Static Pressure and Velocity Pressure
2019-06-19
The pressure in a pipe of still fluid and the pressure in the same pipe with the fluid now moving are different. If a pressure gauge were mounted on the pipe it would read a higher pressure when the fluid was still than when it was flowing through the pipe. When a fluid is in motion the static pressure drops and the velocity pressure rises.
An example of static pressure is the pressure of the air in a room with the windows slightly open while outside there is a howling gale. If you go into the gale you would feel the pressure of the wind on you. That pressure is the velocity pressure of the moving wind. At the same time you would experience the static air pressure acting on you. On the windward side you would experience the static pressure plus the velocity pressure. Behind you there would only be static pressure (less a small amount of vacuum pressure). The static pressure is all around you, whereas the velocity pressure is only in the direction of the wind.
The same situation develops in a pipe full of fluid. Where the static pressure is high, velocities are low and where velocities are high static pressure is low (Recall the change in back-pressure in the kitchen pipes as the tap is closed and opened).