1,720,972 research outputs found
Large-scale structures predicted by linear models of wall-bounded turbulence
No full textThe objective of this article is to determine for which scales stochastic forcing of the linearized Navier-Stokes equations, recast as the resolvent operator, is sufficient to reproduce second-order statistics in turbulent channel flow. Our focus is on the large scales at a friction Reynolds number of Reτ = 2003. We consider a molecular resolvent operator, where only the kinematic viscosity appears, and an eddy resolvent operator, where the kinematic viscosity is augmented with an eddy viscosity profile. The molecular resolvent operator is able to identify the wall-normal height where the maximum energy of a structure is located, but it fails to predict the most energetic wave speed. It also overestimates the streamwise velocity component and underestimates the spatial support of the structures in the wall-normal direction. When the eddy resolvent operator identifies the most energetic wave speed, it also predicts the correct statistics for a given spatial scale. For spatial scales where this criterion is not met, the eddy viscosity overdamps the linear response. As a result, it predicts energetic wave speeds which are too low and velocity structures which are too energetic close to the wall. We conclude that eddy viscosity works best for structures which are most energetic in the wake region while its performance deteriorates for structures that are active in the log region.</p
Experimental flow reconstruction using resolvent analysis and data-assimilation
No full textA flow reconstruction methodology utilizing resolvent analysis and data-assimilation is applied to flow around a NACA 0018 airfoil at an angle of attack α = 10° and chord-based Reynolds number Re = 10250. The underlying principle of the reconstruction is to maximize the value of limited experimental measurements as the only inputs are a rough approximation of the mean profile and two time-resolved probes. The input data may be incomplete, in the sense that measurements near a body are difficult to obtain with techniques such as particle image velocimetry (PIV), or contaminated by noise. The final reconstruction contains less measurement noise compared to the PIV data and obeys the incompressible Navier-Stokes equations (NSE).</p
Effects of integral length scale variations on the stall characteristics of a wing at high free-stream turbulence conditions
No full textThe effect of variations in the integral length scale of incoming free-stream turbulence on a NACA0012 wing is investigated with the use of force, moment and particle image velocimetry measurements. At a chord-based Reynolds number (where c is the chord length, is the free-stream velocity and is the kinematic viscosity) of, an active grid generates turbulence intensities of 15 % at normalised integral length scales ranging from 0.5 to 1. The introduction of turbulence improves the time-averaged performance characteristics of the wing by delaying stall and increasing the peak lift coefficient. It is found that for half-chord integral length scales, the magnitude of the fluctuations in forces and moments is larger than that of full-chord integral length scales, as the former amplifies the naturally occurring unsteadiness in the flow (when there is no free-stream turbulence). The increase in magnitude is ascribed to a larger density of smaller-scale vortices within the separated flow and wake region of the wing. </p
Influence of a local change of depth on the behavior of walking oil drops
No full textBouncing liquid silicone oil drops on a vertically oscillating bath of the same liquid were studied experimentally. The different bouncing regimes previously described in the literature were observed, with transitions depending mainly on the droplet size and the forcing acceleration of the oil bath. In a particular range of forcing amplitudes, just below the Faraday instability threshold where standing waves appear on the free surface, walking drops traveling at a constant velocity over the surface were observed, consistent with previous studies. The influence of a local change of depth on this walking behavior was studied by submerging an obstacle in the oil bath. Notably different to the study of Eddi et al. (2009) [1], the depth change was such that walking was still observed over the obstacle. Previously unobserved drop trajectories, including trapping of a walking drop over the obstacle, crossing for non-normal drop approach to the obstacle, and reflection from the rear face of the obstacle were observed and explained in light of recent results and models in the literature
Resolvent analysis predictions of energy transfer in turbulent channel flow
No full textWe analyse the inter-scale transfer of energy for turbulent channel flow at Reτ = 180 in a minimal flow unit. The dominant energy-producing modes are streamwise-constant streaks with a spanwise spacing of approximately 100 wall units. Since the viscous dissipation for these scales is not sufficient to balance production, the nonlinear terms redistribute the excess energy to other scales. We compare the energy balance to predictions from resolvent analysis and show that it does not model energy transfer well. Nevertheless, we find that the energy transferred from the streamwise-constant streaks can be predicted reasonably well by a Cess eddy viscosity profile. As such, eddy viscosity is an effective model for the nonlinear terms in resolvent analysis and explains good predictions for the most energetic streamwise-constant streaks. Eddy viscosity does not respect the conservative nature of nonlinear transfer and is less effective for scales which receive energy from the nonlinear terms.</p
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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