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A Lagrangian investigation of the small-scale features of turbulent entrainment through particle tracking and direct numerical simulation

Published online by Cambridge University Press:  25 February 2008

MARKUS HOLZNER ,
A. LIBERZON ,
N. NIKITIN ,
B. LÜTHI ,
W. KINZELBACH  and
A. TSINOBER
MARKUS HOLZNER
Affiliation:
International Collaboration for Turbulence Research Institute of Environmental Engineering, ETH Zurich, Wolfgang-Pauli-Str. 15, 8093 Zurich, Switzerland
A. LIBERZON
Affiliation:
International Collaboration for Turbulence Research School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel
N. NIKITIN
Affiliation:
Institute of Mechanics, Moscow State University, 119899 Moscow, Russia
B. LÜTHI
Affiliation:
International Collaboration for Turbulence Research Institute of Environmental Engineering, ETH Zurich, Wolfgang-Pauli-Str. 15, 8093 Zurich, Switzerland
W. KINZELBACH
Affiliation:
International Collaboration for Turbulence Research Institute of Environmental Engineering, ETH Zurich, Wolfgang-Pauli-Str. 15, 8093 Zurich, Switzerland
A. TSINOBER
Affiliation:
International Collaboration for Turbulence Research School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel Institute for Mathematical Sciences and Department of Aeronautics, Imperial College, SW7 2AZ London, UK
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Abstract

We report an analysis of small-scale enstrophy ω2 and rate of strain s2 dynamics in the proximity of the turbulent/non-turbulent interface in a flow without strong mean shear. The techniques used are three-dimensional particle tracking (3D-PTV), allowing the field of velocity derivatives to be measured and followed in a Lagrangian manner, and direct numerical simulations (DNS). In both experiment and simulation the Taylor-microscale Reynolds number is Reλ = 50. The results are based on the Lagrangian viewpoint with the main focus on flow particle tracers crossing the turbulent/non-turbulent interface. This approach allowed a direct investigation of the key physical processes underlying the entrainment phenomenon and revealed the role of small-scale non-local, inviscid and viscous processes. We found that the entrainment mechanism is initiated by self-amplification of s2 through the combined effect of strain production and pressure--strain interaction. This process is followed by a sharp change of ω2 induced mostly by production due to viscous effects. The influence of inviscid production is initially small but gradually increasing, whereas viscous production changes abruptly towards the destruction of ω2. Finally, shortly after the crossing of the turbulent/non-turbulent interface, production and dissipation of both enstrophy and strain reach a balance. The characteristic time scale of the described processes is the Kolmogorov time scale, τη. Locally, the characteristic velocity of the fluid relative to the turbulent/non-turbulent interface is the Kolmogorov velocity, uη.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 598 , 10 March 2008 , pp. 465 - 475
Copyright
Copyright © Cambridge University Press 2008

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References

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