Synthetic jet actuator fabrication : modeling, quantification, and implementation

Abstract

Results were obtained for the displacement of the center of the bimorph's surface and the peak velocity of the air being ingested and expelled through the orifice. These results were compared to values obtained through the mathematical model. The model was seen to have considerable potential for predicting the performance of synthetic jet actuators and their structural resonant frequencies. SJAs have been modeled extensively in literature. However, newer design trends may pose challenges for those models. The model proposed does not rely on treating the actuator cavities as Helmholtz resonators. Numerical analysis demonstrated that the actuator geometries tested experience acoustic resonance at frequencies far removed from those predicted by Helmholtz's equations. Experimental measurements also confirmed the presence of higher mode shapes in the deflection of the actuator's piezoelectric bimorphs under some conditions. Dynamic mode decomposition was used to identify the superimposed presence of the (1,1) and (3,1) modes of a clamped circular membrane as well as the expected (0,1) mode. These modes are also a challenge both to the modeling and performance improvement of SJAs.

An adapted version of the model was able to well predict peak actuator velocity performance, often to within 5 to 15%, as compared to experimental results. It also had success calculating the impact on velocity in cases where higher mode shapes were empirically known to be present. SJA bimorphs with piezoelectric wafer diameter scales of 32, 64, 95, and 127 mm were experimentally tested and compared to the model, which was able to predict their frequencies of maximum velocity performance to within 50 Hz. Fundamental insights into turbulent flow control applications were gained through wind tunnel experiments using stereoscopic particle image velocimetry. The interactions between a single synthetic jet and a turbulent boundary layer were studied. The fluidic structures produced were found to vary greatly depending on the pitch and skew angles of the synthetic jet to the crossflow. A pitch angle of 45 degrees with a skew angle of 0 degrees was found to have the greatest potential for mitigation of separation out of the four configurations tested.

This work advances the understanding of synthetic jet actuators (SJA) for use in active aerodynamic flow control. SJAs are a popular electromechanical flow control device whose application is being widely explored in aerodynamics and fluid mechanics. It addresses their fabrication and modeling, as well as their performance under both quiescent and crossflow conditions. A fabrication process was developed that produced piezoelectric bimorphs for use in the SJAs that were capable of velocity performance at least equal to commercially available products. A mathematical model was derived to represent the behavior of circular piezoelectric bimorphs in a synthetic jet actuator. The actuators' geometries consisted of a cylindrical cavity of low height-to-diameter aspect ratio. The motion of the bimorphs was assumed to be the first mode of the structure. However, under specific conditions, the potential of alternate mode shapes occurring in the bimorphs during operation of the actuator was found and evaluated experimentally. A limited parametric study was conducted, varying the thickness of the piezoelectric wafers used in the bimorphs and the geometry of the cavity and orifice.