*2.3. Simulation Setup*

The dimension and boundary conditions are set up according to the experiment. The inlet velocity is 3.5 m/s. The calculation domain is shown in Figure 2. To get a better result for the wake characteristic, the potential wake region is refined. For the convenience of expression, the calculation domain is subdivided into two regions: the background region and the rotor & wake region. Figure 2 shows the details of the calculation domain. The calculation domain is a little longer than the wind tunnel to avoid the influence of the inlet condition. In summary, the calculation domain is 1.5 m by 1.5 m by 2.8 m and a wake region of 1.0 m by 1.0 m by 1.2 m is refined to get a better simulation of wake characteristics.

**Figure 2.** The calculation domain.

#### *2.4. Experimental Setup*

In this study, an experimental measurement of torque and wake characteristics was conducted to validate the simulations. Although Particle Image Velocimetry (PIV) technology has been proved to be powerful in wind turbine wake measurement [20–22], there are two restrictions for the PIV technology. When using the PIV system, the main direction of flow must be within the laser plane to guarantee that most of particles do not escape and stay illuminated. On the other hand, the measurement of the velocity field in three dimensions can only be achieved by using two cameras and a special laser generator [23,24]. The cost also constrains the use of PIV system. Due to these limitations of PIV system, hot-wire anemometer is also widely used in wake measurement. The wake experiments using hot-wire anemometer carried out by Schümann et al. [25], Lungo et al. [26], Singh et al. [27], and Dou [28] also made good measurements of wake characteristics. In this study, a new method for wind turbine wake measurement were developed using hot-wire anemometer.

Figure 3 shows the wind tunnel used in this experiment. It is composed of a contraction section, test section, diffuser section, and blower section. The length of the test section is 2.2 m and its cross-sectional dimensions are 1.5 m by 1.5 m. Equipped with three screens and two honeycombs and driven by four 11 kW mixed flow motors, the maximum velocity of the wind tunnel can reach 15 m/s and the turbulence intensity is around 0.5%.

A specifically designed two-blade wind turbine model is used in the experiment. The diameter of the rotor is 0.8 m and an NREL S826 airfoil profile is used all along the span for its high lift-drag ratio and low weight. The chord lengths and twist angles of the blade are shown in Figure 4.

**Figure 3.** Wind tunnel used in experiment.

**Figure 4.** Length and twist angle distribution of the aerodynamics significant part of the blade.

The nacelle is equipped with an encoder of 1000 pulse, a torque sensor of 0.1% precision, and a servo-motor. This nacelle is designed mainly based on Anik's [29] equipment. During the experiment, the rotational speed of the rotor is totally controlled by the servo-motor and the relation between

aerodynamic force and the motor force can be determined by the sign of the torque data. The positive sign of the torque data indicates that the rotor is driven by wind and the serves as a load balancing. Furthermore, with this equipment, the friction of the whole system can be measured by a motor-driven experiment without blades. This can help to increase the precision of the experiment. The details of the wind turbine model are shown in Figure 5.

**Figure 5.** Wind turbine model and measurement equipment.

The inlet wind velocity is set to 3.5 m/s during the whole experiment. However, the rotational speed of the wind turbine model varies from 300 RPM to 650 RPM by 50 RPM. Thus, the tip speed ratio correspondingly varies from 3.6 to 7.8. For each rotational speed, the torque and wake characteristic of the model wind turbine are measured. It should be noted that the dimension and boundary condition of the simulation is strictly based on the experiment.
