Visualization of Submerged Annular Jet Exited by Means of a Synthetic Jet

In the present paper the characterization of the flow field generated by means of a submerged gaseous jet exited by means of a coaxial synthetic jet has been investigated. The purpose of this investigation is to infer the aerodynamic interaction of the two fluid structures that play an interesting role in several industrial applications like jet actuator, flame stability in the combustor for mini/micro gas turbine, etc. Substantially two laser techniques have been employed: a Laser Döppler Velocimetry (LDV) featuring a high signal-noise ratio able to extract information from velocity time series in a few location in the jet flow and used, also, for an accurate determination of the Reynolds number. A Particle Image Velocimetry (PIV) system is also employed, in order to obtain instantaneous two dimensional velocity distribution overall the entire gaseous jet. Both techniques have been carried out by stretching the influences of coherent structures generated in the jet, by means of natural aerodynamic effect and by means of perturbation forced in the flow by means of the synthetic coaxial jet. The investigated conditions are ranging from Reynolds number (Re) 1000 to 3000 at several plate distance nozzle diameter ratio (but only the most significant results obtained at h/d=2 are reported in this paper). The synthetic jet has been generated by means of a loudspeaker driven at a pure sinusoidal wave at several frequencies in order to obtain different Strouhal number (St).