Relatore: Prof. Angelo Vulpiani
UniversitÓ di Roma "La Sapienza"

Aula Newton
20 Marzo 2002 - Ore 16.00


Front propagation in two dimensional steady and unsteady 
cellular flows is investigated in the limit of very fast 
reaction and sharp front, i.e. in the geometrical optics 
limit. In the steady case, by means of a simplified model, 
we provide an analytical approximation  for the front
speed, $v_f$, as a function of the stirring intensity, U, 
in good agreement with the numerical results.
The main contribution to $v_f$, comes from the large 
scale dynamics and, for sufficiently high
U-values, $v_f \sim U/\ln U$ closely resembling the 
behavior proposed for turbulent flows.
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 
dynamics is periodic and chaos manifests only in the 
spatially wrinkled structure of the front.