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研究报告目录 |
A wind-tunnel study of windbreak drag |
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| 出 版 社：Agricultural and Forest Meteorology |
| 发表时间：118 (2003) 75–84 |
| 台 站：
长白山森林生态系统定位研究站
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| 作 者：Dexin Guana, Yushu Zhang , Tingyao Zhu |
| 点 击 率：3913 |
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| 关 键 字：Realistic windbreak; Two-dimensional windbreak; Optical porosity; Aerodynamic porosity; Drag coefficient |
| 摘 要：Drag coefficient is an important index of the wind protection effect of a windbreak. Although there have been many studies
on this, there are very few with direct measurements, especially those on realistic windbreaks. We conducted wind-tunnel
experiments on simulated models of narrow and realistic windbreaks with different porosities. The surface boundary layer
on the wind-tunnel floor was simulated with a thickness of 600 mm. The model, with height (H) of 100 mm, was immersed
in the simulated surface boundary layer during measurements. The ratio of the model height to roughness length of the
surface (H/z0) was 774. Two indices, optical porosity (β) and aerodynamic porosity (α), were used. The bleed wind speed
of the model was measured to calculate aerodynamic porosity. The relative bleed speed was generally lower at the canopy
crown and higher at the bottom surface. Dense model canopy had smaller standard deviation (S.D.) in aerodynamic porosity
along the crosswind direction. The relationship between α and β resulted in α = β0.4 for realistic windbreak models. The
divergence of α and β was largest near 0.5 and 0.2, respectively and then decreased as the windbreak became denser or
looser.
The drag (D) and drag coefficient (Cd) of the models decreased with increasing porosity. The model Cd remained nearly
constant within the range of Reynolds number measured in this study. The resultant Cd of our realistic model was lower than
that of other studies using two-dimensional windbreaks with the same optical porosity. Our empirical relationship between Cd
and α (i.e. Cd = 1.08(1 − α1.8)) agreed well with that of two-dimensional windbreaks reported in the literature, suggesting
that α was a better measure as porosity index than β.
© 2003 Elsevier Science B.V. All rights reserved. |
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