OpenFAST input into Ashes


OpenFAST is an open-source wind turbine simulation tool (see https://github.com/OpenFAST ). OpenFAST is based on FAST developed by NREL  (see https://nwtc.nrel.gov/FAST). This section lists the inputs that are necessary to create a wind turbine model in OpenFAST, and their equivalent in Ashes. 

Because of differences in the theories and implementations in OpenFAST and Ashes, not all parameters in OpenFAST are applicable to Ashes, and vice-versa. In this section, the parameters that do not have an equivalent in Ashes or that are not relevant are omitted. More information about OpenFAST can be found in the user guide here https://openfast.readthedocs.io/en/master/

An OpenFAST model is composed of many different input files. Here, the different subsection cover the different input files necessary to create an onshore turbine with OpenFAST .

This section applies to the version OpenFAST 2.1.0, released 2019-03-27

1 Main file (.fst)

This file is the mother file in OpenFAST. It contains some of the information about the simulation and the solver, and links to other files that define the model. This file is composed of the following sections:

1.1 Simulation control


OpenFASTAshesComment
TMaxN/A in time simulation
Duration in the Batch window
Total simulation time
DTAnalysis-> General-> Time stepTime step of the finite element solver

1.2 Feature switches and flags

This section tells OpenFAST which modules will be included in the simulation (for example, a module to include Mooring lines or to compute aerodynamic loads). Ashes is not based on a modular framework, therefore all features are included by default but can be removed during a simulation.
OpenFAST
AshesComment
CompElastN/AIn Ashes, the whole structure is considered by default, which is equivalent to have this parameter set to 2 in FAST. To remove the blades in Ashes, you can remove the RNA in the Support structure files. You can also simulate stiff blades with the parameter Analysis->General->Rotor model. It is also possible to set the number of blades to 0 with the Rotor->Number of Blades parameter. 
CompInflowN/AThe wind type in Ashes is defined by the Atmosphere->Wind type parameter
CompAeroN/ABy default, the aerodynamic loads in Ashes are computed using the Unsteady BEM algorithm. This can be changed in the Aerodynamics->Advanced tab. It is possible to turn off the aerodynamic loads in the Live Window of the Simulation window
CompServoN/ABy default, a controller is included in Ashes.
CompHydroN/ABy default, hydrodynamic loads are included in Ashes, but can be turned off in the Live Window of the Simulation window
CompSubN/ASoil-Structure interaction can be included in Ashes in the Support section files. More info about the soil-structure interaction is given in the theory manual, in the Geotechnical modeling section.
CompMoringN/AMooring lines can be included for floaters in the Support section files. More information about the mooring lines can be found in the theory manual in the Catenary lines section.
CompIceN/AFor the time being, there is no model for Ice on blades in Ashes. The suggested workaround is to modify the characteristics of the blades in the Blade shape file and the Blade structure file.

1.3 Input files

The parameters composing the different FAST files are explained in the rest of the subsections of this section.

OpenFASTAshesComment
EDFileN/ADefines the initial conditions and the model configuration.
BDBldFileN/AThe blades in Ashes are defined in the Blade database. Different blades can be selected with the Rotor->Blade type parameter. 
InflowFileN/AThe wind type in Ashes is defined by the Atmosphere->Wind type parameter
AeroFileN/ADefines the aerodynamic parameters of the simulations.
ServoFileN/ADefines the controller.
HydrofileN/ADefines the hydrodynamic parameters of the simulation
SubFileN/ADefines the soil-structure interaction parameters
MoorignFileN/ADefines the mooring lines
IceFileN/ADefines the input for modeling ice on the blades

1.4 Output

In Ashes, the ouput is managed through the Sensors.

1.5 Linearization

Simulations in Ashes are fully non-linear unless specified otherwise in the Analysis parameters

1.6 Visualization

In Ashes, the model is visualized in the Simulation window.
The data can be visualized in the Sensors pane (see Visualize the data).

2 EDFile (ElastoDyn file)

In OpenFAST, the EDFile (for ElastoDynFile) defines the initial conditions of the simulations and some parameters related to the simulation. This file is composed of the following sections:

2.1 Simulation control

OpenFAST
AshesComment
DTAnalysis->Time stepTime step of the FEM solver. In Ashes, it is not possible to define different time steps for different parts of the simulation

2.2 Environmental conditions

OpenFAST
AshesComment
GravityEnvironment->Gravitational accelerationTo change the gravitational acceleration in Ashes, you also have to change the parameter Environment->Gravity scheme to User defined

2.3 Degrees of Freedom

All degrees of freedom are enabled by default in Ashes. You can block certain degrees of freedom by adding Supports to the Support section files

2.4 Initial conditions

In Ashes, the state of the structure at time t = 0 (i.e. Initial conditions) is specified in the Analysis parameters window, in the Initial conditions tab.
Not all initial conditions from the OpenFAST EDFile can be specified in Ashes. In particular, Ashes does not allow yet for displacements as initial conditions, unless the whole structure's initial conditions is specified as a given mode shape. This can be done by specifying Use mode shape as initial conditions in the Initial conditions tab.

The initial conditions that can be specified in Ashes are given in the following table:
OpenFAST
AshesComment
BlPitch(x)Pitch angleInitial pitch angle of all blades in of the WT. Note that Ashes does not allow for different starting pitch angles for different blades
AzimuthAzimuth angleAzimuth angle for the first blade. An angle of 0 corresponds to the blade pointing upwards
RotSpeedRPMRotational speed of the rotor. Note that in Ashes you can also specify an initial TSR or an initial Blade Tip Speed.
NacYawYaw angleAngle of the RNA around the yaw axis

2.5 Turbine configiration

This section defines several parameter of the model. 
OpenFAST
AshesComment
NumBlRotor->Number of blades
TipRadN/AIn Ashes the tip radius is defined by the blade length and the rotor hub
HubRadHub->RadiusHub Radius. The root of the blade will be placed this distance from the main shaft
PreCone(x)Rotor->Cone angleIn Ashes, the cone angle is the same for all blades of the WT. Note that a negative cone angle in OpenFAST correpsonds to a positive one in Ashes
HubCMN/AIn Ashes, the center of mass of the hub is located at the center of the hub. This can be adjusted by varying the parameters of the main shaft.
OverHangMain shaft->Hori. Distance to HubIn OpenFAST , the distance between the hub and the tower top is given along the main shaft. This means that these parameters will not be equal in Ashes and FAST if the tilt angle is different than 0
ShftGagLMain shaft->Front bearing positionNote that in Ashes this distance is given as a percentage of the shaft length
ShftTiltRNA->Tilt angleNote that a negative tilt angle in OpenFAST corresponds to a positive one in Ashes
NacMxnNacelle frame->CM horizontal offsetHorizontal position of the center of mass of the nacell wrt the tower top. Note that a positive value in OpenFAST corresponds to a negative one in Ashes.
NacMznNacelle frame->CM vertical offsetVertical position of the center of mass of the nacelle wrt the tower top (this time Ashes and OpenFAST have the same sign convention, yey!)
Twr2ShftMain shaft->Vert. Distance to hubNote that OpenFAST requires the distance between the tower top and the shaft, whereas Ashes requires the distance between the tower top and the hub. To get the two to match, you will need to account for the Horizontal distance to hub and the cone angle.
TowerHtTubular tower->HeightFor tubular towers defined by Support section files, this parameter will disappear and the tower height will be defined by the position of the nodes in the file instead
TowerBsHtSupport structure->Vertical offset
PtfmCMxtSupport structure->Initial SurgeThis parameter is only available for Floaters
PtfmCMytSupport structure->Initial SwayThis parameter is only available for Floaters
PtfmCMztSupport structure-> DraftThis parameter is only available for Floaters

2.6 Mass and Inertia

OpenFAST
AshesComment
HubMassHub->MassTo modify the hub mass, you also have to change the parameter Mass scheme to  User defined
HubInerHub->InertiaInerta of the hub around the rotor axis
NacMassNacelle frame->Mass

2.7 Blade

The blades in Ashes are defined in the Blade database
The blades can be selected with the Rotor->Blade type parameter

2.8 Rotor teeter

It is not possible to implement teeters in Ashes..

2.9 Drivetrain

OpenFAST
AshesComment
GboxEffN/ATo account for the efficiency of the Gearbox, you can include it in the generator efficiency with the Generator->Efficiency factor parameter
GBRatioGearbox->Gear ratio

2.10 Furling

It is not possible to implement furling systems in Ashes.

2.11 Tower

OpenFAST
AshesComment
TwrNodesTubular tower->Maximum element lenghtIn Ashes, you cannot input the number of elements of the tower but rather the maximum length of each element. For towers defined by Support section files, this parameter is not available and the number of nodes will be defined by the file itself.
TwrFileTubular tower->Input fileIn Ashes, support structures are modelled with FEM frame elements, as opposed to mode shapes. Therefore, the file describing the tower in Ashes is very different than the one in OpenFAST .

2.12 Output

In Ashes, the ouput are managed in the Sensors pane.

3 BDBldFile (BeamDyn Blade file)

Ashes uses a database system to manage the blades. New blades can be created and stored in the Blade database, and selected for the current model with the Rotor->Blade type parameter.

The aerodynamic properties of a blade are defined by a Blade shape file while the structural properties are defined by a Blade structure file.

Note that to create a new blade, you must make sure that the airfoils composing this blade are added to the Airfoil database

4 TwrFile (Tower file)

In Ashes, support structures are modelled with FEM frame elements, as opposed to mode shapes. This makes Ashes tower input files very different from FAST files. However, some parameters from the OpenFAST tower file can be input to Ashes as well.

Tower input files can be defined as explained in the Support section examples section.

4.1 Tower parameters

OpenFAST
AshesComment
NTwrInpStN/AIn Ashes, the number of tower elements is defined by the elements created in the Members section
TwrFADmp (1)N/ADamping for the tower can be modelled with Rayleigh damping to obtain a given damping ratio at a given eigenperiod (see How to use Rayleigh damping)
TwrFADmp (2)N/ADamping for the tower can be modelled with Rayleigh damping to obtain a given damping ratio at a given eigenperiod (see How to use Rayleigh damping)

4.2 Distributed tower properties

It is possible to define the elements of a tower based on their structural properties rather than their geometrical properties by using Circular shape cross sections (see Keywords) in the Support section files. In that case the following properties can be adjusted:
OpenFAST
AshesComment
HtFractz-coordinateIn Ashes, elements are defined as going from one node to another. It is possible to define the height of an element by adjusting the height of its defining nodes
TMassDenMass
TwFAStif/TwSSStifEIFor Circular shape cross section, only one stiffness can be defined. If the stiffness of the elements is different in the longitudinal and transversal directions, Rectangular shape cross section must be selected.


5 Inflowfile

This file defines the atmospheric environmental conditions.

OpenFAST
AshesComments
WindTypeAtmosphere->Wind Type

5.1 Parameters for steady wind conditions

This wind type corresponds to Live or Uniform in Ashes
OpenFAST
AshesComments
HWindSpeedUniform wind->Initial wind speed
RefHtUniform wind->Reference heightThis parameter only appears if the Reference height Scheme is set to User defined
PLexpUniform wind->Power law exponentThis parameter only appears if Shear is toggled on

5.2 Turbulent wind

Ashes can read turbulent wind files generated with TurbSim or with the IEC Mann turbulence generator (corresponding to the HAWC2 format in OpenFAST). This files can be generated with the Turbulent wind tool and selected as input with the Turbulent wind->Turbulent wind file parameter.

If a Mann turbulence wind file (i.e. HAWC2 format) is selected, the following parameters can be applied in Ashes:
OpenFAST
AshesComments
nx - ny - nzNumber of grid pointsThese can be defined in the Turbulent wind tool
dx - dy - dzDistance between gridsThese are calculated based on the number of grid points and the dimensions of the grid
RefHtTurbulent wind->Reference heightThis parameter only appears if the Reference height Scehem is set to User defined
SigmaFxTurbulent wind->Turbulence intensityAshes requires the turbulence intensity while FAST requires the standard deviation.
UrefTurbulent wind->Avegare wind speed
PLExpTurbulent wind->Power law exponentOnly appears if Shear is toggled on

6 AeroFile

The aerofile contains the parameters of the aerodynamic simulation and of some environmental conditions. The parameters for the aerodynamic computation can generally be found in the Analysis parameters window under the Aerodynamics tab. The parameter for the environmental conditions can generally be found in the Atmosphere part.

6.1 Simulation control

OpenFAST
AshesComment
DTAeroAnalysis->Time stepIn Ashes, the time step for the aerodynamic loads computation is the same as the finite element solver
WakeModeAerodynamics->Advanced->BEM modeIn Ashes you can chose between a Steady BEM or an Unsteady BEM algorithm. You can toggle the aerodynamic loads off in the Live window
AFAeroModN/ADynamic stall is applied by default when using Unsteady BEM
TwrShadowAerodynamics->Tower shadowThe influence of the tower is computed based on potential flow theory
TwrAeroN/AAerodynamic drag loads on the tower are computed by default. You can remove them by setting the drag coefficient to 0, in the Aerodynamics section for parameterized towers or in the Support section files for input file based towers

6.2 Environmental conditions

OpenFAST
AshesComment
AirDensAtmosphere->Air densityOnly visible if the Density scheme is set to User defined
KinViscAtmosphere->Dynamic viscosity (of air)Ashes requires the dynamic viscosity whereas FAST requires the kinematic viscosity. Only visible if the Viscosity scheme is set to User defined.

6.3 Blade-Element/Momentum Theory options

Details about how the BEM theory is implemented in Ashes can be found in the theory manual, in the BEM algorithm section.
OpenFAST
AshesComment
SkewModN/AOnly one type of skewed-wake correction is available in Ashes (see Steady BEM)
TipLossAerodynamics->Tip Loss CorrectionFollowing Prandtl's model, see Steady BEM
HubLossAerodynamics->Hub Loss correctionFollowing Prandtl's model, see Steady BEM
TanIndN/AAshes always includes the tangential induction in the BEM computation
AIDragN/AAshes always includes the drag term in the axial-induction calculations
TIDragN/AAshes always includes the drag term in the tangential-induction calculation

6.4 Airfoil information and Rotor/Blade Properties

In Ashes, the blades and airfoils are defined in the Blade database and the Airfoil database, respectively. The accepted formats for the input files are given in the File formats section.

6.5 Tower Influence and Aerodynamics

In Ashes, the tower can be defined either by parameters or by an input file.
If the tower is defined by parameters, the diameter at different heights is calculated based on the parameters input in the Tubular tower Part. The aerodynamic drag coefficient is then input in Aerodynamics->Aero drag coefs. of circular cs.
If the tower is defined from file, the diameter and the drag coefficient will be input in the Support section files.

7 ServoFile

This file defines the characteristics of the controller. The parameterized controller in Ashes is implemented according to Section 7.2 of the definition of the NREL 5 MW turbine document (Jonkman et al. (2009j), available here https://www.nrel.gov/docs/fy09osti/38060.pdf)

Both OpenFAST and Ashes can read Bladed-style dlls. In Ashes, the path to the dll file is input in the External controller window.