27,458 research outputs found
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Build Time Estimations for Large Scale Modelling
Achieving speedy results in model making is very much desired if not a necessity in ahnost any
manufacturing industry. There is no doubt that rapid prototyping contributes to this process. It is
generally considered that when compared to conventional machining techniques like nlilling, the
current rapid prototyping systems appear to be much faster. This is certainly true for complex,slnall
objects. I-Iowever, this is not alwaysa,pplicable to simple, large and bulky parts.
There are a number of projects and systems concentrating on the fabrication of large models. Work
is being carried out at the University ofHong Kong, using milling. along with slicing technology.
This.report compares some ofthe rapid prototyping systems witl1milling. Milling is an established
technology and recent developments in materials and nlachines used in Inilling nlake it a good
alternative to rapid prototyping when itcomes to largesyale nl0delling.Mechanical Engineerin
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Numerical Study on the Recoating Process in Microstereolithography
Microstereolithography is a promising RP-based micro-fabrication technique that aims to
meet the demands for complex geometry micro-scale parts. Projection microstereolithography
incorporates a Dynamic Pattern Generator to obtain high resolution in the parallel plane.
However, its lateral resolution has been always limited by the final layer thickness and the long
resin settling time, both of which rely on the recoating process. In order to find the critical
factors behind the recoating process, a numerical simulation method (Computational Fluid
Dynamics, CFD) has been used to investigate the relationships among final layer thickness,
settling time, resin viscosity and ratio of object/container size. These results are helpful for the
selection of resin characteristics and the design of the microstereolithography machine.Mechanical Engineerin
Checklist of the Helminth Parasites of South American Bats
My Brazilian co-author paid for this paper to be open--access.Copyright © 2001-2015 Magnolia Press. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The attached file is the published version of the article
Numerical approximation for the infinite-dimensional discrete-time optimal linear-quadratic regulator problem
An abstract approximation framework is developed for the finite and infinite time horizon discrete-time linear-quadratic regulator problem for systems whose state dynamics are described by a linear semigroup of operators on an infinite dimensional Hilbert space. The schemes included the framework yield finite dimensional approximations to the linear state feedback gains which determine the optimal control law. Convergence arguments are given. Examples involving hereditary and parabolic systems and the vibration of a flexible beam are considered. Spline-based finite element schemes for these classes of problems, together with numerical results, are presented and discussed
Shifting the closed-loop spectrum in the optimal linear quadratic regulator problem for hereditary systems
In the optimal linear quadratic regulator problem for finite dimensional systems, the method known as an alpha-shift can be used to produce a closed-loop system whose spectrum lies to the left of some specified vertical line; that is, a closed-loop system with a prescribed degree of stability. This paper treats the extension of the alpha-shift to hereditary systems. As infinite dimensions, the shift can be accomplished by adding alpha times the identity to the open-loop semigroup generator and then solving an optimal regulator problem. However, this approach does not work with a new approximation scheme for hereditary control problems recently developed by Kappel and Salamon. Since this scheme is among the best to date for the numerical solution of the linear regulator problem for hereditary systems, an alternative method for shifting the closed-loop spectrum is needed. An alpha-shift technique that can be used with the Kappel-Salamon approximation scheme is developed. Both the continuous-time and discrete-time problems are considered. A numerical example which demonstrates the feasibility of the method is included
Computational methods for optimal linear-quadratic compensators for infinite dimensional discrete-time systems
An abstract approximation theory and computational methods are developed for the determination of optimal linear-quadratic feedback control, observers and compensators for infinite dimensional discrete-time systems. Particular attention is paid to systems whose open-loop dynamics are described by semigroups of operators on Hilbert spaces. The approach taken is based on the finite dimensional approximation of the infinite dimensional operator Riccati equations which characterize the optimal feedback control and observer gains. Theoretical convergence results are presented and discussed. Numerical results for an example involving a heat equation with boundary control are presented and used to demonstrate the feasibility of the method
X-ray diffraction to probe the kinetics of ice recrystallization inhibition
Understanding the nucleation and growth of ice is crucial in fields ranging from infrastructure maintenance, to the environment, and to preserving biologics in the cold chain. Ice binding and antifreeze proteins are potent ice recrystallization inhibitors (IRI), and synthetic materials that mimic this function have emerged, which may find use in biotechnology. To evaluate IRI activity, optical microscopy tools are typically used to monitor ice grain size either by end-point measurements or as a function of time. However, these methods provide 2-dimensional information and image analysis is required to extract the data. Here we explore using wide angle X-ray scattering (WAXS/X-ray powder diffraction (XRD)) to interrogate 100's of ice crystals in 3-dimensions as a function of time. Due to the random organization of the ice crystals in the frozen sample, the number of orientations measured by XRD is proportional to the number of ice crystals, which can be measured as a function of time. This method was used to evaluate the activity for a panel of known IRI active compounds, and shows strong agreement with results obtained from cryo-microscopy, as well as being advantageous in that time-dependent ice growth is easily extracted. Diffraction analysis also confirmed, by comparing the obtained diffraction patterns of both ice binding and non-binding additives, that the observed hexagonal ice diffraction patterns obtained cannot be used to determine which crystal faces are being bound. This method may help in the discovery of new IRI active materials as well as enabling kinetic analysis of ice growth
Model Dependence of the 2H Electric Dipole Moment
Background: Direct measurement of the electric dipole moment (EDM) of the
neutron lies in the future; measurement of a nuclear EDM may well come first.
The deuteron is one nucleus for which exact model calculations are feasible.
Purpose: We explore the model dependence of deuteron EDM calculations. Methods:
Using a separable potential formulation of the Hamiltonian, we examine the
sensitivity of the deuteron EDM to variation in the nucleon-nucleon
interaction. We write the EDM as the sum of two terms, the first depending on
the target wave function with plane-wave intermediate states, and the second
depending on intermediate multiple scattering in the 3P1 channel, the latter
being sensitive to the off-shell behavior of the 3P1 amplitude. Results: We
compare the full calculation with the plane-wave approximation result, examine
the tensor force contribution to the model results, and explore the effect of
short range repulsion found in realistic, contemporary potential models of the
deuteron. Conclusions: Because one-pion exchange dominates the EDM calculation,
separable potential model calculations will provide an adequate description of
the 2H EDM until such time as a better than 10% measurement is obtained.Comment: 21 pages, 2 figures, submitted to Physical Review
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How Useful are High-Precision Delta ?17O Data in Defining the Asteroidal Sources of Meteorites?: Evidence from Main-Group Pallasites, Primitive and Differentiated Achondrites
High-precision oxygen isotope analysis is capable of revealing important information about the relationship between different meteorite groups. New data confirm that the main-group pallasites are from a distinct source to either the HEDs or mesosiderites
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