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A Turbulent Jet in a Turbulent Background: Implications for Outfall

Author(s): A. Perez-Alvarado; L. M. Mydlarski; S. J. Gaskin

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Keywords: Turbulent jet; Turbulent background; Turbulence; Turbulent mixing

Abstract: Ubiquitous in both natural and industrial settings, turbulent jets are, in most cases, emitted into a turbulent environment. Although well studied in quiescent or laminar background flows, turbulent jets are not fully understood when released into background turbulence. Earlier work on a plane jet released into a turbulent co-flow indicated that the background turbulence broke up the jet structure thereby reducing the entrainment. This indicates that the jet turbulence and the background turbulence effects cannot be superimposed when determining dilution rates. To further investigate this effect, the effect of the external turbulence was isolated by investigating an axisymmetric turbulent jet emitted into a zero mean flow isotropic homogeneous background turbulence. A quasi-homogeneous and isotropic, zero mean flow turbulent background was generated by means of a random jet array (RJA). The axisymmetric turbulent jet, at two Reynolds number, was released into this turbulent background flow. The velocity has been measured using Acoustic Doppler velocimetry and flying hot film (Khorsandi et al., 2013). The mixing of a passive scalar is observed using planar laser induced fluorescence. The results show that while the mean axial velocities decay faster with increasing background turbulence, the axial rms velocities increase with increased background turbulence. The mean concentration decreases near the centreline in the background turbulence as compared to the quiescent case, while the rms concentrations increase with an increased background turbulence. The probability density function of the concentration at points on the radial profile indicate similar levels of peak maximum concentration and a high probability of zero concentration. These results indicate that the background turbulence causes meandering of the jet path, break-up of the jet structure and an apparent lower mean concentration and greater width. Ocean outfalls are buoyant turbulent jets, whose near-field dilution rates are known to depend on the jet momentum and buoyancy. This work indicates that background turbulence affects the jet momentum, hence also affecting the near field dilution of the jet. Therefore the customary assumption of superposition of the jet driven dilution and of the background turbulence driven dilution leads is incorrect and needs further investigation.


Year: 2016

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