with 180 degree). BTW, I drew the rotors for just my convenience

Regards,
Sungwon

Dear Sungwon,

The mooring lines, wind, and rotor are drawn correctly in your updated figure, but the wave direction is still drawn incorrectly. A 0-degree wave propogates along the positive X axis.

In steady-state conditions with regular waves, linear hydrodynamic theory implies that there is no mean offset of the platform. So, the slowly varying mean offset near model initialization is the result of a start-up transient (initial-condition solution). How the mean varies is likely due to the wave direction and initial phase of the wave at time zero, but a positive initial mean offset does not imply that the waves propogate along the positive X axis.

Modeling an upwind turbine with a 180-degree yaw error can be done in multiple ways–by changing the yaw angle of the nacelle or the wind direction. For both cases, you should keep Overhang negative-valued for an upwind rotor.

Best regards,

Dear NREL forum members,

I am trying to understand how the coordinate system is set up in FAST, but after rereading the User guide,
I am in doubt when it comes to the Blade coordinate system, that is presented in FAST User’s guide, page 10.

What I understood from the user guide is that this coordinate system is valid in the case of a downwind turbine, with the blades rotating clockwise. Respectively the sign of the forces affecting the blades on the three coordinates are as follows:

``````        X axis - negative Force
Y axis - negative Force
Z axis - positive Force
``````

How could I understand the sign of these Forces?

I am trying also to get the hold of how is the blade coordinate system defined in the case of an upwind turbine, with blades rotating clockwise. Would it be the same? Meaning that no matter of the nacelles position, the blade coordinate system stays the same. Moreover, I am as well interested in the sign of the forces affecting the turbine on the three axis in this case.

Last, I would like to find out how are the forces affecting the blade on the three axis derived, at the interface between blade and hub, as the forces due to the blade affecting the hub, or the hub affecting the blade?

Best regards,
Attila Veress

Hi again

Concerning my first question with the right coordinate system in case of upwind turbine, rotor rotating clockwise, I have found in this topic [url]http://forums.nrel.gov/t/upwind-and-downwind-turbines/466/1] , that no matter if the turbine is downwind or upwind, the coordinate system does not change.

Having this information, I return with another question regarding the sign of the Forces affecting a blade in case of an upwind turbine. Is the following correct?
X axis - positive Force
Y axis - negative Force
Z axis - positive Force

Thank you!

Best regards,
Attila Veress

Dear Attila,

Yes, the blade coordinate system is unchanged by the upwind or downwind configuration. For the upwind case, the rotor in Figure 9 is unchanged, but the nacelle and tower are downwind of the tower instead of upwind.

Yes, I agree with the updated signs of your forces. Aerodynamic thrust will act along positive x, aerodynamic lift that drives rotation will have a component acting along negative y, and centrifugal forces due to rotation will act along positive z. The blade-root loads output by FAST can be thought of as the loads applied to the hub transmitted by the blade. This convention is consistent throughout the turbine–e.g., the tower-top loads output by FAST are the loads applied to the tower transmitted by the nacelle.

Best regards,

Dear Jason,

I’m confused about load definitions in the coned and blade coordinate systems. According the User Guide for FAST v6 the origin for the coned coordinate system is at the intersection of the rotor axis and the plane of rotation (non-coned rotors) or the apex of the cone of rotation (coned rotors) and the origin for the blade coordinate system is at the intersection of the blade’s pitch axis and the blade root. So the origins are different.

But in the OutListParameters.xls the descriptions for RootMxc1 and RootMxb1 are e.g.:
RootMxc1 - Blade 1 in-plane moment (i.e., the moment caused by in-plane forces) at the blade root - About the xc1-axis
RootMxb1 - Blade 1 edgewise moment (i.e., the moment caused by edgewise forces) at the blade root - About the xb1-axis

In my understanding the moments have different origins and so RootMxc1 is not at the blade root as the description in OutListParameters.xls says but at the apex of the cone of rotation (for coned rotors). Is my understanding correct?

Best regards,
René

Dear René,

RootMxc1 and RootMxb1 are both bending moments at the root of the blade. “c1” and “b1” refer to the orientation, not the origin, of the coordinate system that the bending moments are expressed in.

I hope that clarifies things.

Best regards,

Dear Jason,

thank you for clarification.

Best regards,
René