Front speed enhancement by incompressible flows in three or higher dimensions

TL;DR

This paper investigates the structure of first integrals of incompressible flows in three or more dimensions and their role in linearly enhancing the speed of reaction fronts, revealing new classes of flows with ergodic components.

Contribution

It characterizes first integrals in higher dimensions, introduces flows with ergodic components, and links these to linear front speed-up under specific geometric conditions.

Findings

Existence of ergodic components with positive measure in certain flows.

Conditions under which large amplitude advection linearly speeds up KPP fronts.

New insights into the structure of first integrals in dimensions ≥3.

Abstract

We study, in dimensions $N\geq 3$, the family of first integrals of an incompressible flow: these are $H^{1}_{loc}$ functions whose level surfaces are tangent to the streamlines of the advective incompressible field. One main motivation for this study comes from earlier results proving that the existence of nontrivial first integrals of an incompressible flow $q$ is the main key that leads to a "linear speed up" by a large advection of pulsating traveling fronts solving a reaction-advection-diffusion equation in a periodic heterogeneous framework. The family of first integrals is not well understood in dimensions $N\geq3$ due to the randomness of the trajectories of $q$ and this is in contrast with the case N=2. By looking at the domain of propagation as a union of different components produced by the advective field, we provide more information about first integrals and we give a class of incompressible flows which exhibit `ergodic components' of positive Lebesgue measure (hence are not shear flows) and which, under certain sharp geometric conditions, speed up the KPP fronts linearly with respect to the large amplitude. In the proofs, we establish a link between incompressibility, ergodicity, first integrals, and the dimension to give a sharp condition about the asymptotic behavior of the minimal KPP speed in terms the configuration of ergodic components.