1,720,986 research outputs found
Uncertainty propagation in locally damped dynamic systems
No full textIn the field of stochastic structural dynamics, perturbation methods are widely used to estimate the response statistics of uncertain systems. When large built up systems are to be modelled in the mid-frequency range,perturbation methods are often combined with finite element model reduction techniques in order to considerably reduce the computation time of the response. Existing methods based on Component Mode Synthesis(CMS) allow the uncertainties in the system parameters to be treated independently in each of the substructures and the perturbation in the local parameters to be propagated to the full system global parameters. However, local treatment of damping uncertainty is usually avoided by assuming proportional damping. Here, a perturbation method that includes local modal damping uncertainty and its propagation to the global response is proposed. Local damping is accounted for in the CMS model by use of complex modes. A perturbation relationship between local and global modal properties is stated for non-classically damped systems. In this way, the response statistics for uncertain systems with localized damping can be computed at a lowcomputational cost. A numerical example is presented to illustrate the performance of the proposed method
Response statistics of stochastic built-up structures
No full textCollections of essentially identical manufactured built-up structures display natural variations in their response. These variations arise from many sources including variability in the manufacturing process, variations in the response measurement process and environmental variations. Current techniques for response prediction focus on estimating the response of an ’ideal’ or nominal realisation of the structure. Further understanding is required into the statistics of the response of collections of mass produced structures. An appreciation of the distribution of response enables a cost effective design process with improved knowledge of worst-case behaviour and failure modes. Relevant measures include mean and variance, together with their statistical distributions. This paper considers response statistics in various situations. First, an idealised situation is considered and possible methods of analysis discussed. Then some existing measured response data from industrial products is examined. The availability of such data is extremely limited. Various statistical distributions are fitted to the data and compared using chi-squared tests
Structural response of an aircraft fuselage to hydraulic system — a wave and mobility approach
No full textIn-flight sound pressure levels can sometimes be intense which cause fatigue to the cabin crew, communication failures and discomfort to the passengers. This is often caused by the turbulent boundary layer flow over the aircraft body, engine noise and vibration and internal aircraft systems. Most of these issues have been dealt with; however, little attention was directed towards understanding the effect of the hydraulic system which comprises a pump located at the back of the aircraft and pipes which run along the fuselage. The pipes are connected to the fuselage using rubber mounts. Experimental measurements show that the hydraulic system, on its own, does not contribute to the in-flight sound pressure level in the aircraft cabin; it is its connection to the fuselage that is problematic. In this paper, a relatively simple model is presented to analyse the power flow from the piping of the hydraulic system to the fuselage. Two models of the mount are presented, and various plate models are utilised to represent the fuselage. The models are augmented with experimental measurements of a section of a typical aircraft fuselage. Experimental results show that the internal resonances of the mount interfere with those of the fuselage around the pumping frequency. The proposed model can be further used to conduct sensitivity or optimisation studies at the design stage of the piping or aircraft fuselag
Solutions for the vibration of an axially moving variable length string system: wave propagation versus space-time finite element predictions
No full textThis paper provides numerical and exact solutions for an axially moving stringsystem with variable length by both a space-time finite element and propagating wave model, respectively. Firstly, from the variational form, the dynamic problem for the continuum possessing changing mass is solved by a space-time finite element method. For the problem of a time-varying spatial domain, this finite element method discretizes the spatial and temporal domains simultaneously. Secondly, according to the regularity of propagating wave reflection, an exact solution for a variable-length moving string under uniform motion is derived by a propagating wave method. Subsequently, these two methods proposed are applied to a real-life example, i.e., a high-speed elevator cable. The vibration characteristics of the variable-length moving string with different boundary conditions are analyzed. Compared to the propagating wave method, the space-time finite element method has universality and low computational cost.Keywords: moving string; space-time finite element; changing mass; time-varying spatial domain; propagating wav
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
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
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