# 4.2.1.2. Input Files¶

The user configures the aerodynamic model parameters via a primary AeroDyn input file, as well as separate input files for airfoil and blade data. When used in standalone mode, an additional driver input file is required. The AeroDyn driver and driver input file are detailed in Section 4.2.1.5. The driver file specifies initialization inputs normally provided to AeroDyn by OpenFAST, as well as the per-time-step inputs to AeroDyn.

As an example, the driver.dvr file is the main driver, the input.dat is the primary input file, the blade.dat file contains the blade geometry data, and the airfoil.dat file contains the airfoil angle of attack, lift, drag, moment coefficients, and pressure coefficients. Example input files are included in Section 4.2.1.9.

No lines should be added or removed from the input files, except in tables where the number of rows is specified and comment lines in the AeroDyn airfoil data files.

## 4.2.1.2.1. Units¶

AeroDyn uses the SI system (kg, m, s, N). Angles are assumed to be in radians unless otherwise specified.

## 4.2.1.2.2. AeroDyn Primary Input File¶

The primary AeroDyn input file defines modeling options, environmental conditions (except freestream flow), airfoils, tower nodal discretization and properties, as well as output file specifications.

The file is organized into several functional sections. Each section corresponds to an aspect of the aerodynamics model. A sample AeroDyn primary input file is given in Section 4.2.1.9.

The input file begins with two lines of header information which is for your use, but is not used by the software.

### 4.2.1.2.2.1. General Options¶

Set the Echo flag to TRUE if you wish to have AeroDyn echo the contents of the AeroDyn primary, airfoil, and blade input files (useful for debugging errors in the input files). The echo file has the naming convention of OutRootFile.AD.ech. OutRootFile is either specified in the I/O SETTINGS section of the driver input file when running AeroDyn standalone, or by the OpenFAST program when running a coupled simulation.

DTAero sets the time step for the aerodynamic calculations. For accuracy and numerical stability, we recommend that DTAero be set such that there are at least 200 azimuth steps per rotor revolution. However, when AeroDyn is coupled to OpenFAST, OpenFAST may require time steps much smaller than this rule of thumb. If UA is enabled while using very small time steps, you may need to recompile AeroDyn in double precision to avoid numerical problems in the UA routines. The keyword DEFAULT for DTAero may be used to indicate that AeroDyn should employ the time step prescribed by the driver code (OpenFAST or the standalone driver program).

Set WakeMod to 0 if you want to disable rotor wake/induction effects or 1 to include these effects using the (quasi-steady) BEM theory model. When WakeMod is set to 2, a dynamic BEM theory model (DBEMT) is used (also referred to as dynamic inflow or dynamic wake model). When WakeMod is set to 3, the free vortex wake model is used, also referred to as OLAF (see Section 4.2.2). WakeMod cannot be set to 2 or 3 during linearization analyses.

Set AFAeroMod to 1 to include steady blade airfoil aerodynamics or 2 to enable UA; AFAeroMod must be 1 during linearization analyses with AeroDyn coupled to OpenFAST.

Set TwrPotent to 0 to disable the potential-flow influence of the tower on the fluid flow local to the blade, 1 to enable the standard potential-flow model, or 2 to include the Bak correction in the potential-flow model.

Set the TwrShadow to 0 to disable to the tower shadow model, 1 to enable the Powles tower shadow model, or 2 to use the Eames tower shadow model. These models calculate the influence of the tower on the flow local to the blade based on the downstream tower shadow model. If the tower influence from potential flow and tower shadow are both enabled, the two influences will be superimposed.

Set the TwrAero flag to TRUE to calculate fluid drag loads on the tower or FALSE to disable these effects.

During linearization analyses with AeroDyn coupled OpenFAST and BEM enabled (WakeMod = 1), set the FrozenWake flag to TRUE to employ frozen-wake assumptions during linearization (i.e. to fix the axial and tangential induces velocities, and, at their operating-point values during linearization) or FALSE to recalculate the induction during linearization using BEM theory.

Set the CavitCheck flag to TRUE to perform a cavitation check for MHK turbines or FALSE to disable this calculation. If CavitCheck is TRUE, AFAeroMod must be set to 1 because the cavitation check does not function with unsteady airfoil aerodynamics.

Set the CompAA flag to TRUE to run aero-acoustic calculations. This option is only available for WakeMod = 1 or 2. See section Section 4.2.3 for information on how to use this feature.

The AA_InputFile is used to specify the input file for the aeroacoustics sub-module. See Section 4.2.3 for information on how to use this feature.

### 4.2.1.2.2.2. Environmental Conditions¶

Environmental conditions are now specified in driver input files but are left in the AeroDyn primary input file for legacy compatibility. Use the keyword DEFAULT to pass in values specified by the driver input file. Otherwise, values given in the AeroDyn primary input file will overwrite those given in the driver input file. AirDens specifies the fluid density and must be a value greater than zero; a typical value is around 1.225 kg/m3 for air (wind turbines) and 1025 kg/m3 for seawater (MHK turbines). KinVisc specifies the kinematic viscosity of the fluid (used in the Reynolds number calculation); a typical value is around 1.460E-5 m2/s for air (wind turbines) and 1.004E-6 m2/s for seawater (MHK turbines). SpdSound is the speed of sound in the fluid (used to calculate the Mach number within the unsteady airfoil aerodynamics calculations); a typical value is around 340.3 m/s for air. The last two parameters in this section are only used when CavitCheck = TRUE for MHK turbines. Patm is the atmospheric pressure above the free surface; typically around 101,325 Pa. Pvap is the vapor pressure of the fluid; for seawater this is typically around 2,000 Pa.

The input parameters in this section are not used when WakeMod = 0.

SkewMod determines the skewed-wake correction model. Set SkewMod to 1 to use the uncoupled BEM solution technique without an additional skewed-wake correction. Set SkewMod to 2 to include the Pitt/Peters correction model. The coupled model SkewMod= 3 is not available in this version of AeroDyn.

SkewModFactor is used only when SkewMod = 2. Enter a scaling factor to use in the Pitt/Peters correction model, or enter "default" to use the default value of $$\frac{15 \pi}{32}$$.

Set TipLoss to TRUE to include the Prandtl tip-loss model or FALSE to disable it. Likewise, set HubLoss to TRUE to include the Prandtl hub-loss model or FALSE to disable it.

Set TanInd to TRUE to include tangential induction (from the angular momentum balance) in the BEM solution or FALSE to neglect it. Set AIDrag to TRUE to include drag in the axial-induction calculation or FALSE to neglect it. If TanInd = TRUE, set TIDrag to TRUE to include drag in the tangential-induction calculation or FALSE to neglect it. 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,

IndToler sets the convergence threshold for the iterative nonlinear solve of the BEM solution. The nonlinear solve is in terms of the inflow angle, but IndToler represents the tolerance of the nondimensional residual equation, with no physical association possible. When the keyword DEFAULT is used in place of a numerical value, IndToler will be set to 5E-5 when AeroDyn is compiled in single precision and to 5E-10 when AeroDyn is compiled in double precision; we recommend using these defaults. MaxIter determines the maximum number of iterations steps in the BEM solve. If the residual value of the BEM solve is not less than or equal to IndToler in MaxIter, AeroDyn will exit the BEM solver and return an error message.

### 4.2.1.2.2.4. Dynamic Blade-Element/Momentum Theory Options¶

The input parameters in this section are used only when WakeMod = 2.

Set DBEMT_Mod to 1 for the constant-tau1 model, set DBEMT_Mod to 2 to use a model where tau1 varies with time, or set DBEMT_Mod to 3 to use a continuous-state model with constant tau1.

If DBEMT_Mod=1 (constant-tau1 model) or DBEMT_Mod=3 (continuous-state constant-tau1 model), set tau1_const to the time constant to use for DBEMT.

### 4.2.1.2.2.5. OLAF – cOnvecting LAgrangian Filaments (Free Vortex Wake) Theory Options¶

The input parameters in this section are used only when WakeMod = 3.

The settings for the free vortex wake model are set in the OLAF input file described in Section 4.2.2.4. OLAFInputFileName is the filename for this input file.

### 4.2.1.2.2.6. Unsteady Airfoil Aerodynamics Options¶

The input parameters in this section are used only when AFAeroMod = 2.

UAMod determines the UA model. It has the following options:

• 1: the original theoretical developments of B-L (not currently functional),

• 2: the extensions to B-L developed by González

• 3: the extensions to B-L developed by Minnema/Pierce

• 4: a continuous-state model developed by Hansen, Gaunna, and Madsen (HGM)

• 5: a model similar to HGM with an additional state for vortex generation

• 6: Oye’s dynamic stall model

• 7: Boeing-Vertol model

The models are described in Section 4.2.1.7.

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 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.

FLookup determines how the nondimensional separation distance value, f’, will be calculated. When FLookup is set to TRUE, f’ is determined via a lookup into the static lift-force coefficient and drag-force coefficient data. Using best-fit exponential equations (FLookup = FALSE) is not yet available, so FLookup must be TRUE in this version of AeroDyn. Note, FLookup is not used when UAMod=4 or UAMod=5.

UAStartRad is the starting rotor radius where dynamic stall will be turned on. Enter a number between 0 and 1, representing a fraction of rotor radius, to indicate where unsteady aerodynamics should begin turning on. If this line is omitted from the input file, UAStartRad will default to 0 (turning on at the blade root). All blade nodes that are located at a rotor radius less than UAStartRad will have unsteady aerodynamics turned off for the entire simulation.

UAEndRad is the ending rotor radius where dynamic stall will be turned on. Enter a number between 0 and 1, representing a fraction of rotor radius, to indicate the last rotor radius where unsteady aerodynamics should be turned on. If this line is omitted from the input file, UAEndRad will default to 1 (the blade tip). All blade nodes that are located at a rotor radius greater than UAEndRad will have unsteady aerodynamics turned off for the entire simulation.

### 4.2.1.2.2.7. Airfoil Information¶

This section defines the airfoil data input file information. The airfoil data input files themselves (one for each airfoil) include tables containing coefficients of lift force, drag force, and optionally pitching moment, and minimum pressure versus AoA, as well as UA model parameters, and are described in Section 4.2.1.2.3.

The first 5 lines in the AIRFOIL INFORMATION section relate to the format of the tables of static airfoil coefficients within each of the airfoil input files. InCol_Alfa, InCol_Cl, InCol_Cd, InCol_Cm, and InCol_Cpmin are column numbers in the tables containing the AoA, lift-force coefficient, drag-force coefficient, pitching-moment coefficient, and minimum pressure coefficient, respectively (normally these are 1, 2, 3, 4, and 5, respectively). If pitching-moment terms are neglected with UseBlCm = FALSE, InCol_Cm may be set to zero, and if the cavitation check is disabled with CavitCheck = FALSE, InCol_Cpmin may be set to zero.

Specify the number of airfoil data input files to be used using NumAFfiles, followed by NumAFfiles lines of filenames. The file names should be in quotations and can contain an absolute path or a relative path e.g., “C:\airfoils\S809_CLN_298.dat” or “airfoils\S809_CLN_298.dat”. If you use relative paths, it is relative to the location of the file in which it is specified. The blade data input files will reference these airfoil data using their line identifier, where the first airfoil file is numbered 1 and the last airfoil file is numbered NumAFfiles.

Set UseBlCm to TRUE to include pitching-moment terms in the blade airfoil aerodynamics or FALSE to neglect them; if UseBlCm = TRUE, pitching-moment coefficient data must be included in the airfoil data tables with InCol_Cm not equal to zero.

The blade nodal discretization, geometry, twist, chord, and airfoil identifier are set in separate input files for each blade, described in Section 4.2.1.2.4. ADBlFile(1) is the filename for blade 1, ADBlFile(2) is the filename for blade 2, and ADBlFile(3) is the filename for blade 3, respectively; the latter is not used for two-bladed rotors and the latter two are not used for one-bladed rotors. The file names should be in quotations and can contain an absolute path or a relative path. The data in each file need not be identical, which permits modeling of aerodynamic imbalances.

### 4.2.1.2.2.9. Tower Influence and Aerodynamics¶

The input parameters in this section pertain to the tower influence and/or tower drag calculations and are only used when TwrPotent > 0, TwrShadow > 0, or TwrAero = TRUE.

NumTwrNds is the user-specified number of tower analysis nodes and determines the number of rows in the subsequent table (after two table header lines). NumTwrNds must be greater than or equal to two; the higher the number, the finer the resolution and longer the computational time; we recommend that NumTwrNds be between 10 and 20 to balance accuracy with computational expense. For each node, TwrElev specifies the local elevation of the tower node above ground (or above MSL for offshore wind turbines or above the seabed for MHK turbines), TwrDiam specifies the local tower diameter, TwrCd specifies the local tower drag-force coefficient, and TwrTI specifies the turbulence intensity used in the Eames tower shadow model (TwrShadow = 2) as a fraction (rather than a percentage) of the wind fluctuation. TwrElev must be entered in monotonically increasing order—from the lowest (tower-base) to the highest (tower-top) elevation. Values of TwrTI between 0.05 and 0.4 are recommended. Values larger than 0.4 up to 1 will trigger a warning that the results will need to be interpreted carefully, but the code will allow such values for scientific investigation purposes. See Fig. 4.1.

### 4.2.1.2.2.10. Outputs¶

Specifying SumPrint to TRUE causes AeroDyn to generate a summary file with name <OutFileRoot>.AD.sum. <OutFileRoot> is either specified in the I/O SETTINGS section of the driver input file when running AeroDyn standalone, or by the OpenFAST program when running a coupled simulation. See Section 4.2.1.3.2 for summary file details. If AFAeroMod=2, the unsteady aero module will also generate a file called <OutFileRoot>.UA.sum that will list all of the UA parameters used in the airfoil tables. This allows the user to check what values are being used in case the code has computed the parameters without user input.

AeroDyn can output aerodynamic and kinematic quantities at up to nine nodes specified along the tower and up to nine nodes along each blade. For outputs at every blade node, see Section 4.2.1.2.2.11.

NBlOuts specifies the number of blade nodes that output is requested for (0 to 9) and BlOutNd on the next line is a list NBlOuts long of node numbers between 1 and NumBlNds (corresponding to a row number in the blade analysis node table in the blade data input files), separated by any combination of commas, semicolons, spaces, and/or tabs. All blades have the same output node numbers. NTwOuts specifies the number of tower nodes that output is requested for (0 to 9) and TwOutNd on the next line is a list NTwOuts long of node numbers between 1 and NumTwrNds (corresponding to a row number in the tower analysis node table above), separated by any combination of commas, semicolons, spaces, and/or tabs. The outputs specified in the OutList section determine which quantities are actually output at these nodes.

Fig. 4.1 AeroDyn Tower Geometry

The OutList section controls output quantities generated by AeroDyn. Enter one or more lines containing quoted strings that in turn contain one or more output parameter names. Separate output parameter names by any combination of commas, semicolons, spaces, and/or tabs. If you prefix a parameter name with a minus sign, “-”, underscore, “_”, or the characters “m” or “M”, AeroDyn will multiply the value for that channel by –1 before writing the data. The parameters are written in the order they are listed in the input file. AeroDyn allows you to use multiple lines so that you can break your list into meaningful groups and so the lines can be shorter. You may enter comments after the closing quote on any of the lines. Entering a line with the string “END” at the beginning of the line or at the beginning of a quoted string found at the beginning of the line will cause AeroDyn to quit scanning for more lines of channel names. Blade and tower node-related quantities are generated for the requested nodes identified through the BlOutNd and TwOutNd lists above. If AeroDyn encounters an unknown/invalid channel name, it warns the users but will remove the suspect channel from the output file. Please refer to Appendix E for a complete list of possible output parameters.

### 4.2.1.2.2.11. Nodal Outputs¶

In addition to the named outputs in Section 4.2.1.2.2.10 above, AeroDyn allows for outputting the full set blade node motions and loads (tower nodes unavailable at present). Please refer to the AeroDyn_Nodes tab in the Excel file OutListParameters.xlsx for a complete list of possible output parameters.

This section follows the END statement from normal Outputs section described above, and includes a separator description line followed by the following optinos.

BldNd_BladesOut specifies the number of blades to output. Possible values are 0 through the number of blades AeroDyn is modeling. If the value is set to 1, only blade 1 will be output, and if the value is 2, blades 1 and 2 will be output.

BldNd_BlOutNd specifies which nodes to output. This is currently unused.

The OutList section controls the nodal output quantities generated by AeroDyn. In this section, the user specifies the name of the channel family to output. The output name for each channel is then created internally by AeroDyn by combining the blade number, node number, and channel family name. For example, if the user specifies AxInd as the channel family name, the output channels will be named with the convention of B$$\mathbf{\beta}$$N###AxInd where $$\mathbf{\beta}$$ is the blade number, and ### is the three digit node number.

#### 4.2.1.2.2.11.1. Sample Nodal Outputs section¶

This sample includes the END statement from the regular outputs section.

 1END of input file (the word "END" must appear in the first 3 columns of this last OutList line)
2---------------------- NODE OUTPUTS --------------------------------------------
5              OutList     - The next line(s) contains a list of output parameters.  See OutListParameters.xlsx, AeroDyn_Nodes tab for a listing of available output channels, (-)
6"VUndx"     - x-component of undisturbed wind velocity at each node
7"VUndy"     - y-component of undisturbed wind velocity at each node
8"VUndz"     - z-component of undisturbed wind velocity at each node
9"VDisx"     - x-component of disturbed wind velocity at each node
10"VDisy"     - y-component of disturbed wind velocity at each node
11"VDisz"     - z-component of disturbed wind velocity at each node
12"STVx"      - x-component of structural translational velocity at each node
13"STVy"      - y-component of structural translational velocity at each node
14"STVz"      - z-component of structural translational velocity at each node
15"VRel"      - Relvative wind speed at each node
16"DynP"      - Dynamic pressure at each node
17"Re"        - Reynolds number (in millions) at each node
18"M"         - Mach number at each node
19"Vindx"     - Axial induced wind velocity at each node
20"Vindy"     - Tangential induced wind velocity at each node
21"AxInd"     - Axial induction factor at each node
22"TnInd"     - Tangential induction factor at each node
23"Alpha"     - Angle of attack at each node
24"Theta"     - Pitch+Twist angle at each node
25"Phi"       - Inflow angle at each node
26"Curve"     - Curvature angle at each node
27"Cl"        - Lift force coefficient at each node
28"Cd"        - Drag force coefficient at each node
29"Cm"        - Pitching moment coefficient at each node
30"Cx"        - Normal force (to plane) coefficient at each node
31"Cy"        - Tangential force (to plane) coefficient at each node
32"Cn"        - Normal force (to chord) coefficient at each node
33"Ct"        - Tangential force (to chord) coefficient at each node
34"Fl"        - Lift force per unit length at each node
35"Fd"        - Drag force per unit length at each node
36"Mm"        - Pitching moment per unit length at each node
37"Fx"        - Normal force (to plane) per unit length at each node
38"Fy"        - Tangential force (to plane) per unit length at each node
39"Fn"        - Normal force (to chord) per unit length at each node
40"Ft"        - Tangential force (to chord) per unit length at each node
41"Clrnc"     - Tower clearance at each node (based on the absolute distance to the nearest point in the tower from blade node B#N# minus the local tower radius, in the deflected configuration); please note that this clearance is only approximate because the calculation assumes that the blade is a line with no volume (however, the calculation does use the local tower radius); when blade node B#N# is above the tower top (or below the tower base), the absolute distance to the tower top (or base) minus the local tower radius, in the deflected configuration, is output
42"Vx"        - Local axial velocity
43"Vy"        - Local tangential velocity
44"GeomPhi"   - Geometric phi? If phi was solved using normal BEMT equations, GeomPhi = 1; otherwise, if it was solved geometrically, GeomPhi = 0.
45"Chi"       - Skew angle (used in skewed wake correction) -- not available for OLAF
46"UA_Flag"   - Flag indicating if UA is turned on for this node. -- not available for OLAF
47"CpMin"     - Pressure coefficient
48"SgCav"     - Cavitation number
49"SigCr"     - Critical cavitation number
50"Gam"       - Gamma -- circulation on blade
51"Cl_Static" - Static portion of lift force coefficient at each node, without  unsteady effects -- not available for BEMT/DBEMT
52"Cd_Static" - Static portion of drag force coefficient at each node, without unsteady effects -- not available for BEMT/DBEMT
53"Cm_Static" - Static portion of pitching moment coefficient at each node, without unsteady effects -- not available for BEMT/DBEMT
54"Uin"       - Axial induced velocity in rotating hub coordinates. Axial aligned with hub axis.     rotor plane polar hub rotating coordinates
55"Uit"       - Tangential induced velocity in rotating hub coordinates. Tangential to the rotation plane. Perpendicular to blade aziumth.     rotor plane polar hub rotating coordinates
56"Uir"       - Radial induced velocity in rotating hub coordinates. Radial outwards in rotation plane. Aligned with blade azimuth.     rotor plane polar hub rotating coordinates
57END of input file (the word "END" must appear in the first 3 columns of this last OutList line)
58---------------------------------------------------------------------------------------


## 4.2.1.2.3. Airfoil Data Input File¶

The airfoil data input files themselves (one for each airfoil) include tables containing coefficients of lift force, drag force, and pitching moment versus AoA, as well as UA model parameters. In these files, any line whose first non-blank character is an exclamation point (!) is ignored (for inserting comment lines). The non-comment lines should appear within the file in order, but comment lines may be intermixed as desired for reading clarity. A sample airfoil data input file is given in Section 4.2.1.9.

InterpOrd is the order the static airfoil data is interpolated when AeroDyn uses table look-up to find the lift-, drag-, and optional pitching-moment, and minimum pressure coefficients as a function of AoA. When InterpOrd is 1, linear interpolation is used; when InterpOrd is 3, the data will be interpolated with cubic splines; if the keyword DEFAULT is entered in place of a numerical value, InterpOrd is set to 1.

RelThickness is the non-dimensional thickness of the airfoil (thickness over chord ratio), expressed as a fraction (not a percentage), typically between 0.1 and 1. The parameter is currently used when UAMod=7, but might be used more in the future. The default value of 0.2 if provided for convenience.

NonDimArea is the nondimensional airfoil area (normalized by the local BlChord squared), but is currently unused by AeroDyn.

NumCoords is the number of points to define the exterior shape of the airfoil, plus one point to define the aerodynamic center, and determines the number of rows in the subsequent table; NumCoords must be exactly zero or greater than or equal to three. For each point, the nondimensional X and Y coordinates are specified in the table, X_Coord and Y_Coord (normalized by the local BlChord). The first point must always locate the aerodynamic center (reference point for the airfoil lift and drag forces, likely not on the surface of the airfoil); the remaining points should define the exterior shape of the airfoil. The airfoil shape is currently unused by AeroDyn, but when AeroDyn is coupled to OpenFAST, the airfoil shape will be used by OpenFAST for blade surface visualization when enabled.

BL_file is the name of the file containing boundary-layer characteristics of the profile. It is ignored if the aeroacoustic module is not used. This parameter may also be omitted from the file if the aeroacoustic module is not used.

Specify the number of Reynolds number- or aerodynamic-control setting-dependent tables of data for the given airfoil via the NumTabs setting. The remaining parameters in the airfoil data input files are entered separately for each table.

Re and UserProp are the Reynolds number (in millions) and aerodynamic-control (or user property) setting for the included table. These values are used only when the AFTabMod parameter in the primary AeroDyn input file is set to use 2D interpolation based on Re or UserProp. If 1D interpolation (based only on angle of attack) is used, only the first table in the file will be used.

Set InclUAdata to TRUE if you are including the UA model parameters. If this is set to FALSE, all of the UA model parameters will be determined by the code. Any lines that are missing from this section will have their values determined by the code, either using a default value or calculating it based on the polar coefficient data in the airfoil table:

• alpha0 specifies the zero-lift AoA (in degrees);

• alpha1 specifies the AoA (in degrees) larger than alpha0 for which f equals 0.7; approximately the positive stall angle; This parameter is used when flookup=false and when determining a default value of Cn1.

• alpha2 specifies the AoA (in degrees) less than alpha0 for which f equals 0.7; approximately the negative stall angle; This parameter is used when flookup=false and when determining a default value of Cn2.

• alphaUpper specifies the AoA (in degrees) of the upper boundary of fully-attached region of the cn or cl curve. It is used to compute the separation function when UAMod=5.

• alphaLower specifies the AoA (in degrees) of the lower boundary of fully-attached region of the cn or cl curve. It is used to compute the separation function when UAMod=5. (The separation function will have a value of 1 between alphaUpper and alphaLower.)

• eta_e is the recovery factor and typically has a value in the range [0.85 to 0.95] for UAMod = 1; if the keyword DEFAULT is entered in place of a numerical value, eta_e is set to 0.9 for UAMod = 1, but eta_e is set to 1.0 for other UAMod values and whenever FLookup = TRUE;

• C_nalpha is the slope of the 2D normal force coefficient curve in the linear region;

• 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;

• T_V0 is the initial value of the time constant associated with the vortex lift decay process, used in the expression of Cvn; it depends on Reynolds number, Mach number, and airfoil; if the keyword DEFAULT is entered in place of a numerical value, T_V0 is set to 6.0;

• T_p is the boundary-layer leading edge pressure gradient time constant in the expression for Dp and should be tuned based on airfoil experimental data; if the keyword DEFAULT is entered in place of a numerical value, T_p is set to 1.7;

• T_VL is the time constant associated with the vortex advection process, representing the nondimensional time in semi-chords needed for a vortex to travel from the leading to trailing edges, and used in the expression of Cvn; it depends on Reynolds number, Mach number (weakly), and airfoil; valued values are in the range [6 to 13]; if the keyword DEFAULT is entered in place of a numerical value, T_VL is set to 11.0;

• b1 is a constant in the expression of $$\phi_\alpha^c$$ and $$\phi_q^c$$; this value is relatively insensitive for thin airfoils, but may be different for turbine airfoils; if the keyword DEFAULT is entered in place of a numerical value, b1 is set to 0.14, based on experimental results;

• b2 is a constant in the expression of $$\phi_\alpha^c$$ and $$\phi_q^c$$; this value is relatively insensitive for thin airfoils, but may be different for turbine airfoils; if the keyword DEFAULT is entered in place of a numerical value, b2 is set to 0.53, based on experimental results;

• b5 is a constant in the expression of $$K^{'''}_q$$, $$Cm_q^{nc}$$, and $$K_{m_q}$$; if the keyword DEFAULT is entered in place of a numerical value, b5 is set to 5, based on experimental results;

• A1 is a constant in the expression $$\phi_\alpha^c$$ and $$\phi_q^c$$; this value is relatively insensitive for thin airfoils, but may be different for turbine airfoils; if the keyword DEFAULT is entered in place of a numerical value, A1 is set to 0.3, based on experimental results;

• A2 is a constant in the expression $$\phi_\alpha^c$$ and $$\phi_q^c$$; this value is relatively insensitive for thin airfoils, but may be different for turbine airfoils; if the keyword DEFAULT is entered in place of a numerical value, A2 is set to 0.7, based on experimental results;

• A5 is a constant in the expression $$K^{'''}_q$$, $$Cm_q^{nc}$$, and $$K_{m_q}$$; if the keyword DEFAULT is entered in place of a numerical value, A5 is set to 1, based on experimental results;

• S1 is the constant in the best fit curve of f for alpha0 $$\le$$ AoA $$\le$$ alpha1 for UAMod = 1 (and is unused otherwise); by definition, it depends on the airfoil;

• S2 is the constant in the best fit curve of f for AoA > alpha1 for UAMod = 1 (and is unused otherwise); by definition, it depends on the airfoil;

• S3 is the constant in the best fit curve of f for alpha2 $$\le$$ AoA $$\le$$ alpha0 for UAMod = 1 (and is unused otherwise); by definition, it depends on the airfoil;

• S4 is the constant in the best fit curve of f for AoA < alpha2 for UAMod = 1 (and is unused otherwise); by definition, it depends on the airfoil;

• Cn1 is the critical value of $$C^{\prime}_n$$ at leading-edge separation for positive AoA and should be extracted from airfoil data at a given Reynolds number and Mach number; Cn1 can be calculated from the static value of Cn at either the break in the pitching moment or the loss of chord force at the onset of stall; Cn1 is close to the condition of maximum lift of the airfoil at low Mach numbers;

• Cn2 is the critical value of $$C^{\prime}_n$$ at leading-edge separation for negative AoA and should be extracted from airfoil data at a given Reynolds number and Mach number; Cn2 can be calculated from the static value of Cn at either the break in the pitching moment or the loss of chord force at the onset of stall; Cn2 is close to the condition of maximum lift of the airfoil at low Mach numbers;

• St_sh is the Strouhal’s shedding frequency; if the keyword DEFAULT is entered in place of a numerical value, St_sh is set to 0.19;

• Cd0 is the drag-force coefficient at zero-lift AoA;

• Cm0 is the pitching-moment coefficient about the quarter-chord location at zero-lift AoA, positive for nose up;

• k0 is a constant in the best fit curve of $$\hat{x}_{cp}$$ and equals for $$\hat{x}_{AC}-0.25$$ UAMod = 1 (and is unused otherwise);

• k1 is a constant in the best fit curve of $$\hat{x}_{cp}$$ for UAMod = 1 (and is unused otherwise);

• k2 is a constant in the best fit curve of $$\hat{x}_{cp}$$ for UAMod = 1 (and is unused otherwise);

• k3 is a constant in the best fit curve of $$\hat{x}_{cp}$$ for UAMod = 1 (and is unused otherwise);

• k1_hat is a constant in the expression of Cc due to leading-edge vortex effects for UAMod = 1 (and is unused otherwise);

• x_cp_bar is a constant in the expression of $$\hat{x}_{cp}^{\nu}$$ for UAMod = 1 (and is unused otherwise); if the keyword DEFAULT is entered in place of a numerical value, x_cp_bar is set to 0.2; and

• UACutOut is the AoA (in degrees) in absolute value above which UA are disabled; if the keyword DEFAULT is entered in place of a numerical value, UACutOut is set to 45.

• UACutOut_delta is the AoA difference (in degrees) which, together with UACutOut determines when unsteady aero begins to turn off; if the keyword DEFAULT is entered in place of a numerical value, UACutOut_delta is set to 5 degrees. The unsteady solution is used at angles of attack less than UACutOut - UACutout_delta degrees. Above UACutout, the steady solution is used (i.e., UA is disabled). The steady and unsteady solutions are blended between those two angles.

• filtCutOff is the cut-off reduced frequency of the low-pass filter applied to the AoA input to UA, as well as to the pitch rate and pitch acceleration derived from AoA within UA; if the keyword DEFAULT is entered in place of a numerical value, filtCutOff is set to 0.5. This non-dimensional value corresponds to a frequency of $$\frac{U \times \mathrm{filtCutOff}}{\pi \times \mathrm{chord}}$$ Hz.

NumAlf is the number of distinct AoA entries and determines the number of rows in the subsequent table of static airfoil coefficients; NumAlf must be greater than or equal to one (NumAlf = 1 implies constant coefficients, regardless of the AoA).

AeroDyn will interpolate on AoA using the data provided via linear interpolation or via cubic splines, depending on the setting of input InterpOrd above. If AFTabMod is set to 1, only the first airfoil table in each file will be used. If AFTabMod is set to 2, AeroDyn will find the airfoil tables that bound the computed Reynolds number, and linearly interpolate between the tables, using the logarithm of the Reynolds numbers. If AFTabMod is set to 3, it will find the bounding airfoil tables based on the UserProp field and linearly interpolate the tables based on it. Note that OpenFAST currently sets the UserProp input value to 0 unless the DLL controller is used and sets the value, so using this feature may require a code change.

For each AoA, you must set the AoA (in degrees), alpha, the lift-force coefficient, Coefs(:,1), the drag-force coefficient, Coefs(:,2), and optionally the pitching-moment coefficient, Coefs(:,3), and minimum pressure coefficient, Coefs(:,4), but the column order depends on the settings of InCol_Alfa, InCol_Cl, InCol_Cd, InCol_Cm, and InCol_Cpmin in the AIRFOIL INFORMATION section of the AeroDyn primary input file. AoA must be entered in monotonically increasing order—from lowest to highest AoA; the first row should be for AoA = –180 degrees and the last should be for AoA = +180 (unless NumAlf = 1, in which case AoA is unused). If pitching-moment terms are neglected with UseBlCm = FALSE in the ROTOR/BLADE PROPERTIES section of the AeroDyn primary input file, the column containing pitching-moment coefficients may be absent from the file. Likewise, if the cavitation check is neglected with CavitCheck = FALSE in the GENERAL OPTIONS section of the AeroDyn primary input file, the column containing the minimum pressure coefficients may be absent from the file.

## 4.2.1.2.4. Blade Data Input File¶

The blade data input file contains the nodal discretization, geometry, twist, chord, and airfoil identifier for a blade. Separate files are used for each blade, which permits modeling of aerodynamic imbalances. A sample blade data input file is given in Section 4.2.1.9.

The input file begins with two lines of header information which is for your use, but is not used by the software.

NumBlNds is the user-specified number of blade analysis nodes and determines the number of rows in the subsequent table (after two table header lines). NumBlNds must be greater than or equal to two; the higher the number, the finer the resolution and longer the computational time; we recommend that NumBlNds be between 10 and 20 to balance accuracy with computational expense. Even though NumBlNds is defined in each blade file, all blades must have the same number of nodes. For each node:

• BlSpn specifies the local span of the blade node along the (possibly preconed) blade-pitch axis from the root; BlSpn must be entered in monotonically increasing order—from the most inboard to the most outboard—and the first node must be zero, and when AeroDyn is coupled to OpenFAST, the last node should be located at the blade tip;

• BlCrvAC specifies the local out-of-plane offset (when the blade-pitch angle is zero) of the aerodynamic center (reference point for the airfoil lift and drag forces), normal to the blade-pitch axis, as a result of blade curvature; BlCrvAC is positive downwind; upwind turbines have negative BlCrvAC for improved tower clearance;

• BlSwpAC specifies the local in-plane offset (when the blade-pitch angle is zero) of the aerodynamic center (reference point for the airfoil lift and drag forces), normal to the blade-pitch axis, as a result of blade sweep; positive BlSwpAC is opposite the direction of rotation;

• BlCrvAng specifies the local angle (in degrees) from the blade-pitch axis of a vector normal to the plane of the airfoil, as a result of blade out-of-plane curvature (when the blade-pitch angle is zero); BlCrvAng is positive downwind; upwind turbines have negative BlCrvAng for improved tower clearance;

• BlTwist specifies the local aerodynamic twist angle (in degrees) of the airfoil; it is the orientation of the local chord about the vector normal to the plane of the airfoil, positive to feather, leading edge upwind; the blade-pitch angle will be added to the local twist;

• BlChord specifies the local chord length; and

• BlAFID specifies which airfoil data the local blade node is associated with; valid values are numbers between 1 and NumAFfiles (corresponding to a row number in the airfoil file table in the AeroDyn primary input file); multiple blade nodes can use the same airfoil data.

See Fig. 4.2. Twist is shown in Fig. 4.6 of Section 4.2.1.9.

Fig. 4.2 AeroDyn Blade Geometry – Left: Side View; Right: Front View (Looking Downwind)