Nonlinear effects in buoyancy-driven variable-density turbulence
We consider the time dependence of a hierarchy of scaled L²ᵐ-norms D_m,ω and D_m,θ of the vorticity ω =∇ x u and the density gradient ∇θ, where θ = log.(ρ*/ ρ*₀), in a buoyancy-driven turbulent flow as simulated by Livescu & Ristorcelli (J. Fluid Mech., vol. 591, 2007, pp. 43–71). Here, ρ* (x,t) is the composition density of a mixture of two incompressible miscible fluids with fluid densities ρ*₂ > ρ*₁, and ρ*₀ is a reference normalization density. Using data from the publicly available Johns Hopkins turbulence database, we present evidence that the L²-spatial average of the density gradient can reach extremely large values at intermediate times, even in flows with low Atwood number At = (ρ*₂ - ρ*₁)/(ρ*₂ + ρ*₁) = 0.05, implying that very strong mixing of the density field at small scales can arise in buoyancy-driven turbulence. This large growth raises the possibility that the density gradient ∇θ might blow up in a finite time.