1,721,201 research outputs found
Brownian Motion of Continuous Deformable Bodies
No full textThe fluctuations of continuous elastic bodies are studied, applying the equipartition principle of energy to each normal mode of vibration. The mean square displacement of the particles from their equilibrium configurations is expressed through an infinite series, which is always converging, provided that the elasticity condition is satisfied even at high modes of vibration
Boundary Conditions for Darcy's Flow through Porous Media
No full textA novel formulation for the boundary conditions to be applied at a porous surface is proposed. Interfaces between porous and clear media and porous and solid media are considered. The well known Beavers & Joseph boundary condition is applicable for interfaces betweeen porous and clear media. An equivalent boundary condition is obtained for interfaces between porous and impermeable media, namely v_n = sqrt(k) nabla_t v_t, where v is the velocity field inside the porous medium, the subscripts n and t refer to components normal and tangential to the interface, respectively, while k stands for the permeability. A sample problem is solved for the flow fields exterior to a porous spherical particle and interior to it, assuming that the particle has a rigid concentric core and that the submerging flow field is Newtonian, Stokesian and uniform at infinity. Both Brinkman's equation and Darcy's law are utilized to obtain general forms of the velocity and pressure fiels. Comparison of the two solutions yields the desired boundary conditions applicable to the Darcy problem
Time-Dependent Dispersion of Small Particles in Rectangular Conduits
No full textThe mean velocity and dispersion coefficient of a Brownian particle suspended in a fluid flowing in a rectangular conduit are evaluated, obtaining analytical results over the whole time domain and for any initial position of the particle. These new results are obtained by exploiting the symmetry of the problem, showing that the particle longitudinal motion within the rectangular conduit is equivalent to the one occurring in an unbounded velocity field, which is the even periodic extension of the original velocity profile. Two main parameters are encountered: the aspect ratio eta of the conduit cross section, and the nondimensional time tau_y, scaled by the characteristic time associated with the particle molecular diffusion. Simple asymptotic expressions are obtained for the particle velocity and displacement, the velocity auto-correlation, and the dispersion coefficient. The limits addressed are for small and large times tau_y and small eta ratios. The main results are the following. In the limit of long times the mean displacement strongly depends on the initial position ofthc particle, while the dispersion coefficient is independent of it and, for a fixed cross section area, it attains a maximum value at eta = 0.65. In the limit of vanishingly small eta two distinct timescales tau_y, and tau_z = tau_y/eta^2 are defined, characterizing the particle molecular diffusion along the short and the long side of the conduit cross section, respectively. It is found that for short times t tau_z, it attains a new higher limit value. While the long-time limit value is independent of the initial location of the Brownian particle and is about 7.95 times higher than the two-dimensional dispersivity, the intermediate-time plateau strongly depends on the initial conditions. Namely, when the particle initial location is not too close to the short lateral wall the intermediate-time dispersion coefficient coincides with the two.dimensional dispersivity, indicating that the particle does not "know" of the existence of the lateral boundary. However, when the particle is initiallv introduced near the lateral wall, the intermediate-time dispersion coefficient is higher and is equal to up to four times the two-dimensional value
Lagrangian Approach to Laminar Dispersion in Rectangular Conduits
No full textTime-dependent mean velocities and dispersion coefficients are evaluated for a general two-dimensional laminar flow. A Lagrangian method is adopted by which a Brownian particle is traced in an artificially restructured velocity field. Asymptotic expressions for short, medium and long periods of time are obtained for Couette flow, plane Poiseuille flow and open-channel flow over an inclined flat surface. A new formula is suggested by which the Taylor dispersion coefficient can be evaluated from purely kinematic considerations. Within an error of less than one percent over the entire time domain and for various flow fields, a very simple analytical expression is derived for the time-dependent dispersion coefficient D(t) = D + D^T [1 - (1 - e^(-at))/(at)] where D is the molecular diffusion coefficient, D^T denotes the Taylor dispersion coefficient, t stands for the non-dimensional time pi^2 Dt/Y^2, Y is the distance between walls and a = (N+1)^2 is an integer which is determined by the number of symmetry planes N that the flow field possesses. For Couette and open-channel flow there are no plane of symmetry and a = 1; for Poiseuille flow there is one plane of symmetry and a = 4
Applications of Wiener's Path Integral for the Diffusion of Brownian Particles in Shear Flows
No full textWiener's path integral theory is revisited, stressing that it holds only when the condition of local equilibrium is fulfilled. The diffusion of a particle in a two and three-dimensional shear flow is studied, showing that for small time intervals the particle diffuses freely, without being influenced by the velocity gradient of the incident flow, while for long time intervals it exhibits a time dependent dispersion coefficient
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
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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