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When air flows through a venturi it speeds up, why does it do that? In simplest terms, pressure energy is converted to kinetic energy. In more accurate terms, there is a lot going on, and pressure itself while being the prime mover, is only part of the equation. Part of the reason a venturi even works, is because there is imperfect pressure recovery, due to entropic losses, thus the outlet is always at slightly lower pressure than the inlet. High pressure always flows to low, thus the venturi outlet acts as a favorable pressure gradient for air to follow.

If the pressure in the outlet is lower than the pressure at the inlet, the flow will follow the path of least resistance and naturally seek the outlet.

When you use a shop air hose, the pressure in the hose is much higher than atmospheric, the pressurized air expands into ambient. Now, if you lower the pressure of the ambient air at the discharge point, the air that comes out of the air hose rushes out with even more force, because there is less ambient air pressure acting as a restriction.

This is the principle behind air amplifiers.

https://www.youtube.com/watch?v=6HeagI8Tkh0

Therefore, the effectiveness of a venturi lies not just on the inlet pressure, but the outlet pressure as well.

The taoist principle of yin/yang separation generates power is highlighted here, the higher the separation of high and low pressure, the faster the air moves.

And to add weirdness, if you separate a positive and a negative electrical charge with a non-conductive material, at a certain current and voltage you begin ionizing the air around the charge separation. The so called dielectric barrier discharge, which can be used to influence airflow.

https://www.youtube.com/watch?v=HU0WPt9rKaI

In the previous section I discussed why air tends to curve instead of going in a smooth uninterrupted line. This section I'll talk about the properties of swirling air. Lewis Fry Richardson has/had a famous saying: "Big whirls have little whirls, that feed on their velocity, little whirls have lesser whirls, and so on to viscosity."

What does this mean? It means that turbulence is really just small vortices, and that large vortices, have small vortices, which have still smaller vortices, which terminate due to viscous forces. When a mass displaces air, the fastest most efficient way for that air to be replaced is by turbulence. Turbulence is the most efficient way to mix and fill a volume with air. Take an internal combustion engine, in order to get the most homogenous mixture of air and fuel, turbulence is actually a good thing. After all, turbulence are small vortices, these minimum volume swirls transport the fuel droplets and distribute them in a given volume faster than smooth laminar flow can.

Internal combustion engines are less efficient at burning fuel at idle, than they are at higher engine speeds. This is because the increased piston velocity creates more turbulence in the cylinder, creating a more homogenous mixture of air and fuel.

In order to generate a vortex in an aerodynamic sense, energy must be introduced. The force of a wing slicing through the air requires energy to propel the wing through the air. Energy must be expended to force the wing through the air fast enough to create the pressure difference between the two surfaces of the wing. The vortex that results as the air migrates to fill the low pressure created by the wing, is itself a form of drag. The energy used to move the wing through the air is dissipated as a vortex. This vortex becomes dissipated as it interacts with the air surrounding it, because the large vortex begins to swirl the surrounding air, that surrounding air produces vortices of its own, further dissipating the energy. The swirls and eddies caused by the primary vortex take the energy from the main vortex, and mix it turbulently until the air is completely still, and "static pressure" is recovered.

Even though vortices are a source of drag, they can be used in order to improve the aerodynamic characteristics of wings and a vehicle's aerodynamics. This is because the cyclical momentum of a vortex can help airflow stay attached to a surface.

https://i.stack.imgur.com/O0h2f.jpg

Here the leading edge slats near the cockpit of this F18, generate vortices which help the airflow to stay attached to the wings, and over the fuselage. This is the same principle behind the so called NACA duct. The sides of the NACA duct generate a pair of vortices, which entrain air upstream into the duct.

https://tianyizf1.files.wordpress.com/2013/12/delta-wing-vortex.png

Paper airplanes and other delta wing shaped craft exploit the same principle.

The main vortex rotates away from the center of the delta wing pulling air along with it, lowering the pressure at the center, air upstream then follows that low pressure zone as the path of least resistance. The air won't impede on the vortex, because the walls of the vortex are at or very near atmospheric pressure. This means the air has no need or reason to pass across the vortex.

https://www.centennialofflight.net/essay/Theories_of_Flight/Vortex/TH15G5.jpg

Vortices are partly why wings on aircraft tend to have an elliptical lift distribution.

Next section we'll discuss wing design, and the theory behind the distribution of lift on a wing.