Compressible Turbulent Convection in Highly Stratified Adiabatic Background

Buoyancy-driven turbulent convection leads to a fully compressible flow with a prominent top-down asymmetry of first- and second-order statistics when the adiabatic equilibrium profiles of temperature, density and pressure change very strongly across the convection layer. We report a series of highly resolved three-dimensional fully compressible direct numerical simulations for dimensionless dissipation numbers, $0.1 le D le0.8 $ at fixed Rayleigh number $Ra = 10^6$ and superadiabaticity $epsilon = 0.1$. Strong stratification leads to the formation of an increasingly thicker stabilized sublayer with a slightly negative mean convective heat flux $J_{c}left(z ight)$ at the top of the convection zone. A transition is observed for $D > Dcrit approx 0.65$, when density fluctuations collapse to those of pressure; it is characterized by an up to nearly 50\% reduced global turbulent heat transfer and a sparse network of focused thin and sheet-like thermal plumes falling through the top sublayer deep into the bulk. A systematic comparison of boundary characteristics at top and bottom walls and trends with higher Rayleigh numbers might also be discussed.