Structural and fluidic investigation of piezoelectric synthetic jet actuators for performance enhancements

Synthetic jet actuators are zero-net-mass-flux actuators with a wide range of applications including aerodynamic flow control, jet vectoring and mixing enhancement. They present advantages thanks to their compact size and ease of installation, compared to other active flow control devices. A piezoelectric driven synthetic jet actuator is a structural, fluidic and acoustic device that may in principle afford improvements in engineering applications, such as providing aerodynamic drag reduction over an aircraft wing or vertical tail, although further research is required before potential implementation. This thesis aims to bring a new perspective to the structural mechanics modelling of piezoelectric actuators, as well as modelling of synthetic jet actuators by both analytical and multiphysics methods. The findings presented here may contribute to improved performance metrics of synthetic jet actuators, such as exit jet velocity and power conversion efficiency which was supported by an extensive experimental study. In this thesis, three structural mechanics models were derived for the modal analysis of the unimorph piezoelectric actuator, in order to obtain an accurate model for both natural frequency and displacement. These models use transfer matrix method together with the extensional-flexural deflection of plates, and are derived from first principles of classical vibration theory. The models were then validated with both in-house and previously published experimental data. The mean estimation accuracy of the first mode of oscillation (i.e., natural frequency) is under 1.5% for the set of validation cases. Furthermore, mechanical damping identification is studied for frequency response functions, accurate estimations of displacement (±10%) for which were obtained when compared with the experimental data. A fluidic-acoustic analytical model from the literature was extended by implementing the structural model obtained for the piezoelectric actuator. The main limitation of the existing analytical model was the lack of estimation of the natural frequency and peak diaphragm displacement, which were fixed by the implementation. The new structural-fluidic-acoustic model presented here obtained peak jet velocity estimations within ±10% on the three validation cases of in-house experimental data. Also, within the study, a finite element method based multiphysics model was developed which enabled the accurate modelling of different synthetic jet actuator configurations. Existing CFD models in the literature do not fully model the behaviour of the piezoelectric diaphragm or the Helmholtz resonance, which limits the study to a forcing frequency envelope less than the Helmholtz resonance. The multiphysics model developed here covers the entire actuation frequency including Helmholtz and mechanical diaphragm resonance. It was used for computations of diaphragm deflection profile and exit jet velocity for both opposite and adjacent orifice-diaphragm configuration synthetic jet actuator. The jet velocity estimations fit the experimental data by ±10% on the three validation cases of in-house experimental data. In order to achieve increased transverse displacement, bimorph polycrystalline piezoelectric diaphragms, which consist of two piezoceramic layers, were tested. Despite the enhanced transverse diaphragm displacement and jet velocity compared to a similar overall thickness unimorph, bimorph's current consumption is substantially higher than their counterparts, in turn reducing the power conversion efficiency. With a bimorph piezoelectric driven synthetic jet actuator a peak jet velocity of 92 ms-1 is obtained with an electric-to-fluidic power conversion efficiency of 6.4%, at a peak supply voltage of 40 V. The electromechanical coupling ratio of polycrystalline piezoceramics are inherently low and the effect of using more advanced piezoceramic such as single crystal was also investigated. It was identified that single crystal piezoceramic promotes three times more transverse diaphragm displacement and two times more jet velocity, compared to the polycrystalline piezoelectric actuator for the same input diaphragm voltage. Consequently, employing single crystal piezoceramic enhanced electric-to-fluidic power conversion efficiency. A peak exit jet velocity of 99.5 ms-1 was obtained at 40 V of peak supply voltage which can be classified as a low voltage supply compared to the studies in the literature which obtained similar exit jet velocity. Also, the power conversion efficiency of 70% was achieved corresponding to the Helmholtz resonance dominated actuation region. Different cavity-orifice arrangements, namely, opposite and adjacent configurations are studied which showed that the peak jet velocity drops by approximately 10% when the adjacent configuration is used instead of the similar size opposite configuration synthetic jet actuator, at the same supply voltage. A single modal frequency response synthetic jet actuator was developed and it is identified that 46% of electric-to-fluidic power conversion is attained with an exit jet velocity of 62 ms-1 which is significantly higher in power conversion efficiency compared with the bimodal frequency response synthetic jet actuator