Effect of Microstructural Parameters on the Relative Densities of Metal Foams

A detailed quantitative microstructural analyses of primarily open cell FeCrAlY and 314 stainless steel metal foams with different relative densities and pores per inch (p.p.i.) were undertaken in the present investigation to determine the effect of microstructural parameters on the relative densities of metal foams. Several elements of the microstructure, such as longitudinal and transverse cell sizes, cell areas and perimeters, ligament dimensions, cell shapes and volume fractions of closed and open cells, were measured. The cross-sections of the foam ligaments showed a large number of shrinkage cavities, and their circularity factors and average sizes were determined. The volume fractions of closed cells increased linearly with increasing relative density. In contrast, the volume fractions of the open cells and ligaments decreased with increasing relative density. The relative densities and p.p.i. were not significantly dependent on cell size, cell perimeter and ligament dimensions within the limits of experimental scatter. A phenomenological model is proposed to rationalize the present microstructural observations

Radio observations of two intermittent pulsars: PSRs J1832+0029 and J1841-0500

We present long-term observations of two intermittent pulsars, PSRs~J1832+0029 and J1841$-$0500 using the Parkes 64\,m radio telescope. The radio emission for these pulsars switches "off" for year-long durations. Our new observations have enabled us to improve the determination of the on-off timescales and the spin down rates during those emission states. In general our results agree with previous studies of these pulsars, but we now have significantly longer data spans. We have identified two unexpected signatures in the data. Weak emission was detected in a single observation of PSR~J1832$+$0029 during an "off" emission state. For PSR~J1841$-$0500, we identified a quasi-periodic fluctuation in the intensities of the detectable single pulses, with a modulation period between 21 and 36 pulse periods.Comment: 7 pages, 7 figures, accepted for publication in Ap

Winning versus losing during gambling and its neural correlates

Humans often make decisions which maximize an internal utility function. For example, humans often maximize their expected reward when gambling and this is considered as a "rational" decision. However, humans tend to change their betting strategies depending on how they "feel". If someone has experienced a losing streak, they may "feel" that they are more likely to win on the next hand even though the odds of the game have not changed. That is, their decisions are driven by their emotional state. In this paper, we investigate how the human brain responds to wins and losses during gambling. Using a combination of local field potential recordings in human subjects performing a financial decision-making task, spectral analyses, and non-parametric cluster statistics, we investigated whether neural responses in different cognitive and limbic brain areas differ between wins and losses after decisions are made. In eleven subjects, the neural activity modulated significantly between win and loss trials in one brain region: the anterior insula ($p=0.01$). In particular, gamma activity (30-70 Hz) increased in the anterior insula when subjects just realized that they won. Modulation of metabolic activity in the anterior insula has been observed previously in functional magnetic resonance imaging studies during decision making and when emotions are elicited. However, our study is able to characterize temporal dynamics of electrical activity in this brain region at the millisecond resolution while decisions are made and after outcomes are revealed

The influence of quintessence on the motion of a binary system in cosmology

We employ the metric of Schwarzschild space surrounded by quintessential matter to study the trajectories of test masses on the motion of a binary system. The results, which are obtained through the gradually approximate approach, can be used to search for dark energy via the difference of the azimuth angle of the pericenter. The classification of the motion is discussed.Comment: 7 pages, 1 figur

How can exact and approximate solutions of Einstein's field equations be compared?

The problem of comparison of the stationary axisymmetric vacuum solutions obtained within the framework of exact and approximate approaches for the description of the same general relativistic systems is considered. We suggest two ways of carrying out such comparison: (i) through the calculation of the Ernst complex potential associated with the approximate solution whose form on the symmetry axis is subsequently used for the identification of the exact solution possessing the same multipole structure, and (ii) the generation of approximate solutions from exact ones by expanding the latter in series of powers of a small parameter. The central result of our paper is the derivation of the correct approximate analogues of the double-Kerr solution possessing the physically meaningful equilibrium configurations. We also show that the interpretation of an approximate solution originally attributed to it on the basis of some general physical suppositions may not coincide with its true nature established with the aid of a more accurate technique.Comment: 32 pages, 5 figure

Bottleneck effects in turbulence: Scaling phenomena in r- versus p-space

We (analytically) calculate the energy spectrum corresponding to various experimental and numerical turbulence data analyzed by Benzi et al.. We find two bottleneck phenomena: While the local scaling exponent $\zeta_r(r)$ of the structure function decreases monotonically, the local scaling exponent $\zeta_p(p)$ of the corresponding spectrum has a minimum of $\zeta_p(p_{min})\approx 0.45$ at $p_{min}\approx (10 \eta)^{-1}$ and a maximum of $\zeta_p(p_{max})\approx 0.77$ at $p_{max}\approx 8 L^{-1}$. A physical argument starting from the constant energy flux in p--space reveals the general mechanism underlying the energy pileups at both ends of the p--space scaling range. In the case studied here, they are induced by viscous dissipation and the reduced spectral strength on the scale of the system size, respectively.Comment: 9 pages, 3figures on reques

The energy budget in Rayleigh-Benard convection

It is shown using three series of Rayleigh number simulations of varying aspect ratio AR and Prandtl number Pr that the normalized dissipation at the wall, while significantly greater than 1, approaches a constant dependent upon AR and Pr. It is also found that the peak velocity, not the mean square velocity, obeys the experimental scaling of Ra^{0.5}. The scaling of the mean square velocity is closer to Ra^{0.46}, which is shown to be consistent with experimental measurements and the numerical results for the scaling of Nu and the temperature if there are strong correlations between the velocity and temperature.Comment: 5 pages, 3 figures, new version 13 Mar, 200

Gaussian coordinate systems for the Kerr metric

We present the whole class of Gaussian coordinate systems for the Kerr metric. This is achieved through the uses of the relationship between Gaussian observers and the relativistic Hamilton-Jacobi equation. We analyze the completeness of this coordinate system. In the appendix we present the equivalent JEK formulation of General Relativity -- the so-called quasi-Maxwellian equations -- which acquires a simpler form in the Gaussian coordinate system. We show how this set of equations can be used to obtain the internal metric of the Schwazschild solution, as a simple example. We suggest that this path can be followed to the search of the internal Kerr metric

Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence

A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is presented. This kind of analysis is performed to characterize the scaling behavior of the sign-oscillating flow structures, and their geometrical properties. In particular, it is observed that cancellations between positive and negative contributions of the field inside structures, are inhibited for scales smaller than the Taylor microscale, and stop near the dissipative scale. Moreover, from a simple geometrical argument, the relationship between the cancellation exponent and the typical fractal dimension of the structures in the flow is obtained.Comment: 21 pages, 5 figures (3 .jpg not included in the latex file