Coordinate System in FAST

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SungWon.Kim
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Re: Coordinate System in FAST

Postby SungWon.Kim » Thu Jul 10, 2014 1:01 pm

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

Convention.png
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Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Thu Jul 10, 2014 9:45 pm

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,
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

SungWon.Kim
Posts: 10
Joined: Tue Jun 03, 2014 3:38 pm
Organization: Houston Offshore Engineering
Location: US - Texas

Re: Coordinate System in FAST

Postby SungWon.Kim » Fri Jul 11, 2014 7:43 am

Dear Jason,

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

Code: Select all

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

OC3.png
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Comparison.png
Comparison.png (33.8 KiB) Viewed 4396 times

Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Fri Jul 11, 2014 9:46 pm

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,
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

Attila.Veress
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Re: Coordinate System in FAST

Postby Attila.Veress » Wed Feb 11, 2015 5:32 am

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.
FAST coordinate system.jpg
FAST coordinate system.jpg (15.05 KiB) Viewed 4221 times


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?

Thank you in advance!

Best regards,
Attila Veress
Last edited by Attila.Veress on Wed Feb 11, 2015 6:35 am, edited 1 time in total.

Attila.Veress
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Re: Coordinate System in FAST

Postby Attila.Veress » Wed Feb 11, 2015 6:27 am

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 https://wind.nrel.gov/forum/wind/viewtopic.php?f=4&t=592 , 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
Last edited by Attila.Veress on Wed Feb 11, 2015 7:11 am, edited 1 time in total.

Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Wed Feb 11, 2015 6:40 am

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,
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

Rene.Mebus
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Location: Germany - Baden Wuerttemberg

Coned and blade coordinate systems

Postby Rene.Mebus » Tue Jun 27, 2017 4:15 am

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?

Thank you for your answer.

Best regards,
René

Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Tue Jun 27, 2017 9:30 am

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,
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

Rene.Mebus
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Organization: Hochschule Aalen
Location: Germany - Baden Wuerttemberg

Re: Coordinate System in FAST

Postby Rene.Mebus » Tue Jun 27, 2017 11:22 pm

Dear Jason,

thank you for clarification.

Best regards,
René

Arnold.Ramsland
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Re: Coordinate System in FAST

Postby Arnold.Ramsland » Mon Apr 02, 2018 4:41 pm

Hello,

I'd like to follow up on a post dated June 27, 2016 to René where it was stated that ''c1" and "b1" refer to the orientation, not the origin, of the coordinate system by requesting a further clarification of the shaft (xs, ys, zs) and azimuth (xa, ya and za) coordinate systems. According to the FAST Users Guide, the shaft and azimuth coordinate systems are located at the origin, which is defined as the intersection of the yn-/zn-plane and the rotor axis. According to the diagrams shown, this intersection occurs at the yaw axis at the top of the tower. Many of the output variables indicate rotations about the ys, zs, ya and za axes. Examples include:
TeetVya: Rotor teeter angular velocity about the ya-axis
TeetAya: Rotor teeter angular acceleration about the ya-axis
LSSTipMya: Rotating LSS bending moment at the shaft tip (teeter pin for two-bladed turbines, apex of rotationfor three-bladed turbines) About the ya-axis
LSSTipMza: Rotating LSS bending moment at the shaft tip(teeter pin for two-bladed turbines, apex of rotation for three-bladed turbines) About the za-axis
LSSTipMys: Nonrotating LSS bending moment at the shaft tip (teeter pin for two-bladed turbines, apex of rotation
for three-bladed turbines) About the ys-axis
LSSTipMzs: Nonrotating LSS bending moment at the shaft tip (teeter pin for two-bladed turbines, apex of rotation
for three-bladed turbines) About the zs-axis
LSSGagMys: Nonrotating LSS bending moment at the shaft's strain gage (shaft strain gage located by input ShftGagL) About the ys-axis
LSSGagMzs Nonrotating LSS bending moment at the shaft's strain gage (shaft strain gage located by input ShftGagL) About the zs-axis

I have always assumed that the origin of the ys, zs, ya, and za axes would move along the xs axis to the designated position, e.g, shaft tip, teeter pin, strain gauge location, however I cannot find that stated in the FAST Users Guide and would appreciate your clarification. This interpretation would be consistent with the earlier post regarding "C1" and "b1" where the ys, zs, ya and za axes in the output variables listed above refer to the orientation, rather than the origin of the coordinate systems.

Sincerely,
Arnold Ramsland

Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Mon Apr 02, 2018 5:12 pm

Dear Arnold,

Yes, I agree with your understanding that the origin of the ys, zs, ya, and za axes would move along the xa / xs axis to the designated position, e.g, shaft tip, teeter pin, strain gauge location depending on the output parameter of interest.

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

Arnold.Ramsland
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Re: Coordinate System in FAST

Postby Arnold.Ramsland » Tue Apr 03, 2018 6:45 pm

Thank you,

Arnold

Yingyi.Liu
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Re: Coordinate System in FAST

Postby Yingyi.Liu » Mon Nov 19, 2018 10:13 am

Jason.Jonkman wrote: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,

Can I confirm the following points again for an upwind wind turbine? (check it together with the blade system in the following picture):

(1) NREL-5MW baseline wind turbine is an upwind wind turbine, and its blade rotation direction is clockwise.
(2) The aerodynamic lift that drives rotation has a component acting along negative y, which is opposite to the rotation tangential direction.
(3) Aerodynamic thrust acts along negative x.

Are the above statements correct? If they are correct, I can't understand why the aerodynamic lift force points opposite to the blade rotation direction, could you explain the reason?

Thanks.

Best regards,
Yingyi Liu
Attachments
Blade system.jpg
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Jason.Jonkman
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Re: Coordinate System in FAST

Postby Jason.Jonkman » Mon Nov 19, 2018 10:26 am

Dear Yingyi.Lin,

I agree with points (1) and (2), but point (3) and your picture are not correct. Regarding (3), the thrust for an upwind wind turbine should be directed along positive x. Regarding your picture, it would be correct if you rotate each blade 180 degrees around its z axis, such that x points downwind and the rotor spins clockswise (when looking downwind) opposite to the direction of y.

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


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