Prof. Dr. em. Klaus Weltner
- Misinterpretations of Bernoulli's Law
Abstract: Bernoulli's law and experiments attributed to it are fascinating. Unfortunately some of these experiments are explained erraneously, e.g.: the function of a vaporizer and the soaring of a ping-pong ball in a jet stream of a hair dryer can not be used as applications of Bernoulli's law. The static pressure in a free jet stream is equal to the static pressure in the environmental atmosphere regardless of the streaming velocity of the jet. This can be shown by classroom experiments.
Acceleration of air is caused by pressure gradients. Air is accelerated in direction of the velocity if the pressure goes down. Thus the decrease of pressure is the cause of a higher velocity. It is wrong to say that a lower pressure is caused by a higher velocity.
Pressure gradients perpendicular to the streamlines are caused by the deflection of streaming air. The deflection of air generates regions of lower and higher pressure according to the curvature of the streamlines. Vaporizer, the soaring ping-pong ball as well as the physics of flight are only to be explained regarding the acceleration perpendicular to the streamlines.
- Physics of Flight - reviewed
Abstract: The conventional or standard explanation of aerodynamic lift states the higher streaming velocity at the upper side of the airfoil as cause of the lower pressure, due to Bernoulli's law. But a higher streaming velocity is the effect of a lower pressure and never its cause. The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil - which depends on the angle of attack and its velocity.
In relation to the airfoil the normal acceleration of the air in case of curved streamlines must be regarded which results in pressure gradients perpendicular to the streamlines and reaction forces acting perpendicular on the deflecting surfaces
Belarus Version by Galina Miclosic