## Coordinate System in FAST

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Jason.Jonkman
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### Re: Coordinate System in FAST

Dear Neelabh,

Your image of the blade when viewed from the tip for a rotor spinning counterclockwise looks correct. And yes, a rotor spinning clockwise will have a blade that is the mirror image of what you are showing (the leading edge on the opposite side).

Force directed along x' for a counterclockwise rotor = Spn1FLxb1 from FAST for a clockwise rotor
Force directed along y' for a counterclockwise rotor = -Spn1FLyb1 from FAST for a clockwise rotor
Force directed along z' for a counterclockwise rotor = Spn1Flzb1 from FAST for a clockwise rotor
Moment about x' for a counterclockwise rotor = -Spn1MLxb1 from FAST for a clockwise rotor
Moment about y' for a counterclockwise rotor = Spn1MLyb1 from FAST for a clockwise rotor
Moment about z' for a counterclockwise rotor = Spn1MLzb1 from FAST for a clockwise rotor

(Notice the change of signs related to the force along y' and moment about x'.)

My point regarding TurbSim was that a user cannot specify the horizontal shear when running TurbSim. So, you wouldn't be using TurbSim if you were trying to model a specific horizontal shear. Regarding yaw, the wind direction and nacelle-yaw angle are not specified in TurbSim. If you wish to model a specific yaw error, than you'll have to reverse the sign on the nacelle-yaw angle in FAST because you must model the rotor spinning in the clockwise direction when the actual rotor spins counterclockwise (as described in the YawDyn's User's Guide).

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Neelabh.Gupta
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### Re: Coordinate System in FAST

Dear Jason,

Just to be on the same page, I will reiterate some of the points I understood. Please correct me, if I am wrong:

- x'/y'/z' don't need to follow right hand system for anticlockwise orientation of airfoil.

- While the flap-wise bending moment is positive when the blade is bent in downwind direction for clockwise rotating turbine, its positive when blade is bent in 'upwind' direction for anticlockwise rotating turbine as per the conventions you mentioned (to maintain consistency with y' axis).

- When I am not trying to match a specific horizontal shear (using TurbSim) and yaw error is zero, nothing needs to be changed for this particular scenario.

Sorry for theses long chain of posts, but this coordinate system seems so confusing when I relate it to reality.

Thanks a lot,
Neelabh

Jason.Jonkman
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### Re: Coordinate System in FAST

Dear Neelabh,

While I agree with your third point, I disagree with your first and second points.

I agree with the x'/y'/z' coordinate system you proposed in your Feb 22, 2013 post above whereby you reversed the direction of the y' axis to make the x'/y'/z' coordinate system right-handed.

The flapwise bending moment is positive when the blade deflects downwind whether the rotor spins clockwise or counterclockwise. For a clockwise rotor, the flapwise bending moment is the moment about the positive y' axis that is directed toward the trailing edge. For a counterclockwise rotor, the flapwise moment is moment about the positive y' axis that is directed toward the leading edge. As I said in my prior post, the moment about y' for a counterclockwise rotor = Spn1MLyb1 from FAST for a clockwise rotor.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Neelabh.Gupta
Posts: 45
Joined: Wed Sep 19, 2012 3:35 am
Organization: MECAL
Location: Netherlands

### Re: Coordinate System in FAST

Dear Jason,

I understand your points above and agree to them. One last thing, though (as we agreed upon, I am considering coordinate system from 22nd February post is correct):

While I understand that flapwise and axial moments remain same, what I don't get is why the sign has to be reversed for edgewise bending moment for anticlockwise oriented blade (Moment about x' for a counterclockwise rotor = -Spn1MLxb1 from FAST for a clockwise rotor). From what I know, it is caused by gravity forces and they would always act downwards. Now when the profile of the airfoil changes (LE comes to TE) for anticlockwise orientation of blade, gravity force would still act down, but now the nature of forces in TE and LE should swap. This would require keeping the same sign of edgewise bending moment. If we change the edgewise moment sign, then we have the same nature of LE and TE forces for both orientation of airfoil, which I don't understand?

Thanks and Best Regards,
Neelabh

Jason.Jonkman
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### Re: Coordinate System in FAST

Dear Neelabh,

I agree that the gravity moment would change the nature of the LE/TE forces when considering a clockwise versus counterclockwise blade in the same "time" on the clock (looking downwind). However, as we've discussed here, you must model your counterclockwise rotor spinning clockwise in FAST. To do this, you must mirror the azimuth convention when considering the azimuth angle. For example, the "2 o'clock" position (looking downwind) for a clockwise rotor represents the "10 o'clock" position (looking downwind) for a counterclockwise rotor (the blade is rotating downward in both cases). Because these different "times" represent the same event in both rotors, the sign of the moment about the x' axis is reversed.

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Neelabh.Gupta
Posts: 45
Joined: Wed Sep 19, 2012 3:35 am
Organization: MECAL
Location: Netherlands

### Re: 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

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### Re: Coordinate System in FAST

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

Jason.Jonkman
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### Re: Coordinate System in FAST

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Posts: 12
Joined: Wed Dec 18, 2013 3:41 am
Location: Spain

### Re: Coordinate System in FAST

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

Jason.Jonkman
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### Re: Coordinate System in FAST

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: viewtopic.php?f=4&t=716.

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Posts: 12
Joined: Wed Dec 18, 2013 3:41 am
Location: Spain

### Re: Coordinate System in FAST

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?
http://imgur.com/IZntRTN

Best regards,

Carmen

Jason.Jonkman
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### Re: Coordinate System in FAST

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Posts: 12
Joined: Wed Dec 18, 2013 3:41 am
Location: Spain

### Re: Coordinate System in FAST

Hi Jason,

This is the result from what I understood
http://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

Jason.Jonkman
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### Re: Coordinate System in FAST

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Posts: 12
Joined: Wed Dec 18, 2013 3:41 am
Location: Spain

### Re: Coordinate System in FAST

Hi Jason,

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

Best regards,

Carmen