## 5 Experimental Results
### Wind Tunnel Test  
The modal test could not be used to analyze the vibration caused by the force induced by vortex shedding, which has a direction perpendicular to the wind velocity. In the modal test, the input force was in the same direction as the BWT's vibration, since the modal shaker was physically connected to the mast. Thus, a second experiment was conducted with wind tunnel equipment to analyze the resonance of the BWT as a result of the vortex shedding at a wide range of wind velocities.  
The BWT prototype was placed in front of the wind tunnel equipment, as shown in Figure 8. The direction of the BWT's mast oscillation was set to be perpendicular to the wind velocity. The wind generated by the equipment was blown onto the BWT to induce vortex-shedding vibration, which oscillated the BWT's mast. The wind speed was controlled from 1.6 to 3.6 m/s with an interval of 0.2 m/s. An anemometer was installed near the BWT to record the input wind velocity. For each wind speed, the vibration level of the mast was acquired with an accelerometer at a sampling rate of 1 kHz for 10 s. The experiment was repeated for the BWT without and with the magnetic field applied for the comparison. The root mean square (RMS) of the acceleration was normalized by the lift force for an index metric of the BWT's vibration. The vibration results of the BWT against various wind velocities and different magnetic field levels are plotted in Figure 9.  
According to the experimental results, the BWT with the nominal MRE state (B = OFF) in the Low-K mode had a higher oscillation energy against a wind speed lower than 2.8 m/s. When the wind speed was higher than 2.8 m/s, the BWT could oscillate more if a magnetic field was applied to the MREs (B = ON). In Figure 10, the acceleration and frequency spectrum of the BWT with different MRE states are compared at wind speeds of 2.0 and 3.2 m/s.  
It can be concluded that for low-wind-speed conditions, the BWT should operate in the nominal state. For weather conditions with higher wind speeds, a magnetic field should be applied to maximize the mast oscillations for more efficient energy harvesting. This crossing point at 2.8 m/s was similar to the value observed in the vibration curve in Figure 4, which was 2.6 m/s. This difference was mainly due to the differences in the experimental setup and the input force conditions. The wind speed of the wind tunnel was not perfectly uniform on the BWT's mast. Additionally, there was a wind gradient on the mast surface along the direction of the vertical axis. Thus, the vortex-shedding frequency was not purely sinusoidal in the wind tunnel test, as in the modal test. However, the trend of the vibration-level curves when applying the magnetic field was similar in both the wind tunnel and the modal test results.  
In summary, a higher vibration of the BWT can be maintained for a higher wind velocity by applying a magnetic field. Thus, the proposed MRE-based tuning system can widen the lock-in range of the BWT for efficient energy harvesting.  
Figure 9: Comparison of the BWT’s vibration energy against the wind speed for different MRE stiffnesses. For a wind speed higher than 2.8 m/s, applying a magnetic field to the MREs generated higher vibration energy.