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New aspects of turbulence are uncovered if one considers flow motion from theperspective of a fluid particle (known as the Lagrangian approach) rather thanin terms of a velocity field (the Eulerian viewpoint). Using a new experimentaltechnique, based on the scattering of ultrasounds, we have obtained a directmeasurement of particle velocities, resolved at all scales, in a fullyturbulent flow. It enables us to approach intermittency in turbulence from adynamical point of view and to analyze the Lagrangian velocity fluctuations inthe framework of random walks. We find experimentally that the elementary stepsin the 'walk' have random uncorrelated directions but a magnitude that isextremely long-range correlated in time. Theoretically, we study a Langevinequation that incorporates these features and we show that the resultingdynamics accounts for the observed one- and two-point statistical properties ofthe Lagrangian velocity fluctuations. Our approach connects the intermittentstatistical nature of turbulence to the dynamics of the flow.Comment: 4 pages, 4 figure

Long Time Correlations in Lagrangian Dynamics: A Key to Intermittency in Turbulence