•    When the average depth  k of the surface irregularities is less than laminar sub-layer of the surface δ is called as hydrodynamic ally smooth boundary.

•    The eddy which formed outside of the laminar sub-layer try to penetrate in the laminar sub-layer boundary is called as smooth boundary as shown in

•    When the average depth k of the surface irregularities is greater than laminar sub-layer of surface δ is called as hydrodynamic ally rough boundary.

•    The eddy which formed outside of the laminar sub layer penetrates into the laminar sub-layer. Such boundary is called as rough boundary as shown in.

By Nikuradse’s experiments,

(1)    When k / δ < 0.25 , smooth boundary.
(2)    When k / δ > 6 , rough boundary.
(3)    When 0.25 < k / δ < 6, transition boundary.

In terms of roughness Reynold’s number,

(1)    u.k / v < 4, smooth boundary.
(2)    u.k / v > 100, rough boundary.
(3)    u.k / v lies between 4 to 100, transition boundary.

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