42 research outputs found
Analyse de sous-maille par densité présumée en Simulation des Grandes Echelles
On s'intéresse à la modélisation des termes de réaction en Simulation des Grandes Echelles d'écoulements turbulents. La densité de sous-maille est souvent approchée dans ce cadre par une loi beta. Les estimateurs optimaux pour ce problème sont étudiés. Ils montrent que le choix des lois beta est justifié. L'essentiel de l'erreur commise provient en effet du choix des paramètres fondamentaux du modèle. Nous montrons ainsi que les estimateurs optimaux constituent un outil pertinent pour l'exploration des modèles de sous-maille et la recherche de pistes pour des améliorations futures
Dynamics of spectrally truncated inviscid turbulence
The evolution of the turbulent energy spectrum for the inviscid spectrally
truncated Euler equations is studied by closure calculations. The observed
behavior is similar to the one found in direct numerical simulations
[Cichowlas, Bona\"ititi, Debbasch, and Brachet, Phys. Rev. Lett. 95, 264502
(2005)]. A Kolmogorov spectral range and an equipartition range are observed
simultaneously. Between these two ranges a "quasi-dissipative" zone is present
in the kinetic energy spectrum. The time evolution of the wave number that
marks the beginning of the equipartition range is analyzed and it is shown that
spectral nonlocal interactions are governing this evolution
Large-scale bottleneck effect in two-dimensional turbulence
The bottleneck phenomenon in three-dimensional turbulence is generally
associated with the dissipation range of the energy spectrum. In the present
work, it is shown by using a two-point closure theory, that in two-dimensional
turbulence it is possible to observe a bottleneck at the large scales, due to
the effect of friction on the inverse energy cascade. This large-scale
bottleneck is directly related to the process of energy condensation, the
pile-up of energy at wavenumbers corresponding to the domain size. The link
between the use of friction and the creation of space-filling structures is
discussed and it is concluded that the careless use of hypofriction might
reduce the inertial range of the energy spectrum
Energy Transfer and Triadic Interactions in Compressible Turbulence
Using a two-point closure theory, the Eddy-Damped-Quasi-Normal-Markovian (EDQNM) approximation, we have investigated the energy transfer process and triadic interactions of compressible turbulence. In order to analyze the compressible mode directly, the Helmholtz decomposition is used. The following issues were addressed: (1) What is the mechanism of energy exchange between the solenoidal and compressible modes, and (2) Is there an energy cascade in the compressible energy transfer process? It is concluded that the compressible energy is transferred locally from the solenoidal part to the compressible part. It is also found that there is an energy cascade of the compressible mode for high turbulent Mach number (M(sub t) greater than or equal to 0.5). Since we assume that the compressibility is weak, the magnitude of the compressible (radiative or cascade) transfer is much smaller than that of solenoidal cascade. These results are further confirmed by studying the triadic energy transfer function, the most fundamental building block of the energy transfer
Energy transfer in compressible turbulence
This letter investigates the compressible energy transfer process. We extend a methodology developed originally for incompressible turbulence and use databases from numerical simulations of a weak compressible turbulence based on Eddy-Damped-Quasi-Normal-Markovian (EDQNM) closure. In order to analyze the compressible mode directly, the well known Helmholtz decomposition is used. While the compressible component has very little influence on the solenoidal part, we found that almost all of the compressible turbulence energy is received from its solenoidal counterpart. We focus on the most fundamental building block of the energy transfer process, the triadic interactions. This analysis leads us to conclude that, at low turbulent Mach number, the compressible energy transfer process is dominated by a local radiative transfer (absorption) in both inertial and energy containing ranges
Time-reversibility of Navier-Stokes turbulence and its implication for subgrid scale models
Among existing subgrid scale models for large-eddy simulation (LES) some are
time-reversible in the sense that the dynamics evolve backwards in time after a
transformation at every point in space. In practice,
reversible subgrid models reduce the numerical stability of the simulations
since the effect of the subgrid scales is no longer strictly dissipative. This
lack of stability constitutes often a criterion to reject this kind of models.
The aim of this paper is to examine whether time-reversibility can constitute a
criterion that a subgrid model has to fulfill, or has not to. Thereto we
investigate by direct numerical simulation the time-dependence of the kinetic
energy of the resolved scales when the velocity is reversed in all or part of
the lengthscales of the turbulent flow. These results are compared with results
from existing LES subgrid models. It is argued that the criterion of
time-reversibility to assess subgrid models is incompatible with the main
underlying assumption of LES.Comment: Journal of Turbulence (2012) 1-
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Abstract
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries
Statistical modelling of turbulence and applications to Large-Eddy Simulation
The subject of this presentation is turbulence modelling and applications to Computational Fluid Dynamics (CFD). A special attention is paid to the spectral models of turbulence, also called statistical two-point closures. The main ideas underlying the derivation of the usual two-point closures are reviewed. The Direct Interaction Approximation (DIA) and related closures, such as the Eddy-Damped Quasi Normal Markovian (EDQNM) model or the Closure Based on Fluid Particle Displacements (CBFPD), are briefly presented. Particular emphasis is given to the application of closures to high Reynolds number turbulence. The relations between two-point closures and other types of turbulence models, in particular subgrid models for Large-Eddy Simulations (LES), are discussed. It is stressed that two-point closures constitute useful tools to understand the interaction between large and small scales that is essential for the problem of subgrid modeling in LES