## 2 Research Object and Test Rig  
The experiments were conducted in a closed-loop wind tunnel with a closed test section measuring 700 mm \(\times\) 700 mm \(\times\) 1500 mm. The air speed range at the inlet to the empty test section of this tunnel extends up to 50 m/s. Flow velocity can be adjusted with an accuracy of 0.2 m/s. The turbulence intensity from the inlet to the test section, beyond the boundary layer, reaches values below 0.4% (at 50 m/s). The fan section is powered by a 30 kW three-phase AC motor. A more detailed description of the tunnel can be found in [47]. Figure 1 illustrates the experimental setup for air energy recovery systems, as prepared for the work described in this article. Figure 2 shows the research object mounted within the wind tunnel test section. Figure 1: Experimental setup for energy harvesting systems: _a_—wind tunnel test section with the mounted research object, _b_—set of tested bluff bodies, _c_—computer for recording and data acquisition from the EH system, _d_—computer for controlling the wind tunnel, _e_—data acquisition card, National Instruments USB-6002 (National Instruments, Austin, TX, USA). Data acquisition was performed using a National Instruments USB-6002 data acquisition card. The device features 16-bit resolution and supports up to 8 analog (differential) inputs and 13 digital I/O lines. Its maximum sampling rate is 50 kS/s. The voltage signal generated by the piezoelectric element was fed into an analog input channel with a \(\pm\)10 V range. Data were recorded at a frequency of 800 Hz for at least 30 s. For the selected range, the resulting measurement resolution was \(\pm\)0.3 mV, and the absolute accuracy was 6 mV (typical at full scale). Air velocity in the wind tunnel was measured using a one-piece Prandtl tube (Airflow TSI 06003, TSI Incorporated, Shoreview, MN, USA) connected to a pressure transducer (model APLISENS APR-2000G, APLISENS S.A., Warsaw, Poland). The device measures the difference between static and dynamic pressure at the inlet of the measurement chamber.. These values correspond to the measurement range set from --2500 to 2500 Pa, while the measurement accuracy of the device is 0.1%. Air velocity was set via the wind tunnel control software (Figure 1d). Prior to data acquisition at each measurement point, a minimum of 30 s was allowed for the airflow field inside the test chamber to stabilize. Each measurement was conducted after confirming system stability and signal consistency. Preliminary tests indicated high repeatability under fixed conditions; therefore, each operating point was measured once. System reset and flow stabilization procedures were applied to minimize experimental uncertainty. The main dimensions of the bluff body and the elastic beam, onto which the piezoelectric element was affixed, are presented in Figure 3. The beam, measuring 200 \(\times\) 20 \(\times\) 0.5 mm, was cut from a 1050 aluminum alloy sheet. One end of the beam, 20 mm in length, was inserted into the bluff body, and the other end, of the same length, was secured in the mast. Therefore, the distance between the mast and the bluff body was 160 mm, as shown in Figure 3. The system was constructed with springs, which were attached at one end to the arms and at the other to the beam. The method of attachment to the beam utilized the specially cut slots visible in the drawing. These holes likely influenced the system's stiffness, which will be subject to further analysis. Table 1 presents the technical parameters of the M2807-P1 piezoelectric sensor (Smart Material GmbH, Dresden, Germany) used in the energy harvesting system. This sensor features an active area of 28 \(\times\) 7 mm and is capable of withstanding a maximum blocking force of 62 N, with a free strain of 1035 ppm, both with a tolerance of \(\pm\)20%. It operates within a voltage range of \(-\)500 V to +1500 V and supports frequencies below 1 MHz. With  
Figure 2: View of the research object: 1—bluff body, 2—elastic beam, 3—spring arm, 4—piezoelement, 5—mast, 6—arms.