Wind tunnel test to optimize barrier spacing and porosity to reduce wind damage in horticultural shelter systems

Abstract

New Zealand's yearly horticultural export earnings are over $1 billion NZ. Of this, half is from kiwifruit. Wind damage especially to kiwifruit lowers revenue by about 10%. This research aims to reduce this wind damage and improve export revenue. Correct shelter system geometry may be found by full scale and wind tunnel tests which quantify the complex winds that damage cash crops like kiwifruit. Until now, shelter design for barrier space/height ratio, S/H, was by trial and error using live, trees, artificial cloth, or both. In practice, S/H varies from about 3 to 20. A boundary-layer wind tunnel with 1/100 scaled barriers at five spacings and four porosities modelled full scale shelter systems. A hot-wire anemometer probe measured wind speeds. Its signals were computer analyzed for the wind changes near the tunnel floor (plant canopy). Probe data were then collated into contour plots of isotachs, isoturbs, and turbulence intensity for each barrier S/H and geometric porosity. These contours show evidence of vortices near 0.5H above ground in front of each barrier, from S/H of 4 for 0% porosity to S/H of 10 for all porosities. Spreadsheet bar graphs from the contours show rms turbulence increasing and moving down near the plant canopy, downwind of the first few rows of barriers. Mean and turbulent velocity minima suggest an optimum barrier spacing, S/H of 6 to 8, at 40% geometric porosity. To verify vortex presence, a specially designed hot-wire probe is being built. Turbulence spectra skew, and vortex strength size and number, will be measured in specific regions found from the contour plots. Vertical temperature profiles added to the wind tunnel will test near ground atmospheric stability effects on turbulence and vortex strength. As well, full scale wind studies using an array of sensitive vane anemometers up a mobile tower moved inside a real shelter system will be used to verify the model work. A better knowledge of barrier spacing and porosity for shelter system design will help reduce wind damage to any sheltering plants.