Variable-density buoyancy-driven turbulence with asymmetric initial density distribution

Abstract

The effects of different initial density distributions on the evolution of buoyancy-driven homogeneous variable-density turbulence (HVDT) at low (0.05) and high (0.75) Atwood numbers are studied by using high-resolution direct numerical simulations. HVDT aims to mimic the acceleration-driven Rayleigh-Taylor and shock-driven Richtmyer-Meshkov instabilities and reveals new physics that arise from variable-density effects on the turbulent mixing. Here, the initial amounts of pure light and pure heavy flows are altered primarily to mimic the variable-density turbulence at the different locations of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities' mixing layers where the amounts of the mixing fluids are not equal. It is found that for the low Atwood number cases, the asymmetric initial density distribution has limited effects on both global and local flow evolution for HVDT. However, at high Atwood number, both global flow evolution and the local flow structures are strongly affected by the initial composition ratio. The flow composed of more light fluid reaches higher turbulent levels and the local statistics reach their fully-developed behavior earlier in the time evolution. During the late time decay, where most of the flow is well-mixed, all parameters become independent of the initial composition ratio for both low and high Atwood number cases.

Rayleigh-Taylor Instability: A Status Review of Experimental Designs and Measurement Diagnostics

“…This led the authors to conclude that either a different scaling parameter might be required for higher-At experiments, or an asymptotic value of mass flux has not been reached. This indicates that different parameters may reach self-similarity at different Reynolds numbers; this has been further validated by recent computations of a triply periodic homogeneous variable-density mixing layer using direct numerical simulations [130][131][132]. Spectra of q 0 , v 0 and q 0 v 0 were measured using hot-wire anemometry, both q 0 and v 0 show approximately half to a decade of scales in the inertial range, the q 0 v 0 spectra did not show a clear indication of the-5/3 slope and were similar to the spectra at smaller Atwood numbers in GC1 [46].…”

Acceleration Reversal Effects on Buoyancy-Driven Homogeneous Variable-Density Turbulence

“…2 and 3 are the computational Reynolds number, Re 0 = ρ 0 L 0 U 0 /µ 0 , Schmidt number, Sc = µ 0 /ρ 0 D 0 , and Froude number, Fr 2 = U 2 0 /gL 0 ; where, µ 0 = µ * /rho * is the reference dynamic viscosity, ρ 0 is the reference density, and D 0 is the diffusion coefficient. In this study, we focus on the effects of acceleration reversal, so for all cases, both Sc and Fr numbers are equal to unity consistent with our previous studies on HVDT evolution under constant acceleration [4,5].…”

“…The initial conditions of the simulations are similar to our previous studies [3,4,5], where the initial density field has top- hat energy spectrum between wave-numbers 3 and 5 (see [5] for more details of the initialization). In addition, they all are initialized with a similar mixing state (θ 0 ≈ 0.07), where the non-dimensional mixing state measure (θ) is defined as…”

“…Due to the periodic boundary conditions, the mean pressure gradient needs to be specified and, here, similar to previous studies of HVDT [12,5,4], the mean pressure gradient is chosen to obtain a maximally unstable flow:…”

Rayleigh–Taylor Instability With Varying Periods of Zero Acceleration

“…In addition, recent work HVDT has been used to study structural changes to variable density turbulence when turbulent kinetic energy (KE) evolution is highly nonlinear. In HVDT under constant acceleration, a smooth transition from rapidly increasing to gradually decaying variable-density turbulence was observed that may be expected to persist in the more general case of RTI with constantand variable-acceleration profiles with smoother transitions and different variable-density flows such as Richtmyer-Meshkov instability (RMI), mixing layers, and jet flows [25][26][27].…”