North East Aircraft Museum

Practical Experiments (Under Construction)

Bernoulli Effect


Bernoulli Equation

where p is pressure, rho is density and V is flow velocity

This equation defines the relation between pressure and velocity, and is applicable for an inviscid, incompressible fluid, with no body forces.

For rotational flow, this equation applies only along a streamline , but for irrotational flows the equation is applicable between any two points in the flow.

The physical significance of the equation is that when velocity increases pressure drops, and vice-versa.


Blowing across the top of a piece of paper, held at two corners, will cause the paper to rise up. Increasing the air speed over the top decreases the pressure there.

Holding a spoon in a flow of water from a tap will produce a noticeable pull towards the convex side, because there the flow will be speeded up.

Take a situation as shown, with a ball floating on a fan of air. Anyone attempting to slowly extract the ball in a horizontal direction will experience a force tending to pull the ball back into airflow, because air is moving faster on the surface opposite to the said person.

You can try altered the flow of air from the vertical, and the ball can still be suspended in the flow, for certain angles.

A 'carburetor-type device' or 'simple atomizer' can be constructed from two straws and a glass of water. With one straw immersed in the water, blowing across the top of this straw thru the other straw should (in theory) cause the water to rise, and atomize.

Suspend a couple of table tennis balls, for example, from strings with a small gap between them, and blow thru the middle of them (you could also use a straw for this, if handy). The balls should move together, or at least not move apart as might seem intuitive. More sophisticated arrangements can be devised with heavier balls and hair dryers.

Extra Information

Daniel Bernoulli was a member of a family which produced several well-known mathematicians - there are many laws, formulas etc, named Bernoulli, not all of them connnected with Daniel by any means.

His family came from Basel, and he spent his last years there. In the intermediate period, he was employed at the new Scientific Academy in St. Peterburg, set up by Peter 1., soon after the founding of the city. For further information, see here.

Brian Daugherty