1 Introduction

As an important research method, the wind tunnel experiment can be used to study on the aerodynamics of wind turbine and validate numerical methods. It has become one of the most important bases for the development of analytical and computational models of wind turbine. Due to the distinct difficulties of wind turbine, such as the three-dimensional (3D) character, the rotational feature, and the unsteady phenomenon of the flow field, the wind tunnel experiment is indispensable in the aerodynamic research field.

Generally speaking, the wind turbine aerodynamic experiments can be divided into the field test and the wind tunnel test. In the field test, the wind turbine model is the full scaled and installed in the real wind farm environment. During the test, the inflow wind is measured, and the status of wind turbine is recorded. Based on these results, the load or pressure can be analyzed. The IEA Wind Annex XIV^[1], Annex XVIII^[2], and the DAN-AERO MW^[3] projects are the most influential field tests. Although the results of the field test are more practical, some difficulties are inevitable, for example, the flow field is complex and hard to be measured accurately, the cost is too high, and the measurement consumes too much time.

By contrast, the wind tunnel test is carried out in the controlled flow environment, and lots of the negative factors can be avoided. Thus it is widely used in the wind turbine aerodynamic study. The unsteady aerodynamics experiment (UAE) phase VI experiment^[4], conducted by the national renewable energy laboratory (NREL), is carried out in the 24.4m$\times $36.6m wind tunnel (national full-size aerodynamic comprehensive facility (NFAC)), of NASA Ames. In the test, the load and pressure distribution of the rotor is measured. Another important wind turbine wind tunnel test is the model experiment in controlled conditions (MEXICO) experiment^[5]. The testing model has three blades, and its diameter is 4.5m. It has been tested in the open test section of the 9.5m$\times $9.5m LLF wind tunnel of DNW. Except the load and the pressure distribution, the flow field is measured by particle image velocimetry (PIV) technology in the experiment.

Based on these two wind tunnel experiments and the related experimental research work of China Aerodynamic Research and Development Center (CARDC), the wind turbine experimental techniques and the corresponding research achievements will be reviewed in the following content.

2 Different measurement methods in wind tunnel experiment 2.1 Flow field measurement

The PIV can be used to obtain the detailed flow field information without any influence. It has been widely applied in the wind turbine flow field measurements^[6-14]. The traditional contacting measurement methods, such as the probe and hot-wire anemometer, are rarely used in the measurement of full scale wind turbine flow field.

In the PIV measurement, the data splicing technology is commonly used, because the wind turbine flow field is so large that one single camera window cannot cover all the areas. Some basic work on the splicing technology has been carried out in CARDC. Based on the typical delta wing PIV experiment, the double vortexes on the edge of the delta wing are analyzed, and different PIV data splicing technologies are compared. It is concluded that if the efficiency of the interpolation calculation is not badly affected, the Kriging algorithm is the most optimal interpolation method. Besides, the splicing technology has also been used in the wind turbine PIV experiment. Figure 1 shows the spliced wake flow field of wind turbine captured by the synchronous measurement of two cameras.

It should be noticed that the data procession of the PIV results may cause errors for unsteady analysis. The movement of the vortexes generated by the wind turbine is strongly affected by unsteady characteristics. Even at the same azimuth angle, the vortex centers are changed at different cycles. As a result, it may result in large errors if the raw PIV results at different time are averaged to obtain some steady parameters, such as the vorticities and vortex center trajectory. However, the reliability of the data procession can be significantly improved if the relevant information is firstly extracted from each raw PIV result and then averaged^[14-15].

Besides, in the PIV experiment of wind turbine, the illumination is another important factor, because the light path is usually sheltered by the rotating blade. In order to solve this problem, the light reflection method can be used, and the complete flow field around the rotor is easily obtained (see Fig. 2) .

2.2 Blade pressure distribution measurement

It is meaningful to obtain the pressure distribution of the rotating blade in wind turbine aerodynamic research. However, there are a lot of difficulties to obtain the pressure distribution in the experiment. In order to study the unsteady characteristic of wind turbine, the dynamic pressure is usually measured.

So far, different pressure measurement experiments have been carried out. The absolute pressure sensors are used in the MEXICO projects and the experiments of CARDC (the ranges are 15 psi and 5 psi, respectively). This kind of sensor has good dynamic characteristics, and its correction procedure is simple. Besides, the output of the sensor is electrical signals, which means the reference pressure and the corresponding piezometric tubes are not needed. However, because the range of the sensor is usually large, the pressure results at the 50% inboard span positions are not as accurate as those at the outboard positions if the sensors with the same range are used^[17].

In the UAE phase VI project, the blade pressure is measured by the differential type sensors. In order to reduce the distortion of the dynamic signals, the length of the piezometric tubes must be less than 0.457m^[18]. The multistage pressure transmission line with whirling interface is also needed to provide a steady reference pressure for the system. Apart from that, the pressure data are also needed to be modified according to the complex hydrostatic pressure and centrifugal force corrections^[4]. The advantage of this kind of pressure sensor is that the pressure measured near the blade root is more accurate.

The results of MEXICO project show that the measured pressures at the same span positions of different blades are in good agreement (see Fig. 3) . In the figure, the difference at some positions is mainly caused by the errors of spatial position and measuring system. Therefore, with the space limitation, the measuring points are usually arranged on different blades. This kind of arrangement is also good for reducing the structural strength problem. If the accurate pressure distribution is obtained, the integral of the pressure results is very close to the force measured by balance^[19] (see Fig. 4) .

2.3 Aerodynamic force measurement

In the UAE phase VI and MEXICO experiments, the force of the rotor is measured by the external balance installed at the tower root (see Fig. 5) . The thrust force and the moment can be calculated by the results of the balance. The strain gauge bridges are used at the blade root and low speed shaft to measure the load and torque in the UAE phase VI experiment. However, in the MEXICO experiment, the power characteristics of the wind turbine model of MEXICO experiment are not obtained^[20].

The CARDC has also carried out the force measurement experiment in the test section of the 12m$\times $16m wind tunnel. The testing system consists of 5 force measurement units (see Fig. 6) . Units 4 and 5 are the box balances. The elastic coupling on the drive shaft can only transfer torque, and it is unconstrained at the axis direction. Therefore, the accuracy of the axis force measurement is guaranteed. Unit 3 can be replaced by the torque meter. During the testing, Units 4 and 5 should be protected against the flow. The tower disturbance correction is not needed for results obtained by the system.

3 Research on wind tunnel effect

When the diameter of the rotor is comparative to the size of the wind tunnel test section, the flow field around the model will be disturbed, which makes it difficult for reliable data correction of the wind tunnel results. If the scale effect is not considered in the experiment, the results could only be used for qualitative analysis.

3.1 Effects of closed test section

The wall of the test section has influences on the flow field around and past the rotor of wind turbine model. It is widely accepted that if the blockage, which is defined as the ratio of rotor swept area to that of the cross section, is larger than 2%, the wall interference exists^[21]. The wall interference is more significant when the rotating speed of the rotor is higher. If the blockage reaches up to 10%, the pressure coefficient will be larger than 25%^[25].

In order to make a reasonable correction of wall effects, different methods have been applied. The Glauert method^[23], Maskell method^[24], and the $\varepsilon $-max method^[25] are most commonly used, however, the errors of the these correction methods must be considered. In contrast, the wind tunnel wall pressure correction method^[26] has more advantages, in which the correction is not based on the theoretical model. In the wall pressure correction method, the wall static pressure is measured during the experiment. According to the measured results, the strengths of the virtual surface sources and vortexes at some suitable locations are calculated, so that the induced velocities caused by the wall can be modeled. Based on the wall pressure information matrix method, Jiang in CARDC had created a kind of wall pressure correction method, which has been proved to have good accuracy^[27-28]. So far, the wall pressure correction method is still one of the most commonly used methods to correct the wall interference of the closed test section.

Because the correction quantity in the wall pressure correction method is relatively large, the influence of the accuracy and precision of the wall static pressure is significant. In recent years, Chen et al. in CARDC applied the vortex lattice method to simulate the singular elements. Different from the traditional method, in which the singular elements are usually set on the wall of the test section, the singular elements in the model by Chen et al. are distributed outside of the wall. Based on the no penetration condition on the wall, the strengths of these singular elements can be calculated iteratively. It efficiently avoids the singularity problem of the solution near the wall and improves the correction accuracy. In the future, by taking advantage of the computational fluid dynamic (CFD) technology, the wall correction based on the CFD computation is the new trend.

3.2 Effects of open test section

In the wind tunnel experiment with open test section, a larger blockage is allowed, which means a larger model can be used^[29]. For example, in the MEXICO project and experiments carried out in CARDC, the blockages are up to 17% and 15%, respectively. However, in the UAE phase VI project, in which a closed test section is used, the blockage is 9%. In a similar experiment of CARDC, the blockage of the closed test section is 10%.

The influence of the open test section has been detailed studied in the MEXICO project^[30]. According to the one-dimensional axial momentum theory, it is known that influenced by the open test section, the measured force in the axis direction is larger^[31] , however, the deviation is limited. Shen et al.^[32] analyzed the error and showed that the maximum error is about 5%. R$\acute{\rm e}$thor$\acute{\rm e}$ et al.^[33-34] demonstrated that circular gap in the collection port can make the pressure arise, which may significantly reduce the influence of the open test section. Although these conclusions are derived in the LLF wind tunnel, the effect law is consistent.

4 Determination of equivalent inflow condition oflocal airfoil

In the wind tunnel experiment of rotating wind turbine, how to determine the 2D inflow condition for the local airfoil is an important issue, because it provides effective way to make comparison between 2D and 3D characteristics of the airfoil.

4.1 Determination of local angle of attack (AOA)

As shown in Fig. 7, the stream line at the leading edge of the airfoil is curved. If a probe is set near the leading edge, the local AOA measured by the probe is different with the actual AOA. The difference of the angle is mainly caused by the induction of the attached vortex of the airfoil. The actual inflow AOA can be obtained by subtracting the induced component from the angle measured by the probe at the leading edge. When the 3D effect is not significant, the induced angle component can be determined by comparison of the stagnation location and the pressure distribution between the 2D and 3D conditions. If the 3D effect is considerable, there is no effective method to derive the induced angle component.

Three different methods have been developed to determine the actual AOA in recent years^[17], namely, the inverse BEM method^[35-36], the inverse free wake method^[37], and the direct method^[38-39]. In the inverse BEM method, the normal and tangential forces are obtained by integral of the pressure distribution. Then, the induction factor is calculated by the BEM method iteratively. Finally, the velocity triangle and the actual AOA can be determined.

The inverse free wake method is derived from the inverse free wake model. Firstly, the aerodynamic force of local airfoil is calculated and decomposed according to the initial angle of attack, which is defined as the angle between the inflow direction and the velocity vector of the local airfoil. Secondly, the lift force is used to derive the vorticity of the attached vortex based on the Kutta-Joukowski theory. Thirdly, the shedding vortex can be obtained, and the induced AOA can be determined after several iterations.

Different from the inverse free wake method, the induced AOA in the direct method is obtained by analysis of the basic pressure distribution and the PIV velocity results at some sampling points near the leading edge.

4.2 Non-dimensionalization

The equivalent dynamic and static pressures in the far field are needed in the non- dimensionalization of the test data. Because the static pressure is difficult to be measured directly, it is usually obtained by subtracting the dynamic pressure from the total pressure.

The total pressure is often set as the maximum value of the pressure measured by the leading edge probe or the pressure distribution of the airfoil. In order to make the measured total pressure more accurate, on the one hand, the pressure taps on the airfoil should be set as many as possible. On the other hand, the leading edge probe should be aligned in the inflow direction. In the UAE phase VI project, the total pressure is determined by the algorithms of spherical coordinate system (see Fig. 8) ^[40].

The dynamic pressure is easily obtained by the results of the local velocity. Besides, it can be also calculated by the method proposed by Shipley et al.^[41]. The most simplified method is to determine the dynamic pressure by combination of the wind tunnel inflow velocity and the rotational speed of the local airfoil, however, its errors are larger. All these methods are needed to be further studied.

5 Experimental study on 3D rotational effects

During the operating of wind turbine, the 3D rotational effects may result in larger load and output power than the estimated results based on the static aerodynamic data of airfoil. This is an important reason for the damage of the generator. However, the mechanism of the 3D rotational effects has not been fully understood. It is widely accepted that the existences of the coriolis force and centrifugal force are the main factors that lead to the stall delay of the airfoil.

The National Aeronautical Research Institute of Sweden (FFA) and CARDC had carried out cooperation on the 3D rotational effects study since the 1990s. In this study, the pressures of 232 taps on 8 span positions of the rotating blade are measured in the 12m$\times $16m low speed wind tunnel of CARDC^[42]. The achievements greatly contributed to the study of 3D rotational effects.

In order overcome the limitation of the model size, the pressure distributions of the full scaled wind turbine are measured in the experiments of IEA Wind Annex XIV^[1], Annex XVIII^[2] and DAN-AERO MW ^[3]. However, it has been pointed out that the complexity of the test environment has serious effects on the uncertainty of the results.

In recent years, the 3D rotational effects have been carefully studied in the experiments of the UAE phase VI and the MEXICO projects. The results of UAE phase VI experiment show that for S809 airfoil, the maximum lift coefficient at the 30% span position can reach up to 2.1, and the stall angle is delayed to 26.4°. As a comparison, the 2D wind tunnel experiment carried out by DTU demonstrates the maximum lift coefficient of the S809 airfoil is 1.05, and the stall angle is 15°. It can be concluded that it is the 3D rotational effect that makes the maximum lift coefficient increase and the stall angle delay^[43]. The pressure integral of the blades both in the experiments of UAE Phase VI and MEXICO had been analyzed in Ref. ^[44]. By comparing the difference of the pressure distribution between the rotating and static conditions of the blade, he pointed out that the 3D rotational effects existed in the whole blade, especially in the blade root areas (see Fig. 9) .

In the aerodynamic design and evaluation of wind turbine, the 3D rotational effects corrections are needed by use of the 2D airfoil aerodynamic data. So far, a lot of stall delay correction models have been developed, but there is still a common problem that after correction, the computed load of the wind turbine is too large. Currently, 3D rotation effects are still one of research hotspots. The more sophisticated wind tunnel test data are needed to better understand the mechanism.

6 Experimental study on dynamic inflow effects

When the inflow, pitch angle or rotational speed of the wind turbine has sudden changes, the induced velocities in the rotor plane or the wake cannot be changed at the same time. This delayed effect is usually called dynamic inflow. The dynamic inflow effect often leads to the excessive load and generated power. In the European dynamic inflow project, Snel and Schepers^[45] and Snel et al.^[46] developed several engineering models, which have been applied in their calculation program, such as the ECN model used in the PHATAS code. The field test of the wind turbine is carried out by DTU, and the dynamic inflow problem is studied. In the test, the rapid responses of the flap momentum at the blade root and the torque of the rotor are measured during the pitch movement of the blade^[47]. However, there are a lot of the disturbing factors in the test, and more importantly the relationship between the dynamic inflow and radial positions is not confirmed.

Compared with the field test, the wind tunnel experiment has lots of advantages, for example, the flow in the wind tunnel is more uniform and stable, the turbulence is lower and can be controlled, and the disturbing factors are significantly reduced. In the wind tunnel experiment of the UAE phase VI project, the pressure distribution and the force at 5 different span positions are measured, and the influence of the pitch angle with step changes is analyzed. In order to eliminate some negative effects, such as the disturbance of the dynamic inflow effect, the tower shadow effect, and the fluctuations of wind tunnel condition, the time serials average method is used in the data processing. The experimental results show that the change of the pitch angle caused significant dynamic inflow effect^[48]. The testing results are in good agreement with the simulation of the AWSM, such as the force response during the increase of the pitch angle, as shown in Fig. 10. However, when the pitch angle decreases, the time constant along the blade is almost unchanged, which is different with the theoretical analysis of the ECN. This phenomenon should be further studied.

Pascal^[49] studied the dynamic inflow problem caused by the change of the rotating speed. In his experiment, the pitch angle is unchanged, and the wind speeds are kept at 10m/s, 15m/s, 18m/s, and 22m/s. The rotating speed is changed between 324.5 r/min and 424.5 r/min alternatively. He found out that the induced velocity is influenced by the dynamic inflow effect, and the magnitude is increased with the decrease of the wind speed. The change of the load is more significant at the blade tip. Figure 11 shows the axis force measured by the balance stalled in the blade root at the 10m/s wind speed. It can be found that the dynamic inflow effect is obvious.

7 Experimental study on flow structure and induction effect

In the wake of wind turbine, the vortexes may cause the complex induced velocity, which have important effects on the unsteady aerodynamic characteristics and the aerodynamic noise. If the trajectory, the strength, the age, and other parameters of the vortexes can be accurately measured, it can provide important basic data and validation evidence for the aerodynamic models and vortex methods. The hot-wire anemometer was used to quantitatively measure the wake structures of wind turbine by Ebert and Wood^[50-51], Vermeer et al.^[52], Mast et al.^[53] and Zhu^[11] et al. However, because the flow field data can be only measured point by point, the results obtained by hot-wire anemometer are very few. The PIV technology is utilized to make a more comprehensive measurement of the wake flow by Smith et al.^[54], Whale and Anderson^[6] and Whale^[7-8], Grant et al.^[55], Maeda et al.^[56], Massouh^[57] and Gao et al.^[13]. Dobrev et al.^[12] had measured the wake of a simplified wind turbine model and analyzed the characters of the tip vortex.

In recent years, more accurate and representative wind tunnel results are badly needed for the development of the CFD and vortex methods. CARDC had carried out a large region PIV experiment in the open test section of $\Phi $3.2m wind tunnel. The observation region of the camera is 570mm$\times $380mm. The blade model is the 1/8 scaled NREL UAE phase VI blade. The flow field around the blade and the data of the wake vortexes are obtained in the experiment. Based on the wind tunnel results, the production, movement, and dissipation of the tip vortexes are quantitatively analyzed. Besides, the rolling up and deformation of the vortexes are also captured in the experiment, as shown in Fig. 12. The experimental results are very close to the the analysis of Ref. ^[58]. It can be concluded that the induction effect of the vortex is more significant in the wake near the rotor.

So far, the most complex and detailed wind turbine flow field experiment was the MEXICO project^[59-62]. The 3D PIV technology was used by Schepers et al.^[59] to measure the structure and trajectory of tip vortex as well as the flow filed around wind turbine. Besides, the aerodynamic load was also obtained by the pressure measurement. The results show that there is a good correlation between the strengths of circulation and the tip vortex. It has been demonstrated in this experiment that the deficit of the velocity in the far field of the wake is two times as much as that in the rotor plane, and the velocity deficit is independent of the radial position. The average velocity measured in the rotor plane is the same with the computation result of the blade element momentum (BEM) model. However, the consistent is obtained when the axial force coefficient in the BEM computation is set as 0.89 as shown in Fig. 13. The non-uniformity of the flow field in the rotor plane is also studied in the experiment, which validates the Prandtl tip loss theory.

8 Wind tunnel simulation of large scale wind turbine

The magnitude difference of the Reynolds number is the biggest problem in the wind tunnel experiment of wind turbine. If the incoming flow velocity is $v$, and the rotational speed is $\omega $ for a real wind turbine, in order to keep the kinematic and dynamic similarities simultaneously in the wind tunnel experiment, these values should be changed into kv and 2k$\omega $, respectively, where $k$ is the scaled ratio. The corresponding tip speed ratio (TSR) is $k$ times as much as that of the original wind turbine. With consideration of the limitation of the wind tunnel, even in the biggest wind tunnel, the scaled ratio $k$ will be larger than 10. Generally speaking, the value of $k$ is larger than 20 for most wind tunnels. As a result, the kinematic and dynamic similarities cannot be satisfied at the same time.

The geometry similarity, the kinematic similarity (the TSR), and the incompressible condition (the Mach number) are the most important constrains which should be fulfilled at first. In the experiment, the flow velocity should be increased as much as possible, and the rotational speed of the wind turbine model should be adjusted accordingly to reduce the impact of the Reynolds number effect.

A lot of large scale wind turbine experiments have been carried out in CARDC^[22]. The largest diameter of the prototype wind turbine is 83m, and largest chord length of the blade is 3.198m. The rated rotational speed of the turbine is 17.23 r/min, and the designed wind speed is 10.4 m/s. In the wind tunnel experiment, the scaled ratio of the model is 1:16. According to wind tunnel results, the effect of the Reynolds number is obvious^{[22, 59]}, as shown in Fig. 14.

There are a lot of unique characters for the large scale wind turbine. Firstly, the wind shear effect cannot be ignored. The change of the averaged velocity is significant at different azimuth angles during the rotation. This difference can reach up to 50% for a typical large scale wind turbine. Secondly, the scale of the blade is comparative to that of the turbulence structure. The turbulent fluctuation may have great impacts on the wind turbine and lead to unsteady aerodynamic load of the blade.

In order to simulate the influence of the wind shear effect, the stationary wedge and roughness elements are commonly used at the entrance of the test section in the wind tunnel experiment. This method is good at producing the gradient of wind speed, but fails to simulate the low-frequency turbulence. The decay of the turbulence intensity is so fast that the integral scale of the turbulence is only 1/500-1/300 as much as that of the actual value.

To solve this problem, a new active atmospheric boundary layer simulation device has been designed by CARDC. On the device, the periodic moved wedge is driven by the motor, and the additional turbulent kinetic energy is generated. The produced turbulent kinetic energy is larger and more concentrated in the low frequency range. Near the device, the largest turbulent intensity can reach up to 20%, the integral scale is about 800mm, and the height of the simulated boundary layer is 5~000mm. This device is very suitable to simulate the boundary layer for the wind turbine model with the blade less than 2m. The device can be used to study the wind shear effect and overall dynamic characteristics of wind turbine.

The flow field environment of offshore wind turbine is more complex. By the combination effect of the complicated atmosphere boundary later, nonlinear waves and strong currents, the offshore wind turbine is often in large amplitude motion. Besides, the foundation of wind turbine is also influenced by the waves and currents. In order to make offshore wind turbine operate safely and stably, the issues of the wind, the saves, the currents, and the structure of the wind turbine must be considered simultaneously, and the coupled mechanical mechanism must be fully understood.

So far, some experimental researches have been carried out by some European and American research institutions. In their experiments, the water sink and the wave tank are used to simulate the waves and currents, and the wind is usually driven by fans (see Fig. 16) . However, the control and coverage of the flow field cannot fully meet the research needs.

It is also planned to carry out the wind tunnel simulation of the offshore wind turbine in CARDC. The wind turbine model will install on a base with six degrees of freedom. By specific movements of the base, the effects of the waves and currents will be simulated, and the aerodynamic characters influenced by the motion will be studied in the large scale wind tunnel. Although the coupling effects cannot be simulated, it has more advantages in the research on the interaction of the aerodynamic and hydrodynamic forces.

9 Conclusions

Based on the wind tunnel experiments in the UAE phase VI and MEXICO projects and the related research work carried out by CARDC, the progress in the wind tunnel test technology as well as some important research results are introduced in this paper. A lot of research fields still depend on the refined wind tunnel experiment for wind turbine, such as the flow mechanism study, the development of advanced computational method, and the validation and optimization of engineering model. In order to meet the needs of wind turbine aerodynamics research, the following aspects are very important in the development of wind tunnel experimental techniques. Firstly, various means of measurements should be used to obtain multiple physical parameters, which are useful for quantitatively describing the special flow phenomena of wind turbine. Secondly, the refined wind tunnel correction methods, especially the wall pressure correction method of the closed test section, should be further studied and modified. Thirdly, it is the new trend to integrate different research methods around the same problem, such as the theoretical analysis, the CFD simulation, the large scale wind tunnel experiment, and the field test in the wind farm. Besides, with the fast development of offshore and ultra large scale wind turbines, the comprehensive experimental techniques and aeroelastic wind tunnel techniques will attract more attention in the future.