1,720,975 research outputs found
Disperse two-phase flows, with applications to geophysical problems
Get PDFIn this paper we study the motion of a fluid with several dispersed particles whose concentration is very small (smaller than 10−3), with possible applications to problems coming from geophysics, meteorology, and oceanography. We consider a very dilute suspension of heavy particles in a quasi-incompressible fluid (low Mach number). In our case the Stokes number is small and --as pointed out in the theory of multiphase turbulence-- we can use an Eulerian model instead of a Lagrangian one. The assumption of low concentration allows us to disregard particle--particle interactions, but we take into account the effect of particles on the fluid (two-way coupling). In this way we can study the physical effect of particle inertia (and not only passive tracers), with a model similar to the Boussinesq equations. The resulting model is used in both direct numerical simulations and large eddy simulations of a dam-break (lock-exchange) problem, which is a well-known academic test case. Keywords: Dilute suspensions, Eulerian models, direct and large eddy simulations, slightly compressible flows, dam-break (lock-exchange) problem
A higher-order subfilter-scale model for large eddy simulation
No full textThis paper presents a new subfilter-scale stress model for large eddy simulation. The unknown velocity field is represented in terms of the filtered velocity by using a higher-order Padé approximation of the Fourier transform of the Gaussian filter. This accurate approximation of the velocity field yields an improved subfilter-scale stress tensor accounting for the information lost in the filtering process. The accuracy of the subfilter-scale stress tensor is especially important in the large eddy simulation of complex flows, such as geophysical flows, where the practical grid size is much larger than the scale of turbulent motion. We illustrate our approach through two simple one-dimensional numerical examples. We also present a rigorous mathematical analysis for this new large eddy simulation model
Horizontal Large Eddy Simulation of Stratified Mixing in a Lock-Exchange System
No full textThis paper presents analytical and numerical results for two new anisotropic modifications of the Rational and Clark-alpha LES models. The main difference from their standard form is that in this study horizontal (as opposed to isotropic) spatial filtering is used, which is appropriate for turbulent mixing in stratified flows. We present several mathematical results regarding the horizontal Rational and Clark-alpha LES models. We also present numerical experiments that support the analytical developments and show that both horizontal LES models perform better than their standard, isotropic counterparts in approximating mixing in a 3D lock-exchange problem at Reynolds number Re=10,000
Horizontal Approximate Deconvolution for Stratified Flows: Analysis and Computations
No full textIn this paper we propose a new Large Eddy Simulation model derived by approximate deconvolution obtained by means of wave-number asymptotic expansions. This LES model is designed for oceanic flows and in particular to simulate mixing of fluids with different temperatures, density or salinity. The model -which exploits some ideas well diffused in the community- is based on a suitable horizontal filtering of the equations. We prove a couple of a-priori estimates, showing certain mathematical properties and we present also the results of some preliminary numerical experiments
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Mathematics of large eddy simulation of turbulent flows
No full textTurbulence is ubiquitous in nature and central to many applications important to our life. (It is also a ridiculously fascinating phenomenon.) Obtaining an accurate prediction of turbulent flow is a central difficulty in such diverse problems as global change estimation, improving the energy efficiency of engines, controlling dispersal of contaminants and designing biomedical devices. It is absolutely
fundamental to understanding physical processes of geophysics, combustion, forces of fluids upon elastic bodies, drag, lift and mixing. Decisions that affect our life must be made daily based on predictions of turbulent flows.
Direct numerical simulation of turbulent flows is not feasible for the foreseeable future in many of these applications. Even for those flows for which it is currently feasible, it is filled with uncertainties due to the sensitivity of the flow to factors such as incomplete initial conditions, body forces, and surface roughness. It is also expensive and time consuming-far too time consuming to use as a design tool. Storing, manipulating and post processing the mountain of uncertain data that results from a DNS to
extract that which is needed from the flow is also expensive, time-consuming, and uncertain.
The most promising and successful methodology for doing these simulations of that which matters in turbulent flows is large eddy simulation or LES. LES seeks to calculate the large, energetic structures (the large eddies) in a turbulent flow. The aim of LES is to do this with complexity independent of the Reynolds number and dependent only on the resolution sought. The approach of LES, developed over the last 35 years, is to filter the Navier-Stokes equations, insert a closure approximation (yielding an LES model), supply boundary conditions (called a Near Wall Model in LES), discretize appropriately and perform a simulation. The first three key challenges of LES are thus: Do the solutions of the chosen model accurately reflect true flow averages?
Do the numerical solutions generated by the chosen discretization, reflect solutions of the model?
And, With the chosen model and method, how is simulation to be performed in a time and cost effective manner?
Although all three questions are considered herein, we have focused mostly on the first, i.e. the mathematical development of the LES models themselves. The second and third questions concerning numerical analysis and computational simulation of LES models are essential. However, the numerical analysis of LES should not begin by assuming a model is a correct mathematical realization of the
intended physical phenomenon (in other words, that the model is well posed).
To do so would be to build on a foundation of optimism. Numerical analysis of LES models with sound mathematical
foundations is an exciting challenge for the next stage of the LES adventure.
One important approach to unlocking the mysteries of turbulence is by computational studies of key, building block turbulent flows (as proposed by von Neumann). The great success of LES in economical and accurate descriptions of many building block turbulent flows has sparked its explosive growth. Its development into a predictive tool, useful
for control and design in complex geometries, is clearly the next step, and possibly within reach in the near future. This development will require much more experience with practical LES methods. It will also require fundamental mathematical contributions to understanding ``How", ``Why", and ``When" an approach to LES can work and ``What" is the expected accuracy of the combination of filter, model, discretization and solver.
The extension of LES from application to fully developed turbulence to include transition and wall effects and then to the delicate problems of control and design is clearly the next step in the development of large eddy
simulation. Progress is already being made by careful experimentation. Even as ``[The universe] is written in mathematical language" (Galileo), the
Navier-Stokes equations are the language of fluid dynamics. Enhancing the universality of LES requires making a direct connection between LES models and the (often mathematical formidable) Navier-Stokes equations. One theme of this book is the connection between LES models and the Navier-Stokes equations rather than the phenomenology of turbulence. Mathematical development will complement numerical experimentation and make LES more general, universal, robust and predictive.
We have written this book in the hope it will be
useful for LES practitioners interested in understanding how mathematical development of LES models can illuminate models and increase their usefulness, for applied mathematicians interested in the area and especially for Ph.D. students in computational mathematics trying to make their first contribution. One of the themes we emphasize is that mathematical understanding, physical insight and computational experience are the three foundations of LES! Throughout, we try to present first steps at a theory as simple as possible, consistent with correctness and relevance, and no simpler. We have tried, in this balancing act, to find the right level of detail, accuracy and mathematical rigor.
This book collects some of the fundamental ideas and results scattered throughout the literature of LES and embeds them in a homogeneous and rigorous mathematical framework. We also try to isolate and focus on the mathematical principles shared by apparently distinct methodologies in LES and show their essential role in robust and universal modeling. In part I we review basic facets of on the Navier-Stokes equations; in parts II and III we highlight some promising models for LES, giving details on the mathematical foundation, derivation and analysis. In part IV we present some of the difficult challenges introduced by solid boundaries; part V presents a syllabus for numerical validation and testing in LES.
We are all too aware of the tremendous breadth, depth and scope of the area of LES and of the great limitations of our own experience and understanding. Some of these gaps are filled in other excellent books on LES. In particular, we have learned a lot ourselves from the books of Geurts, John,
Pope, and Sagaut.
We have tried to complement the treatment of LES in these excellent books by developing mathematical tools, methods, and results for LES . Thus, many of the same topics are often treated herein but with the magnifying glass of mathematical analysis. This treatment yields new perspectives, ideas, language and illuminates many open research problems.
We offer this book in the hope that it will be
\textit{useful}
to those who will help develop the field of LES and fill in many of
the gaps we have left behind herein.
It is a pleasure to acknowledge the help of many people in writing this
book. We thank Pierre Sagaut for giving us the initial impulse in the project and for many detailed and helpful comments along the way. We owe our friend and colleague Paolo Galdi a lot as well for many exciting and illuminating conversations on fluid flow phenomena.
Our first meeting came through one such interaction with Paolo. We also thank Volker John, who throughout our LES adventure has been part of our day to day ``battles''.
Our understanding of LES has advanced through working with friends and collaborators Mihai Anitescu, Jeff Borggaard, Adrian Dunca, Songul Kaya, Roger Lewandowski, and Niyazi Sahin.
The preparation of this manuscript has benefited from the financial support of the National Science Foundation and Ministero dell'Istruzione, dell'Universit\`a e della Ricerca
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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