There is an intriguing phenomenon
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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