The Influence of the Slot Aspect Ratio on the Flow Field of Synthetic Jets

Abstract

In the last decade, there has been a surge of research interest in developing flow-controlling devices, which can provide appreciable aerodynamic performance gain, do not require a large energy input from the operating device, and are cheap and lightweight at the same time. The synthetic jet actuator is one such device that has advantages of low cost but also simple structure and easy installation amongst other things, for which it has attracted much research interest. The jet is synthesised from the ambient air, which is indeed a major benefit over a continuous jet that needs a constant, external fluid supply to maintain the jet. The synthetic jet has established itself as a useful fluidic device with increasing potential for a range of other practical applications, such as in heat and mass transfer enhancement and active flow control. The present study investigates the influence of initial conditions primarily that of the orifice aspect ratio, on the flow field of synthetic jets. The flow fields are also examined for dependence on cavity shape and height. The characteristics of synthetic jet flow fields actuated via circular and rectangular orifices (aspect ratios ranging from 1 to 64) are studied experimentally by hot-wire anemometry and flow visualisation methods using smoke and high-speed camera techniques. In addition, unsteady Reynolds-Averaged Navier-Stokes computations, using the hybrid turbulence ‘Scale-Adaptive Simulation’ model, are carried out to complement the data from the physical experiments, and to utilise the power of visualisation, to gain significant insights into the problem. Centreline flow statistics axially downstream and transverse profiles at different axial locations have been evaluated. Furthermore, turbulence intensities have been measured and computed respectively in order to fully investigate the flow fields. The outcomes of this fundamental study contribute to the understanding of synthetic jets from rectangular slots of various aspect ratios. A linear relationship between the slot aspect ratio and the location of axis-switching has been proposed. In addition, the widely accepted vortex formation criterion in SJ research has been extended by a saturation criterion for secondary vortices. Furthermore, several design specifications have been established to assist actuator optimisation. Finally, the usage of the hybrid turbulence model approach has been evaluated and its potential compared to Large-Eddy Simulations or Direct Numerical Simulations has been shown.