Atmospheric-stability effect on windbreak shelter and drag |
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| Authors: | Ido Seginer |
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| Institution: | 1. Faculty of Agricultural Engineering, Technion-Israel Institute of Technology, Technion City, Haifa, Israel
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| Abstract: | Wind speed was measured near the surface, along a line normal to a single, 50% porous windbreak, to determine its wind-reducing effect. Simultaneously, undisturbed wind and temperature profiles were measured to obtain the atmospheric stability. In some of the runs, the drag on a section of the windbreak was also measured. A systematic and significant effect of the atmospheric stability was found. The relative wind speed at any distance from the windbreak could be expressed for unstable conditions as an empirical function of the Richardson number. As an extreme example, the reduction of the surface shear, at a downwind distance 7.5 times the height of the windbreak, was 86 % in adiabatic conditions, and only 62 % when the gradient Richardson number at the windbreak height was ?1.0. The minimum relative effect was found in the neighborhood of the windbreak, where the windbreak-induced turbulence was dominant. The maximum absolute effect was found at a downwind distance of about ten times the height of the windbreak. The drag coefficient of the windbreak was found to bec d =0.77 in a neutral atmosphere, increasing slightly with instability. This trend is contrary to the trend of the surface-shear-reduction coefficient, which decreases significantly with instability. |
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