Front propagation in a laminar cellular flow: Shapes, velocities, and least time criterion

Abstract

We experimentally investigate the propagation of chemical fronts in steady laminar cellular flows at large Péclet numbers and large Damköhler numbers. Fronts are generated in an aqueous solution by an autocatalytic oxydoreduction reaction. They propagate in a channel in which a chain of counter-rotative parallel vortices is induced by electroconvection. We first accurately determine the form, the dynamics and the mean velocity of these fronts in the whole Hele-Shaw regime of the flow. We then address the modeling of the evolution of their mean velocity with the flow amplitude. The structure of the front wakes yields us to reject an effective reaction-diffusion wave as a relevant model for large-scale front propagation. On the other hand, analysis of the role of front heads brings us to introduce a kinematic model at the vortex scale for uncovering the front dynamics. This model addresses the propagation of the front leading point in a chain of vortices whose field is modeled by a two-dimensional solid rotation complemented by a boundary layer. Interestingly, it sensitively relies on the effective trajectory followed by the front leading point. To account for this, a competition is worked out among a one-parameter family of potential trajectories. The actual trajectory is then selected as the fastest one with quite a good agreement with measurements and observations. In particular, the measured effective front velocities are well recovered from the model, including their intrinsic dependence on the boundary layer width. Accordingly, effective front propagation in a laminar steadily stirred medium is thus understood from an optimization principle similar to the Fermat principle of ray propagation in heterogeneous media.

Frozen reaction fronts in steady flows: A burning-invariant-manifold perspective

“…From here on, we use the expression "wind" V w to refer to either the translational speed of the vortices in the lab frame or the speed of the uniform wind in the vortex reference frame. The propagation of a reaction front in the alternating vortex flow in the absence of an imposed wind has been discussed in detail in previous papers [25,26,29,30]. The reaction front is carried around each vortex with the flow and "burns" across the separatrix from one vortex to the next, resulting in long-range propagation that is significantly faster than the reaction-diffusion speed V 0 in a static fluid.…”

Chemical Front Propagation in Periodic Flows: FKPP versus G

“…This is used as a testbed in numerous experimental studies of advection-diffusionreaction (e.g., [37,40,3,32,27]). The classic cellular flow introduced in [38] corresponds to a zero mean velocity U = 0 and to A = 0.…”

Front propagation in cellular flows for fast reaction and small diffusivity

“…2 for c = 0.5). Using (10) gives the integrand in (9) as (cx − x cos y) 2 ≈ 16 exp (−π/c) cosh −2 (σ/c), leading to G (c) ∼ 4 × (2/π)ce −π/c , where c 1 (11) and the factor 4 appears because, for σ ∈ [0 2π], there are 4 regions that are similar to region 1. Inverting (11) and using (8) finally gives…”