Comparison and Uncertainty Quantification of Two-Fluid Models forBubbly Flows with NEPTUNE_CFD and STAR-CCM+

International audienceThe nuclear industry is interested in better understanding the behavior of turbulent boiling flowsand in using modern computational tools for the design and analysis of advanced fuels and reactorsand for simulation and study of mitigation strategies in accident scenarios. Such interests serve asdrivers for the advancement of the 3-dimensional multiphase Computational Fluid Dynamicsapproach. A pair of parallel efforts have been underway in Europe and in the United States, theNEPTUNE and CASL programs respectively, that aim at delivering advanced simulation tools thatwill enable improved safety and economy of operations of the reactor fleet. Results from acollaboration between these two efforts, aimed at advancing the understanding of multiphaseclosures for pressurized water reactor (PWR) application, are presented. Particular attention is paidto the assessment and analysis of the different physical models implemented in NEPTUNE_CFDand STAR-CCM+ codes used in the NEPTUNE and the CASL programs respectively, forapplication to turbulent two-phase bubbly flows. The experiments conducted by Liu and Bankoff(Liu, 1989; Liu and Bankoff 1993a and b) are selected for benchmarking, and predictions from thetwo codes are presented for a broad range of flow conditions and with void fractions varyingbetween 0 and 50percent. Comparison of the CFD simulations and experimental measurements revealsthat a similar level of accuracy is achieved in the two codes. The differences in both sets of closuremodels are analyzed, and their capability to capture the main features of the flow over a wide rangeof experimental conditions are discussed. This analysis paves the way for future improvements ofexisting two-fluid models. The benchmarks are further leveraged for a systematic study of thepropagation of model uncertainties. This provides insights into mechanisms that lead to complexinteractions between individual closures (of the different phenomena) in the multiphase CFDapproach. As such, it is seen that the multi-CFD-code approach and the principled uncertaintyquantification approach are both of great value in assessing the limitations and the level of maturityof multiphase hydrodynamic closures

Spindle Assembly Checkpoint Protein Dynamics Reveal Conserved and Unsuspected Roles in Plant Cell Division

Background: In eukaryotes, the spindle assembly checkpoint (SAC) ensures that chromosomes undergoing mitosis do not segregate until they are properly attached to the microtubules of the spindle. Methodology/Principal Findings: We investigated the mechanism underlying this surveillance mechanism in plants, by characterising the orthogolous SAC proteins BUBR1, BUB3 and MAD2 from Arabidopsis. We showed that the cell cycle-regulated BUBR1, BUB3.1 and MAD2 proteins interacted physically with each other. Furthermore, BUBR1 and MAD2 interacted specifically at chromocenters. Following SAC activation by global defects in spindle assembly, these three interacting partners localised to unattached kinetochores. In addition, in cases of 'wait anaphase', plant SAC proteins were associated with both kinetochores and kinetochore microtubules. Unexpectedly, BUB3.1 was also found in the phragmoplast midline during the final step of cell division in plants. Conclusions/Significance: We conclude that plant BUBR1, BUB3.1 and MAD2 proteins may have the SAC protein functions conserved from yeast to humans. The association of BUB3.1 with both unattached kinetochore and phragmoplast suggests that in plant, BUB3.1 may have other roles beyond the spindle assembly checkpoint itself. Finally, this study of the SAC dynamics pinpoints uncharacterised roles of this surveillance mechanism in plant cell division

Inhibition of IL-10 Production by Maternal Antibodies against Group B Streptococcus GAPDH Confers Immunity to Offspring by Favoring Neutrophil Recruitment

Group B Streptococcus (GBS) is the leading cause of neonatal pneumonia, septicemia, and meningitis. We have previously shown that in adult mice GBS glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an extracellular virulence factor that induces production of the immunosuppressive cytokine interleukin-10 (IL-10) by the host early upon bacterial infection. Here, we investigate whether immunity to neonatal GBS infection could be achieved through maternal vaccination against bacterial GAPDH. Female BALB/c mice were immunized with rGAPDH and the progeny was infected with a lethal inoculum of GBS strains. Neonatal mice born from mothers immunized with rGAPDH were protected against infection with GBS strains, including the ST-17 highly virulent clone. A similar protective effect was observed in newborns passively immunized with anti-rGAPDH IgG antibodies, or F(ab')2 fragments, indicating that protection achieved with rGAPDH vaccination is independent of opsonophagocytic killing of bacteria. Protection against lethal GBS infection through rGAPDH maternal vaccination was due to neutralization of IL-10 production soon after infection. Consequently, IL-10 deficient (IL-10−/−) mice pups were as resistant to GBS infection as pups born from vaccinated mothers. We observed that protection was correlated with increased neutrophil trafficking to infected organs. Thus, anti-rGAPDH or anti-IL-10R treatment of mice pups before GBS infection resulted in increased neutrophil numbers and lower bacterial load in infected organs, as compared to newborn mice treated with the respective control antibodies. We showed that mothers immunized with rGAPDH produce neutralizing antibodies that are sufficient to decrease IL-10 production and induce neutrophil recruitment into infected tissues in newborn mice. These results uncover a novel mechanism for GBS virulence in a neonatal host that could be neutralized by vaccination or immunotherapy. As GBS GAPDH is a structurally conserved enzyme that is metabolically essential for bacterial growth in media containing glucose as the sole carbon source (i.e., the blood), this protein constitutes a powerful candidate for the development of a human vaccine against this pathogen

Bubble Force Balance Formula for Low Reynolds Number Bubbly Flows in Pipes and Channels: Comparison of Wall Force Models

International audienceAbstract In recent work, we investigated analytically low Reynolds number bubbly flows in pipes. We showed that the distribution of bubbles results from a balance between lift, dispersion and wall forces, and exhibited an analytical expression for this void fraction profile. We then performed a comparison of this analytical Bubble Force Balance Formula (BFBF) with an experiment from the literature. Antal’s model was used for the wall force. The objective of the present work is to compare and assess the three main wall force models in the literature: Antal’s, Tomiyama’s and Frank’s models. We begin by deriving two new BFBF, respectively with Tomiyama’s and Frank’s forces. We can see that the choice of the model impacts the velocity with which the analytical void fraction profile goes to zero at the wall. We then compare our three analytical profiles with experimental measurements and DNS simulations of laminar flows from the literature. We restrict ourselves to the near-wall region. The choice of Antal’s wall force model yields the best agreement. The data is also used to estimate the dispersion coefficient at the wall. Interestingly, we obtain the same order of magnitude with the three wall force models

Semi-analytical and numerical investigation of a reactive gaseous film between two evaporating liquids

International audienceReactive gaseous films between two evaporating liquids are studied both analytically and numerically, under simplifying assumptions. For high enough heats of reaction, a semi-analytical solution is exhibited. This solution is used to validate an Arbitrary Lagrangian–Eulerian method. Computations are carried out for low heats of reaction, showing how the system behavior is strongly influenced by the value of the heat of reaction compared to the heats of vaporization of reactants

Cfd study of the transverse migration of bubbles in a narrow rectangular duct assessment of the neptune_cfd code and benchmark of various closure models

International audienceThis paper presents a computational fluid dynamics study of two-phase bubbly flow, and aims ata proper description of the spatial distribution of bubbles observed in narrow rectangular ductsfor various non-uniform inlet conditions. As such the study relies on the air-water experimentsperformed by Liu et al. (2012), and a comparison to their results allows an assessment of theaccuracy of different closure models, such as lift force, wall force and turbulent dispersion.Different interfacial area concentration models are also compared and appraised. The numericalcomputations are performed with the NEPTUNE_CFD code, developed together by AREVA,CEA, EDF and IRSN, and dedicated to the simulation of multiphase flows by means of anEulerian multi-fluid approach

Requirements for cfd-grade experiments for nuclear reactor thermalhydraulics

International audienceWhen looking for a validation experiment for CFD application to reactor simulation for either design or safety studies, it appears that available data often suffer from a lack of local measurements, an insufficient number of measured flow variables, a lack of well-defined initial and boundary conditions, and a lack of information on results uncertainty. Therefore the working group on CFD application to nuclear safety of the OECD-NEA-CSNI-WGAMA decided to establish some requirements for CFD-grade experiments able to validate properly the single phase CFD tools. The SILENCE network also supported this initiative and contributed to this work. This paper is a first attempt to establish such requirements. Several steps of an ideal experimental program are considered from the design to the data analysis in order to give recommendations and guidelines for future experiments. Clear objectives should be first defined with reference to a reactor application, an analysis of the process to be investigated and a selection of the type of turbulence model which may be used for the simulation. Then a discussion between experimentalists and the CFD code practitioners is necessary to define the test section geometry, the initial and boundary conditions, the quantities of interest with their locations, to define the requirements on the measurement uncertainty. The choice of measurement technique stem from these specifications. The acceptance criteria should be defined in accordance with the required accuracy and the sensitivity of the measurements to the uncertainty in the experimental conditions. Preliminary CFD simulations are necessary to confirm the most appropriate measurement locations, experimental conditions and overall the interest of the experiment for the physics to validate. Iterations may be necessary to get an optimized design of the experiment. CFD grade experiments should be able to validate CFD and one important concern is to minimize the validation uncertainty on some selected figures of merit

An analytical relation for the void fraction distribution in a fully developed bubbly flow in a vertical pipe

International audienceThe problem of a steady, axisymmetric, fully developed adiabatic bubbly flow in a vertical pipe is studied analytically with the two-fluid model. The exchange of momentum between the phases is described as the sum of drag, lift, wall and dispersion contributions, with constant coefficients. Under these conditions, we are able to derive an analytical relation between the void fraction, the liquid velocity, and the pressure profiles. This relation is valid independently of the turbulence model in the liquid phase-here, a k-ε model is used-and can serve as a verification case for multiphase flow codes. The analytical void fraction profile vanishes at the wall, as a result of the balance between dispersion and wall forces. This profile is illustrated by calculations performed for upward and downward bubbly flows with the NEPTUNE_CFD code