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Found 6273 publications

Rapid tooling by laser powder deposition: Process simulation using finite element analysis

L. Costa Augusto Moita De Deus Tamás Réti R. Vilar

Publication Name: Acta Materialia

Publication Date: 2005-08-01

Volume: 53

Issue: 14

Page Range: 3987-3999

Description:

Laser powder deposition (LPD) is a rapid manufacturing process, whereby near-net-shape components are fabricated by the successive overlapping of layers of laser melted and resolidified material. As new layers of material are deposited, heat is conducted away from recently resolidified material, through the previously deposited layers, inducing cyclic thermal fluctuations in the part as it is built up. These thermal cycles can activate a variety of metallurgical phenomena, such as solid-state transformations, leading to a progressive modification of the material's microstructure and properties. Since the thermal history of the material in the deposited part will differ from point to point and depends on the deposition parameters and build-up strategy, the finished part may present complex distributions of microstructure and properties. In order to achieve the best properties, the deposition process must be optimized and, given its complexity, this optimization can only be effectively done using mathematical simulation methods. In this paper a thermo-kinetic LPD model coupling finite element heat transfer calculations with transformation kinetics and quantitative property-structure relationships is presented. This model was applied to the study of the influence of substrate size and idle time between the deposition of consecutive layers on the microstructure and hardness of a ten-layer AISI 420 steel wall built by LPD. The results show that the thermal history and, hence, the microstructure and properties of the final part, depend significantly on these parameters. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Open Access: Yes

DOI: 10.1016/j.actamat.2005.05.003

Two-dimensional density and density fluctuation diagnostic for the edge plasma in fusion devices

M. Berta S. Fiedler G. Petravich J. Schweinzer K. McCormick S. Zoletnik A. Bencze

Publication Name: Review of Scientific Instruments

Publication Date: 2005-07-01

Volume: 76

Issue: 7

Page Range: Unknown

Description:

A technique is described for the two-dimensional measurement of electron density profile and fluctuations in edge regions of magnetically confined fusion plasmas. The method is based on existing lithium beam beam emission spectroscopy technique, two-dimensional resolution is achieved by electrostatically scanning the beam. If scanning is performed faster than the lifetime of the turbulent structures in the plasma, the diagnostic is capable of measuring the structure of electron density fluctuations as well. The beam strength of currently available beams makes the detection of single fluctuation events impossible, but the full two-dimensional spatial structure of correlations can still be determined. The article describes the technique and fast beam deflection tests up to 250 kHz. The capabilities of such a diagnostic for fluctuation measurement are explored by simulating measurement signals. Measurement of both the two-dimensional density profile, fluctuation correlation function and poloidal flow velocity are demonstrated at the Wendelstein 7-AS stellarator. The shape of the density profile, the radial and poloidal correlation lengths and the flow velocity are in agreement with expectations and previous Langmuir probe measurement. © 2005 American Institute of Physics.

Open Access: Yes

DOI: 10.1063/1.1947727

In search of the nature of specific nucleic acid-protein interactions

B. Benyo J. C. Biro Z. Benyó C. Sansom

Publication Name: Acta Physiologica Hungarica

Publication Date: 2005-06-23

Volume: 92

Issue: 1

Page Range: 1-10

Description:

The theory of "codon-amino acid coevolution" was first proposed by Woese in 1967. It suggests that there is a stereochemical matching - that is, affinity - between amino acids and certain of the base triplet sequences that code for those amino acids. We have constructed a Common Periodic Table of Codons and Amino Acids, where the Nucleic Acid Table showed perfect axial symmetry for codons and the corresponding Amino Acid Table also displayed periodicity regarding the biochemical properties (charge and hydrophobicity) of the 20 amino acids and the position of the stop signals. The Table indicates that the middle (2nd) amino acid in the codon has a prominent role in determining some of the structural features of the amino acids. The possibility that physical contact between codons and amino acids might exist was tested on restriction enzymes. Many recognition site-like sequences were found in the coding sequences of these enzymes and as many as 73 examples of codon-amino acid co-location were observed in the 7 known 3D structures (December 2003) of endonuclease-nucleic acid complexes. These results indicate that the smallest possible units of specific nucleic acid-protein interaction are indeed the stereochemically compatible codons and amino acids. © 2005 Akadémiai Kiadó.

Open Access: Yes

DOI: 10.1556/APhysiol.92.2005.1.1

On the positivity step size threshold of Runge-Kutta methods

Zoltán Horváth

Publication Name: Applied Numerical Mathematics

Publication Date: 2005-05-01

Volume: 53

Issue: 2-4

Page Range: 341-356

Description:

In the first part of this paper we determine the largest step size of Runge-Kutta (RK) methods for which the corresponding numerical approximations are positive (component-wise non-negative) for arbitrary positive initial vector, whenever the underlying initial value problem (IVP) possesses the related positivity preserving property. We prove that step size thresholds for certain classes of positive IVPs guaranteeing positivity that we derived in a former paper are strict for irreducible and non-confluent RK methods. Investigating the strict positivity step size thresholds we can see that these are rather small if at all positive: often they are, roughly speaking, inverse proportional to the Lipschitz constant of the problem. However, for certain (stiff) IVPs with some particular initial vectors, e.g., for some "smooth" vectors in semi-discretized diffusion problems, we experience preservation of positivity with much larger step sizes than the strict positivity step size threshold. To catch this phenomenon, in the second part of the paper we construct positively invariant sets of positive vectors and derive step size thresholds for the discrete version of the positive invariance. The resulting threshold for discrete positive invariance is, roughly speaking, inverse proportional to the one-sided Lipschitz constant only and is shown in good accordance with some displayed computational experiments. © 2004 IMACS. Published by Elsevier B.V. All rights reserved.

Open Access: Yes

DOI: 10.1016/j.apnum.2004.08.026

The density and biomass of Dreissena polymorpha living on submerged macrophytes in Lake Balaton (Hungary)

Ilona B. Muskó Beáta Bakó

Publication Name: Archiv Fur Hydrobiologie

Publication Date: 2005-04-05

Volume: 162

Issue: 2

Page Range: 229-251

Description:

Dreissena polymorpha appeared and spread throughout Lake Balaton in the 1930s. We studied the density, the body length-body mass relationships and the biomass of D. polymorpha living on the submerged macrophytes in the littoral zone at four sites of different trophic status from May to October 2000. The dominant submerged macrophyte was Potamogeton perfoliatus in May/June and July and Myriophyllum spicatum in October, the fresh biomass of submerged macrophytes ranged between 450.64 and 3,171.51 g/m2, dry biomass ranged between 61.54 and 381.31 g/m2. D. polymorpha represented 2.48-84.85 % of all animals on submerged macrophytes. The density of zebra mussels ranged between 421 and 749,032 ind./m2, between 1 and 282 ind./g macrophyte fresh mass, and between 9 and 2,032 ind./g macrophyte dry mass. The body length-body mass relationships could be described by simple power function equations for fresh body mass with and without shell and dry body mass with and without shell. Zebra mussel biomass in Lake Balaton varied widely between 0.35 and 1,106.55 g fresh mass with shell/m2, between 0.01 and 50.96 g dry mass with shell/ m2, between 0.09 and 260.39 g fresh mass without shell/m 2 and between 0.002 and 6.49 g dry mass without shell/m2. The estimated respiratory organic carbon loss of total D. polymorpha population in the zone of submerged macrophytes ranged between 0.15 and 338.27 mg C/m 2 day, which is 0.04-154 % of planktonic primary production in different parts of the lake, illustrating the important role of this Ponto-Caspian invasive mussel in the matter-energy flux in Lake Balaton. The density of the zebra mussel (ind./m2) significantly depends on the type of submerged macrophytes and on the water depth. The type of submerged macrophytes and water depth have significant additive and interactive effects on the length of animals. Only the water depth determined significantly the biomass of zebra mussel. © 2005 E. Schweizerbart'sche Verlagsbuchhandlung.

Open Access: Yes

DOI: 10.1127/0003-9136/2005/0162-0229

Implicit extension of taylor series method for initial value problems

G. Y. Molnarka

Publication Name: Journal of Computational Methods in Sciences and Engineering

Publication Date: 2005-01-01

Volume: 5

Issue: 4

Page Range: 263-270

Description:

The Taylor series method is one of the earliest analytic-numeric algorithms for approximate solution of initial value problems for ordinary differential equations. The main idea of the rehabilitation of these algorithms is based on the approximate calculation of higher derivatives using well-known technique for the partial differential equations. The approximate solution is given as a piecewise polynomial function defined on the subintervals of the whole interval. This property offers different facility for adaptive error control. This paper describes several explicit Taylor series with implicit extension algorithms and examines its consistency and stability properties. The implicit extension based on a collocation term added to the explicit truncated Taylor series. This idea is different from the general collocation method construction, which conduces to the implicit R-K algorithms [12]. It demonstrates some numerical test results for stiff systems herewith we attempt to prove the efficiency of these new-old algorithms.

Open Access: Yes

DOI: 10.3233/jcm-2005-5404

Partial electron beam hardening of cast iron camshafts

Ágnes Csizmazia Miklós Horváth István Oláh Tamás Réti

Publication Name: Materials Science Forum

Publication Date: 2005-01-01

Volume: 473-474

Issue: Unknown

Page Range: 447-452

Description:

In order to improve the local surface properties (hardness, wear and contact fatigue resistance) of cast iron camshafts, detailed experiments with partial electron beam hardening have been performed. It was found that the required case depth of 0.3-0.5 mm and surface hardness of 600-700 HV can be achieved by using appropriately selected, computer-controlled processing parameters (angular speed, specific energy input, beam deflection). © 2005 Trans Tech Publications, Switzerland.

Open Access: Yes

DOI: 10.4028/0-87849-957-1.447

Reconstruction of myocardial short-scan SPECT images

B. Benyo Ákos Szlávecz Péter Várady Tamás Bükki

Publication Name: Ines 05 IEEE 9th International Conference on Intelligent Engineering Systems Proceedings

Publication Date: 2005-01-01

Volume: 2005

Issue: Unknown

Page Range: 37-41

Description:

In myocardial perfusion SPECT imaging the effect of photon attenuation may introduce artifacts in the reconstructed image due to the highly non-uniform distribution of tissue in the thorax region, potentially resulting in false-positive interpretations. It was the general consideration that the adequate compensation of photon attenuation requires, that the emission data be measured at projection angles over 2π in the case of the attenuation medium is inhomogeneous. The reduction of the scanning angle in SPECT imaging may be desirable because it can reduce scanning time and thereby minimize patient-motion and other artifacts. In SPECT myocardial imaging emission data is measured historically at projection angles over π from the right anterior oblique (RAO) to the left posterior oblique (LPO). This configuration results in better image contrast and, in some cases, betters spatial resolution. However, in this case the reconstructed image may suffer more severely from geometric distortion than 2π angular sampling. It has been proven recently in analytical computer simulation studies that the data function over 2π in SPECT with non-uniform attenuation contains redundant information; therefore the scanning angle theoretically can be reduced from 2π to π without loss of information. In this study our goal was to investigate how the various short-scan SPECT scheme configurations works in a real myocardial SPECT imaging system with highly inhomogeneous attenuating medium using attenuation correction. The measured projection images were reconstructed using the Maximum Likelihood Expectation Maximization algorithm with attenuation correction. The reconstructed slices of the various short-scan configurations and the full-scan slices were compared by a cardiac stress/rest software package. © 2005 IEEE.

Open Access: Yes

DOI: 10.1109/INES.2005.1555127

Speeding up the performance analysis of communication systems

No authors available

Publication Name: 2005 European Simulation and Modelling Conference, ESM 2005 - Proceedings

Publication Date: 2005-01-01

Volume:

Issue:

Page Range: 329-333

Description:

The methods for the performance analysis of communication networks are reviewed. Different ways are shown how the combination of DES and TFA can be made faster by using parallelism. An efficient algorithm for the parallel execution of the combined DES and TFA is presented.

Open Access: No

DOI: DOI not available