  The evolution of buoyancy-driven homogeneous variable-density turbulence
(HVDT) at Atwood numbers up to 0.75 and large Reynolds numbers is studied by
using high-resolution Direct Numerical Simulations. To help understand the
highly non-equilibrium nature of buoyancy-driven HVDT, the flow evolution is
divided into four different regimes based on the behavior of turbulent kinetic
energy derivatives. The results show that each regime has a unique type of
dependency on both Atwood and Reynolds numbers. It is found that the local
statistics of the flow based on the flow composition are more sensitive to
Atwood and Reynolds numbers compared to those based on the entire flow. It is
also observed that at higher Atwood numbers, different flow features reach
their asymptotic Reynolds number behavior at different times. The energy
spectrum defined based on the Favre fluctuations momentum has less large scale
contamination from viscous effects for variable density flows with constant
properties, compared to other forms used previously. The evolution of the
energy spectrum highlights distinct dynamical features of the four flow
regimes. Thus, the slope of the energy spectrum at intermediate to large scales
evolves from -7/3 to -1, as a function of the production to dissipation ratio.
The classical Kolmogorov spectrum emerges at intermediate to high scales at the
highest Reynolds numbers examined, after the turbulence starts to decay.
Finally, the similarities and differences between buoyancy-driven HVDT and the
more conventional stationary turbulence are discussed and new strategies and
tools for analysis are proposed.
