Qing TAO，Wen-1ei SUN（School of Mechanic Engineering，Xinjiang University，Urumqi 830046，China）

The study of large-scale wind turbine blade redesign system

Qing TAO，Wen-1ei SUN*
（School of Mechanic Engineering，Xinjiang University，Urumqi 830046，China）

The large-scale wind turbine blade contour design and the choice of wing section are the core technology in wind power generation；both of them could affect the wind turbine performance and the energy efficiency. This paper presents the redesign principles and methods for large-scale wind turbine blade，based on the analysis of blade CAD model from reverse engineering，looking for geometric features，exploring basic constraint factors on blade shape，and determining parameters of blade cross section calculation formula.Blade design has been completed by using this self-developed blade airfoil automatic generation system.The results showed that the redesign meets all the production requirements.

Wind turbine blade，Reverse engineering，Redesign，Modeling

Hydromechatronics Engineering

http：//jdy.qks.cqut.edu.cn

E-mail：jdygcyw@126.com

1 Introduction

The wind turbine blade is one of the key components in wind turbine power generator，and the cost of wind turbine blade accounts for 20%-30%of the complete machine.The wind turbine blade＇s contour design，wing section aspects and so on will affect the wind turbine performance and the energy efficiency［1］.Because the large-scale wind turbine blade requires high accuracy and the surface is relatively complex，the blade measurement from multiple perspectives must be carried out，then the data obtained from different angles will carry on the splicing conformity by the measurement system［2］.Large-scale wind turbine blade reverse designs are consist of four stages［3-4］.It includes the laser scanning measurement of the blade，reverse remodeling of blade，blade reverse design and evaluation and analysis of blade.Preprocessing of＂points cloud＂data and surface reconstruction are the critical technology of reverse engineering and are the premises of building product CAD model［5］.This paper is a continuation of our previous research which proposed a method that based on reverse engineering CAD technique to redesign the large-scale wind turbine blade，and allowed the measurement of large set of blades and use reversion CAD modeling，then post-processing the large-scale wind turbine blade and looking for ideal geometric features［3］.In the current research，a new system has been developed that can automatically generate blade airfoil by input control parameters，and this system has been applied in industry.The system could support intelligent redesign based on existing product，which include simulation，modeling，automatic generation of redesign and its implementation.

2 Analysis of large-scale wind turbine blade Airfoil

Understanding the original requirement of an existing product prototype is the base of redesign.Blade redesign is to improve the design of an existing one. Through analyzing the existing wind turbine blade shape，it could be found that in the sections of the blade along the vertical direction，the blade shapes are similar.The main difference between the different radiuses of airfoil is the sections of the airfoil which has different size.The blade airfoil at different sections rotated at one point along a different angle and with different thickness to chord ratio.It is necessary to determine the r（camber line length），cr（chord length），β（rotation angle），t（thickness to chord ratio），so that any shape of blades could be designed［6-7］.

The difficulty of the wind turbine blade redesign is how accurately draw the airfoil shape under different r.In order to solve this problem，it is necessary to determine how the coordinates of the corresponding airfoil change with respect to the different r values.From the previous results in studying the blade geometric parameters，factors affecting the airfoil data are mainly the chord length Cr，torsion angle β and thickness to chord ratio t，as shown in Fig.1.

Fig.1 The bIade shape infIuence factors

The calculation and deduction on the relationship between cr（chord length），β（twist），t（thickness to chord ratio）and airfoil shape are described in the following.

Assume a point on the airfoil to be designed is i，the distance of i from rotational center is r.The coordinates of i are Xi，Yiand Zi，respectively，in the X，Y and Z axis.At point i，chord length，torsion angle，thickness to chord ratio are cr，β and t，respectively. X0，Y0and 0，respectively，are the corresponding point data in the unit airfoil database.Therefore，the following could be obtained.

2.1 The relationship between Cr and airfoil data values

The relationship between coordinates，Xi，Yiand Ziin X，Y and Z axis and corresponding points in airfoil database could be determined as follows in（1）：

The airfoil shape change can be obtained from the corresponding relationship，as shown in Fig.2.

Fig.2 InfIuence of chord to airfoiI data

2.2 The relationship between torsional angle β and airfoil data values

The rotational change of the airfoil does not like Fig.3（a）around the zero point coordinate origin，and the rotation is as shown in Fig.3（b）.The distance from the center of rotation to the blade leading edge and the whole length of the blade have a certain proportion.This proportion is determined by the different airfoils，and in general，1/4 of the chord.This could effectively reduce the torque on blade generated by the air force.

Fig.3 The rotation of airfoiI

There are two approaches［8］to deal with the rotating airfoils in the calculation process：calculation based on Fig.3（a），and calculation based on Fig.3（b）.Although the latter approach is direct，due to the fact that the rotating point in unit airfoil database is not the center of rotation，as well as the calculation of this method involves four quadrant，program for calculation will be more complicated.Therefore，the rotational calculation is usually carried out that the part is rotated along data point first and then the translation. The specific method［9］of calculation is shown in（2）：

The results of airfoil are shown in Fig.4.

Fig.4 The infIuence of airfoiI dataβ

2.3 The relationship between thickness to chord ratio t and airfoil data values

The change of the thickness to chord ratio is mainly to make changes in the vertical direction to the airfoil chord［10］.It is that the Y coordinate in airfoil data point should multiply by a ratio，namely the thickness to chord ratio t.Meanwhile，the change of the airfoil thickness to chord ratio should be calculated before the rotational change.Otherwise，additional coordinate changes will be added to the process，resulting in distortion of airfoil，as shown in Fig.5.

Fig.5 The infIuence of airfoiI data cr

3 Redesign of large-scale wind turbine blade

Based on analysis of wind turbine blade airfoil section parameter，a wind turbine blade design system has been developed.Using this system，all kinds of 3D model of the turbine blade under various design conditions could be generated quickly and conveniently.In addition，if the data files could be modified，more personalized blade model design could be obtained.This system will greatly improve the design speed and accuracy of the model.Fig.6 shows the operation interface of the blade redesign system.

Fig.6 The user interface of bIade redesigning

From the user interface of blade redesigning，it can be seen that all kinds of blade parameters are included，i.e.，blade number，distance from root，chord，twist angle，airfoil，pre bending and shell thickness，etc..According to the input values in the table，blade model can be generated automatically，based on the design of wind turbine power，wind field density，drive efficiency，the design wind speed，the tip speed ratio，blade of a wind energy utilization coefficient. An automatically generated three-dimensional CAD model of the blade，under the following conditions：wind turbine design power 750 kW，air density，driving efficiency，designing wind speed，wind energy utilization factor are as follows：1.225 kg/m3，0.81，13 m/s and 0.4，is shown in Fig.7. In order to improve the usability of the system，an Excel output function from the system was set up to enable the output of various calculation results in Excel format.Meanwhile，the Excel data file can also be directly inputted into the system for calculation.This function allows various parameters of the blade can be adjusted freely.The original designing parameters will not be affected by the change of calculation formulas inside the system.

Fig.7 The automaticaIIy generated bIade by the system

4 Results

In order to compare the redesigned bale model with the original 750 kW wind turbine blade，GHBladed software was used to analyze the blade functional parameters，and then evaluate the accuracy and correctness of the redesigned blade.

4.1 Conditions of blade comparison

In this study，GHBladed software was used to simulate turbulent wind at an average speed of 14 m/s，with transverse and radial turbulence intensity at 13.35%，17.08%respectively.Vertical turbulence intensity was 9.45%.The generated turbulent wind at the turbine wheel hub is shown in Fig.8.

Three types of airfoil data，the actual blade data from laser scanning，the ideal blade data from wind turbine theory and the redesigned blade data were all inputted into GH Bladed software，and then three corresponding blade shapes were obtained，as shown in Table 1.

Fig.8 TurbuIent wind veIocity at wheeI hub

TabIe1 GH BIaded bIade modeI

4.2 Blade performance

Below is the parameter analysis for a group of 750 kW blades from actual blade model，ideal blade model and redesigned blade model.The main trends of blade chord length and torsion angle were compared，as shown in Fig.9 and 10.

Fig.9 Chord Iength trend

Fig.10 TorsionaI angIe trend

From the above figures，it can be seen that the chord length of ideal model is smaller than those of both redesigned model and actual blade model，and the redesigned chord length is smaller than the actual model.This indicates that the blade chord length increases in design practice.The actual chord length of 750 kW blade is approximately close to a straight line. It is closer to the blade after redesign，and more material will be reduced in the root of blade.The weight of the redesigned blade is lighter than those of others. The twist angles of the ideal model and the redesigned model have the same trend，both of them get increased slower than the actual blade.That is，the torsional angles at the front are both smaller than the actual blade，and at the root are similar.It shows that the redesigned blade has more torque generation in the front，as the blade twist angle at the front end is reduced.

4.3 Conclusions of blade comparison

The actual blade can produce slightly more electrical power than the redesigned blade，and the redesigned blade output power is greater than that of ideal blade.Ideal blade is close to designing power critical position in theory than others and is consistent with the design objective.

In the premise to achieve designing power output requirements，various loading performance of redesigned blade and the actual blade is similar under low wind speed.When wind speed gets increased，the redesigned blade performance is slightly lower than that of the actual blade，but the performances for the ideal blade and redesigned blade become very similar.

Along the span wise direction，all properties of the redesigned blade are coupled with the actual blade change，but there is a small fluctuation value，and not as smooth as the actual blade，so that further optimization on the selection of each section’s airfoil rotational axis is needed.

Through the above analysis，the results of redesigned blade could meetall the design requirements，and they are similar to the actual blade on aerodynamic performance.

5 Conclusions

In this paper，through the analysis of the large wind turbine blade model，looking for geometric character-istics，determination of calculation formulae for airfoil cross section parameters，the large-scale wind turbine blade redesign principles and methods are proposed. Based on the calculation formulae and database technology，a large-scale wind turbine blade redesign system has been developed.By inputting the design parameters，the system could automatically generate blade airfoil 3D CAD model and complete the design tasks.Results showed that，the newly developed system can quickly and easily generate various three dimensional blade airfoil models according to different designing conditions，and it is proven that it could meetall the design requirements.

Acknowledgements

This paper is supported by National Natural Science Foundation of China（No.51065026；No.51465056）and Xinjiang autonomous region of natural science fund（No.2011211A002）.

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大型風(fēng)能發(fā)電機(jī)組葉片再設(shè)計系統(tǒng)研究

陶 慶，孫文磊*
新疆大學(xué)機(jī)械工程學(xué)院，烏魯木齊 830046

葉片的外形設(shè)計和翼型的選擇是影響風(fēng)力機(jī)性能和產(chǎn)能效率最為核心的技術(shù)。提出了一種大型風(fēng)能發(fā)電機(jī)組葉片再設(shè)計的原理和方法，對葉片模型進(jìn)行剖析，尋找?guī)缀翁卣鳎剿髦萍s葉片形狀的基本因素，確定葉片截面參數(shù)計算公式，利用所開發(fā)的葉片翼型自動生成系統(tǒng)，完成了葉片的再設(shè)計，并得到了實(shí)際應(yīng)用。結(jié)果表明：這種再設(shè)計方法滿足產(chǎn)品設(shè)計需求。

風(fēng)機(jī)葉片；逆向工程；再設(shè)計；建模

10.3969/j.issn.1001-3881.2015.12.004Document code：A

TP302.1

20 August 2014；revised 5 October 2014；accepted 11 January 2015

Qing TAO，Ph.D.，E-mail：xjutao@qq.com

*Corresponding author：Wen-lei SUN，Professor.

E-mail：Sunwenxj@163.com；