Schlieren photography is very much used in wind tunnels to show pressure areas and turbulence. It has been used by aircraft and auto companies using scale and life size models to develop the best aerodynamic shape. I first saw it used on a school trip back in 1970 at a NASA windtunnel. Google it and lots of sites are there.
But now it is easier to use CFD computer modeling instead of having to build an actual model or prototype.
Just yesterday I started a thread about we ecomodders doing it for ourselves. Building a model database to test various aerodynamic modifications for validity before possibly wasting money on something that does not work. Model it and then build it right one time.
See:
http://ecomodder.com/forum/showthrea...gram-9944.html