Small Separation Distances Research Articles

Cascade statistics in the binary collision approximation and in full molecular dynamics

The Binary Collision Approximation (BCA) and Molecular Dynamics (MD) are used to simulate low energy atomic collision cascades in solids. Results are compared and discussed on the example of copper and gold self irradiation. For MD, long range N -body potentials are built, similar to those deduced from the second moment semi-empirical tight binding model. The pair interaction contribution is splined to a Molière screened Coulomb potential at small separation distances. This hybrid potential is checked for consistency with the already assessed N -body potential by means of thermal dynamics calculations in both the canonical (NVT) and the micro canonical (NVE) ensembles. Its use for long time enhanced diffusion simulations is discussed. The BCA calculations are performed with the MARLOWE program, using the same Molière potential as for MD, and modelling the N -body contribution by a binding of the atoms to their equilibrium lattice sites. The scattering integrals are estimated by means of a 4 points Gauss-Mehler quadrature. In MD, the NVT equations of motion are integrated with a constant time step of 2 fs. For the NVE cascade simulations, the Newton equations of motion are solved with a dynamically adjusted time step, kept lower than 2 fs. The influence of the time step on the simulated trajectories is discussed. The mean number of moving atoms with total energy above threshold values ranging from 1 to 100 eV is estimated as a function of time over 300 fs both with MARLOWE and by MD. This estimate is repeated for external primary energies ranging from 250 eV to 1 keV. In the case of copper, the BCA results are found to be in remarkable agreement with MD over about 200 fs cascade development, provided the size of the crystallite used in MD is sufficiently large in order to account for the early mechanical response of the close environment. This agreement between the two methods is found to be the best when the binding energy of the target atoms as modelled in the BCA is adjusted to a value close to the cohesive energy. In the case of gold, the agreement between BCA and MD is reasonable and the results suggest the need of an accurate modelling of linear collision sequences in the BCA.

The effect of underwater explosion shock loading on the fatigue behaviour of grp laminates

This paper reports changes in the fatigue behaviour of glass-reinforced polymer (GRP) laminates after being subjected to increasing shock wave pressures produced by an underwater explosion. The shock testing was performed in a small-scale underwater explosion facility, and by using larger explosive charges and smaller separation distances between the charge and GRP, it was possible to exert higher underwater shock wave pressures on the laminate. The laminate was then fatigue tested in tension-compresion at a stress ratio, R, of — 1 to obtain S-N curves. At low shock pressures the laminate experienced a small amount of cracking in the polymer matrix, but this did not alter the fatigue performance compared with the unshocked laminate. At high shock pressures the laminate experienced gross structural damage in the form of polymer cracking, delamination between the polymer and glass fibre, and fracture of the glass fibres. These effects caused a large reduction in the fatigue resistance.

Effect of magnetization profiles on the torque of magnetic coupling

The torque of magnetic coupling is sensitive to the number of magnetic poles, the magnetization profiles, the magnetic field strength of magnets, and the separation distance between the magnets of the coupling. For different multipole magnetic couplings with the same magnetic field strength, the torque of magnetic coupling is proportional to the number of magnetic poles at small separation distance, but it is inversely proportional to the number of magnetic poles as the separation distance becomes large. The theoretically calculated values of the torque of magnetic coupling are in excellent agreement with the experimentally measured values.

Impingement heat transfer with a single rosette nozzle

This investigation examined the magnitude and uniformity of the convective heat transfer coefficients for a single rosette nozzle impinging air normally to a heated surface. The local heat transfer coefficient was measured using a heated 0.025-mm-thick stainless steel foil impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected light from the liquid crystal with the use of bandpass filters and an electronic digitizer. Measurements were obtained for low nozzle-to-plate spacings ( H = 3.81, 0.635, and 0.159 cm) with and without a confinement surface. Local and average convective coefficients distributions illustrate the uniformity or nonuniformity of the heat transfer and assist in understanding the variations in the magnitude of the average convective coefficient. The convective coefficients for the rosette nozzle were also compared to values obtained with axisymmetric jets. Results show that the local convective coefficients are highly nonuniform for the rosette nozzle. From the comparison of the rosette nozzle with the axisymmetric jet, it would appear that the enhancement of the average heat transfer by this type of specialty nozzle design compared to an axisymmetric jet occurs only for small flow rates, small operating separation distances, and specific averaging areas. Small separation distances ( H ≤ 0.635 cm ) are required to minimize the convective coefficient degradation due to entrainment and velocity decay for the rosette nozzle.

Mixed-mode stress intensity factors for interacting semi-elliptical surface cracks in a plate

Mixed-mode stress intensity factor solutions are presented for two parallel, but not necessarily coplanar, interacting surface cracks in a plate. The plate is subjected to a remote uniaxial stress acting normal to the crack planes. The crack separation distances, measured normal and parallel to the crack planes, are varied to provide 24 crack geometries. The smallest separation distance is equal to a quarter of the plate thickness. The equal sized interacting cracks have fixed depth-to-thickness ratios of 0.5 and fixed half-width-to-depth ratios of 3. The finite element alternating method is used to develop the solutions. The tendency for the cracks to grow apart or to coalesce is discussed.

What color transparency measures.

Color transparency is commonly accepted to be a prediction of perturbative QCD. However it is more a phenomenon probing the interface between the perturbative and nonperturbative regimes, leading to some intricacy in its theoretical description. In this paper we study the consequences of the impulse approximation to the theory in various quantum mechanical bases. We show that the fully interacting hadronic basis, which consists of eigenstates of the exact Hamiltonian in the presence of the nucleus, provides a natural basis to study color transparency. In this basis we can relate the quark wave function at a small transverse separation distance ${\mathit{b}}^{2}$1/${\mathit{Q}}^{2}$ directly to transparency ratios measured in experiment. With the formalism, experiment can be used to map out the quark wave function in this region. We exhibit several loopholes in existing arguments predicting a rise in transparency ratios with energy, and suggest alternatives. Among the results, we argue that the theoretical prediction of a rising transparency ratio with energy may be on better footing for heavy-quark bound states than for relativistic light-quark systems. We also point out that transparency ratios can be constant with energy and not at variance with perturbative QCD.

Creeping axisymmetric flow around a solid particle near a permeable obstacle

AbstractThe hydrodynamic interaction between a solid particle and a porous obstacle, both of spherical shape, the former moving slowly along the line of their centers and the latter held stationary in an external axisymmetrical flow field, is analyzed. Owing to the linearity of the creeping motion equations and the boundary conditions, this general problem can be decomposed into two simpler problems: I. the motion of the solid sphere relative to the porous one in a fluid at rest; II. an axisymmetrical streaming flow past the two spheres held stationary. The solution to problem II requires further decomposition into the problem of undisturbed flow in the absence of the two spheres and that of the two spheres following each other in a fluid at rest (problem III). The above component flow problems are solved analytically using the stream function formulation in bispherical coordinates. The flow and pressure fields, and the drag forces exerted on both spheres are determined as functions of the permeability, the slip factor, the gap length, and the relative size of the two spheres. In problem I it is found that the drag force exerted on the solid particle increases with decreasing permeability for any value of the gap length. The opposite behavior is observed in problems III (and II). In all cases, however, the drag force exerted on the porous sphere increases as the permeability decreases for any separation distance. In the region of very small separation distances the drag forces on the two spheres in problem I attain a weak maximum at a critical gap length which is a function of the obstacle permeability and the sphere size ratio. The behavior of the drag forces in problems II (and III) is more complicated and depends strongly on the sphere size ratio. The effects of the slip velocity and the particle to streaming velocity ratio (in the composite problem) on the values of the drag forces are also examined.

On interface and twin boundaries in a forged alloy

This paper discusses XD (exothermic dispersion) processed TiAl/TiB{sub 2} alloys which are a new class of intermetallic composites with improved toughness, creep resistance and oxidation resistance at high temperatures. They generally have a two-phase {alpha}{sub 2} + {gamma} matrix (Ti{sub 3}Al (DO{sub 19}) and TiAl (L1{sub 0} structures, respectively), which contains monocrystalline TiB{sub 2} particles. Although the deformation mechanisms of these alloys are essentially the same as those of conventional TiAl alloys, dislocation substructures, particularly dislocations at {alpha}{sub 2}/{gamma} interfaces, remain to be unambiguously identified. In a recent paper on a Ti-45Al/TiB{sub 2} alloy, these interfacial dislocations were assumed to be misfit dislocations with 1/6 {l angle}112{r angle}{sub {gamma}}. Burgers vectors and their presence was attributed to boron interstitials in the {gamma} matrix, However, the {alpha}{sub 2} phase has been shown to getter interstitial impurities from the {gamma} phase, thus increasing the ductility of the two phase alloy. Moreover, 1/6 {l angle}112{r angle}{sub {gamma}} partial dislocations generally result from the dissociation of glissile {gamma} dislocations during deformation and cannot be resolved by conventional weak beam electron microscopy because of their small separation distances. Their assumed presence is also unusual, in that lattice mismatch is not normally accommodated by partial dislocations.

Ripplon interaction between two liquid surfaces

Abstract The interaction between two liquid surfaces plays a central role in aerosol microphysics and liquid-drop aggregation phenomena. We study the ripplon (quantized capillary-tension waves) contribution to the interaction energy between two parallel planar liquid surfaces. For 4 He and separations between both liquid surfaces ≳10 A, the effect of ripplon excitation is negligible (smaller than 10 −3 ) compared with the van der Waals energy corresponding to two flat liquid 4 He surfaces. At smaller separation distances, the ripplon contribution to the interaction energy can no longer be considered as a perturbation and it should be included in any detailed analysis of liquid-liquid interactions.

Ripplon interaction between two liquid surfaces

The interaction between two liquid surfaces plays a central role in aerosol microphysics and liquid-drop aggregation phenomena. We study the ripplon (quantized capillary-tension waves) contribution to the interaction energy between two parallel planar liquid surfaces. For 4He and separations between both liquid surfaces ≳10 Å, the effect of ripplon excitation is negligible (smaller than 10−3) compared with the van der Waals energy corresponding to two flat liquid 4He surfaces. At smaller separation distances, the ripplon contribution to the interaction energy can no longer be considered as a perturbation and it should be included in any detailed analysis of liquid-liquid interactions.

Computer simulation of semi-dilute polyelectrolyte solutions

We use Monte Carlo simulation methods to study the conformational properties of flexible partially ionized polyelectrolyte chains and the ordering of the polyelectrolyte chains, for polyelectrolyte concentrations where chain—chain interactions start to become important. In this model the ionic and solvent degrees of freedom are frozen and the charges on the polyelectrolyte chains interact via a screened Coulomb interaction of Debye—Hückel form. Our simulations are restricted to polyelectrolyte ionizations and added-salt concentrations where we know that the screening model is not an unreasonable approximation. For semi-dilute polyelectrolyte concentrations we observe some local anisotropy with some alignment between the nearest polyion chains. This alignment only exists for small polyion separations and generally the chains are isotropically distributed and oriented with respect to one another. The observed interchain correlation at small separation distances disappears for dilute polyelectrolyte concentrations below the overlap threshold concentration c*, and also for high polyelectrolyte concentration where interactions between polyions beyond the nearest neighbours begin to become important. For polyelectrolyte concentrations above this value the results for the two added salt concentrations of 0.1 and 0.01 mol dm −3 are almost identical.

Low-energy extraction with simple two grid accelerator system

An experimental investigation for extending the ion extraction capabilities of a two grid accelerator system to low energy is described. The preliminary results have shown that low-energy ion beams ranging from 15 to 80 eV can be extracted by biasing the screen grid potential to be negative to the ground at small grid separation distance. The beam currents which have been extracted are 20–60 mA. Beam current densities ranging from 0.15 to 0.45 mA/cm2 with good uniformities are measured at a location 20 cm downstream of the grids. It is found that the extraction capabilities are mainly determined by the operation parameters of the ion source, but they can be substantially effected by changing the grid separation distance and the magnetic field geometry and strength in the discharge chamber.

Experimental comparison of two hot-wire techniques in supersonic flow

The performance of two constant-temperature normal hot-wire techniques for resolution of turbulent mass flux and local stagnation temperature in a supersonic flow is examined. The first technique used a single wire and the rapid scanning of multiple overheat ratios. Time averages of the signals at all overheats were used to separate the mean and rms mass flux, stagnation temperature, and their cross-correlation. The second technique used a dual-wire probe with each wire operated at different overheat ratios, giving instantaneous mass flux and stagnation temperature. In spite of a small separation distance (0.18 mm) between the wires in the dual-wire probe and high correlation between their signals, the rms mass flux inferred from the dual-wire technique was a factor of two higher than that from the single-wire technique. A consistency check based on data from one of the wires indicated that the dual-wire method produced results that were too high.

Coupling characteristics of crossover arranged rectangular dielectric waveguides

AbstractThe coupling characteristics of two rectangular crossed‐over dielectric waveguides located on different parallel layers which presented a constant angle of intersection on a projection plane are analyzed. When two open waveguides were situated in proximity, each waveguide is mutually affected by the presence of other waveguide. Distributions of propagating modes in each waveguide were perturbed. This paper considers such perturbations using the authors' analysis method for a coupled system in a parallel arrangement. The coupled part is approximated by a step‐like approximation, and the dependence of the coupled electric power along each waveguide on the angle of intersection and the separation distance between upper and lower guides were clarified. Good agreement was obtained between calculated values and experimental measurements, in particular, the case of a tight coupling for a small separation distance between upper and lower waveguides and for the case of a large separation distance with a small angle of intersection with a long coupling part along the axial direction of the waveguides. This paper has improved greatly the authors' previous calculation results in which variational calculus was used on the assumption of a weak coupling. These results are considered to contribute to designing coupled circuits between upper and lower layers of integrated circuits for optical integrated circuits.

Effect of the Sampling Network on Estimates of the Covariance Function of Stationary Fields

AbstractThe covariance function of the stochastic component of various soil properties plays an essential role in the estimation and interpolation of spatial phenomena using kriging techniques and in the solution of transport problems in heterogeneous media using stochastic continuum transport modeling. In this study the effect of the sampling network on the estimates of covariance functions of two‐dimensional, isotropic, second‐order stationary processes, characterized by different underlying correlation scales, was analyzed. Estimates of the covariance function based on a standard systematic sampling network were compared with estimates of the covariance functions based on a modified sampling network. Results of these comparisons showed that the modified sampling network was better suited than the systematic network to resolving the statistical properties of the underlying stochastic process at small separation distances. This may result in an improvement in structure identification. Analyses of the effect of the sampling network used for estimating the covariance function on the results of conditional inference procedures such as kriging showed that the kriging estimates were essentially insensitive to the geometric configuration of the sampling network. Uncertainties about these estimates were reduced when using the modified sampling network, however. In the case of unconditional inferences, such as with stochastic analyses of transport problems in heterogeneous media, predictions of the head variance provided by using estimates of the parameters of the covariance function of the log saturated hydraulic conductivity based on the modified sampling network fit the theoretical head variance better than those based on the systematic sampling network. The effect of the sampling network used for estimating the covariance function on the predicted head variance was significant, particularly when the underlying correlation scale was relatively small, when the porous medium texture became finer, and when the hydraulic conductivity variations were treated as being three‐dimensional.

Collective Quantum Jumps in Pairs of Atoms with Two or Three States

Abstract We have studied cooperative effects on quantum jumps in two-atom systems. These atoms have either two or three states and are driven by a field of arbitrary intensity and bandwidth. We find that, in two two-state atoms at a small separation distance, quantum jumps appear, while the jump rates in two three-state atoms are modified by the collective interactions. Explicit expressions for jump rates are given in various explicit cases.

Hydrodynamics of Ice Mass Near Large Offshore Structure

The changes in flow field around an ice mass drifting in a current near a large offshore structure are investigated using potential theory. A formulation in two dimensions is adopted here to allow relatively accurate results to be obtained for very small separation distances, and an approximate method to extend the results to threedimensional applications is described. The motions of the ice mass and current interactions are represented by five unit potentials and the hydrodynamic forces are obtained by the Bernoulli equation. The proximity effects are then generalized by introducing a set of hydrodynamic coefficients for the ice mass, which are expressed in terms of geometry of the structure and ice mass. A numerical model based on the boundary integral method is developed to solve the problem. A set of coefficients for the special case of two circular cylinders is presented in this paper. The strengths and weaknesses of the proposed model, as well as its practical applications, are discussed.

Turbulence in an almond orchard: Spatial variations in spectra and coherence

The spatial variability of turbulence in a fully-leafed almond orchard was studied. Two three-dimensional sonic anemometers were used to measure turbulence spectra and coherence at different vertical and lateral separations inside the canopy. Peak frequencies of the horizontal velocity components, normalized by local horizontal wind speed, are greater in the canopy crown than in the trunkspace. Peak-normalized frequencies for the vertical velocity power spectra are similar in the canopy crown and in the subcanopy trunkspace. Spectral slopes in the inertial subrange are more negative than those predicted with Kolmogorov's -2/3 theory. It is thought that the foliage elements act to short-circuit the eddy cascade. Lateral separation of the instruments in the subcanopy trunkspace has little effect on the shape of the velocity spectra. On the other hand, lateral and vertical velocity coherences between spatially separated sensors are low inside the canopy. These low coherences are due to the Eulerian length scales being of the same order of magnitude as the separation distances of the anemometers. Phase angles between velocity components are about zero for small separation distances. When the two instruments are separated by 9 m and one instrument is positioned in a row while the other is between two rows, vertical velocities are about 180 deg out of phase and the streamwise velocities are about 40 to 60 deg out of phase. These data support the contention that preferred differences occur between within- and between-row wind flow regimes.

Some Additional Coherence Data in the Inertial Subrange

Abstract Some coherence measurements, taken at the Nasudden wind energy test site, for small separation distances at a height of 65–77 m, are analyzed in terms of a theoretical coherence model for isotropic turbulence, and for Davenport similarity. The decay parameters for the horizontal wind components are found to be about the same for vertical and horizontal separation in the inertial subrange.

Specific and non-specific interactions in bacterial adhesion to solid substrata

Based on a literature review, a hypothesis is forwarded on the mechanism of initial bacterial adhesion to solid substrata, which accounts both for the role of specific microscopic surface components as well as for the role of non-specific macroscopic surface properties (surface free energy, zeta potential or hydrophobicity). Three distinct regions in the adhesion process are suggested in which at large and intermediate separation distances adhesion is mediated by the macroscopic surface properties as surface free energy and surface charge, respectively. At small separation distances specific short-range interactions can occur, leading to a strong and irreversible bonding, provided the water film present in between the interaction surfaces can be removed. A major role of hydrophobic groups, supposed to be associated with bacterial surface appendages is suggested to be its dehydrating capacity, enabling the removal of the vicinal water film yielding small areas of direct contact between protruberant parts of the cell surface and the substratum.