Coordinate System

Wind & Water General
1

Dear OpenFAST team,
I would like to know whether the coordinate system used in OpenFAST is the same as the one in ‘FAST Users Guide’ released in August 2005

Many thanks to the team

2

Adding to the above, I would like to whether my understandings are correct or not regarding OpenFAST coordinate system. Help me if I am wrong.

  1. Inertial coordinate system origins at the bottom of spar.
  2. The platform (6 DOFs) responses calculated in OpenFAST are the responses at the platform COG

Thanks in advance

3

Dear Akheel,

The coordinate systems documented in the old FAST v7 User’s Guide are the same coordinate systems used within the ElastoDyn module of OpenFAST. Other modules may use other coordinate systems, as described in their respective documentation.

The inertial frame coordinate system used in OpenFAST has the origin at the intersection of the undisplaced tower centerline and mean sea level, with the X-axis along the zero-degree wind/wave direction, the Z-axis vertical (opposite gravity), and the Y-axis directed to the left when looking along the zero-degree wind/wave direction.

The platform DOFs are defined relative to the inertial frame, with an origin determined by PtfmRefzt (when PtfmRefzt = 0, the origin of the platform DOFs is coincident with the origin of the inertial frame coordinate system). This origin is typically not the platform CG.

Best regards,

4

Hi Jason,

I am curious about the difference between inertial frame coordinate system and tower base coordinate system for distributed springs (DS) model modelling foundation in FAST v7, even though I have read FAST user’s guide. And where is the coordinate origin?
Thanks in advance.

Best regards,
Kevin

5

Dear @Kevin.Chang,

In FAST v7, the entire support structure (including the pile in the foundation) is represented by the “tower” model, and with the distributed springs (DS) foundation model, the platform reference point is located at the free end of the pile within the foundation. The tower-base coordinate system orients with this free end of the pile and its origin as the free end. The inertial frame coordinate system does not move with the structure; its origin is the mean sea level at the undisplaced tower centerline.

Best regards,

6

Dear Jason,

Thanks for your reply. I find that TwHtTDxt is equal to TwHtTPxi for default rigid foundation, but TwHtTPxi is larger than TwHtTDxt for distributed sprigns (DS) model. Could you tell the relationship between TwHtTPxi and TwHtDxt? And is TwHtTPxi the actual translational position in x-direction for initial static state (t=0)? Thanks a lot

Best regards,
Kevin

7

Dear @Kevin.Chang,

TwHt#TDxt is the local translation deflection (TD) of a given tower output node (#) relative to the undeflected tower, expressed in tower-base coordinates (t). By “undeflected tower”, I mean the elastic deflection of the tower is neglected, but the rigid-body motion of the tower through the platform DOFs may still exist (TwHt#TDxt would be zero if the elastic deflection of the tower is zero, even if the platform displacement was nozero).

TwHt#TPxi is the global translation position (TP) of a given tower output node (#), expressed in inertial-frame coordinates (i). Both the rigid-body motion of the platform DOFs and the elastic deflection of the tower are considered in the position calculation.

For a rigid foundation, the platform DOFs are disabled, and so, the tower-base and inertial-frame coordinate systems have the same orientation and the origin is the same in the x and y directions, so, TwHt#TDxt should equal TwHt#TPxi.

Best regards,

8

Dear Jason,

Thanks for your clarification.

Best regards,
Kevin