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Effects of stable stratification on turbulent/nonturbulent interfaces in turbulent mixing layers

T. Watanabe1,*, J. J. Riley2, and K. Nagata1

  • 1Department of Aerospace Engineering, Nagoya University, Nagoya, Japan
  • 2Department of Mechanical Engineering, University of Washington, Seattle, USA

  • *watanabe.tomoaki@c.nagoya-u.jp
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Phys. Rev. Fluids 1, 044301 – Published 1 August, 2016

DOI: https://doi.org/10.1103/PhysRevFluids.1.044301

Abstract

Direct numerical simulations are used for investigating the effects of stable stratification on the turbulent/nonturbulent (T/NT) interface in stably stratified mixing layers whose buoyancy Reynolds number Reb on the centerline is large enough for small-scale three-dimensional turbulence to exist. The stratification changes the interface geometry, and a large part of the interface is oriented with normal in the vertical direction in the stratified flows. The structures of the T/NT interface layer are similar between the nonstratified and stratified flows, and the T/NT interface consists of the viscous superlayer and the turbulent sublayer. The stratification is locally strengthened near the T/NT interface as evidenced by the large vertical density gradient, resulting in the decrease in Reb in the T/NT interface layer. Thus, even the small-scale dissipation range is directly affected by the buoyancy near the T/NT interface, although the small scales are somewhat free from the direct effects of the buoyancy in the turbulent core region. The production rates of enstrophy and scalar dissipation, which arise from the strain/vorticity and strain/density-gradient interactions, are decreased near the T/NT interface because the stratification modifies the alignments among the vorticity, density gradient, and strain-rate eigenvectors near the T/NT interface. This influence on the small-scale turbulence dynamics is not observed in the turbulent core region because of the large Reb. A possible explanation is given for the influence of buoyancy on the alignment statistics based on the suppression of the vertical turbulent motions by buoyancy.

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Phys. Rev. Fluids 1, 044301– Published 1 August, 2016
  • Received 23 January 2016
Article part of CHORUS

©2016 American Physical Society

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