Wind energy generation systems - Part 12-4: Numerical site calibration for power performance testing of wind turbines

IEC TR 61400-12-4:2020 summarizes the current state of the art in numerical flow modelling, existing guidelines and past benchmarking experience in numerical model validation and verification. Based on the work undertaken, the document identifies the important technical aspects for using flow simulation over terrain for wind application as well as the existing open issues including recommendations for further validation through benchmarking tests.

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Published
Publication Date
21-Sep-2020
Current Stage
PPUB - Publication issued
Completion Date
22-Sep-2020
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IEC TR 61400-12-4
Edition 1.0 2020-09
TECHNICAL
REPORT
Wind energy generation systems –
Part 12-4: Numerical site calibration for power performance testing of wind
turbines
IEC TR 61400-12-4:2020-09(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 61400-12-4
Edition 1.0 2020-09
TECHNICAL
REPORT
Wind energy generation systems –
Part 12-4: Numerical site calibration for power performance testing of wind
turbines
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180 ISBN 978-2-8322-8781-1

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 61400-12-4:2020 © IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms, definitions, abbreviations and symbols ................................................................. 7

3.1 Abbreviations .......................................................................................................... 7

3.2 Symbols and units................................................................................................... 8

4 Overview of Numerical Flow Simulation Approaches ...................................................... 10

4.1 Linear Flow Models ............................................................................................... 10

4.2 Reynolds-averaged Navier-Stokes (RANS) Models ............................................... 11

4.3 Large Eddy Simulation (LES) and Hybrid RANS/LES Models ................................ 12

5 Existing Guidelines for Numerical Flow Modelling Applications ...................................... 13

5.1 General ................................................................................................................. 13

5.2 AIAA (1998) Guide for the Verification and Validation of Computational Fluid

Dynamics Simulations ........................................................................................... 14

5.3 Standard for Verification and Validation in Computational Fluid Dynamics

and Heat Transfer – ASME V&V 20-2009 .............................................................. 14

5.4 COST Action 732 “Quality Assurance of Microscale Meteorological Models” ......... 15

5.5 Architectural Institute of Japan Guidelines ............................................................ 16

5.5.1 General ......................................................................................................... 16

5.5.2 The guidebook for practical applications of CFD to pedestrian wind

environment around buildings [18] ................................................................. 16

5.5.3 Guidebook of recommendations for loads on buildings 2 [19] ......................... 16

5.6 VDI 3783 Part 9 “Environmental meteorology – prognostic microscale wind

field mode- evaluation of flow around buildings and obstacles” ............................. 16

5.7 International Energy Agency Task 31 Wakebench – Model Evaluation

Protocol for Wind Farm Flow Models ..................................................................... 17

5.8 MEASNET – Evaluation of Site-Specific Wind Conditions ...................................... 17

6 Summary of Benchmarking Validation Tests .................................................................. 17

6.1 General ................................................................................................................. 17

6.2 DEWI Round Robin on Numerical Flow Simulation in Wind Energy ....................... 17

6.3 Bolund Experiment ................................................................................................ 18

6.4 European Wind Energy Association Comparative Resource and Energy Yield

Assessment Procedures I and II (2011, 2013) ....................................................... 18

6.5 IEA Task 31 Wakebench Experiments ................................................................... 19

6.6 New European Wind Atlas Experiments [32] ......................................................... 19

6.6.1 Perdigão (double ridge) ................................................................................ 19

6.6.2 Alaiz (complex terrain with a strong mesoscale component) .......................... 19

6.6.3 Østerild (flow over heterogeneous roughness) ............................................... 19

6.6.4 Kassel (flow over forested hill) ....................................................................... 20

6.7 Wind Forecast Improvement Project 2 [34] ............................................................ 20

6.8 Wind Tunnel Test Validation Data ......................................................................... 20

6.8.1 Compilation of Experimental Data for Validation of Microscale

Dispersion Models [23] .................................................................................. 20

6.8.2 AIJ wind tunnel .............................................................................................. 20

6.8.3 Wind tunnel test for flow over hill ................................................................... 20

---------------------- Page: 4 ----------------------
IEC TR 61400-12-4:2020 © IEC 2020 – 3 –
7 Important Technical Aspects for Performing Flow Simulations over Terrain for

Wind Energy Applications .............................................................................................. 21

7.1 General ................................................................................................................. 21

7.2 Quality of Topographical Input Data ...................................................................... 21

7.3 Computational Domain .......................................................................................... 21

7.4 Boundary Conditions for Computational Domain ................................................... 21

7.5 Mesh Parameters .................................................................................................. 21

7.6 Convergence Criteria ............................................................................................ 21

7.7 Atmospheric Stability ............................................................................................ 21

7.8 Coriolis Effects ..................................................................................................... 22

7.9 Obstacles effects .................................................................................................. 22

7.10 Suggestion on Model Range Applicability for NSC ................................................ 22

8 Open Issues .................................................................................................................. 22

8.1 General ................................................................................................................. 22

8.2 Determination of Flow Correction Factors from Numerical Simulation Results

for Power Curve Testing ....................................................................................... 23

8.2.1 General ......................................................................................................... 23

8.2.2 Correlation check for linear regression .......................................................... 23

8.2.3 Change in correction between adjacent wind direction bins ........................... 23

8.2.4 Site calibration and power performance measurements in different

seasons ......................................................................................................... 23

8.3 Uncertainty quantification ...................................................................................... 23

8.4 Proposal for Validation Campaign for NSC Procedures ......................................... 24

8.4.1 General ......................................................................................................... 24

8.4.2 Assessment of terrain at the test site ............................................................. 24

8.4.3 Experimental layout ....................................................................................... 24

Bibliography .......................................................................................................................... 26

Table 1 – symbols used in this Technical Report ..................................................................... 8

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– 4 – IEC TR 61400-12-4:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –
Part 12-4: Numerical site calibration for power
performance testing of wind turbines
FOREWORD

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all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international

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The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a Technical Report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 61400-12-4, which is a Technical Report, has been prepared by IEC technical committee

88: Wind energy generation systems.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
88/729/DTR 88/774/RVDTR

Full information on the voting for the approval of this Technical Report can be found in the

report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 6 ----------------------
IEC TR 61400-12-4:2020 © IEC 2020 – 5 –

A list of all parts of the IEC 61400 series, under the general title Wind energy generation

systems, can be found on the IEC website.

Future standards in this series will carry the new general title as cited above. Titles of existing

standards in this series will be updated at the time of the next edition.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 7 ----------------------
– 6 – IEC TR 61400-12-4:2020 © IEC 2020
INTRODUCTION

IEC 61400-12-1 [1] is the International Standard for power performance measurements for

electricity producing wind turbines. It specifies that in complex terrain, a site calibration (SC) is

required to find the relation in flow characteristics between the measurement location and the

test turbine. This approach requires – in addition to the permanent measurement mast that is

used to measure the turbine power curve – installing a temporary mast at the location of the

turbine being tested, prior to the turbine installation. The IEC 61400-12-1 approach is frequently

used in industrial practice; however, it has a number of disadvantages:

• additional cost of the second mast and analysis of the site calibration results,

• additional time required for the site calibration in the range of 3 months,
• a site calibration decision has to be made before installing the wind turbine.

Due to these disadvantages, there is interest in the industry to find alternative methods for site

calibration. One alternative is to use numerical simulations to derive flow correction factors

(FCFs), i.e., the relation between wind speed at the wind turbine position and wind speed at the

reference meteorological mast position.

The IEC TC 88 committee, “Wind energy generation systems,” initiated the work on this

document to evaluate the potential application of numerical flow simulations for site calibration,

i.e., numerical site calibration (NSC).

With NSC, the flow correction factors are calculated using numerical simulation of the flow.

Despite eliminating some of the disadvantages mentioned earlier, NSC brings other challenges:

• dependence on simulation models,
• dependence on the setup of these models,
• dependence on the modeler’s expertise,
• uncertainty quantification of the model performance.

The project team (PT 61400-12-4) has outlined the current state of the art in numerical flow

modelling and has summarized existing guidelines and past benchmarking experience of

numerical model validation and verification. Based on the work undertaken, the project team

identified the important technical aspects for using flow simulations over terrain for wind energy

applications as well as the existing open issues including recommendations for further

validation through benchmarking tests. The project team concluded that further work is needed

before a standard for NSC can be issued.
___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 8 ----------------------
IEC TR 61400-12-4:2020 © IEC 2020 – 7 –
WIND ENERGY GENERATION SYSTEMS –
Part 12-4: Numerical site calibration for power
performance testing of wind turbines
1 Scope

This part of IEC 61400, which is a Technical Report, summarizes the current state of the art in

numerical flow modelling, existing guidelines and past benchmarking experience in numerical

model validation and verification. Based on the work undertaken, the document identifies the

important technical aspects for using flow simulation over terrain for wind application as well as

the existing open issues including recommendations for further validation through

benchmarking tests.
2 Normative references
There are no normative references in this document.
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
No terms and definitions are listed in this document.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.2 Abbreviated terms
The following abbreviated terms are used in this document.
AIAA American Institute of Aeronautics and Astronautics
ABL atmospheric boundary layer
AEP annual energy production
AIJ Architectural Institute of Japan
ALEX17 Alaiz experiment 2017
ASME American Society of Mechanical Engineers
CEDVAL Compilation and Experimental Data for Validation of Microscale
DispersionModels
CFD computational fluid dynamics
CHT computational heat transfer
COST European Cooperation in Science and Technology
CREYAP Comparative Resource and Energy Yield Assessment Procedures
DES detached eddy simulation
DDES delayed detached eddy simulation
DEWI Deutsches Windenergie-Institut
---------------------- Page: 9 ----------------------
– 8 – IEC TR 61400-12-4:2020 © IEC 2020
DTU Danish Technical University
EWEA European Wind Energy Association
EWTL Environmental Wind Tunnel Laboratory
FCF flow correction factor
GWh gigawatt-hour
IEA International Energy Agency
IEC International Electrotechnical Commission
LES large eddy simulation
LIDAR light detection and ranging
MEASNET Measuring Network of Wind Energy Institutes
MEP model evaluation protocol
NEWA New European Wind Atlas
NSC numerical site calibration
RANS Reynolds-averaged Navier-Stokes
RNG renormalization group
SC site calibration
SODAR sound detection and ranging
TC technical committee
TR technical report
UQ uncertainty quantification
URANS unsteady Reynolds-averaged Navier-Stokes
V&V verification and validation
VDI Verein Deutscher Ingenieure
WAsP Wind Atlas Analysis and Application Program
WFIP Wind Forecast Improvement Project
WTG wind turbine generator
3.3 Symbols and units
Table 1 shows the symbols used in the text and equations in this document.
Table 1 – Symbols used in this document
Symbol Definition Unit
th m/s
i component of filtered wind speed
filtered pressure Pa
µ molecular viscosity Pa s
turbulence viscosity Pa s
Smagorinsky constant -
κ von Karman constant -
distance to the nearest wall m
local filter size m
turbulence length scale m
---------------------- Page: 10 ----------------------
IEC TR 61400-12-4:2020 © IEC 2020 – 9 –
Symbol Definition Unit
turbulence length scale obtained from RANS model m
RANS
turbulence length scale obtained from LES model m
LES
model constant of DDES model m
m/s
average component of velocity in the direction i
m/s
u i
turbulent component of velocity in the direction
space variable in the direction i
average pressure Pa
ρ density
kg/m
kinematic molecular viscosity
m /s
kg m / s
body forces in the direction
2 2
Reynolds stresses
m /s
i j
Kronecker’s delta -
kinematic turbulence viscosity
m /s
2 2
turbulence kinetic energy
m /s
turbulence length scale m
2 3
m /s
P production of k
2 3
dissipation rate of turbulence kinetic energy
m /s
RANS turbulence model constant -
RANS turbulence model constant -
RANS turbulence model constant -
RANS turbulence model constant -
validation comparison error
error due to the modelling assumptions
model
error due to numerical solution of the equations
num
error due to input parameters
input
error in the experimental values
validation standard uncertainty
val
numerical solution uncertainty
num
input parameters uncertainty
input
experimental value uncertainty
r correlation coefficient -
DDES parameter -
---------------------- Page: 11 ----------------------
– 10 – IEC TR 61400-12-4:2020 © IEC 2020
Symbol Definition Unit
modified DDES constant / stepwise function -
DDES constant -
effective horizontal kinematic viscosity
m /s
effective vertical kinematic viscosity
m /s
m/s
velocity perturbation components in the direction i
pressure perturbation Pa
m/s
U horizontal velocity components of the unperturbed flow in the direction j
rotor diameter m
4 Overview of numerical flow simulation approaches
4.1 Linear flow models

Since the late 1980s, when computing resources were limited, linear wind flow models have

been the standard for wind resource assessment. These models are based on a linearization

of the Navier-Stokes equations, which was originally introduced in reference [2]. They were

designed to be used reliably in neutral atmospheric conditions over terrain with sufficiently

gentle slopes to ensure fully attached flow conditions.
0, for i 1,…,3 (1)
 
  
∂u ∂∂p ∂∂uu
i ii
U =−+ K  + K , for i =1,..,3 and j =1,2 (2)
j hv
 
∂x ∂x ρ ∂∂xx
j i j j
  3
Here, are the horizontal velocity components of the unperturbed flow, are
Uj =1,2 ui( = 1,…,3)
( )
j i

the velocity perturbation components, and 𝑝𝑝� is the pressure perturbation. K and K are the

h v
effective kinematic viscosities in the horizontal and vertical directions.

Linear models perform reasonably well where the wind is not significantly affected by steep

slopes, flow separation, thermally driven flows, low-level jets, and other dynamic and nonlinear

ABL phenomena.

The Wind Atlas Analysis and Application Program (WAsP) [3] has been the most widely used

amongst the linear models. WAsP procedures may be considered as a transfer function model

linking the wind speeds at the reference with those at the predicted locations. Significant

sources of error could be related to the terrain complexity, massive flow separation, wind

direction changes, and varying atmospheric conditions. The latter include, among others,

channeling effects, blocking effects, and thermally driven flows (e.g., diurnal sea breezes,

downslope winds).

Due to their fast and robust performance, linear models are still used in the wind industry.

= =
---------------------- Page: 12 ----------------------
IEC TR 61400-12-4:2020 © IEC 2020 – 11 –
4.2 Reynolds-averaged Navier-Stokes (RANS) models

Due to the limitation of the linear models, as mentioned in 4.1, computational fluid dynamics

(CFD) models became more widely used in the wind industry. CFD application to the

atmospheric boundary layer (ABL) has been influenced by both CFD for mechanical engineering

and mesoscale meteorological modelling. CFD considers momentum and mass conservation

equations with four unknown variables: pressure and three velocity components. Other

variables describing the atmospheric state, such as temperature, humidity, and aerosol

concentration, are usually not considered.

Typical CFD for atmospheric flow simulation applications follow the single wind direction

approach representing a sector from the discretized wind rose. Flow simulations for each sector,

considering the effects of orography and roughness, result in speed-up factors.

In the Reynolds-averaged Navier-Stokes (RANS) approach [4], due to the turbulent nature of

the flow, the variables are described with statistical functions divided into average and

fluctuating (turbulent) components (e.g., U U+ u resulting in RANS equations:
i i i
 
∂∂(ρUU) (ρ ))∂(ρU
∂∂p
ii i
=0 and U =−+ ν − ρuu + F for i,, j =1..,3 (3)
 
j i j i
∂x ∂x ∂∂xx ∂x
 
i j ij j
 

In the RANS equations, turbulent correlations uu (also called turbulent fluxes or stresses)

i j

have to be parameterized to close the equation system. The Boussinesq hypothesis is used to

define the relation between turbulent fluxes and the gradients of mean values by introducing

eddy viscosity (first-order closure):
∂U 2
−=uu νδ+ − k for i, j 1,..,3 (4)
i j T ij
∂∂xx 3

Two basic quantities are introduced to describe the turbulence: the kinematic turbulence

viscosity ν , and the turbulence kinetic energy, k . The kinematic turbulence viscosity depends

12/

on the turbulence kinetic energy, k , and the size of the turbulent eddies, L , as ν = kL .

T TT

There are different types of closures, e.g., one- and two-equations models. In the one-equation

model, the turbulence kinetic energy, k , equation is solved:
ν
∂k k ∂∂k
U =PC−+ for j=1,..,3 (5)
j k µ
∂x νσ∂∂xx
j T j kj

where P is the production of k due to mean wind velocity gradients. The turbulence length

scale, , is deduced from an analytical model, such as a function of the height above the

ground and sometimes the thermal stability [5].

In the two-equation models ( k - ε , RNG k - ε , k - ω , ...), the closure is made through two

transport equations, one for k and one for the turbulence dissipation, ε :
∂∂kk∂ ν
UP=-ε+ for j=1,..,3 (6)
j k
∂x ∂∂xxσ
j j kj
---------------------- Page: 13 ----------------------
– 12 – IEC TR 61400-12-4:2020 © IEC 2020
∂∂ε ε ∂ ν ε
U=CP - C ε+ for j=1,..,3 (7)
( ) 
jk12εε
∂xk ∂∂x σ x
j j  ε  j 
The kinematic turbulence viscosity , is given by the closure equation ν = C .
T T µ

Without the steady-state hypothesis used in the RANS model, the equations of motion can be

described with unsteady RANS (URANS).

Compared to the linear model, the RANS steady-state model is able to predict flow detachment

and reattachments in the separation zone in most cases, but the accuracy of the results in this

region is questionable. This limitation is inherent in the statistical nature of the model. The

RANS model is mostly applied under the
...

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