Diffuser (automobile) – Wikipedia

The diffuser, placed behind the rear axle, of the Renault R29 single -seater.

And Streamer , named previously extractor or Tunnel deporter , is an aerodynamic device born in the 1990s intended, as well as a fin, to reduce the lift – and therefore increase the support – exercising on an automobile.

Generally equipping high performance cars (Formula 1 single -seaters, the 24 Hours of Le Mans trays or the sedans engaged in DTM), the diffuser plays an important role in aerodynamic efficiency and greatly improves stability and adhesion at high speed of the automobile.

The diffuser works according to a simple physical principle, the principle of Bernoulli: the acceleration of a fluid occurs simultaneously with the reduction in pressure. Placed on the rear of the automobile, the diffuser thus evacuates the air flowing under the body and accelerates it so as to create a depression “riding” the automobile on the ground.

The broadcasters are only really of interest in motorsport. This is the reason why they are mainly found on single -seaters and competition trays. Nevertheless, some prestigious sportsmen such as Ferrari are also equipped.

Like most aerodynamic appendages equipping competition cars, the diffuser is intended to thwart part of the harmful aerodynamic forces working on cars. Indeed, in aerodynamics, any fluid flowing around a body generates an aerodynamic force decomposed into two components: the drag, force parallel to the flow of the fluid, and the lift, force perpendicular to the flow ^{[ first ]} .

In the case of an automobile, the first opposes the advancement of the automobile while the second degrades its support. The diffuser is thus intended to decrease the lift, which tends to raise the automobile by limiting adhesion and therefore the power transmitted to the wheels (cf. mechanical links with friction), by creating a force of the same direction but opposite in direction . All the interest of using an aerodynamic appendix is ​​the creation of a fictitious load, that is to say without the addition of mass harmful to dynamic behavior ^{[ first ]} .

THE wing-cars [ modifier | Modifier and code ]

Lotus 79, one of the first wing-cars .

The history of the diffuser actually begins with the wing-cars , the first single -seaters of Formula 1 whose body is fully studied aerodynamically; The pontoons, in particular, take the form of a reverse wing. The first copies, the Lotus 78 and Lotus 79, which appeared in 1977, show an incredible efficiency so that they are quickly copied by the other racing stables ^{[ 2 ] , [ 3 ]} .

Subsequently, Colin Chapman, the engineer responsible for Lotus, decided to take an interest in the base of the single -seaters hitherto largely forgotten. Aware that the creation of deportance generates that of drag, harmful to the progress of the automobile, it is convinced that the base can offer an interesting deportance and minimal trail ^{[ 2 ]} . Chapman then achieves the interest of lowering the floor of the single -seater to take advantage of the floor effect ^{[ 3 ]} .

Following various FIA prohibitions to limit the effectiveness of single -seaters – minimal ground clearance, a flat bottom, etc. – Engineers seek to find the support they have lost with the new rules. This is how the diffuser was born in the 1990s ^{[ 3 ]} .

Brawn GP is revolutionizing [ modifier | Modifier and code ]

If the broadcasters are experiencing constant improvement, the main innovation occurs in 2009. The Brawn GP stable designs that year, a double diffuser offering a considerable advantage to its single-seaters. Larger, therefore more effective, the classic diffuser is divided into two parts (right and left) while a third pipe appears above the two previous parts, reproducing the effect of the diffuser. Vulgarly, two broadcasters overlap ^{[ 4 ]} . The aerodynamic gain offered by the double diffusers is around 6% according to Bob Bell, Technical Director of Renault in 2009 ^{[ 5 ]} .

Faced with this relative domination follows a controversy concerning the legality of this diffuser. On March 25, Red Bull Racing announced its intention to file a complaint against Brawn in case the stable uses this same diffuser in Australia ^{[ 6 ]} . Shortly after, other teams – Ferrari, BMW Sauber and Renault F1 Team – in turn file a complaint with the FIA ^{[ 7 ]} . The technical commissioners of the Australian Grand Prix responsible for examining complaints against Brawn GP ^{[ 8 ]} said that the Brawn GP rear diffuser was in accordance with the 2009 rules ^{[ 9 ]} .

BMW Sauber, Ferrari and Renault, however, decide to interjection, call ^{[ ten ]} which will be examined on April 14 in Paris ^{[ 11 ]} . The FIA ​​Court of Appeal, responsible for validating the classification or downgraging the stables incriminated for the use of an element not in accordance with the 2009 regulation ^{[ twelfth ]} makes its verdict and finally declares that the double diffuser system is authorized ^{[ 13 ]} .

The pressure at point 1 is greater than at point 2; The speed of the fluid at point 2 is greater than at point 1.

The physical principle of the diffuser, moreover, is based on that of Bernoulli, namely that the acceleration of a fluid is necessarily accompanied by a reduction in pressure. This phenomenon is easily demonstrable thanks to a liquid flowing in a venturi; a section pipe

presenting a narrowing

.

The conservation of the flow in the pipeline induces an increase in speed in the narrowed section

, which in turn implies a decrease in pressure

Since, according to Bernoulli’s theorem:

Profile of a wing (not reversed); The lift is represented in red.

But if Bernoulli explains why the pressure decreases, this does not explain how it makes it possible to fight against lift, this force tends to lift the automobile. The example of the profile of an airplane wing is a particularly clear example to understand the interest of the diffuser ^{[ 14 ]} .

The profile of an inverted wing consists of two parts: the extrados (lower part) and the intrados (upper part) unlike a standard wing. The extrados has the distinction of having a higher length than intrados. The air flow passing under the profile having more way to go than that above, it must accelerate what creates, according to Bernoulli, a depression. The pressure difference thus generated between the two faces of the profile generates a force on the inverted wing, from overpressure to depression (down on a car) ^{[ 14 ]} .

This is exactly the same principle on an automotive spoiler since the fin is only an inverted wing. On the other hand, the geometry of a diffuser is more complex ^{[ 14 ] , [ first ]} .

The “fences” channe the air and improve the efficiency of the diffuser.

Depression is not strictly said in the diffuser, but upstream. The air is accelerated at the input of the diffuser, to generate depression under the chassis, and its speed decreases as it travels the diffuser ^{[ 14 ]} .

The visible part of the diffuser, at the rear of the cars, is not intended to generate depression but only to stabilize the flow. This is the reason why the diffuser increases in volume on the rear to create a vacuum, increase the pressure and therefore slow down the speed of the air ^{[ 14 ]} . The vertical walls, traversing the diffuser over the length, are “fences” allowing to optimize the efficiency of the diffusers by channeling the air under the car and therefore preventing that it will spread elsewhere ^{[ 14 ]} . Generally, these fences delimit a central tunnel and two side ^{[ 15 ]} .

The depression values ​​generated by the diffuser are quite significant but difficult to quantify alone. The diffuser is on average at the origin of 40% of the support ^{[ 5 ]} . To give an order of magnitude, the Lotus 79, exploiting the soil effect, generated up to two tonnes of vertical thrust ^{[ 16 ]} . Moreover, the 1990 championship was marked by incredible accidents since the racing trays, equipped with imposing broadcasters, lifted the sewage plates of the Gilles-Villeneuve circuit ^{[ 17 ] , [ 18 ]} .

The geometry of the diffuser is particularly difficult to design, because it is not enough to put any diffuser to obtain support. Not only, if too much air escapes from the diffuser, the automobile is slowed down, but there is a value of the angle formed between the diffuser and the soil for which there is a drop in the boundary layer. Beyond approximately 4 °, the flow of air is no longer laminar but turbulent-trained generation-, which is counterproductive ^{[ 2 ] , [ 15 ]} .

Slow simulations can be made by software such as Symlab or Flor and offer good results. On the other hand, the calculations are complex, require days to be resolved and require impressive calculation capacities, therefore expensive computer equipment. This is the reason why manufacturers work mainly in blower ^{[ 14 ]} .