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.