# Aerodynamics- Theory

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This topic is intended as a guide to the theoretical side of aerodynamics. It seeks to define and explain the basic concepts which help in understanding and improving vehicle aerodynamics. This guide will also aim to progress in order of complexity; beginning with the basics of aerodynamics and progressing through to the more difficult and intellectually challenging aspects so as to appeal to readers of various levels.

Drag: As a vehicle moves along the road, its motion is resisted by the air it is moving through. This resistance to forward motion is known as drag (also know as wind-resistance or air-resistance). If you have ever stuck your hand out the side window and had your arm forced back you have felt the force of drag. Drag is made up of friction forces (between the air and the sides of the vehicle) and pressure forces (a difference in air pressure on either side of an object result in force). In a conventionally shaped car the major contribution is from pressure drag.The force of drag acts in the net direction of airflow, this is usually in direct opposition to the direction of travel (like when your arm hand is blown backwards) but may not be as is it is dependent on the direction of crosswinds (a sailing boat actually uses drag to move forwards). Except in especially favorable winds, drag is always parasitic that is it drains the cars energy without doing anything particularly useful. Hence our goal in building efficient electric cars is to minimize drag whenever possible.

Lift:
Lift is the force on a solid in the direction perpendicular to its motion as it moves through a fluid. Aeroplanes use their (solid) wings as they move through the air (a fluid) to produce a vertical force upward (perpendicular to their horizontal motion). In reference to EVs (and all ground vehicles) lift is the vertical force, either as lift or 'down-force', which is produced by the flow of air around the car. Lift is produced by the deflection of air from its direction of flow; while the principle is simple to understand the physics behind it are somewhat complex (see wikipedia). For any car where efficiency is a priority the net lift on the vehicle should be as close to zero as possible since lift is a form of drag. For cars that require high speed cornering abilities, lift in the form of down-force is desirable as it increases the grip available to the wheels.

Attached Flow:
Attached flow occurs when the path of air flowing past an object directly follows the outer surface of the object without detaching (moving away) from the surface. Generally speaking whenever the airflow separates from the surface of the object, which is known as detached flow, the drag on the object is increased. Therefore when considering the aerodynamics of a vehicle it is important to maintain attached flow over as much of the vehicle as possible to reduce drag.

Boundary Layers:
The boundary Layer is the strata of air which is directly adjacent to the skin of your vehicle,at the boundary of flow. If you imagine the air as a liquid then the boundary layer is the small layer that is making your car 'wet' (interestingly the surface area of the vehicle exposed to the air is called the wetted area).
Pressure Differences: Information needed...

References/resources:
Overview in Fuel Saving Forum
Wikipedia Article
Brilliant discussion of the basics of aerodynamics
Presentation by Phil Knox