doi:10.1017/S0022112009007666 Printed in the United Kingdom 1 Reactive Rayleigh–Taylor turbulence

The Rayleigh–Taylor (RT) instability develops and leads to turbulence when a heavy fluid falls under the action of gravity through a light one. We consider a model in which the RT instability is accompanied by a reactive transformation between the fluids. We study the model using direct numerical simulations (DNSs), focusing on the effect of the reaction (flame) on the turbulent mixing. We discuss ‘slow ’ reactions in which the characteristic reaction time exceeds the temporal scale of the RT instability, τ tinst. In the early turbulent stage, tinst t τ, effects of the flame are distributed over a maturing mixing zone, whose development is weakly influenced by the reaction. At t τ, the fully mixed zone transforms into a conglomerate of pure-fluid patches of sizes proportional to the mixing zone width. In this ‘stirred flame ’ regime, temperature fluctuations are consumed by reactions in the regions separating the pure-fluid patches. This DNS-based qualitative description is followed by a phenomenology suggesting that thin turbulent flame is of a single-fractal character, and thus distribution of the temperature field is strongly intermittent