This loss can be varied as use of different materials i.e. glass tube much smoother than cast iron tube. From a microscopic viewpoint, the pipe wall has an irregular and uneven surface as shown in Figure 3.1. The degree of roughness of the surfaces will depend upon the type of materials.Some fluid particles are trapped between the ‘teeth’ and produce whirls. The random motion of fluid particles forms a stable boundary layer in which the main velocity approaches zero. This phenomenon explains the velocity profile in real fluid flow.