Spreading characteristics of compressible jets from nozzles of various geometries

Abstract

The spreading characteristics of jets from several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number range 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 whereas at supersonic conditions, when 'screech' occurs, they spread much more. The dynamics of the azimuthal vortical structures of the jet, organized and intensified under the screeching condition, are thought to be responsible for the observed effect at supersonic conditions. Curiously, the jet from a 'lobed' nozzle spreads much less at supersonic condition compared to all other 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 slight increase in jet spreading with the asymmetric nozzles contrasts previous observations by others. The present results show that the spreading of most asymmetric jets is not much different from that of a round jet. This inference is further supported by data from the rectangular orifices. In fact, jets from the orifices with small aspect ratio (AR) exhibit virtually no increase in the spreading. A noticeable increase commences only when AR is larger than about 10. Thus, 'shear layer perimeter stretching', achieved with a larger AR for a given cross-sectional area of the orifice, by itself, proves to be a relatively inefficient mechanism for increasing jet spreading. In contrast, the presence of streamwise vortices or 'natural excitation' can cause a significant increase -effects that might explain the observations in the previous investigations. Thus far, the biggest increase in jet spreading 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 eliminated by the tabs. The characteristic spreading of the tabbed jets is explained by the induced motion of the tab-generated streamwise vortex pairs. The tabs, however, incur thrust loss; the flow blockage and loss in thrust coefficient, vis-à-vis the spreading increase, are evaluated for various configurations.

Background Oriented Schlieren Applied to Study Shock Spacing in a Screeching Circular Jet

“…The frequency within a given stage decreases continuously as M j is increased. In previous studies with circular nozzles, 8,9 flow field oscillations in the axisymmetric (stages A 1 and A 2 ), flapping (stages B and D), and helical (stage C) modes have been observed. However, the B and D stages may be unstable, switching between the flapping and helical oscillations.…”

Characterization of turbulent jets from high-aspect-ratio rectangular nozzles

“…A main reason is believed to be that these jets are most widely utilized in technical applications in aerospace and chemical and mechanical engineering due to their relatively simple and flexible configurations. Previous studies, e.g., those by Quinn 2 and Zaman, 3 found that the aspect ratio of nozzle exit, i.e., AR= w / h, where w and h are the long and short sides of rectangle, influences the jet turbulent mixing ͑e.g., momentum transfer͒ process. Based on his velocity measurements for AR= 2, 5, 10, and 20, Quinn 2 concluded that the rate of mixing in the near field of the jet increases with increasing AR as long as the mean flow remains three dimensional.…”

The hydrodynamics of astrophysical jets: scaled experiments and numerical simulations

“…Bulent & Volkan 2002;Belan et al 2010). Studies on the mid-term field of nearly isentropic laboratory jets A&A 554, A99 (2013) up to Mach 2 can be found only in two papers by Zaman (1998Zaman ( , 1999. For these reasons, this work can also help to clarify some aspects of the basic fluid dynamics of these flows.…”