Mechanisms driving shape distortion in two-dimensional flow

In order to elucidate the physical processes governing the evolution of material areas in complex flow, we study the shape dynamics of three-point Lagrangian clusters in an experimental quasi-two-dimensional flow. By comparing our measurements with simulations of triangles evolving purely diffusively, we show that the path taken by the mean triangle shape through a suitably defined phase space is indicative of the underlying flow dynamics. We demonstrate the existence of organizing curves in shape space for the evolution of triangles with different initial shapes. Our results suggest a detailed, multi-step process governing the shape dynamics of clusters in complex flow. Copyright (C) EPLA, 2011