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Fluid-structure interaction for nonlinear response of shells conveying pulsatile flow
No full textCircular cylindrical shells with flexible boundary conditions conveying pulsatile flow and subjected to pulsatile pressure are investigated. The equations of motion are obtained based on the nonlinear Novozhilov shell theory via Lagrangian approach. The flow is set in motion by a pulsatile pressure gradient. The fluid is modeled as a Newtonian pulsatile flow and it is formulated using a hybrid model that contains the unsteady effects obtained from the linear potential flow theory and the pulsatile viscous effects obtained from the unsteady time-averaged Navier-Stokes equations. A numerical bifurcation analysis employs a refined reduced order model to investigate the dynamic behavior. The case of shells containing quiescent fluid subjected to the action of a pulsatile transmural pressure is also addressed. Geometrically nonlinear vibration response to pulsatile flow and transmural pressure are here presented via frequency-response curves and time histories. The vibrations involving both a driven mode and a companion mode, which appear due to the axial symmetry, are also investigated. This theoretical framework represents a pioneering study that could be of great interest for biomedical applications. In particular, in the future, a more refined model of the one here presented will possibly be applied to reproduce the dynamic behavior of vascular prostheses used for repairing and replacing damaged and diseased thoracic aorta in cases of aneurysm, dissection or coarctation. For this purpose, a pulsatile time-dependent blood flow model is here considered by applying physiological waveforms of velocity and pressure during the heart beating period. This study provides, for the first time in literature, a fully coupled fluid-structure interaction model with deep insights in the nonlinear vibrations of circular cylindrical shells subjected to pulsatile pressure and pulsatile flow
Nonlinear dynamics of shells conveying pulsatile flow with pulse-wave propagation. Theory and numerical results for a single harmonic pulsation
No full textIn deformable shells conveying pulsatile flow, oscillatory pressure changes cause local movements of the fluid and deformation of the shell wall, which propagate downstream in the form of a wave. In biomechanics, it is the propagation of the pulse that determines the pressure gradient during the flow at every location of the arterial tree. In this study, a woven Dacron aortic prosthesis is modelled as an orthotropic circular cylindrical shell described by means of the Novozhilov nonlinear shell theory. Flexible boundary conditions are considered to simulate connection with the remaining tissue. Nonlinear vibrations of the shell conveying pulsatile flow and subjected to pulsatile pressure are investigated taking into account the effects of the pulse-wave propagation. For the first time in literature, coupled fluid-structure Lagrange equations of motion for a non-material volume with wave propagation in case of pulsatile flow are developed. The fluid is modeled as a Newtonian inviscid pulsatile flow and it is formulated using a hybrid model based on the linear potential flow theory and considering the unsteady viscous effects obtained from the unsteady time-averaged Navier-Stokes equations. Contributions of pressure and velocity propagation are also considered in the pressure drop along the shell and in the pulsatile frictional traction on the internal wall in the axial direction. A numerical bifurcation analysis employs a refined reduced order model to investigate the dynamic behavior of a pressurized Dacron aortic graft conveying blood flow. A pulsatile time-dependent blood flow model is considered by applying the first harmonic of the physiological waveforms of velocity and pressure during the heart beating period. Geometrically nonlinear vibration response to pulsatile flow and transmural pulsatile pressure, considering the propagation of pressure and velocity changes inside the shell, is here presented via frequency-response curves, time histories, bifurcation diagrams and Poincaré maps. It is shown that traveling waves of pressure and velocity cause a delay in the radial displacement of the shell at different values of the axial coordinate. The effect of different pulse wave velocities is also studied. Comparisons with the corresponding ideal case without wave propagation (i.e. with the same pulsatile velocity and pressure at any point of the shell) are here discussed. Bifurcation diagrams of Poincaré maps obtained from direct time integration have been used to study the system in the spectral neighborhood of the fundamental natural frequency. By increasing the forcing frequency, the response undergoes very complex nonlinear dynamics (chaos, amplitude modulation and period-doubling bifurcation), here deeply investigated
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
Linear and nonlinear vibrations and stability of a periodically simply supported plate in axial flow
No full textLAUREA MAGISTRALELe vibrazioni nonlineari rappresentano uno dei principali problemi nella dinamica strutturale legati a sollecitazioni di varia natura, tra cui quelli dovuti all'interazione fluido-struttura. Infatti, specie in applicazioni di ambito aerospaziale, biomedico, nucleare e navale, le strutture subiscono vibrazioni di grande ampiezza se paragonate allo spessore della struttura stessa vista la necessità di ridurre al minimo tale spessore al fine di minimizzare il peso, aumentare lo scambio termico e ridurre i costi. Sotto tali ipotesi, la struttura non può più essere modellata secondo una teoria lineare visto che le deformazioni fuori dal piano sono sufficientemente ampie tali per cui l'interazione tra gli sforzi membranali e la curvatura della piastra non può più essere trascurata.
Vista la scarsità in letteratura di studi teorici sulle vibrazioni nonlineari e sull'analisi di stabilità di piastre in flussi assiali, l'obiettivo di questo lavoro di tesi è di colmare tale mancanza introducendo in particolare un modello teorico capace di tenere in considerazione anche le imperfezioni della piastra. Ammettendo l'impiego del metodo di Rayleigh-Ritz per discretizzare il sistema, i risultati qui presentati sono esatti dal punto di vista analitico sotto le ipotesi di fluido non-viscoso. Nella fattispecie, le vibrazioni nonlineari di un sistema composto da una piastra con imperfezioni posta in un flusso assiale vengono qui trattate per la prima volta in letteratura.
La parte legata alle vibrazioni lineari della piastra in un flusso assiale presentata nella prima sezione di questo lavoro di tesi è stata pubblicata nel Maggio 2013 dal Journal of Fluids and Structures (Tubaldi & Amabili); la corrispondente parte nonlineare (presentata nella seconda sezione) è stata pubblicata alla conferenza Fourth Canadian Conference on Nonlinear Solid Mechanics (Tubaldi et al.) nel Luglio 2013.
Dal momento che il problema come qui formulato non è mai stato trattato in letteratura, non è possibile effettuare dei confronti numerici diretti con dei risultati di studi antecedenti a tale lavoro. Tuttavia, dal punto di vista qualitativo, il comportamento dinamico e l'analisi di biforcazione della struttura qui presentati sono in accordo con studi precedenti basati su configurazioni simili a quella del sistema analizzato in questa sede.
In particolare, la teoria lineare prevede una perdita di stabilità da parte del sistema all'aumentare della velocità del flusso a causa di una divergenza, ovvero un'instabilità statica (buckling). I modi vibrazionali della piastra accoppiata col fluido sono complessi. Nella fattispecie, il modo fondamentale ha una parte reale con una semionda longitudinale tipico del modo naturale della piastra in assenza di flusso e una parte immaginaria con due semionde longitudinali dovute alla componente convettiva del flusso. Infatti, l'effetto giroscopico è alla base dell'accoppiamento di modi con numero di onde pari e dispari nella direzione del flusso, con angoli di fase diversi, che si manifestano in tali modi di vibrazione complessi (traveling waves) e non modi normali (stationary waves).
Al fine di predire correttamente il comportamento della sistema fluido-struttura per velocità del fluido superiori alla velocità critica di divergenza, la teoria non-lineare di Von Karman viene introdotta per descrivere la piastra. Grazie all'analisi di biforcazione condotta con la velocità del fluido come parametro di biforcazione, una supercritical pitchfork bifurcation viene individuata in corrispondenza della velocità critica di divergenza predetta dalla teoria lineare, mostrando giustamente l'accordo tra le due teorie sulla prima instabilità della struttura. D'altra parte, aumentando la velocità del fluido al di là della velocità critica di divergenza, il coupled mode flutter predetto dalla teoria lineare non viene confermato dalla teoria nonlineare che mostra invece una seconda pitchfork bifurcation.
I risultati numerici mostrano che il picco di risonanza della frequenza naturale della piastra si muove verso frequenze più alte all'aumentare della velocità del fluido accentuando il comportamento hardening del sistema. Tale risultato è in accordo con la teoria che prevede per piastre vincolate trasversalmente sui quattro lati una nonlinearità di tipo hardening per ampiezze vibrazionali dell'ordine dello spessore della piastra.
Un carico concentrato armonico viene applicato in (x, y) dove x = a/4 e y = b/4 essendo "a" e "b" rispettivamente la dimensione longitudinale e laterale della piastra. La risposta del sistema a tale forzante con frequenza di eccitazione fissa e ampiezza variabile viene studiata per diverse velocità del fluido grazie all'utilizzo di un codice basato sulla tecnica di continuazione pseudo-arclength e sulla teoria di biforcazione. L'interazione modale nella risposta del modo fondamentale con armoniche superiori viene individuata in taluni intervalli di frequenze.
Infine, la presenza di imperfezioni geometriche iniziali della piastra -in direzione trasversale e associate a uno sforzo nullo- viene presa in considerazione. A causa di tali imperfezioni, le frequenze proprie e il comportamento nonlineare del sistema accoppiato col fluido vengono modificati. In particolare, la pitchfork bifurcation scompare dal diagramma di biforcazione (in funzione della velocità del fluido) e viene sostituita da una configurazione continua di post-buckling.
Inoltre le imperfezioni giocano un ruolo importante nella risposta in frequenza del sistema dato che la piastra passa dall'essere un pannello perfettamente piatto a un pannello curvo (anche se con curvatura minima) e quindi il sistema presenta un comportamento softening per vibrazioni di piccole ampiezze che diventa hardening per vibrazioni di ampie ampiezze.
Una possibile interessante applicazione ingegneristica del modello teorico qui presentato sarebbe il suo utilizzo per descrivere il comportamento di un fin di un sottomarino (superficie di controllo o di manovra) o di un torpedo (fin di coda). Tale componente potrebbe presentare un comportamento nonlineare conseguentemente a un'esplosione sottomarina, a una manovra improvvisa o a un urto.Vibrations are a major problem due to excitations of many kinds, including flow-induced excitations; vibrations with large amplitude displacements, i.e. geometrically non-linear vibrations, often emerge in structural dynamics. They occur in important applications, including aerospace and aeronautic engineering, biomechanics, bridge dynamics and rotating blades. In particular, a variety of important problems of structural strength and stability of plates, arising in modern aircraft construction, can not be adequately analyzed on the basis of the classical theory since the plate deflections experienced are not small in comparison with the plate thickness. Indeed, due to high levels of acoustic pressure, vibrations of plates with large displacements, introducing geometrical non-linearity, occur in the aeronautical industry.
Here is the reason behind the topic addressed in this thesis that deals with linear and nonlinear vibrations and stability of a thin rectangular plate with immovable edges immersed in axial liquid flow on its upper side. In addition, the literature related to nonlinear studies of plates coupled to flowing fluid is not large and no one before has never addressed the effect of initial geometric imperfections on the dynamical behavior of the coupled system. As a consequence of this lack in the literature, a solution mathematically exact within the hypothesis of inviscid flow and the series solution has been developed in this thesis and the corresponding numerical results are here presented. In addition, nonlinear results for the plate in axial flow with imperferctions are here discussed for the first time in literature.
The linear study here presented has been published in May 2013 in the Journal of Fluids and Structures (Tubaldi & Amabili); the corresponding nonlinear study has been accepted to the Fourth Canadian Conference on Nonlinear Solid Mechanics (Tubaldi et al.).
Since this problem under these hypothesis has never been addressed in literature any comparisons with previous studies can be carried out. Nevertheless, the predicted behavior of the system correctly reproduce the expected general trend according to similar configurations previously studied in literature. In particular, a divergence instability is detected by the linear theory. The vibration modes of the plate coupled to flow are complex. In particular, the fundamental mode has a real part with a predominant longitudinal half-wave, which is the natural mode for zero flow, and an imaginary part with predominant two half-wave terms, due to the convective component. This complex mode travels along the plate with the flow.
In order to correctly predict the system behavior beyond the onset of divergence the use of a geometrically nonlinear plate theory is necessary. Numerical results show a hardening type behavior of the system, which is modified by the flow. Indeed, according to the theory, flat plates with restrained normal displacement at the four edges exhibit a hardening-type non linearity for vibration amplitude of the order of the plate thickness. A stronger hardening behavior is detected by increasing flow velocity. The system loses stability by divergence (pitchfork bifurcation) for high flow speeds in accordance with the linear theory. The nonlinear response at different flow velocities for fixed excitation amplitude by varying the excitation frequency is studied by using a code based on pseudo-arclength continuation method and the bifurcation analysis. The modal interaction in the response of the fundamental mode with higher modes is detected for certain frequency ranges.
Effects of geometric imperfections on the trend of nonlinearity and on natural frequencies are shown. Because of imperfections, the pitchfork bifurcation disappears and the system presents a continuous post-buckling configuration. Geometric imperfections play also an important role in the frequency response behavior of the system since the flat plate becomes a curved panel (even if very shallow), which exhibits an initial softening behavior, turning to strong hardening nonlinearity for larger vibration amplitude. A possible interesting engineering application of this theoretical study could be the simulation of a submarine fin (control surface) or of a torpedo fin (tail fins). The nonlinear behavior (corresponding to vibration amplitude of the thin plate larger of the order of 1/10 the thickness) may represent the reaction of the system to a submarine explosion, a sudden maneuver or to an impact
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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