Synthetic jet actuator performance enhancement and in depth exploration

Abstract

SPIV was also tested at high Reynolds numbers (Reo = 1150, 3450, and 5750). It was found that the velocity and vorticity fields were greatly affected by the Reynolds number, with the lowest Reynolds number case producing the highest relative peak velocities and vorticity. The Q-criterion was utilized for decoupling vortices from the vorticity concentrations in the flow field. This enabled the identification of vortices in the flow field and the reconstruction of the 3-D vortex ring that is formed near the orifice, and to track its evolution and advection. At this range of aspect ratios, secondary flow structures were observed, where for the lower aspect ratio jets, the secondary structures were formed immediately downstream of the orifice; however, for the higher aspect ratio lower Reynolds number jet, near the orifice the flow field was quasi two dimensional along the orifice span, and the formation of secondary structures occurred farther downstream. It is apparent from this work that the effect of the orifice geometry and Reynolds number has a pronounced influence on the evolution of the flow structures and, as a result, may alter the effectiveness of flow control. Furthermore, investigation of the behavior of synthetic jets with applicable aspect ratios ~20 at high Reynolds numbers with peak velocities exceeding 100 m/s, addresses a critical gap in the understanding the synthetic jet flow field. As the synthetic jet actuator transitions to practical applications, better understanding of its flow field will lead to more effective and efficient flow control.

From the SPIV studies it was found that the effect of the aspect ratio is much larger than the effect of the neck length, and as the aspect ratio increased the size of the vortical structures decreased. Moreover, axis switching was observed where its streamwise location increased as the aspect ratio increased. A similarity parameter was suggested for the higher aspect ratio cases; however, it broke down at the lowest aspect ratio case. The effect of the neck length on the flow structures and the evolution of the synthetic jet was found to be secondary, where the effect was only in the very near field (i.e., close to the jet's orifice). In addition, the downstream evolution of the vortex ring, formed at the jet's orifice, was explored and compared to the Dhanak and Bernardinis' analytical model (for an isolated vortex ring in an ideal fluid). It was found that the qualitative evolution of the vortex ring could be predicted using an inviscid model, suggesting that once the vortex ring is formed (due to separation at the orifice lip), its downstream evolution is dominated by an inviscid mechanism.

The effects of different geometries and input parameters on the flow structures and performance characteristics of a finite span synthetic jet were explored in a quiescent fluid. Two scales of actuator apparatuses are used, 32 mm and 64 mm diameter piezoelectric disks. Hotwire, laser displacement sensor, and pressure transducers were used to quantify the performance of a given actuator and the effect of varying neck height, cavity height, orifice length, orifice width, and piezoelectric disk thickness was explored for both single and dual disk configurations. Stereoscopic Particle Image Velocimetry (SPIV) was utilized to explore the effects of actuator neck length, aspect ratio, and the Reynolds number on the flow structures formation and evolution. The different parameters, which are essential to the formation of a synthetic jet, including disk displacement, cavity pressure, and jet exit velocity were isolated and investigated in depth. It is found that changing the geometry had significant effect on the performance characteristics of the synthetic jet (for both scales). For both synthetic jet sizes, the cavity resonances were detected and found to be sensitive to different geometries. Changing the geometry also affected the flow field behavior when analyzing the velocity and vorticity fields surrounding the orifice in both the time average and phase average. A phase difference was found between the disk displacement and the jet exit velocity and is explained in detail in this document. A model was constructed that is successful in predicting the jet velocity at the orifice for an input cavity and orifice geometry and disk deflection. Understanding the effects of geometry and input parameters on the synthetic jet actuator is crucial in taking steps toward improving the design and application of synthetic jets.