1,720,972 research outputs found
Hydrodynamics of wave and flow impacting bridge piers: an experimental and numerical study
Full text linkL'impatto di onde causate da mareggiate potrebbe causare danni significativi alle sottostrutture dei ponti costieri. Inoltre, anche le pile di ponti in alveo fluviale hanno spesso mostrato un livello di vulnerabilità preoccupante a causa dell’impatto con onde di piena. La conoscenza dei meccanismi che caratterizzano l'interazione ondaflusso-struttura è fondamentale per una migliore progettazione di pile da ponte in alveo fluviale o ambiente costiero. Diversi studi hanno documentato che le forze idrauliche aggiungono carichi idrodinamici significativi sulle strutture costruite in ambiente fluviale e marino.
Inoltre, le modifiche indotte dalle pile da ponte sul flusso a valle durante il passaggio di un'onda lunga e la generazione di vortici turbolenti coerenti che si distaccano dalla struttura meritano ulteriori ricerche, considerando che potrebbero influenzare la distribuzione di eventuali sedimenti a valle e l'ecosistema.
A tal fine, l'uso congiunto delle analisi sperimentali e numeriche è utile per studiare l’interazione di un’onda sovrapposta ad un flusso stazionario con una struttura.
Lo scopo di questo studio è lo sviluppo di modelli numerici e di laboratorio su piccola scala per esaminare il comportamento idrodinamico di onde lunghe in acque poco profonde che impattano su cilindri rigidi verticali.
Il modello sperimentale riproduce la propagazione in una canaletta di due onde lunghe, caratterizzate da diversi valori di portata e altezza d'onda, rilasciate su un flusso di base uniforme, in presenza di due cilindri rigidi verticali che simulano due pile da ponte. Le misure di velocità, ottenute con un sistema PIV su piani 2D, sono utilizzate innanzitutto per definire la tipologia delle onde selezionate da un punto di vista analitico. Quindi, si esaminano tali misure di velocità: i) a monte dei cilindri, per rappresentare le distribuzioni verticali variabili nel tempo delle forze di trascinamento e di inerzia agenti sui cilindri durante il passaggio dell'onda; ii) a valle dei cilindri per ottenere le mappe di velocità e vorticità variabili nel tempo su un piano orizzontale; iii) applicando l’analisi Wavelet per rilevare la presenza di vortici turbolenti coerenti che si evolvono durante il passaggio dell'onda, a valle dei cilindri. I principali risultati sperimentali evidenziano che la forza di trascinamento è il contributo prevalente che
agisce sul cilindro durante l'intero passaggio dell'onda, raggiungendo un valore massimo in corrispondenza della cresta dell'onda. A valle di ogni cilindro, è evidente una zona d'ombra, come previsto, oltre a vortici di distacco nella scia del cilindro.
Infine, utilizzando l’analisi Wavelet, viene valutata la presenza di strutture coerenti turbolente in alcuni punti target, sia internamente che esternamente alla scia del cilindro, fornendo alcune informazioni utili sulle scale di lunghezza turbolenta e sulla frequenza di tali vortici coerenti, anche in funzione della fase dell'onda.
Con riferimento all’attività numerica, vengono eseguite due simulazioni numeriche con il solver olaFlow per riprodurre il modello sperimentale. Per ciascuna onda vengono studiati i profili di velocità variabili nel tempo durante il passaggio dell'onda e i campi orizzontali di velocità e vorticità a valle del cilindro. In questo modo è possibile confrontare i risultati numerici con quelli sperimentali, al fine di validare il modello numerico. In generale, gli errori relativi stimati tra dati sperimentali e risultati numerici rivelano una migliore prestazione del modello a monte dei cilindri sul piano verticale, mentre si osserva una minore capacità del modello di riprodurre il campo di flusso a valle dei cilindri, soprattutto nelle loro scie, richiedendo quindi ulteriori approfondimenti futuri.The impact of waves caused by storm surges could lead to significant damage to coastal bridges’ substructure. As well, also piers of river-crossing structures have shown an often worrying level of vulnerability due to flooding wave impact. The knowledge of the mechanisms characterizing the interaction wave-flow-structure plays a key role in better design of coastal and fluvial bridges. Several studies have documented that hydraulic forces add significant hydrodynamic loads on structures built in fluvial and marine environment.
As well, the modifications induced by the vertical structures of the bridge on the downstream flow during the transit of a long wave and the generation of coherent turbulent eddies from the structure deserve further research, considering that they could affect sediment and ecosystem downstream.
For these purposes, the joint use of experimental and numerical analysis is a useful method for studying the complex behaviour of the wave superimposed to a steady flow in presence of a structure.
The aim of this study is to develop small-scale laboratory and numerical models to examine the hydrodynamic behaviour of long waves released on a shallow current when impacting vertical rigid cylinders.
The experimental model reproduces the propagation of two long waves in a flume, characterized by different values of flow rate and wave height, overlapped on a uniform base flow, in the presence of two rigid vertical cylinders simulating the bridge’s piers. The velocity measurements, obtained with a PIV (Particle Image Velocimetry) system on 2D planes, are firstly used to define the typology of the selected waves from an analytical point of view. Then, they are examined: i) upstream the cylinders, to depict the time-varying vertical distributions of the drag and inertia forces acting on the cylinders during the transit of the wave; ii) downstream the cylinders to obtain the time-varying velocity and vorticity maps in a horizontal plane; iii) by applying the continuous wavelet technique to detect the presence of coherent turbulent eddies downstream the vertical obstructions, evolving during the wave transit.
The main experimental findings highlight that the drag force is the prevalent contribution acting on the cylinder during the whole wave transit, reaching a maximum value at the wave crest. Downstream each cylinder, a shadow zone is evident, as expected, as well as detaching vortexes in the cylinder’s wake. Finally, using the continuous wavelet technique, the presence of coherent turbulent eddies in some target points of the cylinder’s wake is assessed, providing some useful information about the turbulent lengths scales and the frequency of such coherent structures, also depending on the wave phase.
As for the numerical aspect, two numerical simulations with the olaFlow solver are executed to reproduce the experimental model. For each wave, the time-varying velocity profiles during the wave transit and the velocity and vorticity horizontal fields downstream the cylinder are investigated. In this way it is possible to compare the numerical results with the experimental ones, in order to validate the numerical model. Generally, the estimated relative errors between experimental data and numerical outputs reveal a better performance of the model upstream the cylinders in the vertical plane, while we observe a lower capability of the model to reproduce the flow field downstream the cylinders, especially in their wakes, thus requiring further future research
Flow Induced Excitation on basic shape structures
No full textThe study of flow-induced excitation on structures and obstacles is one of the main topics of fluid dynamics related to the practical interests in a large number of engineering applications e.g. aerodynamic, mechanical, civil, naval, etc. New design and project techniques have offered hazardous solutions, resulting in structures that are even more slender and flexible. This has led to a number of situations of self-excited vibration due to the interaction between flow fields and structures.
Forces coming from this mechanism depend upon both the incoming flow and the structure motion, giving rise to a strong non-conservative force field, which may eventually lead to a growing structure motion. The aim of this chapter is to offer an overture about the phenomenon of the fluid–structure interaction. Because of the importance that the cylindrical and spherical shapes have in the practical applications and the generalizations that these shapes allow, in this chapter the fluid–structure interaction is mainly referred to these basic shapes
Analysis of effects of the free surface on the movement of a tethered sphere immersed in a steady flow
No full textIn this work, we study the effects of the free surface on the sphere immersed in a steady flow through the analysis of stream wise and transverse displacements. The range of relative submergence, h*, is between 0 and 0.75 and the tethered sphere is characterized by a low value of the mass ratio m*~1 and a low value of damping ratio (ζ=0.007). The movements of the sphere have been measured using two methods: (a) by means of an analogue laser displacement sensor, and (b) by image analysis of the sphere movement CCD acquisitions. The second method provided the 2D reconstruction of the trajectory. The experimental data have highlighted a significant influence of free surface flow both on the transverse oscillation amplitudes and on the oscillation frequencies of the sphere
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
Boundaries effects on the movements of a sphere immersed in a free surface flow
No full textThis work analyses the influence of boundary conditions on the movements of a sphere immersed in a steady free surface flow. The sphere is free to move in both transverse and stream wise directions and it is characterized by values of the mass ratio, m*, equal to 1.34 and of the damping ratio, ζ, equal to 0.006. In all the experiments the blockage coefficient is kept constant, while the sphere is located at different distances from the free surface and from the bottom wall of the channel. The movements of the sphere have been measured by means of the image analysis of CCD camera which provides the 2D (stream wise and transverse) displacements of the sphere with a temporal resolution of 0.02. The experimental data show a significant influence of the boundaries on the sphere movement and highlight a different behaviour of the amplitude response between the three different experimental set-ups considered
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
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
- …
