The role of unsteadiness within a central jet on the structure of a combined central and co-annular jet flow

Abstract

Qualitative and quantitative assessments of the effect of the unsteadiness in a central precessing jet (PJ) on a co-annular jet were made using experimental methods. The central jets were an unsteady PJ, a simple axial jet, a simple jet directed at 45° to the nozzle axis and a conical jet directed at 45° to the nozzle axis. The central jets were compared on the basis of similar axial momentum. The simple directed jet corresponds to case in which the PJ flow has a precession frequency of zero and the conical jet corresponds to the case with a precession frequency of infinity. Experimental investigations of the near-nozzle region were performed in water using a two-colour planar laser induced fluorescence visualisation technique. The visualisations showed that, like the PJ flow, both the directed and the conical jets increased the initial spread of the combined flow. Nevertheless the PJ flow is fundamentally different from the steady analogues. The PJ flow was the only flow to increase the scale of the large-scale, visually coherent, motions in the combined flow. This observation was quantified by measurements of jet half-width. The central PJ flow increased the annular jet half width by 66%, while the steady jets reduced it by 7% - 32 %.