The Rayleigh-Taylor Instability driven by an accel-decel-accel profile

We describe numerical simulations of the miscible Rayleigh-Taylor (RT) instability driven by a complex acceleration history, g(t), with initially destabilizing acceleration, g $>$ 0, an intermediate stage of stabilizing deceleration, g $<$ 0, and subsequent destabilizing acceleration, g $>$ 0. Initial perturbations with both single wave-number and a spectrum of wave-numbers (leading to a turbulent front) have been considered with these acceleration histories. We find in the single-mode case that the instability undergoes a so-called phase inversion during the first acceleration reversal from g $>$ 0 to g $<$ 0. If the zero-crossing of g(t) occurs once the instability growth has reached a state of nonlinear saturation, then hitherto rising bubbles and falling spikes reverse direction and collide, resulting in small-scale structures. For multi-mode perturbations, we find that bubbles and spikes collide during phase inversion, the interfacial region is turbulent, and undergoes a period of enhanced structural breakdown. This is accompanied by a rapid increase in the rate of molecular mixing, and increasing isotropy within the region. During the final stage of g $>$ 0 acceleration, self-similar RT mixing re-emerges, together with a return to anisotropy.