In light of the Fukushima accident, significant research effort has been focused on containment thermal- hydraulics and gaseous mixing. In one such effort, the recent OECD/NEA-PSI Computational Fluid Dynamics (CFD) Benchmark "PANDA", an initially stratified air-helium layer is gradually eroded by a turbulent round jet (Re = 20000) of primarily air. Mole fraction and temperature readings were taken at various points throughout the domain to record the erosion behavior, and mean and RMS velocity profiles were averaged over a long transient time. These data were the basis for comparison with CFD results from Unsteady Reynolds-Averaged Navier-Stokes (URANS) and Large-Eddy (LES) simulations. URANS two-species simulations were run with STAR-CCM+ focusing on the Realizable k-s turbulence model and low-Mach approximation under ideal gas conditions. The LES calculations were performed with the spectral-element open-source highly-scalable code Nek5000 using both a Boussinesq and low- Mach-number approximation that involved a new computational framework for handling multi-species diffusion and transport. URANS results for the mole fractions over time and averaged flow profiles showed reasonable agreement with the available experimental results for nearly all available measurements. The initial LES results demonstrated the new code capability and some early-time behavior restricted by long integration time due to a vast range of spatial and temporal scales. Nevertheless, these results can be used to improve turbulence modeling for the URANS approach that as shown here is an appropriate tool for future analyses of this type of transient flow.