*2.5. Frozen Rotor Method*

Although PIV technology has been widely used in wind turbine experiments, it still has some limitations: the measuring plane must along the main flow direction to ensure the particle being illuminated, 2D-PIV can only measure the two-dimensional component of the wind field and 3D-PIV equipment is very expensive.

The frozen rotor method is widely used in CFD simulations for a rotating machine. When considering from the ground coordinates, the fluid field of a rotating machine is a transient problem. However, it can be turned into a static problem by considering form rotating coordinates. In this study, a new measurement method for wind turbine wakes was developed borrowing the idea of frozen rotor. A three dimensional hot-wire anemometer and encoder were employed to measure the velocity field in the wake region. At each measurement point, the wind velocity and the rotor azimuth were collected simultaneously. According to the axial symmetry of the wind turbine flow, this measurement can be considered as the hot-wire probe gathering velocity data around the axis of the rotor while the rotor and fluid field are frozen. Mean velocity was calculated according to the rotor azimuth to avoid the influence of small-scale vortex.

Figure 6 shows the experimental setup of hotwire anemometer probe. In this study, the probe was moved by an auto-controlled platform with the precision of 25 μm to scan a whole plane. Figure 7 shows all the locations of measured points. For each point, the wind velocity and encoder data are collected simultaneously. Using the idea of frozen rotor, with each wind speed marked by a rotor azimuth, the wake characteristic can be reconstructed for the whole wake region. During the experiment, more measurement points were added to the potential tip vortex region, as shown in Figure 7, to get more precise results. By this way, we rebuild the wake region inexpensively but accurately with a hot-wire anemometer system.

**Figure 6.** (**a**) Wind turbine model and coordinate system (**b**) hotwire anemometer probe location.

**Figure 7.** Locations of measurement points.
