ON 3-DIMENSIONALITY OF SHELTERBELT STRUCTURE AND ITS INFLUENCES ON SHELTER EFFECTS

Natural shelterbelts, unlike planar barriers, have a certain width, wi thin which interactions among wind speed, drag force and pressure pert urbations create a net sheltering effect. The variations of flow, drag force, permeability, and pressure perturbation for shelterbelts of di fferent widths and different horizontal structures are numerically stu died, and their influences on shelter efficiency are discussed. Compar isons are made of fourteen medium-dense shelterbelts, with the same ov erall leaf-area, that differ only in width or horizontal distribution of leaf-area density. The simulated results are consistent with both f ield observations and wind-tunnel measurements. The simulations demons trate that the total drag force of the entire shelterbelt varies littl e with changes in width and structure. The results also show that shel ter distance and the overall average wind speed reduction decrease onl y by 15-18% as width increases by a factor of 100, and changes even le ss for different internal structure. However, width greatly affects th e location of minimum wind speed, pressure perturbation, and the perme ability of shelterbelts. Horizontal changes of wind speed inside the u niform shelterbelts have four different patterns, which depend on shel terbelt width and height. The absolute pressure perturbation significa ntly decreases with increasing width. A possible cause of the insensit ivity of shelter efficiency to width and internal inhomogeneous struct ure is the compensation between the effects of permeability and pressu re perturbation on shelter efficiency.