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This article was first published in 2006.
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Vortex generators are now being used on a variety
of high performance cars, with the best known being the latest Mitsubishi Evo
Lancer. Because the same approach can be used on other cars, let’s take a
detailed look at how the vortex generators on the Evo were developed, how they
work and what their benefits are.
How they Work
As with many booted sedans, on the Lancer the
airflow tends to separate form the body at the trailing end of the roof. In
other words, the streamline doesn’t stick to the body in the roof/rear window
transition but instead tends to leave at this point. The result is a larger wake
of disturbed air and a lack of pressure recovery on the rear window. (If you’re
lost, see last week’s article at
Blowing the Vortex, Part 1).
This separation is in part caused by the thickness
of the boundary layer which increases in depth as you move towards the rear of
the vehicle. A thick boundary layer means – by definition – that the airflow is
no longer moving across the surface of the vehicle at the airspeed at which the
car is travelling. In other words, the airflow close to the car has lost its
flow energy.
However, if vortex generators are placed just
ahead of the separation point, they can be used to put airspeed back into the
boundary layer. The boundary layer then becomes energises and as a result, the
airflow is more likely to stick to the body.
This diagram shows the flow velocities at
different points on the roof/rear window transition of a sedan. The longer the
arrow, the faster the airspeed. The direction of the arrows show the direction
of the airflow. At ‘A’, which is ahead of the rear window on the roof, the air
speed is slowest near the roof and then increases in speed as you move upwards
and away from the roof. At ‘B’, which is at the beginning of the rear window,
you can see that the speed of airflow at the surface of the car is zero. This is
indicative of the thickening boundary layer. At ‘C’, which is part way down the
rear window, you can see that the airflow on the body is actually heading in the
opposite direction to the airflow past the car! The flow has separated.
However, if a vortex generator is placed ahead of
the point of separation, energy is put into the otherwise slow-moving boundary
layer, as can be seen by the longer velocity arrows closer to the body surface.
Furthermore, the airflow keeps going in the right direction!
If as a result of the presence of the vortex
generators the wake is reduced in size, or increased in pressure, or if there is
increased pressure acting on vertical or angled rear surfaces, drag will be
reduced. However, the vortex generators themselves will develop some drag so the
end result is the balance of the decreased car body drag minus the increased
vortex generators’ drag.
Designing the Evo Vortex Generators
Mitsubishi did extensive testing of the Evo vortex
generators in their full-sized wind tunnel. The test speed was 180 km/h.
As described, it’s possible for the vortex
generators – even when functioning correctly – to add an excessive amount of
drag by their very presence. A rule of thumb is that the height of the vortex
generators should therefore not be much greater than the thickness of the
boundary layer.
This diagram shows the velocity profile measured
100mm upstream of the rear window. As can be seen, at a height of about 3mm
above the roof, the air is moving at 0.6 times (or 60 per cent) of the forward
speed of the car, by 10mm above the roof the airspeed is about 85 per cent of
the speed of the car, and by the time a height of 30mm is reached, the airflow
is 100 per cent of the speed of the car. This indicates that at this roof
position, the boundary layer is about 30mm thick.
(Cross reference this data with last week’s
example of how high an ant would have to lift their head before being blown off
the surface of the car!)
The vortex generators on the roof of the Lancer
were placed immediately upstream of the flow separation point, a position which
was 100mm ahead of the rear window (although this appears to have changed on the
production model).
Two completely different designs of vortex
generators were trialled by Mitsubishi. The first was a bump-shaped device that
looked a bit like an upside-down spoon. However, at the rear of the ‘spoon’ a
flat surface was placed with a rear slope angle of 25 – 30 degrees.
The other design was modelled on an aircraft’s
delta wing (although it looks more like a wedge doorstop than a wing). Note how
the shape is not mounted in-line with the airflow but instead at a 15 degree
angle to it – this means that the airflow direction must be carefully mapped
right across the roof if the vortex generators are to be angled correctly.
Test Results
So, how well did these shapes work on the roof of
the Evo Lancer?
This graph shows the test results for the bump
shaped vortex generators. Three different height vortex generating bumps were
tested – 15mm, 20mm and 25mm. Looking firstly at the change in drag coefficient
(delta Cd), it can be seen that the 15mm high bumps caused a decrease in overall
drag of about 0.001 and the 20mm and 25mm bumps a decrease of about 0.003.
Changes in coefficient of lift showed that the higher the bumps, the greater
reduction in the coefficient of lift to a maximum of a 0.005 decrease for the
30mm bumps.
This graph shows the test results of the delta
shaped vortex generators. The tested height of these vortex generators were
again 15, 20 and 25mm. The test results showed a similar decrease in drag and
lift coefficients of 0.006 for 15 and 20mm delta vortex generators. For 25mm
high deltas the drag decrease remained much the same but the coefficient of lift
decreased by 0.007.
The delta-shaped vortex generators therefore gave
clearly better results than the bumps. However, Mitsubishi do not quote drag or
lift coefficients for the car, so assessing the percentage changes are
difficult. If a drag coefficient of 0.35 is assumed, the drag reduction with the
delta-shaped vortex generators is only 1.7 per cent. That is an important figure
to keep in mind when later in this series we look at fuel consumption
improvement claims made by sellers of aftermarket vortex generators!
Mitsubishi performed a number of other tests to
see the effect of the vortex generators. These included computational fluid
dynamics (CFD) simulations and actual pressure measurements on the surface of
the vehicle body.
This shows the CFD-calculated velocities of the
airflow without the vortex generators, modelled along the centreline of the car.
From slowest to fastest air speeds, the colours are: dark blue, light blue,
green, yellow, red.
With the vortex generators fitted, it can be seen
that the calculated velocity past the rear wing is increased.
The measured pressure distribution on the boot lid
is an even clearer indication of the positive changes. From lowest to highest
pressures the colours are: dark blue, light blue, green, yellow, red. The
greater area of red (high pressure) can be clearly seen on the boot lid and the
rear window. However, note the lower pressure (dark blue) behind the vortex
generators themselves, indicative of the drag being created by their presence.
Conclusions
In their technical paper, Mitsubishi’s engineers
made the following conclusions:
(1) Vortex generators (VGs) were installed
immediately upstream of the flow separation point in order to control separation
of airflow above the sedan’s rear window and improve the aerodynamic
characteristics. It was found that the optimum height of the VGs is almost
equivalent to the thickness of the boundary layer (15 to 25 mm) and the optimum
method of placement is to arrange them in a row in the lateral direction 100 mm
upstream of the roof-end at intervals of 100 mm. The VGs are not highly
sensitive to these parameters and their optimum value ranges are wide. Better
effects are obtained from delta-wing-shaped VGs than from bump-shaped
VGs.
(2) Application of the VGs of the optimum shape
showed a 0.006 reduction in both the drag coefficient and lift coefficient of
the Mitsubishi Lancer Evolution.
(3) It is confirmed that VGs create streamwise
vortices, the vortices mix higher and lower layers of boundary layer and the
mixture causes the flow separation point to shift downstream, consequently the
separation region is narrowed. From this, we could predict that VGs cause the
pressure of the vehicle’s entire rear surface to increase therefore decreasing
drag, also the velocity around the rear spoiler to increase, and the lift to
decrease.
Reference: Masaru Koike,Tsunehisa Nagayoshi
& Naoki Hamamoto; Research On Aerodynamic Drag Reduction By Vortex
Generators, Mitsubishi Motors Technical Review, 2004, No 16.
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Next week: sourcing pre-made vortex
generators
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