  The effects of isothermal stratification strength on vorticity dynamics for
single-mode Rayleigh-Taylor instability (RTI) are examined using two
dimensional fully compressible wavelet-based direct numerical simulations. The
simulations model low Atwood number ($A=0.04$) RTI development for four
different stratification strengths, corresponding to Mach numbers from 0.3
(weakly stratified) to 1.2 (strongly stratified), and for three different
perturbation Reynolds numbers, from 5,000 to 20,000. All simulations use
adaptive wavelet-based mesh refinement to achieve very fine spatial resolutions
at relatively low computational cost. For all stratifications, the bubble and
spike go through the exponential growth regime, followed by a slowing of the
RTI evolution. For the weakest stratification, this slow-down is then followed
by a re-acceleration, while for stronger stratifications the suppression of RTI
growth continues. Bubble and spike asymmetries are observed for weak
stratifications, with bubble and spike growth rates becoming increasingly
similar as the stratification strength increases. For the range of cases
studied, there is relatively little effect of Reynolds number on bubble and
spike heights, although the formation of secondary vortices becomes more
pronounced as Reynolds number increases. The underlying dynamics are analyzed
in detail through an examination of the vorticity transport equation, revealing
that incompressible baroclinicity drives RTI growth for small and moderate
stratifications, but increasingly leads to the suppression of vorticity
production and RTI growth for stronger stratifications. These variations in
baroclinicity are used to explain the suppression of RTI growth for strong
stratifications, as well as the anomalous asymmetry in bubble and spike growth
rates for weak stratifications.
