初论地表粗糙度

粗糙度是流体力学引进的一个重要参数,是现代地球表面各种物质流运动研究中不可或缺的一个重要概念。它在定床流动中曾获得巨大的成功,但在动床及所谓零位移较大的粗糙面上,表现出其局限性。文章在系统阐述了空气动力学意义上粗糙度概念的由来及其物理意义;在总结某些学科领域中粗糙度的应用成果基础上,发现对于定床,地表粗糙具有地表阻力系数特性,它比地表物体群落平均高度小一个量级;而对于动床,粗糙度则更具有阻力系数的特性,它与超出临界摩阻之值成正比;至于植被地表的粗糙度则近似于定床,但它要从零平面位移高度算起。因此,地表粗糙度概念的进一步完善应从地表阻力系数和实验研究入手,并加强自然界和室内实验室的观测和实验以获得相应系数的变化规律,最终解决粗糙度本身和流体力学相关的理论和实践问题。

Discussion on the terrain aerodynamic roughness

Terrain roughness (or roughness length) is an important parameter borrowed from fluid mechanics to describe the aerody- namic characteristics of various terrain surfaces. It is an essential and critical concept in the study of the flow of various materials over the earth's land surfaces. In spite of achieving great success in the research of the immovable bed flow, it exhibits certain limita- tions to the movable bed and the so called zero displacement with large roughness. In this review paper, we discussed the origin of the aerodynamic roughness concept, described the derivation of aerodynamic roughness and its physical significance, and summa- rized the applications of terrain roughness in the research fields of soil and wind erosions. We pointed out that the roughness length for immovable bed flow is more related to surface resistant characteristics, which is usually one order less than the height of the sur- face object; whereas the roughness length for movable bed flow is more related to the surface resistance coefficient itself, which is positively proportional to the friction coefficient. The roughness length for vegetation surfaces is similar to that of movable bed flow, but the calculations of it should consider the height of the zero displacement. There is still a need to strengthen field observations and indoor experiments in the future, so as to obtain the corresponding coefficients, understand their complex regularity, and finally solve the scientific issues concerning the roughness itself and the fluid mechanics.