The sides of a horizontal pipe carrying dirty water get dirty. Explain.

Explanation

when dirty water is flowing through the horizontal pipe, the velocity of the water is maximum along the axis of pipe, i.e. a relative motion exists between the various layers of the liquid flowing through the pipe. As a result of it, a viscous drag comes into play which opposes the relative motion between the adjacent layers. Due to it, the dirt particles presuent in dirty water move outwards and get struck on the walls of pipe, making the pipe dirty.