FRONT PROPAGATION IN LAMINAR AND TURBULENT FLOWS

The problem of front propagation in a stirred medium is addressed
in the case of laminar and turbulent flows in three different regimes:
slow reaction, fast reaction and geometrical optics limit. It is well
known that a consequence of stirring is the enhancement of front speed
with respect to the non-stirred case. By means of numerical
simulations and theoretical arguments we describe the behavior of
front speed as a function of the stirring intensity. The large scale
of the velocity field mainly rules the front speed behavior even in
the presence of smaller scales. In the unsteady (time-periodic) case,
the front speed displays a phase-locking on the flow frequency and,
albeit the Lagrangian dynamics is chaotic, chaos in front dynamics
only survives for a transient. Asymptotically the front evolves
periodically and chaos manifests only in the spatially wrinkled
structure of the front.