Han Cao

Current wind tunnels possess a large space volume and high manufacturing cost, which are not suitable for investigating micro energy harvesters. This paper aims to design and fabricate a small, portable and low-speed wind tunnel for energy harvesting. A wind tunnel structure was first designed, a finite element analyses is then utilized to obtain the airflow velocity and turbulence intensity at the testing section, and the influence of the structural parameters of the wind tunnel on the flow field performance is finally investigated to achieve better performance. An experimental prototype of the wind tunnel was fabricated to verify the simulation results. Results demonstrated that the distribution uniformity and average turbulence intensity at the test section decrease first and then increase with the increase of both the diffuser and contraction lengths. The rectifying and damping effect of the honeycomb increase with increasing porosity and thickness. When the diffuser and contraction lengths are 850 mm and 480 mm, respectively, a better distribution uniformity and a lower turbulence intensity can be achieved. Experimental results were in good agreement with the simulation values. The maximum airflow velocity can reach up to 24.74 m/s, and the minimum error was only 1.23%. The designed wind tunnel achieved low-speed, small, portable and stable functions. This work provides an important guidance for further investigating the piezoelectric energy harvesting.

This paper presents a novel airfoil-based piezoelectric energy harvester (EH) with two small square prisms attached to an airfoil. This harvester can achieve a two degree-of-freedom (DOF) plunge–pitch motions. Several prototypes of energy harvester were fabricated to explore the nonlinear aerodynamic response and the output performance in a wind tunnel. The experimental results showed that the longer the flexible spring was, the lower the critical velocity and frequency of the harvester were, and the better aerodynamic response and output performance could be achieved. The initial disturbance, the following limit-cycle oscillation, and the ultimate chaos of nonlinear response occurred, as increasing airflow velocity was increased. The overall output performance of the harvesters with a flexible spring having a thickness of 1 mm outperformed than that of the harvesters with a flexible spring having a thickness of 0.5 mm at a higher airflow velocity, while the tendency was opposite at a lower velocity. An optimum output voltage of 17.48 V and a power of 0.764 mW were harvested for EH-160-1 at 16.32 m/s, which demonstrated it possessed better performance than the other harvesters. When the capacitor was charged for 45 s and directly drove a sensor, it could maintain working for 17 s to display temperature and humidity in real time.

This paper proposes a novel and efficient energy harvester (EH) system, for capturing simultaneously flutter and vortex-induced vibration. There exists a coupling effect between flexible spring energy harvester (FSEH) and cantilever beam energy harvester (CBEH) in aerodynamic response and output characteristic. Many prototypes of the harvester were manufactured to explore the coupling effect in a wind tunnel. The experimental results demonstrate that FSEH is mainly subjected to flutter-induced vibration and CBEH undergoes vortex-induced vibration. Disturbance of FSEH first takes place, a limited oscillation cycle then occurs, and chaos ultimately happens as airflow velocity increase. Root mean square voltages are more than 11 V for FSEH at beyond 10.52 m/s, which shows the better output performance over the existing harvesters. Vibration response and output voltage of various harvesters are mutually enhanced with each other. An enhancing ratio for FSEH-130-25 is up to 69.6% over FSEH-130-0, while the enhancing ratio for CBEH-130-30 is 198.3% compared to CBEH-0-30. Field application testing manifests that discharging time to power the pedometer is almost twice as long as the charging one for FSEH-130-25 at 14.48 m/s. The current research offers a suggestive guidance for promoting future practical application in micro airfoil aircrafts.