Revisiting the resolution requirements for turbulence simulations in nuclear heat transfer

Direct numerical simulations (DNSs) require the resolution of all relevant turbulence scales in space and time, whereas large eddy simulations (LESs) need only to resolve the dominant energy carrying large scales. Important influences from physics and numerics on the small-scale resolution are discussed. Quantitative criteria for turbulent flows are re-evaluated. Experience shows, resolving the microscales is usually by far not achieved in DNS; this is less relevant than the adequate resolution of the anisotropic coherent fine flow structures. These structures depend on the flow type, so that general criteria cannot be given. Resolving the large scales is a serious problem. When the computational domain covers only part of the flow domain, the large-scale resolution is coupled to the artificial boundary conditions for open boundaries. Each measure and criteria have to be carefully considered to ensure that the simulations meet the expectations. Special emphasis is given to liquid metal flows because related nuclear applications are often in the transition range between LES and DNS of the temperature field. A new model is given to predict local turbulent Prandtl numbers for subgrid scale heat flux modeling. It covers the required most important influences: local resolution, a local turbulence parameter, and Reynolds and Prandtl numbers.