Bernoulli’s Principle tells us that the sums of pressure and kinetic energy at
various points in a system must be constant, if flow is constant. When a fluid
flows through areas of different diameters as shown in Figure 23, there must be
corresponding changes in velocity. At the left, the section is large so velocity
is low. In the centre, velocity must be increased because the area is smaller.
Again, at the right, the area increases to the original size and the velocity again
decreases.