Fluid flow is ubiquitous in nature and in technological processes. It is observed in scales ranging from the very large (cyclones in the atmosphere, ...) to the very small (blood flow in microscopic capillaries, bacterial swimming, ...). The research activities of DFI-IMEDEA during 1999 in this field addressed two fronts: on the one hand we studied basic processes in transport by fluid flows, namely in the context of chaotic advection; on the other hand, applications to oceanographic flows and to the transport of biologically active substances in the sea were developed.

The figure displays particle trajectories leading to global diffusion induced by resonances in a flow between two concentric spheres.

Fluid motion transports particles and substances contained in the fluid. The efficiency of mixing processes strongly depends on the characteristics of the fluid motion.  Chaotic advection is a phenomenon by which simple flow patterns can produce very complex trajectories for the particles suspended in it.    We have obtained novel results for properties of chaotic advection in threedimensional flows, and published a review on the subject (Cartwright, Feingold and Piro, An Introduction to Chaotic Advection, in the book Mixing: Chaos and Turbulence, edited by Chaté, Villermaux and  Chomaz, Kluwer, (1999).

 
It is a long standing observation that, in the absence of strongest forcings such as wind stress or instabilities, marine currents tend to follow topographic features. Although several mechanisms have been proposed to explain this fact, a detailed understanding was still lacking.
 
 

We have identified a mechanism that explains the observations of marine currents associated to topographic features (submarine mountains or valleys). It is a kind of `noise rectification’ mechanism related to the one appearing in other fields of science, The direction of the currents generated is determined by the statistical properties of the small scales of turbulence  A. Álvarez, E. Hernández-García, J. Tintoré, Phys. Lett. A 261, 179 (1999); Geophys. Res. Lett. 27, 739 (2000).

An eddy appears on top of a submarine mountain as a result of the interaction between turbulence and the topography. The rotation sense of the eddy changes from anticyclonic (top) to cyclonic (bottom) as the proportion of small scale turbulence is increased.