Journal of Fluid Mechanics

Papers

Turbulence structure above a vegetation canopy

JOHN J. FINNIGANa1 c1, ROGER H. SHAWa2 and EDWARD G. PATTONa3

a1 CSIRO Marine and Atmospheric Research, GPO Box 3023, Canberra, ACT 2601, Australia

a2 University of California, Davis, CA 95616, USA

a3 National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA

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.

(Received July 22 2008)

(Revised May 21 2009)

(Accepted May 21 2009)

Correspondence:

c1 Email address for correspondence: john.finnigan@csiro.au

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