Abstract
The turbulent diffusion of Lagrangian tracer particles has been studied in a flow on the surface of a large tank of water and in computer simulations. The effect of flow compressibility is captured in images of particle fields. The velocity field of floating particles has a divergence, whose probability density function shows exponential tails. Also studied is the motion of pairs and triplets of particles. The mean-square separation ⟨Δ(t)2⟩ is fitted to the scaling form ⟨Δ(t)2⟩ ∝ tα, and in contrast with the Richardson-Kolmogorov prediction, an extended range with a reduced scaling exponent of α = 1.65 ± 0.1 is found. Clustering is also manifest in strongly deformed triangles spanned within triplets of tracers.