Originally Posted by
italia458
So what we see is air entering a BIGGER area and expanding to fill the area, and hence, decreasing the static pressure. But Bernoulli says that if air in a closed system goes into an area of bigger dimensions the air will expand to fill the area and INCREASE static pressure! The difference being that air over an airfoil in flight is not in a closed system, energy IS being added to the system and that energy is added to the air.
Yes, expansion means increasing pressure. But increasing can mean increasing back to free stream static pressure. No discrepancy to the 'closed' system here. The same applies for the energy. Also in a closed system you will convert energy if viscosity and friction exists. Those two don't care much if there is a tube around. You still have boundary layer, speed gradient, etc.
So I still do not really see where the fundamental difference between a closed system and an open system lies that would conflict with the general principle.
The expansion happens as any gas is always looking for equilibrium. So does air. That makes gases try to fill voids.
Re: Push vs. Pull: Pull is the consequence of less Push on ine side than on the other.
Knowing that we don't live in a vaccum we consider any pressure lower than ambient pressure as Pull although technically it is simply less Push as ambient.
Still in an ambient pressure of 1013 HPa we generally consider anything below as Pull or Suction in daily life.