  It is shown how the variable density model (VDM) that governs the
Rayleigh-Taylor instability (RTI) for the miscible mixing of two incompressible
fluids can be transformed into a diffusive version of the inhomogeneous,
incompressible Navier-Stokes equations forced by gradients of the composition
density $\rho$ of the mixing layer. This demonstrates how buoyancy-driven flows
drive and enhance Navier-Stokes turbulence. The role of the potential vorticity
$q = \omega\cdot\nabla\rho$ is also discussed.
