Dispersion in freely decaying and forced 2D turbulence
We report experimental results obtained on freely decaying and forced two-dimensional turbulence. The flow is produced in a thin stratified layer of electrolyte, using an electromagnetic forcing. The velocity and vorticity fields are measured using a particle image velocimetry (PIV) technique. The dispersion of passive particles is studied by numerical computation of lagrangian trajectories from the experimentally obtained velocity fields. We find that the properties of the decaying and forced systems are completely different. In the decaying case, the passive particles move hyperdiffusively, with an exponent identical to that obtained for the vortex motion (Hansen et al. 1998) and their dispersion appears to be controlled by Lévy flights. In the forced case, in the presence of an inverse cascade range with k −5/3 scaling, the dispersion of pairs of passive particles is found to be controlled by Richardson law (Richardson 1926) throughout the inertial range of scales revealed by the analysis of the flow field. Moreover, we find that the probability density functions of pair separations at times corresponding to the Richardson scaling law are self-similar and that their shape is a stretched exponential, thus suggesting that the relative dispersion is not controlled by Lévy flights.
KeywordsProbability Density Function Particle Image Velocimetry Vorticity Field Inertial Range Passive Particle
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- Hansen, A.E., Marteau, D. and Tabeling, P. (1998): Two-dimensional turbulence and dispersion in a freely decaying system, submitted to Phys. Rev. EGoogle Scholar
- Paret, J. and Tabeling, P. (1998): Intermittency in the 2D inverse cascade of energy: experimental observations, submitted to Phys. FluidsGoogle Scholar