Expert: Mark Janus - 5/1/2007

Question
which is more aerodynamically efficient, car driving close to a wall or in the open? and would head/cross winds affect the results?I'm working for a motoracing team and boss always tells the drivers to run close to the pitwall because 'the air is cleaner' so there will be less drag.  

Answer
The aerodynamics of the situation is a bit complicated, but I have read that the car will experience a slight increase in drag along with an increase in downforce.
The wall basically becomes a plane of symmetry for the vehicle.  By this I mean because the flow can not cross through the wall it acts as a streamline and thus the appearance is though another car is on the other side of the wall (in a mirrored fashion).  As the car approaches the wall I would expect there to be a slight side force (probably drawing the car closer to the wall) owing to a venturi effect. Also there would be probably be a little more sideways flow off of the roofline (also due to the lowered pressure on the wall side of the wehicle) which may cause slightly more separation near the deck lid...

A headwind wouldn't change anything, but a crosswind would.  This is an interesting topic....   

I'm trying to answer you quickly, but I'll try to setup and run a quick CFD demo of a wedge (simulating the car) next to a wall and e-mail it to you if you drop me your e-mail