Indoor airflows CFD modelling requires relevant prediction of natural convective heat transfer at wall boundaries. On the one hand, widely used eddy viscosity turbulent models such as k-E or k-!SST combined with standard wall function perform poorly for the prediction of walls heat fluxes. Improvement of wall heat fluxes prediction requires wall-resolved meshes combined with advanced turbulence models which are generally computationally demanding while not always feasible for complex geometries. On the other hand, zonal and nodal models are widely used to predict building thermal behaviour. These models rely on integral scale correlation to predict room wall heat fluxes but give very limited information about local behaviours. The present study investigates the use of integral scale based wall functions to reach a reasonable compromise between the computational cost of wall-resolved models and the required accuracy for fields of interest such as temperature or velocity.

Electricity generation relies on a set of complex facilities where mechanical engineering plays an essential role in ensuring the performance, reliability, safety, and sustainability of systems. This document presents a non-exhaustive overview of the contribution of mechanics in the nuclear, hydraulic, and wind power sectors, from a fundamental understanding of the phenomena to advanced methods of design, experimentation, and numerical modeling. We show how fluid mechanics, structural mechanics, and materials science complement each other in order to design, dimension, control, and optimize the components and structures necessary for electricity generation. This complementarity of mechanical sciences is made possible by advanced scientific work providing detailed knowledge of flows, heat transfer, mechanical stresses, vibration phenomena, and damage mechanisms, which occur in various contexts such as reactor cooling, dam resistance, wind turbine stability, and nuclear fuel performance. This summary highlights an important feature of mechanical sciences, namely the parallel evolution of experimental and numerical approaches, which complement each other in understanding the complex phenomena affecting energy facilities. Scale model testing, real-world measurements, and modern imaging techniques provide data that is essential for validating three-dimensional simulations. Numerical models, on the other hand, make it possible to explore extreme conditions that are difficult to reproduce in the laboratory and to test multiple design variants. These models are becoming bigger thanks to high performance computing and allow today chaining or coupling different physics and scales. The decarbonization of energy and the resulting increase in electricity production are associated with several scientific challenges that must be addressed: realistic consideration of dynamic phenomena, improvement of physical models, control of material aging, management of fluid-structure interactions, simulation of two-phase flows, and evaluation of uncertainties in calculations. These challenges will be met more easily if industry and research actively collaborate to maintain a high level of innovation and thus guarantee the safety of production facilities.

Pressure vessels submitted to turbulent flows are prone to fluid-structure interactions and vibrations. The design of a nuclear power plant comes along with the prediction of the large scale vibration pattern generated by turbulent flows exerted upon large areas of the core barrel containing the fuel assemblies.The present paper focuses on turbulent forcing in annular gaps with impinging inlets, in view of assessing the relevance of traditional models of reactor vessel studies and of improving future calculations. An analytical reference case is designed to test the pressure field homogeneity hypothesis of the literature models. Pressure fluctuations associated to the turbulent flow are measured in an experimental mock-up and calculated in CFD simulations, at a gap Reynolds number of 105. The global flow pattern in the annular gap is first provided. The Power Spectrum Density of the pressure field and its convection and coherence parameters are obtained both experimentally and numerically. A fair agreement is found between the measurements and the simulations, and the flow pattern appears inhomogenous in large proportions, contrary to the traditional representation. Furthermore, the first mode of vibration of the inner cylinder is measured under turbulent forcing, and compared to the predictions of the simplified model and of CFD calculations: a fair agreement is observed. Finally, the literature model is revisited in the light of these findings, and some potential improvements are discussed.

This study investigates the physical mechanism of laminar-to-turbulent transition in natural convection by developing algebraic Local-Correlation-based Transition Models (LCTMs) through a physics-based, data-driven framework combining Bayesian optimisation and Symbolic Regression. Accurate transition prediction is critical for high-Rayleigh-number natural convection applications, where standard RANS models and existing transition correlations, calibrated for flows without buoyancy effects, fail to capture the transition onset and boundary layer development. We employ DNS data from differentially heated rectangular cavities (Rayleigh number (Ra) = 10^10 to 10^11 ) to calibrate model parameters by Bayesian optimisation, then use Symbolic Regression with buoyancy and transition-specific invariants to discover explicit algebraic expressions that generalise across flow conditions. Two models are developed: ML-1, optimised for high Rayleigh-number accuracy, and ML-2, designed for broader generalisation across laminar and turbulent regimes. Validation against a natural convection vertical boundary layer, tall rectangular cavities, and a square cavity demonstrates substantial improvements over baseline RANS models, with the data-driven LCTMs accurately predicting transition location, Nusselt number, and velocity and temperature profiles, showing promising capability for enhancing natural convection predictions while maintaining RANS efficiency.

This work evaluates hybrid RANS-LES models for high-Rayleigh-number natural convection, focusing on laminar-turbulent transition and the DDES switching function, independently and their interactions. A sensitivity analysis is first performed on the shielding function of the DDES formulation to adjust the RANS-LES switch and thus the extent of the near-wall RANS region, revealing that the standard formulation activates LES too close to the wall and degrades the prediction of wall quantities. An algebraic data-driven transition model, optimised using DNS data of natural convection flows, is embedded in the DDES framework to form the DDES-T model. The new formulation is demonstrated to predict transition location and wall heat transfer in a differentially heated cavity with small deviations from DNS. The transition model and hybrid switch interact to lock the RANS-LES transition to the physical onset of turbulence and thereby reduce sensitivity to ad-hoc shielding modifications, motivating the need for improved boundary-layer shielding and explicit transition modelling in hybrid RANS-LES for high-Rayleigh-number buoyancy-driven flows.

Direct Numerical Simulations are performed to provide a fine description of the turbulent mixed convection flow in an upward heated pipe. The introduction of buoyancy forces in this type of flow gives rise to different non-trivial phenomena such as laminarisation and a bi-stable behaviour, capable of modifying the flow characteristics in a deep and non-monotonic way. The present work aims therefore to establish a highfidelity and publicly accessible database, containing first and second-order accurate turbulence statistics, as well as budgets of various one-point correlations of turbulent quantities, namely the turbulent kinetic energy and its associated dissipation rate, the temperature variance and its associated dissipation rate, the Reynolds stress tensor and the turbulent heat flux vector. Besides, the coexistence of two and very distinct statistically stationary solutions fora same set of flow parameters is also investigated. These two states are referred to as weak- and strong-turbulence regimes. The budget analyses performed here, such as the original ones presented for the budgets of the Reynolds stresses, contribute to gaining further understanding of the physics of mixed convection flows. The results issued from the present study shall also support the validation of lower-fidelity numerical models, such as Large Eddy Simulation, RANS and hybrid RANS/LES approaches.

The interaction between cooling fluid and solid structures (rods, tubes) in nuclear power plants may lead to flow-induced vibrations (FIV), causing material fatigue, fretting wear, and eventually loss of component integrity. This can cause further safety issues as well as substantial standstill costs due to longer or unplanned outages. With the growing computational power, the application of modern 3D numerical simulation tools for the accurate prediction of FIV phenomena is rapidly increasing. In 2022, the GO-VIKING ( G athering expertise O n V ibration I mpa K t I n N uclear power G eneration) project received a grant within the Horizon Europe research and innovation funding program. Sixteen European and two US partners started their collaboration in the field of FIV experiments and analysis. Over four years, the GO-VIKING project investigates FIV phenomena occurring in nuclear reactor cores and steam generators under single- and two-phase flow conditions. The project’s main objectives are to expand the expertise in the field of FIV through generation of new experimental and high-resolution numerical data; development, improvement, and validation of fluid-structure interaction (FSI) methods for FIV evaluation; training stakeholders in the application of these methods; and synthesizing guidelines for the prediction and assessment of FIV phenomena in nuclear reactors. This paper provides an overview of the GO-VIKING objectives, scientific program, as well as of the main scientific achievements in the first project year.

A new formulation of the Elliptic Blending Differential Flux Model extended the original model of Dehoux et al.[1] to various wall thermal boundary conditions for the turbulent heat flux and the temperature variance [2]. An assessment of this new model coupled with Conjugate Heat Transfer (CHT) is proposed for several values of fluid-solid thermal diffusivity and conductivity ratios. A careful attention is paid to the discountinuity of the dissipation rate associated with the temperature variance at the fluid-solid interface. The present analysis is supported by the Direct Numerical Simulations of an incompressible channel flow at a friction Reynolds number and a Prandtl number equal to 150 and 0.71, respectively [3].

This study analyses the heat transfer and flow characteristics of cross-flow over two heated infinite cylinders in a tandem (in-line) configuration. Non-isothermal Large Eddy Simulations (LES) using the dynamic Smagorinsky model were conducted at a fixed Reynolds number of 3, 000 (based on the free stream velocity and the cylinder diameter). A range of cylinder gap ratios (1.0 <= L/D <= 5.0) was investigated (in increments of 0.25) with two different Prandtl numbers Pr = 0.1 and 1.0. Results show that the flow structures vary according to the order of the patterns: (i) Extended body regime: without attachment for low L/D (1.0-1.25) where cylinders behave as a single bluff body with top-bottom vortex shedding, (ii) Shear layer reattachment regime: with reattachment for moderate L/D (1.5-3.75) where the detached shear layer from the upstream cylinder reattaches to the down-stream cylinder, and (iii) Co-shedding regime: for high gap ratios (3.75 <= L/D <= 5.0) a phenomenon called "jumping", where the two cylinders behave as isolated bluff bodies. Furthermore, it was observed that the average Nusselt number of both cylinders experience a drastic variation at a critical spacing ratio (between 3.75 <= L/D <= 4.0). For L/D <= 3.0, the average Nusselt number of the upstream cylinder was found to be higher than that of the downstream one. However, for spacing ratios L/D > 3.0, the average Nusselt number was similar for both cylinders. For the downstream cylinder, the maximum Nusselt number was located at the separation angle and was found to be independent of the spacing ratio.

Dealing with complex geometries for industrial applications is challenging in computational fluid dynamic workflows. Current developments in scan devices offer the possibility to represent very complex solid geometries in fluid dynamic solvers. This paper proposes a novel approach for reconstructing solid geometry from 3-D scans and flow simulation. Based on a 3-D point cloud, the approach automatically reconstructs the solid surface by including local solid planes in any convex computational cell. An immersed boundary method is then used to impose appropriate boundary conditions on the solid surfaces in the co-located finite volume context. The present approach avoids the complex and time-consuming manual/assisted meshing typical of body-fitted mesh workflows while showing satisfactory robustness and accuracy.

The present work evaluates the performance of different RANS turbulence models for the prediction of a natural convection air vertical boundary layer at Prandtl numbers 0.71. The assessment on the prediction of the boundary layer is characterized in terms of the wall normal heat flux, wall friction as well as mean and turbulent flow fields at four locations. It is observed that the chosen model used to close the mean momentum equation is crucial in order to correctly capture the flow field. From the thermal point of view, advanced models such as transport equations of the turbulent heat fluxes accurately reproduce the heat fluxes in the fully turbulent part along the wall. Moreover, the laminar-turbulent transition is also more accurately reproduced with the advanced RANS turbulence models.

In this study, we present a robust conservative time‐staggered scheme for variable density flow. This pressure correction scheme uses the compressible Navier–Stokes equations and is implemented in the collocated finite‐volume open‐source computational fluid dynamics solver code_saturne. The Helmholtz equation is solved for the pressure increment, taking the thermodynamic pressure into account and avoiding the acoustic time step limitation. The internal energy equation is used and completed by a source term derived from the discrete kinetic energy equation, thus enforcing total energy conservation and consistency for irregular solutions. A numerical analysis providing conditions ensuring the positivity of the thermodynamic variables is proposed. The scheme is verified and validated against analytical and experimental test cases. Its ability to reproduce the pressure variation while conserving the mass is demonstrated. Its conservative property and time convergence order are also verified. An irregular shock solution is studied, emphasizing the importance of the source term in the internal energy equation. Finally, the scheme is validated against reference numerical results on a two‐dimensional natural convection cavity and experimental data on a three‐dimensional ventilation test case. The comparison against experimental data is made using first‐and second‐order turbulent simulations.

Characterization of parallel flow through rod bundles is of key importance in assessing the performance and safety of several engineering systems, including a majority of nuclear reactor concepts. Inhomogeneities in the bundle cross-section can present complex flow phenomena, including varying local conditions of turbulence. With the ever-increasing capabilities of high-performance computing, Direct Numerical Simulation (DNS) of turbulent flows is becoming more feasible. Through resolving all scales of turbulence, DNS can serve as a “numerical experiment,” and can provide substantial insight into flow physics, but at considerable computational cost. Thus to date, the DNS in open literature for rod bundle flows is relatively scarce, and largely limited to unit-cell domains. Since wall effects are important in rod bundle flows, a multiple-pin DNS study can expand understanding of rod bundle flows while providing valuable reference data for evaluating reduced-resolution techniques. In this work, DNS of a 5x5 square bare rod bundle representative of typical light water reactor fuel dimensions was performed using the spectral element code Nek5000. Turbulent microscales based on an advanced Reynolds-Averaged Navier–Stokes model were used to establish the required DNS resolution. Velocity and Reynolds stress fields are analyzed in detail, and invariant analysis is used for further investigation into flow physics. The results show stark changes in the structure of turbulence in the edge gaps, suggesting the presence of gap vortices in these regions. In addition, turbulent kinetic energy budgets are presented to more fully illustrate the various turbulent processes. These data can prove useful for rigorous evaluation of lower-fidelity turbulence modeling approaches.

Rod bundle flows are prevalent in nuclear engineering for both light water reactors (LWR) and advanced reactor concepts. Unlike canonical channel flow, the flow in rod bundles presents some unique characteristics, notably due to the inhomogeneous cross section which can present different local conditions of turbulence as well as localized effects characteristic of external flows. Despite the ubiquity of rod-bundle flows and the decades of experimental and numerical knowledge acquired in this field, high-fidelity datasets of multiple-pin configurations are still relatively sparse due to the computational cost involved in generating them at relevant Reynolds numbers. These datasets are of great value as they allow for assessment of the reliability of various turbulence models, as well as allow for a deeper understanding of the flow physics. We present Large Eddy Simulations (LES) of the flow at Reynolds number 19,000 in a square 5 x 5 rod bundle representative of LWR fuel generated by the flow solver Nek5000. We consider standard configurations as well as configurations where the central pin is replaced with a control rod guide thimble, and analyze the flow fields in detail. The accuracy of the LES of the base 5 x 5 configuration is established by comparing against Direct Numerical Simulation (DNS) results. We compare the LES results from Nek5000 with an advanced Reynolds Stress model from STAR-CCM+, which shows good agreement in the wide gaps with larger discrepancies in the narrow gaps. In particular, evidence of gap vortex streets and changes in the turbulence structure are seen in the edge subchannels and in the narrow thimble gaps in LES, but are not predicted by STAR-CCM+.