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The local entrainment velocity is a viscous quantity

DOI: 10.1615/ICHMT.2009.TurbulHeatMassTransf.600
10 pages

Markus Holzner
Institute of Environmental Engineering, ETH - Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland

Beat Luthi
Institute of Environmental Engineering, ETH Zurich, CH-8093 Zurich, Switzerland

Alexander Liberzon
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Ramat Aviv 69978, Israel

Arkady Tsinober
Department of Fluid Mechanics and Heat Transfer, School of Mechanical Engineering, Tel-Aviv University, Tel-Aviv, Israel; and Institute for Mathematical Sciences and Department of Aeronautics, Imperial College, London, United Kingdom

Wolfgang Kinzelbach
Institute of Hydromechanics and Water Resources Management, Swiss Federal Institute of Technology, ETH Honggerberg, CH 8093 Zurich, Switzerland

Abstrakt

In this contribution we analyze properties of the turbulent/nonturbulent interface (TNTI) in a flow without mean shear. The approach is numerical and based on a Direct Numerical Simulation (DNS). The focus is on the local entrainment velocity, vn, defined as the velocity of the TNTI relative to the fluid and directed towards the irrotational ambient. We provide an expression for vn that consists of two parts: a viscous and an inviscid one. The results show that the viscous contribution is stronger than the inviscid part and governs the local entrainment velocity towards the ambient. We also propose a measure to characterize the characteristic length scale or 'thickness' of the TNTI. In a second part, the spatial distribution of various quantities with respect to the TNTI is analyzed. The main finding is that inviscid and viscous part of vn contribute together to higher entrainment velocities mostly within localized areas at the top of outward facing billows of the TNTI.

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