Abstract :
In wind tunnel research, experiments are carried out with invariable wind speed and wind direction. These ideal circumstances are never encountered in the field. To evaluate the effect of wind barriers in a practical situation a field experiment was set up. A half-standard tree orchard was selected for that purpose. The orchard was protected against excessive winds mainly by a natural barrier of alder bushes combined with a synthetic screen, although alder bushes only and conifers were used as well on some places. The methodology to determine wind speed reduction in full-scale experiments consisted of measuring average wind speed leeward of a barrier during a given time interval simultaneously with the average reference wind speed and the mean wind direction. From these measurements, wind speed reduction and incidence angle of wind direction with the barrier could be derived.
In spite of the fact that the results of on-field measurements are not directly comparable with those of wind tunnel research, because of different materials, similar trends were found. An overall wind speed reduction of about 40% at a height of 1 m between the fruit trees was obtained relative to the reference wind speed at a height of 4 m while at the height of 2 m the overall wind speed reduction decreases to 20%. Wind speed reduction is mostly small at a height of 3 m and non-existing at 4 m. Furthermore, the three-dimensional natural barriers in the field experiment act differently than the two-dimensional artificial screens of the wind tunnel research due to the thickness and suppleness of the natural barrier, especially with oblique wind. The intensity of wind speed also affects the wind speed reduction: a higher wind speed results in a smaller wind speed reduction leeward of the natural barrier compared to a lower wind speed. No substantial differences were observed between the three types of barriers. Although measurements were performed in winter to avoid damage during the flowering and fruit bearing season, an appropriate picture of the sheltering effect of the barriers on the orchard could be constituted in spite of the fact that alder bushes and fruit trees had no leaves.
