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Reduced Entrainment into a Turbulent Jet Due to Turbulence in the Ambient

Author(s): Susan Gaskin; Khashayar F. Kohan; Rana Sahebjam

Linked Author(s): Susan Gaskin

Keywords: Turbulent jets; Ambient turbulence; Entrainment mechanisms; Turbulent interface

Abstract: An approximately homogeneous isotropic turbulence in an ambient with negligible mean flow hastens the evolution of an axisymmetric turbulent jet reducing its entrainment and resulting in early breakup. The passive scalar field of two jets (Re=10600 and 5800) was obtained using PLIF. An analysis conditioned on the jet centroid removed the effect of advection by the large scales of the ambient turbulence to assess the impact of increasing relative turbulent intensity, ξ. Before jet break-up, ξ≤0.5, the momentum driven jet dynamics are perturbed, while after jet break up, ξ>0.5, the scalar is only diffused by the ambient turbulence. In first region, the mean velocity decays reducing entrainment, resulting in a more rapid decay of the scalar concentration, an increase in width growth, and a decrease in mean scalar flux, while the jet remains self-similar and selfpreserving. The entrained ambient turbulence increases the RMS concentration resulting in a greater portion of the total momentum attributable to the turbulent fluctuations, and a decreasing mean momentum flux with distance. The interface between the jet and ambient in a turbulent ambient, the turbulent/turbulent interface (TTI), is longer and more contorted with a tortuosity that increases with increasing ξ. The fractal dimension of the TTI also increases with ξ, reflecting the multi-scale nature of the turbulence. A scalar jump defines the location of the turbulent/non-turbulent interface (TNTI) and TTI, which is steeper as ξ increases due to the enhanced radial transport of scalar from the jet core to the edges.


Year: 2023

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