From:
Automotive Aerodynamics - Sport Compact Car Magazine
Side ducts
Side ducts are primarily seen on race cars for two reasons, because brake and engine cooling is crucial, and because most serious race cars will use a front underbody diffuser that channels airflow toward the rear of the front wheel well. Conventional fender designs trap much of the turbulent air coming off the top and back of the tire generated by the counter rotation of the tires and wheels. Combined with hot air moving through the engine bay and brakes, this generates losses and drag. Side ducting not only provides a smooth outlet for these hot and turbulent gasses, but also turns the flow to exit smoothly along the side of the car instead of directly outward, which would interfere with the turbulent curtain generated by the canards. This reduction of air stagnation inside the bay also helps pull more fresh air through the cooling system.
