Last week in Part 1 of this series we looked at practical and easy techniques that allow you to directly measure the flow restrictions of intakes (air filters, air filter boxes, etc) and exhausts (including cat converters, mufflers and pipe-work).
In other words, techniques that put a giant, very low cost, flow bench at your disposal!
This week we’ll look at how you can access what is effectively a full-size wind tunnel. Again, all without costing much at all...
Think ‘aerodynamic testing’ and most people immediately conjure-up visions of racing cars, downforce and drag. But in addition to these things, aero testing is also vital if you want to place the engine air intake in the best location, or install bonnet vents or scoops that actually work. We’ll get to those latter ideas later in this series, but first: how do you see the way in which air flows over a car?
As a car move forward, air passes over its body. But, unlike a boat that pushes water aside, in the case of cars, the fluid (air) is invisible. So in most cases people don't really have any idea of the paths that the air takes over, under and around their car – they just guess.
But help is at hand: it is a near zero cost process to wool-tuft a car and then drive it down the road, inspecting the pattern of airflow from another moving car!
The idea is deceptively simple: cut up lots of short lengths of wool, selecting a colour that contrasts well with the car's paint colour. Using good quality masking tape (good quality so that it won't lift the paint when you take it off!), stick the tufts all over the car, keeping them far enough apart that they can't touch together. (If they're too close, they tend to stick to one another.)
Once you've done that, drive the car (or have someone else drive the car) down an empty, multi-lane road at about 70-80 km/h. From another car, shoot video or still pics of the tufts, including close-up details and overall shots.
If you are concerned that the buffeting of the chase car will upset the target's aerodynamics, get further away and use a telephoto lens.
I first did this testing about 20 years ago when I owned a VL Commodore Turbo. I’d been reading about aerodynamics – and knew the old-shape Commodore was reputedly quite poor – but I was quite flabbergasted when I did the testing and saw the mass of air turbulence over the rear window and boot lid. I even mounted vertical sticks at the end of the bootlid and tested how high a wing would need to be before it was in clean air and could work. But I’m getting ahead of myself...
Types of Airflow
Airflow over cars can be basically split into two types: attached and non-attached. The slightly less precise alternative terms are also more descriptive: laminar and turbulent.
In laminar flow, the air slides over the surface of the car in layers. Where this type of flow is present, the wool tufts will line up end to end, the ends fluttering just a little. On the other hand, where turbulence is present, the movement of the tufts is nearly random - they'll wave around in the air, sometime writhe upwards at 90 degrees to the surface that they're stuck to, and quite often even angle forward into the direction of flow!
Here attached flow can be seen on the bonnet (tufts lined up in neat rows) and turbulence is visible directly behind the scoop.
At the front of a modern car, the flow across the surface of the bonnet will be almost always be characterised by attached flow: the tufts will be lined up beautifully. The transition up the windscreen will also be attached, but the transition from the windscreen to the roof may not be so good. If the flow detaches itself at this transitional change of angle, there will be turbulence across the leading edge of the roof. The tufts here will be whirling around, not lying flat and nearly still.
If this is the case, you can imagine that the frontal area of the car which is disturbing the air is actually larger than that indicated by the body dimensions - the turbulence on the leading edge of the roof making the car "appear" larger, resulting in more drag.
On a sedan, the airflow will generally be attached at the trailing edge of the roof - but then it has to make the transition onto the rear glass. It's the back of the car which is most important in determining overall lift and drag, so it's really important what happens here.
If the flow remains attached right down the back window and onto the boot, the car's doing well. Why? Because when the air finally leaves the trailing edge of the boot, the cross-sectional area of disturbed air being pulled along behind (called the "wake") will be small. And a small wake equals lower drag.
But if the air becomes detached (say) halfway down the rear window, then the wake is made much larger. If the flow leaves the car at the top of the rear window (eg all hatchbacks and the vast majority of old-shape sedans like that VL Commodore) you can see that the drag will be even larger again.
So a small wake is important to low drag, and this can be gained by the presence of attached flow as long as possible. A small wake is to be strived for if you want low drag - but it has a major downside. If you still want to be able to fit people in the cabin, the centre part of the car will be much higher than the rear proportions - so the airflow will have to wrap up and over curves. Hmm - an aircraft wing generates lift by having a curved upper surface and a much flatter lower surface.... and in much the same way, a car body generates lift as well.
So, the more attached the flow is from the front of the car to the rear and the lower the rear surface can be made - the smaller the wake and therefore probably the drag. But shapes like this invariably generate lots of lift because of the flow wrapping itself around those upper curved surfaces....
And even worse, if the curved surface is at the very rear of the car (eg the classic Porsche 911 or the New Beetle shapes), that airflow will generate both lift and drag.
Instead, you want the airflow to finally depart the car body without wrapping over any final curves - the reason for raised lips and sharp changes of angle on the trailing edge of the boots of modern aero sedans, and the roof extension spoilers of hatches. So a trade-off is necessary - keep the wake small but generate as little lift at the rear of the car as possible. (Rear lift leads to instability - again, the rear is a vital area.)
That’s a lot jammed into a few paragraphs, so where does it leave us?
By looking at the patterns of airflow revealed by wool tufting, you can accurately estimate:
Using some rules of thumb, you can then estimate:
By placing the tufts appropriately, you can also clearly see airflow patterns through:
Further, you can generalise as to:
In addition to examining the airflow pattern over the standard car, you can also assess whether modifications are altering that pattern.
For example, vortex generators are being increasingly used to energise the boundary layer, so keeping the airflow attached to car past the point at which it would normally separate.
I did some testing of vortex generators on an NHW10 Toyota Prius.
The first step was to track the airflow pattern over the rear window, using wool-tufting. On this occasion I photographed the wool tufts from the side of the road, using a telephoto lens and a fast shutter speed.
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 were 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 and probably able to be seen by wool tufts placed appropriately.
So the testing clearly showed that the vortex generators could certainly energise the boundary layer, so promoting attached flow where previously there was turbulence.
As you can see, those bits of waving wool are actually an enormously powerful tool!
Next week: directly measuring aerodynamic pressures – find the best places for bonnet vents and the engine air intake, and measure the effectiveness of spoilers and other modifications