Benchmark solution for a three-dimensional mixed convection flow - Part 1: reference solutions

International audienceAbstract A solution to a benchmark problem for a three-dimensional mixed convection flow in a horizontal rectangular channel heated from below and cooled from above (Poiseuille-Rayleigh-Bénard flow) is proposed. This flow is a steady thermoconvective longitudinal roll flow in a large aspect ratio channel at moderate Reynolds and Rayleigh numbers (Re=50, Ra=5000) and Prandtl number Pr=0.7. The model is based on the Navier-Stokes equations with Boussinesq approximation. We propose reference solutions resulting from computations on large grids, Richardson extrapolation (RE) and cubic spline interpolations. The solutions obtained with one finite difference, one finite volume and two finite element codes are in good agreement and reference values for the flow and thermal fields and the heat and momentum fluxes are given with 4 to 5 significant digits

The Positive Impact of the Early-Feeding of a Plant-Based Diet on Its Future Acceptance and Utilisation in Rainbow Trout

Sustainable aquaculture, which entails proportional replacement of fish-based feed sources by plant-based ingredients, is impeded by the poor growth response frequently seen in fish fed high levels of plant ingredients. This study explores the potential to improve, by means of early nutritional exposure, the growth of fish fed plant-based feed. Rainbow trout swimup fry were fed for 3 weeks either a plant-based diet (diet V, V-fish) or a diet containing fishmeal and fish oil as protein and fat source (diet M, M-fish). After this 3-wk nutritional history period, all V- or M-fish received diet M for a 7-month intermediate growth phase. Both groups were then challenged by feeding diet V for 25 days during which voluntary feed intake, growth, and nutrient utilisation were monitored (V-challenge). Three isogenic rainbow trout lines were used for evaluating possible family effects. The results of the V-challenge showed a 42% higher growth rate (P = 0.002) and 30% higher feed intake (P = 0.005) in fish of nutritional history V compared to M (averaged over the three families). Besides the effects on feed intake, V-fish utilized diet V more efficiently than M-fish, as reflected by the on average 18% higher feed efficiency (P = 0.003). We noted a significant family effect for the above parameters (P,0.001), but the nutritional history effect was consistent for all three families (no interaction effect, P.0.05). In summary, our study shows that an early shortterm exposure of rainbow trout fry to a plant-based diet improves acceptance and utilization of the same diet when given at later life stages. This positive response is encouraging as a potential strategy to improve the use of plant-based feed in fish, of interest in the field of fish farming and animal nutrition in general. Future work needs to determine the persistency of this positive early feeding effect and the underlying mechanisms

Rosina - Rosetta Orbiter Spectrometer for Ion and Neutral Analysis

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will answer important questions posed by the mission's main objectives. After Giotto, this will be the first time the volatile part of a comet will be analyzed in situ. This is a very important investigation, as comets, in contrast to meteorites, have maintained most of the volatiles of the solar nebula. To accomplish the very demanding objectives through all the different phases of the comet's activity, ROSINA has unprecedented capabilities including very wide mass range (1 to >300 amu), very high mass resolution (m/Δ m > 3000, i.e. the ability to resolve CO from N2 and 13C from 12CH), very wide dynamic range and high sensitivity, as well as the ability to determine cometary gas velocities, and temperature. ROSINA consists of two mass spectrometers for neutrals and primary ions with complementary capabilities and a pressure sensor. To ensure that absolute gas densities can be determined, each mass spectrometer carries a reservoir of a calibrated gas mixture allowing in-flight calibration. Furthermore, identical flight-spares of all three sensors will serve for detailed analysis of all relevant parameters, in particular the sensitivities for complex organic molecules and their fragmentation patterns in our electron bombardment ion source

Influence of container shape and size on surface-tension-driven Benard convection

International audienceThe aim of this study is to investigate the influence of the container size and shape on the main fluid flow characteristics of Surface-Tension-Driven B,nard Convection. Computations have been performed for high Prandtl number fluids and realistic boundary conditions in various configurations either at steady state when it exists or unsteady one for Ma (c) a parts per thousand currency sign Ma a parts per thousand currency sign 2.5Ma (c) . The threshold value, its associated pattern and secondary bifurcation one are presented for each configuration. For very small aspect ratios, it turns out that the threshold value is determined by the friction coefficient whereas for medium size aspect ratios both size and shape enters the game in a more subtle way. Some containers have been found to induce a quasi-perfect hexagonal pattern in their core region provided they satisfy shape and size compatibility conditions. Otherwise, dynamical regimes may appear even close to the threshold so their peculiar characteristics have been reported and analyzed as they seem to be intrinsic to small aspect ratio configurations

Asymptotic Numerical Method applied to stability and bifurcation analysis of a boiling flow at low pressure in a pipe

International audienceIn the framework of the RetD on GEN IV Sodium-cooled Fast Reactors (SFRs) at CEA, a hypothetical Unprotected Loss of Flow (ULOF) is investigated and could lead to sodium boiling if one assumes the non-activation of passive complementary safety devices. As a qualification basis, the CEA is currently developing and coupling reference codes at different scales. Supposing a complete integrity of hydraulic channels geometry, reactor case studies have shown the possibility of a periodic boiling regime the density wave mechanism, which is believed to be enhanced by low void worth core design distinctive of a GEN IV SFR, is interestingly characterized by pins claddings cooling possibility at sodium saturation temperature (instead of dry-out). In that scientific framework, this paper presents the ongoing development of an innovative semi-analytical methodology to perform stability and bifurcation analyses of boiling flows. While equations for a 1D multiphase model based on a drift model are classically used, the Asymptotic Numerical Method (ANM) is implemented to solve steady-state equations whose non linearities drive the observed dynamic phenomena. Inline, elements of the Theory of Dynamical Systems are first recalled, such as the Hopf bifurcation linked to the appearance of a periodic solution and its stability, to provide the necessary mathematical background. The added-value for this specific study of the ANM over a zero-order continuation method is illustrated on a test case. Some simulation results are then reported to investigate flow phenomenology that was addressed along boiling stability experiments led by Saha in 1970s. On one hand, the runs show our model's numerical ability to efficiently handle a non-linear set of equations while imposing a numerically stringent pressure difference for flow boundary conditions; on the other hand, no qualitative evidence of unstability onset has been observed while such a phenomenon is the central point of Saha experimental work. Even if only eigenvalues analysis, to be next carried out, could assert a miss of the model, this point could reveal some limitations regarding the set of closure laws or geometrical description that is up to now considered in our model. To support this statement, some additional simulation results on an academic case are given which exhibit how a non-linearity of the model, not engaged in the Saha experimental conditions, could lead to an oscillating response in the bifurcation diagram

Méthodes numériques et thermique

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Mixed convection flow in a horizontal rectangular channel subjected to a horizontal thermal gradient

International audienceThis paper deals with a mixed convection water flow in a horizontal rectangular duct, uniformly heated from one lateral vertical wall and thermally insulated elsewhere. The supplied heat flux induces a secondary flow, which structure is constituted of one longitudinal roll in the considered aspect ratio (Gamma = 1.9), embedded into a return flow of possibly large stream wise extension (up to twenty channel heights). Such situation induces helicoidal trajectories for the fluid flow particles, which contributes to a heat transfer enhancement compared to purely forced convection flow

Numerical simulation of welding positions in Gas Tungsten Arc Welding

International audienceThe fluid flow in weld pool can play a prominent role in the weld quality as well as in the shape of the weld bead seam. It is influenced by several process parameters and operating conditions such as current intensity, welding speed, chemical composition of the material and welding positions. A coupled model that accounts for electromagnetism, heat transfer and fluid flow with moving free surface has been developed. It aims at predicting temperature field, fluid flow velocity in the melting pool and shape of weld joints for various operating parameters. Numerical simulations are carried out with our in-house finite element code Cast3M to study the influence of welding positions flat, horizontal, vertical up, vertical down and overhead. It turns out that there exists a strong sensitivity of the molten pool shape to welding positions due to different gravity direction. The present work enables us to undertake the development of a more advanced numerical model that will include metal filling