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

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, = 0.80 h,, where h, is the canopy height. A sensor assembly consisting of three coplanar hot wires and one cold wire was used to make simultaneous measurements ofthe temperature and the streamwise and vertical velocity components. It was found that:(i) The thermal layer consisted of two parts with different length scales, an inner sublayer (scaling with h, and h,) 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.(ii) Below 2h,, the vertical eddy diffusivity for heat from the plane source (KHP) 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. (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, and h, and a principal gain mechanism below h,, as also observed in the budgets for turbulent energy and shear stress. (iv) Quadrant analysis of the vertical heat flux showed that sweeps and ejections contributed about equal amounts to the heat flux between h, and h,, though among the more intense events, sweeps were dominant. Below h,, almost all the heat was transported by sweeps.

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

“…This is necessary to relate the measurements to the actual O 3 removal (i) at plant and soil surfaces, or (ii) by chemical reactions within the canopy. Assuming horizontal homogeneity (see Coppin et al, 1986;Raupach and Shaw, 1982), the O 3 mass balance equation is given by:…”

A Lagrangian Solution to the Relationship between Source Strength and Concentration Profile Under Conditions of Local Advection

“…This section compares the model with the experimental data of Rider et al (1963) for evaporation under advective conditions from a ground/surface source in the field, and of Coppin et al (1986) for heat advection from a plane source in a wind-tunnel canopy.…”

“…However, the good agreement between modelled and measured humidity profiles suggests that it is valid to apply this model to inhomogeneous flow, at least for small streamwise flow variations. Coppin et al (1986) reported a wind-tunnel study on the dispersion of heat from a planar source within a plant canopy, conducted in the CSIRO Pye Laboratory wind tunnel in Australia. The canopy consisted of 60 mm tall metal strips spaced in a regular diamond array, with the planar heat source located at z/h c = 0.8, where h c is the canopy height.…”

“…The canopy consisted of 60 mm tall metal strips spaced in a regular diamond array, with the planar heat source located at z/h c = 0.8, where h c is the canopy height. The heat source strength was 275 W m −2 , and the temperature profiles were digitized from Coppin et al (1986) for different fetches. The profiles of u and σ w /u * for the above wind-tunnel experiment were reported by Raupach et al (1986) and used in this calculation.…”

“…In addition, the concentration profile was computed from the highest point based on the gradients, and so an error in one point could also affect all the calculations below it (Warland and Thurtell 2000). Lee (2004) reported a mathematic model for scalar advection inside canopies, and the model was also tested using data from the Coppin et al (1986) wind-tunnel heat dispersion experiment. In general, the two models (this model and Lee's model) show similar good overall agreement with the wind-tunnel measurements.…”