Introduction

A particle of water near the conduit wall will not move as fast as a particle toward the center.  To understand this, we need to look at the molecules of moving liquids.  The first layer of molecules stick to the wall of the conduit.  The next layer will move by sliding across the first layer.  This happens throughout the flow with each successive layer moving at a faster velocity.  The change in velocity is greater near the conduit wall than it is toward the center.  If velocity measurements of each layer could be taken, a velocity profile similar to the one in Figure 1-1 would be produced.  Notice that the velocity decreases near the surface.  Since most flows fit this profile, this is called the typical profile.  There are, however, situations which will cause other profile shapes and it is usually more difficult to calculate flow with these shapes.

Figure 1-1

To calculate flow, an average or mean of all the varying velocities must be determined.  Since it is not practical to measure the velocity of each layer of molecules, methods have been developed by which a mean velocity can be determined from velocity measurements taken at a number of positions in the flow.