Geometrical influences on the fluid mechanics of oscillating cantilevers

Recent advances in microelectronics integration have resulted in electronic devices with high power densities and significantly high heat fluxes (~102 - 103W/m2). The thermal management challenges posed by these miniaturised devices are driven by strict size, space, power, and noise limitations to which existing active cooling solutions may struggle to adhere. Oscillating cantilever beams, or piezoelectric fans, have garnered considerable research interest in recent years due to their potential to augment or replace existing rotary fan thermal solutions in next generation electronic devices. To date, the majority of the literature on the thermal performance of oscillating fans has focused on the influence of oscillatory parameters, as well as positions and orientations of the fan relative to a heat source. There has been comparatively little research on the influence of the geometrical properties of the fan structure itself. The objective of this thesis is to investigate the geometrical influences on the fowfields that are induced by oscillating fans, with a view towards implementation as air movers for active cooling solutions. The fans that were considered in this thesis were rigid, cantilevered structures - thereby obviating the requirement to oscillate at resonance. The influence of fan thickness was investigated using two rectangular cantilevers with thicknesses of 1 mm and 3.7 mm. In addition to this, an experiment investigating the influence of geometrical asymmetry was conducted on a cantilever whose cross-sectional geometry featured a stepped combination of the two previously mentioned thicknesses. Experiments were conducted at an oscillatory frequency and amplitude of 40 Hz and 16 mm respectively. Phase-locked Particle-Image Velocimetry (PIV) was utilised to record and visualise the induced flowfields, as well as the underlying fluidic mechanisms that drive their development. Numerical models that emulated the experimental conditions were also employed to validate the experimental methods, and to provide additional insights into the ow behaviour. It was found that increasing the thickness of geometrically similar fans from 1 mm to 3.7 mm resulted in markedly different time-averaged flowfields. Out- of-plane measurements, as well as numerical data, revealed that fluid separation from the thicker 3.7 mm fan was inhibited due to the more dominant influence of viscous friction on the larger, upper and lower surfaces of the rigid fan structure. The numerical analysis also revealed the significant influence of shed, counter- rotating vortex pairs on the pressure fields around the fan structures during the subsequent half-stroke. For the asymmetric fan, both experimental and numerical methods showed the presence of a strong downwash generated by the fan in the direction of its thinner side. It was found that an imbalance in the strength of the vortices generated around the fan structure induced the downward ow bias. Thereafter, the asymmetric fan exhibited similar characteristics to a plunging plate or airfoil in a freestream fluid flow, whereby a thrust-producing wake is generated by the asymmetric shedding of counter-rotating vortices. The finndings of this thesis highlight the potential impact of geometric modifications on the air moving capabilities of oscillatory fan cooling solutions.