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Will Davies

Since before he can remember Will^ has watched F1 – he would always be sat down to watch the start before going out and continuing to follow on the radio. His first F1 memory is of his parents snoozing to what they considered the soporific tones of Murray Walker. Having lost touch somewhat during the years at university, Will^ has now reconnected with the sport and is a bigger fan than ever; often combining his interest for the more technical aspects with his love for maths for some 'interesting' results.

The eXhaust-Factor // How the Coandă effect is used in conjunction with exhausts

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Ever since teams realised the importance of generating downforce, cars have been designed so that they trade off maximum speed and create these forces that help hold the car on the track and improve cornering speeds, therefore ultimately improving lap times. The wings are the parts of the car that first come to mind, but they are not the full story – it is often forgotten that the car itself can be used for producing downforce and for many years teams have been looking for ways to use the under-floor to produce much of this desired downforce. Not having to rely on wings alone means they can be run at shallower angles, which means that they create less drag and therefore allow a faster top speed to be reached.

Ground effect – its physics and its history

The principle behind the ability for the under-floor of a car to generate downforce is called ground effect. It states that as the flat bottom runs close to the track surface the proximity to the two boundaries cause the already fast flowing air to speed up. When the air moving below the car is travelling at a faster speed than that over it the car is essentially sucked down onto the track. (A similar effect creates lift in planes.) The effect is increased as the distance between the floor and track is reduced (to a point – too little distance would mean that the air between the car and track would generate friction, which is not ideal in a machine built for racing.)

Ground effect is an application of Bernoulli’s Principle and essentially states that a change in speed is inversely proportional to the change in pressure, which in itself derived from the principle of conservation of energy – it is named after the Swiss mathematician Daniel Bernoulli. When the high pressure air ahead of the floor is forced under it and towards the low pressure area of the diffuser, this net force accelerates the air under the car, which then in turn creates the pressure differential and a positive feedback loop which generates the desired downforce.

The first example of a car to take ground effect to the next level was the Lotus 78. It started out with sidepods that had inversions underneath to better channel airflow. This increased the force generated over that of a standard under-floor of the time, allowing the car to extract more grip from the tyres as well as sticking it to the road, vastly increasing the potential to corner like the car was on rails. The shape of the top of the bodywork was too improved so that the car now resembled an upturned wing (of sorts).

Now the benefit of this development was realised, the design was pushed further and side-skirts were added. The skirts started life as brushes, but soon were upgraded to plastic skirts and again to movable rubber skirts with metal rubbing strips on the bottom. The skirts all but sealed the airflow under the car and created a much larger vacuum effect than before. Soon the whole grid was copying this, now essential, development.

The wind machine

Another very prominent exploitation of these effects was the Gordon Murray designed Brabham BT48 and more specifically the B-spec known as the "fan-car". (The team at the time was owned by a chap by the name of Bernie Ecclestone – I’m sure he’s gone on to do other things.) The B-spec car was used only in the 1978 Swedish Grand Prix as it was declared illegal before the next race. It too had skirts to prevent air from leaking out from under the chassis, but the main feature of the car, as the name suggests, was a large fan situated behind the engine. The ruse was that this was there just to aid cooling; however its secondary function (which some argued was actually the main reason for it being on the car) was to suck through air that was passing under the car, accelerating its passage further. This heightens the effect and increases the downforce generated.

One downside to using the under-floor to generate a large proportion of a car’s down-force is an effect known as porpoising; where the changing forces, speed and ground clearance, as the car travels round the track, can rapidly alter the centre of pressure around the car. These changes interact with the suspension causing the cars to begin to resonate and oscillate. They begin to dive, bounce, rock and roll, which is no good for the performance of either the car or the feelings in the stomachs of the drivers. It is suggested that one of the reasons (along with the safety aspect) behind Mario Andretti and Alan Jones leaving the sport was that they didn’t like the unpleasant ride.

Another and more major disadvantage to driving cars that rely so much on this technology, in particular side-skirts, is that when the vacuum is disturbed or the car goes over the bump and the seal made with the skirts is broken then the car will lose virtually all of the force that is holding it onto the track, sending it careering off the circuit. You can find a more detailed look into the history, downsides and eventual outlawing of ground effect cars in the Sidepodcast archives. The current regulations are designed to reduce the possibilities for exploitation of this effect – with the aim of ensuring driver safety.

Modern under-floors

Running cars too close to the ground can start to create friction as the slow-moving or stationary air in direct contact with the car and floor interacts. The fact that the air is slow moving might also cause the car to generate lift. The rules permitted the use of a diffuser which contrary to what the name suggests is there to collect the airflow from under the car and direct it out of the back. It ensures that air does not become stagnant beneath the car and allows the floor to work as efficiently as possible. Additionally, as it has an upturned outer edge, it has the benefit of acting like an upturned wing itself creating downforce.

The next step in the battle for downforce in this area of the car came in the form of Exhaust Blown Diffusers

Then in 2009 came the introduction of double diffusers. Some air that was passing under the floor was ducted up so that it ran on an upper level of the diffuser amplifying its efficiency. After that season double diffusers were banned, so the next step in the battle for downforce in this area of the car came in the form of Exhaust Blown Diffusers (EBDs). Through the 90s similar solutions had been used by various teams, but in 2010 Red Bull revisited the idea. Hot and energised exhaust gasses were fed directly into the diffuser. The high speed of these gasses further accelerated the pace of other flowing air through the diffuser, producing more downforce than the un-blown configurations.

After being used for a year things changed and in came a year of regulation as to how much the engine was allowed to ‘blow’ when it was off throttle, as it was thought that teams were gaining too much of an advantage. Specifically, it was noted, when they were making their engines rev hard when the driver was off-throttle, with the sole purpose of charging the diffuser and boosting rear downforce. Restrictions on the placements of exhaust outlets were brought in to effect at the start of the 2012 season – so that they were no longer allowed to directly blow exhaust gasses over the diffuser. Exhausts had to be positioned in a ‘neutral’ location so that the gases could no longer be ingested by the rear bodywork.

Just because the exhausts were placed in the new location, on top of and towards the rear of the sidepods, didn’t mean that teams gave up trying to use the high-speed plumes to energise the diffuser, and this is where the effect in question comes into play.

Spoon-bending

The Coandă effect is when an air flow is drawn to follow the surface that it is passing – interaction between particles on the edge of the flow and those air particles that are stuck to the surface (forming the boundary layer) cause the flow to bend. This effect can be seen if you hold the back of a spoon to the stream of water from a running tap. The stream of falling water is bent to follow the curvature of the spoon, in this case redirecting part of the flow away from the vertical.

F1 Coandă exhaust diagram
Tap to view larger

In the case that we are looking at the flow over the sidepod (1) adheres somewhat (as we now know because of the Coandă effect), so as the sidepod slopes down it follows. The exhaust (2) now in its ‘neutral’ position is unable to directly blow any bodywork; however, the action of the flow over the sidepod causes the gasses that are leaving it to be bent downwards so they are redirected along the streamlines of the flow. Teams utilise a channel behind the exhaust exit to help direct the energised flow and so that less energy is lost when the two air-streams converge.

These now follow the line of the bodywork and the floor until they reach the diffuser (5) where the fast and hot exhaust is able to transfer some of its energy to that of the air passing under the floor (as well as under any sidepod-overhang) (4), speeding that up and thereby lowering its pressure (and hence increasing downforce). On most cars it has the effect of also sealing the diffuser – stopping leakage and further improving the efficiency of the system.

Frequently you can see cars now running an addition to the front of the sidepod where a vane passes vertically up next to the front edge and then horizontally over the top surface.

Sidepod vanes on the Lotus
Sidepod vanes on the LotusCredit: LAT Photographic

It works to ensure that the flow over the sidepods attaches more uniformly to them. Improving the flow profile over the bodywork is another method of enhancing the Coandă effect.

Some Sauber subtleties

Most cars on the grid have sidepods that are being packaged as small and tight as possible as teams try to squeeze out the maximum amount of cooling from as small a space as possible – so as to reduce the car’s cross section and minimise drag. Most try to keep a reasonable width over the top so that there remains a good flow of air down over the exhaust and on towards the diffuser, however Sauber have gone one step further. They have created a car with a much narrower (10-15cm) profile than that of the other teams.

It is said that it was inspired by the crash Sergio Pérez suffered in 2011, coming out of the tunnel in Monaco where he ended up with the side of his car compressed to its tightest possible form by the Techpro barriers. The appearance is somewhat of an illusion, and the sidepods gain back volume as they are not as harshly undercut as in previous seasons. If they can manage to extract the desired amount of cooling as well as still feeding the rear of the car with enough air-flow and potentially feeding less disturbed air to the rear wing, then the reduced drag that this configuration offer definitely has promise for the whole car’s performance. This year’s Sauber is sporting some of the more ‘interesting’ design features on the grid and I’m not sure whether it’s paying off yet!

The tightly packaged Sauber
The tightly packaged SauberCredit: Sauber AG

With the regulations staying fairly stagnant this year (compared to the virtual overhaul that is coming in 2014) I don’t feel that teams will be experimenting too much in this area. They have all had a year to look around and see what works, and have spent most of 2012 devising and testing their own solutions. There may be some work done to perfect systems, but when it will probably become redundant at the end of the season, investing too much time, effort and money may turn out to be a waste in the long run – some of the bigger teams will already have an eye on next season’s car. (And you can bet Adrian Newey has already run the first round of CFD-simulations in his head.) However, having said all that, a good system could well be the key to a great car. I’ve said it before, but it’s worth saying again: more downforce here means less wing angle needed causing less drag and a higher top speed (both on the straights and through corners).

It is worth mentioning what we are in for in 2014 with respect to the way exhausts are to be configured. It is currently the intention for the exits to be located no more than 25cm from the centre line of the car, 3-5cm forward of the centre line of the rear wheels, and 35-55cm above the reference plane. From this position it will be all but impossible for the gasses to blow at the edges of the floor as they currently do, so some other way will have to be found to energise the diffuser and generate more downforce, in the face of the other changes to the regulations regarding wings.

In an ideal world

It is my opinion that this is the area that Formula 1 should be looking at in order to push the sport further. If cars were able to gather greater levels of downforce from the under-floor rather than using wings then they could race closer together – there would no longer be the penalty of the loss of downforce that a car following closely behind experiences, and more wheel to wheel racing could be expected.

Currently the diffuser region generates approximately 50% of the rear downforce which can be improved on if cars were allowed to use the gadgets like double or exhaust-blown diffuser technology. And if developments from the late 1970s, like ground effect tunnels, could be incorporated as well then there is a possibility that both front and rear wings would have to do little or no work.

Cars being able to run closer together would mean there would be a bigger benefit (compared to currently) in being in the slipstream. A greater amount of drag would be removed and so the speed increase that would be experienced would be greater. This might mean there would no longer be the need to ‘artificially’ help drivers to overtake, which will come as good news for some, as we could do away with the acronym-that-shall-not-be-named... but until then it is here to stay.

All it would take to make a jump to an all but wingless formula would be a set of regulations that could adequately prevent the problems of the past incarnations of this technology, as well as a decent amount of time in the hands of the teams, the designers and their super-computers/wind-tunnels.