The specific objectives with the wind tunnel experiments were the following:
- Verify and investigate the difference in 2D airfoil characteristics measured in three different wind tunnels: at Delft University (NL), at LM Glasfiber (DK) and at Velux (DK), which in the past have been used for testing airfoils for wind turbines
- Investigate the turbulence characteristics in the wind tunnels and investigate the correlation with boundary layer transition and surface pressure spectra
- Measure the 2D airfoil characteristics on the four specific sections on the LM 38.8 m blade for comparison with 3D airfoil characteristics of the NM80 rotor.
One problem often encountered in the design of a new wind turbine rotor is that the airfoil data from different wind tunnels deviate. It was therefore decided to include the test of two airfoils, the Risø-B1-18 and the NACA 633-418 airfoil, in the three wind tunnels mentioned above. The turbulence characteristics in the tunnels are an important parameter with influence on the transition characteristics on the airfoils and measurements with hotwires have been performed to determine the turbulence spectra in two of the tunnels.
Position of transition has been determined by analyzing the PSD spectra of the high frequency surface pressure fluctuations measured with microphones mounted close to the blade surface, Figure 1.
Figure 1 Microphones with a diameter of around 4 mm installed about 1 mm below the blade section surface and connected through a boring of 1.5 mm.
In total around 60 microphones were positioned around the surface of the airfoil section at one spanwise position, both on the pressure and suction side. However, the microphone positions were shifted slightly in spanwise direction in order to avoid disturbance from one microphone to the next.
The tests performed in the LM wind tunnel, Figure 2 were carried out at a number of different Reynolds numbers from 1.6 million to 6 million. Influence of increased turbulence from a turbulence grid in the inlet to the test section in the LM wind tunnel was also measured Figure 2.
Figure 2 To the left one of the airfoil sections with pressure taps and microphones mounted in the LM wind tunnel. To the right a view into the inlet to the test section with a turbulence grid installed.