1,720,991 research outputs found
Triangular rod bundle simulations of a CFD-turbulence model for heavy liquid metals k-e-kt-et heat transfer
No full textHeavy liquid metals are often considered as coolant fluids for fast nuclear reactors because of their physical and neutronic properties. Nevertheless, these fluids are not well known as ordinary fluids, such as air or water, and the use of reliable numerical tools for the simulation of turbulent heat transfer inside reactor cores refrigerated with liquid metals is still an open issue. In the present work we propose the use of a k-e-kt-et turbulence model to improve the prediction of turbulent heat transfer for liquid metal flows in triangular rod bundle geometry with different pitch-to-diameter ratios and different Peclet numbers. Nusselt number comparisons between experimental correlations and different heat turbulence models are reported for many test cases
A k-Ω-kθ-Ωθ four parameter logarithmic turbulence model for liquid metals
No full textTurbulence modeling is one of the main issues when dealing with Computational Fluid Dynamics simulations. Models based on the Reynolds Averaged Navier Stokes equations are commonly used in several applications but, for liquid metals, the constant turbulent Prandtl number approximation leads to overestimate the heat transfer. In this work we propose a new k-Ω-kθ-Ωθ turbulence model which improves the k-ε-kθ-εθ four parameter model presented in [1]. The main difficulties encountered in the numerical implementation of the k-ε-kθ-εθ model are the nonlinear boundary conditions in kθ-εθ and the evaluation of the characteristic near wall times. By introducing the logarithmic variables Ω and Ωθ in order to avoid negative values, we can easily set appropriate boundary conditions and improve numerical stability. The new formulation of the model is validated and compared with Direct Numerical Simulation data and experimental correlations
Optimal control problems for the Navier-Stokes system and k-ω turbulence model
No full textIn this work turbulence is considered by coupling the k-w two-equation turbulence model with the average Navier-stokes system. The complete optimality system is derived and the existence of a weak solution proven. Some numerical examples are reported
Hemodynamics of the heart's left atrium based on a Variational Multiscale-LES numerical method
Full text linkIn this paper, we investigate the haemodynamics of a left atrium (LA) by proposing a computational model suitable to provide physically meaningful fluid dynamics indications and detailed blood flow characterization. In particular, we consider the incompressible Navier–Stokes equations in Arbitrary Lagrangian Eulerian (ALE) formulation to deal with the LA domain under prescribed motion. A Variational Multiscale (VMS) method is adopted to obtain a stable formulation of the Navier–Stokes equations discretized by means of the Finite Element method and to account for turbulence modelling based on Large Eddy Simulation (LES). The aim of this paper is twofold: on one hand to improve the general understanding of blood flow in the human LA in normal conditions; on the other, to analyse the effects of the turbulence VMS-LES method on a situation of blood flow which is neither laminar, nor fully turbulent, but rather transitional as in LA. Our results suggest that if relatively coarse meshes are adopted, the additional stabilization terms introduced by the VMS-LES method allow to better predict transitional effects and cycle-to-cycle blood flow variations than the standard SUPG stabilization method
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Numerical validation of a four parameter logarithmic turbulence model
No full textComputational Fluid Dynamics codes are used in many industrial applications in order to evaluate interesting physical quantities, such as the heat transfer in turbulent flows. Commercial CFD codes use only turbulence models with an imposed constant turbulent Prandtl number Prt, which can give accurate results only for simulations when a strong similarity between the velocity field and the temperature field can be assumed. For fluids with a low Prandtl number, as for heavy liquid metals, a constant turbulent Prandtl number leads to an overestimation of the heat transfer, so experimental results and Direct Numerical Simulation cannot be reproduced. In this work we propose a new k-Ω-kθ-Ωθ turbulence model as an improvement of the k-ω-kθ-ωθ turbulence model, already validated by the authors, where Ω and Ωθ are calculated as the natural logarithm of the variables ω and ωθ. With this reformulation of the previous turbulence model we obtain some important advantages in numerical stability and robustness of the code. Results for the simulations of fully developed turbulent flows in two and three dimensional geometries are reported and compared with experimental correlations and DNS data, when available
Coupling of fluid structure interaction solver with a VOF method for multiphase structure interaction
No full textIn this work we propose a preliminary model to study the deformation of solid structures induced by the interaction with a two-phase flow. The study of Fluid Structure Interaction and multiphase problems is of great interest because of many potential applications ranging from the biomedical field to the pressure tank design. We use a monolithic approach for the FSI problem while a Volume Of Fluid method (VOF) is considered for the reconstruction and the advection of the multiphase interface. An unstructured, time dependent computational grid and a fine Cartesian mesh are used for the FSI and the VOF problem, respectively. The interaction between the two different grids is obtained by projecting the velocity and the displacement field into the Cartesian grid and the color function into the unstructured mesh. This projection is performed with the MEDmem libraries included in the Salome platform. Concerning the VOF method, for an accurate reconstruction of the interface a huge number of computational elements are required and a multilevel algorithm coupled to an efficient compression-expansion technique is developed to reduce computational costs and memory requirements. After the mathematical description of the problems we test the proposed algorithm with different cases where the solid domain undergoes to both small and large deformation
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
