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Fix BD + AD linearization indexing (negative damping results) #2060

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merged 3 commits into from
Feb 29, 2024
Merged

Fix BD + AD linearization indexing (negative damping results) #2060

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8e140b9
Lin: incorrect BD index in dUdy for AD motion
andrew-platt Feb 28, 2024
db8d795
Lin: sum BD blade root motion to hub moment m_ud term (consistent wit…
andrew-platt Feb 28, 2024
8269984
Update reg test 5MW_Land_BD_Linear_Aero
andrew-platt Feb 28, 2024
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7 changes: 4 additions & 3 deletions modules/openfast-library/src/FAST_Lin.f90
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Original file line number Diff line number Diff line change
Expand Up @@ -3331,8 +3331,8 @@ SUBROUTINE Linear_ED_InputSolve_dy( p_FAST, y_FAST, SrvD, u_ED, y_ED, y_AD, u_AD

! ED translation displacement-to-ED moment transfer (dU^{ED}/dy^{ED}) from BD root-to-ED hub load transfer:
ED_Start = Indx_u_ED_Hub_Start(u_ED, y_FAST) + u_ED%HubPtLoad%NNodes*3 ! start of u_ED%HubPtLoad%Moment field (skip forces)
ED_Out_Start = Indx_y_ED_Hub_Start(y_ED, y_FAST) ! start of y_ED%HubMotion%TranslationDisp field
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Note that the documentation has this as a summation of all the terms, not terms at each ED root node to the hub. So this change makes it more consistent with the documentation and doesn't affect the resulting frequencies.

This also brings this closer in line with the tight coupling implementation.

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Based on the rest of the code, it looks like the original implementation where it connected to the blade root was a bug.... that apparently didn't affect the results. Thanks for fixing it!

DO k=1,p_FAST%nBeams
ED_Out_Start = Indx_y_ED_BladeRoot_Start(y_ED, y_FAST, k) ! start of y_ED%BladeRootMotion(k)%TranslationDisp field
call SumBlockMatrix( dUdy, MeshMapData%BD_P_2_ED_P(k)%dM%m_ud, ED_Start, ED_Out_Start)
END DO

Expand Down Expand Up @@ -3733,8 +3733,9 @@ SUBROUTINE Linear_AD_InputSolve_NoIfW_dy( p_FAST, y_FAST, u_AD, y_ED, BD, MeshMa

DO k=1,p_FAST%nBeams
AD_Start = Indx_u_AD_Blade_Start(u_AD, y_FAST, k) ! start of u_AD%BladeMotion(k)%TranslationDisp field
BD_Out_Start = y_FAST%Lin%Modules(Module_BD)%Instance(k)%LinStartIndx(LIN_OUTPUT_COL)

BD_Out_Start = y_FAST%Lin%Modules(MODULE_BD)%Instance(k)%LinStartIndx(LIN_OUTPUT_COL) & ! start of BD%y(k)%BldMotion%TranslationDisp field
+ BD%y(k)%ReactionForce%NNodes * 6 ! 2 fields with 3 components
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This change was the problem that likely was giving rise to negative damping.


CALL Assemble_dUdy_Motions(BD%y(k)%BldMotion, u_AD%rotors(1)%BladeMotion(k), MeshMapData%BDED_L_2_AD_L_B(k), AD_Start, BD_Out_Start, dUdy, skipRotAcc=.true.)
END DO

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2 changes: 1 addition & 1 deletion reg_tests/r-test
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Submodule r-test updated 2 files
+689 −689 glue-codes/openfast/5MW_Land_BD_Linear_Aero/5MW_Land_BD_Linear_Aero.1.lin
+ − glue-codes/openfast/5MW_Land_BD_Linear_Aero/5MW_Land_BD_Linear_Aero.outb