Kai Yang

This study investigates the generation mechanism of high-frequency whistling noise in the pipeline system of air-conditioner compressors using Large Eddy Simulation (LES) combined with numerical computation and experimental analysis. The research finds that the L-shaped insert structure induces vortex shedding due to the abrupt reduction in cross-sectional area, generating flow-induced noise. Moreover, when the vortex shedding frequency is close to the natural frequency of the pipeline, it excites deep-cavity vortex-sound coupling resonance, further amplifying the noise. Simulations and experiments demonstrate that increasing the insertion depth (e.g., from 6 mm to 9 mm) significantly increases the noise intensity, while using a T-shaped tee structure can effectively eliminate the noise. The study proposes optimization recommendations: limiting the insertion depth to 0.3 times the exhaust pipe diameter and replacing the right-angle welded structure with a streamlined T-shaped tee to suppress noise. The results provide a theoretical basis for high-frequency noise control in engineering applications.

This paper proposed an improved metamaterial piezoelectric beam with the acoustic black holes (ABHs) for low-frequency energy harvesting. The ABHs are embedded in the local resonant beams distributed on both sides of the primary beam. Piezoelectric plates are attached to the ABHs, leveraging the energy-focusing effect to enhance the energy harvesting performance. Finite element simulation results indicate that the energy harvesting electrical energy has improved by 6.92 times compared to the traditional metamaterial resonant beam without ABHs. Parameter analysis indicates that increasing the number of local resonators and extending the length of the piezoelectric patches can further enhance energy harvesting performance. However, reducing the truncation thickness of the ABH boosts energy harvesting but compromises structural strength. Extending the ABH length and increasing load resistance within the appropriate range are conducive to energy harvesting. Besides, the optimal load resistance varies with frequency. These findings provide useful guidance for future applications of improved metamaterial beam.

This study aims to improve the performance of the galloping-based piezoelectric energy harvester (GPEH) by benefiting from two half-cylinder obstacles. Two half-cylinder obstacles are employed to construct a variable cross-section tube which changes the airflow characteristics near the GPEH placed inside. The performance of the GPEH in the variable cross-section tube is tested by wind tunnel experiments. The experimental results demonstrate that compared with the traditional GPEH, the cut-in speed of the GPEH with the two half-cylinder obstacles at the downstream position −1.5D is reduced by 43.87%, from 2.23 m/s to 1.55 m/s. For the highest experimental wind speed of 4.29 m/s, the output voltage of the GPEH with the two half-cylinder obstacles is increased by 109.28%, from 4.31 V to 9.02 V. Moreover, the performance of the GPEH with the obstacles can be improved by appropriately increasing the obstacle curvature and reducing the throat width. The computational fluid dynamics (CFD) simulations are conducted to qualitatively interpret the experimental discoveries. The results demonstrated that the downstream area is beneficial to the GPEH, whereas the upstream area degrades the performance.