1,720,979 research outputs found
Modelling the rocking dynamics of objects with irregular geometry
Get PDFFree-standing objects are all around us. Computer monitors and towers, bottles, kitchen containers, museum artifacts, tables, chairs, and medical equipment are just a few of the objects which may be prone to rocking and toppling due to external forcing or support excitation. Many objects with irregular geometry are able to rock about their edges and vertices, twist, topple, and impact their support, and could thus pose hazards to the integrity of the object and also to surrounding structures, objects, and even human life. However, previous rocking models are not adequate to handle objects with irregular geometry due to their assumptions. Thus, this thesis aims to fill the knowledge gap of the dynamics of irregular objects by introducing new rocking models for planar and 3-D rocking which take into account the full irregular geometry of such objects. The planar rocking model assumes a rigid body and rigid support and
fully considers sliding, free fight, and a variable location for the impulses at impact. The 3-D model similarly uses a rigid body and rigid support with a variable location for the impulses, but assumes no sliding or free fight, as it focuses on the pure rocking response of such systems and how this can lead to substantially more complicated responses of the body when taking into account the geometry. Examples of objects in both planar and 3-D rocking are provided and conclusions are drawn on the importance of irregular geometry, sliding, free fight, and a variable location for the impulse. Finally, the 3-D model is used to describe the rolling dynamics of various objects with curved geometry, offering a new dynamic model to describe these motions that is efficient and qualitatively similar to the experimental dynamic responses
Robust frameworks for the observability and lie symmetries of structural dynamical systems
Get PDFSystem identification is an important technique in reconstructing and estimating dynamic states, unknown parameters and unmeasured inputs of dynamical systems using measured input-output signals, and in minimizing the gaps between real engineering systems and their mathematical models. Whether a system for a given setup of sensors can be, in theory, successfully identified is associated with its observability properties.
This thesis is overall devoted to two research directions: 1) developing efficient observability algorithms for handling large and complex dynamical systems and 2) incorporating unmeasured or unknown inputs into robust observability computation and tool. The research is motivated by the need to relax the computational limitation of the existing observability methods that is associated with their high physical memory requirements when used for large and complex real systems, as for example large civil infrastructures encountered in Structural Health Monitoring (SHM). Moreover robust observability computation with the consideration of unmeasured inputs is needed to account for joint state-parameter-input identification problems which have gained increasing attention in recent years.
In particular, two efficient and robust algorithms are proposed to test observability properties in Chapter 2 and Chapter 3. The first algorithm applies to large linear systems with unknown parameters, based on the efficient implementation of the Observability Rank Condition (ORC) method. The second algorithm applies to rational nonlinear systems with unmeasured inputs, based on the extended use of the extended Observability Rank Condition (EORC-DF) and a power series-based computational framework.
In Chapter 4, computational frameworks are developed for Lie symmetries of nonlinear systems with unmeasured inputs. The obtained Lie symmetries can provide an alternative path to approach the observability properties of a system for a given setup of sensors. More importantly, Lie symmetries imply the mathematical relationship between the true solutions of the system’s states, parameters and unmeasured inputs and their other possible solutions.
Finally in Chapter 5, the application of observability and Lie symmetry analyses is illustrated through a complex, nonlinear and non-smooth mechanical model. The model is successfully reduced and identified using suitably chosen identification methods
Dynamic modelling of floating wind turbines
No full textFloating Wind Turbines (FWTs) offer a promising solution to harnessing substantial offshore wind energy in deep waters. However, accurately modelling their complex dynamics is challenging due to the complex coupling between aerodynamics, hydrodynamics, structural elasticity, and controls. This thesis proposes an integrated FWT simulation framework in Simulink to investigate these dynamics, with a particular focus on nonlinear wave-platform interactions induced by large platform motions.
A comprehensive FWT model is developed in Simulink, featuring efficient state-space representations for linear wave radiation and excitation effects, a multibody formulation to account for large geometrically nonlinear blade deformations, a modified blade-element momentum method for unsteady aerodynamics, and a multi-loop ROSCO control strategy for power regulation and platform stabilization. Validation against the widely-used OpenFAST simulation tool demonstrates the ability of the developed Simulink model to capture the dominant FWT dynamic couplings under realistic environmental conditions.
To address the limitations of the conventional linear hydrodynamic model, which assumes small platform motion around the equilibrium position, an analytical solution to the 2D linear wave-platform boundary value problem (BVP) linearized at an arbitrary platform pose is proposed. The analytical method is compared against a numerical boundary element method, and the influence of platform pose on the hydrodynamic behaviour is investigated.
Additionally, a novel linear parameter varying (LPV) modelling framework is developed to capture the geometrically nonlinear wave radiation effect. The LPV system is constructed using state-space models identified from the BVPs linearized at various platform positions, with the instantaneous platform pose used to interpolate the state-space matrices. To ensure state-basis coherency across all linear models, a black-box method based on the balanced realization and a gray-box method that ties the state vector to physical parameters are proposed. Both LPV models are validated on the benchmark van Daalen floating cylinder, and they are then integrated into the Simulink FWT model. Results indicate that the LPV models effectively capture nonlinear hydrodynamic effects and offer significant computational efficiency, making them practical for early-stage FWT design and optimization
Analytical models for laterally flexible rocking bodies
No full textRocking motion often arises due to dynamic excitations, such as earthquakes. To understand the rocking behaviour of non-rigid structures, specifically laterally flexible rocking bodies, this thesis develops analytical models of flexible rocking oscillators and validates them using experimental tests. With regards to the analytical models, an important contribution of this thesis relates to improving the modelling of impact during rocking motion. Based on the principle that spring and damper elements do not transfer impulses during an infinitesimal duration impact, a new impact model is developed. Numerical studies demonstrate this model’s ability to unify divergent modelling assumptions within a consistent framework. Another key area of research is the development of an analytical model capturing the sliding and free-flight phases of motion, which were overlooked in previous studies on laterally flexible rocking bodies. An impact model is formulated based on a hierarchical choice of post-impact phases for given impact parameters. Within this context, a new procedure is developed to choose new impact parameters if admissible solutions cannot be found for the original impact parameters. The inclusion of sliding behaviour allows the examination of how the overturning stability of flexible bodies is influenced by the coefficient of friction between the support medium and the structure. Additionally, failures of structures due to excessive sliding motion were also examined. The third area of focus in this thesis is an experimental investigation of the dynamic response of laterally flexible rocking bodies. An experimental setup is designed which allows examining bodies with variable lateral flexibility, slenderness, and coefficient of friction. Dynamic responses of the specimen under free rocking and pulse excitations were recorded using a 3D digital image correlation (DIC) system and accelerometers. Analysis of the data allows direct identification of the rigid body (including rocking and sliding motions) and flexible (lateral elastic deformation) components of the motion. Negligible impulse transfer to the flexible body during impact is experimentally validated, and key observations from the analytical models regarding overturning stability and sliding are confirmed experimentally
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
Recommended from our members
The Dynamics of Rigid Bodies on Moving Deformable Support Media
Get PDFThe rocking motion of a solid block on a moving deformable base is a dynamic problem, that despite its apparent simplicity, involves a number of complex dynamic phenomena such as impacts, sliding, geometric and material nonlinearities and, under some circumstances, chaotic behavior. For that reason, since the first model proposed by G. W. Housner in 1963, a number of alternative models have been proposed for its mathematical simulation. Although, with very few exceptions, the previous models in the literature make the simplified assumption that this motion is planar, this is usually not true since a body will probably not be aligned with the direction of the ground motion. Thus, even in the case where the body is fully symmetric, the rocking motion involves three dimensional rotations and displacements. Moreover, for reasons more related to functionality than safety, it is not uncommon for heavy mechanical and electrical equipment to be placed on wheels.
Examples of such devices are medical carts, mechanical equipment in hospitals, electrical transformers and recently even supercomputers. Although wheels facilitate the operation of these devices, they also affect the response of these objects during earthquakes; not necessarily in a beneficial way. This dissertation develops suitable models for simulating the previous dynamic problems. The equations of motion and suitable contact models are developed for each case. The importance of phenomena often neglected in the literature is stressed. Suitable examples illustrate the complex dynamic character of the problems examined. Finally, a static contact problem is examined. A model is developed for systems of multiple jointed elastic beams, using exact shape functions. A special application of the method for the definition of pressure loads in the wires of the main cable of a suspension bridge is presented. Examples illustrate the robustness of the method and the special properties associated with pressure loads
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
- …
