Aerodynamics is the study of flow of air over any object. It is the study of dynamics of air. Any object which undergoes any motion has an aerodynamic effect in it. We can harness the power of air through aerodynamics. In general aerodynamics has a degrading feature which is called drag. 

Drag opposes the motion of the object or the vehicle. So, there will be more consumption of fuel by the engine in order to overcome this drag. The amount of drag increases with the speed. So car designers need to design cars in such a way that there should be a minimum effect of drag. There is also one effect of aerodynamic forces, which is lift. For the aircraft lift is useful, as it makes the aircraft fly in the sky. However, for the case of a car, it is not useful. The air which is blowing beneath the car can uplift the vehicle when the car is moving at a very high speed.

Air flow over car

So, with respect to a car we need to see both lift and drag. Nowadays cars are designed which are of aerodynamic design. Aerodynamic design means the car is designed to overcome both lift and drag forces or to utilize it for the enhancement of car performance. Cars are now made of streamlined shape. By streamline means there is a smooth flow of air over the car body. When there is a smooth flow of air and no restriction to it, then there is a very less creation of drag force. We have cars with door handles and wipers as well as flush headlights and glasses designed in such a way that it provides least resistance to the flow of air. So now in a car there is more concentration on its design with every detail. Similarly bumpers with spoilers are provided in order to channelize the flow of air beneath the car and create a downforce such that the car does not leave the ground at a very high speed. Similarly a rear wing is also present in racing cars in order to create a downward force at high speeds. These are adjustable which means it can move up and down in order to adjust and create the required downforce. 

Smooth car design

Earlier designs of the cars used to have a blunt body. A blunt body can create a huge amount of drag.  At lower maximum speed of cars this does not create a significant amount of drag, however at  high speed it can create substantial amount of drag and can affect the performance of cars. 

So this is how aerodynamics work on a car.