Aerodynamics & Aeroelasticity: Certification of Wind Turbines Σπύρος Βουτσινάς / Spyros Voutsinas

Εθνικό Μετσόβιο Πολυτεχνείο National Technical University of Athens Aerodynamics & Aeroelasticity: Certification of Wind Turbines Σπύρος Βουτσινάς / Spyros Voutsinas

Άδεια Χρήσης Το παρόν εκπαιδευτικό υλικό υπόκειται σε άδειες χρήσης Creative Commons. Για εκπαιδευτικό υλικό, όπως εικόνες, που υπόκειται σε άδεια χρήσης άλλου τύπου, αυτή πρέπει να αναφέρεται ρητώς. Aerodynamics & Aeroelasticity Certification of WT 2

Outline 1. What is certification who needs it and why? certification for designers and customers 2. What is the basic philosophy of standards? design classes, site assessment, modelling requirements 3. How can we describe real (turbulent) wind? description of turbulent wind, modelling principles, extremes 4. What can realistic simulations offer? spectral analysis, cumulative spectra & fatigue analysis 5. Review of the IEC standard ultimate and fatigue analysis, normal and extreme operation, faults 6. Some indicative examples Aerodynamics & Aeroelasticity Certification of WT 3

1. What is certification who needs it and why? Certification is standard in engineering. It guarantees the operational reliability and safety of a product, in our case a wind turbine. Both aspects are closely related to external conditions, in our case basically the wind flow (but also the grid, ice, earthquake etc) In order to meet its targets, certification needs to define standards. Standards aim at answering in the best possible way questions like: Will the wind turbine last? Will it produce what it is designed for? Is its operation free of risk? To this end we need measurements and theory. Theory is important because we cannot measure everything. Measurements are necessary because our theories but also manufacturing are subjected to uncertainties Aerodynamics & Aeroelasticity Certification of WT 4

What is certification who needs it and why? Certification is addressed to designs and customers. The designer will certify his product while the customer will certify/secure his investment. Both aspects share one common thing: the conditions. There are normal conditions and extreme conditions. Wind turbines will operate in normal conditions for most of their life-time. Still however, since normal conditions are not steady, wind turbines are subjected to failure because of fatigue Extreme conditions are related to safety. In fact they test the wind turbine for strength. The designer choose the conditions against which he wishes to test his machine On the contrary the customer chooses wind turbines for given conditions Aerodynamics & Aeroelasticity Certification of WT 5

2. What is the philosophy of standards? The central point of all standards is classification. The current version of the IEC standard defines 3 main classes. All other cases are included in a special class which is free. Classes are defined with respect to basic wind conditions: a reference velocity and a reference turbulence intensity I ref There are three different values for I ref allowing to account for site specific conditions (as experience has shown) Aerodynamics & Aeroelasticity Certification of WT 6

What is the philosophy of standards? Based on V ref and I ref theory allows to define all wind dependent information we need. It includes: The long terms statistics The turbulent spectrum of the wind The extreme wind conditions Describing real wind plays a central part in assessing designs or selecting wind turbines. After all wind is our excitation mechanism Aerodynamics & Aeroelasticity Certification of WT 7

What is the philosophy of standards? The designer is therefore asked to produce for the specific wind characteristics of the class he has chosen, all the necessary information. It includes both calculations and tests. The customer is asked to perform a site assessment and define the class corresponding to it. He also will present measurements and calculations. In case the site assessment concludes that the definition of the classes is not appropriate, then an S class is defined and the designer is asked to check his design for these special conditions. The standard besides specifying the content of the information, also specifies in either a normative or informative way the theory which should/could be used. Wind modelling, wake effects in clusters, fatigue analysis are among the topics covered. Aerodynamics & Aeroelasticity Certification of WT 8

3. How can we describe real (turbulent) wind? The significance of scale In nature wind flows vary in space and time in an arbitrary (stochastic) way. Variability can be due to flow particularities, such as terrain effects, atmospheric conditions, wake effects but also turbulence. Because of arbitrariness, wind is described in a probabilistic/ statistical/ stochastic context. In this connection time scale is important. For wind potential assessment we need long term statistics: how wind is distributed over the year. The Weibull or the Rayleigh distributions for the 10min averaged wind velocity, are used: k V o P(V V o) 1exp, k 2 kvave V 0.2V For strength and fatigue analysis this is not appropriate. We need to take into account smaller scales: namely those contained within a 10min period. A different but very important issue is also the estimation of extremes. Aerodynamics & Aeroelasticity Certification of WT 9 ave ref

How can we describe real (turbulent) wind? The turbulent wind model We need to describe wind in the form of time series for the all three components over a box which covers the rotor diameter and has enough length so as to cover the duration of a simulation. This box will move and pass through the rotor disk at a speed equal to the mean speed at hub-height. So for a 10min simulation the length should be at least 60*U Hub. In order to include the turbulent scales needed, in one of the methods we can use, we assume: a spectrum at one point (e.g. the hub height), for the wind velocity vector, a coherence function in space So the idea is to extract from these data time series covering the length of our box. The method is based on factorization Coh( f,δr,u) G(f ) Aerodynamics & Aeroelasticity Certification of WT 10

How can we describe real (turbulent) wind? 1. We first define the discrete spectral matrix: f S jj(f m ) G jj(f m ) 2 S (f ) Coh (f, r,u ) S (f )S (f ) jk m jk m jk jj m kk m S(f ) H(f )H (f ) *T m m m In case we assume that H is assumed to be lower triangular there is simple algorithm for evaluating H 2. We factorise the spectral matrix: 3. The elements of H can be regarded as the weighting factors for the linear combination of independent, unit-magnitude, white-noise inputs that yield correlated outputs with the correct spectral matrix. So in order to produce the desired time series we just need to add random phases: j j i V(f j m) H jk(f m)x k(f m) H jk(f m)e k1 k1 km Aerodynamics & Aeroelasticity Certification of WT 11

How can we describe real (turbulent) wind? It is important to know that phases are uniformly distributed values over [0,2p] This means that we can have several realizations. The standard specifies at least 6 realizations. The reason is shown in the figures Aerodynamics & Aeroelasticity Certification of WT 12

How can we describe real (turbulent) wind? Evaluation of extremes We are interested on the following extremes: Extreme wind speed over a certain period Extreme direction change over a certain period Extreme gust (max change within a short time interval) over a certain period The scale or the period within which we search extremes is large (say 1 year) so in this case we are based on long term statistics, e.g. based on Weibull: k1 ku U p(u) W[U;C,k] exp CC C To this end we need: A properly defined probability density function of the event A theory to evaluate extremes k Aerodynamics & Aeroelasticity Certification of WT 13

How can we describe real (turbulent) wind? 2.5 2.4 2.3 C1 CU U p(u) W[U;A,C] exp AA A C shape parameter, c 2.2 2.1 2.0 1.9 1.8 Class IV Class III Extreme wind speed (m/s) during 50 year versus the Weibull parameters. Confidence level 95%, averaging time 3 sec. 1.7 1.6 1.5 Class II Class I 1.4 1.3 4 5 6 7 8 9 10 11 12 scale parameter, A Aerodynamics & Aeroelasticity Certification of WT 14

How can we describe real (turbulent) wind? 0.9 time (sec) 0 2 4 6 8 10 12 14 16 18 20 22 C1 CU U p(u) W[U;A,C] exp AA A C correlation coefficient 0.8 0.7 0.6 L/U = 20 Extreme 50 year gust (m/s). Weibull A=8 m/s, c=1.7, 10% turbulence intensity and 95% confidence level 0.5 L/U = 10 0.4 L/U = 5 0.3 6 8 10 12 14 16 18 20 V1 (m/s) Aerodynamics & Aeroelasticity Certification of WT 15

4. What can realistic simulations offer? Realistic simulations are performed with turbulent wind inflow in the time domain. The duration of a simulation is 10min, which is in fact quite long! For given wind characteristics: one point wind spectrum and spatial correlation time series are produced covering the complete duration of the simulation plus an initial margin. All results (basically internal loads & stresses, deformations and electrical output), are produced in the form of time series. Spectral analysis of the signals can provide mean, min & max values as well as spectra. In particular for structural design, fatigue loads are of particular importance. They provide for each range the number of cycles contained in the signals. Ranges are related to fatigue limits directly Aerodynamics & Aeroelasticity Certification of WT 16

What can realistic simulations offer? Load spectra Mode description Calc. Meas. Lateral Tower bemding 0.94 - mode Windwise Tower bending 0.95 0.94 mode Torsional mode 1.15 1.16 Assymetric flap (yaw) 1.69 1.70 Assymetric flap (tilt) 1.81 1.80 Symmetric flap 2.07 2.08 Asymmetric edge 3.69 3.50 Asymmetric edge 3.70 - Aerodynamics & Aeroelasticity Certification of WT 17

What can realistic simulations offer? Load spectra Mode description Calc. Meas. Lateral Tower bemding 0.94 - mode Windwise Tower bending 0.95 0.94 mode Torsional mode 1.15 1.16 Assymetric flap (yaw) 1.69 1.70 Assymetric flap (tilt) 1.81 1.80 Symmetric flap 2.07 2.08 Asymmetric edge 3.69 3.50 Asymmetric edge 3.70 - Aerodynamics & Aeroelasticity Certification of WT 18

What can realistic simulations offer? Fatigue loads Aerodynamics & Aeroelasticity Certification of WT 19

What can realistic simulations offer? Fatigue loads The effect of design L eq i L N m i eq n i 1/ m Normalized L eq 100 Leq 3 R σ u Aerodynamics & Aeroelasticity Certification of WT 20

What can realistic simulations offer? Fatigue loads The effect of wind conditions Α: High Turbulence Intensity Β: High Turbulence Intensity Aerodynamics & Aeroelasticity Certification of WT 21

5. Review of the IEC standard Power Production (+fault) Start-up or Stop Normal operation NTM=Normal Turbulent Wind or NWP=Normal Wind Profile Extreme conditions ETM=Extreme Turbulent Model ECD=Extreme Coherent Gust + Direction Change EOG=Extreme Operation Gust EDC=Extreme Direction Change Fatigue or Ultimate Safety Factors Aerodynamics & Aeroelasticity Certification of WT 22

Review of the IEC standard Aerodynamics & Aeroelasticity Certification of WT 23

Review of the IEC standard Safety Factors Safety factors are introduced on both loads and materials. Their aim is to secure that we are on the safe side! For loads we increase the unfavourable loads and decrease the favourable Aerodynamics & Aeroelasticity Certification of WT 24

Review of the IEC standard For the material strength we wish to account uncertainties of all types: Assumptions on the strength itself Inaccuracies in assessment the strength of compound parts Geometrical uncertainties during manufacturing Uncertainties resulting from any difference between the actual structure and the specimen which were tested. The meterial safety factor >1/1.1 As an indication, the values depend on whether the component is fail-safe or non fail safe The type of loading (1/1.2 for buckling, 1/1.3 for exceeding tensile or compression strength For fatigue analysis the safety factor for loads is 1 while for the material properties is more severe and depending on the data these properties are derived from, it can reach 1/1.7 Aerodynamics & Aeroelasticity Certification of WT 25

6. Some examples Extreme Coherent gust Directional Change Vhub= Vout Aerodynamics & Aeroelasticity Certification of WT 26

Some examples Shut Down plus 1 year Extreme Operating Vhub= Vout Aerodynamics & Aeroelasticity Certification of WT 27

Some examples Aerodynamics & Aeroelasticity Certification of WT 28

END Aerodynamics & Aeroelasticity Certification of WT 29

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