Turbulent times for fluids
Feb 21, 2001
Physicists in the US have borrowed technology normally used in high-energy physics to gain a better picture of turbulence - a phenomenon that is still not well understood. Eberhard Bodenschatz and colleagues at Cornell University found that particles in swirling fluids undergo a flabbergasting range of accelerations within extremely short distances and times (A La Porta et al 2001 Nature 409 1017).
Bodenschatz and co-workers had studied turbulence for several years using conventional methods based on tracking 'tracer particles' in the fluid. But it was impossible to accurately track particle motion in highly turbulent conditions because the method only revealed changes in acceleration over relatively long periods. "We were about to call it quits", Bodenschatz told PhysicsWeb, "when I spoke to Jim Alexander at the Cornell Electron-Positron Collider". The collider at Cornell uses arrays of light-sensitive silicon strips capable of gathering up to 70 000 images every second. Bodenschatz and co-workers adapted the system for their experiment and were delighted to find that it could image particle motion with vastly improved time resolution.
Bodenschatz's team studied highly turbulent flows with Reynolds numbers - a measure of turbulence - of up to 63 000. The Reynolds number is related to the density and velocity of the fluid, and a value of 2000 marks the transition from streamline or 'laminar' flow to turbulence. In the new set-up, three arrays of silicon detectors image the motion of tracer particles in each dimension, and a computer program calculates the three-dimensional acceleration measurements. The team was surprised by the enormous range of accelerations they found, from zero to 12 000 m s-2. Statistically, this is an increase from zero to 30 times greater than the root mean square velocity and back again - all within fractions of a millisecond and hundreds of micrometres.
Studies of turbulence underpin our understanding of cloud formation, atmosphere and pollution transport as well as numerous industrial processes involving chemical mixing and combustion. "On the academic side, however, we still have no understanding of the universal properties of turbulence", says Bodenschatz, "but such accurate particle tracking opens up a whole new avenue".
Bodenschatz adds that these extremely chaotic conditions may already be well understood - by mosquitoes, which are known to cling perilously to blades of grass when the wind exceeds a certain speed, presumably to avoid being swept up in the turbulent atmosphere.