Mathematical modelling of surface segregation in aluminum DC casting caused by exudation

A mathematical model for the development of a segregated layer of exudated droplets during DC casting of aluminum ingots is established. The model accounts for the metallostatic pressure driven interdendritic melt flow through the mushy zone by a Darcy type equation, the surface segregation due to this melt flow, and the decrease of the total solute concentration in different positions of the mush as a result of the exudation. The solution domain for the governing differential equations is constituted by the mushy zone of the cast. The main physical phenomena included in the model have been studied in a simple one dimensional case study.     

A new boundary-layer wind tunnel at the Danish Maritime Institute

The paper describes a new boundary-layer wind tunnel at the Danish Maritime Institute in Lyngby. The wind tunnel is of the open-circuit type and is integrated into a building raised for the purpose. The design was constrained by the need to provide a simulation of the atmospheric boundary layer (without thermal stratification) and alternatively a uniform flow without excessive turbulence.The wind tunnel has a working section of principal dimensions: length 20.8 m, width 2.6 m and height 1.8 m (adjustable from 1.8 to 2.3 m). The maximum wind speed in the working section when empty is about 26 m s^?1.Data acquisition, analysis and instrumentation in relation to the new wind tunnel are discussed and, finally, an example of a simulated natural wind is presented.     

On the off-design of a natural gas-fired combined cycle with CO2 capture

During the last 15 years cycles with CO₂ capture have been in focus, due to the growing concern over our climate. Often, a natural gas fired combined cycle with a chemical absorption plant for CO₂ capture from the flue gases have been used as a reference in comparisons between cycles. Neither the integration of the steam production for regeneration of amines in the combined cycle nor the off-design behaviour of such a plant has been extensively studied before.     

Room electromagnetics

In analogy with the established discipline of room acoustics, various aspects of diffuse wideband microwave propagation in a room are treated. It is shown that an equivalent to Sabine's equation for reverberation time in a room is valid for the completely diffused field, depending only on the volume, the surface area, and an effective absorption coefficient. An exponential decay of the power as a function of the delay is a consequence of the assumptions. Furthermore, the concept of a reverberation distance is also valid. This is the distance from a transmitting antenna where the received diffuse, randomly scattered power equals the direct line-of-sight received power, such that the diffuse power dominates for distances larger than the reverberation distance. A number of measurements in a large room support the theory with an effective absorption coefficient of 0.5. The power delay profiles around the room from a transmitter in the ceiling vary only in the first arriving part of the impulse, whereas the tail, being dominated by the diffuse field, has the same power level for a given delay and the same decay rate all over the room. It is also a consequence of the theory that the diffuse fields incident on an antenna are uniformly distributed in angle.     

The Rician inverse Gaussian distribution: a new model for non-Rayleigh signal amplitude statistics.

In this paper, we introduce a new statistical distribution for modeling non-Rayleigh amplitude statistics, which we have called the Rician inverse Gaussian (RiIG) distribution. It is a mixture of the Rice distribution and the inverse Gaussian distribution. The probability density function (pdf) is given in closed form as a function of three parameters. This makes the pdf very flexible in the sense that it may be fitted to a variety of shapes, ranging from the Rayleigh-shaped pdf to a noncentral chi2-shaped pdf. The theoretical basis of the new model is quite thoroughly discussed, and we also give two iterative algorithms for estimating its parameters from data. Finally, we include some modeling examples, where we have tested the ability of the distribution to represent locale amplitude histograms of linear medical ultrasound data and single-look synthetic aperture radar data. We compare the goodness of fit of the RiIG model with that of the K model, and, in most cases, the new model turns out as a better statistical model for the data. We also include a series of log-likelihood tests to evaluate the predictive performance of the proposed model.     

Static and Dynamic Spins in Superconducting La2−x Sr x CuO4: The Risø Years

One important area of modern condensed matter research is the investigation of the nature of the superconducting cuprates. Much progress in this field has been obtained with the technique of neutron scattering. We here present a review of neutron scattering studies of the high-temperature superconductor La_2?xSr_xCuO₄, performed at Risø National Laboratory. We review the work on the mapping of the incommensurate spin fluctuations, the investigation of the gap in the fluctuation spectrum, the magnetic properties of the vortices appearing in an applied field, and the quantum critical behaviour of the system. We discuss our findings in the light of results of neutron scattering from other groups, on other cuprate systems, and results from other experimental methods, e.g. NMR, μSR, STM, X-ray diffraction, and ARPES. We end with a discussion on the implications of the experimental results for the progress in the general understanding of high-temperature superconductivity. PACS numbers: 61.12.Ex, 74.20.Mn, 74.72.Dn, 75.25.+z, 78.70.Nx.     

The effect of cyclic stress on the physical properties of a poly(dimethylsiloxane) elastomer

The dynamic creep behavior of a filled poly(dimethylsiloxane) elastomer was studied under cyclic stress. The stress level was chosen such that the increase in the internal temperature was small and that microcracks were not observed. This work has demonstrated that cyclic stress in combination with high temperature accelerates the degradation of the elastomer. The results suggest that because of the applied force, breaks in the load-bearing chains of the network occur. These breaks, while relieving the mechanical stress, create highly reactive ionic fragments. It is believed that because of the subsequent reactions of the ionic fragments, changes in the specific gravity, storage modulus, effective crosslink density, and length of the sample (creep) are observed. The observed decrease in the storage modulus is thought to occur because of the reaction of the ionic fragments with moisture, which results in the formation of silanol chain ends that reduce the effective crosslink density. The results also show that contrary to the prediction of the Boltzmann's Superposition Principle, the rate of creep is greatly enhanced when the sample is subjected to a sinusoidally varying dynamic load as compared to a comparable static load. The polymer weight loss was found to be linear with time and strongly dependent on the level of applied dynamic and static force. In addition, the weight loss and rate of creep were also found to be strongly dependent upon temperature.     

Comparative antagonism of kainate-activated kainate and AMPA receptors in hippocampal neurons

Native kainate receptors expressed by cultured hippocampal cells were studied in the whole-cell configuration of the patch-clamp technique by using a fast perfusion system. About 80% of the neurons expressed kainate receptors independently of the time in culture (0-4 days), which coincided with the number of cells immunoreactive for a monoclonal antibody against the GluR5/6/7 subunits. Three types of cells were considered: neurons in which the rapid application of kainate induced a rapidly desensitizing current, cells in which kainate induced a more slowly rising, non-desensitizing, response and those in which a mixture of both responses was apparent. Steady responses induced by 300 microM kainate were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a dose-dependent manner (IC50 = 0.92 microM). CNQX was less potent in blocking transient kainate-induced responses (IC50 = 6.1 microM). Responses to kainate, whether steady or transient, were also inhibited by NS102, showing poor selectivity for the transient response (IC50 = 4.1 and 2.2 microM respectively). The new alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptor antagonist NS394 was very potent in inhibiting steady kainate-induced currents (IC50 = 0.45 microM), but was even more effective in preventing peak responses (IC50 = 0.13 microM). In contrast, cyclothiazide did not affect transient kainate-induced responses but did potentiate current induced by activation of AMPA receptors by AMPA or kainate. These results demonstrate the lack of complete selectivity amongst some available competitive antagonists for AMPA and kainate receptors, and indicate that kainate receptors expressed by hippocampal cells lack the cyclothiazide modulatory site present at AMPA receptors. In addition, the present data support the idea that low-affinity kainate binding sites in the brain correspond to receptor channels selectively activated by kainate.     

Numerical analysis of IPIRG cracked pipe experiments subjected to dynamic and cyclic loading

This report contains results of a finite element study aiming to identify the influence of loading history and geometry for cracked pipes subjected to complex loading. The experiments have been performed within the International Piping Integrity Research Group (IPIRG) Program. The majority of the numerically analyzed experiments were conducted on straight pipes with an outside diameter of 168 mm and containing a large circumferential through-wall crack. The considered pipes were loaded in four-point bending under displacement control and at a temperature of 288°C. The types of loading were combinations of either quasi-static or dynamic and also monotonic or cyclic loading with different loading ratios R. Some analyses were also performed on surface-cracked pipes subjected to slow, monotonic loading.

In the finite element study, 20-node solid elements were used for the through-wall cracked pipes and a combination of shell and non-linear line spring elements for the surface-cracked pipes. Stable crack growth was simulated by gradual node relaxation and crack closure is accounted for by using simple contact elements. The J-integral for a remote contour is calculated and used as a characterizing fracture parameter although the cyclic loading violates the theoretical basis for this procedure. The near-tip J can not be used for growing cracks because of the weak energy singularity. The results of the numerical study confirm the trends from the experiments in that a high loading rate has a negative influence on the fracture properties of the studied carbon steel and that large cyclic loading, especially at R = −1, lowers the apparent J_R-curve for both carbon and stainless steels. To some extent geometry effects appear to be present when comparing the results from pipes containing surface cracks and through-wall cracks with results from CT specimens. These effects are more pronounced for large amounts of stable crack growth than at initiation.