Multi-fidelity validation of variable-density turbulent mixing models*

In this study, ensembles of experimental data are presented and utilized to compare and validate two models

used in the simulation of variable density (Atwood = 0.22), compressible turbulent mixing. Though models of

this kind (Reynolds Averaged Navier-Stokes and Large-Eddy Simulations) have been validated extensively with

more canonical flows in previous studies, the present approach offers novelty in the complexity of the geometry,

the ensemble based validation, and the uniformity of the computational framework on which the models are

tested. Moreover, all experimental and computational tasks were completed by the authors which has led to

a tightly coupled experimental configuration with its "digital twin.” The experimental divergent-shock-tube

facility and its data acquisition methods are described and replicated in simulation space. A 2D Euler model

which neglects the turbulent mixing at the interface is optimized to experimental data using a Gaussian process.

This model then serves as the basis for both the 2D RANS and 3D LES studies that make comparisons to the

mixing layer data from the experiment. RANS is shown to produce good agreement with experimental data

only at late flow development times. The LES ensembles generally show good agreement with experimental

data, but display sensitivity to the characterization of initial conditions. Resolution dependent behavior is also

observed for certain higher-order statistics of interest. Overall, the LES model successfully captures the effects

of divergent geometry, compressibility, and combined non-linear instabilities inherent to the problem. The

successful prediction of mixing width and its growth rate highlight the existence of three distinct regimes in the

development of the instability, each with similarities to previously studied instabilities.