717 research outputs found
Stick-slip behavior of ice interacting with concrete surfaces
No full textWhen sea or lake ice interacts with concrete offshore structures in Arctic regions, the frictional forces between the ice and the structure cause abrasion of the concrete surface of the structure. This may endanger the structural integrity when the steel reinforcement gets exposed and experiences corrosion, and must therefore be taken into account in the design process. For the design of concrete offshore structures in Arctic conditions, an accurate description and prediction of ice-structure interaction is required. The interaction between moving ice and concrete surfaces is mainly governed by friction and the so-called stick-slip phenomenon. This phenomenon has been observed during laboratory and field testing and, although the physics of this phenomenon are believed to be well understood, the corresponding static and kinetic friction coefficients reported in literature have a widespread range and are inconclusive. This thesis aims at a more accurate identification of the ice-concrete friction coefficients.For this graduation project, an experimental set-up was designed and stick-slip tests were carried out at Memorial University of Newfoundland, Canada. Additionally, a numerical model describing stick-slip behavior between ice and concrete was created. For the experimental set-up, a cylindrical fresh water columnar ice sample with a 50 mm radius and 50 mm height was attached to four springs with the same stiffness. The springs were attached to a support structure and throughout the test campaign, the stiffness of these springs was varied between 20 - 70 N/m per spring. To simulate one-dimensional ice-concrete interaction, the ice sample was placed near the edge of a rotating concrete slab. The normal load on the ice sample was varied from 0.7 to 2 kg by adding weight. In addition, the concrete velocity as experienced by the ice was varied between 0.15 and 0.82 m/s by increasing the rotational rate of the concrete slab. The static and kinetic friction coefficients were obtained from the experimental data and their dependence on normal load, velocity and spring stiffness was analyzed as well. The static friction coefficients found over the whole range of tests varied from 0.1 to 0.5. The analysis showed that the static friction coefficient decreases with increasing normal load and with increasing velocity. The kinetic friction coefficient was found to be in the range of 0.08 to 0.4 and may on average be obtained as 0.7 times the static friction coefficient. The kinetic friction coefficient, too, decreases with an increase in normal load and velocity. The influence of the spring stiffness was not clearly identified.The friction coefficients that were calculated using the experimental data were provided as input to the numerical stick-slip model. An analysis was performed to verify that the model displays similar regression with the varied mass, velocity and spring stiffness, compared to what was observed in the experiment. The output was compared to the experimental data, and it was found that the model describes the stick-slip behavior as seen during the experiment with an accuracy between 84 and 99%. Although some further improvements to the model can be implemented, in general it is concluded that under the made assumptions, the model is valid for the prediction of stick-slip behavior as observed during the experiment.Offshore and Dredging Engineerin
Mixtures of Stick-Breaking Processes
No full textWe consider mixtures of stickbreaking processes as a generalization of the mixture of Dirichlet process model. We provide a sampling algorithm which covers all such models provide specific reasons for using particular choices of prior. Numerical illustrations involving real data sets are presented
The spatial and temporal distribution of avian stick nests across a managed forest
No full textStick nests (as created by several forest dwelling birds) are valuable habitat features. Consequently, forest management practices in Western Canada often call for stick nests and the surrounding habitat to be conserved where possible. I examined historical distributions of stick nests across a working-forest landscape in west-central Alberta, to determine if locations as amassed by forest workers (1999 -2017) appeared randomly-distributed across the landscape or were biased towards specific habitat metrics, and if so, did these metrics change over time? I worked with three sets of data compiled from 1999, 2003, and 20152017, respectively. Biologically relevant and important management habitat metrics were compiled using the most relevant GIS layers corresponding to the years of stick-nest reporting. These metrics were calculated at five spatial scales: 25 m, 50 m, 100 m, 250 m, and 500 m. Identical data were collected from generated random (reference) sites paired with each stick nest site.
I used conditional logistic regression to isolate the best predictors of stick nest occurrence in each time period, at each spatial scale. Models were successfully fitted for four of five spatial scales only in the 1999 time period. Deciduous cover was found to be a strong explanatory variable for stick nest locations at the 25 m and 50 m scale. Increased area of land-use (primarily oil and gas developments) and a high component of deciduous cover were found significant at the 100 m scale. The model generated for the 500 m scale indicated an increased likelihood of stick nests in areas with increased area of land-use, probably a result of both nesting behaviour and observer effects. The results of this study did not support the notion that habitat metrics associated with stick nests have remained constant (or changed) between 1999 and 2017 in the forest management area. A consistent and more thorough stick-nest monitoring program is likely required to fully understand the factors (natural and anthropogenic) linked to the conservation of stick nests across a working-forest landscape. Moreover, investing in these monitoring programs may help improve sustainable management practices over time by enhancing understanding of the complex influences of landscape management on raptor nesting behaviour
On the lifetimes of evaporating droplets
No full textThe complete description of the lifetime of a droplet on a solid substrate evaporating in a 'stick–slide' mode is obtained. The unexpectedly subtle relationship between the lifetime of such a droplet and the lifetimes of initially identical droplets evaporating in the extreme modes (namely the constant contact radius and constant contact angle modes) is described and summarised in an appropriate master diagram. In particular, it is shown that the lifetime of a droplet is not, in general, constrained by the lifetimes of the extreme modes
Hidden dynamics in models of discontinuity and switching
No full textSharp switches in behaviodr, like impacts, stick slip motion, or electrical relays, can be modelled by differential equations with discontinuities. A discontinuity approximates fine details of a switching process that lie beyond a bulk empirical model. The theory of piecewise-smooth dynamics describes what happens assuming we can solve the system of equations across its discontinuity. What this typically neglects is that effects which are vanishingly small outside the discontinuity can have an arbitrarily large effect at the discontinuity itself. Here we show that such behaviour can be incorporated within the standard theory through nonlinear terms, and these introduce multiple sliding modes. We show that the nonlinear terms persist in more precise models, for example when the discontinuity is smoothed out. The nonlinear sliding can be eliminated, however, if the model contains an irremovable level of unknown error, which provides a criterion for systems to obey the standard Filippov laws for sliding dynamics at a discontinuity. (C) 2014 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).</p
The worldwide status of phasmids (Insecta: Phasmida) as pests of agriculture and forestry, with a generalised theory of phasmid outbreaks
No full text© 2015 Baker. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided
you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate
if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/
zero/1.0/) applies to the data made available in this article, unless otherwise stated. The file attached is the published version of the article.NHM Repositor
HRD and its critical factors according to practitioners in the training division of Telkom Indonesia
No full textThis study of an Indonesian company, Telkom Indonesia, identifies how human resource development (HRD) practitioners view or define HRD, what factors they consider critical for HRD practice, and specific issues related to training. Interviews with 20 practitioners in the company's Training Division were used as the primary vehicle for gathering information, while observations and documents supplemented the interviews. The practitioners essentially equate HRD with training and development (T&D), but the company's practice reflects the presence of considerable attention not only to training and development but also to organization development (OD) and career development (CD). The practitioners' comments about critical factors for HRD can be categorized into four broad groups: corporate, workplace, supplier, and internal; and the company's training interventions are classified into three major clusters: telecommunications technology, business management, and leadership. This study also underscores the presence of some forces that lead HRD practice to stick around the training area and some other forces that promote a movement toward a more strategic HRD orientation
Theoretical Study of Stick-Slip Behaviour in Ice-Structure Interaction
No full textStructures built in lakes and rivers or at sea are under huge forces induced by ice. These cause friction at the ice-structure interface that may pose a threat to the structural integrity of the construction. For the detailed design and realisation of structures in such regions an understanding is required of this friction process. The friction process at the ice-structure interface is driven by a phenomenon called the stick-slip phenomenon, where during stick mode the structure and the ice move simultaneously with the same velocity, and during slip mode the ice and structure interface slide over one another. This phenomenon has been observed and researched extensively, yet the corresponding static and kinetic friction coefficients reported show a wide range and the relation between the involved parameters and the friction coefficients is inconclusive. The most common theory used in describing general friction processes is Coulomb's Law of Friction. This thesis aims to determine whether and when ice-structure interaction can or cannot be described by Coulomb's Law of Friction in particular cases.For this graduation project, first an analysis of several test set-ups was carried out to determine which set-up was best suited for experiments with stick-slip behaviour. The chosen test set-up consists of a one-dimensional rotating conveyor belt made from sandpaper on which an ice sample would be placed. This sample would be held in place by four springs attached to the back and front of the sample. This would allow the ice sample to move over the rotating belt, but remain relatively in its place. This would enable to the ice to displace in one direction, the direction of the rotating conveyor belt. This test set-up would be compact and it would be easy to switch the belt or ice sample if they had deteriorated too much. However, the use of a 1D conveyor belt prohibits construction materials such as steel or concrete to be used. Using sandpaper with similar roughness as concrete or steel would serve as a substitute to overcome this problem. Ultimately, it proved to be impossible to realise the set-up and the focus of this thesis switched towards modelling the two-dimensional stick-slip behaviour based on data from previous research. For this a three degree of freedom (3DOF) model was created. As the forces exerted by the springs on the mass act under an angle that is depending on the position of the mass four different approaches were tested to see how this angle could best be incorporated. Furthermore, the friction force acting on the ice-structure interface was modelled using the friction coefficients found by previous research, and using two different approaches to find out whether the friction force should be modelled using the relative sliding velocity at the interface or not. It proved that the angle between the springs’ original position and it’s instantaneous position should be updated every step during slip mode, and that the friction force should be modelled using the relative sliding velocity. Both are cause for higher computational time. When validating this model against the experimental data it was found that friction coefficients were overestimated substantially for the 3DOF model. Using this the 3DOF model was updated and it was found that it now matches with the experimental data.After analysing the output of the model, it was found that the kinetic friction coefficients are now in the range of 0.05-0.11 for the used parameters (mass 0.67 kg – 2.03 kg, slab velocity 0.15 m/s – 0.82 m/s, stiffness 20.17 N/m – 68.22 N/m). Also, the friction coefficients show a linear dependency with mass, and an slight inverse linear relationship with the slab velocity. Furthermore, with a low stiffness varying friction coefficients were found when varying the slab velocity, but when increasing the stiffness the friction coefficients converge and this variation decreases. Although further improvements can be made to create a better model, in general it is concluded that under the made assumptions, the 3DOF model describes the two-dimensional behaviour well, and shows how research into friction can be carried out while still using construction materials in an experimental set-up.Offshore and Dredging Engineerin
Assessment of bronchodilator responsiveness in preschool children using forced oscillations
No full textBackground: The forced oscillation technique (FOT) requires minimal patient cooperation and is feasible in preschool children. Few data exist on respiratory function changes measured using FOT following inhaled bronchodilators (BD) in healthy young children, limiting the clinical applications of BD testing in this age group. A study was undertaken to determine the most appropriate method of quantifying BD responses using FOT in healthy young children and those with common respiratory conditions including cystic fibrosis, neonatal chronic lung disease and asthma and/or current wheeze. Methods: A pseudorandom FOT signal (4-48 Hz) was used to examine respiratory resistance and reactance at 6, 8 and 10 Hz; 3-5 acceptable measurements were made before and 15 min after the administration of salbutamol. The post-BD response was expressed in absolute and relative (percentage of baseline) terms. Results: Significant BD responses were seen in all groups. Absolute changes in BD responses were related to baseline lung function within each group. Relative changes in BD responses were less dependent on baseline lung function and were independent of height in healthy children. Those with neonatal chronic lung disease showed a strong baseline dependence in their responses. The BD response in children with cystic fibrosis, asthma or wheeze (based on both group mean data and number of responders) was not greater than in healthy children. Conclusions: The BD response assessed by the FOT in preschool children should be expressed as a relative change to account for the effect of baseline lung function. The limits for a positive BD response of -40% and 65% for respiratory resistance and reactance, respectively, are recommended
Sampling from Dirichlet process mixture models with unknown concentration parameter: mixing issues in large data implementations
No full textWe consider the question of Markov chain Monte Carlo sampling from a general stick-breaking Dirichlet process mixture model, with concentration parameter (Formula presented.). This paper introduces a Gibbs sampling algorithm that combines the slice sampling approach of Walker (Communications in Statistics - Simulation and Computation 36:45-54, 2007) and the retrospective sampling approach of Papaspiliopoulos and Roberts (Biometrika 95(1):169-186, 2008). Our general algorithm is implemented as efficient open source C++ software, available as an R package, and is based on a blocking strategy similar to that suggested by Papaspiliopoulos (A note on posterior sampling from Dirichlet mixture models, 2008) and implemented by Yau et al. (Journal of the Royal Statistical Society, Series B (Statistical Methodology) 73:37-57, 2011). We discuss the difficulties of achieving good mixing in MCMC samplers of this nature in large data sets and investigate sensitivity to initialisation. We additionally consider the challenges when an additional layer of hierarchy is added such that joint inference is to be made on (Formula presented.). We introduce a new label-switching move and compute the marginal partition posterior to help to surmount these difficulties. Our work is illustrated using a profile regression (Molitor et al. Biostatistics 11(3):484-498, 2010) application, where we demonstrate good mixing behaviour for both synthetic and real examples. © 2014 The Author(s)
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