1,720,958 research outputs found
Analysis of Free and Forced Ship Vibrations Using Finite Element Method
No full textWith the increase of ship size and speed, shipboard vibration becomes a great concern in the design and construction of the vessels. Excessive ship vibration is to be avoided for passenger comfort and crew habitability. In addition to undesired effects on humans, excessive ship vibration may result in the fatigue failure of local structural members or malfunction of machinery and equipment. The propeller induces fluctuating pressures on the surface of the hull, which induce vibration in the hull structure. These pressure pulses acting on the ship hull surface above the propeller as the predominant factor for vibrations of ship structures are taken as excitation forces for forced vibration analysis. Ship structures are complex and may be analyzed after idealization of the structure. Several simplifying assumptions are made in the finite element idealization of the hull structure. In this study, a three-dimensional finite element model representing the entire ship hull, including the deckhouse and machinery propulsion system, has been developed using a solid modeling software for local and global vibration analyses. Vibration analysis has been studied under two conditions which are free-free (dry) and in-water (wet). Wet analysis has been implemented using acoustic elements. The total damping associated with overall ship hull structure vibration has been considered as a combination of the several damping components. As the result of global ship free vibration analysis, global natural frequencies and mode shapes have been determined. Besides, responses of local ship structures have been determined as the result of propeller induced forced vibration analysis.</jats:p
The microstructure effects on fracture toughness of pressureless sintered silicon carbide
Get PDFBu çalışmada, farklı içerikte SiC karışımlar hazırlanmış, tek yönlü ve soğuk izostatik presleme tekniği ile preslenmişlerdir. Şekillendirilen numunelerin yaş yoğunlukları tespit edilmiş, piroliz işlemi silisyum karbür numunelerde sinterleme öncesi ara işlem olarak kullanılmıştır. Numuneler, katı - katı ve katı - sıvı difüzyonu tekniği ile açık atmosfer ve argon gaz atmosferinde farklı sıcaklık ve sürelerde sinterlenerek üretilmişlerdir. Elde edilen numunelerin sinterleme sonrasında sahip oldukları, porozite, yoğunluk, sinterleme küçülmesi, sertlik ve kırılma tokluğu ölçümleri yapılmıştır. Elde edilen sonuçların literatürde verilen değerlerle uyum içinde olduğu belirlenmiştir.Anahtar Kelimeler: Silisyum karbür (SiC), sinterleme, yaş yoğunluk, yoğunluk, küçülme, kırılma tokluğu.In this study, eight group silicon carbide compositions were prepared and pressed using by uni-axially and cold isostatically pressing techniques. Green density has been observed for the pressed sample and Pyrolysing process is used as pro-process for carbonizing the Phenolic resins. Phenolic resins have been used to avoid the decomposition of silicon carbide and stabilize the structure. Solid state and liquid phase sintering techniques have been used for the sintering. Temperature and time are used as sintering parameters. Densities and porosity have been measured by Archimedes method and fracture toughness have been tested by indentation fracture toughness. CAB12 Coded composition (B4C + Al2O3+Y2O3 + carbon) sintered at 2150 oC. The density of samples has been measured as 3.02 gr/cm3 indentation fracture toughness were obtained 4.57 MPa.m1/2 after sintering. CAJ09 Coded composition (HP1 SiC + Al2O3 + Y2O3 + carbon additives) has been sintered at 2000 oC. The density of sintered samples was measured as 3.12 gr/cm3 and 4,39 MPa.m1/2 indentation fracture toughness values were obtained after sintering. Al2O3 and Y2O3 reaction resulted in a phase being Y5Al3 observed by XRD analysis.CAK01- CAK07 Coded compositions (#70 SiC + HP1 SiC + Al2O3 + Y2O3 + carbon additive) has been sintered at 2000 oC. The density of sintered samples was measured as 2.48 ? 2.62 gr/cm3. Fracture toughness cannot be measured due to the 20.61 - 24.76 % porosity of the sample. Al2O3 and Y2O3 reaction resulted in a phase being Al5.Y3.O12 observed by XRD analysis. The obtained results have been observed in a good harmony compared to literature survey.Keywords: Silicon carbide (SiC), sintering, green density, density, indentation fracture toughnes
Finite element analysis of global ship vibrations
Get PDFGemi boyut ve hızlarındaki büyük artış nedeniyle gemi titreşimleri, gemi tasarımı ve yapımında büyük önem taşır hale gelmiştir. Gemi tasarımındaki son gelişmeler, daha büyük stroklu ve daha güçlü dizel motorların kullanıldığı, daha büyük boyutlarda, daha hafif, daha esnek teknelerin yapılmasına yol açmıştır. Deniz taşımacılığında artan talebi karşılamak için ihtiyaç duyulan bu yapıların daha esnek olması titreşim problemlerini de beraberinde getirmektedir. Aşırı gemi titreşimleri, yolcu konforunu ve mürettebat yaşamını önemli ölçüde etkilemektedir. İnsan üzerindeki istenmeyen etkilerinin yanında, aşırı gemi titreşimleri, makina ve cihazlarda bozulmalara neden olmakla birlikte yapısal elemanlarda da yorulma hasarına neden olmaktadır. Gemilerde titreşim problemlerinin çözüm kaynağının erken safhadaki tasarım aşamasında belirlenmesinin önemi ve sonradan yapılacak olan düzeltmelerin çok ağır maliyetler gerektirdiği bilinmektedir. Tasarım aşamasında yapılacak olan bir takım basit çalışmalarla ileride ortaya çıkacak büyük titreşim problemlerinin önlenmesi sağlanabilmektedir. Gemi titreşimleri bakımından, yerel titreşim problemlerinin yanı sıra geminin bütününde ortaya çıkan global titreşimler de gemi emniyeti açısından büyük önem taşımaktadır. Bu çalışma kapsamında tüm gemi, sonlu eleman analizi yöntemiyle global olarak serbest – serbest (susuz) ve su içinde olmak üzere serbest titreşim açısından incelenerek, tüm geminin doğal frekanslarının ve mod şekillerinin belirlenmesine çalışılmıştır. Bu şekilde geminin hangi frekanslarda rezonansa gireceği ve nasıl davranışlar sergileyeceği belirlenmiştir. Ayrıca sonlu eleman yöntemiyle hesaplanan doğal frekans değerleri, klas kuruluşları tarafından kullanılan ampirik formüllerle hesaplanan doğal frekans değerleri ile karşılaştırılarak, sonlu eleman analizinin gerekliliği üzerinde durulmuştur. Anahtar Kelimeler: Gemi titreşimi, sonlu eleman analizi, modal analiz.With the increase of ship size and speed, shipboard vibration becomes a great concern in the design and construction of the vessels. The flexibility of vessels which are needed to meet the demand in sea transportation, causes ship vibrations. In addition to undesired effects on passenger comfort and crew habitability, excessive ship vibration may result in the fatigue failure of local structural members or malfunction of machinery and equipment. The importance of the solutions of vibration problems which are addressed at the earliest design stage and the great cost in later correction efforts are clear. It is possible to avoid great vibration problems by means of simple studies in the early design stage. Besides local vibrations, global vibrations are of great importance to ship safety. In this study, global ship hull free vibration problems are studied under two conditions which are free - free (dry) and in water (wet) using finite element analysis. Excessive ship vibration is of great importance to the performance of precious navigation equipments and usually cause them to malfunction. In the condition of resonance which the propulsion frequency collides with the global natural frequency of the ship and during manoeuvres, the amplitude of oscillations on local structures exceeds certain limits. Concept design is where the vibration avoidance process must begin. It is clear that if the vibration problems, repeatedly identified by experience as the most important, are addressed at the earliest design stage, ultimately serious problems, involving great cost in correction efforts, may be avoided. The focus is on planning for vibration early at the concept design stage, where there has been no development of details. If as much as possible can be done in concept design with the simple tools and rules of thumb available at that level, it will help to avoid major vibration problems. The major potential problems may often be present in the crude concept design definition. Just identifying and addressing those potential problems in terms of the minimal technology available at the concept design stage is considered very important to the success of ship design. The design and construction of a ship free of excessive vibration continues to be a major concern and, as such, it is prudent to investigate, through analysis, the likelihood of vibration problems early in the design stage. Vibration analysis is aimed at the confirmation of the many design considerations associated with stern configuration, main propulsion machinery, propeller/shafting system and location and configuration of major structural assemblies. The ship hull structure includes the outer shell plating and all internal members, which collectively provide the necessary strength to satisfactorily perform the design functions in the expected sea environment. The hull structure responds as a free - free beam (both ends free) when subjected to dynamic loads. The vibration induced by the propulsion system is a common source of ship vibration. The vibration from this source manifests itself in several ways. Dynamic forces from the shafting system are transmitted to the hull through shaft bearings. The propeller induces fluctuating pressures on the surface of the hull, which induces vibration in the hull structure. The main and auxiliary engines can directly cause vibrations through dynamic forces transmitted through their supports and foundations. The response to this forcing can cause the vibration of the hull girder, deckhouse, deck and other structures, local structures and equipment. When attempting to determine the source of vibration, it is necessary to establish the frequency of excitation and to relate the frequency of excitation to the shaft rotational frequency by determining the number of oscillations per shaft revolution. Ship structures are complex and may be analyzed after idealization of the structure. Several simplifying assumptions are made in the finite element idealization of the hull structure. The modeling requirements are that all significant structural sections are to be captured and deflection/ velocity/acceleration are to be sufficiently predicted. A three-dimensional finite element model representing the entire ship hull, including the deckhouse and machinery propulsion system, needs to be developed for vibration analysis. If a global model exists from any previous tasks such as stress analysis, it needs to be conditioned for vibration analysis. In the section which the global vibrations are studied, a three dimensional ship model is prepared using a solid modelling software and the natural frequencies and mode shapes are determined using finite element analysis. In this way, the resonance frequencies and behaviour of the ship are determined. Keywords: Ship vibration, finite element analysis, modal analysis
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
Theoretical and Experimental Vibration Analyses of Trapezoidal and Sinusoidal Corrugated Plates
No full textVariations 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
- …
