TerminologyAn object moving through a gas or liquid is dragged by a force pointing to the opposite direction of movement. If the object is moving through a gas (air) the drag force is called air resistance.
There is no change in the magnitude of the drag force if the object is moving through the air or the air is flowing around the resting object. Like explained at the chapter about velocity, movement is always relative to a (fixed) point. For measurements, the last mentioned method is commonly used. Cars (or scaled models of cars) are placed inside a wind tunnel, where at the resting vehicle is flowed by air of the accordant speed. You have to differentiate between linear and turbulent flow. Linear flow means all particles (air molecules) are moving in parallel to the surface of the object. In this case the drag force is dependent from the viscosity of the gas or fluid. If the particles of the medium are moving crossway's, perpendicular or partly contrary to the main direction, the flow is called turbulent. The drag caused by turbulence is dependent from the viscosity and density of the air as well as the shape and the surface properties of the object.
In practiceLinear flow takes place just at a few specially shaped objects. The wings of airplanes are designed to avoid turbulent flow. The surfaces have to be designed without edges or corners. As far as no car is built without edges, turbulences are inevitable. The computation of turbulent flow is very complex. Things that can't be calculated are captured by measurements. A complete car or just a scaled model of it is placed on a platform, equipped with force sensors, inside a wind tunnel. Purpose of the measurements is the
drag coefficientThe drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object moving through air or a fluid. Knowing the drag coefficient, the drag force can be calculated by using the formula:
Fd - drag force, ρ - mass density of the air, CW - drag coefficient
A - reference area perpendicular to the direction of movement, v - flow velocity
Drag of a carThe formula shown above, clarifies that the drag force is primely dependent from the shape of the car (A and CW) and the driving speed (v). The air density varies insignificantly within the temperature. At a pressure of 1013,25 hPa it is 1.395kg/m3 with -20°C and 1.146kg/m3, with +35 °C. Finally this is an environmental factor we can't alter while driving a car. Things we can influence are the shape of the car (CW-value and reference area), as well as the speed we are driving with. The drag coefficient is lower the more drop-shaped the car is designed. The reference area becomes lower, the smaller we design our car. Uncongenial the drag force grows quadratic with the driving speed. You have to overpower the quadruple force to move with 100km/h instead of 50km/h.
At the next chapter we will examine the correlations between work, energy and power. There you will get to know, why rolling friction and air resistance are such unpleasant symptoms for car designers.