roughness constant粗糙常数是什么含义？

guowei14

请问在k-e模型中壁面条件的roughness constant是什么含义?
帮助文件这么说明
The default roughness constant was determined so that, when used with   -   turbulence models, it reproduces Nikuradse’s resistance data for pipes roughened with tightly-packed, uniform sand-grain roughness. You may need to adjust the roughness constant when the roughness you want to model departs much from uniform sand-grain.
我看不懂，有高手给解释一下吗，万分感谢！