While the amount of total energy within a closed system (the
tube) does not change, the form of the energy may be altered.
Pressure of flowing air may be compared to energy in that the
total pressure of flowing air always remains constant unless
energy is added or removed. Fluid flow pressure has two
components—static and dynamic pressure. Static pressure
is the pressure component measured in the flow but not
moving with the flow as pressure is measured. Static pressure
is also known as the force per unit area acting on a surface.
Dynamic pressure of flow is that component existing as a
result of movement of the air. The sum of these two pressures
is total pressure. As air flows through the constriction, static
pressure decreases as velocity increases. This increases
dynamic pressure. Figure 2-5 depicts the bottom half of the
constricted area of the tube, which resembles the top half of
an airfoil. Even with the top half of the tube removed, the air
still accelerates over the curved area because the upper air
layers restrict the flow—just as the top half of the constricted
tube did. This acceleration causes decreased static pressure
above the curved portion and creates a pressure differential
caused by the variation of static and dynamic pressures