Experimental Study on Aerodynamic Characteristics of the Model Wind Rotor System and on Characterization of A Wind Generation System

In order to investigate the aerodynamic characteristics of 6-MW wind turbine, experimental study on the aerodynamic characteristics of the model rotor system and on characterization of a wind generation system is carried out. In the test, a thrust-matched rotor system and a geometry-matched rotor system, which utilize redesigned thrust-matched and original geometry-matched blades, respectively, are applied. The 6-MW wind turbine system is introduced briefly. The proper scaling laws for model tests are established in the paper, which are then implemented in the construction of a model wind turbine with optimally designed blades. And the parameters of the model are provided. The aerodynamic characteristics of the proposed 6-MW wind rotor system are explored by testing a 1:65.3 scale model at the State Key Laboratory of Ocean Engineering at Shanghai Jiao Tong University. Before carrying out the wind rotor system test, the turbulence intensity and spatial uniformity of the wind generation system are tested and results demonstrate that the characterization of the wind generation system is satisfied and the average turbulence intensity of less than 10% within the wind rotor plane is proved in the test. And then, the aerodynamic characteristics of 6-MW wind rotor system are investigated. The response characteristic differences between the thrust-matched rotor system and the geometry-matched rotor system are presented. Results indicate that the aerodynamic characteristics of 6-MW wind rotor with the thrust-matched rotor system are satisfied. The conclusion is that the thrust-matched rotor system can better reflect the characteristics of the prototype wind turbine. A set of model test method is proposed in the work and preparations for further model basin test of the 6-MW SPAR-type floating offshore wind turbine system are made.


Introduction
With the depletion of fossil fuels , the wind turbine has been developed rapidly. The wind turbine is a kind of equipment that can convert wind energy into electric energy. For the development and validation of large-scale wind turbine systems, scaled model tests have been commonly employed as a refined scientific approach (Zhao et al., 2016a(Zhao et al., , 2018. Compared with CFD method and other numerical models, scale model tests generate accurate data on wind turbine system responses while taking fewer risks and requiring less time and resources than prototype data collection methods do. Also, wind load is a very important factor for the aerodynamic characteristics of wind turbine (Nejad et al., 2015;Salehyar and Zhu, 2015). The wind generation system is the necessary equipment for wind turbine. Wind load is generated by wind generation system in the model test. Thus, the characterization of wind generation system becomes vital. The aerodynamic characteristics of the model wind rotor system and the characterization of the wind generation system should be researched together. Characterization of a wind generation system used for wind turbine development has been studied in recent years (Urbina et al., 2017). The Maritime Research Institute Netherlands (MARIN) (De Ridder et al., 2013), OCEANIDE, Ecole Centrale de Nantes (Courbois et al., 2013), IH Cantabria, IFREMER (Ohana et al., 2014), Technical University of Denmark (Bredmose et al., 2015), and the Norwegian Marine Technology Research Institute (Nielsen et al., 2006) have constructed the wind generation systems used to generate airflow at facilities. The initial de-velopment of wind generation system is made for the DeepC wind Consortium. These wind characteristics are accomplished by using a bank of 35 fans with a honeycomb front plate to reduce swirl and a nozzle to reduce turbulence (Roddier et al., 2010). However, this wind generation system has some shortcomings, for example a high enough place for the bank of fans is needed to prevent fans from interacting with the water. The nine axial fans in a 3×3 stacked square configuration installed on a towing carriage are used in the model test of Duan et al. (2016a) and Zhao et al.(2018). The wind generation system is simple. However, the average turbulence intensity is smaller than 20%. The average turbulence intensity smaller than 20% is acceptable but not very satisfied. In this work, a new wind generation system is applied with the average turbulence intensity of smaller than 10% within the wind rotor plane. Also, due to the scale effect in the model test, two different wind rotor systems are used to research the aerodynamic characteristics, which are geometry scale model rotor (geometry-matched rotor) and redesigned model rotor (thrust-matched rotor). Redesigned wind rotor meets the thrust similarity (Duan et al., 2016b). Compared with NREL (National Renewable Energy Laboratory) 5-MW (Jonkman et al., 2009), 6-MW wind turbine has longer blade and a greater output power, and it is in line with the development trend of the wind turbine. Also, 6-MW wind turbine belongs to multiple megawatt levels. A complete experimental research process of 6-MW wind rotor system is presented in the paper.
In the following parts, fixed wind turbine system is introduced firstly. Secondly, scaling laws and parameters in the tests are displayed. And then, model and experimental setups are shown. Finally, the test result are given and conclusions are drawn.

6-MW fixed wind turbine system
Wind turbine and tower are the important components of wind turbine system. Wind turbine is installed at the upper side of the wind turbine system, which can transform wind energy into electrical energy. In this paper, the wind turbine is a conventional three-bladed, upwind, variablespeed, variable-blade-pitch turbine. The most important performance parameters of 6-MW horizontal axis wind turbine are given in Table 1.
Tower is a key equipment which supports the upper equipment. Consequently, the selection of tower is vitally critical. In the paper, a tower is applied according to the upper wind turbine (Meng et al., 2017b). The tower bottom section is designed with a 40-mm-thick wall and the diameter at tower bottom is 8 m (International Electrotechnical Commission, 2009). An amplification factor is added into tower density considering the existence of bolts and welds. The factor takes 1.05-1.1 under normal conditions, and the tower density in the paper is 1.08, i.e. 8500 kg/m 3 . Table 2 is the tower performance parameters. A three-dimensional figure of rotor and tower is shown in Fig. 1. Fig. 1a shows the tower schematic figure and Fig. 1b is the rotor system schematic figure, including the blade, hub and nacelle.

Scale methodology
In order to predict the aerodynamic characteristics of the prototype wind turbine accurately, geometrical similarity, kinematical similarity, dynamical similarity, and structural stiffness similarity should be satisfied (Zhao et al., 2016a(Zhao et al., , 2016b. In summary, the following scaling relationships between the prototype and the model are considered. (1) Geometrical similarity In the model test, the linear dimensions all need to meet the geometric similarity, such as length, thickness, diameter and other parameters.
λ where is the scale parameter, and the subscript s refers to the prototype and m to the model.
(2) Kinematical similarity In a wind turbine system, system excitation frequency is generated due to wind rotor rotation and imbalance. In addition, an aerodynamic interaction between the wind turbine and the tower exists, which is the tower shadow effect (Meng et al., 2016). In order to ensure that the system excitation frequency and the tower shadow effect between the model wind turbine and the prototype wind turbine are the same, the model wind turbine needs to meet certain similarity criteria. The tip speed ratio (TSR) is similar, and the tip speed ratio is the ratio of the tangential speed of the wind turbine outer diameter to the wind speed before the wind turbine. To ensure this similarity, the wind turbine TSR should be maintained as follows: where V is the incoming wind velocity, ω is the wind rotor rotational velocity, and R is the wind rotor radius.
(3) Structural stiffness similarity The tower of a single-column wind turbine system belongs to a towering flexible structure. The flexibility of the tower has a great influence on the movement of the wind turbine system, such as the pitch motion response. Therefore, the model and prototype tower have to meet similar structural stiffness.
where E is the Young's modulus of the tower structure, and I is the sectional moment of inertia.
In order to ensure that the natural frequency and deformation of the model tower are the same as those of the prototype tower, the structural rigidity of the model and the prototype tower is similar.
Given these scaling relationships, the proposed scaling ratios are summarized in Table 3.

Scale effect of model blades
The performance of wind turbine is mainly represented by the power coefficient C P and thrust coefficient C T , which are characterized by the following formulae: (4) where P denotes rotor power, T denotes rotor thrust, ρ is the density of the air, v 0 is the velocity of incoming wind, and A is the swept area of the rotor. Normally, these two coefficients are functions of the tipspeed ratio (TSR), which is defined in Eq. (2).
The Reynolds numbers of the full and model scales can be expressed as: λ where the scale parameter , which represents the ratio between the lengths of the prototype and the model, is set to 65.3 in this model test.
Therefore, the Reynolds number for model scale is 1/528 of that for full scale. Utilizing this geometry-matched scaled model blade under low Reynolds number conditions will lead to drastic changes in wind rotor performance (Martin, 2011;Du et al., 2013Du et al., , 2016, as depicted in Fig. 2. Fig.  2a shows the comparison between TSR-C T performance and Fig. 2b is the comparison of TSR-C P performance. The negative values of geometry-matched rotor C P indicate that the wind cannot make it run and it needs external force to operate. As the existence of model scale effect, the blade is redesigned in the paper, and the details of the designed method can refer to Moriarty and Hansen (2005) and Guo et al. (2017).

Coordination system definition
The right-handed coordinate system is applied in this work. The coordinate system is shown in Fig. 3. The X-direction is vertical to the wind rotor plane and parallel to wind direction.

Wind rotor model
In this paper, two sets of experimental lightweight blades are made from carbon fiber, as shown in Fig. 4. Fig. 4 shows the geometry-matched blade and thrust-matched blade. The weight of the blades is strictly controlled. A set of blades is obtained according to the scale ratio using the 6-MW prototype blade, and the other set of blades is the blades that are redesigned in accordance with similar thrust. The NACA4412 airfoil with a low Reynolds number is used. The test model diagram is shown in Fig. 5. 4.3 Nacelle model A set of nacelle models is designed in this paper. The real products of torque sensor, motor, gearbox and hub are all shown in Fig. 6.

Tower model
Structural stiffness similarity and geometrical similarity in lengths are satisfied between model tower and prototype tower. The inner diameter of the model tower is set to a fixed value of 25 mm and is divided into 5 sections as shown in Fig. 7.

Wind generation system
In the paper, a new wind generation system is applied (Meng et al., 2017a). The device consists of 4×4 frequencyconverting electric axial flow fans equipped with controlling devices and software to generate the wind field required for wind turbine model tests as shown in Fig. 8. The dimensions of the effective wind output area are 3 m×3 m. In order to improve the inflow wind quality of wind turbine experiments and reduce turbulent flow components, rectifier fins and grids have been installed behind the variable-     frequency motor-driven axial fans. Correctly setting the rectifier fins not only improves the quality of wind generated by the fan, but also improves the efficiency of the axial fan.

Instrument calibration
The six-component force sensor is an important instrument and it is calibrated before the experiment. Three direction forces and three direction torques can be measured by the six-component force sensor. The mass of 1 kg and 2 kg is used to calibration the sensor. The instrument calibration is carried out as shown in Fig. 9.

Test and test results
5.1 Investigation of the characterization of the wind generation system Before carrying out the wind rotor thrust and torque tests, the characterization of the wind generation system is investigated. The array is designed to measure the timeseries of the wind at forty-nine measurement points and is illustrated in Fig. 10. The anemometer of the series EE65 type is used to measure spatial points. The array, as is shown, consisted of seven measurement points along the horizontal direction arranged at seven different vertical elevations, where the measurement point No. 25 is the center po-    sition of the wind rotor hub. Test points are always in the same plane and are perpendicular to the incoming wind speed. The main test contents are as follows:

Relationship between rotation and wind speed
In this part, the relationship between the wind generation rotation, which is the axial flow fan rotation, with wind speed generated by wind generation system is investigated.
In the test, 16 frequency-converting electric axial flow fans start and work simultaneously. The maximal rotation of the frequency-converting electric axial flow fan is 3000 rpm with the frequency of 50 Hz. The wind generation system can produce wind speed above 15 m/s. The relationship is shown in Fig. 11 and Eq. (7) is obtained by quadratic curve fitting. v = −6.07143 × 10 −7 Ω 2 + 0.00625Ω − 0.07214, v Ω where represents the generated wind speed by wind generation system and denotes the wind generation rotation.
In the test, a motor installed in the wind turbine nacelle controlled the wind rotor rotation. The wind rotor rotation is set to a required value for each case. This approach means that the rotation of the wind rotor is induced by the motordrive but not by the wind loads. However, the wind-drive is another modeling approach and is used by Duan et al. (2016a). Thus, the relationship between the wind speed and wind rotor rotation under the condition of wind rotor free rotation is also studied in the test. Under the wind rotor free rotation condition, the thrust-matched needs to be satisfied and the blade pitch is fixed 0 deg. The result is displayed in Table 4. Wind rotor thrust of 3.2 N is the rated thrust condition. The result can be used to compare with motor-drive modeling approach to research the dynamic characteristics of platform when the further basin test is carried out.

Turbulence intensity and spatial uniformity
The distances from the plane where the measuring point is located to the wind generation system are 2 m, 3 m, 4 m, 5 m, and 6 m. The link between array position and turbulence intensity is displayed in Fig. 11. Nine points are measured in every measured plane. Distribution of measured points is shown in Fig. 10.
According to Fig. 12a and Fig. 12c, the lowest turbulence intensity is observed at the 4-m-horizontal position from measuring plane to wind generation system. However, as can be seen in Fig, 12b, the turbulence intensity decreases as the distance increases. But, the decreasing trend of turbulence intensity slows down when the distance from the measured plane to the wind generation system exceeds 4 m. Thus, the distance from the measured plane to the wind generation system of 4 m is a better choice taking all the factors into considerations. Consequently, the array is positioned 4 m down-stream from the wind generation system during the model testing.
The uniformity of the wind speed and the turbulence intensity are presented in Fig. 13. The wind speed surface as shown represents the smoothed mean wind velocity values of the time histories, and the turbulence intensity surface is the corresponding temporal standard deviation of the time histories divided by the mean wind speed of the time history at each point in the grid. The solid black circle is used to define the wind rotor plane in the wind field, and the black cross at its center indicates the positioning of the hub center. As observed from Fig. 13a, the spatial uniformity of the wind field is basically fair and some uneven phenomenon exists in the rotor plane. The uneven phenomenon is caused by the rotation effect of wind generators. Also, as shown in Fig. 13a that high wind speed exists near the bottom of the wind generation system, which can be explained by the wall effect. As the wind generation system is placed directly on the ground, the wind speed at ground surface will become high. As can be seen from Fig. 13b, in most areas of the rotor plane, the turbulence intensity is smaller Fig. 11. Relationship between wind generation rotation and wind speed. 2.3 3.2 1.9 1.6 1.2 Frequency of wind generation drive (Hz) 6.9 7.5 6.8 5.9 5.1 than 10%. In the reference of Duan et al. (2016a) the turbulence intensity is generally smaller than 20% within the rotor plane. Thus, the wind generation system applied in the work is satisfied and acceptable.

Turbulent wind
Kaimal wind spectrum and von Karman wind spectrum are all tested with the vertical shear index of 0.14 and the surface roughness length of 0.03 m. Wind speed of wind turbulence is generated by NREL TurbSim software.
Turbulent wind generated by the wind generation system with Kaimal and von Karman wind spectrum are shown in Fig. 14 and Fig. 15, respectively, and comparisons with target values are carried out. Mean wind speed of 1.30 m/s is the rated wind speed condition, and below the rated wind speed condition is mean wind speed of 0.99 m/s. Mean wind speed of 1.73 m/s and 2.10 m/s are above the rated wind speed condition, and the cut-out wind speed condition is 3.09 m/s mean wind speed condition. As the five wind speed conditions are used by the test of aerodynamic characteristics of wind rotor, the five wind speed conditions are all measured in the test. From Fig. 14 and Fig. 15, two main phenomena can be obtained. Firstly, experimental results are slightly smaller than the target values. Secondly, compared with the target values, hysteresis phenomenon exists in the experimental results. The reason may be that rectifier fins and grids installed behind the variable-frequency motor-driven axial fans have a certain influence on wind speed. Wind speed decreases due to the existence of rectifier fins and grids, and a small part of wind energy is dissipated to a certain degree. Also, the time to reach the measuring point becomes longer. Consequently, small experimental results and hysteresis phenomenon appeared. Although small deviation between experimental results and target values exists, the experimental results can be acceptable. The wind generation system can be used to conduct the further experimental study.

Aerodynamic characteristics test of model wind rotor
In the fixed wind turbine working condition test, the wind load parameters are shown in Table 5, five conditions in total. No. 2 is the rated wind speed condition. The different blade pitches are used by the geometry-matched blade and thrust-matched blade.
In using geometry-matched and thrust-matched rotor systems, the wind rotor thrust and torque are compared as shown in Fig. 16. The reference values are provided by the manufacturer as shown in Table 6. As can be seen from Fig. 16, the aerodynamic characteristics of thrust-matched rotor system agree well with the reference values and simulation results. However, the geometry-matched rotor system has lower experimental results than the thrust-matched ro-  MENG Long et al. China Ocean Eng., 2019, Vol. 33, No. 2, P. 137-147 tor system does. The geometry-matched model blade is geometrically scaled down from the 6-MW reference wind turbine blade which is designed for a much higher full-scale Reynolds number. Consequently, utilizing this geometryscaled model rotor under low Reynolds number conditions leads to the lower rotor thrust and torque.
The maximal deviation of thrust between experimental results of thrust-matched rotor system with reference value is 16% at the mean wind speed of 2.10 m/s, and the deviation is 6% at the rated wind speed condition, that is at the mean wind speed of 1.3 m/s condition. The reason of the deviation existence can be explained as following: (1) The reference value is obtained at the steady wind condition and the experimental results are obtained by the turbulent wind condition; (2) During the test, vibration phenomenon of the nacelle components is observed as the flexibility tower is applied. The range of rotor torque deviation between experimental results of thrust-matched rotor system with the reference value is from 8% to 25%. The deviation is acceptable with the explanations mentioned above. Results indicate that thrust-matched rotor system has better properties, and also prove that the blade design method is satisfied and reliable. The thrust-matched rotor system can be used for the further basin model test.
Time history curve comparison of wind rotor thrust between experimental results and target values with Kaimal wind spectrum is carried out, as shown in Fig. 17. Three typical operating conditions are listed, namely, rotor thrust below the rated wind speed condition, rated wind speed condition and above the rated wind speed condition. Compared with the target values, experimental results have larger oscillation as seen from Fig. 17. It can also be found that the higher the wind speed is, the larger the oscillation of wind rotor thrust will be. The reason can be explained by the flexible tower. In the test, the flexible tower is applied and the obstruction effect of tower on the incoming wind cannot be ignored. Thus, the vibration of tower is higher as wind speed increases. Although the test results have large oscillation, same trend is obtained by comparing the experimental results and target values, thus the experimental res-ults can be accepted.

Conclusions
The characterization of the wind generation system applied in the work is investigated and aerodynamic characteristics of model wind rotor system are presented in the paper. Results indicate that the spatial uniformity of the wind field is fair, and the average turbulence intensity is generally lower than 10% within the rotor plane. The dataset has provided information on the area in which the turbine can be operated with a uniform and low turbulent flow. The ex-   perimental data presented in this paper will serve as a dataset to aid the further design of wind generation systems. Besides, comparison between thrust-matched rotor system and geometry-matched rotor system is made and it indicates that the thrust-matched rotor system can better reflect the characteristics of the prototype wind turbine. Also, a set of model test methods are proposed in the work and preparations for the following basin model test are made. Future work will focus on more thorough investigations on the wind turbine. The wind generation system and 6-MW floating offshore wind turbine proposed by the reference Meng et al. (2017b) will be used to perform a scaled model basin test.