I am struggling to understand what is going on in the wheel well area. Lots of posts indicate there is a low pressure area there. This is supported by the dirt accumulation theory.
If true, why?
On one hand, Bernoulli would suggest that the pressure is higher than in the air flow passing outside it. ( ie. the simple idea is that the air in the wheel well is stationary WRT to the car while outside it is moving). Of course the simple explanation ignores the fact that the wheel is spinning, plus the fact that there is a pressure area under and inside the engine compartment.... but whether this is higher or lower.... again Bernoulli would suggest....
If the dirt theory is correct... I would expect that one side of the wheel would be dirtier than the other (is this the case?)... else where would the air be going? In other words, dirt would explain an airflow from the outside in, through the wheel slots.... then.... where? under and in? Around and out?
sorry for bringing this up but from what i know there is a high pressure zone( i think because it acts almost like a blunt front end) behind the wheel and a low pressure in front, and one of the things you can do is put a hole through the fenders to let that air excape or as if seen with Le Mans cars put in a duct for cooling something (i was thinking split water radiators for the engine for a car made from scratch)
Next time it rains, go for a drive on the freeway and observe the spray coming out of the front wheel well and front tire area. I've noticed a spray pattern that tells me the front of the front tires have high pressure, probably due to oncoming air hitting the front of the tire and being dispersed around it; up, down and around the oncoming tire.
I've also seen many race cars with vents on top of the wheel well to vent high pressure air and aid downforce. The Mazda RX7 from the mid 1990's and the Mitsubishi EVO X have vents inside the wheel well that channel air from the top of the wheel well to the low pressure area behind the front tire.
In most cars the wheel well area is not connected to the engine compartment and the airflow is basically around the fender giving rise to a negative pressure zone within the wheel well itself and hence the dirt shadows.
The original Mini took advantage of this and vented the radiator into the LHS wheel well. The engine bay was positively pressured and the airflow was enhanced as a result.
A minor aside: It also helped keep snow out of the wheel well area since there was a constant flow of warm air.
Some Citroen models (the DS and the GS) has vents to allow airflow from the engine compartment to the wheel well areas to aid aero and help cooling.