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The paper focuses on a study of turbulence decay in flow with streamwise gradient. For the first time, an analytical solution of this problem was obtained based on the k-epsilon model of turbulence in one-dimensional (1D) approximation, as well as on the symmetry properties of the system of differential equations. Lie group technique enabled reducing the problem to a linear differential equation. The analytical solution enabled parametric studies, which are computationally cheap in comparison to CFD based simulations. The lattice Boltzmann method (LBM) in two-dimensional approximation (2D) was used to validate the analytical results. Large eddy simulation (LES) Smagorinsky approach was used to close the LBM model. Computations revealed that the rate of turbulence decay is significantly different for the cases of positive and negative streamwise pressure gradient. The further comparisons showed that the analytical solution underpredicts the predictions by the numerical methodology, which can be attributed to the simplified problem statement used to derive the closed-form analytical solution. Comparisons of calculations with experiments revealed that the theoretical models used in the study underpredict the measurements for flows with a positive pressure gradient. Hence it can be concluded that the LBM technique combined with the LES Smagorinsky model requires the further modification.

To understand the behavior of commercial nuclear fuel during its storage phase in spent fuel dry storage casks, simulations are necessary and to validate the simulation models, full-scale experiments are required. Therefore, an experimental facility called the dry cask simulator (DCS) was built in Sandia National Laboratories with the purpose of producing validation-quality data that can be used to test the accuracy of the modeling used to determine cladding temperatures in modern vertical dry casks. The DCS has been created under the framework of the Extended Storage Collaboration Programme (ESCP), coordinated by EPRI. The ultimate goal of the DCS was to study the thermal-hydraulic response of a single boiling water reactor (BWR) fuel assembly under variety of heat loads, internal vessel pressures, and external configurations. In this article, the simulations conducted by ENUSA Industrias Avanzadas S.A., S.M.E and UPM (Universidad Politecnica de Madrid, Spain) using STAR-CCM+ are validated against the experiments conducted at Sandia National Laboratories. In order to validate the simulation models several sensitivity analyses have been performed: a grid convergence index to estimate the discretization error in the simulations, a turbulence model sensitivity for the inner part of the cask, and three different approaches to model the fuel assembly: Case 1 (model with no spacers and no bottom tie plate), Case 2 (model with spacers and no bottom tie plate and Case 3 (model with spacers and bottom tie plate). Additionally, a thermal-hydraulic analysis is also presented, not only to explain the difference between the three ways to model the fuel assembly, but also to obtain a better understanding of the behavior of the dry cask according to pressure and heat output changes. Among the three cases, the simplest model (Case 1) has proven to obtain the most accurate results, being able to predict the temperature distribution in the fuel assembly and the air mass flow compared to the experiment measurements for all four cases of study. Meanwhile, the two other cases have shown that for steady state solutions the spacers and the bottom tie plate are not necessary and can introduce unwanted errors. Nonetheless, all cases have provided insight into the behavior of a dry storage cask system under different conditions showing the different heat transfer mechanisms interactions.