Vortex dynamics of a jet at the pressure node in a standing wave

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.

The response of an axisymmetric jet placed at various positions in a standing wave

“…A schematic of the experimental set-up is shown in figure 1 and is similar to the set-up reported in Worth et al. (2020). An axisymmetric jet of exit diameter was placed at the base of a long rectangular box with side-mounted speakers designed to produce approximately one-dimensional plane waves which propagate normally to the streamwise flow direction of the jet.…”

“…These correspond to planar cuts of a vortex ring wrapped around the nozzle (see Worth et al. (2020) or Gohil & Saha (2019) for three-dimensional views). The trajectory of the vortices is indicated schematically by the red arrows in the particle images.…”

“…For example, the flapping mode caused by transverse forcing in the numerical simulations of Danaila & Boersma (2000) and Gohil & Saha (2019) and the experimental studies of Corke & Kusek (1993) and Worth et al. (2020) is a combination of two counter-rotating azimuthal modes, , that induce transverse motions along a plane leading to asymmetric vortex formation in the near field followed by bifurcation of the far field. These studies have shown that the jet response is most amplified when forced at or near the preferred mode.…”

“…2004; Kasagi 2006; Worth et al. 2020) and numerical simulations (Urbin & Métais 1997; Danaila & Boersma 2000; da Silva & Métais 2002; Tyliszczak & Geurts 2014; Gohil, Saha & Muralidhar 2015; Tyliszczak 2015; Gohil & Saha 2019).…”

Proper Orthogonal Decomposition of the Unsteady Pressure Field in the Three-Dimensional Wall Jet

“…Since it appears that specific modes are connected to lateral jet development, determining the characteristic frequencies of these modes would potentially allow control of lateral growth in the jet. Identification of key frequencies in the flow would provide a basis for targeted active flow control, similar to the work of [26].…”