Aerodynamics of passenger vehicles - bluff bodies

Artikel i övrig tidskrift, 2012

Vehicle manufacturers are today struggling to develop more energy efficient vehicles that will meet future emissions targets of CO2 (Carbon Dioxide). To do so it is essential to improve both efficiency of driveline and reduce resistance forces of all kind. The aerodynamic drag force is the single biggest resistance force for a passenger car to overcome at highway speeds and it is a significant part of the total resistance forces that the vehicle has to overcome in average use, city and highway driving. Since a reduction in fuel consumption directly will lead to a reduction of CO2 the best way is to reduce the energy required for propulsion of the system. Aerodynamic drag force can be divided into two parts, pressure and friction. Pressure forces on the exterior body of a passenger car dominate the total drag force. A rule of thumb says that 80-90% of the aerodynamics drag forces of a passenger car are pressure forces and the rest is friction. For a finite wing for instance we will have the opposite, about 95% friction and 5% pressure forces. When the pressure forces are the dominating part of the aerodynamic drag we use to call the body bluff, and if the friction forces are dominating we call it a streamlined body. This then, classifies a passenger car aerodynamically as a bluff body. The biggest pressure forces are associated to wake formations at the rear end of a bluff body.