Synthetic Jet Actuators for Flow Control

Abstract

Among the various active flow control techniques, Synthetic Jet (SJ) actuators represent a very promising technology due to their short response time, high jet velocity and absence of traditional piping, that matches the requirements of reduced size and low weight. Therefore, understanding in depth the basic physical aspects driving the operation of these actuators is a key point. A practical tool, employed for design and manufacturing purposes, consists in the definition of a low-order model, lumped element model (LEM), which is able to predict the dynamic response of the actuator in a relatively quick way and with reasonable fidelity and accuracy. The research activity focused by the present author has tried to tackle different aspects to achieve various goals. A major task has concerned the development of LEMs to predict the behavior of two types of actuators: piezo-driven SJ and Plasma Synthetic Jet (PSJ) actuators. These models share the same philosophy: they represent valuable tools useful not only for design purposed, but also to obtain useful insights on the devices performances. A second crucial task has consisted in the design, manufacturing and characterization of various prototypes, which have been used to validate LEMs results. As an additional task of the research activity, applications both in automotive and aerospace fields have been considered. As regards the piezo-driven synthetic jets, the main activity was concerned with the extension of an already available lumped-element model in order to: derive a non-dimensional form of the governing equations; identify the main design quantities; estimate the device performances; shed light on the actuator energy efficiency with reference to the different stages involved in the operation process. Several issues have been faced including the technology required for bonding the piezo-element over the metallic shim (so as to realize the so-called diaphragm), the design and manufacturing of the experimental mock-up, the production of the different parts of the device, the post-processing analysis. A very interesting application of the piezo-driven SJ technology, which can have outcomes in the automotive sector, has regarded the manipulation of a continuous water spray. Experimental data, taken within a chamber test rig at two injection pressures, for different SJ positions, have been acquired. Starting from the flow field velocity distributions, detected with Particle Image Velocimetry (PIV), the effective influence region of the jet on the spray has been computed through a T-Test algorithm and corroborated by a vorticity analysis. Another innovative LEM, able to predict the temporal evolution of the main fluid-dynamic variables involved, has been developed for PSJ actuators. It is fully based on the gasdynamics equations, it includes viscous losses as well as radiative and convective heat transfer mechanisms at walls, and it considers real gas effect for air. OpenFOAM numerical computations have been carried out to perform a first calibration of the lumped model through the determination of key fitting parameters. Results for both single pulse mode and repetitive working regimes have been analyzed, providing insights on the major actuation characteristics. To validate the LEM model, a home-made PSJ actuator has been designed and manufactured. The overall experimental mock-up has been also designed, together with the control electrical system. Experimental measurements of the jet velocity, obtained with Hot-wire anemometer and Pitot tube, have completed the actuator characterization and have allowed the model validation.