Elliptic jets. part 1. characteristics of unexcited and excited jets

This paper summarizes experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions, and effects of excitations at selected frequencies and amplitudes. The dominance of coherent structures in the jet far field is evident from the finding that switching over of the cross-section shape continues at least up to 100 equivalent diameters D_e. The locations and the number of switchovers are strongly dependent on the initial condition, on the aspect ratio, and, when excited, on the Strouhal number and the excitation level. The authors studied jets with constant exit momentum thickness θ_e all around the perimeter, thus separating the effects of azimuthal variations of θ_e (typical of elliptic jets) and of the shear-layer curvature. Also investigated are the instability characteristics, and enhanced entrainment caused by bifurcation as well as pairing of vortical structures. The paper documents some time-average measurements and their comparison with those in circular and plane jets. Additional aspects of the study are discussed.