hkross · Sep 28, 2026 · Sep 28, 2026 · Sep 28, 2026 · Sep 28, 2026 · Sep 28, 2026
diff --git a/docs/source/user/aerodyn/bibliography.bib b/docs/source/user/aerodyn/bibliography.bib
@@ -107,6 +107,28 @@ @techreport{ad-Murray:2011
   institution={49th AIAA Aerospace Sciences Meeting, Orlando, Florida}
 }
 
+@article{ad-Bangga:2020,
+    author = {Galih Bangga and Thorsten Lutz and Matthias Arnold},
+    title = {An improved second-order dynamic stall model for wind turbine airfoils},
+    year = {2020},
+    journal = {Wind Energy Science},
+    volume = {5},
+    number = {3},
+    pages = {1037--1058},
+    doi = {10.5194/wes-5-1037-2020}
+}
+
+@article{ad-Bangga:2023,
+    author = {Galih Bangga and Jason Parkinson and William Collier},
+    title = {Development and Validation of the IAG Dynamic Stall Model in State-Space Representation for Wind Turbine Airfoils},
+    year = {2023},
+    journal = {Energies},
+    volume = {16},
+    number = {10},
+    pages = {3994},
+    doi = {10.3390/en16103994}
+}
+
 
 @article{ad-hammam2022,
     author = {Mohamed M. Hammam and David H. Wood},

diff --git a/docs/source/user/aerodyn/examples/ad_primary_example.dat b/docs/source/user/aerodyn/examples/ad_primary_example.dat
@@ -48,7 +48,7 @@ False         SectAvg           - Use sector averaging (flag)
 "unused"      OLAFInputFileName  - Input file for OLAF [used only when Wake_Mod=3]
 ======  Unsteady Airfoil Aerodynamics Options  ====================================================
 True          AoA34              - Sample the angle of attack (AoA) at the 3/4 chord or the AC point {default=True} [always used]
-          3   UA_Mod             - Unsteady Aero Model Switch (switch) {0=Quasi-steady (no UA), 2=B-L Gonzalez, 3=B-L Minnema/Pierce, 4=B-L HGM 4-states, 5=B-L HGM+vortex 5 states, 6=Oye, 7=Boeing-Vertol}
+          3   UA_Mod             - Unsteady Aero Model Switch (switch) {0=Quasi-steady (no UA), 2=B-L Gonzalez, 3=B-L Minnema/Pierce, 4=B-L HGM 4-states, 5=B-L HGM+vortex 5 states, 6=Oye, 7=Boeing-Vertol, 9=IAG}
 True          FLookup            - Flag to indicate whether a lookup for f' will be calculated (TRUE) or whether best-fit exponential equations will be used (FALSE); if FALSE S1-S4 must be provided in airfoil input files (flag) [used only when UA_Mod=2 or UA_Mod=3]
           3   IntegrationMethod  - Switch to indicate which integration method UA uses (1=RK4, 2=AB4, 3=ABM4, 4=BDF2)
           0   UAStartRad         - Starting radius for dynamic stall (fraction of rotor radius [0.0,1.0]) [used only when UA_Mod>0; if line is missing UAStartRad=0]

diff --git a/docs/source/user/aerodyn/input.rst b/docs/source/user/aerodyn/input.rst
@@ -358,8 +358,13 @@ Most ``UA_Mod`` will require `AoA34` to be set to true. But when using quasi-ste
 - ``5``: 5-states continuous-time B-L model similar to HGM with an additional state for vortex generation
 - ``6``: 1-state continuous-time developed by Oye
 - ``7``: discrete-time Boeing-Vertol (BV) model
+- ``9``: 5-states continuous-time IAG model (first-order, state-space), with a vortex state
 
-Linearization is supported with ``UA_Mod=4,5,6`` (which use continuous-time states) but not with the other models. The different models are described in :numref:`AD_UA`.
+Linearization is supported with ``UA_Mod=4,5,6,9`` (which use continuous-time states) but not with the other models. The different models are described in :numref:`AD_UA`.
+
+.. note::
+    For ``UA_Mod=9``, only the first four states are linearized; the fifth (vortex) state
+    is excluded, exactly as for ``UA_Mod=5``.
 
 .. note::
     Link to old inputs: If `UA_Mod>0`, then this is equivalent to the old `AFAeroMod=2`. 
@@ -841,6 +846,37 @@ or calculating it based on the polar coefficient data in the airfoil table:
 -  ``C_lalpha`` is the slope of the 2D normal lift coefficient curve
    in the linear region; Used for ``UA_Mod=4,6``.
 
+-  ``Ka`` is the impulsive (non-circulatory) normal-force gain of the IAG
+   model; used only when ``UA_Mod=9``. If the keyword ``DEFAULT`` is entered
+   in place of a numerical value, ``Ka`` is set to 0.75.
+
+-  ``Kv`` is the amplitude of the vortex center-of-pressure travel in the
+   IAG model; used only when ``UA_Mod=9``. If the keyword ``DEFAULT`` is
+   entered in place of a numerical value, ``Kv`` is set to 0.2.
+
+-  ``dCNdA`` is the slope of the static :math:`C_n` curve used by the IAG
+   model; used only when ``UA_Mod=9``. If the keyword ``DEFAULT`` is entered
+   in place of a numerical value, ``dCNdA`` is obtained from a linear fit to
+   the attached-flow region of the supplied polar. Note that ``UA_Mod=9``
+   uses ``dCNdA`` rather than ``C_nalpha`` or ``C_lalpha``, and that it
+   builds its **own** separation function by tabulating
+   :math:`dCNdA\,\sin(\alpha-\alpha_0)`, rather than using the
+   piecewise-linear fully-attached curve shared by the HGM/HGMV models. The
+   ``f_st`` values written for ``UA_Mod=9`` are therefore **not** on a common
+   basis with those written for ``UA_Mod=5``.
+
+-  ``CnMax`` and ``CnMin`` are not airfoil-file inputs. They are the maximum
+   and minimum of the static :math:`C_n` polar, calculated at initialization,
+   and they set the positive and negative vortex-shedding thresholds of the
+   IAG model. They are reported in the unsteady-aero summary table written
+   when ``UA_Mod=9`` and ``SumPrint = TRUE``. A reported value of
+   ``999.00000`` for ``CnMax`` (and ``-999.00000`` for ``CnMin``) is a
+   sentinel, not a physical coefficient: it means the vortex logic has been
+   disabled for that table, either because the model is not IAG or because
+   the polar has no identifiable stall peak, as for a cylinder-like table at
+   the blade root. Because :math:`|C_n|` can never reach 999, the shedding
+   trigger simply never fires and the model runs without vortex lift.
+
 -  ``T_f0`` is the initial value of the time constant associated with
    *Df* in the expressions of *Df* and *f’*; if the keyword ``DEFAULT`` is
    entered in place of a numerical value, ``T_f0`` is set to 3.0;

diff --git a/docs/source/user/aerodyn/theory_ua.rst b/docs/source/user/aerodyn/theory_ua.rst
@@ -494,6 +494,75 @@ The moment coefficient is calculated based on values at the aerodynamic center a
 where :math:`\alpha_{50}` is computed the same way as :math:`\alpha_{34}` (using the velocity at the aerodynamic center and the rotational rate of the airfoil) but using the distance from the aerodynamic center to the mid-chord (see :numref:`ua_notations`).
 
 
+IAG model (UAMod=9)
+~~~~~~~~~~~~~~~~~~~
+
+The IAG model :cite:`ad-Bangga:2020,ad-Bangga:2023` is a five-state, continuous-time
+Beddoes-Leishman variant. Like the HGMV model (``UA_Mod=5``) it carries two downwash memory
+states :math:`x_1,x_2`, a lagged attached-flow state :math:`x_3`, a separation state
+:math:`x_4`, and a vortex state :math:`x_5`. Linearization is supported, but only
+:math:`x_1`-:math:`x_4` are linearized; the vortex state is excluded.
+
+The model differs from HGM/HGMV in four ways that matter when comparing output:
+
+**1. It uses a sinusoidal attached-flow curve, not a piecewise-linear one.** The circulatory
+normal force is
+
+.. math::
+   C_N^C = \frac{\mathrm{d}C_N}{\mathrm{d}\alpha}\,\sin(\alpha_E - \alpha_0)
+
+so the model is driven by the airfoil input ``dCNdA`` rather than by ``C_nalpha`` or
+``C_lalpha``, neither of which it reads. ``dCNdA`` is obtained by a linear fit to the
+attached-flow region of the supplied polar unless the user overrides it.
+
+**2. It builds its own separation function.** The ``f_st`` column used by ``UA_Mod=9`` is
+tabulated by inverting the squared-Kirchhoff relation against the sinusoidal curve above,
+
+.. math::
+   C_N^f = \frac{\mathrm{d}C_N}{\mathrm{d}\alpha}
+           \left(\frac{1+\sqrt{x_4}}{2}\right)^{2}\sin(\alpha_F-\alpha_0) + C_N^I
+
+rather than against the piecewise-linear ``FullyAttached`` curve shared by the HGM/HGMV
+models. **The** ``f_st`` **values written for** ``UA_Mod=9`` **are therefore not on a common
+basis with those written for** ``UA_Mod=5``, and the two should not be compared directly.
+
+**3. The effective and separation angles do not coincide.** Because the attached-flow
+relation is sinusoidal, inverting it for :math:`\alpha_F` gives
+:math:`\alpha_F = \alpha_0 + \sin(\alpha_E-\alpha_0)` under the model's linearized
+inversion, so :math:`\alpha_F \neq \alpha_E` in general. For HGM the two collapse to the
+same value.
+
+**4. The** ``Cc`` **output channel is a different quantity than for the other models.**
+When unsteady-aero outputs are enabled, ``UA_Mod=9`` writes the *viscous* chordwise force
+evaluated at :math:`\alpha_F` with :math:`C_{d0}` removed,
+
+.. math::
+   C_c = C_l^{st}(\alpha_F)\sin\alpha_F - \left[C_d^{st}(\alpha_F) - C_{d0}\right]\cos\alpha_F
+
+which is the :math:`C_T^D` of the IAG formulation. The HGM and HGMV models instead write
+:math:`C_c = C_l\sin\alpha - C_d\cos\alpha` evaluated at the instantaneous :math:`\alpha`
+and **without** subtracting :math:`C_{d0}`. The two are not the same quantity and **the**
+``Cc`` **column should not be compared between** ``UA_Mod=9`` **and the other models.**
+This affects the reported channel only: :math:`C_l`, :math:`C_d` and :math:`C_m` are the
+quantities passed to the rest of AeroDyn, and for the IAG model those are reconstructed
+from :math:`C_N^D` and :math:`C_T^D` before they are returned, so loads are unaffected.
+
+The impulsive (non-circulatory) normal force is scaled by the airfoil input ``Ka``, and the
+vortex center-of-pressure travel by ``Kv``. Vortex shedding is triggered on the calculated
+``CnMax``/``CnMin`` thresholds rather than on ``Cn1``/``Cn2``, which the model does not use.
+``CnMax`` and ``CnMin`` are the extrema of the static :math:`C_n` polar and are reported in
+the unsteady-aero summary table; values of :math:`\pm 999` there are a sentinel indicating
+that no stall peak could be identified for that table and that vortex shedding is therefore
+disabled for it (see :numref:`airfoil_data_input_file`).
+
+Beyond roughly 30 degrees of incidence the separated-flow construction loses validity, so
+the dynamic :math:`C_d` and :math:`C_m` are faded linearly back to their static values
+between 30 and 45 degrees. This is applied before the shared UA cutout blend, so the two
+compose. :math:`C_l`, :math:`C_n` and :math:`C_c` are deliberately not faded separately,
+since :math:`C_l` is reconstructed from :math:`C_n` and :math:`C_c` and blending it
+independently would make the three mutually inconsistent.
+
+
 
 
 

diff --git a/docs/source/user/api_change.rst b/docs/source/user/api_change.rst
@@ -18,6 +18,8 @@ The generalized support-structure (GS) influence model was added to AeroDyn. Thi
 
 OLAF now selects the regularization function for the vortex particles separately from the vortex segments. The new ``RegFunctionPart`` input on line 26 of the OLAF input file controls the particle kernel, and ``RegFunction`` now applies only to the segments. Previously the particle kernel was inferred from ``RegFunction``: any regularized value gave an exponential particle kernel, while ``RegFunction=0`` gave an unregularized one. The default ``RegFunctionPart=1`` reproduces the exponential kernel, so results are unchanged for decks with ``RegFunction`` greater than 0. Decks with ``RegFunction=0`` will change: the particles are now regularized with the exponential kernel unless ``RegFunctionPart=0`` is also set. Setting ``RegFunctionPart=0`` restores an unregularized particle kernel, but with ``VelocityMethod=2`` the results will still differ from previous versions because the particle-tree far-field cutoff is no longer padded by the particle core radius when the particle kernel is unregularized.
 
+The IAG dynamic stall model was added to AeroDyn as a new ``UA_Mod=9`` option. This change is **backwards compatible**: no lines are added to or removed from the AeroDyn primary input file (only the comment text of the existing ``UA_Mod`` line changes), and the three new airfoil-file inputs it introduces (``Ka``, ``Kv``, ``dCNdA``) are all optional and accept ``DEFAULT``, so existing airfoil files continue to work unchanged. The new airfoil inputs are placed after ``x_cp_bar`` and before ``UACutout``, and are read only when ``UA_Mod=9``.
+
 ============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
 Added in OpenFAST `5.1.0`
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

diff --git a/modules/aerodyn/src/AeroDyn.f90 b/modules/aerodyn/src/AeroDyn.f90
@@ -4894,8 +4894,13 @@ SUBROUTINE ValidateInputData( InitInp, InputFileData, NumBl, calcCrvAngle, ErrSt
          call SetErrStat( ErrID_Fatal, 'Wake_Mod must be 0 or 1 for linearization.', ErrStat, ErrMsg, RoutineName )
       endif
 
-      if (InputFileData%UA_Init%UAMod /= UA_None .and. InputFileData%UA_Init%UAMod /= UA_HGM .and. InputFileData%UA_Init%UAMod /= UA_HGMV .and. InputFileData%UA_Init%UAMod /= UA_OYE) then
-         call SetErrStat( ErrID_Fatal, 'UA_Mod must be 0, 4, 5, or 6 for linearization.', ErrStat, ErrMsg, RoutineName )
+         ! NOTE: this is a chain of exclusions, not an allowed-list, so a new model is
+         ! REJECTED for linearization unless it is named here. UA_IAG is admitted on the
+         ! same basis as UA_HGMV: both carry a 5th (vortex) state that is not linearized,
+         ! and both linearize x1-x4 only (see p%lin_nx in UA_SetParameters). UA_HGMV360 is
+         ! deliberately still absent.
+      if (InputFileData%UA_Init%UAMod /= UA_None .and. InputFileData%UA_Init%UAMod /= UA_HGM .and. InputFileData%UA_Init%UAMod /= UA_HGMV .and. InputFileData%UA_Init%UAMod /= UA_OYE .and. InputFileData%UA_Init%UAMod /= UA_IAG) then
+         call SetErrStat( ErrID_Fatal, 'UA_Mod must be 0, 4, 5, 6, or 9 for linearization.', ErrStat, ErrMsg, RoutineName )
       end if
 
       select case(InputFileData%DBEMT_Mod)

diff --git a/modules/aerodyn/src/AeroDyn_IO.f90 b/modules/aerodyn/src/AeroDyn_IO.f90
@@ -981,7 +981,7 @@ SUBROUTINE ParsePrimaryFileInfo( PriPath, InitInp, InputFile, RootName, NumBlade
    ! UAMod (Legacy)
    call ParseVar( FileInfo_In, CurLine, "UAMod", UAMod_Old, ErrStat2, ErrMsg2, UnEc )
    UAModProvided = legacyInputPresent('UAMod', CurLine, ErrStat2, ErrMsg2, 'UA_Mod=0 (AFAeroMod=1), UA_Mod>1 (AFAeroMod=2 and UA_Mod=UAMod')
-   ! UA_Mod - Unsteady Aero Model Switch (switch) {0=Quasi-steady (no UA),  2=Gonzalez's variant (changes in Cn,Cc,Cm), 3=Minnema/Pierce variant (changes in Cc and Cm)} 
+   ! UA_Mod - Unsteady Aero Model Switch (switch) {0=Quasi-steady (no UA),  2=Gonzalez's variant (changes in Cn,Cc,Cm), 3=Minnema/Pierce variant (changes in Cc and Cm), 4=HGM, 5=HGM+vortex, 6=Oye, 7=Boeing-Vertol, 9=IAG}
    call ParseVar( FileInfo_In, CurLine, "UA_Mod", InputFileData%UA_Init%UAMod, ErrStat2, ErrMsg2, UnEc )
    if (newInputMissing('UA_Mod', CurLine, errStat2, errMsg2)) then
       ! We'll deal with it when we deal with AFAeroMod
@@ -2089,6 +2089,8 @@ SUBROUTINE AD_PrintSum( InputFileData, p, p_AD, u, y, NumBlades, BladeInputFileD
          Msg = 'Stieg Oye dynamic stall model'
       case (UA_BV)
          Msg = 'Boeing-Vertol dynamic stall model (e.g. used in CACTUS)'
+      case (UA_IAG)
+         Msg = 'IAG dynamic stall model (first-order, state-space)'
       case default
          Msg = 'unknown'
    end select