## 3 Results and Discussion
The FFT (Fast Fourier Transform) results were consistent, showing a gradual shift in resonance frequency depending on the bluff body mass--specifically, the highest resonance frequency was observed for mass \(M=1\), while the lowest was observed for mass \(M=5\), for each bluff body diameter. This trend confirms the expected physical behavior of the system, where increased mass leads to lower natural frequencies due to higher inertia. It is also worth noting that the spacing between frequency peaks became narrower with increasing mass, suggesting reduced sensitivity to excitation conditions. These findings may be crucial for  
Figure 5: Voltage signal for configuration \(m_{2}\), \(d=25\) mm, and \(U=10\) 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, \(\mu=0.9294\) V, and the standard deviation, \(\sigma=0.3675\) V, on the normal distribution, respectively. tuning the energy harvesting system to specific flow conditions, especially in applications where the available excitation frequency range is limited. The next step in the analysis, following the initial verification of the system dynamics, was to calculate the RMS (Root Mean Square) value of the voltage generated by the system under varying wind flow velocities ranging from 1 to 10 m/s, with the RMS calculated as the average value over the entire 30 s signal. In the case of the smallest diameter of 20 mm, all tested masses resulted in oscillatory behavior, and the system was the most voltage-efficient for the three smallest masses. For the two largest masses, oscillations still occurred, but with very small amplitudes, which translates into a low value of the generated voltage. This change may have been caused by the increased inertia of the larger masses, which required more force to generate the same displacements, resulting in smaller oscillation amplitudes and lower voltage generation. For the 25 mm bluff body, oscillatory response appeared at low airflow velocities from 1 m/s and generally diminished around 7-8 m/s. This configuration produced higher voltage than the 20 mm case, with peak output occurring between 4-6 m/s. For the 30 mm diameter, maximum voltage was observed at 3-4 m/s across all masses, with oscillations fading at 6 m/s. Although the system showed the least flexibility at this size, it operated in a narrow and more predictable velocity range. Studies by [PERSON] et al. [48; 49] on an energy harvester with a square prism bluff body mounted transversely to the longitudinal axis of a cantilever beam with a piezoelectric element showed that the RMS voltage increased with increasing wind speed. In selected cases, RMS values exceeded 25 V at a wind speed of 5 m/s. Moreover, it was demonstrated that increasing the length of the protrusion on the leeward side of the bluff body generally led to higher vibration amplitudes and output power at higher wind speeds. A similar  
Figure 6: Characteristic frequency (maximum) components of the voltage signal in the frequency domain for different bluff body diameters: (**a**) \(d\) = 20 mm, (**b**) \(d\) = 25 mm, (**c**) \(d\) = 30 mm. relationship between voltage and wind speed was observed for an apple-shaped bluff body mounted on a cantilever beam with a piezoelectric element [50]. As wind speed increased, the RMS voltage also increased. The maximum value approached 8 V at 5 m/s. The effect of bioinspired metasurfaces on cylindrical bluff bodies was also investigated by [PERSON] et al. [51]. A wavy surface proved beneficial at low wind speeds, increasing the output voltage by over 94% (0.44 V) at 0.5 m/s compared to a traditional cylindrical bluff body. A cylindrical bluff body with a modified snowflake-shaped surface, presented in [52], allowed for RMS voltage values close to 20 V at a wind speed of 2 m/s within the VIV range.