Today's New York Times has an article and slideshow featuring this visualization method. I'm fascinated by images showing fluid mechanics, and those using this technique are beautiful, even haunting. I'm not sure if this method can be used in water (all these images are in air), but I'd like to try.
Essentially, the Schlieren photography shows variations in air density and thus turbulence, shock waves and other fluid phenomenon.
The method is complicated, and I don't fully understand it. Here's a good link (from which the diagram was taken).
Labels: fluid mechanics, NY Times, photography, Schlieren