Blowing the Vortex, Part 4

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 wing.

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 boot-lid.

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 remained.

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 ‘filling-the-wake’ idea.

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 world!)

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 worse.

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 www.airtab.com.

Front Undertray

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 consumption...

Conclusions

Some very important conclusions can be drawn from the four parts of this series.

1. 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.

2. 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.

3. 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.

3. The use of vortex generators under cars has zero visual impact and looks to have excellent potential for reducing lift without increasing drag.

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

Interested in do-it-yourself car aerodynamics? You’re sure then to be interested in the Amateur Car Aerodynamics Sourcebook, available now.