Abstract
In this paper we investigate the vortex structure and dynamics formed in the near field of a turbulent axisymmetric jet subjected to transverse acoustic forcing. Full three-dimensional phase-averaged velocity measurements were obtained to elucidate the coherent structures formed when the jet is positioned at the pressure node of a plane standing wave oriented transversely to the streamwise flow direction, which creates a plane symmetry about the nodal line dissecting the jet exit. Due to the change in phase that occurs across the nodal line, it was found that axisymmetry is broken and the jet undergoes a periodic transverse flapping motion consistent with a sinuous mode. This was accompanied by a periodic train of interconnected vortex structures, resembling inverted hairpin (or horseshoe) vortices, formed as the shear layers rolled up in anti-phase either side of the jet, and propagated a few diameters downstream before breaking up. An inviscid vortex model employing inverted hairpin line vortices is shown to capture both the dynamics of the vortex structures and the fluctuating velocity fields. Overall, the jet response and resulting vortex dynamics observed represent a significant departure from the axisymmetric flow structures observed with conventional longitudinal forcing and more closely resemble the phenomenon of bifurcating jets.
Highlights
It has long been known that the Kelvin–Helmholtz instability leads to the formation of organised coherent structures in the near field of turbulent jets (Becker & Massaro 1968; Crow & Champagne 1971)
In the present paper we have studied a turbulent jet subject to transverse acoustic excitation using tomographic particle image velocimetry (PIV)
The jet was located at the pressure node location in an enclosure, and experienced a strong transverse flapping motion which was found to dominate its structure and dynamics
Summary
It has long been known that the Kelvin–Helmholtz instability leads to the formation of organised coherent structures in the near field of turbulent jets (Becker & Massaro 1968; Crow & Champagne 1971). To produce the asymmetric disturbances, complex forcing methods were used such as mechanically oscillating the jet nozzle (Lee & Reynolds 1985) and acoustically by phased oscillations directed around the nozzle circumference (Parekh, Leonard & Reynolds 1988). This induced either a slight staggering, or inclination change in successive ring structures which, through mutual induction, proceed to alter their relative orientation and phase as they advect downstream, effectively amplifying the asymmetry, with dramatic consequences for the far-field jet development. Similar dynamics has been produced by employing alternative methods of active forcing (Suzuki, Kasagi & Suzuki 2004; Kasagi 2006) or passive control (Longmire & Duong 1996)
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