## 3 Results and Discussions
### Behavioral Characteristics of the Flap  
The behavioral characteristics of the flap, according to the experimental conditions, can be classified into five patterns, as illustrated in Figure 7. The solid gray line in the figure denotes the rotation angle \(\theta_{x}\) of the flap, and the brown and light blue dotted lines denote the output voltages of \(V_{on}\) and \(V_{off}\). Each pattern is defined as follows:  
* Pattern A (stable, 108 cases): the flap rotated stably with incident waves. Figure 6: Results of free fluctuation tests of the flap for (**a**) the time variation of the rotation angle \(\theta_{x}\) measured by the gyro sensor and (**b**) the natural frequency. L1–6 and R1–6 denote the cases of the flap initially tilted to the onshore (left side) and offshore sides (right side), respectively. Figure 7: Behavioral characteristics of the flap for (**a**) pattern A (stable), (**b**) pattern B (fluctuation), (**c**) pattern C (unstable), (**d**) pattern D (loss), and (**e**) pattern E (\(P_{invc}=0\)). Capitals of each subtitle indicate experimental conditions; e.g., D109_h42_5_T30_H12 denotes the distance from the reflection wall \(D\) (=1.09 m), water depth \(h\) (=0.425 m), wave period \(T\) (=3.0 s), and wave height \(H_{i}\) (=0.12 m). * Pattern B (fluctuation, 66 cases): the flap behavior changed with time. * Pattern C (unstable, 40 cases): the behavior of the flap changed randomly during the experiment. * Pattern D (loss, 65 cases): the flap did not return to its original position after moving several times so that the flap lost its restoring force and remained stationary in the water. * Pattern E (\(P_{\text{WEC}}\) = 0, 49 cases): the flap did not move during experiments. It can be confirmed that in all patterns, \(V_{on}\) and \(V_{\mathit{off}}\) increased and decreased with the rotational direction of \(\theta_{x}\). \(V_{\mathit{off}}\) becomes maximum when \(\theta_{x}\) is maximum, whereas \(V_{on}\) becomes maximum when \(\theta_{x}\) is minimum. Pattern A depicts the behavior of the flap rotating periodically with a constant angle, as illustrated in Figure 7a. In contrast, \(\theta_{x}\) in pattern B fluctuates owing to the long period component contained in the main period component (see Figure 7b). The long period fluctuation in pattern B is a particular phenomenon induced by the closed experimental region in which an undoubtedly long period, such as resonance, occurs. This pattern can be generated by the interactions of the incident waves, the reflected waves from the reflecting wall, and the extra waves generated by the behavior of the flap. Pattern B also includes the cases in which the rotation angle \(\theta_{x}\) gradually increases or decreases over time. In pattern C, the rotation of the flap becomes irregular over time, as depicted in Figure 7c, and in some cases, the rotation of the flap is halted for a while. In pattern D, as depicted in Figure 7d, the flap does not rotate after it has tilted into the water and remains stationary. In pattern E, the pendulum does not rotate from the beginning. As a result, the output voltages \(V_{on}\) and \(V_{\mathit{off}}\) were 0 for the whole experimental time, as depicted in Figure 7e. Hereafter, patterns A and B are valid, and patterns C, D, and E are invalid, in terms of generating electricity. The influences of the distance from the reflection wall and the wavelength ratio \(D/L\), wave steepness \(H_{i}/L\), initial water depth \(h\), and wave energy power \(P_{wave}\) on the behavioral characteristics of the flap were investigated. Figure 8a,b depicts the tendency of the behavior of the flap depending on \(D/L\) and \(H_{i}/L\), and \(D/L\) and \(P_{wave}\), respectively. The results in Figure 8 include all experimental cases of different initial water depth. \(H_{i}/L\) was calculated from the generated wave height \(H_{i}\) and the wavelength \(L\) of the linear wave theory for respective experimental conditions.