This article was first published in 2006.
Over the first three parts of this series we’ve
seen how vortex generators work, looked in detail at their use on the Mitsubishi
Evo Lancer, and found where they can be bought for aftermarket use. And now’s
the time to put them to the test – to see if they can be used to improve fuel
economy by reducing drag, or used to make a car more aerodynamically stable.
Changing Flow Direction
As we’ve already covered, in the Lancer Evo
application vortex generators are used to put energy back into the boundary
layer, so causing the airflow to better stick to the body on the transition from
the roof to the rear window. As a result, there is increased pressure on the
rear glass (good for reducing drag and lift) and also better airflow to the rear
So, can commercially available vortex generators
perform the same function on other ‘three box’ sedans? Using a bunch of AirTabs,
we decided to find out.
The guinea pig car was one that is already highly
aero-efficient - an NHW10 Toyota Prius. The first step was to track the airflow
pattern over the rear window, using the very effective (and cheap!) approach of
wool-tufting. Unlike the many other occasions where we’ve used similar
wool-tufting techniques at AutoSpeed, this time we photographed the wool tufts
from the side of the road, using a telephoto lens and a fast shutter speed. (On
other occasions we’ve used a second car to carry the photographer.)
This is the airflow pattern over the rear window
of the Prius at about 50 km/h. (Click on pictures to enlarge them.) As can be
seen, there is attached flow across the transition from roof to rear window (ie
the wool tufts all nicely line up). The attached flow continues down the window
at both ends of the rear glass, however, in the lower middle area (circled)
there is turbulence. In other words, a separation bubble forms at the
middle/base of the rear window which would adversely affect the flow onto the
To see if the separation bubble at the base of the
rear glass could be eradicated, four AirTab vortex generators where centred at
the trailing edge of the roof. (They’re hard to see because they’re clear/white
and there’s glare on the top of the roof.) With the four vortex generators in
place, the difference in airflow was immediately apparent. This time, the
airflow down the middle of the rear window remained attached to the glass
(circled). This change in flow pattern is directly downstream of the vortex
generators. However, either side of this path of influence, the turbulence
Another two vortex generators were added, giving a
total of six centred on the trailing edge of the roof. Again, the difference was
obvious. As can be seen here, the airflow pattern is completely transformed,
with no separation bubble forming at all. However, with such good airflow,
any turbulence becomes more visible and some can be seen at the base of
the window at each extreme end. Would fitting another two vortex generators (so
extending the line across the whole width of the roof) fix this problem?
The answer is ‘no’. With eight vortex generators
placed on the roof, the separation at the lower edges of the rear glass remains
– perhaps caused by airflow wrapping around the C-pillars. This may be able to
be addressed by fitting a pair of vortex generators part way down the rear
glass, one each side. However, we decided to go with six vortex generators and
run with the small edge turbulence remaining.
So, was there a fuel economy gain (indicative of a deacrease in drag) from the fitting of the six vortex generators? Unfortunately,
we don’t know. The NHW10 Prius doesn’t run a trip fuel consumption display and
we simply don’t think that filling the tank each time is a sufficiently accurate
way of checking the fuel consumption – not when we’re talking a car where just
250ml difference in tank fill volume would dramatically change the results. This
is one case where the fuel consumption records would need to be accurately kept
over a long period if valid data were to be gained.
However, one thing is clear from this test – the
AirTab vortex generators can certainly energise the boundary layer, so promoting
attached flow where previously there was turbulence.
But what about the idea that vortex generators can
pull extra air into the low pressure wake, increasing its pressure and so
decreasing drag? As far as we’re aware, there is no independent scientific
evidence for this idea (as opposed to the energising of boundary layers, where
there is 50+ years of experience on aircraft!). But as it happened, we had
available a vehicle and a daily route that was perfect for checking out this
Reducing Drag by Filling the Wake
The test car in this case was a Honda Insight.
Each week for four weeks it travelled an identical morning route of 86
kilometres, comprising mostly freeway travel at 80 – 110 km/h. The car runs a
trip fuel consumption display which was reset each day. The fuel consumption was
incredibly consistent over this distance, every trip being 2.9 litres/100 km
except for one that was 2.8 litres/100km and another that was 3.0 litres/100km.
(Yes, those figures aren’t mistakes: this is the most fuel-efficient car in the
Five AirTab vortex generators were placed across
the trailing edge of the rear hatch.
After fitting the vortex generators, the measured
trip fuel consumption immediately rose to 3.0 - 3.1 litres/100 km, a 3-7 per
cent increase over the 2.9 litres/100km average. No change in the feel of the
car could be felt - there was no apparent improvement in stability, reduction in
wake noise or any other positives. Simply put, the vortex generators made things
On the basis of the scientific evidence that is
available, and the results of this test, we very much doubt whether vortex
generators fitted to the trailing edges of vehicles will reduce drag. That said,
it is well worth closely reading the testimonials page for AirTabs at
A further test was performed on the Honda.
The Honda runs a short front undertray with a
slightly raised section in the front bumper flowing air to it. Further back
under the car there are some in-fill panels, but the underside is certainly not
a smooth, flat and continuous surface.
I thought that if the boundary layer of air under
the car was energised, the flow might better ‘jump the gaps’ on the underside.
Additionally, it might accelerate air past the frontal undertray, reducing lift.
Or, to be honest, fitting the vortex generators to the undertray might do
something – I doubt if anyone in the world could say what the outcome
would be without first trying it!
Four AirTab vortex generators were placed at the
leading edge of the undertray.
Immediately noticeable was improved aerodynamic
stability. The Insight is not an aerodynamically stable car at speed – it is
discombobulated by, especially, the bow waves of trucks. This can be felt when
slowly passing a truck heading in the same direction on a multi-lane road. As
the Honda draws adjacent to the front of the truck, the very light car is pushed
away from the truck. (This affect, once recognised, can be felt to a degree in
lots of cars – but it’s quite clear in the Honda.) With the undertray vortex
generators in place, the affect of the truck bow waves was diminished. The bow
wave affect could still be felt, but it needed less steering correction.
OK, so the car was experiencing better stability,
probably through reduced frontal lift. But would there be a trade-off in extra
drag, resulting in increased fuel consumption? The answer to that is – no!
With the undertray vortex generators in place, the
fuel consumption averaged 2.9 litres/100. In fact, on one trip the car equalled
its best-ever consumption at 2.8 litres/100km – however, the traffic flow that
day was particularly kind and so I don’t think that the vortex generators can be
said to have helped in achieving this. But they certainly weren’t harming fuel
Some very important conclusions can be drawn from
the four parts of this series.
Vortex generators on cars can achieve measurable,
scientifically proven improvements in car aerodynamics – reducing both lift and
drag. The Lancer Evo is probably the shape of things to come – expect vortex
generators (of whatever design) to be used on new cars, especially to improve
the flow around corners towards the rear of the body.
Commercially available (and cheap) vortex
generators like AirTabs can achieve a provable change in car aerodynamic
behaviour. Even a single vortex generator will alter local flow behaviour,
something to keep in mind when considering airflow into bonnet scoops and at
specific problem areas.
Positive gains can be achieved only by practical
experimentation. That’s the downside – the upside is that the vortex generators
are easy to temporarily stick into place with masking tape, and just as easily
removed if they are not achieving the desired results. They’re also cheap enough
that buying ten or so for experimentation is a good investment.
The use of vortex generators under cars has zero
visual impact and looks to have excellent potential for reducing lift without
The devices and the potential are there – so get
out and try them!
Blowing the Vortex, Part 1
Blowing the Vortex, Part 2
Blowing the Vortex, Part 3