Experimental study of chaotic advection regime in a twisted duct flow

The generation of Lagrangian chaos has been studied experimentally in a twisted duct flow, a configuration representing a three-dimensional steady open flow in which various signatures of Lagrangian chaos are documented. The twisted duct consists of four 90 degrees bends of square cross-section; the plane of curvature of each bend is at 90 degrees to that of its neighbors. Dean roll-cells, generated by centrifugal forces and the geometrical perturbation due to the change in curvature plane, are the source of the irregular trajectories of the fluid particles. The Eulerian nature of the flow was investigated using a laser Doppler velocimeter (LDV). From the Eulerian point of view, the flow is completely regular. We show by laser-induced fluorescence (LIF) visualization that many characteristics of a Lagrangian chaotic system are present in this flow: - strong stretching and folding of material lines and surfaces; - sensitivity to initial conditions; - exponential growth of stretching in some flow regions. It is also shown that in some regions of the flow stretchings grow linearly with space, indicating non-chaotic behavior. Due to the chaotic nature of the flow, an equalization of the fly-rime of fluid particles was observed during their passage through the twisted duct. (C) 2001 Editions scientifiques et medicales Elsevier SAS.