Estimated aleatoric uncertainty from initial and inlet conditions for variable density mixing

Abstract

Variable density flows occur in a variety of different systems with a wide range of scales, from astrophysics to atmospheric flows to inertial confinement fusion or reacting flows. Given the inherent limitations of RANS simulations, it is important to find ways to quantify the uncertainty in the predictions. The aleatoric uncertainty of two different variable density flows to inlet and initial conditions, respectively, is studied. The two cases are the turbulent mixing of a jet in a co-flow at small Atwood number $A$, and the Rayleigh-Taylor mixing in a tilted rocket rig at medium $A$. Uncertainty estimates are made for simulations using the BHR-1 turbulence model and compared to reference data. Estimated distributions of measurement uncertainties from an experimental report are used to do Monte-Carlo sampling for some inlet parameters of the jet in a co-flow case. The results show only little sensitivity to all inlet parameter perturbations, and even with increased perturbations the estimated uncertainty band does not fully match the experimental results. Therefore, the model predictions are very robust with respect to the inlet parameter variability. For the tilted rocket rig case, a perturbation to the initial interface location is varied over an expected range of parameter values. The simulation results are significantly more sensitive to the initial conditions than the jet in co-flow simulation results are to the inlet conditions. The estimated uncertainty band of the tilted rocket rig matches quite well the experimental data at nominal perturbation amplitude. The predictions of this Rayleigh-Taylor instability case hence are less robust to the initial interface perturbations.