1,721,147 research outputs found
When Does Eddy Viscosity Damp Subfilter Scales Sufficiently?
Full text linkLarge eddy simulation (LES) seeks to predict the dynamics of spatially filtered turbulent flows. The very essence is that the LES-solution contains only scales of size ≥Δ, where Δ denotes some user-chosen length scale. This property enables us to perform a LES when it is not feasible to compute the full, turbulent solution of the Navier-Stokes equations. Therefore, in case the large eddy simulation is based on an eddy viscosity model we determine the eddy viscosity such that any scales of size <Δ are dynamically insignificant. In this paper, we address the following two questions: how much eddy diffusion is needed to (a) balance the production of scales of size smaller than Δ; and (b) damp any disturbances having a scale of size smaller than Δ initially. From this we deduce that the eddy viscosity νe has to depend on the invariants q = ½tr(S^2) and r =−⅓tr(S^3) of the (filtered) strain rate tensor S. The simplest model is then given by νe = 3/2(Δ/π)^2|r|/q. This model is successfully tested for a turbulent channel flow (Reτ = 590).
A new spectral model for shear-driven homogeneous anisotropic turbulent flows
No full textA new system of governing equations for spherically-averaged descriptors, which allows to calculate incompressible homogeneous turbulent flows, is derived in the present study. Remarkable features of this model are that it makes a distinction between directional and polarization anisotropies, which are treated separately, and that no heuristic tuning of arbitrary constants is required. Spherical averaging allows to obtain a model for anisotropic turbulence which is as versatile as the classical Eddy-Damped Quasi-Normal Markovian (EDQNM) model for isotropic turbulence, i.e. this model can calculate anisotropic turbulent flows at both very high and low Reynolds numbers, with good resolution of both large and small scales and over very long evolution times. The present model is particulary suited for the study of shear-driven turbulent flows and their return to isotropy
Temperature dynamics in decaying isotropic turbulence with Joule heat production
No full textThis paper presents an extension of existing works dealing with the dynamics of a passive scalar in freely decaying isotropic turbulence, by accounting for a production mechanism of the passive scalar itself. The physically relevant case of the temperature dynamics in the presence of Joule heating via the dissipation of the turbulent kinetic energy is selected and analysed by theoretical and numerical means. In particular, the sensitivity of the temperature decay to the non-dimensional parameters Prandtl number (Pr) and Eckert number (Ec), the latter measuring the intensity of the internal energy production mechanism, is investigated. The time behaviour of the global quantities such as the temperature variance -2.t/ and its destruction rate "- .t/ is analysed, and a detailed analysis of the temperature variance spectrum E- .k/ is provided. In the case of a very strong heating mechanism, some important modifications of the temperature dynamics are observed. The time-decay-law exponents of the global physical quantities assume new values, which are governed only by features of the kinetic energy spectrum, while they depend on the shape of E- .k/ in the classical free-decay case. The temperature variance spectrum E- .k/ exhibits two new spectral ranges. One is a convective-production range such that E- .k/ / k1=3 is observed for a finite time at all values of Pr. In the case of very diffusive fluids with Pr - 1, a convective-diffusive-production range with E- .k/ / k7=3 is also detected. © 2013 Cambridge University Press
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
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