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Classical control of a magnetorheological squeeze film damper
No full textAn adaptive squeeze film damper is one whose properties can be continuously adapted to minimize the amplitude of vibration of the rotor in any working condition. This paper concerns an adaptive squeeze film damper with a magnetorheological fluid (MR): a change of its apparent viscosity and stiffness is obtained by applying a magnetic field through the fluid. Electric coils generate the magnetic field and hence simply varying the electric current intensity in the coils can vary the damping effect of the device.
The device has been manufactured and preliminary tests have been carried out showing its susceptibility to be automatically controlled. The paper is focused on the study of the control chain of the rotor whirling orbit amplitude in the neighborhood of its critical speed.
The rotor is modeled as a flexible shaft supported at the ends with the rotor mass concentrated in the middle and with the damper journal mass close to one support. The damper reaction force is linearized so that the system dynamic equilibrium is described by a set of linear differential equations. Models with different combinations of springs and dashpots in series and parallel have been also considered and compared so as to have a better representation of the magnetorheological fluid behavior. Models of the power supply and of the measurement system are also taken into account.
A regulator classical synthesis is then performed to limit the shaft orbit amplitude keeping the damper journal displacement well within the range of the film thickness.
Finally simulation results are presented and discussed to show the validity of the proposed solution
Identificazione sperimentale dei coefficienti dinamici di uno smorzatore a squeeze-film magnetoreologico
No full textNegli ultimi anni c’è stata una crescente attenzione nei confronti dei fluidi magnetoreologici (MR), che reagiscono ai campi magnetici con variazioni di viscosità e rigidezza, per la loro applicazione in dispositivi semi-attivi anche nel campo del controllo delle vibrazioni. Presso il Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione dell’Università di Pisa è stato realizzato uno smorzatore a squeeze-film MR per alberi rotanti, controllabile attivamente agendo sul campo magnetico che attraversa il meato fluido. Un controllo di tipo classico presuppone tuttavia la completa conoscenza del modello dinamico dello smorzatore e la sua validazione sperimentale. E’ stata quindi progettata un’apparecchiatura, interfacciata con il banco prova esistente, finalizzata a validare il modello teorico dello smorzatore e identificarne i relativi parametri, al variare della frequenza di eccitazione e dell’intensità della corrente. Il sistema di misura è costituito da trasduttori di spostamento e da una cella di carico triassiale montata tra smorzatore e basamento in modo da misurare direttamente le forze esercitate dal meato sull’albero. Sulla base della misura di tali forze e della risposta dinamica del rotore all’applicazione di una forza rotante sincrona, dovuta allo sbilanciamento, è possibile determinare i coefficienti dinamici di rigidezza, smorzamento e massa aggiunta. I primi risultati mostrano la particolare importanza, nel campo di funzionamento considerato, dei coefficienti di rigidezza e smorzamento diretti
Experimental Model of a Semi-Actively Controlled Rotor
No full textExperimental models are an important means to gather knowledge about the behaviour of physical systems. In particular a reduced scale rotor bench can help in testing the performance of rotor vibration control devices such as magnetorheological squeeze-film dampers. In order to implement a classical control the dynamic model of the damper must be completely known and validated. Therefore a suitable apparatus aimed at the identification of the parameters of the devised model as function of the excitation frequency and of the electric current generating the magnetic field was set up and interfaced to a modified Bently-Nevada Rotor-Kit. Eddy current displacement sensors measured the dynamic response of the shaft to the synchronous rotating unbalance force while a triaxial load cell placed between the damper and the Rotor-Kit pedestal measured directly the forces transmitted by the shaft to the damper. On the basis of such measurements the dynamic stiffness, damping and adjoint mass coefficients, related to a linear model of the MR fluid were determined by means of a parametric identification technique
Investigation on the dynamic characteristics of a magnetorheological squeeze-film damper for the design of a classical rotor control strategy
No full textIn the last decades there has been an increasing attention towards magnetorheological fluids (MR), which
react to magnetic fields undergoing variations of viscosity and stiffness, due to their possible application in semiactive
devices also in the field of vibration control. A lab magnetorheological squeeze-film damper for the
control of the vibrations of a rotating shaft was designed and constructed. In order to implement a classical
control the dynamic model of the damper must be completely known and validated. Therefore a suitable
apparatus aimed at the identification of the parameters of the devised model as function of the excitation
frequency and of the electric current generating the magnetic field was set up and interfaced to a modified
Bently-Nevada Rotor-Kit. Eddy current displacement sensors measured the dynamic response of the shaft to the
synchronous rotating unbalance force while a triaxial load cell placed between the damper and the Rotor-Kit
pedestal measured directly the forces transmitted by the shaft to the damper. On the basis of such measurements
the dynamic stiffness, damping and adjoint mass coefficients, related to a linear model of the MR fluid were
determined by means of a parametric identification technique. The non-linear response was also 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
Studio di fattibilità di un orecchio virtuale
No full textLa ricerca intrapresa dagli autori ha come obiettivo la realizzazione di un “orecchio virtuale”, ovvero di un modello “completo” psico-acustico della percezione uditiva umana, che tenga conto di contributi alternativi alla via aerea normale quali contributi derivanti da vie tissutali (in particolare ossee) e fluide e contributi a frequenze ultrasoniche ed infrasoniche. Dato che risulta necessario studiare tecniche specifiche di modellazione e simulazione per ciascuna parte anatomica, nel presente lavoro sono stati approfonditi ed applicati a geometrie semplificate bidimensionali e tridimensionali dell’orecchio esterno, alcuni metodi (principalmente metodo ad elementi finiti spettrali (SFE), metodo ad elementi finiti generalizzato (GFEM)) di generalizzazione ed estensione del metodo agli elementi finiti classico (FEM) (principalmente metodo ad elementi finiti spettrali (SFE), metodo ad elementi finiti generalizzato (GFEM)). Tali metodi permettono di ottenere un’elevata accuratezza dei risultati delle simulazioni di trasmissione acustica con numero inferiore di nodi della meshdiscretizzazione rispetto alla regola convenzionalmente accettata per il FEM di dieci punti per lunghezza d’ondaun costo computazionale inferiore rispetto ai metodi tradizionali
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
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