Bugatti Bolide's Aero Trick Explained
Guide to Active Bubbles
Last year, Bugatti announced their latest concept car, the Bolide. It is an experimental hypercar built around Bugatti's iconic 8-litre, quad-turbo W16 engine that is used in the Chiron. It weighs just over 1200kg with the legendary engine cranked up to produce over 1800 horsepower. According to Bugatti's simulation, the Bolide can go from 0-100km/h in just over 2 seconds and has a maximum speed of just over 500 km/h.
The Bolide is built for breaking lap records. With its racecar suspension, Michelin slick tyres and racecar aerodynamics, Bugatti claim that it can lap the Nordschleife in 5 minutes 23 seconds (lap record: 5 minutes 19 seconds by the Porsche 919 Evo) and the La Sarthe circuit 7 seconds faster than the fastest LMP1 car.
However, besides its streamlined aerodynamics, the Bolide's most fascinating aerodynamic feature is the 60 inflatable bubbles on top of the air intake. The bubbles, which can extend upto 10mm from the surface, help reduce drag by 10% while also improving agility and efficiency. Bugatti say that the 'active bubbles' can do this by working like the dimples on a golf ball. This seems quite contradictory since bubbles are the exact opposite of dimples. So how do the active bubbles really work? Let's start with some science.
The Science of Aerodynamics
Aerodynamics is based on two simple physics principles, the Bernoulli Principle, which states that an increase in speed of a fluid is accompanied simultaneously by a decrease in pressure and the Venturi effect, which states that any fluid flowing through a constricted space speeds up and hence, has low-pressure. Since air is a fluid, it follows both these principles. This means that slow-moving air has higher pressure than fast-moving air.
Bernoulli Principle and Venturi effect. Photo: https://physics4frca.com/2018/05/01/bernoulli-venturi-and-coanda/
Aerodynamic devices like the rear wing, undertray and diffuser work on these principles. The rear wing is shaped as an aerofoil, meaning that air has to travel a longer path below the wing than above. The air passing below the rear wing is faster than the air passing above and thus there is a high-pressure region above the wing and a low-pressure region below. The undertray and diffuser channel air through constricted passages which increases the speed, thus reducing pressure under the car leading to ground effect.
All these devices create a downward pressure differential that pushes the car down due to the high pressure being on top. This produces the downforce required to keep the car stuck to the ground even when it goes fast through corners, preventing it from taking off like an aeroplane. It also increases grip which allows the car to go faster around corners meaning faster lap times.
Aerodynamics of a wing. Photo: buildyourownracecar.com
Though these aerodynamic devices increase cornering speeds, they reduce a car's top speed since they produce lots of drag. Though the apparent drag that a car needs to overcome is air resistance, there is another type of drag that needs to be countered. This is called pressure drag.
Pressure Drag. Photo: sciencedirect.com
When a car with such aerodynamic devices moves fast, it leaves a low-pressure region behind it, called wake. This happens because aerodynamic structures send high-pressure air higher up, leaving fast-moving air directly behind the car. This pressure differential caused by detached airflow pulls the car backwards causing loss of top speed. Pressure drag also reduces efficiency of the rear wing since it has access to less air for producing downforce. This is where the active bubbles come in for Bugatti. But first, we need to look at the aerodynamics of their inspiration, a golf ball.
Golf Ball Aerodynamics
The golf ball has one of the most iconic shapes amongst all sports equipment. On its surface, there are hundreds of small dimples to reduce aerodynamic drag and make the ball go further. With a smooth ball, the airflow becomes detached and creates pressure drag behind the ball. This happens because the air close to the surface sticks to the fast-moving air above it instead of to the surface.
Aerodynamics of dimpled Golf Ball. Photo: aerospaceweb.org
Dimples change the direction of airflow around the ball. As air moves over the ball, the dimples create small pockets of turbulence or disturbed airflow as the air detaches and reattaches multiple times. These pockets help the air move closer and tighter to the ball. This leads to a smaller region of wake which reduces drag. It is estimated that a golf ball with dimples will go twice as far as one without them. Now that we know the science, we can look at the active bubbles on the Bolide.
As mentioned above, the Bolide has 60 inflatable bubbles on top of the engine's intake. But what is their function?
Since the Bolide is heavily dependent on aerodynamics, it suffers from the same problem of wake as the golf ball. Like the golf ball, the bubbles produce multiple regions of turbulence to keep the airflow closer to the car so that the aerodynamic devices can continue to function effectively while also reducing pressure drag.
Active Bubbles on the Bolide's engine intake. Photo: Bugatti
The term 'active' in active bubbles means that they are not always inflated but become inflated when the car goes faster than 80km/h. This is because below 80 km/h, the air moves slow enough to remain stuck to the car by itself. If the bubbles remained inflated at low speeds, they would create drag in the form of air resistance.
Functioning of the Active Bubbles. Photo: Bugatti
Finally, Bugatti used bubbles instead of dimples because dimples produce lift which is not desirable in a racecar. Thus, Bugatti have used the exact opposite of dimples, bubbles, to produce downforce instead of lift.
I would like to conclude by saying that all the speeds, lap times and the effectiveness of the bubbles has not been tested in real life and are based on Bugatti's simulations. Till the Bolide actually goes out and achieves this, the active bubbles will remain a fascinating experiment whose result has only been seen on a computer. However, if they can make them work, it would be another testament to the French marque's ability to innovate.