# Unsteady Flow Characteristics of an Oscillating Piezoelectric Fan Blade at High Reynolds Numbers
###### Abstract
They claimed that the interaction of side vortices and tip vortices produced by confined piezoelectric fans lead to the velocity defect near the end wall, which agreed with the experimental data. [PERSON] et al. [15] performed 3D phase-locked PIV measurements for the flow structure of an unconfined piezoelectric fan working at its first vibration mode. The structures of the vortex generated by the oscillation were constructed by interpolated PIV data, which initially appears as a horseshoe; the vortices generated by side edges form the two legs of the horseshoe vortex, then the tip-induced vortex separate from the trailing edge and formed into a hairpin shape with two legs still connect the blade tip. Numerical simulations were also performed to confirm the PIV result. [PERSON] et al. [16] conducted 2D and 3D PIV measurements and numerical simulations to obtain detailed jet flow and vortex structures generated by the oscillation of a PZT fan with different oscillating frequency, amplitude and geometric parameter. They found that the breakdown of the shed vortex from the cantilever tip and the reorientation of the substructures are the primary factors that affect the shape of the induced jet. From the literature mentioned above, effects of the thickness, frequency and amplitude of a PIZ fan blade on the flow field have been investigated in two-dimensional cases. Most 3D flow field measurements mainly focus on the evolution of the flow structures within one oscillating period for the same parameter. Few three-dimensional measurements have been conducted to investigate effects of fan blade geometry and oscillating frequency and amplitude. With the increase in the oscillating Reynolds number, the flow produced by the oscillating blade becomes more unsteady. In the current study, three-dimensional flow fields induced by a PZT fan blade in an unconfined quiescent air environment are measured at Reynolds numbers up to 550. The higher Reynolds number investigated in this study is more consistent with the piezoelectric fan operating condition in real applications. The level of turbulence in the flow field induced by a piezoelectric fan is closely related to the performance of the piezoelectric fan. A high level of turbulence indicates a large kinetic energy loss, resulting in a less efficient fan to drive the fluid. However, a high level of turbulence can also increase the heat transfer coefficient. The overall role of turbulence generated by the piezoelectric fan blade on the heat dissipation is very complicated and still not clear. So far, there have been few studies focused on the turbulence characteristics generated by a piezoelectric fan blade. In the current study, the turbulence intensity levels at different Reynolds number conditions are evaluated, the maximum turbulence intensity at high Reynolds numbers up to 14. The paper is organized as below. Section 2 introduces the PIV experiment setup and the measurement procedures. Section 3 presents the experiment results and discussion of the flow characteristics induced by the oscillating PZT fan blade. Section 4 concludes this study.