Experimental study of chaotic advection regime in a twisted duct flow

The generation of Lagrangian chaos has been studied experimentally in a twi sted duct flow, a configuration representing a three-dimensional steady ope n 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 pertu rbation due to the change in curvature plane, are the source of the irregul ar trajectories of the fluid particles. The Eulerian nature of the flow was investigated using a laser Doppler velo cimeter (LDV). From the Eulerian point of view, the flow is completely regu lar. We show by laser-induced fluorescence (LIF) visualization that many charact eristics of a Lagrangian chaotic system are present in this flow: - strong stretching and folding of material lines and surfaces; - sensitivi ty to initial conditions; - exponential growth of stretching in some flow r egions. 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 d uring their passage through the twisted duct. (C) 2001 Editions scientifiqu es et medicales Elsevier SAS.