Near field of a synthetic jet-controlled sweeping jet

Abstract

In the present study, the free flow field generated by a synthetic jet-controlled jet is experimentally investigated. The device under examination consists of two synthetic jets, which, being driven with a phase shift of $180$°, force a main steady jet emitted by a square exit nozzle to sweep. Besides the baseline configuration (i.e. without control), nine control configurations are studied by varying frequency and momentum of the synthetic jets, resulting in different values of the Strouhal number $St$ and momentum coefficient $C_{\mu }$. For each tested configuration, the main jet Reynolds number is set at $5.79\times 10^3$. The external flow field is measured by employing the planar Particle Image Velocimetry (PIV) technique. The triple decomposition technique is used to analyse the time- and phase-averaged mean and turbulent statistics of the controlled sweeping jet. It is observed that the main jet sweeps a wider angle when the synthetic jet control parameters, above all the momentum coefficient, are increased. The configurations characterized by the largest values of the actuation parameters feature also a faster streamwise decay of the centreline velocity. On the other hand, the increase of $C_{\mu }$ and $St$ leads to higher spreading rate. Furthermore, by fixing $St$, the increase of $C_{\mu }$ leads to the increase of the values of phase-correlated kinetic energy in correspondence of the jet most deflected positions and to the reduction of the potential core-like region. Conversely, by fixing $C_{\mu }$ and increasing $St$, the highest values of turbulent kinetic energy are attained nearby the jet centreline. For the highest $C_{\mu }$, it can be also noticed that the extent of the region marked by high values of phase-correlated kinetic energy reduces in the streamwise direction. Such a behaviour is explained through the phase-average analysis, which reveals that the Strouhal number strongly affects the jet oscillating pattern, specifically its curvature, thus reducing coherent fluctuations and promoting the increase of turbulent activity.