# Unsteady Flow Characteristics of an Oscillating Piezoelectric Fan Blade at High Reynolds Numbers
###### Abstract  
Piezoelectric fans have started to play an essential role in small-scale heat removal applications in recent years due to their high reliability and efficiency. In this study, an experimental study on the flow field characteristics produced by an oscillating piezoelectric fan at various Reynolds numbers (\(140<Re<550\)) in a quiescent air environment is investigated. Time resolved particle image velocimetry (PIV) measurements are performed for the flow field visualization. The flow pattern generated by the oscillating fan blade in the longitudinal plane changes as the Reynolds number increases. The ratio between the trailing edge velocity and side edge velocity increases as the Reynolds number increases. As a result, the trailing edge plays a more important role in driving fluid at a higher Reynolds number. Multiple vortexes are shed from the trailing edge during one oscillation cycle and is observed only at a high Reynolds number. This vortex shedding increases the unsteadiness of velocity field significantly, resulting in a turbulence intensity level beyond 100%. This result implies that turbulence models used in numerical studies need to be carefully validated as some might struggle at this highly turbulent flow regime. piezoelectric fan; flow visualization; turbulence; time resolving PIV measurement 1  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], J. Unsteady Flow Characteristics of an Oscillating Piezoelectric Fan Blade at High Reynolds Numbers. _Appl. Sci._**2021**, _11_, 9510. [[https://doi.org/10.3390/appl1209510](https://doi.org/10.3390/appl1209510)]([https://doi.org/10.3390/appl1209510](https://doi.org/10.3390/appl1209510))  
More detailed studies on the characteristics of the flow field induced by PZT fans and the underlying mechanism have been performed both theoretically and experimentally. [PERSON] et al. [9] investigated the velocity field of a vibrating cantilever plate using PIV. A Y-shape pseudo-jet flow dominated by vortical structures was found beyond the plate tip. They obtained individual vortex information such as the size, strength, distribution and position by wavelet analysis for single phase data. Meanwhile, they applied the proper orthogonal decomposition (POD) technique to deduce the transient evolution process of the regional vortex. [PERSON] et al. [10] conducted a phase-locking PIV experiment on a root-fixed piezoelectric flapping wing in a wind tunnel. Different vortex behaviors are found at different wingspans. The inner half close to the wing root produces drag, whereas the outer half was found to produce thrust. Besides, the flapping wing was also found to lift producing when it was mounted with a positive static angle of attack. [PERSON] et al. [11] applied PIV technique and numerical method to study the effect of the thickness of a cantilever plate on the flow structure. They concluded that as the beam became thicker, the wake was dominated by the lateral jet region and the propagation of vortices was prevented. Moreover, they suggested vortical structures were consistent with bluff body aerodynamic theory. Multi-angle PIV measurements are also adopted to show the flow field details. [PERSON] et al. [12] investigated the flow field in both longitudinal and transverse planes of the fan blade using PIV measurement. Vortices and jet-like flow generated at the PZT fan tip were presented under different plane positions correlated with the Keulegan-Carpenter (KC) numbers. [PERSON] and [PERSON] [13] pointed out that the flow in the region near the corner of the oscillating cantilever beam would become extremely three-dimensional, i.e., two-dimensional measurements can hardly reflect the real flow structure around these regions. They made PIV measurements in both longitudinal and transverse planes at multiple locations to obverse the two-dimensional flow affected by the sharp edges. The three-dimensional flow field induced by PZT fans is studied recently. [PERSON] et al. [14] developed a three-dimensional simulation model for the confined vibrating flat plate.