Bernoulli effect why does the paper lift?

[scilearner]

if I get a paper and blow on top of that. The air would move faster and it would lift but why is it that only air in the bottom pushes the paper, what about the air above the region I'm blowing can't that push down on the paper. Thanks

 

PS: While we are at it why exactly does air move faster in long distance, like aeroplane wing?

[swansont]

Faster moving air has a lower pressure. The pressure differential means there is a force. The air moves faster because of continuity — you can't have "gaps" in the fluid.

[Amr Morsi]

That's right.

Also, when the paper is raised enough, it is got affected by the dynamic pressure of the blown air, and it get pushed down again..... That's why it vibrates.

[J.C.MacSwell]

if I get a paper and blow on top of that. The air would move faster and it would lift but why is it that only air in the bottom pushes the paper, what about the air above the region I'm blowing can't that push down on the paper. Thanks

 

PS: While we are at it why exactly does air move faster in long distance, like aeroplane wing?

 

It's the Coanda effect. While Bernoulli's effect is involved, the air above and below the paper are in different flow regimes and Bernoulli's equation does not apply.

 

The blown air entrains and sweeps the dead air from above the paper, which the paperers shape/proximity deflects downward , reducing the pressure on the paper and causing lift.

 

For the PS: The air in the high pressure zone accelerates toward the lower pressure zone in the velocity and pressure field around the wing, which in turn effects the velocity and pressure field around the wing and the pressure and thus net forces on the wing.

 

This increases at greater angles of attack, until the Coanda effect is somewhat lost, and stall occurs, sometimes abruptly. So it is all related.

Edited March 30, 2010 by J.C.MacSwell