## 3 Results and Discussion
Although the presented study focused on the variability of mass and diameter, the orientation of the bluff body relative to the flow also plays a crucial role. According to the findings in [29], the optimal orientation varies with flow velocity. As the cited study showed, at low velocities (<2 m/s), the best performance was achieved when the bluff body's axis was aligned parallel to the beam, whereas at higher velocities (>2 m/s), better results were obtained when the body was oriented perpendicularly. This indicates that the aerodynamic forces acting on the body are highly dependent on its orientation, which affects the amplitude and frequency of the beam's vibrations and, consequently, the energy harvesting efficiency. In the investigated setup, the cylindrical bluff body was mounted transversely to the longitudinal axis of the beam. In this configuration, the aerodynamic force acts directly in the transverse direction with respect to the beam's axis, promoting the generation of maximum bending moments and, therefore, strong deformations in the region where the piezoelectric element is located. Transverse mounting of the bluff body is effective \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Diameter**} \\
**Mass** & \begin{tabular}{c} **20 mm** \\ **Threshold** \\ **U [m/s]** \\ \end{tabular} & \
\begin{tabular}{c} **25 mm** \\ **Threshold** \\ **U [m/s]** \\ \end{tabular} &
\begin{tabular}{c} **30 mm** \\ **Threshold** \\ **U [m/s]** \\ \end{tabular} \\ \hline  & K1(c) & K2(c) & K1(c) & K2(c) & K1(c) & K2(c) \\ \hline \(m_{1}\) [g] & 10 & 10 & 8 & 8 & 6 & 6 \\ \(m_{2}\) [g] & N/D & N/D & 8 & 8 & 7 & 7 \\ \(m_{3}\) [g] & 10 & 10 & 9 & 9 & 6 & 6 \\ \(m_{4}\) [g] & N/D & 6 & 9 & 9 & 6 & 6 \\ \(m_{5}\) [g] & N/D & 10 & 6 & 6 & 7 & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Airflow velocity thresholds (\(U\) [m/s]) for the onset of chaos (\(K\geq 0.4\)) identified using MSD-based (\(K1\)) and correlation-based (\(K2\)) 0–1 test approaches. for exciting vortex-induced vibrations (VIV), provided that frequency synchronization (lock-in) occurs between the vortex shedding and the system's natural frequency. However, at higher flow velocities (around 10 m/s) or for heavier bluff bodies (M4 or M5), the system may experience so-called potential well trapping, which results in the suppression of vibrations. This is most likely due to increased aerodynamic forces that dampen rather than excite oscillations. The lack of asymmetry stemming from the cylindrical shape of the bluff body leads to energy harvesting via classical VIV mechanisms. This does not result in the so-called galloping effect, which typically occurs with asymmetrical cross-sections. Therefore, in the examined configuration, no synergistic VIV-galloping phenomenon was present. In the presented experiment, the mass of the bluff body was uniformly distributed throughout its volume, which directly resulted from the use of 3D printing technology with a homogeneous material. This configuration provided good control over the total mass and its repeatability; however, it did not allow for an analysis of how the distribution of mass along the body's axis affects the system's dynamic behavior. The mass distribution of the bluff body can significantly influence the dominant vibration modes and their energy efficiency. Concentrating mass near the attachment point of the bluff body to the beam will affect the moment of inertia of the system with respect to the fixed point. It can be expected that this will impact the dynamic response (amplitude, frequency), the possibility of achieving resonance with the vortex shedding frequency, and the efficiency of voltage generation by the piezoelectric element. Therefore, a promising direction for further research could involve introducing controlled anisotropy in the mass distribution--for example, through the use of gradient infill structures in 3D printing or additional point loads.