Watching turbulence

With a particle-physics detector. There are two distinct ways to observe fluid flow: In the Eulerian approach, one watches the fluid flow past a specific point in space, whereas in the Lagrangian approach, one follows a specific fluid element as it is carried along. The Lagrangian view is much favored by theorists who model such phenomena as fluid mixing or the dispersal of contaminants, but laboratory data of the Eulerian type have been much easier to obtain. Now, a group at Cornell University led by Eberhard Bodenschatz and Jim Alexander has brokered an experimental marriage between fluid dynamics and particle physics. The physicists modified silicon-strip detectors from Cornell’s electron–positron collider to make them work as optical-imaging elements able to track minute tracer particles being buffeted about in all three dimensions within a turbulent fluid. With that setup, the researchers could track the particles’ positions with 0.7-µm accuracy, at up to 70‥000 frames per second. The Lagrangian data showed that a particle’s acceleration was highly intermittent: It could vary from 0 to 12‥000 m/s² and back to 0 in a fraction of a millisecond and within a few hundred microns. The group found that highly turbulent fluids, with Reynolds numbers up to 63‥000, agreed well with predictions based on the scaling theory put forth in 1941 by Andrei Nikolayevich Kolmogorov. The experimental technique can be used to test recent theoretical predictions related to the geometry of turbulence. (A. La Port et al., Nature 409, 1017, 2001 http://dx.doi.org/10.1038/35059027 .)