  Growth of the single-fluid single-mode Rayleigh-Taylor instability (RTI) is
revisited in 2D and 3D using fully compressible high-resolution simulations. We
conduct a systematic analysis of the effects of perturbation Reynolds number
($Re_p$) and Atwood number ($A$) on RTI's late-time growth. Contrary to the
common belief that single-mode RTI reaches a terminal bubble velocity, we show
that the bubble re-accelerates when $Re_p$ is sufficiently large, consistent
with [Ramaparabhu et al. 2006, Wei and Livescu 2012]. However, unlike in
[Ramaparabhu et al. 2006], we find that for a sufficiently high $Re_p$, the
bubble's late-time acceleration is persistent and does not vanish. Analysis of
vorticity dynamics shows a clear correlation between vortices inside the bubble
and re-acceleration. Due to symmetry around the bubble and spike (vertical)
axes, the self-propagation velocity of vortices points in the vertical
direction. If viscosity is sufficiently small, the vortices persist long enough
to enter the bubble tip and accelerate the bubble [Wei and Livescu 2012]. A
similar effect has also been observed in ablative RTI [Betti and Sanz 2006]. As
the spike growth increases relative to that of the bubble at higher $A$,
vorticity production shifts downward, away from the centerline and toward the
spike tip. We modify the Betti-Sanz model for bubble velocity by introducing a
vorticity efficiency factor $\eta=0.45$ to accurately account for
re-acceleration caused by vorticity in the bubble tip. It had been previously
suggested that vorticity generation and the associated bubble re-acceleration
are suppressed at high $A$. However, we present evidence that if the large
$Re_p$ limit is taken first, bubble re-acceleration is still possible. Our
results also show that re-acceleration is much easier to occur in 3D than 2D,
requiring smaller $Re_p$ thresholds.

Fluid Dynamics