# 4.2.1.1. Introduction

AeroDyn is a time-domain wind turbine aerodynamics module that is coupled in the OpenFAST multi-physics engineering tool to enable aero-elastic simulation of horizontal-axis turbines. AeroDyn can also be driven as a standalone code to compute wind turbine aerodynamic response uncoupled from OpenFAST. When coupled to OpenFAST, AeroDyn can also be linearized as part of the linearization of the full coupled solution (linearization is not available in standalone mode). AeroDyn was originally developed for modeling wind turbine aerodynamics. However, the module equally applies to the hydrodynamics of marine hydrokinetic (MHK) turbines (the terms “wind turbine”, “tower”, “aerodynamics” etc. in this document imply “MHK turbine”, “MHK support structure”, “hydrodynamics” etc. for MHK turbines). Additional physics important for MHK turbines, not applicable to wind turbines, computed by AeroDyn include a cavitation check and buoyant forces and moments on the blades, tower, hub, and nacelle. This documentation pertains version of AeroDyn in the OpenFAST github repository. The AeroDyn version released of OpenFAST 1.0.0 is most closely related to AeroDyn version 15 in the legacy version numbering. AeroDyn version 15 was a complete overhaul from earlier version of AeroDyn. AeroDyn version 15 and newer follows the requirements of the FAST modularization framework.

AeroDyn calculates aerodynamic loads on both the blades and tower. Aerodynamic calculations within AeroDyn are based on the principles of actuator lines, where the three-dimensional (3D) flow around a body is approximated by local two-dimensional (2D) flow at cross sections, and the distributed pressure and shear stresses are approximated by lift forces, drag forces, and pitching moments lumped at a node in a 2D cross section. Analysis nodes are distributed along the length of each blade and tower, the 2D forces and moment at each node are computed as distributed loads per unit length, and the total 3D aerodynamic loads are found by integrating the 2D distributed loads along the length. When AeroDyn is coupled to OpenFAST, the blade and tower analysis node discretization may be independent from the discretization of the nodes in the structural modules. The actuator line approximations restrict the validity of the model to slender structures and 3D behavior is either neglected, captured through corrections inherent in the model (e.g., tip-loss, hub-loss, or skewed-wake corrections), or captured in the input data (e.g., rotational augmentation corrections applied to airfoil data).

AeroDyn assumes the turbine geometry consists of a one-, two-, or three-bladed rotor atop a single tower. While the undeflected tower is assumed to be straight and vertical, an undeflected blade may consider out-of-plane curvature and in-plane sweep. For blades, the 2D cross sections where the aerodynamic analysis take place may follow the out-of-plane curvature, but in-plane sweep is assumed to be accomplished by shearing, rather than rotation of the 2D cross section. Aerodynamic imbalances are possible through the use of geometrical differences between each blade.

When AeroDyn is coupled to OpenFAST, AeroDyn receives the instantaneous (possibly displaced/deflected) structural position, orientation, and velocities of analysis nodes in the tower, hub, and blades. As with curvature and sweep, the 2D cross sections where the blade aerodynamic analysis takes place will follow the out-of-plane deflection, but in-plane deflection is assumed to be accomplished by shearing, rather than rotation of the 2D cross section. AeroDyn also receives the local freestream (undisturbed) fluid velocities at the tower and blade nodes. (Fluid and structural calculations take place outside of the AeroDyn module and are passed as inputs to AeroDyn by the driver code.) The fluid and structural motions are provided at each coupling time step and then AeroDyn computes the aerodynamic loads on the blade and tower nodes and returns them back to OpenFAST as part of the aero-elastic calculation. In standalone mode, the inputs to AeroDyn are prescribed by a simple driver code, without aero-elastic coupling.

AeroDyn consists of six submodels: (1) rotor wake/induction, (2) blade airfoil aerodynamics, (3) tower influence on the fluid local to the blade nodes, (4) tower drag, (5) aeroacoustics, and (6) buoyancy on the blades, hub, nacelle, and tower (for MHK turbines). Nacelle, hub, and tail-vane fluid influence and loading (with the exception of nacelle and hub buoyant loads) and wake and array effects between multiple turbines in a wind plant are not yet available in AeroDyn. Aeroacoustics are not available for MHK turbines.

For operating wind and MHK turbine rotors, AeroDyn calculates the influence of the wake via induction factors based on the quasi-steady Blade-Element/Momentum (BEM) theory, which requires an iterative nonlinear solve (implemented via Brent’s method). By quasi-steady, it is meant that the induction reacts instantaneously to loading changes. The induction calculation, and resulting inflow velocities and angles, are based on flow local to each analysis node of each blade, based on the relative velocity between the fluid and structure (including the effects of local inflow skew, shear, turbulence, tower flow disturbances, and structural motion, depending on features enabled). The Glauert’s empirical correction (with Buhl’s modification) replaces the linear momentum balance at high axial induction factors. In the BEM solution, Prandtl tip-loss, Prandtl hub-loss, and Pitt and Peters skewed-wake are all 3D corrections that can optionally be applied. When the skewed-wake correction is enabled, it is applied after the BEM iteration. Additionally, the calculation of tangential induction (from the angular momentum balance), the use of drag in the axial-induction calculation, and the use of drag in the tangential-induction calculation are all terms that can optionally be included in the BEM iteration (even when drag is not used in the BEM iteration, drag is still used to calculate the nodal loads once the induction has been found). The wake/induction calculation can be bypassed altogether for the purposes of modeling rotors that are parked or idling, in which case the inflow velocity and angle are determined purely geometrically. During linearization analyses with AeroDyn coupled to OpenFAST and BEM enabled, the wake can be assumed to be frozen (i.e., the axial and tangential induces velocities, \(-V_x a\) and \(V_y a'\), are fixed at their operating-point values during linearization) or the induction can be recalculated during linearization using BEM theory. Dynamic wake that accounts for induction dynamics as a result of transient conditions are not yet available in AeroDyn v15 and newer.

The blade airfoil aerodynamics can be steady or unsteady, except in the
case that a cavitation check is requested for MHK, in which case only
steady aerodynamics are supported. In the steady model, the supplied
static airfoil data — including the lift force, drag force, and optional
pitching moment and minimum pressure coefficients versus angle of attack
(AoA) — are used directly to calculate nodal loads. The
AirfoilPrep preprocessor can be
used to generate the needed static airfoil data based on uncorrected 2D
data (based, e.g., on airfoil tests in a wind tunnel or
XFoil), including
features to blend data between different airfoils, apply 3D rotational
augmentation, and extrapolate to high AoA. The unsteady airfoil
aerodynamic (UA) models account for flow hysteresis, including unsteady
attached flow, trailing-edge flow separation, dynamic stall, and flow
reattachment. The UA models can be considered as 2D dynamic corrections
to the static airfoil response as a result of time-varying inflow
velocities and angles. Three semi-empirical UA models are available: the
original theoretical developments of Beddoes-Leishman (B-L), extensions
to the B-L developed by González, and extensions to the B-L model
developed by Minnema/Pierce. **While all of the UA models are documented
in this manual, the original B-L model is not yet functional. Testing
has shown that the González and Minnema/Pierce models produce reasonable
hysteresis of the normal force, tangential force, and pitching-moment
coefficients if the UA model parameters are set appropriately for a
given airfoil, Reynolds number, and/or Mach number. However, the
results will differ a bit from earlier versions of AeroDyn, (which was
based on the Minnema/Pierce extensions to B-L) even if the default UA
model parameters are used, due to differences in the UA model logic
between the versions. We recommend that users run test cases with
uniform wind inflow and fixed yaw error (e.g., through the standalone
AeroDyn driver) to examine the accuracy of the normal force, tangential
force, and pitching-moment coefficient hysteresis and to adjust the UA
model parameters appropriately.** The airfoil-, Reynolds-, and
Mach-dependent parameters of the UA models may be derived from the
static airfoil data. These UA models are valid for small to moderate AoA
under normal rotor operation; the steady model is more appropriate under
parked or idling conditions. The static airfoil data is always used in
the BEM iteration; when UA is enabled, it is applied after the BEM
iteration and after the skewed-wake correction. The UA models are not
set up to support linearization, so, UA must be disabled during
linearization analyses with AeroDyn coupled to OpenFAST. The interpolation
of airfoil data based on Reynolds number or aerodynamic-control setting
(e.g., flaps) is not yet available in AeroDyn v15 and newer.

The influence of the tower on the fluid flow local to the blade is based on a potential-flow and/or a tower-shadow model. The potential-flow model uses the analytical potential-flow solution for flow around a cylinder to model the tower dam effect on upwind rotors. In this model, the freestream (undisturbed) flow at each blade node is disturbed based on the location of the blade node relative to the tower and the tower diameter, including lower velocities upstream and downstream of the tower, higher velocities to the left and right of the tower, and cross-stream flow. The Bak correction can optionally be included in the potential-flow model, which augments the tower upstream disturbance and improves the tower wake for downwind rotors based on the tower drag coefficient. The tower shadow model can also be enabled to account for the tower wake deficit on downwind rotors. This model includes an axial flow deficit on the freestream fluid at each blade node dependent on the location of the blade node relative to the tower and the tower diameter and drag coefficient, based on the work of Powles. Both tower-influence models are quasi-steady models, in that the disturbance is applied directly to the freestream fluid at the blade nodes without dynamics, and are applied within the BEM iteration.

The aerodynamic load on the tower is based directly on the tower diameter and drag coefficient and the local relative fluid velocity between the freestream (undisturbed) flow and structure at each tower analysis node (including the effects of local shear, turbulence, and structural motion, depending on features enabled). The tower drag load calculation is quasi-steady and independent from the tower influence on flow models.

The primary AeroDyn input file defines modeling options, environmental conditions (except freestream flow), airfoils, tower nodal discretization and properties, tower, hub, and nacelle buoyancy properties, as well as output file specifications. Airfoil data properties are read from dedicated inputs files (one for each airfoil) and include coefficients of lift force, drag force, and optional pitching moment and minimum pressure versus AoA, as well as UA model parameters. (Minimum pressure coefficients versus AoA are also included in the airfoil input files in case that a cavitation check is requested.) Blade nodal discretization, geometry, twist, chord, airfoil identifier, and buoyancy properties are likewise read from separate input files (one for each blade).

Section 4.2.1.3 describes the AeroDyn input files. Section 4.2.1.4 discusses the output files generated by AeroDyn; these include an echo file, summary file, and the results file. Section 4.2.1.5 provides modeling guidance when using AeroDyn. Example input files are included in Section 4.2.1.11.1. A summary of available output channels are found Section 4.2.1.11.2.