BD RefChange: add short note about changes

Some minor cleanup to better match parent branch (deleted blank lines, etc)

modules/beamdyn/src/BeamDyn.f90

@@ -53,7 +53,12 @@ MODULE BeamDyn
    PUBLIC :: BD_UpdateGlobalRef      !< update the BeamDyn reference.  The reference for the calculations follows u%RootMotionMesh
                                                !  and therefore x%q must be updated from T -> T+DT to include the root motion from T->T+DT
 
-   ! do we change the reference frame at each State update?
+   ! The original formulation kept all states in the inertial reference frame.  This has been leading to convergence issues
+   ! when there is a large rotational change from the reference frame (i.e. large turbine yaw, large blade pitch).  During
+   ! the development of the tight coupling algorithm for OpenFAST, we decided to try changing all the states in BeamDyn to
+   ! follow the moving BladeRootMotion mesh.  This requires changing the states after an UpdateStates call to be relative to
+   ! the new BladeRootMotion mesh orientation and position.
+   ! Upadate the reference frame after each State update (or use the old method)?
    LOGICAL, PARAMETER :: ChangeRefFrame = .true.
 
 CONTAINS
@@ -6825,7 +6830,6 @@ END SUBROUTINE BD_WriteMassStiffInFirstNodeFrame
 !> Update the state information to follow the blade rootmotion mesh.
 !!    - move the state information in x from the previous reference frame at time T (u(2)%rootmotion) to the new reference frame at T+dt (u(1)%rootmation)
 !!    - the GlbRot, GlbPos, and Glb_crv values are stored as otherstates and updated
-!!    - 
 subroutine BD_UpdateGlobalRef(u, p, x, OtherState, ErrStat, ErrMsg)
    type(BD_InputType),           intent(in   ) :: u          !< Inputs at utimes
    type(BD_ParameterType),       intent(in   ) :: p          !< Parameters

modules/beamdyn/src/BeamDyn_Subs.f90

@@ -656,8 +656,6 @@ SUBROUTINE Set_BldMotion_NoAcc(p, x, OtherState, m, y)
 
                ! Calculate the translational displacement of each GLL node in the FAST coordinate system,
                ! referenced against the DCM of the blade root at T=0.
-!            y%BldMotion%TranslationDisp(1:3,temp_id2) = OtherState%GlbPos - y%BldMotion%Position(1:3,temp_id2) + &
-!                                                        matmul(OtherState%GlbRot, p%uuN0(1:3, j, i) + x%q(1:3, temp_id))
             y%BldMotion%TranslationDisp(1:3,temp_id2) = OtherState%GlbPos + matmul(OtherState%GlbRot, p%uuN0(1:3, j, i) + x%q(1:3, temp_id)) - &
                                                         y%BldMotion%Position(1:3,temp_id2)
 
@@ -719,9 +717,12 @@ SUBROUTINE Set_BldMotion_NoAcc(p, x, OtherState, m, y)
          ENDDO
       ENDDO
 
+
+
    CASE (BD_MESH_STATIONS)
    END SELECT
 
+
 END SUBROUTINE Set_BldMotion_NoAcc
 !-----------------------------------------------------------------------------------------------------------------------------------
 !> This routine calculates values for the y%BldMotion mesh.