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Density currents or density wedges: boundary-layer influence and control methods

Published online by Cambridge University Press:  21 April 2006

Gerhard H. Jirka  and
Masamitsu Arita
Gerhard H. Jirka
Affiliation:
DeFrees Hydraulics Laboratory, Cornell University, Ithaca, NY 14853, USA
Masamitsu Arita
Affiliation:
DeFrees Hydraulics Laboratory, Cornell University, Ithaca, NY 14853, USA
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Abstract

Density currents and density wedges are two observed manifestations of interactions between an ambient flow and a horizontal buoyant intrusion. In a density current the buoyant pressure force is primarily balanced by the local form drag of the current head which has a blunt shape and abrupt depth change. In a density wedge a distributed interfacial drag is the primary balancing force, leading to a stretched-out shape and long-distance intrusions. A perturbation analysis of the approach flow to the inclined front of a density current shows that slight momentum changes caused by viscous effects in the ambient flow determine which of these two flow types is established. In a uniform ambient channel flow, any momentum deficit relative to the inviscid case will lead to a local flattening of the front and ultimate breakdown into a density wedge. On the other hand, a momentum surplus will support a steady-state density current. Several exploratory experiments on control of the ambient boundary layer through local non-uniformities were performed with the objective of achieving stable density-current forms with limited intrusion lengths. These methods include a small step, a barrier and suction and are applied for intrusions at either the bottom or surface of an ambient water flow. In all cases, good agreement is found with the force balances predicted by Benjamin's (1968) theory and its extension by Britter & Simpson (1978) which accounts for entrainment in the wake zone of the head.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 177 , April 1987 , pp. 187 - 206
Copyright
© 1987 Cambridge University Press

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