On the convection velocity of wall-bounded turbulence resolved by ZDES mode III at Re θ = 13 000

International audienceWMLES simulations of a flat-plate zero-pressure-gradient boundary layer are done with the Zonal Detached Eddy Simulation Mode III technique over a wide range of Reynolds numbers 3150≤Reθ≤14000. A WMLES field is compared with the WRLES interpolated onto the WMLES mesh. Two interface heights are considered, yinterface=0.1δ and y+interface=3.9√Reτ. The prediction and resolved fraction of mean skin friction is discussed, as well as remaining issues, especially in the logarithmic layer. An excess of high-wavelength streamwise velocity fluctuations is observed below the RANS/LES interface with y+interface=3.9√Reτ, and studied by a spectral convection velocity analysis, leading to the suggestion that it may be a footprint of coherent structures located further away from the wall.ZONAL DETACHED EDDY SIMULATION, REYNOLDS NUMBERS, RANS/LES INTERFACE, CONVECTION VELOCITY

A rapid and low noise switch from RANS to WMLES on curvilinear grids with compressible flow solvers

International audienceA turbulent inflow for a rapid and low noise switch from RANS to Wall-Modelled LES on curvilinear grids with compressible flow solvers is presented. It can be embedded within the computational domain in practical applications with WMLES grids around three-dimensional geometries in a flexible zonal hybrid RANS/LES modelling context. It relies on a physics-motivated combination of Zonal Detached Eddy Simulation (ZDES) as the WMLES technique together with a Dynamic Forcing method processing the fluctuations caused by a Zonal Immersed Boundary Condition describing roughness elements. The performance in generating a physically-sound turbulent flow field with the proper mean skin friction and turbulent profiles after a short relaxation length is equivalent to more common inflow methods thanks to the generation of large-scale streamwise vorticity by the roughness elements. Comparisons in a low Mach-number zeropressure-gradient flat-plate turbulent boundary layer up to Reθ = 6 100 reveal that the pressure field is dominated by the spurious noise caused by the synthetic turbulence methods (Synthetic Eddy Method and White Noise injection), contrary to the new low-noise approach which may be used to obtain the low-frequency component of wall pressure and reproduce its intermittent nature. The robustness of the method is tested in the flow around a three-element airfoil with WMLES in the upper boundary layer near the trailing edge of the main element. In spite of the very short relaxation distance allowed, self-sustainable resolved turbulence is generated in the outer layer with significantly less spurious noise than with the approach involving White Noise. The ZDES grid count for this latter test case is more than two orders of magnitude lower than the Wall-Resolved LES requirement and a unique mesh is involved, which is much simpler than some multiple-mesh strategies devised for WMLES or turbulent inflow

Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star

Discoveries from the prime Kepler mission demonstrated that small planets (< 3 Earth-radii) are common outcomes of planet formation. While Kepler detected many such planets, all but a handful orbit faint, distant stars and are not amenable to precise follow up measurements. Here, we report the discovery of two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located 65$\pm$6 pc from Earth. We detected the transiting planets in photometry collected during Campaign 3 of NASA's K2 mission. Analysis of transit light curves reveals that the planets have small radii compared to their host star, 2.60 $\pm$ 0.14% and 3.15 $\pm$ 0.20%, respectively. We obtained follow up NIR spectroscopy of K2-21 to constrain host star properties, which imply planet sizes of 1.59 $\pm$ 0.43 Earth-radii and 1.92 $\pm$ 0.53 Earth-radii, respectively, straddling the boundary between high-density, rocky planets and low-density planets with thick gaseous envelopes. The planets have orbital periods of 9.32414 days and 15.50120 days, respectively, and have a period ratio of 1.6624, very near to the 5:3 mean motion resonance, which may be a record of the system's formation history. Transit timing variations (TTVs) due to gravitational interactions between the planets may be detectable using ground-based telescopes. Finally, this system offers a convenient laboratory for studying the bulk composition and atmospheric properties of small planets with low equilibrium temperatures.Comment: Updated to ApJ accepted version; photometry available alongside LaTeX source; 10 pages, 7 figure

Mass Constraints of the WASP-47 Planetary System from Radial Velocities

We report precise radial velocity (RV) measurements of WASP-47, a G star that hosts three transiting planets in close proximity (a hot Jupiter, a super-Earth, and a Neptune-sized planet) and a non-transiting planet at 1.4 au. Through a joint analysis of previously published RVs and our own Keck-HIRES RVs, we significantly improve the planet mass and bulk density measurements. For the super-Earth WASP-47e (P = 0.79 days), we measure a mass of 9.11 ± 1.17 M_⊕, and a bulk density of 7.63 ± 1.90 g cm^(−3), consistent with a rocky composition. For the hot Jupiter WASP-47b (P = 4.2 days), we measure a mass of 356 ± 12 M_⊕ (1.12 ± 0.04 M_(Jup)) and constrain its eccentricity to $\lt 0.021$ at 3σ confidence. For the Neptune-size planet WASP-47d (P = 9.0 days), we measure a mass of 12.75 ± 2.70 M_⊕ and a bulk density of 1.36 ± 0.42 g cm^(−3), suggesting that it has a thick H/He envelope. For the outer non-transiting planet, we measure a minimum mass of 411 ± 18 M_⊕ (1.29 ± 0.06 M_(Jup)), an orbital period of 595.7 ± 5.0 days, and an orbital eccentricity of 0.27 ± 0.04. Our new measurements are consistent with but two to four times more precise than previous mass measurements

La couche limite turbulente en développement spatial

International audienceSpatially developing boundary layers at high Reynolds numbers play a key role in many engineering applications e.g. for determining friction drag on a body moving relative to a fluid. This enables performance prediction (e.g. fuel consumption)

Simulations avancées de turbulence pariétale à haut nombre de Reynolds sur des géométries curvilignes par une approche hybride RANS/LES

La capacité de simuler la dynamique de la couche limite turbulente représente aujourd hui un enjeu important pour la prévision de l aérodynamique instationnaire et de l aéroacoustique des aéronefs et des véhicules terrestres. Aussi, les travaux présentés dans ce manuscrit proposent une méthode originale de simulation de la dynamique des écoulements turbulents pariétaux, à haut nombre de Reynolds, sur des géométries curvilignes. L approche ZDES, dans son mode Wall Modelled Large Eddy Simulation , est ici retenue. Dans un premier temps, une méthode de réactivation turbulente, par combinaison de l injection de turbulence synthétique et de l application de termes de forçage, est développée afin de permettre une résolution WMLES locale des régions d intérêt au sein d une simulation majoritairement RANS. Puis l étude des interactions entre la physique et la méthode de résolution numérique (ZDES), sur le cas d une couche limite turbulente sans gradient de pression en développement spatial jusqu à Re 13 000, a conduit à une généralisation des conditions d emploi du mode WMLES de la ZDES. Des post-traitements instationnaires avancés ont permis de démontrer la capacité de la méthode à simuler la dynamique particulière de la zone externe à ces grands nombres de Reynolds. Enfin, les différentes méthodes développées au cours de cette étude ont été appliquées à la simulation d une manche à air coudée de section rectangulaire. Cette nouvelle méthode a permis de simuler avec succès la dynamique multi-échelles de cet écoulement et des distorsions dynamiques associées, pour un coût environ 50 fois inférieur à celui d une approche LES classiqueThe ability to simulate the dynamics of turbulent boundary layers is essential for the prediction of the unsteady aerodynamic and aeroacoustic properties of both land and air vehicles. Therefore, the present work proposes an innovative method to simulate near wall flows, around curvilinear geometries at high Reynolds numbers. The Wall Modelled Large Eddy Simulation mode of the ZDES method is considered here. First of all, a turbulence reactivation method, based on the combination of a synthetic turbulence injection method and of a body forcing one, is developed in order to allow a local WMLES resolution of the regions of interest within an essentially RANS calculation. Therefore, the study of the interactions that exist between the physics and the ZDES numerical method, in the case of a zero pressure gradient turbulent boundary layer developing over a flat plate until Re =13 000, led to a generalization of the guide lines to use the WMLES mode of the ZDES. Advanced unsteady post-processing enabled to demonstrate the ability of the method to simulate the specific dynamics of the outer region at high Reynolds numbers. Finally, the various methods that have been developed during this study were applied to the simulation of a S-shaped duct of rectangular section. The multi-scale dynamic of this flow as well as its resulting distortions were successfully simulated thanks to this new method, for an overall cost reduction of about 50 times compared to a traditional LES approachPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

On the convection velocity of wall-bounded turbulence resolved by ZDES mode III at Re θ = 13 000

International audienceThe convection velocity of resolved streamwise velocity fluctuations by a ZDES mode III WMLES simulation of high-Reynolds-number ZPG flat plate turbulent boundary layer is spectrally assessed. The physical nature of the fluctuations is then discussed

Simulation numérique des charges latérales instationnaires sur des configurations de lanceur

ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Simulation numérique, analyse physique et contrôle d'écoulements massivement décollés (Application au buffeting culot et à l'ovalisation de la tuyère sur des configurations de lanceur)

Le développement de l'accès à l'espace s'inscrit dans le contexte scientifique et économique actuel comme un enjeu majeur de l'industrie et de la recherche. Un des objectifs principaux est d'augmenter la capacité et le confort de la charge utile pour réduire les coûts de transport vers l'espace. L'exploitation des données en vol du lanceur Ariane 5 a mis en évidence la présence de fluctuations de pression pouvant induire des efforts instationnaires repris par les vérins du moteur Vulcain. Ces efforts s'exercent dans la zone décollée du culot d'un lanceur normalement à l'axe de la poussée, et sont qualifiés de charges latérales. Cette observation conduit à l'étude de deux aspects. Dans un premier temps, la dynamique des écoulements massivement décollés d'arrière-corps, à haut nombre de Reynolds et en régime compressible est considérée au moyen de simulations numériques ZDES. Une analyse approfondie des champs instantanés, moyens et fluctuants est réalisée au moyen de post-traitements avancés. Notamment, le champ tri-dimensionnel de pression fluctuante dans la région du culot fait l'objet d'analyses spectrales (Fourier) et modales (DMD) massives. Ensuite, le contrôle des phénomènes potentiellement nuisibles au confort de la charge utile à savoir le buffeting culot et l'ovalisation de tuyère est abordé. L'analyse physique de l'écoulement sur une géométrie tri-corps permet la conception d'un dispositif de contrôle adapté. Enfin, l'effet de deux dispositifs retenus (quatre jets équirépartis en azimut sur le corps central et augmentation de la section de passage entre les trois corps) est évalué au moyen des outils d'analyse de la dynamique de la configuration non contrôlée.In the current scientific and economic framework, access to space has become a great challenge for both the industry and research. One of the main purpose is to increase both the payload capacity and safety in order to reduce space transportation costs. The analysis of flight data from the Ariane 5 launcher has put forward the occurrence of pressure fluctuations leading to unsteady loads on the actuators of the Vulcain engine. These loads occur in the separated zone of the launcher base flow and act normally to the thrust axis. They are referred to as side loads. In the frame of such observations, two aspects are investigated. First, the dynamics of separating/reattaching flows at high Reynolds number and compressible regime is considered by means of numerical simulations of ZDES type. The instantaneous, mean and fluctuating flowfields are scrutinized using advanced post-processing. In particular, massive spectral (Fourier) and modal analyses (DMD) have been performed on the three-dimensional fluctuating pressure field of the base flow. Then, the control of phenomena which results in the buffeting phenomenon and nozzle ovalization is discussed. The physical analysis of the flow around a three-body geometry feeds into the design of an appropriate flow control device. Besides, the same analysis as for the uncontrolled configuration is undertaken in order to determine the effects of the tested devices (four jets equally distributed around the main body and increase of the cross section between the three bodies).PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Towards a physical scale decomposition of mean skin friction generation in the turbulent boundary layer

International audienceThe physical decomposition of mean skin friction generation in the ZPG flat plate turbulent boundary layer is discussed with the emphasis put on the physical mechanisms at play. The decomposition is then applied to a WRLES database obtained by ZDES up to Re_\theta = 13000, where a spectral decomposition of the TKE production is performed. This term is indeed identified with the generation of the excess of mean skin friction by turbulence compared with the laminar case in the limit of very high Reynolds numbers