387 research outputs found
A mathematical model for describing the mechanical behaviour of root canal instruments
No full textZhang EW, Cheung GSP, Zheng YF. A mathematical model for describing the mechanical behaviour of root canal instruments. International Endodontic Journal, 44, 72-76, 2011.Aim The purpose of this study was to establish a general mathematical model for describing the mechanical behaviour of root canal instruments by combining a theoretical analytical approach with a numerical finite-element method.Method Mathematical formulas representing the longitudinal (taper, helical angle and pitch) and cross-sectional configurations and area, the bending and torsional inertia, the curvature of the boundary point and the (geometry of) loading condition were derived. Torsional and bending stresses and the resultant deformation were expressed mathematically as a function of these geometric parameters, modulus of elasticity of the material and the applied load. As illustrations, three brands of NiTi endodontic files of different cross-sectional configurations (ProTaper, Hero 642, and Mani NRT) were analysed under pure torsion and pure bending situation by entering the model into a finite-element analysis package (ANSYS).Results Numerical results confirmed that mathematical models were a feasible method to analyse the mechanical properties and predict the stress and deformation for root canal instruments during root canal preparation.Conclusions Mathematical and numerical model can be a suitable way to examine mechanical behaviours as a criterion of the instrument design and to predict the stress and strain experienced by the endodontic instruments during root canal preparation. © 2010 International Endodontic Journal.link_to_subscribed_fulltex
A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments
No full textCheung GSP, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. International Endodontic Journal. Aim To evaluate the bending fatigue lifetime of nickel-titanium alloy (NiTi) and stainless steel (SS) endodontic files using finite element analysis. Methodology The strain-life approach was adopted and two theoretical geometry profiles, the triangular (TR) and the square cross-sections, were considered. Both low-cycle fatigue (LCF) lifetime and high-cycle fatigue (HCF) lifetime were evaluated. Results The bending fatigue behaviour was affected by the material property and the cross-sectional configuration of the instrument. Both the cross-section factor and material property had a substantial impact on fatigue lifetime. The NiTi material and TR geometry profiles were associated with better fatigue resistance than that of SS and square cross-sections. Conclusions Within the limitations of this study, finite element models were established for endodontic files to prejudge their fatigue lifetime, a tool that would be useful for dentist to prevent premature fatigue fracture of endodontic files. © 2010 International Endodontic Journal.link_to_subscribed_fulltex
Graph Spectral Image Processing
No full textRecent advent of graph signal processing (GSP) has spurred intensive studies of signals that live naturally on irregular data kernels described by graphs (e.g., social networks, wireless sensor networks). Though a digital image contains pixels that reside on a regularly sampled 2-D grid, if one can design an appropriate underlying graph connecting pixels with weights that reflect the image structure, then one can interpret the image (or image patch) as a signal on a graph, and apply GSP tools for processing and analysis of the signal in graph spectral domain. In this paper, we overview recent graph spectral techniques in GSP specifically for image/video processing. The topics covered include image compression, image restoration, image filtering, and image segmentation
Higher Hida theory and p-adic L-functions for GSp(4)
No full textWe use the "higher Hida theory" recently introduced by the second author to p-adically interpolate periods of non-holomorphic automorphic forms for GSp(4), contributing to coherent cohomology of Siegel threefolds in positive degrees. We apply this new method to construct p-adic L-functions associated to the degree 4 (spin) L-function of automorphic representations of GSp(4), and the degree 8 L-function of GSp(4) x GL(2)
Generic Analysis Methods for Gas Turbine Engine Performance: The development of the gas turbine simulation program GSP
No full textNumerical modelling and simulation have played a critical role in the research and development towards today’s powerful and efficient gas turbine engines for both aviation and power generation. The simultaneous progress in modelling methods, numerical methods, software development tools and methods, and computer platform technology has provided the gas turbine community with ever more accurate design, performance prediction and analysis tools. An important element is the development towards generic tools, in order to avoid duplication of model elements for different engine types. This thesis focuses on the development of generic gas turbine system performance simulation methods. This includes the research required to find the optimal mathematical representation of the aero-thermodynamic processes in the gas turbine components in terms of fidelity, accuracy and computing power limitations. The results have been applied in the development of the Gas turbine Simulation Program GSP. GSP is a modelling tool for simulation and analysis of gas turbine system performance. This involves 0-D (i.e. zero-dimensional or parametric) component sub-models that calculate averaged values for parameters such as pressures and temperatures at the gas path stations between the components. The component sub-models are configured (‘stacked’) corresponding to the gas turbine configuration. Component performance is determined by both aero-thermodynamic equations and user specified characteristics, such as turbomachinery performance maps. If higher fidelity is required at a specific location in the system model, 1-D component models can be added to predict the change in gas state or other parameters as a function of a spatial (usually in the direction of a streamline) parameter. Non-linear differential equations (NDEs) are used to represent the conservation laws and other relations among the components. The sets of NDEs are automatically configured depending on the specific gas turbine configuration and type of simulation. Simulation types include design point (DP), steady-state off-design (OD) and transient simulations. The research and development challenge lies in the development of generic, accurate and user friendly system modelling methods with sufficient flexibility to represent any type of gas turbine configuration. The accuracy and fidelity is enhanced by the development of modelling methods capturing secondary effects on component and system performance in 0-D or 1-D sub-models. Object oriented software design methods have been used to accomplish the flexibility objectives, also resulting in a high degree of code maintainability. This allows easy adaptation and extension of functionalities to meet new requirements that are emerging since the start of the development of GSP in its current form (1997). The object oriented architecture and how it relates to the system and component modelling and the ensuing solving of the NDEs, is described in the thesis. An important element has been the development of the gas model with chemical equilibrium and gas composition calculations throughout the cycle. Fuel composition can be specified in detail for accurate prediction of effects of alternative fuels and also detailed emission prediction methods are added. The gas model uses a unique and efficient method to iterate towards chemical equilibrium . The object oriented architecture enabled the embedding of a generic adaptive modelling (AM) functionality in the GSP numerical process and NDEs, providing best AM calculation speed and stability. With AM, model characteristics are adapted for matching specified (often measured) output parameter values for engine test analysis, diagnostics and condition monitoring purposes. The AM functionality can be directly applied to any GSP engine model. The recent trend towards the development of micro turbines (with very high surface-to-volume ratios in the gas path) requires accurate representation of thermal (heat transfer) effects on performance. For this purpose, GSP has been extended with an object oriented thermal network modelling capability. Also, a 1-D thermal model for representing the significant heat soakage effects on micro turbine recuperator transient performance has been developed. For real-time transient simulation, the Turbine Engine Real-Time Simulator (TERTS) modelling tool has been derived from GSP. In TERTS, the methods from GSP are used with fidelity reduced to some extent in order to meet the real-time execution requirements. GSP has been applied to a wide variety of gas turbine performance analysis problems. The adaptive modelling (AM) based gas path analysis functionality has been applied in several gas turbine maintenance environments. Isolation of deteriorated and faulty turbofan engine components was successfully demonstrated using both test rig data and on-wing data measured on-line during flight. For a conceptual design of a 3kW recuperated micro turbine for CHP applications, design point cycle parameters were optimized based on careful component efficiency and loss estimates. Worst and best case scenarios were analysed with GSP determining sensitivity to deviations from the estimates. The predictions have proven very accurate after a test program showing 12% (electric power) efficiency on the first prototype. For increasing the efficiency towards 20%, GSP was used to predict the impact of several design improvements on system efficiency. GSP was used to study the effects on performance and losses of scaling micro turbines in the range of 3 to 36 kW. At small scales, turbomachinery losses become relatively large due to the smaller Reynolds number (larger viscous losses) and other effects. The scale effects have been analysed and modelled for the turbine and compressor and GSP has been used to predict the effects on system efficiency. Other applications include prediction of cumulative exhaust gas emissions of the different phases of commercial aircraft flights, simulation of thermal load profiles for hot section lifing studies, alternative fuel effect studies, performance prediction of vertical take-off propulsion systems and reverse engineering studies. The object oriented design of GSP has proven its value and has provided the building blocks for an ever increasing number of component models, adaptations and extensions. The flexibility of GSP is demonstrated with the modelling of novel cycles, including a parallel twin spool micro turbine with a single shared combustor, a rotating combustor micro turbine concept, a modern heavy duty gas turbine with a second (reheat) combustor and a multi-fuel hybrid turbofan engine, also with a reheat combustor. Several new capabilities have been developed following new requirements from the user community, using the original object oriented framework and component model classes. In the future, new technologies may replace today’s simulation tools. Maybe even the concept of modelling and simulation as we know it today will entirely change. However, as long as gas turbines and related systems will be developed and operated, there will be a need to understand their behaviour. The fundamental physics behind this will not change nor will the equations describing the processes. In that sense, GSP can be seen as a phase in the development of gas turbine modelling and simulation technology. An interesting question would be, how long will GSP remain before it is left behind for new ways. A lot will depend on the ability of GSP and its developers to adapt to future needs and also future opportunities emerging from new modelling, simulation, and computer and software technologies. So far however, GSP has proven a remarkable track record and will be around for quite a while, serving many scientists and engineers interested in gas turbine system performance analysis and simulation.AWEPAerospace Engineerin
On -adic -functions for
No full textWe use higher Coleman theory to construct a new -adic -function for
. While previous works by the first author,
Pilloni, Skinner and Zerbes had considered the -adic variation of classes in
the of Shimura varieties for , in this note we explore the
interpolation of classes in the , which allows us to access to a different
range of weights. Further, we show an interpolation property in terms of
complex -values using the algebraicity results established in previous work
by the authors.Comment: 16 pages, comments welcome
Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis
No full textKim TO, Cheung GSP, Lee JM, Kim BM, Hur B, Kim HC. Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis. International Endodontic Journal, 42, 14-21, 2009.AbstractAim To compare and evaluate the stress distribution of three NiTi instruments of various cross-sectional configurations under bending or torsional condition using a finite-element analysis model. Methodology Three NiTi files (ProFile, ProTaper and ProTaper Universal) were scanned using Micro-CT to produce a three-dimensional digital model. The behaviour of the instrument under bending or torsional loads was analysed mathematically in software (ABAQUS V6.5-1), taking into consideration the nonlinear mechanical characteristic of NiTi material. Results ProFile showed the greatest flexibility, followed by ProTaper Universal and ProTaper. The highest stress was observed at the surface near the cutting edge and the base of (opposing) flutes during cantilever bending. Concentration of stresses was observed at the bottom of the flutes in ProFile and ProTaper Universal instruments in torsion. The stress was more evenly distributed over the surface of ProTaper initially, which then concentrated at the middle of the convex sides when the amount of angular deflection was increased. Conclusion Incorporating a U-shaped groove in the middle of each side of the convex-triangular design lowers the flexural rigidity of the origin ProTaper design. Bending leads to the highest surface stress at or near the cutting edge of the instrument. Stress concentration occurs at the bottom of the flute when the instrument is subjected to torsion. © 2009 International Endodontic Journal.link_to_subscribed_fulltex
Endodontic treatment outcomes: patient based assessments
No full textpublished_or_final_versionDentistryDoctoralDoctor of Philosoph
Background of the nickel-titanium materials, its properties and an overview of various instrumentation tehniques
No full textEndodontic failures - Changing the approach
No full textThe underlying reason for endodontic failures is almost invariably due to bacterial infection. The bacteria may be situated within a previously missed or uninstrumented portion of a root canal, infiltrate via a leaky coronal restoration and root filling, or cause contamination from an extra-radicular infection. Management of the failing root canal filling begins with the identification of the source of persistent infection. Should the infection be present within the root canal system, such as a missed canal, orthograde retreatment is the choice of treatment. This is also true for asymptomatic cases which had been inadequately obturated and which require the placement of a dowel into the canal for restorative reasons. Periapical surgery is best reserved for cases with no sign of healing after orthograde retreatment and those with extra-radicular infection. This paper discusses the relationship between endodontics and restorative dentistry, treatment planning for endodontic failures, and the reported rates of success with orthograde and surgical retreatments.link_to_subscribed_fulltex
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