Spectral imbalance and the normalized dissipation rate of turbulence

The normalized turbulent dissipation rate $C_\epsilon$ is studied in decaying and forced turbulence by direct numerical simulations, large-eddy simulations, and closure calculations. A large difference in the values of $C_\epsilon$ is observed for the two types of turbulence. This difference is found at moderate Reynolds number, and it is shown that it persists at high Reynolds number, where the value of $C_\epsilon$ becomes independent of the Reynolds number, but is still not unique. This difference can be explained by the influence of the nonlinear cascade time that introduces a spectral disequilibrium for statistically nonstationary turbulence. Phenomenological analysis yields simple analytical models that satisfactorily reproduce the numerical results. These simple spectral models also reproduce and explain the increase of $C_\epsilon$ at low Reynolds number that is observed in the simulations

Un nouvel Indice Diatomique Pratique pour l'évaluation de la qualité des eaux en réseau de surveillance

L'Indice de Polluosensibilité Spécifique (IPS) est considéré comme l'un des indices diatomiques les plus performants pour l'évaluation de la qualité des cours d'eau. Son utilisation en réseau de surveillance reste cependant limitée en raison de la nécessité de travailler au niveau spécifique voire infraspécifique et de la systématique en perpétuelle évolution. A l'opposé, l'Indice Diatomique Générique (IDG) est plus accessible dans sa mise en oeuvre mais ne permet pas d'obtenir des résultats très fiables. Un nouvel Indice Diatomique Pratique (IDP) a donc été mis au point sur un bassin versant expérimental à partir d'un jeu de 86 relevés. Dans un premier temps, les inventaires ont été classés en fonction des écarts observés entre IPS et IDG. Dans un second temps, ont été identifiées les espèces responsables de ces écarts en prenant en compte celles présentant une abondance relative supérieure à 5 % et une différence de polluosensibilité avec le genre correspondant supérieure ou égale à 0,4. Plusieurs IDP ont été mis au point et leurs performances, par rapport à l'IPS, étudiées. Il apparaît que la prise en compte des espèces responsables des écarts supérieurs ou égaux à 2 constitue le meilleur compromis entre fiabilité et applicabilité en réseau. Cette méthodologie a été appliquée aux 480 relevés effectués dans le bassin Artois - Picardie et aux 550 espèces inventoriées. Elle permet de proposer un indice diatomique pratique basé sur l'identification de 45 genres et 91 espèces.Macroinvertebrates constitute the main biological support for an evaluation of the quality of water courses and are, therefore, widely put to use in monitoring networks. However, for major water courses and canalized waterways the use of other methodologies is imperative. Diatoms and diatom indices are well adapted to the study of these environments. Among these, the Specific Polluosensitivity Index (SPI) established by CEMAGREF seems to be one of the better performing diatom indices. Calculation of this index relies on the Zelinka & Marvan formula derived from the saprobic system: SPI=[Epsilon]A[inf]j v[inf]j i[inf]j / [Epsilon] A[inf]j v[inf]j where A[inf]j is the relative abundance of the species j, v j is its indicative value ( 1 [smaller or equal] v[inf]j [smaller or equal] 3) and i[inf]j its pollution sensitivity (1 [smaller or equal] i[inf]j [smaller or equal] 5). The values initially falling in the range between 1 and 5 are transformed into values comprised between 1 and 20, in order to make comparisons between the various existing indices easier. Five categories of water quality can be distinguished according to the value of the index: SPI [Bigger or equal] 16: zero pollution or low eutrophication; 13.5 [smaller or equal] SPI < 16: moderate eutrophication; 11 [smaller or equal] SPI < 13.5: moderate pollution or heavy eutrophication; 7 [smaller or equal] SPI < 11: high pollution; SPI < 7 : very heavy pollution. However, the SPI index is rarely used because of two main obstacles: it requires data at a specific or even infraspecific level, and it is based on constantly changing systematics. Progress towards increased accessibility and, therefore, larger application was made with the elaboration of the Generic Diatomic Index (IDG) based on the same principle as the SPI. However, this GDI does not yield reliable results, in so far as certain genera, such as Navicula and Nitzschia, contain species with a widely differing ecologies. In order to provide a methodology that can be used as a matter of routine, a protocol for the elaboration of a Practical Diatomic Index (PDI) was established and tested on 86 inventories from the water basin of the river Aa (North of France). These were first classified into four categories according to the variations observed between SPI and GDI: category 1: |SPI-GDI| [bigger or equal] 3 ; category 2: 2 [smaller or equal] |SPI-GDI|; category 3: 1 [smaller or equal] |SPI-DGI| < 2 ; category 4: |SPI-DGI| < 1. For each of the first three categories, the species responsible for the variations were identified, taking into consideration those with a relative abundance of more than 5%, the pollution sensitivity of which showed, compared to the corresponding genus, a variation higher than or equal to 0.4. Thus, three indices corresponding respectively to category 1 (PDI1), 2 (PDI2), and 3 (PDI3) were proposed and tested against the SPI taken as reference index. The results of this comparative study can be summarized as follows:- GDI=0.57 SPI + 5.47 r=0.801 (242 species), - PDI1=0.86 SPI + 1.12 r=0.972 ( 27 species), - PDI2=0.95 SPI + 0.55 r=0.991 ( 39 species), - PDI3=0.96 SPI + 0.45 r=0.994 ( 42 species). To test the implications of replacing the presently used SPI by this practical index, a comparative study of the classification of inventories in four categories of hydrobiological quality was also carried out. This study shows that the mean, at - 1.76 ± 2.25 for the GDI, is reduced to 0.14 ± 0.94 for PDI1, to - 0.07 ± 0.51 for PDI2, and to - 0.07 ± 0.45 for PDI3. Given the variability of the index at one and the same site and in one sampling, PDI2 considered to be the best compromise between reliability and network applicability. The methodology corresponding to PDI2 was applied to the 480 samplings carried out in the Artois-Picardie basin and a new Practical Diatom Index is thus proposed for the monitoring of the 200 sites making up the monitoring network of the Artois-Picardie water basin. This PDI, built on a base of more than 550 species and varieties, rests on the joint determination of 45 genera and 91 species of which the pollution sensitivity coefficients and the indicative values are given

Genetic Rat Models of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by a specific loss of dopaminergic neurons. Although the vast majority of PD cases are idiopathic in nature, there is a subset that contains genetic links. Of the genes that have been linked to PD, α-synuclein and leucine-rich repeat kinase 2 have been used to develop transgenic rat models of the disease. In this paper we focused on the various transgenic rat models of PD in terms of their ability to mimic key symptoms of PD in a progressive manner. In general, we found that most of these models provided useful tools for the early stages of PD, but the development of new transgenic rats that present significant neuropathologic and motoric deficits in a progressive manner that more accurately mimics PD is needed

Numerical studies towards practical large-eddy simulation

Large-eddy simulation developments and validations are presented for an improved simulation of turbulent internal flows. Numerical methods are proposed according to two competing criteria: numerical qualities (precision and spectral characteristics), and adaptability to complex configurations. First, methods are tested on academic test-cases, in order to abridge with fundamental studies. Consistent results are obtained using adaptable finite volume method, with higher order advection fluxes, implicit grid filtering and "low-cost" shear-improved Smagorinsky model. This analysis particularly focuses on mean flow, fluctuations, two-point correlations and spectra. Moreover, it is shown that exponential averaging is a promising tool for LES implementation in complex geometry with deterministic unsteadiness. Finally, adaptability of the method is demonstrated by application to a configuration representative of blade-tip clearance flow in a turbomachine

Effect of sonication conditions: solvent, time, temperature and reactor type on the preparation of micron sized vermiculite particles

International audienceThe effects of temperature, time, solvent and sonication conditions under air and Argon are described for the preparation of micron and sub-micron sized vermiculite particles in a double-jacketed Rosett-type or cylindrical reactor. The resulting materials were characterized via X-ray powder diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR) Spectroscopy, BET surface area analysis, chemical analysis (elemental analysis), thermogravimetry analysis (TGA) and Laser Granulometry. The sonicated vermiculites displayed modified particle morphologies and reduced sizes (observed by scanning electron microscopy and laser granulometry). Under the conditions used in this work, sub-micron sized particles were obtained after 5 h of sonication, whereas longer times promoted aggregation again. Laser granulometry data revealed also that the smallest particles were obtained at high temperature while it is generally accepted that the mechanical effects of ultrasound are optimum at low temperatures according to physical/chemical properties of the used solvent. X-ray diffraction results indicated a reduction of the crystallite size along the basal direction [001]; but structural changes were not observed. Sonication at different conditions also led to surface modifications of the vermiculite particles brought out by BET surface measurements and Infrared Spectroscopy. The results indicated clearly that the efficiency of ultrasound irradiation was significantly affected by different parameters such as temperature, solvent, type of gas and reactor type

Convergence of the Generalized Volume Averaging Method on a Convection-Diffusion Problem: A Spectral Perspective

A mixed formulation is proposed and analyzed mathematically for coupled convection-diffusion in heterogeneous medias. Transfer in solid parts driven by pure diffusion is coupled with convection-diffusion transfer in fluid parts. This study is carried out for translation-invariant geometries (general infinite cylinders) and unidirectional flows. This formulation brings to the fore a new convection-diffusion operator, the properties of which are mathematically studied: its symmetry is first shown using a suitable scalar product. It is proved to be self-adjoint with compact resolvent on a simple Hilbert space. Its spectrum is characterized as being composed of a double set of eigenvalues: one converging towards −∞ and the other towards +∞, thus resulting in a nonsectorial operator. The decomposition of the convection-diffusion problem into a generalized eigenvalue problem permits the reduction of the original three-dimensional problem into a two-dimensional one. Despite the operator being nonsectorial, a complete solution on the infinite cylinder, associated to a step change of the wall temperature at the origin, is exhibited with the help of the operator’s two sets of eigenvalues/eigenfunctions. On the computational point of view, a mixed variational formulation is naturally associated to the eigenvalue problem. Numerical illustrations are provided for axisymmetrical situations, the convergence of which is found to be consistent with the numerical discretization