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]. Currently only options [1] and [5] are implemented. 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
nNWPanels [-] specifies the number of near-wake (NW) panels (i.e. FVW time steps, DTfvw) used for the extent of the near-wake lattice. See Section 4.3.3.2 for recommendations on setting this parameter.
nNWPanelsFree [-] specifies the number of near-wake panels (in FVW time steps), for which the wake is convected as “free.” If nNWPanelsFree is equal to nNWPanels, then the entire near wake is free. Otherwise, the Lagrangian markers located within the buffer zone (“frozen near wake”) delimited by nNWPanelsFree and nNWsPanel are all convected with a common and decaying induced velocity but with a varying free-stream. (see Section 4.3.6.6). This option can be used to speed up the simulation and stabilize the end of the “near-wake” region. It can potentially remove the need for the far wake region. Currently, the induced velocity of the frozen near wake is arbitrarily determined as the average over the last 20 panels of the free near wake. The decay of the convection velocity is such that the induced velocity is about 50% at the end of the frozen near wake. The convection velocity of the frozen wake requires additional verification and validation, and might change in future releases. If a “frozen” near-wake region is used then the “free” far-wake region needs to be of zero length (nFWPanelsFree=0) By default, this variable is set to nNWPanels (no frozen wake). See Section 4.3.3.2 for recommendations on setting up this parameter.
nFWPanels [-] specifies the number of panels (FVW time steps) used for the far wake (where the tip and root vortex are rolled-up to speed up computational time). See Section 4.3.3.2 for recommendations on setting this parameter. Default value: 0.
nFWPanelsFree [-] specifies the number of far-wake panels (in FVW time steps), for which the wake is convected as “free.” If nFWPanelsFree is equal to nFWPanels, then the entire far-wake is free. Otherwise, the Lagrangian markers located within the buffer zone (“frozen far wake”) delimited by nNWPanelsFree and nNWPanels are all convected with a common induced velocity but with a varying free-stream. Currently, the induced velocity for the frozen far wake is determined as the average over the free far-wake when nNWPanelsFree=nNWPanels (i.e. no frozen near wake), or, using the same convection as the end of the frozen near wake otherwise. By default, this variable is set to nFWPanels. See Section 4.3.3.2 for recommendations on setting up this parameter.
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].
RegDeterMethod [switch] specifies which method is used to determine the regularization parameters. There are four options: 1) constant [0] and 2) optimized [1], 3) chord [2], and 4) span [3]. The optimized option determines all the parameters in this section for the user. The optimized option is still work in progress and not recommended. The constant option requires the user to specify all the parameters present in this section. The default and recomment option is [3].
When RegDeterMethod==2, the regularization parameters is set according to the local chord:
When RegDeterMethod==3, the regularization parameters is set according to the spanwise discretization:
where \(Delta r\) is the length of the spanwise station. See Section 4.3.3.2 for recommendations on setting up this parameter.
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 Section 4.3.6.8.3. 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 4.3.6.8.4. The default option is [3].
WakeRegFactor [m, or -] 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. See Section 4.3.3.2 for recommendations on setting up this parameter.
WingRegFactor [m, or -] specifies the bound vorticity regularization parameter, which is the regularization value used for the vorticity elements bound to the blades. See Section 4.3.3.2 for recommendations on setting up this parameter.
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 4.3.6.8.4. 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 four options: 1) \(N^2\) Biot-Savart computation on the vortex segments [1], 2) Particle-Tree formulation [2], 3) \(N^2\) Biot-Savart computation using a particle representation, 4) Segment-Tree formulation. Option [2] and [3] requires the specification of PartPerSegment (see below). Option [4] is expected to give results close to option [1] while offering significant speedup, and this option does not require the specification of PartPerSegment. The default option is [2].
TreeBranchFactor [-] specifies the dimensionless distance, in branch radius, above which a multipole calculation is used instead of a direct evaluation. Only used when VelocityMethod = [2,4]. Default value: 1.5.
PartPerSegment [-] specifies the number of particles that are used when a vortex segment is represented by vortex particles. Only used when VelocityMethod = [2,3]). 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 VTK files at time steps defined by VTK_fps.
WrVTK*= *[2], outputs at time steps defined by VTK_fps, but ensures that
a file is written at the beginning and the end of the simulation (typically
used with VTK_fps=0 to output only at the end of the simulation).
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.
Default value: 0.
nVTKBlades [-] specifies how many blade VTK files are to be written out. nVTKBlades \(= n\) outputs VTK files for \(n\) blades, with \(0\) being an acceptable value. The default value is \(0\).
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 \(1/dt_\text{aero}\). If VTK_fps<0, then no outputs are created, except if WrVTK=2.
nGridOut [-] specifies the number of grid outputs. The default value is 0. The grid outputs are fields (velocity, vorticity) that are exported on a regular Cartesian grid. They are defined using a table that follows on the subsequent lines, with two lines of headers. The user needs to specify a name (GridName) used for the VTK output filename, a grid type (GridType), a start time (TStart), an end time (TEnd), a time interval (DTOut), and the grid extent in each directions, e.g. XStart, XEnd, nX. When GridType is 1, the velocity field is written to disk, when GridType is 2, both the velocity field and the vorticity field (computed using finite differences) are written. It is possible to export fields at a point (nX=nY=nZ=1), a line, a plane, or a 3D grid. When set to “default”, the start time is 0 and the end time is set to the end of the simulation. The outputs are done for \(t_{Start}\leq t \leq t_{End}\) When the variable DTOut is set to “all”, the AeroDyn time step is used, when it is set to “default”, the OLAF time step is used. An example of input is given below:
3 nGridOut Number of grid outputs
GridName GridType TStart TEnd DTOut XStart XEnd nX YStart YEnd nY ZStart ZEnd nZ
(-) (-) (s) (s) (s) (m) (m) (-) (m) (m) (-) (m) (m) (-)
"box" 2 default default all -200 1000. 5 -150. 150. 20 5. 300. 30
"vert" 1 default default default -200 1000. 100 0. 0. 1 5. 300. 30
"hori" 1 default default 2.0 -200 1000. 100 -150. 150. 20 100. 100. 1
In this example, the first grid, named “box”, is exported at the AeroDyn time step, and consists of a box of shape 5x20x30 and dimension 1200x300x295. The grid contains both the velocity and vorticity. The two other grids are vertical and horizontal planes containing only the velocity.
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.