Toward an improved wall treatment for multiple-correction k-exact schemes

Improved wall boundary treatments are investigated for a family of high-order Godunovtype finite volume schemes based on k-exact polynomial reconstructions in each cell of the primitive variables, via a successive corrections procedure. We focus more particularly on the 1-exact and 2-exact schemes which offer a good trade-off between accuracy and computational efficiency. In both cases, the reconstruction stencil needs to be extended to the boundaries. Additionally, information about wall curvature has to be taken into account, which is done by using a surface model based on bicubic Bézier patches for the walls. The performance of the proposed models is presented for two compressible cases, namely the inviscid flow past a Gaussian bump and the viscous axisymmetric Couette flow

Laboratory Studies of Mantle Convection

International audienc

Adaptive remeshing for industrial unsteady CFD

FLUSEPA is an advanced simulation tool which performs a large panel of aerodynamicstudies. It is the unstructured finite-volume solver developed by Airbus Defence& Space tocalculate compressible, multidimensional, unsteady, viscous and reactive flows around bodies in relative motion. The numerical strategy in FLUSEPA is designed for highly compressible flow and keeps its accuracy regardlessof the grid. According the desired accuracy, a second-order accurate shock-capturing scheme is generally used for RANS and URANS simulations and a fourth order accurate vortex-centredscheme is used for hybrid RANS/LES simulations[1].In this paper we introducean adaptive meshing approach to accurately represent unsteady flows in FLUSEPA.The meshing strategy is based on amulti-overlapping grid intersection which is conservative and allows to quickly and properly mesh 3D complex geometries. It can be seen as a CHIMERA strategy without interpolation. Each part of the bodiesis meshed independently and immersed in background grids.This technique will be largely described for it differs from the commonly used interpolation-based CHIMERA methods.The adaptive mesh construction is based on a simple 2-1 balanced octree approach. Different criteria for refinement will be tested:one relying on the Ducros[2]sensor, another is a basic hessian and the last one is based on the resolved kinetic energy. The aim of this study is to obtain a versatile module for industrial applications combining different criteria. The test case is a transonic turbulent flow around a square cylinder at a Mach number of 0.9 and Reynolds number of 4.105based on the experimental conditions of Nakagawa[3]. Computations are carried out using a CFL of 0.8 and on a Cartesian grid.The study will be focused on theinteraction between the von Karman eddy street and theshock wave

Cavitation bubbles: a tracer for turbulent mixing in large rivers

International audienceOn the Seine river in Paris, the high frequency of tourist boats traffic may exert a significant impact on transport of sediments and thus on transport and residence time of pollutants. To have a better understanding of anthropogenic effects and more generally to study rivers suspended sediments dynamics, it is essential to quantify the river transport capacity. Turbulent mixing is one of these transport mechanisms and we present here a simple technique to estimate lateral coefficient using ADCP backscatter signal analysis. We realized several static measurements during low water discharge (Q = 140 cubic meters per second) in which we can see a strong correlation between high backscatter values and the passage of boats. We argue that these high backscatter values, in the center of Paris city, are not due to a sediment plume but to cavitation bubbles. These high values suggest that resonant, ~10 micron bubbles are present in the flow in agreement with LISST grain size measurements. Given their slow ascent velocity these particles can be used as passive markers to estimate the lateral turbulent mixing coefficient. For this purpose we develop a dimensionless form of the sonar equation that, when coupled to a simple lateral turbulent diffusion model, allows to compute lateral diffusion coefficients. These estimates of lateral diffusion coefficient have an important implication for representative river water sampling and can be very useful to calibrate numerical models of river flow and sediment transport

A fluid dynamics perspective on the interpretation of the surface thermal signal of lava flows

International audienceEffusion rate is a crucial parameter for the prediction of lava-flow advance and should be assessed in near real-time in order to better manage a volcanic crisis. Thermal remote sensing offers the most promising avenue to attain this goal. We present here a ‘dynamic’ thermal proxy based on laboratory experiments and on the physical framework of viscous gravity currents, which can be used to estimate the effusion rate from thermal remote sensing during an eruption. This proxy reproduces the first-order relationship between effusion rate measured in the field and associated powers radiated by basaltic lava flows. Laboratory experiments involving fluids with complex rheology and subject to solidification give additional insights into the dynamics of lava flows. The introduction of a time evolution of the supply rates during the experiments gives rise to a transient adjustment of the surface thermal signal that further compromises the simple proportionality between the thermal flux and the effusion rate. Based on the experimental results, we conclude that a thermal proxy can only yield a minimum and time-averaged estimate of the effusion rate

Lignin plasticization to improve binderless fiberboard mechanical properties

International audienceMechanical properties of binderless fiberboard are correlated to the hydrothermal history of the fibers during the board process. Water and temperature thickness gradients allow to calculate a gradient of T-Tg difference (Tg, local glass transition temperature, function of local water content; T, local temperature). This gradient explains the variation of mechanical properties within the thickness and in time. The in situ plasticization of lignin, which is supposed to increase the T-Tg difference, improves the macroscopic mechanical properties of fiberboards. POLYM. ENG. SCI., 45:809-816, 2005

An experimental study of the surface thermal signature of hot subaerial isoviscous gravity currents: Implications for thermal monitoring of lava flows and domes

International audienceManagement of eruptions requires a knowledge of lava effusion rates, for which a safe thermal proxy is often used. However, this thermal proxy does not take into account the flow dynamics and is basically time-independent. In order to establish a more robust framework that can link eruption rates and surface thermal signals of lavas measured remotely, we investigate the spreading of a hot, isoviscous, axisymmetric subaerial gravity current injected at constant rate from a point source onto a horizontal substrate. We performed laboratory experiments and found that the surface thermal structure became steady after an initial transient. We develop a theoretical model for a spreading fluid cooled by radiation and convection at its surface that also predicts a steady thermal regime. We show that, despite the model's simplicity relative to lava flows, it yields the correct order of magnitude for the effusion rate required to produce the radiant flux measured on natural lava flows. For typical thermal lava properties and an effusion rate between 0.1 and 10 m3 s-1, the model predicts a steady radiated heat flux ranging from 108 to 1010 W. The assessed effusion rate varies quasi-linearly with the steady heat flux, with much weaker dependence on the flow viscosity. This relationship is valid only after a transient time which scales as the diffusive time, ranging from a few days for small basaltic flows to several years for lava domes. The thermal proxy appears thus less reliable to follow sharp variations of the effusion rate during an eruption

The influence of wind on the estimation of lava effusion rate from thermal remote-sensing

International audienceEffusion rate is a key parameter to model lava flow advance and associated risks. Estimation of effusion rate from thermal remote-sensing using satellite data has matured to the point where it can be an operational monitoring tool, notably for volcanoes without a ground observatory. However, robust physical models, as required for quantitative interpretations, have not yet been adequately developed. The current and widely used method relates the satellite-measured radiated power to the flow effusion rate through the lava area, with an empirical fit that assumes a low surface cooling efficiency. Here we use novel fluid dynamic laboratory experiments and viscous flow theory to show that assuming low convective cooling at the surface of the flow leads to a systematic underestimation of the effusion rate. This result, obtained for the case of a hot isoviscous gravity current which cools as it flows, relies only on the respective efficiency of convection and radiation at the flow surface, and is independent of the details of the internal flow model. Applying this model to lava flows cooling under classical wind conditions, we find that the model compares well to data acquired on basaltic eruptions within the error bars corresponding to the uncertainties on natural wind conditions. Hence the thermal proxy deduced from the isoviscous model does not seem to require an additional fitting parameter accounting for internal flow processes such as crystallization. The predictions of the model are not correct however for thick lava flows such as highly viscous domes, because a thermal steady state is probably not reached for these flows. Furthermore, in the case of very large basaltic flows, extra cooling is expected due to self-induced convection currents. The increased efficiency of surface cooling for these large eruptions must be taken into account to avoid a gross - and dangerously misleading - underestimate of the effusion rate

Influence de l’aluminium dans la protection par le zinc de l’acier contre la corrosion atmosphérique

La consommation mondiale de zinc par an est de l’ordre de 6 millions de tonnes, le marché le plus important étant la protection des aciers contre la corrosion. Dans les pays développés, le pourcentage de zinc destiné à la galvanisation — technique de revêtement de loin la plus utilisée — dépasse 40 %.Depuis une vingtaine d’années, de nombreuses recherches ont été entreprises afin d’améliorer la résistance à la corrosion des revêtements à base de zinc. Ces études ont conduit à mettre en évidence, en particulier, l’intérêt de certains alliages zinc-aluminium. Ceux-ci, actuellement au stade industriel, nouvellement introduits sur le marché, conduisent les décideurs à s’interroger sur leurs possibilités réelles.Il a paru utile de publier une série d’articles qui permettront de situer l’intérêt de ces produits grâce à la présentation des résultats et observations connus jusqu’alors