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Thermally driven exchange flow between open water and an aquatic canopy

Published online by Cambridge University Press:  27 July 2009

XUEYAN ZHANG  and
HEIDI M. NEPF
XUEYAN ZHANG*
Affiliation:
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
HEIDI M. NEPF
Affiliation:
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
*
†Email address for correspondence: [email protected]
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Abstract

Differential solar heating can result from shading by rooted emergent aquatic plants, producing a temperature difference between vegetated and unvegetated regions of a surface water body. This temperature difference will promote an exchange flow between the vegetation and open water. Drag associated with the submerged portion of the plants modifies this exchange, specifically, changing the dominant velocity scale. Scaling analysis predicts several distinct flow regimes, including inertia-dominated, drag-dominated and energy-limiting regimes. After a constant heat source is initiated, the flow is initially inertial, but quickly transitions to the drag-dominated regime. The energy-limiting regime is not likely to occur in the presence of rooted vegetation. Laboratory experiments describe the exchange flow and confirm the scaling analysis. Particle Imaging Velocimetry (PIV) was used to quantify the velocity field. Once the exchange flow enters the drag-dominated regime, the intrusion velocity uV is steady. The intrusion velocity decreases with increasing density of vegetation. The thickness of the intruding layer is set by the length scale of light penetration.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 632 , 10 August 2009 , pp. 227 - 243
Copyright
Copyright © Cambridge University Press 2009

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