A wind-tunnel study of windbreak drag

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.