Visualized Turbulence (Image 2) The distribution of vorticity in developing Mach 1 turbulence, as computed with the PPM gas dynamics code on the TeraGrid cluster at the National Center for Supercomputing Applications (NCSA) in 2003 using a computational grid of over 8 billion cells. This series of images shows the vorticity as it evolves through the transition of fully developed turbulence. The vorticity, which measures the amount of shear in the flow, highlights thin surface-like regions in the flow across which the flow speed changes very rapidly. Later in the development of the turbulence, these sheets of vorticity roll up to form a large number of vortex tubes or filaments, which, especially given the high flow speeds here, are somewhat akin to tornadoes.

http://www.nsf.gov/news/mmg/mmg_disp.cfm?med_id=62873&from=mmg