1,721,087 research outputs found
Vortex-induced vibration of spar platforms for floating offshore wind turbines
No full textAn experimental study of flow-induced oscillations of a floating model spar-type wind turbine platform was conducted. The model spar consisted of a floating uniform cylinder moored in a water tunnel test section and free to oscillate about its mooring attachment point near the center of mass. For the bare cylinder, periodic oscillations with figure-eight trajectories were observed over a range of reduced velocities, resembling a lock-in region. This was shown to be a vortex-induced vibration response since the oscillation frequencies and the shedding frequencies stayed equal to each other during the lock-in region. While the lower part of the cylinder was moving upstream at the 2 crossflow extremes of the trajectory, referred to as a counterclockwise (CCW) motion, its upper part experienced clockwise motion. It was hypothesized that the portion of the spar undergoing CCW figure eights is the portion where the flow excites the structure. This hypothesis was then validated by showing that adding helical strakes to the portion of the cylinder with CCW orbital motion suppresses vortex-induced vibration almost entirely, while adding strakes to the portion with clockwise orbital motion had a minimal influence on the amplitude of response
Vortex-induced vibration of a single degree-of-freedom flexibly-mounted horizontal cylinder near the free surface
No full textThis paper reports findings from an experimental study on vortex-induced vibration of a horizontal, flexibly-mounted cylinder positioned near the free surface of fluid flow. The cylinder is allowed to oscillate in the vertical direction only. Oscillation amplitude and frequency data are presented as a function of reduced velocity and dimensionless depth from the free surface. Data are presented for cases of decreasing depth from where the resting cylinder is fully-submerged until its centerline is even with the free surface of the water. For a fully-submerged cylinder, VIV behavior is consistent with published findings of similar systems, producing a well-documented lock-in region with initial, upper, and lower branches. Broadly, as the cylinder is raised from the fully-submerged case, the amplitudes diminish and the lock-in region decreases in range and shifts toward higher reduced velocities. When the cylinder is very near the surface, a second region of oscillations occurs at higher reduced velocities. This region emerges in cases where the cylinder’s top edge is half a diameter below the free surface. For a short range of depths, the lock-in region and the non-zero amplitude region at higher reduced velocities coexist. But after a critical depth, the lock-in region disappears and oscillations are observed only at the higher reduced velocity region. Multi-frequency oscillations and regions of hysteresis are observed for some cases of low depth and high reduced velocity
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
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
Exploring multi-stability in three-dimensional viscoelastic flow around a free stagnation point
Full text linkFluid elements passing near a stagnation point experience finite strain rates over long persistence times, and thus accumulate large strains. By the numerical optimization of a microfluidic 6-arm cross-slot geometry, recent works have harnessed this flow type as a tool for performing uniaxial and biaxial extensional rheometry (Haward et al., 2023 [5,6]). Here we use the microfluidic ‘Optimized-shape Uniaxial and Biaxial Extensional Rheometer’ (OUBER) geometry to probe an elastic flow instability which is sensitive to the alignment of the extensional flow. A three-dimensional symmetry-breaking instability occurring for flow of a dilute polymer solution in the OUBER geometry is studied experimentally by leveraging tomographic particle image velocimetry. Above a critical Weissenberg number, flow in uniaxial extension undergoes a supercritical pitchfork bifurcation to a multi-stable state. However, for biaxial extension (which is simply the kinematic inverse of uniaxial extension) the instability is strongly suppressed. In uniaxial extension, the multiple stable states align in an apparently random orientation as flow joining from four neighbouring inlet channels passes to one of the two opposing outlets; thus forming a mirrored asymmetry about the stagnation point. We relate the suppression of the instability in biaxial extension to the kinematic history of flow under the context of breaking the time-reversibility assumption
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
Microtomographic particle image velocimetry measurements of viscoelastic instabilities in a three-dimensional microcontraction
No full textViscoelastic flow through an abrupt planar contraction geometry above a certain Weissenberg number () is well known to become unstable upstream of the contraction plane via a central jet separating from the walls and forming vortices in the salient corners. Here, for the first time, we consider three-dimensional (3-D) viscoelastic contraction flows in a microfabricated glass square–square contraction geometry. We employ state-of-the-art microtomographic particle image velocimetry to produce time-resolved and volumetric quantification of the 3-D viscoelastic instabilities arising in a dilute polymer solution driven through the geometry over a wide range of but at negligible Reynolds number. Based on our observations, we describe new insights into the growth, propagation and transient dynamics of an elastic vortex formed upstream of the 3-D microcontraction due to flow jetting towards the contraction. At low we observe vortex growth for increasing , followed by a previously unreported vortex growth plateau region. In the plateau region, the vortex circulates around the jet with a period that decreases with but an amplitude that is independent of . In addition, we report new out-of-plane asymmetric jetting behaviour with a phase-wise dependence on . Finally, we resolve the rate-of-strain tensor and ascribe local gradients in as the underlying driver of circulation via strain hardening of the fluid in the wake of the jet
Flow-induced vibrations of a square prism free to oscillate in the cross-flow and inline directions
No full textVortex-induced vibration and galloping of prisms with triangular cross-sections
No full textFlow-induced oscillations of a flexibly mounted triangular prism allowed to oscillate in the cross-flow direction are studied experimentally, covering the entire range of possible angles of attack. For angles of attack smaller than α=25∘ (where 0∘ corresponds to the case where one of the vertices is facing the incoming flow), no oscillation is observed in the entire reduced velocity range tested. At larger angles of attack of α=30∘ and α=35∘, there exists a limited range of reduced velocities where the prism experiences vortex-induced vibration (VIV). In this range, the frequency of oscillations locks into the natural frequency twice: once approaching from the Strouhal frequencies and once from half the Strouhal frequencies. Once the lock-in is lost, there is a range with almost-zero-amplitude oscillations, followed by another range of non-zero-amplitude response. The oscillations in this range are triggered when the Strouhal frequency reaches a value three times the natural frequency of the system. Large-amplitude low-frequency galloping-type oscillations are observed in this range. At angles of attack larger than α=35∘, once the oscillations start, their amplitude increases continuously with increasing reduced velocity. At these angles of attack, the initial VIV-type response gives way to a galloping-type response at higher reduced velocities. High-frequency vortex shedding is observed in the wake of the prism for the ranges with a galloping-type response, suggesting that the structure’s oscillations are at a lower frequency compared with the shedding frequency and its amplitude is larger than the typical VIV-type amplitudes, when galloping-type response is observed
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