I've been reading on wikipedia that the Bernoulli principle is often incorrectly applied to the example of blowing on pieces of paper because

A third problem is that it is false to make a connection between the flow on the two sides of the paper using Bernoulli’s equation since the air above and below are different flow fields and Bernoulli's principle only applies within a flow field.


I'm confused about what it means by "different flow fields" and what the implications of this are. Another common example of the Bernoulli principle is wind from a tornado or hurricane blowing over the top of a house, causing the roof to come off because the pressure inside the house is still 1 atm, and the pressure on the roof is less because the wind is moving faster and therefore has a lower pressure due to Bernoulli. Is this a misapplication, or is the difference between the inside of the house and the outside not the same as the difference between the two sides of the piece of paper. Does it have something to do with the fact that you are adding energy to the air when you blow with your mouth?

  • $\begingroup$ By "different flow fields" what they probably meant was that the flows above and beneath the paper are composed of different streamlines. Unless the flow domain is free of vorticity, Bernoulli equation can be applied only between two points on the same streamline and not just to any two arbitrary points. $\endgroup$ – Deep Nov 18 '17 at 9:19

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