HydroDyn Input/Output meshes: change from SWL to MSL for consistency with OF glue code #831
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Internally, the HydroDyn calculations use the Still Water Line as the Z=0 reference, and this was being applied to the Morison input and output mesh data. This was a bug. The mesh data should have been referenced to the Mean Sea Level, which corresponds to OpenFAST's inertial coordinate system. This code, corrects the node positions for the Morison input/output meshes. Addionally, the Marine growth depth values in the HD input file were being shifting using the MSL2SWL parameter, but this was also a bug. The new code no longer shifts these input file depth values. Internally, this change requires the Morison module's CalcOutput routine to shift the input mesh positions to the Still Water Line reference system for the hydrodynamics equations.
Internally, the HydroDyn calculations use the Still Water Line as the Z=0 reference, and this was being applied to the Morison input and output mesh data. This was a bug. The mesh data should have been referenced to the Mean Sea Level, which corresponds to OpenFAST's inertial coordinate system. This code, corrects the node positions for the Morison input/output meshes. Addionally, the Marine growth depth values in the HD input file were being shifting using the MSL2SWL parameter, but this was also a bug. The new code no longer shifts these input file depth values. Internally, this change requires the Morison module's CalcOutput routine to shift the input mesh positions to the Still Water Line reference system for the hydrodynamics equations.
… F/HD_MSL2SWL_fix
jjonkman
approved these changes
Sep 22, 2021
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I've reviewed these changes with @HaymanConsulting and the implementation looks good to me.
andrew-platt
changed the title
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This pull request is ready to merge. Please examine why AeroDyn regression is failing for MacOS [@rafmudaf ]. We should add a future test for MSL2SWL.
Feature or improvement description
In OpenFAST, the reference positions of all nodes of all module-level input and output meshes are stored in the inertial frame coordinate system, which for offshore wind turbines is defined as the intersection of the undisplaced tower centerline and the mean sea level (MSL). These reference positions are used in the mesh-mapping within OpenFAST to transfer motions and loads between modules, including rigid-body behavior between nodes that are offset across modules.
Within HydroDyn, there is an input MSL2SWL that defines the offset between the MSL and the still water level (SWL), e.g., to represent the change in water depth associated with tidal variations or storm surge. This change in water depth does not change the inertial frame coordinate system, which is relative to MSL regardless. However, in HydroDyn, it is important to know the position of hydrodynamic nodes relative to the SWL, e.g. for the calculation of local wave kinematics and hydrostatic pressure. It has been discovered that the module-level input and output meshes within HydroDyn stored the reference position relative to SWL instead of relative to MSL. This allowed HydroDyn to use the proper relative water depth in its internal calculations, but leads to offset in the mesh mappings between HydroDyn and other modules like ElastoDyn and SubDyn. Thus, any module-to-module coupling will result in errors when MSL2SWL is nonzero.
Related issue, if one exists
Bug report: Incorrect Treatment of MSL2SWL within HydroDyn #830
Impacted areas of the software
HydroDyn, Morison
Additional supporting information
None
Test results, if applicable
This feature was never part of the OpenFAST test suite