No abstract

A numerical technique has been used to determine the free-molecule drag on a straight chain of uniform spheres moving parallel and perpendicular to its axis. For such a concave-shaped body, certain surface elements are inaccessible to molecules having certain velocity components because these areas are screened by neighboring elements. Likewise, reflected molecules do not always escape directly but may experience more than one reflection due to screening by neighboring elements. Because an analytical solution for the drag seems hopelessly complex, we have used a Monte Carlo method to determine the rate of momentum transfer (drag force) due to the molecular collisions and reflections of large numbers of molecules by following their individual dynamics. The results are discussed in terms of the translation of straight chains having fixed orientation, straight chains undergoing Brownian rotations, and branched and kinked chains.

The critical dynamics of dislocation avalanches in plastic flow is examined using a phase field crystal model. In the model, dislocations are naturally created, without any ad hoc creation rules, by applying a shearing force to the perfectly periodic ground state. These dislocations diffuse, interact and annihilate with one another, forming avalanche events. By data collapsing the event energy probability density function for different shearing rates, a connection to interface depinning dynamics is confirmed. The relevant critical exponents agree with mean field theory predictions.

We extend the phase field crystal model to accommodate exact atomic configurations and vacancies by requiring the order parameter to be non-negative. The resulting theory dictates the number of atoms and describes the motion of each of them. By solving the dynamical equation of the model, which is a partial differential equation, we are essentially performing molecular dynamics simulations on diffusive time-scales. To illustrate this approach, we calculate the two-point correlation function of a liquid.PACS numbers: 02.70.Ns, 05.70.Ln Molecular dynamics (MD) has long been a powerful tool to study statistical mechanical systems (for an introduction, see (e.g.) Ref.[1]). By postulating the interaction between atoms and solving the resulting equations of motion, precise information about each atom is known. One of the drawbacks of MD, however, is that too much information is captured. For example, atomic motions in MD simulations are resolved on atomic time scales, whereas in many systems the relevant time scales are diffusive. This makes MD computationally demanding, if not completely inapplicable, in many cases of interest where long time scales are required. In this paper we pursue a novel approach to attain long time scales, starting not from individual particles but from a continuum description of matter known as the phase field crystal (PFC) model [2,3,4,5,6].The starting point of the PFC model is that crystalline materials are governed by a free energy functional that penalizes departures from periodicity of the density in the same way that the Landau theory of phase transitions uses a functional that penalizes spatial gradients of the order parameter. The PFC model is formulated in terms of an order parameter representing the local density, and is constructed so that the free energy functional is minimized by a periodic order parameter configuration. Despite its simplicity and minimal physical input, the PFC model can reproduce both qualitative and semiquantitative (i. [5,10], and can be related to density functional theory [5]. Recent applications of the renormalization group technique [11,12,13] and adaptive mesh refinement have improved the computational efficiency of the model, with resultant computational times several orders of magnitude times faster than MD [11,14].Although the PFC model represents microscopic configurations, it is not MD. The model describes the collective properties of the crystal, but it does not attempt to describe the motion of each individual atom. One can regard the peaks in the order parameter as representing local density maxima, and thus be identified as PFC 'atoms'. However, although the order parameter, ρ( x, t), tends to form PFC 'atoms' in order to minimize the total energy of the system, their number is not conserved. This neglect of the actual atomic configuration, and the resulting absence of vacancies in the description, prevents us from using the model to describe faithfully microscopic phenomena that involve atomic hopping and vacancy diffusion.The goal of this p...

Auditory 4.0 is the only model known to date that provides the full TTS and PTS dose-response curves, including a TTS recovery model. The model shows good agreement with historical data.

From a heuristic calculation of the leading order essential singularity in the distribution of YangLee zeroes, we obtain new scaling relations near the ferromagnetic-Griffiths transition, including the prediction of a discontinuity on the analogue of the critical isotherm. We show that experimental data for the magnetization and heat capacity of La0.7Ca0.3MnO3 are consistent with these predictions, thus supporting its identification as a Griffiths ferromagnet.PACS numbers: 75.40. Cx, 75.47.Lx The influence of disorder on ferromagnets remains, after more than 30 years of effort, a complex and poorlyunderstood phenomenon. In its simplest form, disorder can be represented as a random spatial variation of the exchange interaction J in the bonds between neighbouring sites on a regular lattice. If a great enough fraction p > p c of the bonds have J = 0, then one would expect that there is a vanishingly small probability of finding a percolating pathway of bonds throughout the system, and the cooperative ferromagnetic phase would cease to exist. For smaller values of p, we would expect that the ferromagnetic phase will exist in a form weakened by the shortage of percolating paths; hence thermal fluctuations will destroy the ferromagnetic phase at a temperature T c which is lower than the critical temperature T G of the pure ferromagnet. However, as Griffiths showed[1] it is not the case that the phase for T c < T < T G is purely paramagnetic, because in the thermodynamic limit, there can exist arbitrarily large volumes of the system that are devoid of disorder, with a probability exponentially sensitive to the volume. As a result, the free energy is non-analytic in external field, h, throughout the whole Griffiths phase. The effect of disorder is to partition the pure system into small ferromagnetic clusters. Depending on the size, each cluster has a different value of T c , so that the system as a whole exhibits a spectrum of T c , spanning from the critical temperature of the pure system, T G , due to arbitrarily large clusters, to some value of T c , contributed by smaller clusters.Here we are concerned with the phase transition between the ferromagnetic and Griffith's phases. Just as in the case of a pure ferromagnet, one would like to predict the critical phenomena, but the non-analytic nature of the Griffith's phase makes it difficult to apply off-the-shelf renormalization group techniques [2,3,4] or to posit simple scaling laws, despite recent theoretical progress [5,6,7,8,9,10]. Indeed, it is currently controversial whether or not there is clear experimental evidence [11,12,13,14] supporting the existence of the Griffiths phase. From the practical perspective, perhaps the most unsatisfactory aspect of efforts to relate theory to experiment is that critical exponents derived from conventional scaling laws are unrealistically large: for example, the critical isotherm exponent was recently [15] estimated as δ = 17. The breakdown of conventional scaling strongly suggests that the functional form of the scaling r...

Four impulse noise auditory injury criteria adopted by NATO countries, namely, the MIL-STD-1474D (USA), Pfander (Germany), Smoorenburg (Netherlands), and L(Aeq8) (France), are evaluated against human volunteer data. Data from subjects wearing single-hearing protection exposed to increasing blast overpressure effects were obtained from tests sponsored by the US Army Medical Research and Material Command. Using logistic regression, the four criteria were each correlated with the test data. The analysis shows that all four criteria are overly conservative by 9.6-21.2 dB for the subjects as tested. The MIL-STD-1474D for single-hearing protection is 9.6 dB lower than the observed injury threshold for 95% protection with 95% confidence for this particular group of subjects as tested. Similar conclusions can be drawn for the other three criteria.

We formulate and model the dynamics of spatial patterns arising during the precipitation of calcium carbonate from a supersaturated shallow water flow. The model describes the formation of travertine deposits at geothermal hot springs and rimstone dams of calcite in caves. We find explicit solutions for travertine domes at low flow rates, identify the linear instabilities which generate dam and pond formation on sloped substrates, and present simulations of statistical landscape evolution.