  New aspects of turbulence are uncovered if one considers flow motion from the
perspective of a fluid particle (known as the Lagrangian approach) rather than
in terms of a velocity field (the Eulerian viewpoint). Using a new experimental
technique, based on the scattering of ultrasounds, we have obtained a direct
measurement of particle velocities, resolved at all scales, in a fully
turbulent flow. It enables us to approach intermittency in turbulence from a
dynamical point of view and to analyze the Lagrangian velocity fluctuations in
the framework of random walks. We find experimentally that the elementary steps
in the 'walk' have random uncorrelated directions but a magnitude that is
extremely long-range correlated in time. Theoretically, we study a Langevin
equation that incorporates these features and we show that the resulting
dynamics accounts for the observed one- and two-point statistical properties of
the Lagrangian velocity fluctuations. Our approach connects the intermittent
statistical nature of turbulence to the dynamics of the flow.
