Spreading characteristics of compressible jets from nozzles of various geometries

The spreading characteristics of jets from several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number rang e of 0.3-2.0. The effect of 'tabs' for a rectangular and a round nozzle is also included in the comparison. Compared to a round jet, the jets from the asymmetric nozzles spread only slightly more at subsonic conditions wherea s at supersonic conditions, when 'screech' occurs, they spread much more. T he dynamics of the azimuthal vortical structures of the jet, organized and intensified under the screeching condition, are thought to be responsible f or the observed effect at supersonic conditions. Curiously, the jet from a 'lobed' nozzle spreads much less at supersonic condition compared to all ot her cases; this is due to the absence of screech with this nozzle. Screech stages inducing flapping, rather than varicose or helical, flow oscillation cause a more pronounced jet spreading. At subsonic conditions, only a slig ht increase in jet spreading with the asymmetric nozzles contrasts previous observations by others. The present results show that the spreading of mos t asymmetric jets is not much different from that of a round jet. This infe rence is further supported by data from the rectangular orifices. In fact, jets from the orifices with small aspect ratio (AR) exhibit virtually no in crease in the spreading. A noticeable increase commences only when AR is la rger than about 10. Thus, 'shear layer perimeter stretching', achieved with a larger AR for a given cross-sectional area of the orifice, by itself, pr oves to be a relatively inefficient mechanism for increasing jet spreading. In contrast, the presence of streamwise vortices or 'natural excitation' c an cause a significant increase - effects that might explain the observatio ns in the previous investigations. Thus far, the biggest increase in jet sp reading is observed with the tabs. This is true in the subsonic regime, as well as in the supersonic regime, in spite of the fact that screech is elim inated by the tabs. The characteristic spreading of the tabbed jets is expl ained by the induced motion of the tab-generated streamwise vortex pairs. T he tabs, however, incur thrust loss; the flow blockage and loss in thrust c oefficient, vis-a-vis the spreading increase, are evaluated for various con figurations.