Experimental Investigation and Visualization of Helical Structures in a Novel Swirling Jet

This paper describes an experimental study carried out to investigate the dynamics of helical structures in an unforced swirling jet and in a forced swirling jet that is excited by acoustic methods. An experimental rig that consists of a novel swirling jet and acoustic generation chamber is utilized to study the characteristics and mutual interaction of helical structures. For the excited jet, the helical structures are acoustically triggered in the jet-mixing layer. The acoustic excitation is applied transversely by an array of eight loudspeakers using radial wave-guides. The loudspeakers are driven by harmonic acoustic signals mutually phase shifted by a quarter of the period between each two neighbors so as to act in effect to produce two rotating waves. To generate swirl in the flow, novel swirl generators are utilized. The approach is to substitute the usual vanes set at an angle relative to the incoming flow by fixed vanes at a zero angle of attack, with the flow deflection based on super-circulation produced by the Coanda effect. The particle image velocimetry method was employed to analyze, quantify and visualize the flow field.