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Experiments on scalar dispersion within a model plant canopy part II: An elevated plane source

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Abstract

This paper describes a wind-tunnel experiment on the dispersion of trace heat from an effectively planar source within a model plant canopy, the source height being h s = 0.80 h c , where h c is the canopy height. A sensor assembly consisting of three coplanar hot wires and one cold wire was used to make simultaneous measurements of the temperature and the streamwise and vertical velocity components. It was found that:

  1. (i)

    The thermal layer consisted of two parts with different length scales, an inner sublayer (scaling with h s and h c ) which quickly reached streamwise equilibrium downstream of the leading edge of the source, and an outer sublayer which was self-preserving with a length scale proportional to the depth of the thermal layer.

  2. (ii)

    Below 2h c , the vertical eddy diffusivity for heat from the plane source (K HP ) was substantially less than the far-field limit of the corresponding diffusivity for heat from a lateral line source at the same height as the plane source. This shows that dispersion from plane or other distributed sources in canopies is influenced, near the canopy, by turbulence ‘memory’ and must be considered as a superposition of both near-field and far-field processes. Hence, one-dimensional models for scalar transport from distributed sources in canopies are wrong in principle, irrespective of the order of closure.

  3. (iii)

    In the budgets for temperature variance, and for the vertical and streamwise components of the turbulent heat flux, turbulent transport was a major loss between h s and h c and a principal gain mechanism below h s , as also observed in the budgets for turbulent energy and shear stress.

  4. (iv)

    Quadrant analysis of the vertical heat flux showed that sweeps and ejections contributed about equal amounts to the heat flux between h s and h c , though among the more intense events, sweeps were dominant. Below h s , almost all the heat was transported by sweeps.

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Author notes

  1. B. J. Legg

    Present address: Head, Horticultural Engineering Division, National Institute of Agricultural Engineering, Wrest Park, MK45 4HS, Silsoe, Bedford, UK

Authors and Affiliations

  1. CSIRO Division of Environmental Mechanics, Canberra, A.C.T., Australia

    P. A. Coppin, M. R. Raupach & B. J. Legg

Authors
  1. P. A. Coppin
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  2. M. R. Raupach
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  3. B. J. Legg
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Coppin, P.A., Raupach, M.R. & Legg, B.J. Experiments on scalar dispersion within a model plant canopy part II: An elevated plane source. Boundary-Layer Meteorol 35, 167–191 (1986). https://doi.org/10.1007/BF00117307

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