Most people are familiar with the notion of turbulence. Whether it is the chaotic swirls that appear as you add milk to your morning coffee, the branching and twisting of cigarette smoke that causes it to linger in front of your face, or the unpredictable motion of the atmosphere all too familiar to frequent fliers. However, despite this ubiquity, it is exceptionally hard to pin down in precise mathematical terms, with the most successful descriptions simple rules-of-thumb, derived either empirically or through dimensional analysis.
Subsonic turbulence – like the random and chaotic motions of air and water in our everyday lives – is something we have only a limited understanding of. Our knowledge of supersonic turbulence, where shock waves and compressibility rule, is even more tenuous. In part this is because, although we can observe snapshots of supersonic turbulence in astronomical settings like the Orion Nebula shown above, we cannot watch it evolve. On these scales, features simply don’t change appreciably on human timescales.
This has limited scientists to mostly numerical and theoretical studies of supersonic turbulence, but that is starting to change. Researchers are now building experimental set-ups that collide laser-driven plasma jets to generate boundary-free turbulence at Mach 6. Thus far, the observations are consistent with what’s been seen in nature: at low speeds, the turbulence is consistent with Kolmogorov’s theories, with energy cascading from large scales to smaller ones predictably. But as the Mach number increases, the nature of the turbulence shifts, moving toward the large density fluctuations seen in nebulae and other astrophysical realms.