A LES-Langevin model for turbulence

We propose a new model of turbulence for use in large-eddy simulations (LES). The turbulent force, represented here by the turbulent Lamb vector, is divided in two contributions. The contribution including only subfilter fields is deterministically modeled through a classical eddy-viscosity. The other contribution including both filtered and subfilter scales is dynamically computed as solution of a generalized (stochastic) Langevin equation. This equation is derived using Rapid Distortion Theory (RDT) applied to the subfilter scales. The general friction operator therefore includes both advection and stretching by the resolved scale. The stochastic noise is derived as the sum of a contribution from the energy cascade and a contribution from the pressure. The LES model is thus made of an equation for the resolved scale, including the turbulent force, and a generalized Langevin equation integrated on a twice-finer grid. The model is validated by comparison to DNS and is tested against classical LES models for isotropic homogeneous turbulence, based on eddy viscosity. We show that even in this situation, where no walls are present, our inclusion of backscatter through the Langevin equation results in a better description of the flow.Comment: 18 pages, 14 figures, to appear in Eur. Phys. J.

A Bayesian fusion model for space-time reconstruction of finely resolved velocities in turbulent flows from low resolution measurements

The study of turbulent flows calls for measurements with high resolution both in space and in time. We propose a new approach to reconstruct High-Temporal-High-Spatial resolution velocity fields by combining two sources of information that are well-resolved either in space or in time, the Low-Temporal-High-Spatial (LTHS) and the High-Temporal-Low-Spatial (HTLS) resolution measurements. In the framework of co-conception between sensing and data post-processing, this work extensively investigates a Bayesian reconstruction approach using a simulated database. A Bayesian fusion model is developed to solve the inverse problem of data reconstruction. The model uses a Maximum A Posteriori estimate, which yields the most probable field knowing the measurements. The DNS of a wall-bounded turbulent flow at moderate Reynolds number is used to validate and assess the performances of the present approach. Low resolution measurements are subsampled in time and space from the fully resolved data. Reconstructed velocities are compared to the reference DNS to estimate the reconstruction errors. The model is compared to other conventional methods such as Linear Stochastic Estimation and cubic spline interpolation. Results show the superior accuracy of the proposed method in all configurations. Further investigations of model performances on various range of scales demonstrate its robustness. Numerical experiments also permit to estimate the expected maximum information level corresponding to limitations of experimental instruments.Comment: 15 pages, 6 figure

Efficient Retrieval and Ranking of Undesired Package Cycles in Large Software Systems

International audienceMany design guidelines state that a software system architecture should avoid cycles between its packages. Yet such cycles appear again and again in many programs. We believe that the existing approaches for cycle detection are too coarse to assist developers to remove cycles from their programs. In this paper, we describe an efficient algorithm that performs a fine-grained analysis of cycles among application packages. In addition, we define multiple metrics to rank cycles by their level of undesirability, prioritizing cycles that are the more undesired by developers. We compare these multiple ranking metrics on four large and mature software systems in Java and Smalltalk

The three-dimensional structure of large scale motions in turbulent boundary layer

Turbulent boundary layer, a natural phenomenon occurring at the vicinity of bounding surface for almost any flow condition, is important and remains a challenging engineering problem in many domains such as transportation, energy or health. Recently, the large scale coherent structures have been shown to play a significant role in the statistics of friction drag. Moreover, dynamic of such structures are essential in an attempt to control such turbulent flows. The detailed description of 3D structures in physical space are mainly accessible from large Direct Numerical Simulation (DNS). Few DNS at significant Reynolds number have already been performed. Schlatter and Orlu (2010) have shown that rigorous approach to set up simulation is substantial since different simulations (for same canonical flow case) give inconsistent measures even for basic quantities. Sillero et al. (2011) investigated large scale structures in detail and compared with channel flows. However, the statistics of very large scales can be affected by the simulation parameters. In the present study, DNS of zero pressure gradient turbulent boundary layer up to Re? = 2550 is conducted with the code ""Incompact3d"" on a simulation domain of size 600?_inlet × 40?_inlet × 20?_inlet. Transition to turbulence from the inlet Blasius profile is forced at Re?=300 with a tripping suggested by Schlatter and Orlu (2010). The advantage of this method over recycling methods is to avoid some spurious streamwise correlations that could act upon large scale turbulence statistics for not long enough simulation domain. Up to 500, 3-dimensional velocity and pressure fields have been collected within 20 characteristic time based on u? and ? at the middle of the simulation domain. In addition, time resolved data in four planes perpendicular to the flow at Re? = 922, 1522, 2063 and 2365 also recorded in order to compare space and time statistics. The statistics of the current DNS is in good agreement with other state of the art TBL simulations. The aim of the present study is to investigate large scale structures by using the same methodology used with data acquired from particle image velocimetry (PIV) on a large field of view at higher Reynolds numbers (Srinath et al. 2017). The statistics of structures extracted from 2-dimensional and 3-dimensional data, later differences between those are investigated. Besides, large scale structures are extracted from both spatial and time resolved data. Low and high momentum regions are extracted by applying a threshold on streamwise velocity fluctuations and morphological treatments. Moreover, the 3-dimensional examination is strengthened with quadrant analysis of Reynolds shear stress. Aspect ratio for different directions are presented and compared to similar results obtained in channel flow by Duran et al. (2014). This study notifies the difficulties to extract statistics of large scale motions and to propose detection procedures which allow a fair comparison of statistics of different wall turbulent flows between 3-dimensional numerical data at moderate Reynolds numbers and high Reynolds experiments data

Mullite interaction with bismuth oxide from minerals and sol-gel processes

International audienceThe high temperature treatment of kaolinite-muscovite alternate layers doped by bismuth oxide was studied by TGA and DTA, X ray diffraction, and electron microscopy (SEM). Thermal analyses shows that the two main transformation stages are the dehydroxylation of phyllosilicates and the structural reorganization of the whole assembly. During dehydroxylation, a progressive decrease of the structural order of kaolinite and muscovite occurs. It is more significant at temperature above 1000°C and mullite, glass and Alrich oxides are formed. Mullite exhibits an accentuated acicular morphology along 3 preferential orientations in relation to the remaining structure in the basal (001)musc planes of the former muscovite. With addition of bismuth oxide, SEM observations point to the strong accentuation of mullite growth along the c axis, at the very low temperature of 1050°C. This behavior was studied with various mullite-bismuth oxide compounds at temperatures ranging from 1000°C to 1400°C. Solgel processing were performed with TEOS (C2H5O)4Si, aluminum nitrate Al(NO3)3.9H2O and bismuth nitrate Bi(NO3)3. From Rietveld refinements of X ray diffraction patterns it was found that bismuth doesn't form a specific phase with mullite during the mullite nucleation and growth. At low Bi2O3 content, mullite coexists with amorphous silico-aluminate phases, even at high temperature. When the Bi2O3 content increases above 12mol%, mullite is no longer crystallized. Above 750°C, aluminum bismuth oxide coexists with silicon bismuth oxide. The liquidus temperature is below 1100°C and decreases with Bi2O3 additions. Above the liquidus, only corundum and a liquid coexist. A tentative mullite-Bi2O3 binary phase diagram is then proposed. These results evidence the Bi2O3 role on the early and large mullite growth in kaolinite-muscovite materials. Particularly, a very low quantity of a low viscous phase favor the local mobility of diffusing species and promotes the nucleation and growth of very large mullite crystals. In this way, highly textured and dense ceramics are obtained at low sintering temperature. Thin substrates (<500μm) present specific mechanical behaviors with the deviation of cracks along mullite layers

Détection de tourbillons longitudinaux dans la zone tampon de la couche limite turbulente

La zone tampon est une région qui joue un rôle très important dans le processus d'auto-génération de la couche limite turbulente. Cette région contient des structures cohérentes : Tourbillons, Streaks, Sweep, Ejections, ... Une expérience de PIV Stéréoscopique a été réalisée à la soufflerie du LML pour caractériser l'organisation de cette région. Dix plans de PIV ont été enregistrés très prés de la paroi (14.5<y+<48) contenant chacun 500 champs de vitesse à trois composantes. Après avoir validé cette base de données, une analyse de la structure à l'aide de la reconnaissance de forme a été effectuée qui a permis de détecter et de caractériser statistiquement les Streaks et les tourbillons longitudinaux. Une organisation de la zone tampon a pu être déduite à partir de la corrélation spatiale double des fonctions indicatrices des différentes structures

Caractérisation expérimentale du décollement instationnaire autour d'un obstacle 2D

Nous présentons les résultats expérimentaux concernant la caractérisation de l'écoulement turbulent autour d'un obstacle 2D à la fois en écoulement confiné de type canal plan et en couche limite. L’objectif est d’étudier d’une part les effets Reynolds menant au décollement et d’autre part les mécanismes d'interaction entre les structures énergétiques et la zone décollée instationnaire. L’originalité de l’étude réside dans l’emploi de la méthode électrochimique polarographique en vue de caractériser expérimentalement le frottement pariétal sur l’obstacle. Ces résultats, et notamment les statistiques du frottement pariétales sont comparé favorablement avec des simulations numériques (DNS) récentes pour la même configuration d’essai. Les aspects instationnaires ainsi que les instabilités liées à la zone décollée ont également été clairement identifiées

Experimental Characterization of a High Reynolds Number Turbulent Boundary Layer subjected to an Adverse Pressure Gradient

The observation of large-scale coherent structures in turbulent boundary layers has sparked great experimental and numerical interest. While most studies have focussed on channel flows and zero pressure gradient (ZPG) boundary layer flows, our understanding of wall turbulence under the influence of a streamwise pressure gradient is still quite limited due to the lack of sufficiently high Reynolds number data and large facilities to reach an equilibrium state [1], where theoretical scaling laws can be relevant. The length of these structures (7-14? [2]) requires a large field of view and a high spatial resolution to measure all relevant spatial scales. To resolve and characterize the structures in an adverse pressure gradient (APG) boundary layer, a set-up in the LML boundary layer wind tunnel was built using 16 sCMOS cameras (a consortium involving 4 teams under the framework of EuHIT) in order to perform large scale turbulent boundary layer measurements with appropriate spatial resolution [3]. The length of the 2D2C PIV measurement domain was 3.5m long and 0.25m high to ensure the possibility of capturing very large-scale structures with lengths more than 10?. A total of 30000 images were recorded for two free-stream velocities of 5m/s and 9 m/s (corresponding to Re??10600 and Re??17700 at station 4 respectively) [4]. The topology and dynamics of the large-scale turbulent structures under an APG will be presented. As a similar experiment with a large field of view (1.16m long and 0.3m high) was earlier conducted on a ZPG turbulent boundary layer at LML, the influence of an APG on these structures will be shown. References [1] L. Castillo and W. K. George. Similarity analysis for turbulent boundary layer with pressure gradient: outer flow. AIAA journal, 39.1:41-47, 2001 [2] N. Hutchins and I. Marusic. Evidence of very long meandering features in the logarithmic region of turbulent boundary layers. J Fluid Mech, 579:1-28, 2007 [3] Cuvier et al. Under preparation. [4] Database of the large field APG experiments: https://turbase.cineca.it/turbase/default/#/view_dataset/2