4.3.4. Input Files¶
No lines should be added or removed from the input files, except in tables where the number of rows is specified.
4.3.4.1. Units¶
OLAF uses the International System of Units (e.g., kg, m, s, N). Angles are assumed to be in degrees unless otherwise specified.
4.3.4.2. OLAF Primary Input File¶
The primary OLAF input file defines general free wake options, circulation model selection and specification, near- and far-wake length, and wake visualization options. Each section within the file corresponds to an aspect of the OLAF model. For most parameters, the user may specify the value “default” (with or without quotes), in which case a default value, defined below, is used by the program.
See Section 4.3.10 for a sample OLAF primary input file.
4.3.4.2.1. General Options¶
IntMethod [switch] specifies which integration method will be used to convect the Lagrangian markers. There are four options: 1) fourth-order Runge-Kutta [1], 2) fourth-order Adams-Bashforth [2], 3) fourth-order Adams-Bashforth-Moulton [3], and 4) first-order forward Euler [5]. The default option is [5]. These methods are specified in Section 4.3.6.4.
DTfvw [sec] specifies the time interval at which the module will update the wake. The time interval must be a multiple of the time step used by AeroDyn15. The blade circulation is updated at each intermediate time step based on the intermediate blades positions and wind velocities. The default value is \(dt_{aero}\), where \(dt_{aero}\) is the time step used by AeroDyn.
FreeWakeStart [sec] specifies at what time the wake evolution is classified as “free.” Before this point is reached, the Lagrangian markers are simply convected with the freestream velocity. After this point, induced velocities are computed and affect the marker convection. If a time less than or equal to zero is given, the wake is “free” from the beginning of the simulation. The default value is \(0\).
FullCircStart [sec] specifies at what time the blade circulation reaches its full strength. If this value is specified to be \(>0\), the circulation is multiplied by a factor of \(0\) at \(t=0\) and linearly increasing to a factor of \(1\) for \(t>\textit{FullCircStart}\). The default value is \(0\).
4.3.4.2.2. Circulation Specifications¶
CircSolvMethod [switch] specifies which circulation method is used. There are three options: 1) \(C_l\)-based iterative procedure [1], 2) no-flow through [2], and 3) prescribed [3]. The default option is [1]. These methods are described in Section 4.3.6.3.
CircSolvConvCrit [-] specifies the dimensionless convergence criteria used for solving the circulation. This variable is only used if CircSolvMethod = [1]. The default value is \(0.001\), corresponding to \(0.1\%\) error in the circulation between two iterations.
CircSolvRelaxation [-] specifies the relaxation factor used to solve the circulation. This variable is only used if CircSolvMethod = [1]. The default value is \(0.1\).
CircSolvMaxIter [-] specifies the maximum number of iterations used to solve the circulation. This variable is only used if CircSolvMethod = [1]. The default value is \(30\).
PrescribedCircFile [quoted string] specifies the file containing the prescribed blade circulation. This option is only used if CircSolvMethod = [3]. The circulation file format is a delimited file with one header line and two columns. The first column is the dimensionless radial position [r/R]; the second column is the bound circulation value in [m\(^2\)/s]. The radial positions do not need to match the AeroDyn node locations. A sample prescribed circulation file is given in Section 4.3.11.
4.3.4.2.3. Wake Extent and Discretization Options¶
nNWPanel [-] specifies the number of FVW time steps (DTfvw) for which the near-wake lattice is computed. In the future, this value will be defined as an azimuthal span in degrees or a downstream distance in rotor diameter.
WakeLength [D] specifies the length, in rotor diameters, of the far wake. The default value is \(8\). 1
FreeWakeLength [D] specifies the length, in rotor diameters, for which the turbine wake is convected as “free.” If FreeWakeLength is greater than WakeLength, then the entire wake is free. Otherwise, the Lagrangian markers located within the buffer zone delimited by FreeWakeLength and WakeLength are convected with the average velocity. The default value is \(6\). 2
FWShedVorticity [flag] specifies whether shed vorticity is included in the far wake. The default value is [False], specifying that the far wake consists only of the trailed vorticity from the root and tip vortices.
4.3.4.2.4. Wake Regularization and Diffusion Options¶
DiffusionMethod [switch] specifies which diffusion method is used to account for viscous diffusion. There are two options: 1) no diffusion [0] and 2) the core-spreading method [1]. The default option is [0].
RegDetMethod [switch] specifies which method is used to determine the regularization parameters. There are two options: 1) manual [0] and 2) optimized [1]. The manual option requires the user to specify the parameters listed in this subsection. The optimized option determines the parameters for the user. The default option is [0].
RegFunction [switch] specifies the regularization function used to remove the singularity of the vortex elements, as specified in Section 4.3.6.4. There are five options: 1) no correction [0], 2) the Rankine method [1], 3) the Lamb-Oseen method [2], 4) the Vatistas method [3], and 5) the denominator offset method [4]. The functions are given in . The default option is [3].
WakeRegMethod [switch] specifies the method of determining viscous core radius (i.e., the regularization parameter). There are three options: 1) constant [1], 2) stretching [2], and 3) age [3]. The methods are described in Section 4.3.6.7.4. The default option is [1].
WakeRegParam [m] specifies the wake regularization parameter, which is the regularization value used at the initialization of a vortex element. If the regularization method is “constant”, this value is used throughout the wake.
BladeRegParam [m] specifies the bound vorticity regularization parameter, which is the regularization value used for the vorticity elements bound to the blades.
CoreSpreadEddyVisc [-] specifies the eddy viscosity parameter \(\delta\). The parameter is used for the core-spreading method (DiffusionMethod = [1]) and the regularization method with age (WakeRegMethod = [3]). The variable \(\delta\) is described in Section 4.3.6.7.4. The default value is \(100\).
4.3.4.2.5. Wake Treatment Options¶
TwrShadowOnWake [flag] specifies whether the tower potential flow and tower shadow have an influence on the wake convection. The tower shadow model, when activated in AeroDyn, always has an influence on the lifting line, hence the induction and loads on the blade. This option only concerns the wake. The default option is [False].
ShearVorticityModel [switch] specifies whether shear vorticity is modeled in addition to the sheared inflow prescribed by InflowWind. There are two options: 1) no treatment [0] and 2) mirrored vorticity [1]. The mirrored vorticity accounts for the ground effect. Dedicated options to account for the shear vorticity will be implemented at a later time. The shear velocity profile is handled by InflowWind irrespective of this input. The default option is [0].
4.3.4.2.6. Speedup Options¶
VelocityMethod [switch] specifies the method used to determine the velocity. There are two options: 1) Biot-Savart law applied to the vortex segments [1] and 2) tree formulation using a particle representation [2]. The default option is [1].
TreeBranchFactor [-] specifies the dimensionless distance, in branch radius, above which a multipole calculation is used instead of a direct evaluation. This option is only used in conjunction with the tree code (VelocityMethod = [2]).
PartPerSegment [-] specifies the number of particles that are used when a vortex segment is represented by vortex particles. The default value is \(1\).
4.3.4.2.7. Output Options¶
WrVTK [flag] specifies if Visualization Toolkit (VTK) visualization files
are to be written out. WrVTK = [0] does not write out any VTK files. WrVTK
= [1] outputs a VTK file at every time step. The outputs are written in the
folder, vtk_fvw. The parameters WrVTK, VTKCoord, and VTK_fps are
independent of the glue code VTK output options.
VTKBlades [-] specifies how many blade VTK files are to be written out. VTKBlades \(= n\) outputs VTK files for \(n\) blades, with \(0\) being an acceptable value. The default value is \(1\).
VTKCoord [switch] specifies in which coordinate system the VTK files are written. There are two options: 1) global coordinate system [1] and 2) hub coordinate system [2]. The default option is [1].
VTK_fps [\(1\)/sec] specifies the output frequency of the VTK files. The provided value is rounded to the nearest allowable multiple of the time step. The default value is \(1/dt_\text{fvw}\). Specifying VTK_fps = [all], is equivalent to using the value \(1/dt_\text{aero}\).
4.3.4.3. AeroDyn15 Input File¶
4.3.4.3.1. Input file modifications¶
As OLAF is incorporated into the AeroDyn15 module, a wake computation option has been added to the AeroDyn15 input file and a line has been added. These additions are as follows.
WakeMod specifies the type of wake model that is used. WakeMod = [3] has been added to allow the user to switch from the traditional BEM method to the OLAF method.
FVWFile [string] specifies the OLAF module file, the path is relative to the AeroDyn file, unless an absolute path is provided.
4.3.4.3.2. Relevant sections¶
The BEM options (e.g. tip-loss, skew, and dynamic models) are read and discarded when WakeMod = [3]. The following sections and parameters remain relevant and are used by the vortex code:
general options (e.g., airfoil and tower modeling);
environmental conditions;
dynamic stall model options;
airfoil and blade information;
tower aerodynamics; and
outputs.