## 3 Results and Discussion  
Tests using the piezoelectric energy harvesting system were conducted to evaluate its performance under various geometric and dynamic configurations. The experiments involved three different bluff body diameters, five different bluff body masses, and a variable airflow velocity ranging from 1 to 10 m/s. This diversity of parameters enabled a comprehensive analysis of the operating conditions' impact on the energy harvesting system's efficiency. During the tests, two main dynamic states were observed at different airflow velocities. At lower airflow velocities, the system typically exhibited full oscillations. In contrast, at higher velocities, the system either became trapped in a potential well, performing minimal motion, or entered a transient state--initially oscillating for a short period before the motion was gradually damped out. To illustrate this phenomenon, two representative cases were selected, one in which the system exhibited full oscillations (Figure 4) and another in which transient states were observed (Figure 5). Additionally, to highlight the distinct dynamic behavior of the system under the same geometry, the peak-to-peak values were calculated by identifying individual maxima and minima over time series of equal length. In the case of the oscillatory response, 140 peaks were identified, whereas, for the transient response, the number of such observations was approximately four times higher. Moreover, in the oscillatory solution, the calculated peak-to-peak values consistently ranged between 10 and 11 V throughout the entire time series. In contrast, under transient conditions, these values fluctuated between 0.2 V and 2 V. These results have a direct impact on the amount of harvested energy and emphasize the importance of selecting appropriate excitation conditions for optimal harvester performance. Figure 4: Voltage signal for configuration \(m_{2}\), \(d=25\) mm, and \(U=3\) m/s: (**a**) raw voltage signal recorded from the piezoelectric element, (**b**) peak-to-peak voltage calculated based on local minima and maxima in the signal, (**c**) histogram for the mean value, \(\mu=10.6437\) V, and the standard deviation, \(\sigma=0.2126\) V, on the normal distribution, respectively. For the analyzed cases, the mean and standard deviation values of the peak-to-peak voltage were calculated. For the selected case representing full oscillations (Figure 4), the mean peak-to-peak voltage was 10.6437 V and the standard deviation was 0.2126 V. A total of 140 peaks were considered for this signal. In contrast, for the case representing transient states (Figure 5), the mean and standard deviation of the peak-to-peak voltage were 0.9294 V and 0.3675 V, respectively. A larger number of 600 peaks was included in the analysis due to the more irregular and less periodic nature of the signal. The accompanying histograms further illustrate the statistical distribution of the calculated peak-to-peak values for each case. In the full oscillatory regime (Figure 4c), the histogram shows a tightly clustered distribution centered around the mean value, indicating a highly stable and periodic signal. In contrast, the transient regime (Figure 5c) exhibits a broader distribution with a higher standard deviation, reflecting increased signal variability and the absence of stable oscillatory patterns. The next step in observing the system's dynamics was the identification of its characteristic frequency at different airflow velocities (Figure 6). In this case, the dominant amplitude peak in the frequency spectrum was taken as the resulting frequency value. The results did not reveal a clear distinction between the oscillatory and transient responses; however, a slight decrease in frequency was observed at higher airflow velocities.