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Title: Air flow through and above a forest of widely spaced trees
Author: Green, Steven Robert
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1990
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Agroforestry systems are currently being advocated for the uplands of the UK, consisting of widely spaced trees on grassland utilized by grazing sheep or cattle. One of the aims of agroforestry is to provide wind shelter which will benefit the animals and plants and lead to overall increases in productivity. Practical information on various aspects of the canopy microclimate, such as air flow, is needed to design the optimum agroforestry system. This thesis reports on a series of field experiments, wind tunnel experiments and numerical experiments which wereundertaken to examine and predict the properties of turbulent air flow through a forest of widely spaced trees. The field experiment was carried out at Cloich farm forest, 32 km south of Edinburgh, in three stands of 8 m tall Sitka spruce trees (Picea sitchensis (Bong.) Carr) at spacings of 4 m, 6 m and 8 m between tree centres. Turbulent statistics associated with the air flow were measured using a vertical array of 3-component propeller anemometers, at heights of between 0.25h to 1.25h, h being the mean tree height. Mean wind speed in the forest trunk space increased with increasing tree spacing, and was 46% (8 m), 29% (6 m) and 16% (4 m) of mean wind speed in an adjacent, open-paddock. Zero plane displacement, d, decreased with increasing tree spacing, and was 0.74h (8 m), 0.80h (6 m) and 0.87h (4 m) during daytime. Thermal stability acted to reduce turbulence velocities and momentum stresses at night by between 10% and 25%. Turbulence events within the widely spaced forest canopies were less extreme than reported elsewhere for closed forest stands. Slopes of the u-spectra in the trunk space were greater than -2/3 suggesting a bypass of the normal eddy cascade process. The wind tunnel experiment was carried out in an open jet wind tunnel at the Civil Engineering Department, Edinburgh University, using 1:75 scale model forest made from 20 cm tall bottle-brush elements at spacings of 1.3h, 1/2h and 2/3h, extending a distance of 10h and 20h in the downwind dimension. The area densities matched approximately those of the Cloich forest study. Turbulence statistics were mapped from extensive measurements obtained using a 3-hot-wire probe. The wind tunnel study was successful in stimulating many of the features of canopy flow identified in the field experiment. In addition, the experimental study resulted in a comprehensive set of measurements suitable for testing the predictions from the numerical experiment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available