If a fluid or a gas which is not being compressed is flowing in a steady state , then the pressure depends on the speed of the fluid or the gas. The faster the fluid is flowing, the lower is the pressure at the same height. Phenomena which can be understood with the help of Bernoulli's equation include the Pitot tube , the Venturi effect , atomizers, hurricanes, flapping flags, etc. A Venturi tube may be used as a fluid flow meter. To accelerate the water in a circuit and to overcome frictional forces while maintaining a steady flow, you use a pump .
Pumps create pressure gradients.
To accelerate a column of water, a pump either increases the pressure on one side of the column, or decrease the pressure on the other side of the column. Consider a simple U-shaped pipe filled with water. The water level is the same in each leg and the pressure on each surface is the atmospheric pressure. You add air molecules. The density of the air in the left leg increases.