RESUMO
The advection of a passive substance by a turbulent flow is important in many natural and engineering settings. The concentration of such a substance can exhibit complex dynamic behaviour that shows many phenomenological parallels with the behaviour of the turbulent velocity field. Yet the statistical properties of this so-called 'passive scalar' turbulence are decoupled from those of the underlying velocity field. Passive scalar turbulence has recently yielded to mathematical analysis, and such progress may ultimately lead to a better understanding of the still intractable problem of fluid turbulence itself.
RESUMO
An important scientific and technological goal in the field of optical communications is the achievement of the clarity limit in optical fibres--that is, ensuring that the SiO2 glass from which fibres are made is as transparent as possible. The clarity of the wavelength transmission window (and the width of that window) in existing fibres is already sufficient to form the basis of a world-wide optical communication system, yet it is still limited by contamination of the fibre by water. Here we measure the spatial distribution of water in the glass rods from which optical fibres are drawn and explain the distribution quantitatively with a mathematical model of diffusion in a medium with essentially perfect cylindrical symmetry. Our analysis shows that the water enters from the outside of the rod and diffuses into the molten, flowing glass much faster than is expected from extrapolation of low-temperature measurements. Our elucidation of the physics underlying the contamination process has already led to the fabrication of dry fibres, which have a clarified and broadened communications window. The improved operational range of wavelengths should yield applications for new lasers, optical amplifiers and detectors, and should substantially increase the information-carrying capacity of optical communications systems.
