Chaotic advection and relative dispersion in an experimental convective flow

Lagrangian motion in a quasi-two-dimensional, time-dependent, convective fl ow is studied at different Rayleigh numbers. The particle tracking velocime try technique is used to reconstruct Lagrangian trajectories of passive tra cers. Dispersion properties are investigated by means of the recently intro duced finite size Lyapunov exponent analysis. Lagrangian motion is found to be chaotic with a Lyapunov exponent which depends on the Rayleigh number a s Ra-1/2. The power law scaling is explained in terms of a dimensional anal ysis on the equation of motion. A comparative study shows that the fixed sc ale method makes more physical sense than the traditional way of looking at the relative dispersion at fixed times. (C) 2000 American Institute of Phy sics. [S1070-6631(00)00112-4].