Secrets of Underbody Tunnels, Rear Diffusers and Venturis
Underbody tunnels, rear diffusers and venturis are common terms used to describe the contouring of a racing car's underbody. While largely hidden from view, these devices are the secret weapons in an arsenal of aerodynamic features for generating downforce on racing cars.
Pressure coefficient contours, where red is high and blue is low
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Downforce is an essential ingredient in keeping a racing car glued to a racing circuit, especially through corners. By using the motion of the car through the surrounding air it is possible to induce a force perpendicular to the direction of travel. In aircraft the force is directed upward to enable flight; in motor racing the force is directed downward to press the car onto the race circuit.
The origins of the rear diffuser can be traced back to the 1977 Lotus Type 78 F1 car conceived by Colin Chapman, Peter Wright and Tony Rudd. In a brilliant example of lateral thinking the Lotus team applied the well known "airplane in ground effect" principle (reduced drag) to a racing car and found a significant increase in downforce with minimal increase in drag as a result. By incorporating inverted (compared to an aircraft) airfoil sections into the sidepods of their car, the era of ground effects in Formula 1 was ushered in. A side skirt was connected to the edge of the sidepods and extended down to the road surface. This skirt helped maintain 2D flow characteristics that provide increased downforce and reduced drag compared to a typical 3D wing. The side skirts also hid the new device from the prying eyes of competitors.
The increased cornering speeds encouraged by the first generation of racing car ground effects and the reliability of the side-skirt sealing with the ground raised safety concerns that culminated in a near universal ban of side skirts across all forms of motor sport. However, having had a taste of ground effects, the racecar designers weren't ready to give them up.
Racecar aerodynamicists found that without side skirts it was still possible to induce downforce by sculpting the underbody of a car into 2 tunnels either side of the engine-gearbox assembly. The tunnels ideally start close to the middle of the car, where the maximum downforce will be generated, and then gradually rake upwards (between 4-14 degrees) towards the rear of the car. The overall effect is similar to a venturi in that the air is first accelerated by gradually decreasing the cross-sectional area and then decelerated back to its original speed and pressure by gradually increasing the cross-sectional area. At the highest velocity (smallest cross section) the lowest pressure is produced according to Bernoulli's principle.
Airflow through a venturi
The effect of such a tunnel on the air is similar to a diffuser. The air enters the diffuser in a low-pressure, high-velocity state after accelerating under the car. By gradually increasing the cross-sectional area of the diffuser, the air gradually slows down and returns to its original free-stream speed and pressure. The diffuser's aim is to decelerate the air without it separating from the tunnel walls, which would cause a stall, reducing the downforce and inducing a large drag force. By installing an inverted wing close to the diffuser exit it is possible to create a low-pressure area, which essentially sucks the air from the diffuser. The diffuser and wing combination permits a higher air-mass-flow rate through the diffuser, thus resulting in higher downforce. Sharp edges on the vertical tunnel walls generate vortices from entrained air and help confine the air through the diffuser and reduce the chance it will separate. Fully optimized tunnels are found on many closed-wheel racing cars such as those used in the American Le-Mans Series.
Even so-called "flat bottomed" racing cars, such as those in F1, can create significant downforce from a well-shaped small diffuser. Cars without scope for a diffuser can still generate downforce from a slanted lower surface at the rear of the car in combination with a rear wing.
If you want to investigate ground effects for yourself, try downloading and viewing for free the following airflow simulations in Caedium: