## 3 Results and Discussion
A galloping triboelectric nanogenerator using a square-section bluff body mounted transversely to the axis of the beam enabled average voltage values ranging from 20 to 90 V, depending on the Reynolds number and damping ratio [53]. Hybrid bluff bodies combining square and semicircular cross-sections were studied by [PERSON] et al. [54]. It was shown that the proposed configuration can achieve maximum RMS output voltages exceeding 30 V at a cut-in wind speed of 1.5 m/s. The analysis of the RMS of generated voltage under varying airflow velocities revealed distinct behaviors depending on the diameter of the bluff body. For the smallest diameter of 20 mm (Figure 7a), the highest peak-to-peak voltage values (exceeding 10 V) were obtained exclusively for the smallest masses (M\({}_{1}\) and M\({}_{2}\)), particularly at velocities ranging from 5 to 8 m/s. As mass increased (M\({}_{4}\) and M\({}_{5}\)), there was a clear reduction in the generated voltage. This can be attributed to the increasing inertia, which dampened the beam's vibration amplitudes. For M5, the voltage remained low (<5 V) across the entire velocity range. In the case of the 25 mm diameter (Figure 7b), the system achieved the best overall performance. Maximum peak-to-peak voltages (exceeding 12 V) were recorded for masses M\({}_{1}\) and M\({}_{3}\) within the velocity range of 4-7 m/s. Mass M\({}_{2}\) also yielded good results;  
Figure 7: Peak-to-peak voltage values calculated from the entire voltage signal for different bluff body diameters: (**a**) \(d\) = 20 mm, (**b**) \(d\) = 25 mm, (**c**) \(d\) = 30 mm. however, the efficiency of voltage generation deteriorated more rapidly at higher velocities (7 and 8 m/s). Unlike the 20 mm configuration, even a larger mass (M\({}_{4}\)) produced favorable results, suggesting a beneficial matching of the system's natural frequency with the vortex shedding frequency (lock-in). Above 7 m/s, a drop in voltage occurred, which may indicate a transition to chaotic behavior or a suppression of vibrations. For the largest diameter of 30 mm (Figure 7c), the system's response was more limited and predictable. Effective operation occurred exclusively within a narrow velocity range of 3-4 m/s, regardless of the mass used. The best voltages (exceeding 14 V) were achieved for masses M\({}_{1}\) and M\({}_{2}\). At higher velocities (from 5 to 10 m/s), the system frequently transitioned into a state of suppressed vibrations, likely due to aerodynamic damping or potential well trapping, resulting in an almost complete voltage dropout. These results highlight the balance between voltage output, system flexibility, and stability across different geometric configurations. Table 3 summarizes the identified flow velocity thresholds at which the system transitions from an oscillatory to a transient response for various mass-diameter configurations. The results indicate a clear dependence of the transition point on both the mass of the bluff body and its diameter. For all tested masses, the lowest threshold velocities are consistently observed for the largest diameter (30 mm), suggesting that increased geometric blockage promotes an earlier onset of transient behavior. Conversely, for smaller diameters (20 mm and 25 mm), the transition occurs at higher flow velocities, and in some cases (e.g., M4 and M5 at 20 mm), no distinct transition is detected within the tested range, as indicated by the N/D (not detected) entries. This suggests that for lighter or more streamlined configurations, the system remains in an oscillatory regime over a wider range of flow velocities. These observations highlight the interplay between mass and geometric parameters in shaping the system's dynamic response and support the hypothesis that both inertial and aerodynamic effects contribute to the onset of transient behavior. To further investigate the dynamic response of the system, the 0-1 test for chaos was applied.