1,720,976 research outputs found
From two-dimensional to three-dimensional turbulence through two-dimensional three-component flows
Full text linkThe relevance of two-dimensional three-components (2D3C) flows goes well beyond their occurrence in nature, and a deeper understanding of their dynamics might be also helpful in order to shed further light on the dynamics of pure two-dimenional (2D) or three-dimensional (3D) flows and vice versa. The purpose of the present paper is to make a step in this direction through a combination of numerical and analytical work. The analytical part is mainly concerned with the behavior of 2D3C flows in isolation and the connection between the geometry of the nonlinear interactions and the resulting energy transfer directions. Special emphasis is given to the role of helicity. We show that a generic 2D3C flow can be described by two stream functions corresponding to the two helical sectors of the velocity field. The projection onto one helical sector (homochiral flow) leads to a full 3D constraint and to the inviscid conservation of the total (three dimensional) enstrophy and hence to an inverse cascade of the kinetic energy of the third component also. The coupling between several 2D3C flows is studied through a set of suitably designed direct numerical simulations (DNS), where we also explore the transition between 2D and fully 3D turbulence. In particular, we find that the coupling of three 2D3C flows on mutually orthogonal planes subject to small- scale forcing leads to stationary 3D out-of-equilibrium dynamics at the energy containing scales. The transition between 2D and 3D turbulence is then explored through adding a percentage of fully 3D Fourier modes in the volume
Multiscale properties of Large Eddy Simulations: correlations between resolved-scale velocity-field increments and subgrid-scale quantities
Full text linkWe provide analytical and numerical results concerning multi-scale correlations be-tween the resolved velocity field and the subgrid-scale (SGS) stress-tensor in largeeddy simulations (LES). Following previous studies for Navier-Stokes equations(NSE), we derive the exact hierarchy of LES equations governing the spatio-temporalevolution of velocity structure functions of any order. The aim is to assess the influ-ence of the sub-grid model on the inertial range intermittency. We provide a seriesof predictions, within the multifractal theory, for the scaling of correlation involvingthe SGS stress and we compare them against numerical results from high-resolutionSmagorinsky LES and froma-priorifiltered data generated from direct numericalsimulations (DNS). We find that LES data generally agree very well with filteredDNS results and with the multifractal prediction for all leading terms in the balanceequations. Discrepancies are measured for some of the subleading terms involvingcross-correlation between resolved velocity increments and the SGS tensor or theSGS energy transfer, suggesting that there must be room to improve the SGS mod-elisation to further extend the inertial range properties for any fixed LES resoluti
Nonuniversal behaviour of helical two-dimensional three-component turbulence
Full text linkThe dynamics of two-dimensional three-component (2D3C) flows is relevant to describe the long-time evolution of strongly rotating flows and/or of conducting fluids with a strong mean magnetic field. We show that in the presence of a strong helical forcing, the out-of-plane component ceases to behave as a passive advected quantity and develops a nontrivial dynamics which deeply changes its large-scale properties. We show that a small-scale helicity injection correlates the input on the 2D component with the one on the out-of-plane component. As a result, the third component develops a nontrivial energy transfer. The latter is mediated by homochiral triads, confirming the strong 3D nature of the leading dynamical interactions. In conclusion, we show that the out-of-plane component in a 2D3C flow enjoys strong nonuniversal properties as a function of the degree of mirror symmetry of the small-scale forcing
Energy transfer in turbulence under rotation
Full text linkt is known that rapidly rotating turbulent flows are characterized by the emergence of simul-taneous upscale and downscale energy transfer. Indeed, both numerics and experiments show theformation of large-scale anisotropic vortices together with the development of small-scale dissipa-tive structures. However the organization of interactions leading to this complex dynamics remainsunclear. Two different mechanisms are known to be able to transfer energy upscale in a turbulentflow. The first is characterized by two-dimensional interactions among triads lying on the two-dimensional, three-component (2D3C)/slow manifold, namely on the Fourier plane perpendicular tothe rotation axis. The second mechanism is three-dimensional and consists of interactions betweentriads with the same sign of helicity (homochiral). Here, we present a detailed numerical study ofrotating flows using a suite of high Reynolds number direct numerical simulations (DNS) withindifferent parameter regimes to analyze both upscale and downscale cascade ranges. We find thatthe upscale cascade at wave numbers close to the forcing scale is generated by increasingly dominanthomochiral interactions which couple the three-dimensional bulk and the 2D3C plane. This couplingproduces an accumulation of energy in the 2D3C plane, which then transfers energy to smaller wavenumbers thanks to the two-dimensional mechanism. In the forward cascade range, we find thatthe energy transfer is dominated by heterochiral triads and is dominated primarily by interactionwithin the fast manifold wherekz6= 0. We further analyze the energy transfer in different regionsin the real-space domain. In particular, we distinguish high-strain from high-vorticity regions andwe uncover that while the mean transfer is produced inside regions of strain, the rare but extremeevents of energy transfer occur primarily inside the large-scale column vortice
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
A-priori study of the subgrid energy transfers for small-scale dynamo in kinematic and saturation regimes
Full text linkThe statistical properties of the subgrid energy transfers of homogeneous small-scale dynamo areinvestigated during the kinematic, nonlinear and statistically saturated stages. We carry out ana priorianalysis of data obtained from an ensemble of direct numerical simulations on 5123gridpoints and at unity magnetic Prandtl number. In order to provide guidance for subgrid-scale (SGS)modelling of different types of energy transfer that occur in magnetohydrodynamic dynamos, weconsider the SGS stress tensors originating from inertial dynamics, Lorentz force and the magneticinduction separately. We find that all SGS energy transfers display some degree of intermittencyas quantified by the scale-dependence of their respective probability density functions. Concerningthe inertial dynamics, a depletion of intermittency occurs in presence of a saturated dyna
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
Effect of filter type on the statistics of energy transfer between resolved and subfilter scales from a-priori analysis of direct numerical simulations of isotropic turbulence
Full text linkThe effects of different filtering strategies on the statistical properties of the resolved-to-subfilter scale (SFS) energy transfer are analysed in forced homogeneous and isotropic turbulence. We carry out a-priori analyses of the statistical characteristics of SFS energy transfer by filtering data obtained from direct numerical simulations with up to 2048(3) grid points as a function of the filter cutoff scale. In order to quantify the dependence of extreme events and anomalous scaling on the filter, we compare a sharp Fourier Galerkin projector, a Gaussian filter and a novel class of Galerkin projectors with non-sharp spectral filter profiles. Of interest is the importance of Galilean invariance and we confirm that local SFS energy transfer displays intermittency scaling in both skewness and flatness as a function of the cutoff scale. Furthermore, we quantify the robustness of scaling as a function of the filtering type
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
- …
