Stretch and Fold

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Science  05 Jul 2002:
Vol. 297, Issue 5578, pp. 17
DOI: 10.1126/science.297.5578.17c

Mixing in fluid systems is normally associated with turbulent flows, where the high energy of the system causes neighboring fluid elements to diverge. However, mixing can be produced in less energetic, time-periodic systems by introducing chaos into the agitation pattern, which causes stretching and folding of the fluid elements. Voth et al. have monitored experimentally the local flow dynamics in two dimensions, using approximately 800 fluorescent particles to track the fluid elements. From the trajectories of the particles, a Poincaré map of the local velocities can be constructed; of particular interest are the elliptic (circles) and hyperbolic (crosses) fixed points where there is no net fluid motion. Introducing a fluorophore revealed regions where the fluid elements are stretched, and, by analyzing snapshots, the authors are able to correlate lines of stretching with the elliptic fixed points. — MSL

Phys. Rev. Lett.88, 254501 (2002).

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