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The Boundary Layer
To most of us air is just there. Although we can't survive
more than a couple of minutes without it, we take it for granted.
Airflow, actually, is pretty simple as long as you travel at
a walk or a human r0un or even at the speed of a galloping horse.
But, if you're designing or racing motorcars, air is a big challenge.
One of the challenging characteristics of fluid flow is the boundary
The boundary layer is a simple concept--when a fluid flows
next to a fixed surface, friction slows down the air molecules
close to that surface. If you measure the speed of the air right
at that surface, you'll find the air is still; its velocity is
zero, as in the graph. If you measure further out from the wall,
you'll see increasing speed until it reaches the maximum speed,
which is called the free stream velocity. A difference in flow
velocity across a fluid stream means there is shear--differences
in speeds between adjacent parts of the stream. If the fluid
is a viscous material, as all real-world fluids are, then shear
means friction losses in the flow and that's where things get
Here's a familiar analogy to demonstrate a viscous fluid boundary
layer. Let's say you're driving your car on a crowded freeway
with four lanes in your direction. Cars in the left lane are
moving at about 60 mph and are spaced out comfortably with six
or seven car lengths in between each vehicle. But the far right
lane is completely stopped because of a constant stream of cars
coming onto the freeway from a crowded on-ramp.
Upstream of the on-ramp the cars in the right lane are close-packed
bumper to bumper, almost stopped. That creeping right lane slows
cars in the lane next to it because drivers aren't comfortable
whizzing by stopped cars, and also because drivers in the right
lane are pulling out into the second lane in an attempt to make
some progress. This drags down the speed of the second lane which,
of course, means those drivers try to move into the third lane,
which drags down its speed also.
If you were to stand on an overpass and get the big picture
on this situation you'd see a dense, almost stopped right lane
and a loose, speedy left lane. In between, the second lane is
moving a little faster than the right lane but is dragging down
the speed of the third lane. The reluctance of drivers to travel
a lot faster than an adjacent lane is analogous to friction in
a viscous fluid. The vehicle velocity profile across the road
would look like the graph.
We talked about Bernoulli's Theorem in the Race
Tech 101 page, learning that Total Pressure, the sum of Static
Pressure and Dynamic Pressure, is a constant in any fluid. A
fluid gains dynamic pressure as it speeds up and has to lose
static pressure to keep the sum the same. The spacing of the
cars on the freeway is analogous to dynamic pressure. In the
fast lane the cars are strung out but, as the speed decreases
in lanes to the right, the spacing closes up just as the dynamic
pressure decreases in a fluid.
In a real-world freeway situation, a driver in the right lane
seeing a crowded on-ramp ahead would (if the driver was paying
attention) change lanes to avoid slowing down. The driver senses
a pressure increase ahead. If changing lanes is difficult, the
driver is forced to slow and now wants to change lanes even more.
That's higher pressure!
When an air molecule slows almost to a stop because it's up
against a boundary wall, it might encounter a completely stopped
molecule in front of it at a slightly higher pressure. Air flows
away from a higher pressure so the moving molecule moves away
from the wall and gets in the way of a molecule going a little
faster than itself. The slow, higher-pressure molecules in front
may actually move against the flow and reverse direction. This
is what causes circular eddies in boundary layer flow. You can
see something similar in smoke rising from a cigarette (do people
still do that?) and in water flowing around rocks.
As you can see from the freeway analogy flow separation is
caused by a reverse pressure gradient due to air slowing next
to a fixed boundary. You don't have separation when air is accelerating
as on the front of a moving vehicle where the flow is still attached
to the surface. It's on the back of the car, where you have the
air slowing down, that you get separation and the drag that goes
Racecar designers fight flow separation in three main ways-gradual
transitions, spoilers, and creating turbulence. At the rear of
the car after the maximum body cross section, body-panel angles
are kept small, less than 10 degrees. This gives the familiar
teardrop, streamlined shape.
If a large transition is needed a lip can spoil the air into
the low-pressure area lowering drag. Look again at these sketches
illustrating what a Gurney does. Air flowing over the bottom
of a wing at low angles of attack speeds up over the thick part
of the wing and then slows down until, at the trailing edge,
it's at the same speed as the air that went over the top of the
wing. As the air slows down, its static pressure rises and the
boundary layer thickens. At higher angles of attack, the boundary
layer can develop well enough to generate eddies and flow separates
from the surface. When you have separated flow, as in the top
sketch, drag goes up and the wing can stall.
A Gurney flap allows a wing to operate at higher angles of
attack than the same wing without a Gurney. Look at the streamlines
on the Gurney sketch. You can see the air has to move up to go
around the Gurney lip. That creates a lower pressure right behind
the flap that translates around the corner to the bottom of the
wing. This guarantees a high-to-low pressure gradient along the
bottom of the wing and prevents separation. This is why you see
a lip on the back of almost every shape on a racecar.
The third drag-lowering device is turbulence. If the air close
to a fixed boundary is swirling, at any moment some of it is
moving toward the boundary making separation more difficult.
When you see vanes and barge boards on a racecar think about
how the air sees this obstacle and tries to move from areas of
high pressure in front toward lower pressure behind the feature.
Often the result is a turbulent, swirling motion that sweeps
along the body panel retarding separation.