Thermal fluctuations in low-Prandtl number fluid flows over a backward facing step

The backward facing step geometry (BFS) is a representative geometry for sudden expansions in pipe, duct and channel flows. While this type of geometry by itself is not a part of engineering components, the flow separation and the accompanying flow features present in a BFS are of great importance when designing manifolds, heat exchangers or fuel bundles. In the frame of EU Horizon 2020 project SESAME, an extensive effort has been put forward to gain more insights into the flow and thermal features in a BFS geometry for low-Prandtl number fluids. The main motivation behind this effort is two-fold: to generate a reference database by means of experiments and high fidelity simulations, and accordingly utilize the reference database to validate and/or improve the turbulence models in engineering applications. In this paper, we present a broad description of the experimental facility and its expected capabilities, as well as the results of numerical efforts. The experimental results will be obtained in the DITEFA 2 facility of KIT with a GaInSn eutectic alloy. The expansion ratio of the BFS in the experiment is set to 2 and the geometry has one heated wall. Unlike the vast majority of the BFS experiments found in the literature, the present BFS experiment has an outflow in a shape of a square, that is, the width and height of the outflow are approximately the same. Second, a direct numerical simulation (DNS) is performed with a passive scalar and for an expansion ratio of 2.25. Similar to the experiment, the shape of the outflow is a square and the average flow is three-dimensional. Conjugate heat transfer DNS is performed for the heated solid walls, while the unheated walls were neglected. Finally, this reference DNS data is used to validate a LES and an advanced RANS modelling approach.