when you closely examine the science behind airfoils.

Why does the air above the airfoil flow much faster than the air below?

How come the two never meet?

The answer is right there in the pressure gradient.

Before explaining the reason,

we will first describe how the pressure gradient is developed.

In the first part of the airfoil video

we learned that the flow gets curved as shown to the kiwanda effect.

You can explain the pressure distribution

by keeping in mind that in a curved flow pressure is higher at the outside.

There are three main flow curvatures in this flow.

The biggest is at the top of the airfoil.

Far away from the airfoil the pressure is atmospheric

so due to this high curvature

pressure will decrease as we move toward the airfoil.

The second curvature is at the bottom of the airfoil near the tail,

this is also curved downward

so here if we move toward the airfoil, the pressure should increase.

The last flow curvature is also at the bottom of the airfoil

close to the leading edge.

This is a very small curvature

this curvature, however, is curved slightly upward.

This means that the pressure should decrease in this region

as we move toward the airfoil.

Due to the very small curvature

there will be a very small drop in pressure.

We know that far away from the upstream and downstream

the pressure is atmospheric.

At the leading edge of the airfoil,

a high-pressure region is generated as the flow directly hits this portion

so we can easily construct the pressure distribution as shown

The CFD results could form exactly two are logical conclusions.

Now back to the initial question

to facilitate the analysis

we can neglect this very small drop in pressure.

You can see that at the top

the pressure decreases almost to the midpoint before it increases

at the bottom the pressure keeps on increasing

until it reaches the tail

only after that does it decrease.

Pause for a moment now

and consider two fluid particles starting at the same speed

but in different pressure gradients.

The top particle is surrounded by a decreasing pressure condition

while the bottom particle sits in an increasing pressure condition.

For the top particle pressure on the right side is less than at the left side

so there will be a net force in the same direction of the velocity

and the particle will speed up.

However, the reverse is true for the bottom particle

here the net forces against velocity direction

so it will decelerate.

In short, in a decreasing pressure filled the fluid particle will accelerate

and in an increasing pressure filled the fluid particle will decelerate.

This is exactly what happens in an airfoil also

the bottom particle will keep on decelerating

the top particle will accelerate up to the midpoint.

This means that the speed of the top particle

will be higher at any point in time

and the two particles will never meet.

The bottom particle also experiences of pressure decreasing scenario,

However, it is almost after the trailing edge and it happened suddenly

such a sudden drop in pressure will not considerably increase the particle speed.

In short for this particular problem

the pressure distribution makes the particles flow at different speeds

but the reverse argument does not hold

the different speeds of the particles are not what make the pressure distribution

because for the second textbook argument

there is no logical explanation for what causes this speed difference?

These two arguments are not too different ways of looking at the same thing.

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