Accurate simulation of pulled and pushed fronts in the nonautonomous Fisher-KPP equation

TL;DR

This paper presents a new numerical method for accurately simulating front propagation in the nonautonomous Fisher-KPP equation, capturing complex dynamics and improving velocity measurement precision across various regimes.

Contribution

The authors develop a novel boundary-coupled numerical approach that accurately simulates nonautonomous Fisher-KPP fronts, including pulled and pushed types, on small domains.

Findings

Improved front velocity accuracy in simulations.

Revealed deviations from linear theory in time-dependent diffusion.

Demonstrated adiabatic following of velocity in slowly varying parameters.

Abstract

We introduce a novel numerical method for direct simulation of front propagation in the Fisher-KPP equation with a time-dependent parameter on an infinite domain. The method computes a time-dependent boundary condition that accurately captures the leading-edge dynamics by coupling the nonlinear simulation region to a linear approximation region in which the dynamics can be solved exactly via the Green's function of the linearized equation. This approach enables precise front velocity measurements on relatively small computational domains for a variety of nonautonomous regimes and initial conditions for which existing numerical methods break down. We apply the method to pulled and pushed fronts in the Fisher-KPP equation with quadratic and quadratic-cubic nonlinearities, finding that it improves the accuracy of the simulated front velocity even for constant parameters and a fixed domain size. For pulled fronts with a diffusion coefficient that increases algebraically in time, our results reveal a deviation from the natural asymptotic velocity predicted by linear theory, whose explanation requires nonlinear theory. For pushed fronts with constant parameters, the method reproduces the exponential convergence to the theoretical asymptotic front speed and profile with improved precision. For a slowly time-varying linear growth parameter, we find that the pushed front velocity follows the changing parameter adiabatically if the asymptotic pushed velocity remains faster than the natural asymptotic pulled velocity. As the growth parameter moves toward the pushed--pulled transition point, the competition between the pushed and pulled fronts can result in both delayed and even premature onset of the pushed--pulled transition, depending on the form of parameter growth. The numerical method presented here proves to be an effective tool for analyzing front propagation in nonautonomous systems.