Statistical Uncertainty of Turbulent Heat Flux in a Flow of Low- Prandtl Fluid over a Backward Facing Step

In this paper, we present the profiles, together with statistical uncertainties of turbulent heat flux, obtained in the recently performed Direct Numerical Simulation (DNS) of flow of two liquid metals over a confined backward facing step. This work is a continuation of work that was performed within the European project SESAME. The presented profiles are obtained in the whole domain, while the uncertainty is analysed in 49 pre chosen points throughout the domain. The analysis is performed over at most 10 million time step or equivalently of around 5000 dimensionless time units. The turbulent heat flux is important for development of new turbulence models with less accurate methods for simulating flow properties, such as RANS methods, based on averaged Navier-Stokes equations. The DNS was performed with the Nek5000 code. The most notable feature of this code is the use of spectral elements to solve for velocity, temperature and any other passive scalar. It is an open source code developed by the Argonne National Laboratory. Two Prandtl fluids were simulated (Pr = 0.005 and Pr = 0.1), although due to brevity, we only show uncertainty results for the lower Prandtl number. We found significant differences in results for turbulent heat flux between the two fluids. The positions of minimal and maximal values differ significantly. The estimated relative uncertainty remains high even after 10 million time steps. Nevertheless, within the uncertainty bounds, we can observe symmetries and anti-symmetries, which are appropriate for the nature of the flow in a confined backward facing step.