AN AXISYMMETRICAL FLUIDIC NOZZLE TO GENERATE JET PRECESSION

A continuously unstable precessing flow within a short cylindrical cha mber following a large sudden expansion is described. The investigatio n relates to a nozzle designed to produce a jet which achieves large-s cale mixing in the downstream field. The inlet flow in the plane of th e sudden expansion is well defined and free from asymmetry. Qualitativ e flow visualization in water and semi-quantitative surface flow visua lization in air are reported which identify this precession within the chamber. Quantitative simultaneous measurements from fast-response pr essure transducers at four tapping points on the internal walls of the nozzle chamber confirm the presence of the precessing field. The inve stigation focuses on the flow within the nozzle chamber rather than th at in the emerging jet, although the emerging flow is also visualized. Two flow modes are identified: a 'precessing jet' mode which is insta ntaneously highly asymmetric, and a quasi-symmetric 'axial jet' mode. The precessing jet mode, on which the investigation concentrates, pred ominates in the geometric configuration investigated here. A topologic ally consistent flow held, derived from the visualization and from the fluctuating pressure data, which describes a three-dimensional and ti me-dependent precessing motion of the jet within the chamber is propos ed. The surface flow visualization quantifies the axial distances to l ines of positive and negative bifurcation allowing comparison with rel ated flows involving large-scale precession or flapping reported by ot hers. The Strouhal numbers (dimensionless frequencies) of these flows are shown to be two orders of magnitude lower than that measured in th e shear layer of the jet entering the chamber. The phenomenon is demon strated to be unrelated to acoustic coupling.