Flat Barrier Research Articles

Constant Flux Diffusion Across a Corner

The temperature distribution of a curved barrier heated by constant flux on one side is studied. Both the right-angled corner and the rounded corner are solved by the method of domain decomposition and matching. It is found that hot spot temperatures may reach 2.5 times that of a flat barrier but may be tempered with appropriate rounding at the corner.

Surface roughness effects onmicroparticle adhesion

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.

A combinatorial approach for pricing Parisian options

This paper provides a discrete time algorithm, in the framework of the Cox–Ross–Rubinstein analysis (1979), to evaluate both Parisian options with a flat barrier and Parisian options with an exponential boundary. The algorithm is based on a combinatorial tool for counting the number of paths of a particle performing a random walk, that remains beyond a barrier constantly for a period strictly smaller than a pre-specified time interval. As a result, a binomial evaluation model is derived that is very easy to implement and that produces highly accurate prices.

Measurement of fusion excitation functions of 27, 29, 31Al + 197Au

A systematic study of the sub-barrier fusion reactions with neutron-rich projectiles has been carried out for three isotopes 27,29,31Al bombarding a 197Au target. A target chamber equipped with a target stack and sets of MWPC was employed in order to enhance the efficiency of the radioactive beam experiment. Coupled-channel calculations including the quadrupole excitations do not well fit the measured fusion excitation functions, whereas flat barrier distributions to represent the coupling to the neutron transfer largely account for the observed enhancement of the sub-barrier fusion cross-sections.

Atomistic Simulations of Steps in Bimetallic Interfaces as Barriers to Interface Slip Transmission

ABSTRACTAtomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency stresses at the interface. The (111)-steps afford a larger barrier to slip than the flat, coherent interface. The coherent flat interface dislocation barrier is largely due to the large compressive stresses in the Cu layer that must be overcome by applied tensile stresses. Additional Koehler forces are present as the dislocation in the elastically softer Cu approaches the stiffer Ni layer. The step, however, possesses a small residual edge dislocation with a Burgers vector equal to the difference of bCu and bNi times the height of the (111)-step in (111)-layers. We find that these steps are potent slip barriers, which suggests that homogeneous slip is preferred in such systems.

The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes-Direct Observation of Transition States of Degenerate Cope Rearrangements.

The UV/Vis spectra of thermochromic semibullvalenes 1 and barbaralanes recorded at various temperatures yield enthalpy differences between the two degenerate classical structures 1 and 1' and the less stable species (1*) that absorb at long wavelengths. The latter species are interpreted in terms of higher, delocalized states that are located just above the flat potential energy barrier between the ground states (ΔH(0) =11 (R=H; in butyronitrile), 1 kJ mol(-1) (R=Ph)).

Epitaxial Growth of Atomically Flat Spin Dependent Tunneling Junctions

ABSTRACTSpin dependent tunneling junctions with epitaxially grown underlayers have been investigated to examine the possibility of achieving very flat and uniform barrier layers. Pt/Ni80Fe20 /Fe50Mn50/Ni80Fe20layers were deposited on sapphire (0001) substrates at different temperatures and monitored by in-situ reflection high energy electron diffraction (RHEED). The surface morphology has been found to depend strongly on the growth temperature. X-ray diffraction and magnetic hysteresis loop measurements were also performed to characterize the film structures

High-energy channeling in protein folding.

Recent controversy about the role of populated intermediates in protein folding emphasizes the need to better characterize other events on the folding pathway. A complication is that these involve high-energy states which are difficult to target experimentally since they do not accumulate kinetically. Here, we explore the energetics of high-energy states and map out the shape of the free-energy profile for folding of the two-state protein U1A. The analysis is based on nonlinearities in the GdnHCl dependence of the activation energy for unfolding, which we interpret in terms of structural changes of the protein-folding transition state. The result suggests that U1A folds by high-energy channeling where most of the conformational search takes place isoenergetically at transition-state level. This is manifested in a very broad and flat activation barrier, the top of which covers more than 60% of the reaction coordinate. The interpretation favors a folding mechanism where the pathway leading to the native protein is determined by the sequence's ability to stabilize productive transition states.

Pathways, pathway tubes, pathway docking, and propagators in electron transfer proteins.

The simplest views of long-range electron transfer utilize flat one-dimensional barrier tunneling models, neglecting structural details of the protein medium. The pathway model of protein electron transfer reintroduces structure by distinguishing between covalent bonds, hydrogen bonds, and van der Waals contacts. These three kinds of interactions in a tunneling pathway each have distinctive decay factors associated with them. The distribution and arrangement of these bonded and nonbonded contacts in a folded protein varies tremendously between structures, adding a richness to the tunneling problem that is absent in simpler views. We review the pathway model and the predictions that it makes for protein electron transfer rates in small proteins, docked proteins, and the photosynthetic reactions center. We also review the formulation of the protein electron transfer problem as an effective two-level system. New multi-pathway approaches and improved electronic Hamiltonians are described briefly as well.

Macrodynamics of Microparticles

A primary goal of this paper is to describe the development of two, independent engineering models for the oblique mechanical impact dynamics of solid aerosol particles, treated as microspheres, in the presence of adhesion forces. One model is algebraic and is based on rigid body impact theory* * In dynamics, a distinction is usually made between a point mass and a rigid body; for a rigid body, rotations and the extent of the mass are taken into account. The phrase rigid body impact theory in this paper implies the latter and does not imply an absence of deformation. using coefficients such as the coefficient of restitution and the impulse ratio. This model is augmented by an energy conservation expression. Being algebraic and based directly on Newton's laws, the model offers a rigor and simplicity that makes it ideal for analyzing, displaying and interpreting experimental data. Dealing with impulses, this model does not require a detailed knowledge of the forces to analyze energy loss. The second model takes the form of a simulation using the differential equations of planar motion of a sphere in contact with a flat barrier. It uses Hertzian theory for the normal restoring force, an idealized tensile line force around the periphery of the contact region to represent adhesion and Coulomb friction for the force tangent to the surface. Damping with a form of velocity-dependent hysteresis is used both for the material dissipation as well as the energy loss associated with adhesion. Original experimental data from normal and oblique impacts of poly-disperse aerosol particles are used to illustrate and compare both impact models. The rigid body model's segregation of dynamic material dissipation (or restitution) and adhesion dissipation allows the latter to be observed as a function of initial normal velocity. Results of this model follow the normally incident data trends quite well. The model also facilitates the interpretation of tangential motion, particularly the conditions of sliding and rolling at separation. Experimental data analyzed with this model indicate that initial angular velocities of microspheres of the order of magnitude of 105 rad/s are common and significantly affect the rebound velocities for ordinary levels of friction. Physical constants, calculations and experimental data for silver coated glass spheres colliding with a stainless steel surface are used to estimate the parameter values of the simulation. With these parameters held fixed, the results of the simulation compare quite well to a broad range of experimental data including the transition region from rebound to attachment. The two models predict the impact dynamics almost identically but provide different estimates of the work of adhesion. Improvements of the models in this area are needed and are ongoing.

Distribution of fusion barriers

Heavy ion fusion cross sections and compound nucleus average spin values obtained from distribution of fusion barriers are discussed. Various shapes of distribution functions are studied using a truncated Gaussian distribution function (TGD). It is shown that fusion cross section and average spin values are less sensitive to different parametrization of TGD function, whereas the second derivative of the product of energy and fusion cross sections (w.r.t. energy), obtained from the corresponding TGD functions are significantly different depending on the shape of the barrier distribution function. It is also shown byχ 2 analysis of fusion cross section data that some systems favour a narrow Gaussian distribution function whereas others, for which the vibrational and rotational collective states are less important, favour a flat barrier distribution. A physical interpretation of the dynamical process that gives rise to different barrier distribution is given in the framework of microscopic coupled channel calculations.

What is a realistic frontal-offset test procedure?

With an increasing number of vehicles satisfying the FMVSS 208 Flat Barrier Impact Test and increasing belt usage, a high reduction of occupant injuries in general and a change from acceleration-induced to intrusion-induced injury mechanisms has been observed in Europe. The significance of intrusion-induced injury mechanisms associated with offset frontal impacts is assuming greater importance. Beginning in 1978, in response to this challenge, Mercedes-Benz has emphasized front structural countermeasures that have been found to be effective in the frequent asymmetrical frontal impacts. As an internal test, an offset impact with 40% overlap against a rigid barrier was defined. The relationship between actual accident frontal-overlap damage and barrier-test overlap values is explored in the paper. The results support the view that if a test of this type and associated design countermeasures were adopted by all vehicle manufacturers, a significant reduction of injuries would take place.

Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials

The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−-type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self-consistent field (HF-SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long-lived species formally existing in a metastable state. The stability of the gas-phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF-SCF and Green’s function methods. The inclusion of many-body effects by the latter was found to decrease the vertical binding energy of the extra electrons with respect to the Koopmans’ theorem HF-SCF result, but the extra electrons remain bound. At the ab initio level, the compounds studied in this work are predicted to be extremely ionic species. The variation of the properties through a series of MXmq− species has been investigated. Motivated by the overall large ionic character—in particular of the MX32− molecules—the properties of the systems have been studied using a previously derived theoretical approach, referred to as ionic model, based on (classical) electrostatic terms. The versatility of the ionic model scheme has been extended here to obtain information on the tendency of an ionic molecule to accomodate an additional electron.

Effect of the face dimensions of a flat barrier on the self-oscillations generated in the interaction with a supersonic underexpanded jet

Effect of the face dimensions of a flat barrier on the self-oscillations generated in the interaction with a supersonic underexpanded jet

Transport Properties of Inhomogeneous Schottky Contacts

We review an analytical model for the interpretation of transport measurements on spatially inhomogeneous Schottky contacts. The comparison of barriers from current/voltage- as well as from capacitance/voltage-curves permits a quantitative analysis of spatially distributed Schottky barriers. We reveal that the ideality coefficient n of abrupt Schottky contacts reflects the deformation of the barrier distribution under applied bias; a general temperature dependence for the ideality n is predicted and observed. An extension of our model includes the so-called flat band barrier of current/voltage curves. Here we demonstrate the interdependence of flat band Schottky barrier, ideality n and the homogenization of the barrier distribution under the application of a bias voltage. Effective photoresponse barriers and electrical noise at inhomogeneous Schottky diodes are also discussed.

The application of integral equation methods to the calculation of sound attenuation by barriers

Diffracted fields by thin rigid or absorbent barriers are investigated theoretically by using integral equations derived from Helmholtz-Kirchhoff's formula and from its normal derivative. The surface of the barrier is divided into elements to solve the integral equations numerically and the potential is assumed to be uniform on each element. For the numerical treatment of the integral equations in three dimensions, the integral including the strongly singular kernel is evaluated by the line integral along the edge of the element; this reduces the amount of computation and avoids the difficulty due to singularity. A numerical method which removes the problem of singularity is also presented to solve the integral equations in two dimensions. Some numerical examples are presented to demonstrate the effectiveness of this method. Moreover, for a flat rigid barrier, farfield solutions in the high frequency range are evaluated asymptotically on the basis of Kirchhoff's boundary conditions and compared with exact solutions.

Models for Cutaneous Gas Exchange and Transport

SYNOPSIS. Models suited for quantitative analysis of cutaneous gas exchange and transport with particular emphasis on diffusion limitation are discussed. I. The diffusive transfer of gases between body and environment can be compared to diffusion across a homogeneous flat tissue sheet characterized by a diffusion conductance or diffusing capacity (D). II. A flat barrier separating the medium from blood flow Q ˙ () is a widely useful model. The characteristic parameter is the ratio Q ˙ D/(rβ), where rβ is the effective solubility of the gas in blood. A high Q ˙ D/(rβ) value means predominant perfusion limitation; low Q ˙ D/(rβ}) signifies diffusion limitation. III. In some conditions, as in still water or at body-soil contact, a considerable diffusion resistance in the medium may become important. IV. Unequal distribution of D to Q ˙ may strongly reduce gas exchange efficiency. V. Diffusive arteriovenous shunting of gases may lead to additional reduction of gas exchange efficiency. VI. The parallel arrangement of skin circulation with respect to tissue circulation (in contrast to the arrangement in series present in fish gills and in tetrapod lungs) leads to mixing of arterialized with venous blood and reduced efficiency of blood flow in gas transport.

Impact response of thin-walled frame structures

This paper describes a finite-element method which was developed to predict the impact response of three-dimensional thin-walled frame structures. The softening effect of the load-deformation characteristics for thin-walled beams under combined axial compression, bending and torsional loads during impact is simulated by making use of a nonlinear Maxwell model. The large deformation of a structure is calculated by using updated nodal coordinates and internal forces, and the flexibility matrix for a curved beam element. Crash tests against a flat barrier were made on three-dimensional frame models. Comparisons between the calculated and experimental force-time relations and the deformed shape of the frame are discussed.

Amplification in the ultraviolet range due to transitions in ions of a carbon plasma interacting with a barrier

The observation of stimulated emission in the ultraviolet due to C II and C IV transitions in an electric-discharge plasma interacting with a flat barrier is reported. The results of measurements of the gain are presented.

Impact response of thin-walled plane frame structures

A numerical study of the impact response of plane frame structures with thin-walled members was performed to predict the deformation and absorbed energy of automobiles under crash loading. The collapse characteristics of thin-walled members under axial compression or bending loads are considered in the present analysis. The load-displacement relations are given for curved beam members, and the inelastic deformations of members are described in terms of equivalent internal loads and their histories. Crash tests on simplified plane frame models impacted against a flat barrier were performed to verify the proposed method. Numerical analyses were also made of an oblique barrier impact of an automobile underframe structure and a rear end collision of a vehicle structural system. The analytical predictions were compared with the corresponding test results.