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Register a new userDevelopment of a finite element firn densification model for converting volume changes to mass changes
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In regions where ice sheets are increasing in mass, there is a 50-200 m layer of old snow called firn which does not melt in the summer months. The density of firn tracks the transformation of snow into glacial ice at approximately 917 kg m^-3. The process of firn densification is important in at least two ways: 1) it can be a dominant component in the observed rate of change of the surface elevation, and 2) storage of liquid water in the lower density firn layer is now considered a critical component in the mass balance of ice sheets. If the rate of change of surface elevation can be equated with the rate of change in the mass of the ice sheet, we would have an excellent means of monitoring ice sheet mass balance. However, knowledge of firn densification rates is needed to make the inference of mass rate of change from volume rate of change. Several firn models have been created for areas without melt. We have reformulated these models with the finite-element software package FEniCS and integrated them with an enthalpy-formulation. This integration allows us to account for the melting and subsequent re-freezing of firn layers into ice lenses.
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To accurately restore interdecadal oscillations from the length of day variation ({Delta}LOD) and the polar motion (PM), we propose a normal time-frequency transform (NTFT) combing with curve fitting scheme. Compared with the NTFT, the combined NTFT with a boundary extreme point mirror-image-symmetric extension (BEPME) process, and singular spectrum analysis (SSA) in some simulated tests, the superiority and reliability of this new scheme have been confirmed; and we further verified the validity of it in a mature case analysis from the Earths free oscillation modes 0S0 and 1S0. After then, we use it to restore the ~5.9yr oscillation (referred to as SYO) and ~8.5yr oscillation (referred to as EYO) from the {Delta}LOD and the PM records. Our results reconfirm that the SYO and EYO in the {Delta}LOD (and PM) have no stable damping trend (which is different from results in some previous studies), and for the first time, we find that the SYOs (and EYOs) contained in the {Delta}LOD and the PM show very good consistency. Such consistency demonstrates that the SYOs/EYOs in the {Delta}LOD and the PM must come from the same source. As the external excitation sources of the Earth rotation contain no such oscillations, we suggest that core motions are possible sources.
In this work, the finite elements method (FEM) is used to analyse the growth of fretting cracks. FEM can be favourably used to extract the stress intensity factors in mixed mode, a typical situation for cracks growing in the vicinity of a fretting contact. The present study is limited to straight cracks which is a simple system chosen to develop and validate the FEM analysis. The FEM model is tested and validated against popular weight functions for straight cracks perpendicular to the surface. The model is then used to study fretting crack growth and understand the effect of key parameters such as the crack angle and the friction between crack faces. Predictions achieved by this analysis match the essential features of former experimental fretting results, in particular the average crack arrest length can be predicted accurately.
Agile processes are now widely practiced by software engineering (SE) teams, and the agile manifesto claims that agile methods support responding to changes well. However, no study appears to have researched whether this is accurate in reality. Requirements changes (RCs) are inevitable in any software development environment, and we wanted to acquire a holistic picture of how RCs occur and are handled in agile SE teams in practice. We also wanted to know whether responding to changes is the only or a main reason for software teams to use agile in their projects. To do this we conducted a mixed-methods research study which comprised of interviews of 10 agile practitioners from New Zealand and Australia, a literature review, and an in-depth survey with the participation of 40 agile practitioners world-wide. Through this study we identified different types of RCs, their origination including reasons for origination, forms, sources, carriers, and events at which they originate, challenging nature, and finally whether agile helps to respond to changes or not. We also found that agile teams seem to be reluctant to accept RCs, and therefore, they use several mitigation strategies. Additionally, as they accept the RCs, they use a variety of techniques to handle them. Furthermore, we found that agile allowing better response to RCs is only a minor reason for practicing agile. Several more important reasons included being able to deliver the product in a shorter period and increasing team productivity. Practitioners stated this improves the agile team environment and thus are the real motivators for teams to practice agile. Finally, we provide a set of practical recommendations that can be used to better handle RCs effectively in agile software development environments.
Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 $mathrm{M}_{mathrm{W}}=9.0$ Tohoku earthquake generated coseismic stresses of $10^2-10^5$~Pa that could have induced changes in motion of small (radius $sim100$~km) crustal blocks within Honshu. Analysis of time-dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to $sim3$ mm/yr between $sim3.75$-year epochs bracketing this earthquake, consistent with an upper mantle viscosity of $sim 5times10^{18}$Pa$cdot$s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as $10^-8$~Hz.
This work reports on a study to develop a patient-specific finite element model of the Transcatheter Aortic Valve Implantation procedure, using a model of a balloon-expandable percutaneous prosthetic aortic valve as a framework for the prediction of its performance. An experimentally measured left ventricle and aortic pressure profile of a single systolic-diastolic cycle of a resting heart are used to assess the performance of the stent after its deployment. The results of the simulation show that the stent maintains its structural integrity after deployment, and successfully pushes the native leaflets back to keep the aortic root clear of all impediments.
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