- 2019 Elsevier B.V. Modeling and simulation performed with advanced tools are important for thorough understanding of existing power plant response to accidents; for life extension decisions of existing plants; and to support licensing activities for new power plants. The use of computational fluid dynamics (CFD) tool in nuclear R&D has gained significantly due to its capabilities to predict complex flow phenomena at very fine resolutions. High-fidelity numerical simulations including direct numerical simulation (DNS) and large-eddy simulation (LES) have been considered as reliable CFD tools for the development and validation of turbulence models along with experiments. Compared to other CFD techniques, DNS is the most computationally expensive approach, and limited to flow studies at low to moderate Reynolds numbers. LES subgrid-scale (SGS) models require the specification of model coefficients that cannot be generally used to simulate a wide spectrum of flows. Performances of LES with modified SGS model coefficients need to be verified and validated versus high-resolution experimental database or DNS results. In this paper, we present the current state-of-the-art experimental measurements in complex geometries of advanced nuclear reactors, such as turbulent flows in wirewrapped fuel bundle for liquid metal reactors and randomly packed beds for gas-cooled and molten-salt reactors. It is important to achieve an in-depth understanding of flow phenomena and complex flow characteristics within these reactor cores because they are related to the safety and design scenarios. High-fidelity experimental measurements of velocity fields are acquired featuring a combination of time-resolved particle image velocimetry (TR-PIV) and matching-index-of-refraction approaches. Experimental results are obtained at high spatial and temporal resolutions of velocity fields and the first- and second-order flow statistics are suitable for the verification and validation of CFD codes currently used in nuclear engineering applications.