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Entrainment and structure of negatively buoyant jets

Published online by Cambridge University Press:  25 January 2021

L. Milton-McGurk Open the ORCID record for L. Milton-McGurk [Opens in a new window] ,
N. Williamson Open the ORCID record for N. Williamson [Opens in a new window] ,
S.W. Armfield ,
M.P. Kirkpatrick Open the ORCID record for M.P. Kirkpatrick [Opens in a new window]  and
K.M. Talluru Open the ORCID record for K.M. Talluru [Opens in a new window]
L. Milton-McGurk*
Affiliation:
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia
N. Williamson
Affiliation:
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia
S.W. Armfield
Affiliation:
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia
M.P. Kirkpatrick
Affiliation:
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia
K.M. Talluru
Affiliation:
School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia
*
†Email address for correspondence: lmil3787@sydney.edu.au
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Abstract

Turbulent negatively buoyant jets occur when the buoyancy of a jet directly opposes its momentum, and will decelerate until its mean momentum is reduced to zero. Here, the flow reverses direction and, for an axisymmetric flow originating from a round inlet, returns annularly towards the source, mixing with the opposing fluid and forming a fountain. This investigation focuses on the initial stage of the flow, before the return flow is established. Data are obtained experimentally using two-dimensional particle image velocimetry and planar laser induced fluorescence for saline/freshwater negatively buoyant jets with source Froude number $Fr_o=30$ and Reynolds numbers $5500\lesssim Re_o\lesssim 5900$ at axial locations $18\lesssim z/D\lesssim 30$, and compared to a neutral jet. The development of the mean and turbulence profiles with local $Fr$ are investigated, and it is found that, unlike neutral jets and plumes, the turbulence intensity in negatively buoyant jets does not scale with the mean flow. Additionally, the ratio of widths of the buoyancy and velocity profiles, $\lambda$, increases along the jet. The entrainment coefficient, $\alpha$, was estimated for a negatively buoyant jet, and was found to decrease with local $Fr$, eventually becoming negative, indicating fluid is being ejected from the jet. These observations differ to neutral or buoyant jets and plumes, which approach a constant $\lambda$ and $\alpha$ in the far field. This different behaviour in negatively buoyant jets is a natural consequence of the strongly decelerating mean flow as a result of opposing buoyancy, which is demonstrated in the context of the integral model framework developed by Morton et al. (Proc. R. Soc. Lond. A, vol. 234, no. 1196, 1956, pp. 1–23).

JFM classification

Wakes/Jets: Jets Convection: Plumes/thermals

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JFM Papers
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Journal of Fluid Mechanics , Volume 911 , 25 March 2021 , A21
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© The Author(s), 2021. Published by Cambridge University Press

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