What is a Boundary Layer?
Unless this was the first page you came to, chances are that you’ve
seen a lot of pages here talk about something called a boundary layer.
Simply put, the boundary layer is a skin of air around a flying plane.
A very fundamental aerodynamic principle states that the velocity of
any fluid is zero exactly next to a moving body, such as an airplane.
According to this principle, the boundary layer can be defined as the
area close to the airplane where the airflow’s velocity goes from zero
to the surrounding airspeed.
Boundary Layer Sections
When the airflow first hits the airfoil, the flow separates to go
around the wing. The boundary layer produced at the leading edge is
laminar flow. This is the flow that is uniform and its velocities are
predictable. However, somewhere along the airfoil, the laminar flow
becomes turbulent at the transition point. The turbulent flow is what
increases the friction drag and ultimately decreases overall flight
Another factor in efficient flight is the concept of attached and
separated flows. Wings with separated flows have detached areas of low
pressure. This doesn’t help as much as it would if it were attached,
since low pressure regions behind a plane will propel it forward, and
low pressure regions below a wing will lift it upwards. This is the same
principle at work as in Bernoulli's
Principle. [View the
Creating Laminar Flows
Flying at higher speeds or increased angles of attack all push the
transition point closer towards the leading edge. This is bad news
since that means there will be more turbulent flow that usual. In order
to keep the flow laminar as long as possible, the boundary layer must
be re-energized so that it does not break off and become turbulent.
There are essentially three engineering feats in use that delay the
- Vortex Generators
Vortex generators employ the use of small strips of metal that are
positioned at an angle of attack across the span of the wings. These
strips create small vortices that help energize the boundary layer and
keep the flows from separating. Keep in mind, the actual wings produce
vortices also, but these are drastically different than the ones
produced by vortex generators. These generators work by redirecting the
energy of the high speed airflow above the boundary layers into the
boundary layer to energize it.
Vacuums suck the air from the boundary layer into small slits in the
airfoil. These vacuums are located at about halfway down the chord
length. This is where the laminar flow is thickest and tends to break
off into turbulent flows. By sucking the air from the boundary layer,
the flow is forced to follow the curvature of the airfoil, thus
delaying the transition into turbulent flow.
Finally, the most direct method of creating laminar flows are pumps.
Flows generally follow the surface of the
body that is immersed in that fluid, given a reasonable curve. These
pumps operate on that premise. Since turbulent flows are chaotic and
relatively low in kinetic energy due to their loss in momentum, a pump
near the middle of the airfoil blows out a uniform
jet of high pressure air onto the surface of the airfoil. This
energizes the parts of the boundary layer that lack energy.