How air dam reduce drag and lift?

Question

Few questions about air dam(flexibile vertical plate on front bumper)

1. What is main purpose of air dam in car industry, reduce drag or reduce lift(increase downforce)?

2. How dam reduce drag if : a)increase frontal area, b)increase stagnation pressure at the front of car, c)cause high
   turbulent air behind dam, d) decrease air speed under the car(if gap is zero)?

3. Air dam or flat undertray, does air dam has sense when car has flat smooth undertray?

4. Is better to put dam all the way to the road or must have some gap to work?

5. Make sense to install front dam and side skirts with zero gap(zero lekage) and leave back open to achieve maximum suction under the car?

Please dont use simple jurnalist explanation, I want scientific proves how air dam works, maybe some CFD-study or wind tunnel testing or so...

Answer 1

Depending on exactly how the entire system is designed, it reduces drag and/or reduces lift.

The undercarriage isn't faired, so reducing air flow through the bottom of the car reduces drag. You will see that modern cars have plastic panels under the car to help this.

If air is properly directed away from the underside it can reduce lift that a lot of cars see at higher speeds. This is all extremely complicated and is affected by airflow into & out of the engine compartment. Doing this stuff on your own without a wind tunnel is quite likely to give you an opposite effect than intended.