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On the entrainment coefficient in negatively buoyant jets

Published online by Cambridge University Press:  16 October 2008

PANOS N. PAPANICOLAOU ,
ILIAS G. PAPAKONSTANTIS  and
GEORGE C. CHRISTODOULOU
PANOS N. PAPANICOLAOU
Affiliation:
Department of Civil Engineering, University of Thessaly, Pedion Areos, 38334 Volos, [email protected]
ILIAS G. PAPAKONSTANTIS
Affiliation:
School of Civil Engineering, National Technical University of Athens, 5 Heroon Polytechniou Street, 15780 Zografou, Athens, Greece
GEORGE C. CHRISTODOULOU
Affiliation:
School of Civil Engineering, National Technical University of Athens, 5 Heroon Polytechniou Street, 15780 Zografou, Athens, Greece
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Abstract

Integral models proposed to simulate positively buoyant jets are used to model jets with negative or reversing buoyancy issuing into a calm, homogeneous or density-stratified environment. On the basis of the self-similarity assumption, ‘top hat’ and Gaussian cross-sectional distributions are employed for concentration and velocity. The entrainment coefficient is considered to vary with the local Richardson number, between the asymptotic values for simple jets and plumes, estimated from earlier experiments in positively buoyant jets. Top-hat and Gaussian distribution models are employed in a wide range of experimental data on negatively buoyant jets, issuing vertically or at an angle into a calm homogeneous ambient, and on jets with reversing buoyancy, discharging into a calm, density-stratified fluid. It is found that geometrical characteristics such as the terminal (steady state) height of rise, the spreading elevation in stratified ambient and the distance to the point of impingement are considerably underestimated, resulting in lower dilution rates at the point of impingement, especially when the Gaussian formulation is applied. Reduction of the entrainment coefficient in the jet-like flow regime improves model predictions, indicating that the negative buoyancy reduces the entrainment in momentum-driven, negatively buoyant jets.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 614 , 10 November 2008 , pp. 447 - 470
Copyright
Copyright © Cambridge University Press 2008

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