Vortical structures in a laminar V-notched indeterminate-origin jet

Abstract

A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a V-notched indeterminate-origin jet with two peaks and two troughs. The laminar jet (Re=2000) was studied under forcing and nonforcing conditions to investigate the resultant dynamics of coherent large- and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex pairs which spread outwards into the ambient fluid and the troughs generating similar vortex pairs but entraining ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex pairs. Laser cross sections further showed that the subsequent formation of azimuthal ring vortices causes these streamwise vortex pairs to be entrained. This entrainment causes the streamwise vortex pairs to "roll-up" together with the ring vortices, leading to intense flow interactions between them. Interestingly, in a comparison with the experimental study reported by Longmire et al. ["Control of jet structure by crown-shaped nozzles," AIAA J. 30, 505 (1992)] using higher Reynolds number air jet (Re=19 000), it was found that forced jet flows with four peaks and four troughs yielded practically the same flow observations as the present nozzles with two peaks and two troughs. An updated flow model based on instantaneous and time-averaged evidence is presented to explain how the interaction of the vortex structures will give rise to the present new observations. © 2005 American Institute of Physics.