Put a light fluid underneath a heavy one and the interface between them goes unstable: bubbles rise, spikes fall, and small wrinkles merge into larger ones. That merging, an inverse cascade in wavenumber, is what makes the classical Rayleigh–Taylor mixing layer forget its initial conditions and settle into self-similar growth. We wanted to know whether a burning interface forgets in the same way.
It does not, or at least not straightforwardly. A reacting front can stabilize itself: perturb a model flame with a single mode and it stops growing, locking into a steady traveling wave that can persist almost indefinitely. We perturbed it with two instead, a large-amplitude primary mode k1 and a smaller secondary mode k2, and looked at what it takes to break that stalemate.
Early on the two modes ignore each other and the flame propagates as a metastable traveling wave. Once the secondary mode has grown large enough they couple, the traveling wave is destabilized, and the bubbles take off. What the coupling produces is a long-wavelength mode whose wavenumber is the greatest common divisor of the two you started with.
