## 4 Conclusions  
This paper presents a comprehensive experimental and numerical analysis of the influence of bluff body mass and diameter on the efficiency of a piezoelectric energy harvesting system under airflow conditions. The conducted studies allowed for a detailed evaluation of the system's dynamics depending on its geometrical configuration and varying flow conditions. The experimental results demonstrated that the investigatedsystem exhibited a diverse dynamic response--ranging from regular oscillatory vibrations to a state of being trapped in a potential well, where the vibration amplitude was severely limited. This effect is plausibly due to left-to-right symmetry breaking by a small deflection of the bluff body beam. It is sometimes called the stagnation effect with a smaller amplitude of vibration. The frequency lock limit is almost reached. The natural frequency in the stagnation limit is still present as the relaxation; however, the alternative scenario with the increasing shedding frequency [x] and larger response amplitude of vibration is possible. The competition of two solutions may lead to the chaotic response [35]. Effective energy harvesting was only achievable within a specific range of flow velocities, which enabled a full oscillatory response. This velocity range varied depending on the bluff body's diameter and mass. The highest voltage values were obtained for configurations with a 25 mm diameter bluff body, suggesting an optimal compromise between mass, geometry, and the system's resonant frequency. Conversely, the largest diameter (30 mm) provided predictable behavior but with a significantly narrower range of effective operation. Notably, an increase in bluff body mass led to a decrease in generated voltage, which was attributed to a reduction in vibration amplitude resulting from the system's increased inertia. For the smallest masses, the most efficient conversion of mechanical to electrical energy was observed, especially for the 20 mm diameter bluff body, where full oscillations occurred almost throughout the entire range of tested flow velocities. Frequency spectrum analysis revealed the expected effect of bluff body mass on lowering the resonant frequency: as mass increased, system inertia rose, resulting in a decrease in resonant frequency and reduced generated voltage. Additionally, a decrease in the distance between peaks in the frequency spectrum indicated reduced system sensitivity to excitation conditions at larger masses. For a bluff body diameter of 20 mm, the system operated effectively with smaller masses (M\({}_{1}\), M\({}_{2}\)) within the velocity range of 5 to 8 m/s. Larger masses restricted the amplitude of vibrations, even though the system remained dynamically active. A diameter of 25 mm provided the best compromise between mass and VIV excitation. The system demonstrated peak efficiency (exceeding 10 V peak-to-peak voltage) even with medium masses and moderate velocities (4-7 m/s). For the largest bluff body diameter (30 mm), the system was more sensitive to changes in flow velocity and exhibited the narrowest effective operating range (3-4 m/s), despite an initially stable response. A crucial aspect of the analysis was the implementation of the 0-1 test for chaos, which enabled the identification of transitions between regular and chaotic system behavior. Specifically, the approach based on the Pearson correlation coefficient (_K2_) proved more effective in distinguishing dynamic states compared to the classical mean square displacement (MSD) method. High values of the \(K\) index correlated with the occurrence of chaotic, irregular vibrations, which are not conducive to efficient energy harvesting. The system effectively harvests energy only within specific, often narrow, wind speed ranges that induce full oscillations. Outside this range, the system may become trapped in a potential well or transition into chaotic states, significantly reducing efficiency.