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Courtesy of Dr. David Kohlman, Engineering Systems Inc.
Over the past few decades, the subject of aircraft icing has gained more
and more attention from the FAA in the United States, which has dedicated
one full section to the certification of aircraft under icing conditions.
At the FLUENT Users' Group Meeting in June, Dr. David Kohlman, an
internationally recognized expert in the analysis of aircraft icing, presented
a short course on the subject. His course covered many aspects of icing,
from a description of how it happens under a variety of environmental
and flight conditions to how it can be prevented by a number of deicing
and anti-ice systems currently in use.
An anti-ice system that uses engine bleed air heats the wing skin using
a linear array of jets
Icing occurs when liquid droplets in the air strike the front of the
oncoming wing and either freeze on contact or flow along the surface and
then freeze. Many factors influence the likelihood that ice build-up will
occur. For example, large droplets are more apt to strike the wing than
small ones, since the former have more mass and are therefore more likely
to deviate from the local flow stream as it curves around the wing. The
ice collects in different patterns on the wing, depending on the local
conditions. It is easy to understand why icing is a leading cause of airplane
crashes, since all of these patterns distort the profile of the wing to
some degree. The distorted profiles cause the lift and drag characteristics
of the wing to change drastically. In particular, the stall speed of the
aircraft increases as the amount of accreted ice increases, making landings,
where the aircraft speed must reduce, especially dangerous.
Icing on the front and under-side of a commercial aircraft wing with the
high lift devices extended, displayed using contours of collection efficiency
The main method used by aircraft manufacturers to certify their aircraft
is to test a model of the aircraft in simulated icing conditions using
an icing tunnel. Another method is to perform a tanker test, where a tanker
aircraft sprays water droplets onto a following aircraft. Since both of
these tests are expensive and time consuming, computational methods are
gaining in popularity. In FLUENT, the collection efficiency, a measure
of the amount of droplets in the oncoming air that strike the aircraft,
can be obtained using the discrete phase model (DPM), which tracks the
trajectories of droplets in the Lagrangian frame. The Eulerian multiphase
model can also be used, in which the droplets and air are treated as separate
fluids. The collection efficiency is computed through the use of a user-defined
function (UDF). NASA has developed a computer program called LEWICE, which
is designed to predict the ice shape that forms on the fuselage over time.
This code consists of modules that calculate the flow field around the
body, compute the trajectories of the water droplets, solve for the mass
and energy balance of the liquid layer formed by the impinging droplets,
and compute the ice shape. Several European agencies also have developed
codes with similar capabilities.
The collection efficiency on the nacelle of a Boeing 737
Two mechanisms for protecting aircraft wings from icing are commonly
used. Deicing systems act to remove ice that has already developed on
the wing. This is done by the application of deicing fluid prior to take-off
or by jarring the wing surface slightly, either through the application
of an electromagnetic pulse that translates into a mechanical pulse, or
through the use of pneumatic boots that expand and contract. Anti-ice
systems act to prevent ice from developing in the first place. Bleed air
from the engine can be used for this purpose. A small tube carrying the
hot air runs the length of the wing, and the air is injected through tiny
holes towards the front wing surface,or skin. The hot air keeps the skin
above freezing temperatures, preventing ice from forming. Other methods
make use of resistive heating on the skin or the seepage through a porous
layer of a special liquid that lowers the freezing point of water. Through
the use of icing analysis and prevention, it is hoped that the number
of accidents due to this hazard will decrease in the years to come.
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