[Bug] HydroDyn strip-theory member-based coefficients and output locations #3044
Description
Bug description
In HydroDyn, the starting joint of a strip-theory member is always assumed to be below the ending joint, so that the member axial direction, pointing from the starting joint to the ending joint, has a positive vertical z-component. If the member is horizontal, the axial direction is assumed to point toward the +x-direction. Finally, if the member is aligned with the y-axis, the axial direction is assumed to be in the +y-direction.
If a member defined in the input file does not follow this convention, HydroDyn swaps the starting and ending joints of the member internally to satisfy the assumed convention. The section geometries of the starting and ending joints are also swapped. However, when using member-based hydrodynamic coefficients (model 3), the hydrodynamic coefficients associated with the start and the end of the member are not swapped. As a result, the variation of hydrodynamic coefficients along the member, if any, is effectively backward.
A similar issue also affects strip-theory member output locations defined in the input file. If HydroDyn swaps the start and end of a member internally, the output locations defined by the user are effectively measured from the end point toward the starting joint as defined by the user in the input file.
To Reproduce
Any strip-theory member using member-based hydrodynamic coefficients (model 3) or with output locations defined will have this issue if the starting and ending joints do not follow the HydroDyn member orientation convention.
Expected behavior
If a member is reversed in HydroDyn internally, the member-based hydrodynamic coefficients and the member output locations should also be reversed for consistency.
Temporary workaround
Ensure the starting and ending joints of each member in HydroDyn follow the convention described above.
OpenFAST Version
This bug affects all existing versions of OpenFAST, both released and under development.