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Turbulent heat transfer in buoyancy-dominated flows is a challenging problem for computational fluid dynamics (CFD). Many authors attribute model error in these conditions to the Reynolds analogy. We leverage a brand-new direct numerical simulation database to evaluate the performance of several popular turbulence models in buoyant diabatic channel flow. We find that heat transfer results are relatively accurate, with a Nusselt number error less than 20%. However, the turbulent flow solution is very inaccurate, with wall shear overpredicted by up to 100%. This indicates significant turbulence model error in such flows. We determined that the dominant sources of model error are missing physics in the algebraic Reynolds stress framework and the simple buoyancy production term used in industrial CFD. We suggest that future modeling efforts focus on these two sources of model error. We demonstrate that the Reynolds analogy is not the dominant source of model error. [ABSTRACT FROM AUTHOR]

This paper presents the development of a benchmark for predicting thermal striping through simulation. This work utilized large eddy simulation and will be used to benchmark future models. The testing domain was created using both the STRUCT and the Reynolds-averaged Navier-Stokes turbulence models and is based on an earlier design of the General Atomics Fast Modular Reactor upper plenum. The plenum features two adjacent, identical hexagonal bundles each with a center-placed axial rod drive, with a hot left coolant stream and a cold right coolant stream. The simulation solves the nondimensional Navier-Stokes equations, with temperature accounted as a passive scalar. First- and second-order flow statistics were obtained after 600 convective time units of averaging. The first-order statistics reveal that the hot jet is damped by a recirculatory flow from the near wall. At the same location, the second-order statistics show strong oscillations both in velocity and temperature. The power spectral density was utilized to determine that a low-frequency oscillation occurs here that is within the range of interest for thermal striping. Furthermore, proper orthogonal decomposition was used to identify coherent structures that confirm the oscillatory behavior, indicative of thermal striping. Overall, this benchmark can aid in the development of future models for predicting thermal striping in nuclear reactors, potentially leading to improved reactor safety and performance. [ABSTRACT FROM AUTHOR]

Turbulent mixing of coolant streams can result in an oscillatory mixing phenomenon called thermal striping. These fluctuations have the potential to lead to anticipated thermal fatigue failures in advanced nuclear reactors. To predict thermal striping, robust and computationally affordable modeling tools that are capable of accurately representing complex turbulence are needed. Hybrid turbulence approaches, such as detached-eddy simulation and scale-adaptive simulation, have shown some success in resolving complex unsteady turbulence for massively separated flows, however the applicability of these models to internal flows is limited. A STRUCTure-based (STRUCT) second-generation Unsteady Reynolds-Averaged Navier–Stokes turbulence model was recently proposed at the Massachusetts Institute of Technology to robustly extend the applicability of hybrid closures. In this work, the STRUCT model is evaluated using experimental data taken at the Reactor Cavity Cooling System separate-effects test facility at the University of Michigan. The experiments observed the interaction of parallel symmetric rectangular jets, and include measurements for mean profiles of velocity and Reynolds stresses. In the present work, the simulation results are assessed against mean profiles of velocity and Reynolds stresses, demonstrating the ability to reproduce the unsteadiness of the jets in close agreement with the measurements at considerably reduced computational cost. [ABSTRACT FROM AUTHOR]

Turbulent flow inside a modified differentially heated cavity at high Rayleigh number (Ra ~ 109) has been studied through fully resolved direct numerical simulation (DNS) using the high-order spectral element method code Nek5000. The flow configuration includes two separate physical phenomena: the natural recirculation itself, and the flow inside a curved channel. Simulations have been carried out using both the Boussinesq approximation and the low-Mach compressible formulation. Significant discrepancies between the two methods inform of the extreme caution that should be exercised when using the Boussinesq approximation in the limits of its applicability. The DNS solutions are analyzed in terms of polynomial-order convergence and Reynolds stress budgets, and the turbulence quantities and velocity profiles are presented as a reference for the validation of turbulence models. [ABSTRACT FROM AUTHOR]

Building on the strong belief that the advancement and consistent adoption of cutting-edge simulation tools is critical to the future of nuclear power, three-dimensional thermal-hydraulic methods in the form of computational fluid dynamics (CFD) have made enormous advancement and promise to transform the way we approach the design of more efficient and reliable systems. The success of these methods hinges on the accuracy and predictive ability of the underlying models, which must, at the same time, limit the computational cost and allow optimal scalability. A large effort at the Massachusetts Institute of Technology has been devoted to the development of a second-generation of multiphase-CFD (M-CFD) closures and to leveraging the continuous progression in the experimental techniques. Among the numerous objectives, the central challenge that has driven the overall approach is the prediction of departure from nucleate boiling. This work focuses on deriving the fundamental meso-scale mechanisms from the CFD-grade experiments and incorporates them in the M-CFD framework as subgrid-scale models. A more complete representation of lateral lift force and near-wall effects are proposed, in combination with direct numerical simulation-driven understanding of bubble-induced turbulence effects. The improved description of the multiphase flow distribution is coupled to a novel representation of boiling heat transfer, which aims at introducing all the physical mechanisms that are encountered at the boiling surface. Starting from the improved representation at the wall, this work concentrates on the micro-hydrodynamics of the thin liquid film on the heated surface, which governs the critical heat flux limit. [ABSTRACT FROM AUTHOR]