OpenFAST · andrew-platt · Mar 10, 2026 · Dec 30, 2025 · Feb 19, 2026 · Feb 26, 2026
diff --git a/docs/changelogs/v4.2.1.md b/docs/changelogs/v4.2.1.md
@@ -0,0 +1,141 @@
+**Feature or improvement description**
+Pull request to merge `dev` into `main` for release version 4.2.1
+
+See the milestone and project pages for additional information
+
+    https://github.com/OpenFAST/openfast/milestone/27
+
+No test results change
+
+### Release checklist:
+- [ ] Update the documentation version in docs/conf.py
+- [ ] Update the versions in docs/source/user/api\_change.rst
+- [ ] Update version info in openfast\_io/pyproject.toml (`openfast_io` package)
+- [ ] Update version info in glue-codes/python/pyproject.toml (`pyOpenFAST` package for testing)
+- [ ] Verify readthedocs builds correctly
+- [ ] Create an annotated tag in OpenFAST during merge (mark as most recent if necessary)
+- [ ] Create a merge commit in r-test and add a corresponding annotated tag
+- [ ] Upload Docker image
+- [ ] Compile executables for Windows builds
+    - [ ] `AeroDisk_Driver_x64.exe`
+    - [ ] `AeroDyn_Driver_x64.exe`
+    - [ ] `AeroDyn_Driver_x64_OpenMP.exe`
+    - [ ] `AeroDyn_Inflow_c_binding_x64.dll`
+    - [ ] `AeroDyn_Inflow_c_binding_x64_OpenMP.dll`
+    - [ ] `BeamDyn_Driver_x64.exe`
+    - [ ] `DISCON.dll (x64)`
+    - [ ] `DISCON_ITIBarge.dll (x64)`
+    - [ ] `DISCON_OC3Hywind.dll (x64)`
+    - [ ] `FAST.Farm_x64.exe`
+    - [ ] `FAST.Farm_x64_OMP.exe`
+    - [ ] `FAST_SFunc.mexw64`
+    - [ ] `HydroDynDriver_x64.exe`
+    - [ ] `HydroDyn_C_Binding_x64.dll`
+    - [ ] `IinflowWind_c_binding_x64.dll`
+    - [ ] `InflowWind_Driver_x64.exe`
+    - [ ] `InflowWind_Driver_x64_OpenMP.exe`
+    - [ ] `MoorDyn_Driver_x64.exe`
+    - [ ] `MoorDyn_c_binding_x64.dll`
+    - [ ] `OpenFAST-Simulink_x64.dll`
+    - [ ] `openfast_x64.exe`
+    - [ ] `SeaStateDriver_x64.exe`
+    - [ ] `SeaState_c_binding_x64.dll`
+    - [ ] `SimpleElastoDyn_x64.exe`
+    - [ ] `SubDyn_x64.exe`
+    - [ ] `Turbsim_x64.exe`
+    - [ ] `UnsteadyAero_x64.exe`
+
+
+
+# Release Overview
+------------------
+
+
+
+### Contribution Acknowledgements
+
+We are grateful for a first time code contributions from @IrisMeasure.
+
+
+
+# Changelog (from 4.2.1)
+------------------------
+
+
+## Solvers
+
+### FAST.Farm
+
+#3243 Add high res windslice at `T=t_low - DT_high` (Fix for FAST.Farm issue #3183) (@andrew-platt)
+
+
+
+## Modules
+
+## AeroDyn
+
+#### AeroDyn Driver / AeroDyn\_Inflow\_C\_Bindings interface
+
+#3214 [BugFix] ADI c-bind: comment out code that causes segfaults with interface debugging (@andrew-platt)
+
+#3227 [BugFix] AD: DTAero from input file was ignored (@andrew-platt)
+
+#3230 [BugFix] AD driver: hub acceleration value incorrect in RotMotionType==1 (@andrew-platt)
+
+
+### NWTC-Library
+
+#3123 Switch from xPPTRF to xPOTRF to improve TurbSim speed on macOS (@IrisMeasure)
+
+#3229 Re-add the xPPTRF routines (rename xPOTRF routines from #3123) (@andrew-platt)
+
+
+### SeaState
+
+#3235 backport of #3231 - SS WaveTp logic (@andrew-platt, @RBergua)
+
+#3236 Add SeaSt\_C\_GetFluidDynP routine to SeaState\_C\_Binding interface (@andrew-platt)
+
+
+### TurbSim
+
+#3224 Fix TurbSim compile with OpenMP and IFX compiler (@deslaughter)
+
+
+
+## Testing and input file processing
+
+### Regression and Unit testing
+
+#3228 regtest: check if destination dirs exist before copyTree call (@andrew-platt)
+
+
+
+## Input file changes
+
+No input file changes since v4.2.0
+
+
+## Known issues
+There are several issues that have not been addressed in this release due to time constraints, but will be addressed in future releases.  These include:
+
+- No visualization of rectangular members from _HydroDyn_ or _SubDyn_ through the VTK output options
+- Missing and broken features from several c-binding library interfaces:
+   - the _AeroDyn\_Inflow\_c-binding_ library interface does not allow for coupling to the tower.  This will require an interface update.
+   - the _HydroDyn\_c-binding_ library interface does not currently support vizualization.  This will require an interface update.
+   - the `InitNodePositions` input to _HydroDyn\_c-binding_ library interface does not currently work with any non-zero `x` or `y` coordinates (non-zero `z` is ok)
+   - the _MoorDyn\_c-binding_ library interface does not currently support vizualization.  This will require an interface update.
+- Documentation on the new _pyOpenFAST_ module is incomplete.  Partial documentation exists on how to use it in regression testing, but no documentation or examples exist on using it to call c-bindings modules from Python.
+- Documentation is incomplete for _HydroDyn_, _SubDyn_, and a few other modules.
+
+
+# Precompiled Windows Binaries
+The binary files in this release were built with the Visual Studio solution files distributed with OpenFAST (not using cmake), using
+
+- Intel Fortran Essentials 2025.3.0.333
+- Microsoft Visual Studio 2022 Version 17.14.23.
+- MATLAB 2025.2.999 (R2025b)
+- Executables with `_OpenMP` or `_OMP` in the name are built with OpenMP libraries and linked with dynamic libraries.
+   - You will need [this Intel Fortran redistributable package](https://registrationcenter-download.intel.com/akdlm/IRC_NAS/0dc56e76-d2c0-4bb8-9c83-c2ee3952b855/w_ifx_runtime_p_2025.2.1.1001.exe) installed to use these executables if you do not already have Intel Fortran OneAPI 2024 installed. See the installation instructions [here](https://software.intel.com/content/www/us/en/develop/articles/redistributable-libraries-for-intel-c-and-fortran-2022-compilers-for-windows.html).
+
+**The other OpenFAST executables DO NOT require these redistributable libraries to be installed. Instead, they were built with static libraries.**
diff --git a/docs/conf.py b/docs/conf.py
@@ -139,7 +139,7 @@ def runDoxygen(sourcfile, doxyfileIn, doxyfileOut):
 # The short X.Y version.
 version = f'4.2'
 # The full version, including alpha/beta/rc tags.
-release = f'v4.2.0'
+release = f'v4.2.1'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.

diff --git a/docs/source/user/api_change.rst b/docs/source/user/api_change.rst
@@ -10,6 +10,13 @@ The line number corresponds to the resulting line number after all changes are i
 Thus, be sure to implement each in order so that subsequent line numbers are correct.
 
 
+OpenFAST v4.2.0 to OpenFAST v4.2.1
+----------------------------------
+
+No input file changes were made.
+
+
+
 OpenFAST v4.1.x to OpenFAST v4.2.0
 ----------------------------------
 

diff --git a/glue-codes/fast-farm/src/FASTWrapper.f90 b/glue-codes/fast-farm/src/FASTWrapper.f90
@@ -44,7 +44,7 @@ MODULE FASTWrapper
 
    PUBLIC :: FWrap_t0                             !  call to compute outputs at t0 [and initialize some more variables]
    PUBLIC :: FWrap_Increment                      !  call to update states to n+1 and compute outputs at n+1
-   PUBLIC :: FWrap_SetInputs  
+   PUBLIC :: FWrap_SetWindTStart
    PUBLIC :: FWrap_CalcOutput  
 
 
@@ -119,7 +119,7 @@ SUBROUTINE FWrap_Init( InitInp, u, p, x, xd, z, OtherState, y, m, Interval, Init
    ExternInitData%windGrid_n(1) = InitInp%nX_high
    ExternInitData%windGrid_n(2) = InitInp%nY_high
    ExternInitData%windGrid_n(3) = InitInp%nZ_high
-   ExternInitData%windGrid_n(4) = InitInp%n_high_low
+   ExternInitData%windGrid_n(4) = InitInp%n_high_low+1      ! include a step at t-dt_high
 
    ExternInitData%windGrid_delta(1) = InitInp%dX_high
    ExternInitData%windGrid_delta(2) = InitInp%dY_high
@@ -392,7 +392,7 @@ SUBROUTINE FWrap_Increment( t, n, u, p, x, xd, z, OtherState, y, m, ErrStat, Err
    !ELSE
    !   
          ! set the inputs needed for FAST
-      !call FWrap_SetInputs(u, m, t)   <<< moved up into FAST.Farm FARM_UpdateStates
+      !call FWrap_SetWindTStart(u, m, t)   <<< moved up into FAST.Farm FARM_UpdateStates
 
       ! call FAST p%n_FAST_low times   (p%n_FAST_low is simply the number of steps to make per wrapper call. It is affected by MooringMod)
       do n_ss = 1, p%n_FAST_low 
@@ -434,7 +434,7 @@ SUBROUTINE FWrap_t0( u, p, x, xd, z, OtherState, y, m, ErrStat, ErrMsg )
    ErrMsg  = ''
 
       ! set the inputs needed for FAST:
-   call FWrap_SetInputs(u, m, 0.0_DbKi)
+   call FWrap_SetWindTStart(u, m, 0.0_DbKi)
 
       ! compute the FAST t0 solution:
    call FAST_Solution0_T(m%Turbine, ErrStat2, ErrMsg2 )
@@ -685,16 +685,18 @@ SUBROUTINE FWrap_CalcOutput(p, u, y, m, ErrStat, ErrMsg)
 END SUBROUTINE FWrap_CalcOutput
 !----------------------------------------------------------------------------------------------------------------------------------
 !> This subroutine sets the inputs needed before calling an instance of FAST
-SUBROUTINE FWrap_SetInputs(u, m, t)
+SUBROUTINE FWrap_SetWindTStart(u, m, t)
 
    TYPE(FWrap_InputType),           INTENT(INOUT)  :: u           !< Inputs at t
    TYPE(FWrap_MiscVarType),         INTENT(INOUT)  :: m           !< Misc variables for optimization (not copied in glue code)
    REAL(DbKi),                      INTENT(IN   )  :: t           !< current simulation time
 
    ! set the 4d-wind-inflow input array (a bit of a hack [simplification] so that we don't have large amounts of data copied in multiple data structures):
-      m%Turbine%IfW%p%FlowField%Grid4D%TimeStart = t
+   ! NOTE: the wind data starts at `t - DT_high` as one extra slice of wind data is added at start.  If AeroDyn is updated to not require the `t-DT_high`
+   !        timestep, this can be changed
+   m%Turbine%IfW%p%FlowField%Grid4D%TimeStart = t - m%Turbine%IfW%p%FlowField%Grid4D%delta(4)
 
-END SUBROUTINE FWrap_SetInputs
+END SUBROUTINE FWrap_SetWindTStart
 !----------------------------------------------------------------------------------------------------------------------------------
 END MODULE FASTWrapper
 !**********************************************************************************************************************************
diff --git a/glue-codes/fast-farm/src/FAST_Farm_Subs.f90 b/glue-codes/fast-farm/src/FAST_Farm_Subs.f90
@@ -1204,7 +1204,7 @@ subroutine FARM_UpdateStates(t, n, farm, ErrStat, ErrMsg)
 
    ! set the inputs needed for FAST (these are slow-varying so can just be done once per farm time step)
    do nt = 1,farm%p%NumTurbines
-      call FWrap_SetInputs(farm%FWrap(nt)%u, farm%FWrap(nt)%m, t)
+      call FWrap_SetWindTStart(farm%FWrap(nt)%u, farm%FWrap(nt)%m, t)
    end do
 
 

diff --git a/glue-codes/python/pyproject.toml b/glue-codes/python/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pyOpenFAST"
-version = "4.2.0"
+version = "4.2.1"
 description = "Python interface to OpenFAST FAST Library and physics modules."
 readme = "README.md"
 requires-python = ">=3.9"

diff --git a/modules/aerodyn/src/AeroDyn.f90 b/modules/aerodyn/src/AeroDyn.f90
@@ -370,7 +370,7 @@ subroutine AD_Init( InitInp, u, p, x, xd, z, OtherState, y, m, Interval, InitOut
       if (Failed()) return;
       ! -----------------------------------------------------------------
       ! Read the AeroDyn blade files, or copy from passed input
-   call ReadInputFiles( InitInp%InputFile, InputFileData, interval, p%RootName, NumBlades, AeroProjMod, UnEcho, calcCrvAngle, ErrStat2, ErrMsg2 )
+   call ReadInputFiles( InitInp%InputFile, InputFileData, p%RootName, NumBlades, AeroProjMod, UnEcho, calcCrvAngle, ErrStat2, ErrMsg2 )
       if (Failed()) return;
 
       ! override some parameters to simplify for aero maps

diff --git a/modules/aerodyn/src/AeroDyn_Driver_Subs.f90 b/modules/aerodyn/src/AeroDyn_Driver_Subs.f90
@@ -820,7 +820,7 @@ subroutine Set_Mesh_Motion(nt, dvr, ADI, FED, errStat, errMsg)
          call interpTimeValue(wt%hub%motion, time, wt%hub%iMotion, hubMotion)
          !print*,hubMotion
          wt%hub%rotSpeed  = hubMotion(2)
-         wt%hub%rotAcc    = hubMotion(2)
+         wt%hub%rotAcc    = hubMotion(3)
          wt%hub%azimuth = MODULO(hubMotion(1)*R2D, 360.0_ReKi )
       else if (wt%hub%motionType == idHubMotionUserFunction) then
          ! We call a user-defined function to determined the azimuth, speed (and potentially acceleration...)

diff --git a/modules/aerodyn/src/AeroDyn_IO.f90 b/modules/aerodyn/src/AeroDyn_IO.f90
@@ -588,14 +588,12 @@ end subroutine Calc_WriteOutput_FVW
 
 END SUBROUTINE Calc_WriteOutput
 !----------------------------------------------------------------------------------------------------------------------------------
-SUBROUTINE ReadInputFiles( InputFileName, InputFileData, Default_DT, OutFileRoot, NumBlades, AeroProjMod, UnEcho, calcCrvAngle, ErrStat, ErrMsg )
+SUBROUTINE ReadInputFiles( InputFileName, InputFileData, OutFileRoot, NumBlades, AeroProjMod, UnEcho, calcCrvAngle, ErrStat, ErrMsg )
 ! This subroutine reads the input file and stores all the data in the AD_InputFile structure.
 ! It does not perform data validation.
 !..................................................................................................................................
 
       ! Passed variables
-   REAL(DbKi),              INTENT(IN)    :: Default_DT      ! The default DT (from glue code)
-
    CHARACTER(*),            INTENT(IN)    :: InputFileName   ! Name of the input file
    CHARACTER(*),            INTENT(IN)    :: OutFileRoot     ! The rootname of all the output files written by this routine.
 
@@ -624,7 +622,6 @@ SUBROUTINE ReadInputFiles( InputFileName, InputFileData, Default_DT, OutFileRoot
    ErrStat = ErrID_None
    ErrMsg  = ''
 
-   InputFileData%DTAero = Default_DT  ! the glue code's suggested DT for the module (may be overwritten in ReadPrimaryFile())
    calcCrvAngle = .false.             ! initialize in case of early return
 
       ! get the blade input-file data

diff --git a/modules/aerodyn/src/AeroDyn_Inflow_C_Binding.f90 b/modules/aerodyn/src/AeroDyn_Inflow_C_Binding.f90
@@ -935,18 +935,20 @@ subroutine ShowPassedData()
       call WrScr("       ADinputFilePassed_C            "//TmpFlag )
       call WrScr("       ADinputFileString_C (ptr addr) "//trim(Num2LStr(LOC(ADinputFileString_C))) )
       call WrScr("       ADinputFileStringLength_C      "//trim(Num2LStr( ADinputFileStringLength_C )) )
-      if (ADinputFilePassed==0_c_int) then
-         i = index(ADinputFileString, char(0))     ! skip anything after c_null_char
-         call WrScr("       ADinputFileString_C            "//ADinputFileString(1:i))
-      endif
+!FIXME: This causes a seg fault
+!      if (ADinputFilePassed==0_c_int) then
+!         i = index(ADinputFileString, char(0))     ! skip anything after c_null_char
+!         call WrScr("       ADinputFileString_C            "//ADinputFileString(1:i))
+!      endif
       TmpFlag="F";   if (IfWinputFilePassed==1_c_int) TmpFlag="T"
       call WrScr("       IfWinputFilePassed_C           "//TmpFlag )
       call WrScr("       IfWinputFileString_C (ptr addr)"//trim(Num2LStr(LOC(IfWinputFileString_C))) )
       call WrScr("       IfWinputFileStringLength_C     "//trim(Num2LStr( IfWinputFileStringLength_C )) )
-      if (IfWinputFilePassed==0_c_int) then
-         i = index(IfWinputFileString, char(0))     ! skip anything after c_null_char
-         call WrScr("       IfWinputFileString_C           "//trim(IfWinputFileString(1:i)))
-      endif
+!FIXME: This causes a seg fault
+!      if (IfWinputFilePassed==0_c_int) then
+!         i = index(IfWinputFileString, char(0))     ! skip anything after c_null_char
+!         call WrScr("       IfWinputFileString_C           "//trim(IfWinputFileString(1:i)))
+!      endif
       call WrScr("       OutRootName                    "//trim(OutRootName) )
       call WrScr("   Interpolation")
       call WrScr("       InterpOrder_C                  "//trim(Num2LStr( InterpOrder_C )) )