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Turbulence structure above a vegetation canopy

Published online by Cambridge University Press:  07 October 2009

JOHN J. FINNIGAN ,
ROGER H. SHAW  and
EDWARD G. PATTON
JOHN J. FINNIGAN*
Affiliation:
CSIRO Marine and Atmospheric Research, GPO Box 3023, Canberra, ACT 2601, Australia
ROGER H. SHAW
Affiliation:
University of California, Davis, CA 95616, USA
EDWARD G. PATTON
Affiliation:
National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA
*
†Email address for correspondence: [email protected]
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Abstract

We compare the turbulence statistics of the canopy/roughness sublayer (RSL) and the inertial sublayer (ISL) above. In the RSL the turbulence is more coherent and more efficient at transporting momentum and scalars and in most ways resembles a turbulent mixing layer rather than a boundary layer. To understand these differences we analyse a large-eddy simulation of the flow above and within a vegetation canopy. The three-dimensional velocity and scalar structure of a characteristic eddy is educed by compositing, using local maxima of static pressure at the canopy top as a trigger. The characteristic eddy consists of an upstream head-down sweep-generating hairpin vortex superimposed on a downstream head-up ejection-generating hairpin. The conjunction of the sweep and ejection produces the pressure maximum between the hairpins, and this is also the location of a coherent scalar microfront. This eddy structure matches that observed in simulations of homogeneous-shear flows and channel flows by several workers and also fits with earlier field and wind-tunnel measurements in canopy flows. It is significantly different from the eddy structure educed over smooth walls by conditional sampling based only on ejections as a trigger. The characteristic eddy was also reconstructed by empirical orthogonal function (EOF) analysis, when only the dominant, sweep-generating head-down hairpin was recovered, prompting a re-evaluation of earlier results based on EOF analysis of wind-tunnel data. A phenomenological model is proposed to explain both the structure of the characteristic eddy and the key differences between turbulence in the canopy/RSL and the ISL above. This model suggests a new scaling length that can be used to collapse turbulence moments over vegetation canopies.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 637 , 25 October 2009 , pp. 387 - 424
Copyright
Copyright © Cambridge University Press 2009

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