Suction
Suction is the everyday term for a suction effect that objects experience in the vicinity of moving gases and liquids ( fluids ).
The term “suction” is misleading and assumes the air pressure present on earth , because the difference in pressure compared to another volume is always decisive.
The term suction is also used to describe a strong current of the surf flowing back below the surface towards the sea .
Mode of action
According to Bernoulli's law, different static and dynamic pressures result from locally different flow velocities . These pressure differences disturb the equilibrium of forces around a body. The result is a force that tries to move it: the body is pushed by the higher pressure into the lower pressure , so the term suction is actually wrong.
The following thought experiments with a jet pump should make it clear that "suction" is a very superficial description for creating a negative pressure in a room , so that a gas or a liquid is driven in by the overpressure of an adjacent room:
- If a jet pump were operated on the moon (there is a vacuum there ), the suction medium would remain in the container because there is no (air) pressure that pushes the molecules of the suction medium out of the container into the jet pump. Since one has got used to the effect of air pressure on earth, its effect of pushing a liquid out of a container against the effect of gravity into a room with lower pressure is often assumed to be "normal".
- A gas or a liquid can in principle neither suck nor draw because the cohesive forces ( Van der Waals forces ) are far too low to accelerate other molecules appreciably.
- The necessity of actually having to push the suction medium into the pump can be seen if, in a second attempt at thought, one imagines that the suction medium is water that is more than 10 m deep in a well. In this case you cannot pump water with a suction pump ( see air pressure # experiments and measurements ).
Applications
Suction effects are used, for example, with the water jet pump, in which a negative pressure can be generated by means of a Venturi nozzle .
In the case of boats, the analogous process is called pumping . The water flowing around the boat creates negative pressure around the boat body, the higher air pressure in the boat body pushes water or air out through the bilge flap. If the speed is too slow, the external negative pressure is insufficient; the water must then be prevented from flowing back into the boat. Even with ships with a great deal of draft, the speed is not sufficient to achieve the necessary negative pressure in a few meters of water near the keel .
Ships are vented without a fan when the airstream creates negative pressure around a Dorade fan . A comparable effect can be observed if you turn "into the wind" with your mouth open during a storm.