Controlling the unsteady analogue of saddle stagnation points

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It is well known that saddle stagnation points are crucial flow organizers in steady (autonomous) flows due to their accompanying stable and unstable manifolds. These have been extensively investigated experimentally, numerically, and theoretically in situations related to macroand micromixers in order to either restrict or enhance mixing. Saddle points are also important players in the dynamics of mechanical oscillators, in which such points and their associated invariant manifolds form boundaries of basins of attraction corresponding to qualitatively different types of behavior. The entity analogous to a saddle point in an unsteady (nonautonomous) flow is a timevarying hyperbolic trajectory with accompanying stable and unstable manifolds which move in time. Within the context of nearly steady flows, the unsteady velocity perturbation required to ensure that such a hyperbolic (saddle) trajectory follows a specified trajectory in space is derived and shown to be equivalent to that which can be obtained via a heuristic approach. An expression for the error in the hyperbolic trajectory's motion is also derived. This provides a new tool for the control of both fluid transport and mechanical oscillators. The method is applied to two examples-a four-roll mill and a Duffing oscillator-and the performance of the control strategy is shown to be excellent in both instances.

Original languageEnglish
JournalSIAM Journal on Applied Mathematics
Volume73
Issue number2
Pages (from-to)1038-1057
Number of pages20
ISSN0036-1399
DOIs
Publication statusPublished - 23.04.2013
Externally publishedYes

    Research areas

  • Flow control, Hyperbolic trajectory, Nonautonomous flows, Oscillator control, Saddle point, Stagnation point
  • Mathematics

DOI