I'm new to the forum, please forgive if this is a re-post (chances are it may be).
Surfing around I found a nice article about GM's Wind Tunnel in Warren, MI.:
General Motors' wind tunnel turns 30, more important than ever
The article was written around the tunnel's 30th anniversary in 2010 and demonstrates what an ENORMOUS undertaking it is to design and construct such a facility.
There are also a bunch of nice high-res photos taken inside the tunnel, a bunch of which demonstrate smoke trail testing on a Cruze Eco. These images might be of some interest to people looking to understand how air moves around a vehicle at speed.
I spent several nights/mornings working in this tunnel when I worked for a supplier. I held a GM Design Engineer position for an integrated sealing products supplier and was present in the tunnel for several tests to validate changes to our products, making sure the changes did not negatively impact wind noise. The tunnel was booked solid 7 days a week, and all of my parts were tested between 2am and 6am, and scheduling was done several weeks to months in advance.
The acoustic insulation inside the test chamber is really impressive... it is really eerie to be standing there with a light breeze blowing in absolute silence, and when you speak there are no detectable sounds reflected from the walls.
For those who haven't seen this article before, enjoy!