Suppose you are standing beside a road. A bus is running on the road, when it is crossing you, you feel a push of wind. Why its happen? Is there any mathematical relation?
As a bus passes you at speed, it displaces the air that occupied the volume which the bus fills. The displaced air flows past the bus, creating a high pressure area near the bus which pushes you away. After the midpoint of the bus has passed you, the displaced air starts to flow into the area vacated by the bus as it leaves. This creates an area of low pressure which tends to draw you toward the bus as it leaves.
Aerodynamics is the study of airflow, especially in relation to objects moving through the air. Vehicle aerodynamics specifically describes the effect on autos, trucks, and buses of air which they displace. Here is a reference that explains some of the aerodynamic factors that are considered, and the mathematical equations used to describe them: http://www.kasravi.com/cmu/tec452/aerodynamics/vehicleaero.htm