Aerodynamic drag is the restraining force that acts on any moving body in the direction of the freestream flow. From the body's perspective (near-field approach), the drag comes from forces due to pressure distributions over the body surface, symbolized Dpr, and forces due to skin friction, which is a result of viscosity, denoted Df. Alternatively, calculated from the flowfield perspective (far-field approach), the drag force comes from three natural phenomena: shock waves, vortex sheet and viscosity.
The pressure distribution over the body surface exerts normal forces which, summed and projected into the freestream direction, represent the drag force due to pressure Dpr. The nature of these normal forces combines shock wave effects, vortex system generation effects and wake viscous mechanisms all together.
When the viscosity effect over the pressure distribution is considered separately, the remaining drag force is called pressure (or form) drag. In the absence of viscosity, the pressure forces on the vehicle cancel each other and, hence, the drag is zero. Pressure drag is the dominant component in the case of vehicles with regions of separated flow, in which the pressure recovery is fairly ineffective.
The friction drag force, which is a tangential force on the aircraft surface, depends substantially on boundary layer configuration and viscosity. The calculated friction drag Df utilizes the x-projection of the viscous stress tensor evaluated on each discretized body surface.
The sum of friction drag and pressure (form) drag is called viscous drag. This drag component takes into account the influence of viscosity. In a thermodynamic perspective, viscous effects represent irreversible phenomena and, therefore, they create entropy. The calculated viscous drag Dv use entropy changes to accurately predict the drag force.
When the airplane produces lift, another drag component comes in. Induced drag, symbolized Di, comes about due to a modification on the pressure distribution due to the trailing vortex system that accompanies the lift production. Induced drag tends to be the most important component for airplanes during take-off or landing flight. Other drag component, namely wave drag, Dw, comes about from shock waves in transonic and supersonic flight speeds. The shock waves induce changes in the boundary layer and pressure distribution over the body surface. It is worth noting that not only viscous effects but also shock waves induce irreversible phenomena and, as a consequence, they can be measured through entropy changes along the domain as well. The figure below is a summary of the various aspects previously discussed.
Automobile drag coefficient
Tatra T77 maquette by Paul Jaray, 1933
The drag coefficientis a common metric in automotive design pertaining to aerodynamic effects. As aerodynamic drag increases as the square of speed, a low value is preferable to a high one. As about 60% of the power required to cruise at highway speeds is used to overcome aerodynamic effects, minimizing drag translates directly into improved fuel efficiency.
For the same reason aerodynamics are of increasing concern to truck designers, where greater surface area presents substantial potential savings in fuel costs.
Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. However, there are other important aspects of aerodynamics that affect cars designed for high speed, including racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car becoming airborne. Increasing the downforce pushes the car down onto the race track—allowing higher cornering speed. It is also important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, i.e. flipping over, when hitting a bump or experiencing turbulence from other vehicles (most notably the Mercedes-Benz CLR). For best cornering and racing performance, as required in Formula One cars, downforce and stability are crucial and these cars must attempt to maximize downforce and maintain stability while attempting to minimize the overall Cd value.
Typical drag coefficients
The average modern automobile achieves a drag coefficient of between 0.30 and 0.35. SUVs, with their typically boxy shapes and larger frontal area, typically achieve a Cd of 0.35–0.45. A very gently inclined windshield gives a lower drag coefficient but has safety disadvantages, including reduced driver visibility. Certain cars can achieve figures of 0.25–0.30, although sometimes designers deliberately increase drag to reduce lift.
Some examples of Cd follow. Figures given are generally for the basic model. Some "high performance" models may actually have higher drag, due to wider tires and extra spoilers.
0.7 to 1.1
typical values for a Formula One car (downforce settings change for each circuit)