Basis Of Simulations Research Articles

First adiabatic invariant of a charged particle modified in a time-dependent magnetic field

The mechanics of the change of the first adiabatic invariant of a nonrelativistic charged particle due to the linear change with time of the spatially-uniform magnetic induction is studied with the aid of an approximate analytic formalism. The same formalism enables one also to undertake an analytic study of the motion of the guiding center of the gyrating particle. The accuracy of the formalism is checked by comparing it with numerical calculations. The results obtained within the formalism are thoroughly compared with a former study of the mechanics of the change of the first adiabatic invariant done by Borovsky and Hansen [Phys. Rev. A 43, 560 (1991)] mainly on the basis of computer simulations. The examined changes of the magnetic field are small compared to the original field and occupy a time period much longer than one gyroperiod.

Theory and practice of isoelectric focusing of interacting systems.

The theory of mass transport coupled to reversible protein interactions forms the basis for computer simulation of the isoelectric focusing behavior of several model systems. These include pH-dependent conformational transition, carrier ampholyte-induced interactions and protein-ligand interactions. The computational results compare favorably with experimental observations. In addition, a method is formulated for an isoelectric focusing procedure which enables determination of intrinsic ligand-binding constants for statistical binding of a charged ligand, binding to heterogeneous sites, and cooperative binding.

On the nature of lattice distortion near the γ/γ′ interfaces in nickel-base superalloys

Nickel base superalloys are strengthened by ordered {gamma}{prime} precipitates of L1{sub 2} structure, which are coherently embedded in the {gamma} matrix (fcc). The precipitate and the matrix are in a cube/cube orientation relationship. The mismatch in lattice parameter ({delta}) between the two phases influences the mechanical properties of these alloys. The value and the sign of {delta} also control the morphological changes in {gamma}{prime} (rafting), which occur during deformation at high temperatures and at low strain rates. In designing these superalloys, the mismatch can be controlled through a modification of the composition. For these reasons measurement of {delta} is important in order to understand the alloy behavior. Several experimental techniques have been used for the measurement of {delta} which include X-ray diffraction (XRD), neutron diffraction (ND) and convergent beam electron diffraction (CBED). In the present paper the authors present results from CBED measurements in [014] zone axis to show that the distortion is not only tetragonal, there is also angular distortion in the unit cell in addition to the dilation/contraction in some of the lattice dimensions. The experimental results are verified on the basis of computer simulations of the HOLZ patterns from crystals subjected to different distortions. A geometrical modelmore » of the lattice across the {gamma}/{gamma}{prime} interface is also presented.« less

Optical evaluation of fractal surfaces using array illumination

A new optical method using an array illumination is introduced in order to analyze fractal surfaces on the basis of computer simulations. In computer simulations, some different self-affine surfaces are employed as models of fractal surfaces, and their scaling factors are evaluated by the proposed method. The results are compared with true values to make clear the effectiveness of the method. The effect of shadowing on the surfaces is discussed. The method is applied to three different actual rough surfaces, and their scaling properties are evaluated experimentally.

Polymer Chain Dynamics III. Intermittent Orientational Relaxation of Terminal Segment on Polystyrene Chain

Polystyrene having an anthryl group attached at the end of a chain, whose degree of polymerization is 550, was used. We have found that the most appropriate orientational auto-correlation function for a terminal segment on a chain is hyperbolic function r(t)∼r0(t/τeff)−β. This behavior indicates the primary process of orientational relaxation of chain segments to be the intermittent process. The distribution of the state density of a terminal segment is in agreement with that of “inner” segment according to the reference temperature Tr, which reflects to the characteristic activation energy of orientational relaxation of segments. Therefore the characteristic activation energy of orientational relaxation of a segment is independent of position on a chain. The values of minimum reaction radii ξ0 of a terminal segment were about twice times larger than that of a central segment. This behavior can be reduced to the degree of freedom of segmental motion on the basis of computer simulation. Therefore ξ0 corresponds not only the apparent activation energy of solvent molecules but the degree of freedom of segmental motion on a polymer chain.

High-resolution electron microscopy of Al–Ni–Fe decagonal quasicrystal

The structure of a stable decagonal quasicrystal with 0.4 nm periodicity, formed in a conventionally solidified alloy with composition Al71.6Ni23.7Fe4.7, was investigated by high-resolution electron microscopy. It was found that this decagonal phase is one of the best-ordered quasicrystals, without any detectable linear phason strain and with only weak random phason strain. The structure is interpreted as an aperiodic tiling of decagonal atom columns with a diameter of about 1.2 nm. The atomic arrangements in special local regions are discussed on the basis of computer simulations.

Design and measurements of a novel subharmonically pumped millimeter-wave mixer using two single planar Schottky-barrier diodes

A novel design of a subharmonically pumped millimeter-wave mixer operating at room temperature was developed and realized. The double sideband conversion loss and mixer noise temperature were measured to be 6.2 dB and 930 K, respectively, at a local oscillator frequency of 73 GHz and an IF of 1.5 GHz. These results are comparable to the best published results measured for subharmonically pumped mixers at similar frequencies. The mixer shows good performance even at IF's up to 9.5 GHz resulting in a useful RF range from 136 GHz to 156 GHz. For the first time a subharmonically pumped millimeter-wave mixer was designed without the use of any scale model measurements or other high-frequency measurements. The whole design process occurred on the basis of computer simulations. Two single low-capacitance planar air-bridge type Schottky-barrier diodes are used as the mixing elements.

Computer Simulation Predicts Unfavorable Mud Rate and Optimum Air Injection Rate for Aerated Mud Drilling

Summary In certain areas, the use of aerated mud as a circulating medium for drilling oil and gas wells is becoming an attractive practice. This is because aerated drilling has many advantages over conventional mud drilling, such as a higher penetration rate, less formation damage, minimized lost circulation, and lower drilling cost. The importance of maintaining adequate air and mud flow rates is generally recognized in aerated drilling operations. However, it remains unclear to drilling operators as to what constitutes an "adequate flow rate." On the basis of computer simulation, this paper discusses carrying capacity of an aerated mud and the optimum air-injection rate that ensures a maximum penetration rate. It is found in this study that the carrying capacity of an aerated mud is very different from that of both the conventional mud and pure air. There is an unfavorable range of mud flow rate that provides lower carrying capacity of the aerated fluid for a given air injection rate. As a unique characteristic of multiphase flow, there exists an air injection rate that gives the lowest flowing annulus pressure for a given well geometry and a mud rate. By considering both the carrying capacity and flowing annulus pressure, an optimum combination of mud and air rates can be determined. This optimum combination of flow rates should ensure a maximum penetration rate for a given well geometry. This paper provides drilling operators with a means of optimizing aerated mud drilling.

Some considerations regarding optimum flow fields for centrifugal air classification

Centrifugal air classifiers, or classifiers with some sort of centrifugal field, are used in classifying particulate materials in the cut size range between a few hundred microns to below one micron. The sharpness of cut in these devices depends on adequate dispersion of the feed material and the maintenance of identical forces on the particles, regardless of their position, in the classifying zone. This paper demonstrates, on the basis of computer simulations, that the latter condition is not as easy to realize as one might wish.

Peformance of RZ- versus NRZ-transmission on standard single-mode fibers

The huge amount of standard single-mode fibers installed in terrestrial systems makes the analysis of return-to-zero (RZ) modulation format an interesting proposal. On the basis of computer simulations, we investigate the RZ-transmission for the optimum system performance taking into account the dispersion, Kerr nonlinearity, attenuation and ASE noise. Parameters as bit rate, distance and power levels are estimated for the lowest system degradation. The work considers different duty ratios including NRZ-pulses.

Experimental Study on Control of Autopilot System(I)

This paper presents a design method for autopilot control system in course change to the specified direction based on a robust digital servo controlmelthod incorporating the concept of the annihilator polynormial. The mathematicalmodel of ship turning motion is very complex in the view of practical control because it has time varying parameters, nonlinear and dead time terms. To apply the digital servo control method based on computer control, the model is linearized at an equilibrium point and discretized with appropriate sampling time. The control algorithm was evaluated on the basis of computer simulation for a model ship and the practical experiment was carried out with an image processing method for measurement of ship position in a water tank. The results of overall experiments show that the proposed control method will be one of good way to keep a track plotted in the map.

Modulation of the redox state of the copper sites of human ceruloplasmin by chloride

Incubation of human ceruloplasmin with physiological concentrations of chloride at neutral pH invariably caused dramatic changes of both the spectroscopic and the functional properties of the protein. The optical intensity at 610 nm increased up to 60%, with a concomitant decrease at 330 nm and the appearance of new bands between 410 and 500 nm. Signals previously undetectable appeared in the EPR spectrum. On the basis of computer simulations, they were interpreted as stemming from an oxidized type 1 copper site and from a half-reduced type 3 copper pair. Removal of chloride completely restored the original optical and EPR lineshapes. Hydrogen peroxide, added to ceruloplasmin in the presence of chloride, was able to capture the electron of the half-reduced type 3 site and to yield a protein insensitive to subsequent removal and readdition of the anion. As a whole, the spectroscopic data indicate that a blue site is partially reduced in the resting protein and that, upon binding of chloride, human ceruloplasmin undergoes a structural change leading to displacement of an electron from the reduced type 1 site to the type 3 site pair. Chloride dramatically affected the catalytic efficiency of human ceruloplasmin. At neutral pH, the anion was an activator of the oxidase activity, being able to enhance up to tenfold the catalytic rate. At pH < 6, in line with all previous reports, chloride strongly inhibited the activity. At intermediate pH values, i.e., around 6, the effect was composite, with an activating effect at low concentration and an inhibitory effect at higher concentration. Since chloride is present at very high concentrations in the plasma, these results suggest that human ceruloplasmin is, in the plasma, under control of this anion.

Agreement percolation and phase coexistence in some Gibbs systems

We extend some relations between percolation and the dependence of Gibbs states on boundary conditions known for Ising ferromagnets to other systems and investigate their general validity: percolation is defined in terms of the agreement of a configuration with one of the ground states of the system. This extension is studied via examples and counterexamples, including the antiferromagnetic Ising and hard-core models on bipartite lattices, Potts models, and many-layered Ising and continuum Widom-Rowlinson models. In particular our results on the hard square lattice model make rigorous observations made by Hu and Mak on the basis of computer simulations. Moreover, we observe that the (naturally defined) clusters of the Widom-Rowlinson model play (for the WR model itself) the same role that the clusters of the Fortuin-Kasteleyn measure play for the ferromagnetic Potts models. The phase transition and percolation in this system can be mapped into the corresponding liquid-vapor transition of a one-component fluid.

Computational Simulation of Flow Localization Behavior

The present article provides a perspective for computational predictions relating to flow localization under a wide range of deformation rates for a variety of materials characterized by constitutive equations incorporating the strain-rate and temperature sensitivities, and strain-rate-history, temperature-history, and strain-gradient dependences of the flow stress. The regularization schemes to remedy the problems associated with spurious mesh sensitivity and incorrect convergence in finite-element prediction of flow localization behavior are reviewed. Various aspects of flow localization behavior of plane strain blocks with different shapes and sizes under a wide range of tension rate are discussed on the basis of computational simulations.

Fluctuation of the number of particles deposited on a flat surface by a random sequential adsorption mechanism.

The problem of the fluctuations of the number n of particles adsorbed on surfaces through a random sequential adsorption process is discussed. Attention is paid, in particular, to the effect of the size of the adsorbing surfaces upon the variance sigma2(n) of this number. On the basis of computer simulations, it is shown that sigma2(n) is not proportional to the area a of the surface but can be written as a sum of three contributions, which are proportional to a, a1/2, and a0. A theoretical estimate based on the relation between the radial distribution function g(r) and the fluctuation is presented and provides a basis for these findings. This analysis is of general validity and can, in particular, also be applied to the equilibrium case (in the absence of a phase transition) and to the ballistic deposition process.

Edge-preserving tomographic reconstruction from gaussian data using a gibbs prior and a generalized expectation-maximization algorithm

The problem of edge-preserving tomographic reconstruction from Gaussian data is considered. The problem is formulated within a Bayesian framework, where the image is modeled as a pair of Markov Random Fields: a continuous-valued intensity process and a binary line process. The a priori information considered here enforces constraints both on the local regularity of the image and on the line configurations. The solution, defined as the maximizer of the posterior probability, is obtained using a Generalized Expectation-Maximization (GEM) algorithm, in which both the intensity and the line processes are iteratively updated. The simulation results show that introducing suitable priors on the line configurations improves the quality of the reconstructed images, and is particularly useful when the data record is small. The relationships with other approaches for managing discontinuities are outlined. A comparison between the GEM algorithm and an algorithm based on mixed-annealing is made on the basis of computer simulations.©1994 John Wiley & Sons Inc

Active Control Of The Transmission Of Sound Through A Thin Cylindrical Shell, Part II: The Minimization Of Acoustic Potential Energy

By using the model of an aircraft fuselage discussed in Part I, an analysis is presented which demonstrates that the total acoustic potential energy (Ep) within the cylindrical model is minimized by a unique vector of secondary force inputs. Similar analysis is used to derive an expression for the optimum vector of secondary forces that minimize the sum of the squared pressures at a number of microphone positions (Jp) within the fuselage model. These results are used as the basis of computer simulations in which Ep and Jp are minimized at 88 Hz and 176 Hz. It is shown that large global reductions in the acoustic energy within the cylinder model can be obtained by using very few secondary forces. This is achieved, however, at the cost of increasing the vibrational energy in the cylinder.

Capillarity-induced filling of carbon nanotubes

THE recent discovery1 and bulk synthesis2 of nanometre-scale carbon tubules has led to much speculation about possible uses of these graphitic structures3–5. Broughton and Pederson predicted on the basis of computer simulations that open nanotubes may be filled with liquid by capillary suction6. Here we describe experiments in which annealing of the tubules in the presence of liquid lead results in opening of the capped tube ends and subsequent filling of the tubes with molten material through capillary action. The nanotubes thus act as moulds for the fabrication of (possibly metallic) wires, some of which are less than two nanometres in diameter.

Computer analysis of side-band holography in STEM

The feasibility of a recently proposed method for direct reconstruction of a wave function at the exit plane of an object is tested on the basis of computer simulation. It is assumed that the point electron source of a STEM instrument is ideally coherent. Optimum imaging conditions for recording of a hologram are found with respect to a restoration error as a function of the desired spatial resolution of a restored wave function and the accelerating voltage. The highest spatial resolution that can be achieved for 100 and 300 kV holography is determined. The size of a region is determined for which the wave function can be restored for any one beam position. Accuracy requirements to which the experimental imaging conditions should be known are worked out.

Aspects concerning the choice of measuring points and specification of parameters needed for a proper determination of magnetic fields

Laplacian fields approximated by a finite series based on an orthogonal set of spherical harmonics are treated. A procedure for estimating the optimal shape of an approximation model, showing not only the variety of degrees but also the highest significant order and a possible order absence of involved field functions, is developed. The restrictions on the distribution of field observations are specified, and a way to ensure a distribution respecting these restrictions that leads to a reduction of necessary field observations is proposed. On the basis of computer simulation, rules for the estimation of the minimal number and distribution of observation points necessary for a proper analysis of a field determination by a particular approximation model are specified. The approach substantially reduces time and calculation effort for magnetic field analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>