The paper discusses the most accurate methods for description of turbulent flows: computationally very expensive Direct Numerical Simulation (DNS) and, slightly less accurate and slightly less expensive, Large Eddy Simulation (LES) method. Both methods have found the way into the nuclear thermal hydraulics as tools for studies of the fundamental mechanisms of turbulence and turbulent heat transfer. In the first Section of the paper, both methods are briefly introduced in parallel with the basic properties of the turbulent flows. The focus is on DNS method, the so-called quasi-DNS approach, and the coarsest turbulence modelling approach discussed in this work, which is still on the very small scale, wall-resolved LES. Other, coarser turbulence modeling approaches (such as wall-modelled LES, RANS/LES hybrids, or RANS) are beyond the scope of the present paper. Section 2. answers the question: "How do DNS and LES methods work?", with a short discussion of the computational requirements, numerical approaches and computational tools. Section 3. is about the interpretation of the DNS and LES results and statistical uncertainties. Sections 4. and 5. give some examples of the DNS and wall-resolved LES results relevant for nuclear thermal hydraulics. The last section lists the conclusions and some of the challenges, which might be tackled with the most accurate techniques like DNS and LES.
