Visualization of three-dimensional structures shed by an oscillating beam

Piezoelectric fans have been studied extensively and are seen as a promising technology for thermal management due to their ability to provide quiet, reliable cooling with low power consumption. The fluid mechanics associated with a piezoelectric fan are similar to that for a flapping bird wing, which are known to be complex. This paper is the first to investigate the three-dimensional fluid mechanics of an unconfined fan operating in its first vibration frequency mode. A custom built experimental facility was developed to capture the fan's flow field using two-dimensional phase locked Particle Image Velocimetry (PIV). The fluid-structure interaction was also captured through unique two-way coupled three-dimensional simulations of an oscillating beam in which the beam is actuated by a shear force at its resonant frequency and interacts with the surrounding air. This forgoes the need for temporal beam displacement data from experiments as in previous studies, allowing the numerical technique to be used independently. A finite element method is used for the simulations which allows the two-way coupling while maintaining computational efficiency. A three dimensional λ2 criterion constructed from interpolated PIV measurements as well as numerical data was used to identify a horse shoe vortex in the vicinity of the fan and its evolution into a hairpin vortex before it breaks up due to a combination of vortex shedding and flow along the fan blade. The experimental and numerical data are comparatively in agreement, confirming that the methods presented are valid for capturing the complex flow fields generated by this fluid-structure interaction. The results provide both a fundamental understanding on the formation and break-up of vortices from an oscillating beam, and demonstrate a validated approach which can be applied in the development of high efficiency piezoelectrically driven air moving devices and extended to the study of flapping bird and UAV wings.