A Common Pre- and Post-processor for Wind Farm Simulations

This project has received funding from the European

Union’s Horizon 2020 research and innovation

programme under grant agreement No 727477

Closed Loop Wind Farm Control

DELIVERABLE

A Common Pre- and Post-processor for Wind

Farm Simulations

DOCUMENTATION

Deliverable No. D1.3 Work Package No. WP1 Task/s No. Task 1.3

Work Package Title Wind farm control-oriented model development

Linked Task/s Title Model classification

Status Draft Final (Draft/Draft Final/Final)

Dissemination level Public (PU-Public, PP, RE-Restricted, CO-Confidential)

(https://www.iprhelpdesk.eu/kb/522-which-are-

different-levels-confidentiality)

Due date deliverable 2017-08-31 Submission date 2017-08-31

Deliverable version CL-Windcon-D1.3-DraftFinal-PrePostProcessing_Documentation

Ref. Ares(2017)4257564 - 31/08/2017

D1.3 - A Common Pre- and Post-processor for Wind Farm

Simulations: Documentation Public

Copyright CL-Windcon Contract No. 727477 Page 2

DOCUMENT CONTRIBUTORS

DOCUMENT HISTORY

Deliverable responsible USTUTT

Contributors Organization Reviewers Organization

Matthias Kretschmer USTUTT Fritz Wilts DEWI

Carsten Fichtner USTUTT Irene Eguinoa Erdozain CENER

Yusik Kim USTUTT Matti Scheu RAMBOLL

Jiangang Wang TUM

Version Date Comment

0.1 2017-06-16 Chapters defined; content for pre-processing written

0.6 2017-07-04 Filled in text for post-processing

0.8 2017-07-20 Added information for post-processing

0.9 2017-08-09 Added examples

1.0 2017-08-30 Considering feedback from reviewers

Draft Final 2017-08-31 Final check by CENER




TABLE OF CONTENTS

1 EXECUTIVE SUMMARY ...........................................................................................................7

2 INTRODUCTION .....................................................................................................................8

3 COORDINATE SYSTEMS ..........................................................................................................9

3.1 INERTIAL FRAME ......................................................................................................................... 9

3.1.1 Wind Farm ............................................................................................................................. 9

3.1.2 Wind Turbine ......................................................................................................................... 9

3.2 TOWER BASE ............................................................................................................................. 10

3.3 TOWER TOP .............................................................................................................................. 10

3.4 NACELLE/YAW .......................................................................................................................... 11

3.5 HUB ........................................................................................................................................... 11

3.6 BLADE ....................................................................................................................................... 12

3.6.1 Blade Reference System (BRS) ............................................................................................ 12

3.6.2 Blade Element Reference System (ERS) .............................................................................. 12

4 PRE-PROCESSING ................................................................................................................. 13

4.1 STRUCTURE OF PRE-PROCESSING FRAMEWORK ..................................................................... 13

4.1.1 PreProcessing_Main.m ........................................................................................................ 14

4.2 REFERENCE STRUCTURE FORMAT ............................................................................................ 14

4.2.1 Wind Turbine Reference Structure ..................................................................................... 14

4.2.2 Wind Farm Reference Structure .......................................................................................... 15

4.3 SIMULATION TOOL INPUT FORMATS ....................................................................................... 15

4.3.1 FAST 8 Input ........................................................................................................................ 16

4.3.2 SOWFA Input ....................................................................................................................... 16

4.3.3 Implementation of New Tools ............................................................................................. 16

4.4 EXAMPLE ................................................................................................................................... 17

5 POST-PROCESSING ............................................................................................................... 19

5.1 STRUCTURE OF POST-PROCESSING FRAMEWORK ................................................................... 19

5.1.1 PostProcessing_Main.m ...................................................................................................... 19

5.2 DATA STRUCTURE FORMAT OF POST-PROCESSING ................................................................. 20

5.3 ANALYSIS FUNCTIONS ............................................................................................................... 20

5.3.1 Timeseries Plots................................................................................................................... 20

5.3.2 Sectional Analysis Plots ....................................................................................................... 20

5.3.3 Flow field Analysis ............................................................................................................... 20

5.4 SUPPORTED TOOLS ................................................................................................................... 22




5.4.1 FAST 8 .................................................................................................................................. 22

5.4.2 SOWFA ................................................................................................................................. 22

5.5 POST-PROCESSING: EXAMPLE (SMALL WIND FARM) ............................................................... 23

6 REFERENCES......................................................................................................................... 27

7 APPENDIX A ......................................................................................................................... 28

7.1 REFERENCE STRUCTURE FORMAT: DEFINITIONS ..................................................................... 28

7.1.1 Wind Turbine ....................................................................................................................... 28

7.2 POST-PROCESSING SENSOR TYPES ........................................................................................... 35




LIST OF FIGURES

Figure 1: Scheme of common pre- and post-processing [1] ................................................................... 8

Figure 2: Calling routines for output ..................................................................................................... 16

Figure 3: Pre-processing main input file ................................................................................................ 18

Figure 4: Post-processing: SOWFA flowfield file structure ................................................................... 21

Figure 5: Post-processing: flowfield analysis parameters ..................................................................... 22

Figure 6: Post-processing main input file .............................................................................................. 24

Figure 7: Post-processing example: User input analysis type ............................................................... 25

Figure 8: Post-processing Example: component selection.................................................................... 25

Figure 9: Post-processing example: timeseries plot rotor speed .......................................................... 26

Figure 10: Post-processing example: flowfiled contour plot of TI ........................................................ 26

Figure 11: Reference structure: top level .............................................................................................. 28

Figure 12: Reference structure: blade definition .................................................................................. 28

Figure 13: Definition of blade geometry parameters and coordinate systems in the reference

structure [8] ........................................................................................................................................... 30

LIST OF TABLES

Table 1: Definition CS, inertial frame wind farm ..................................................................................... 9

Table 2: Definition CS, inertial frame wind turbine ................................................................................. 9

Table 3: Definition CS, tower base ........................................................................................................ 10

Table 4: Definition CS, tower top .......................................................................................................... 10

Table 5: Definition CS, nacelle/yaw ....................................................................................................... 11

Table 6: Definition CS, hub .................................................................................................................... 11

Table 7: Definition CS, blade reference system .................................................................................... 12

Table 8: Definition CS, blade element reference system ...................................................................... 12

Table 9: Pre-processing example user input ......................................................................................... 17

Table 10: Post-processing: flowfield parameters .................................................................................. 22

Table 11: Reference structure: blade structural properties [7] ............................................................ 30

Table 12: Reference structure: blade aerodynamic properties ............................................................ 31

Table 13: Post-processing sensor types ................................................................................................ 35




LIST OF ABBREVIATIONS

Abbreviation Description

BRS Blade Reference System

CG Center of Gravity

CS Coordinate System

ERS Blade Element Reference System

MSS Medium Speed Shaft

TB Tower Base

TI Turbulence Intensity

TT Tower Top

WF Wind Farm

WT Wind Turbine




1 EXECUTIVE SUMMARY

A software toolbox is developed which is for the pre- and post-processing of wind farm simulations.

As a basis a new reference structure format is developed in which all wind turbine/farm specific

information are stored as well as simulation specific information for tools of different fidelity. This

reference structure format fulfills the need for having a common definition of wind farms/turbines to

be able to exchange wind farm/turbine information without having format constraints.

With the pre-processing toolbox the input files for different wind farm simulation tools can be

created by using the information of a wind farm/turbine in the reference structure. New tools can be

added to the pre-processing toolbox besides the two examples which are already implemented.

The simulation data can be analyzed by using the post-processing toolbox. In the post-processing

toolbox the simulation data of different tools are fed in and analysis functions for the comparison of

data are applied; e.g. aeroelastic turbine data or the flow field of a wind farm can be plotted.




2 INTRODUCTION

With the growing share of wind energy in the total electricity generation capacity the analysis of

wind farm effects becomes more important and numeric simulations of wind farms have been

proved to be an appropriate way in order to understand the complex phenomena within a wind

farm. Hereby a variety of tools are used by different institutions implying the fact that each tool uses

its own pre- and post-processing. Also in the CL-Windcon project multiple tools with varying model

fidelity are used in order to develop new control strategies for complete wind farms. This has raised

the demand for having a common pre-and post-processing framework to have the possibility to

compare the simulation results of different tools. The provided toolbox aims to standardize the pre-

and post-processing of wind farm simulations as shown in Figure 1. A common pre-processing is

established which generates the inputs for each simulation tool. Similar to this a common post-

processing toolbox is developed to analyze the simulation data of the tools in one framework.

Figure 1: Scheme of common pre- and post-processing [1]

This manual describes the general features and definitions of the pre- and post-processing toolbox.

In chapter 3 the coordinate systems are defined which are used throughout the entire toolbox.

Chapter 4 is about the application of the pre-processing routines for wind farm simulations. The post-

processing analysis functions are explained in chapter 5.

As a starting point the simulation tools FAST 8 and SOWFA are already implemented in this toolbox.

The relevant examples are provided and described in this documentation.




3 COORDINATE SYSTEMS

The coordinate systems defined in this section are the basis of the entire pre- and post-processing

toolbox.

3.1 Inertial Frame

3.1.1 Wind Farm

For defining the wind farm layout an inertial coordinate system is needed. Here it is given as a

Cartesian coordinate system having the following properties:

Origin Position of Turbine 1 in wind farm array at Mean Water Sea

Level or Tower Base.

xWF-axis Pointing to the East direction, perpendicular to the Y-axis

yWF-axis Pointing to the North direction

zWF-axis Pointing upwards

Table 1: Definition CS, inertial frame wind farm

3.1.2 Wind Turbine

The wind turbine inertial frame is used to have a fixed reference coordinate system for the wind

turbine. It has got the same orientations as the wind farm WF system.

Origin The point about which the translational motions of the

support platform (surge, sway, and heave) are defined.

xi-axis Pointing to the East direction, perpendicular to the Y-axis

yi-axis Pointing to the North direction

zi-axis Pointing upwards

Table 2: Definition CS, inertial frame wind turbine




3.2 Tower Base

This coordinate system is fixed in the support platform so that it translates and rotates with the

platform.

Origin Intersection of the center of the tower and the tower base

connection to the support platform.

xTB-axis When the support platform has no pitch or yaw displacement,

it is aligned with the xi axis

yTB-axis When the support platform has no roll or yaw displacement, it

is aligned with the yi axis

zTB-axis Pointing up from the center of the tower

Table 3: Definition CS, tower base

3.3 Tower Top

This coordinate system is fixed to the top of the tower. It translates and rotates as the platform

moves and the tower bends, but it does not yaw with the nacelle.

Origin A point on the yaw axis at the maximum height of the tower.

xTT-axis When the tower is not deflected, it is aligned with the xTB axis.

yTT-axis When the tower is not deflected, it is aligned with the yTB axis.

zTT-axis When the tower is not deflected, it is aligned with the zTB axis.

It is also the yaw axis.

Table 4: Definition CS, tower top




3.4 Nacelle/Yaw

This coordinate system translates and rotates with the top of the tower, plus it yaws with the nacelle.

Origin The origin is the same as that for the tower-top/base-plate

coordinate system.

xn-axis Pointing horizontally toward the nominally downwind end of

the nacelle.

yn-axis Pointing to the left when looking toward the nominally

downwind end of the nacelle.

zn-axis Coaxial with the tower/yaw axis and pointing up.

Table 5: Definition CS, nacelle/yaw

3.5 Hub

The hub coordinate system rotates with the rotor.

Origin Intersection of the rotor axis and the plane of rotation (non-

coned rotors) or the apex of the cone of rotation (coned

rotors).

xH-axis Pointing along the hub centerline in the nominal downwind

direction.

yH-axis Orthogonal with the xH and zH axes such that they form a

right-handed coordinate system.

zH-axis Perpendicular to the hub centerline with the same azimuth as

Blade 1.

Table 6: Definition CS, hub




3.6 Blade

3.6.1 Blade Reference System (BRS)

It is used to localise the blade stations. It is the reference system for the twist angle, pre-bend and

pre-sweep (s.a. Figure 13 in the appendix).

Origin Intersection of the blade’s pitch axis and the blade root.

xRRS-axis Orthogonal with the yBRS and zBRS axes such that they form a

right-handed coordinate system.

yBRS-axis Pointing towards the trailing edge of the blade and parallel

with the chord line at the zero-twist blade station.

zBRS-axis Pointing along the pitch axis towards the tip of blade.

Table 7: Definition CS, blade reference system

3.6.2 Blade Element Reference System (ERS)

The blade element reference system is defined at each blade station of the blade. In the ERS system

the local properties of the airfoil are defined, i.e. inertia, center of gravity, elastic center, shear

center etc. (s.a. Figure 13 in the appendix).

Origin Intersection of blade station and blade’s pitch axis. For blades

with pre-bend and pre-sweep the ERS-system is shifted from

the pitch axis by pre-bend (in yBRS direction) and pre-sweep (in

xBRS direction).

xERS-axis Parallel to xBRS axis

yERS-axis Parallel to yBRS axis

zERS-axis Parallel to zBRS axis

Table 8: Definition CS, blade element reference system




4 PRE-PROCESSING

In general pre-processing of a simulation includes the steps which are required before running the

actual simulation. The pre-processing can be split up into two parts which is on the one hand the

setup of the numerics of the simulation tool (e.g. solver settings, timestep etc.) and on the other

hand the definition of the simulation case (e.g. geometry, physical conditions etc.). In the case of

wind farm simulations the preprocessing consists for instance of defining the layout of the wind farm

and the atmospheric conditions such as wind speed and turbulence intensity.

Pre-processing is usually tool dependent which means that different tools require an individual

setup. This includes varying input formats but also a variety of input parameters which are typically

changing with the model fidelity; tools of high model fidelity require generally other input

parameters as tools of medium or low fidelity.

In CL-Windcon different simulation tools of different model fidelity for wind farm simulations are

used by the partners in the consortium. Since the simulation results of the various tools are going to

be compared to each other it is in a first step required to have a common basis for the setup of all

the simulation tools. So the goal of this pre-processing toolbox is to have a standardization of the

pre-processing, especially for the above mentioned definition of the simulation cases meaning the

geometries and physical conditions. This is done in two steps: At first a new reference structure

format is developed which is more deeply explained in section 4.2. In this structure wind

turbine/farm information is defined in an easy readable way. Second a toolbox is developed which

takes the reference structure format as input and translates the general format into the input

formats of different tools.

4.1 Structure of Pre-Processing Framework

The framework is mainly realized with Matlab [2] scripts and has got a specific folder structure which

is organized in the following way:

• 01_Functions: The Matlab functions for converting the reference structure format into

different tools are stored here.

• 02_Input: Storage location of reference structure wind turbine/farm files

• 03_Output: The generated output (input files) for the different simulation tools is stored here

after executing the conversion scripts.

• PreProcessing_Main.m: This is the main Matlab script which must be executed in order to

achieve the conversion of the reference structure format into the simulation tool input.




A high priority is given to a modular structure of the framework which means that the functions can

easily be extended in order to incorporate the capability of handling new tools by the framework.

4.1.1 PreProcessing_Main.m

The file “PreProcessing_Main.m” is located on the highest level of the folder structure. This Matlab

script contains the user interaction and calls the relevant functions in order to convert the reference

data structures into the desired output format. It is split up into four blocks (s. Figure 3):

• User input: User interaction takes place by defining the folder paths and desired output

formats.

o .yaml or .mat files are allowed as input (s. section 4.2 for more information)

• Global definitions: Relevant functions are incorporated.

• Load input files: The reference data structures are read in, either as .yaml file or .mat file.

• Write output files: The functions are called which translate the reference structures into the

desired output.

4.2 Reference Structure Format

The reference structure format aims the purpose of providing a general format for the definition of

wind turbines as well as wind farms. It is of major interest to have a common data structure format

which is tool independent and can be read by at least every institution in the CL-Windcon project.

With this regard it is mentioned that initial work has already been done in defining such a data

structure within the IEA Task 37 [3]. In this task it was decided to use YAML [4] as the file format

which offers the capability to define structures in a text file being at the same time easily readable by

humans.

In this project YAML is also used in order to define the reference structure for wind turbines/farms.

Additionally to the YAML format a Matlab structure is defined which can be used in parallel for this

project. Within the framework routines are provided which allow the conversion from YAML to

Matlab and vice versa.

The definition of a wind turbine and a wind farm is separated in two files allowing the flexibility of

using the same wind turbine in different wind farms. In the following sections the reference structure

format for wind turbines and wind farms is described.

4.2.1 Wind Turbine Reference Structure

The wind turbine reference structure includes all geometric/structural/aerodynamic definitions of a

specific wind turbine. With the information in this structure the tools at least used within the project




can be fed and simulations be defined. An example file is provided containing the definitions for the

DTU 10 MW wind turbine which has been selected as one of the reference turbines for the project

[5].

The format has got a hierarchical structure ensuring a clear overview. On the highest level the

definition of the wind turbine is split up into main components of a wind turbine such as tower,

blade, nacelle etc. Also on this level one substructure is defined which is dedicated to specific

simulation parameters since still every tool has got parameters being only relevant for the

corresponding tool. All wind turbine information given in this structure is defined with respect to the

coordinate systems described in chapter 3.

In the appendix the data structure and parameters respectively are explained in more detail.

4.2.2 Wind Farm Reference Structure

Similar to the wind turbine reference structure the wind farm structure includes all information of a

specific wind farm site. This covers information about the layout of the wind farm where for instance

the turbine type and positions are defined. In other substructures the terrain and the atmospheric

conditions can be defined. Furthermore in the substructure “simulation” tool specific information

can be stored. All wind farm information given in this structure is defined with respect to the

coordinate systems described in chapter 3.

In the appendix the data structure and parameters respectively are explained in more detail.

4.3 Simulation Tool Input Formats

The Pre-Processing framework is used to generate different input formats for specific simulation

tools from a general definition of wind farms and wind turbines. The Pre-Processing framework is

built in a way to allow the implementation of new tools. For example currently the Pre-Processing

framework is able to generate the input files to run the simulation tools FAST 8 [6] and SOWFA [7].

These capabilities can be extended for other tools by implementing the necessary routines to convert

from the reference structure format to the tool input format.

The simulation tool inputs is written into the folder “03_Outputs” of the framework where a new

directory having the wind farm label is created. In this folder the simulation tool input files are

written sorted by tool name.




4.3.1 FAST 8 Input

By specifying “FAST816” as User.OutFormat in the main script the input files for a FAST v8.16

simulation are written. The information stored in the wind farm and wind turbine reference

structures is used to create the wind farm/turbine specific parameters for a FAST 8 simulation.

The files for the controller are not included and must be linked by the user.

4.3.2 SOWFA Input

By specifying “SOWFA” as User.OutFormat in the main script the input files for a SOWFA run

are written. Here only the files are adapted which are related to wind farm/turbine specific

parameters such as location of the turbines or turbine diameter. Any other simulation/numeric

specific files or parameters have to be revised by a user who has experience in SOWFA. This also

includes the run files for OpenFoam and the job system which can be found in the SOWFA example

cases.

4.3.3 Implementation of New Tools

The Pre-Processing framework can be extended to allow the generation of input files for other

simulation tools. Hereby the following steps have to be done:

1. Generate a new folder for the functions in the directory

“02_PreProcessing\01_Functions\<toolname>”

2. Add a command in the file “WriteOutput.m” to call those functions as it is done for FAST 8 or

SOWFA (s. Figure 2):

3. Add the toolname in the “PreProcessing_MAIN.m” script under User.OutFormat to

specify the demanded output for the new tool.

Figure 2: Calling routines for output




4.4 Example

An example for the tools SOWFA and FAST8 is given and can be executed. It consists of a generic

wind farm with 3 turbines in a row. The type of the wind turbine is the DTU 10 MW [5].

Open the main Matlab input file “PreProcessing_MAIN.m” (s.a. Figure 3) and use the following

information for the setup:

Parameter

User.InpFile_Format 'Mat'

User.InpFile_Turbine '.\..\02_RefStructure\WindTurbine\DTU10MW\DTU10MW_CL-

Windcon.mat'

User.InpFile_WindFarm '.\..\02_RefStructure\WindFarm\GenericWindFarm1x3.mat'

User.OutPath '.\03_Output'

User.OutFormat {'FAST816','SOWFA','YAML'}

Table 9: Pre-processing example user input

After having specified the parameters in the main script the script can be executed. Now the tool

specific input files for a FAST run or a SOWFA simulation are written in the specified output directory.

It is noted that the files for the FAST simulation is only for single turbines and cannot handle wind

farm effects.




Figure 3: Pre-processing main input file




5 POST-PROCESSING

Post-processing includes all steps which are required after having run a simulation. This starts with

converting the simulation results data into new formats and ends with analyzing the processed data

by the user.

The post-processing of wind farm simulations is a complicated procedure and needs a standardized

process in order to manage the complex and non uniform data. This is even more important when

extending the post-processing of wind farm simulations for multiple tools. For the realization of the

provided toolbox the following process is chosen:

At first the simulation results are read in by the toolbox. Then the simulation results are converted to

a common data structure which is the basis for all further steps of the post-processing. Using the

common data structure allows then the creation of secondary analysis functions such as calculating

minima or maxima. Also simple timeseries plots can be generated from this basis.

5.1 Structure of Post-Processing Framework

The framework is mainly realized with Matlab scripts and has got a specific folder structure which is

organized in the following way:

• 01_Functions: Here the Matlab functions for loading and converting the simulation data as

well as plotting the simulation data are stored.

• 02_Input: Here the simulation results of the used tools are stored.

• 03_Output: Generated plots are written in this directory.

• PostProcessing_Main.m: This is the main Matlab script which must be executed to start the

post-processing routines.

5.1.1 PostProcessing_Main.m

The file “PostProcessing_Main.m” is located on the top level of the folder structure. This Matlab

script contains the user interaction and calls the relevant functions. The user input section is divided

into 2 blocks. The first block contains information about the input files and has got the following

parameters:

• filenameOutput: The result files containing the aeroelastic and operating information of the

wind turbines in the wind farm is linked. For each turbine in a wind farm array a file-path has

to be set.

• simTool: The simulation tool corresponding to the “filenameOutput” file is filled in.




• filenameInput: Contains the reference structure format for each wind turbine. This is needed

to get information where the sensors of the wind turbine are located.

• pathFlowfield: Sets the path to the directory containing the flow field results.

In the second block the user has to set information in order to analyse the flow field of the windfarm.

This block is described in more detail in chapter 5.3.

5.2 Data Structure Format of Post-Processing

The post-processing toolbox is coded in Matlab object oriented language. The main classes are the

wind turbine class and the wind farm class. These classes define the data structure of the simulation

tool outputs. The results of each tool are filled into the post-processing data structure in order to be

able to apply general analysis functions on a uniform format. For each simulation tool a subclass of

the main classes is set which inherits the properties of the main classes.

In the attached Excel-file “PostProcessing_SensorList.xlsx” an overview about the structure and

possible sensors for analysis is given.

5.3 Analysis Functions

Currently rudimentary plotting functions are supported. The plotting functions are divided into

analysis of aeroelastic and wind farm operating data as well as flow field data.

5.3.1 Timeseries Plots

The category timeseries plots defines the type of 2D-plot where in a diagram the x-axis is the time

and on the y-axis a specified sensor is plotted. In Table 13 the possible sensor types are listed which

can be chosen by the user in order to be analyzed.

5.3.2 Sectional Analysis Plots

For some components of a wind turbine it makes sense to make an analysis of multiple sections. For

example the blade has usually different sections distributed along the blade. In each section there is

information about for instance the angle of attack or the tangential force. In a sectional analysis the

chosen sensor can be plotted along the blade for a specific timestep. In Table 13 the possible sensor

types are listed which can be chosen by the user for a sectional analysis.

5.3.3 Flow field Analysis

The flow field analysis is currently working for SOWFA simulations. Slices of the flow field containing

information about velocity or turbulence intensity can be analyzed. These slices are written by the

SOWFA simulation and have to be defined before running a SOWFA simulation. An example is given

in chapter 5.5.




The flow field data of the SOWFA simulation has to follow specific file format conventions. The

general file structure for flow field data can be seen in Figure 4. On the top level the timestep of the

output files is given. Then different slices are defined containing the files which include the output

information of SOWFA.

Figure 4: Post-processing: SOWFA flow field file structure

In the main script of the post-processing toolbox “PostProcessing_Main.m” parameters for the flow

field analysis must be set as can be seen in Figure 5. The explanation of the parameters is given in

Table 10. Contour plots are generated for all variables which are available in the corresponding

directory. Optionally 2D plots can be defined in which a specific quantity is plotted along a line on a

slice.

time Simulation time of the slices

which are to be analyzed; Must

correspond to the folder name

as can be seen in Figure 4.

sliceName Name of the slice; Must

correspond to the folder name

as can be seen in Figure 4.

numTime Number of time

numSlice Number of slice

Parameters for 2D plot

quantity Possible options:

- Velocity components (u,v,w)

or magnitude (mag)




- Power density (pd)

- Turbulence intensity (TI)

endPointCooA Coordinate of end point A of the

line

endPointCooB Coordinate of end point B of the

line

res Resolution of line, i.e. how many

points are used for the line

Table 10: Post-processing: flow field parameters

Figure 5: Post-processing: flow field analysis parameters

5.4 Supported Tools

5.4.1 FAST 8

FAST 8 format is supported and result files of a FAST 8 simulation can be analyzed. An example is

given in chapter 5.5.

5.4.2 SOWFA

A SOWFA simulation can be analyzed which is coupled to FAST 8. Also specific 2D-slices of the flow

field can be plotted. The example in chapter 5.5 can be taken as reference.




5.5 Post-processing: Example (small wind farm)

An example for the post-processing of a small generic wind farm simulation is set up and can be

executed. It is a 3 turbine case where all turbines are placed in a row behind each other. The

simulation is done in SOWFA which is coupled to FAST.

The example files are located in the folder “04_PostProcessing\02_Input\Example”. The folder itself

has got the following structure:

1. Definition: Here the wind turbine information used in the simulation is stored. The wind

turbine information is stored in the reference structure format as defined for the pre-

processing.

2. Aeroelastic: In this folder the aeroelastic simulation results must be copied. In this specific

case the FAST output files of the SOWFA-simulation are stored here. Each of the 3 turbines

has got its own FAST file.

3. Flowfield: Flow field data of the SOWFA run is stored here.

After having copied all relevant files into those directories the post-processing main script can be

opened. This script should look like Figure 6 having the following properties:

• filenameOutput: The aeroelastic output files of the SOWFA-simulation are linked

• simTool: The type of the simulation tool which has been used for the calculation of the

aeroelastic wind turbine data is written. This has to be correspond to the inputs of

“filenameOutput”. In this case FAST is the tool which is linked to SOWFA.

• filenameInput: The reference structure format file of the wind turbine has to be linked for

each turbine of the wind farm. In this case this is the DTU 10MW turbine.

• pathFlowfield: The path for the flow field information must be filled.

The parameters specifying the analysis of the flow field data are following. Note: For this example the

flow field data is not corresponding to the aeroelastic data. Nevertheless the values which are filled

in can be used.




Figure 6: Post-processing main input file




Now the script can be executed. At first a window pops up in which the type of the analysis must be

chosen. Choose “rotational speed” as the parameter to be analyzed and “Timeseries” as the type of

analysis (s.a. Figure 7).

Figure 7: Post-processing example: User input analysis type

In a second prompt it is asked which component is to be analyzed. Type in “Rotor” to get the rotor

speed of the turbines.

Figure 8: Post-processing Example: component selection




Now a plot is created in which the rotor speed of all 3 turbines in the wind farm are plotted (s. Figure

9).

Figure 9: Post-processing example: timeseries plot rotor speed

Running the script again and setting the “type” to “Flowfield” activates the plotting of 2D-contour

plots for different flow field quantities such as velocity or turbulence intensity. Hereby the

parameters which are described in chapter 5.3.3 are taken.

Figure 10: Post-processing example: flow field contour plot of TI




6 REFERENCES

[1] C. Fichtner, "Entwicklung eines Standardisierten Frameworks zur Auswertung und zum Vergleich

von Simulationsdaten und Messdaten," Stuttgart, 2017.

[2] The MathWorks, Inc., "MATLAB Release 2015b," Natick, Massachusetts, United States.

[3] "IEA Task 37," [Online]. Available: https://sites.google.com/site/ieawindtask37/. [Accessed 07

June 2017].

[4] "YAML Format," [Online]. Available: http://yaml.org/. [Accessed 18 May 2017].

[5] Bak C, Zahle F, Bitsche R, Kim T, Yde A, Henriksen L C, Natarajan A, Hansen M, , " Description of

the DTU 10 MW Reference Wind Turbine," DTU Wind Energy Report-I-0092, 2013.

[6] J. Jonkman, "NWTC design codes (FAST)," 2017. [Online]. Available:

http://wind.nrel.gov/designcodes/simulators/fast. [Accessed 20 08 2017].

[7] M. Churchfield and S. Lee, "NWTC design codes (SOWFA)," 2017. [Online]. Available:

http://wind.nrel.gov/designcodes/simulators/SOWFA. [Accessed 20 08 2017].

[8] Simpack, "Simpack 9.9.1 Documentation".




7 APPENDIX A

7.1 Reference Structure Format: Definitions

7.1.1 Wind Turbine

The reference structure format has got a hierarchical structure. This is implemented in Matlab via

Matlab-structures. On the top level the wind turbine is split into its main components and a few

further categories (s. Figure 11). Those fields are explained in the following sections.

Figure 11: Reference structure: top level

Blade

The definitions for the blade are split into aerodynamic and structural properties. Furthermore

integrated quantities such as the total blade mass are given (s. Figure 12).

Figure 12: Reference structure: blade definition

In the field “Structure” the parameters describing the blade structure are stored. These are explained

in Table 11 more thoroughly and are referring to [8]. The convention for the definitions follows

Figure 13.




Name Description Unit

Eta Dimensionless distance of section along the pitch axis from blade root [-]

Chord Chord length of blade section [m]

PreBend Prebend of blade-section in flapwise direction given in BRS-CS. [m]

PreSweep Presweep of blade-section in edgewise direction given in BRS-CS. [m]

LeadingEdge This is calculated from the position of the element reference system

and the chord length.

[%]

EA Tensile Stiffness. The local tensile stiffness per unit length [N m]

EIxx Flexural Stiffness in plane. The in plane flexural stiffness per unit length

is given about the xERS-axis at the elastic center.

[kg m]

EIyy Flexural Stiffness out of plane. The out of plane flexural stiffness per

unit length is given about the yERS-axis at the elastic center.

[kg m]

EIxy Flexural Stiffness Cross. The cross flexural stiffness per unit length is

given about the x-y-axis at the elastic center.

[kg m]

GJ Torsional Rigidity. The local torsional rigidity per unit length. [N m^2]

CenterOfMass Center of Gravity. The position of the center of gravity w.r.t. the ERS-

CS.

[m]

ShearCenter The position of the shear center w.r.t. ERS-CS. [m]

ElasticCenter The position of the elastic center w.r.t. ERS-CS. [m]

MassPerLength The parameter describes the mass per unit length distribution for this

station.

[kg/m]

TwistStiff Structural Twist [deg]

TwistInertia Inertia Twist [deg]

Jxx Inertia in plane [kg m]

Jyy Inertia out of plane [kg m]




Jxy Inertia cross [kg m]

Table 11: Reference structure: blade structural properties [7]

Figure 13: Definition of blade geometry parameters and coordinate systems in the reference structure [8]

The aerodynamic parameters of the blade are stored in the field “Aerodynamic” and explained in

Table 12. Those definitions can be seen independent of the structural definitions.


Eta Dimensionless distance of section along the pitch axis from blade root [-]

Chord Chord length of blade section [m]

Twist Aerodynamic twist angle of section [deg]

NFoil Number of airfoil from field “Airfoils” [-]




AeroCenter Aerodynamic center of section. The position of the aerodynamic center is

calculated from the parameters ’chord’ and ’leading edge’

[%]

Thickness The thickness of the airfoil can be scaled as a percentage with respect to the

chord.

[%]

Airfoils Field including the aerodynamic properties of each airfoil used:

Eta = position of airfoil on blade

Thick = thickness in percent

AirTable = Polar data of airfoil

Coord = 2D coordinates of airfoil

[-]

Table 12: Reference structure: blade aerodynamic properties

PerformanceGlobal

The field “PerformanceGlobal” contains the information about the designed performance of the wind

turbine. This includes a normal power curve defining the power output depending on the wind speed

and blade pitch angle. Furthermore in the field “CP_TSR” the dimensionless performance coefficients

cP, cT and cQ of the wind turbine are stored.

Controller

In the field “Controller” the basic parameters are stored which are used to describe the Controller

behaviour. At the moment this field basically includes the information which controller “.dll” file is

used and where it is stored.

Drivetrain

Information about the drivetrain is located in the field “Divetrain”.


MSSInertia Inertia about medium speed

shaft (MSS)

[kg m^2]

MSSFullyDeployedBrakeTorque Torque of fully deployed MSS [N m]

MSSBrakeTimeConstant Time for MSS-brake to reach

full deployment once initiated

[s]

StructuralDamping [%]




GenRatedSpeed [rpm]

GearboxRatio [-]

EffTablePower Efficiency table of generator [%]

GenEfficiency Average generator efficiency [%]

GenSyncSpeed [rpm]

GenPowerRated [W]

GearboxEff Average of Gearbox efficiency [%]

Hub


InertiaAboutShaft Inertia of hub around shaft [kg m^2]

InertiaOffShaft [kg m^2]

MassCenter [m]

Mass [kg]

Nacelle


InertiaYaw Inertia of nacelle around z-axis

of nacelle-CS

[kg m^2]

InertiaRolling Inertia of nacelle around x-axis

of nacelle-CS

[kg m^2]

InertiaNodding Inertia of nacelle around y-axis

of nacelle-CS

[kg m^2]

Length Length of nacelle (x-direction) [m]

Width Width of nacelle (y-direction) [m]

Height Height of nacelle (z-direction) [m]




Twr2Shft Vertical distance from the

tower-top to the rotor shaft

[m]

DragCoeff Drag coefficient of nacelle [-]

Mass Nacelle mass [kg]

CGDownwindYA Downwind distance from the

yaw-axis to the nacelle CG

[m]

CGaboveTT Vertical distance from the yaw-

axis to the nacelle CG

[m]

CGSide Lateral distance from the yaw-

axis to the nacelle CG

[m]

YawSpring Torsional spring of yaw joint [N m rad^-1]

YawDamp Torsional damper of yaw joint [N m rad^-1 s^-1]

RotorGlobal


Orientation 1 for clockwise [-]

NbBlades Number of blades [-]

RotorDiameter [m]

HubHeight [m]

ConeAngle [°]

UpTilt [°]

Overhang Distance from yaw axis to rotor

apex

[]

BladeRoot Radius of blade root [m]

JRotorTotal Total inertia of rotor [kg m^2]




HubDiameter [m]

RatedSpeedRPM Rated rotor speed [rpm]




7.2 Post-Processing Sensor Types

A variety of sensor types can be selected for post-processing. In Table 13 the sensor types are listed

and it is defined which analysis type can be applied on the sensor.

Sensor Type Description Timeseries

Analysis

Sectional

Analysis

power Power output of wind turbine x

rotational speed Rotational speed (e.g. rotor, generator) x

thrust Total thrust of rotor x

torque Torque of rotor shaft x

azimuth Azimuth position (e.g. rotor) x

total load Total load on rotor x

pitch Blade pitch x

yaw Yaw position of nacelle x

force x x

moment x x

deflection

Displacement of component in specified

direction

x

x

acceleration x x

coefficients

Coefficients (e.g. cP, cT); also coefficients for

blade sections x

x

dynamic pressure Dynamic pressure at blade section x x

reynolds number Reynolds number at blade section x x

induction Induction at blade section x x

motion Motion of nacelle x

flowfield Flow field analysis

Table 13: Post-processing sensor types

Documents

A Common Pre- and Post-processor for Wind Farm Simulations