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By Kimiyoshi Takada, Fuji Techno-Service; Toru Komoriya, Yukio Furugori,
and Kazuhiro Kaminaga, Fuji Heavy Industries; Gunma, Japan
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SUBARU LEGACY
As SUBARU invests in its IMPREZA
for the World Rally Championship
(WRC), the know-how and
technology developed are fed back into
their commercial line of cars, such as
the SUBARU LEGACY. In the WRC, competitions
take place under a variety of
weather, road, and driving conditions,
and ensuring visibility is one of the primary
requirements for safe and fast driving.
One issue at the forefront of wiper
design is the tendency of wipers to lift
off the surface of the windshield, or
float, especially at high driving speeds.
When this occurs, the wipers are not
as effective in keeping the windshield
clear. The problem has become more
pronounced in recent years, with the
reductions in aerodynamic drag on automobiles
as a whole. In the same manner
that a rear-mounted spoiler
increases the down force on a performance
car, a wiper-spoiler can also
be used to increase the down force on
a windshield wiper. By optimizing the
spoiler design, the threshold speed at
which floating begins can be increased.
At SUBARU, FLUENT was recently
used to calculate the airflow around
a stationary wiper arm, with and without
a spoiler, on a car moving at a range
of speeds. The entire car, as well as the
road, was included in the solution
domain. The mesh of approximately
4.6 million cells was refined in the vicinity
of the wiper. A uniform inlet velocity
boundary condition was specified
to simulate the moving car. The realizable
k-e model was used to calculate
the separation vortices generated
behind the wiper. Laboratory measurements
were made to validate the
CFD models.
Comparison of
down force and
drag force
predicted by
FLUENT and
measured results
Experimental tests demonstrated that
for all speeds tested (from 100 to 180
km/h), the spoiler greatly increased the
down force of the wiper on the windshield.
For this range of driving conditions,
FLUENT predictions of both
down force and drag force were found
to be in excellent agreement with experimental
data for a wiper equipped with
a spoiler. Both forces were found to
increase in proportion to the square
of the car speed, in accordance with
the experimental results, and the agreement
between the CFD calculations and
experiment improved as the speed of
the car increased. The detailed flow field
surrounding the wiper was not measured
experimentally, but the validation
of the down and drag forces led designers
to have confidence in the CFD predictions
of the flow field for subsequent
design changes that were proposed.
Pressure
distribution on
the wiper arm
CFD predictions of pressure in the
vicinity of a wiper with a spoiler showed
a larger pressure on top of the wiper
than below it, even at the highest speed
considered. The results showed that
where air hits the top of the spoiler, a
region of high pressure develops.
Vortices generated behind the spoiler
cause a corresponding drop in pressure
in that region. The interaction of the
wiper blade and spoiler was assumed
to exist, but was not investigated.
Velocity vectors
on a slice
through the
wiper arm
SUBARU engineers have also been
using FLUENT to study windshield washer
designs. Using the discrete phase
model (DPM), the trajectories of
water droplets were followed for a variety
of car speeds and nozzle designs.
The locations where each of the trajectories
splashed onto the windshield
in the experiments were compared to
the CFD predictions. For two nozzle
designs, the agreement was very
good for driving speeds ranging from
100 to 180 km/h. The advances made
using CFD for windshield wiper and
washer design will be realized in the
newest line of SUBARU LEGACY automobiles.
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