Small Accommodation Research Articles

Inverted velocity profile in the cylindrical Couette flow of a rarefied gas.

The cylindrical Couette flow of a rarefied gas is investigated, under the diffuse-specular reflection condition of Maxwell's type on the cylinders, in the case where the inner cylinder is rotating whereas the outer cylinder is at rest. The inverted velocity profile for small accommodation coefficients, pointed out by Tibbs, Baras, and Garcia [Phys. Rev. E 56, 2282 (1997)] on the basis of a Monte Carlo simulation, is investigated extensively by means of a systematic asymptotic analysis for small Knudsen numbers as well as the direct numerical analysis of the Boltzmann equation, and the parameter range in which the phenomenon appears is clarified.

Quantitative bathymetric analyses of selected deepwater siliciclastic margins: receiving basin configurations for deepwater fan systems

Comparative bathymetric analyses of four siliciclastic continental margins reveal important distinguishing characteristics in deepwater receiving basin configurations. These characteristics have potential impact on deepwater sedimentation patterns and associated fan development. Present-day bathymetry was analysed quantitatively, using advanced software and digital bathymetry data. Various physical attributes of dip, basin geometry, and accommodation were quantified and mapped in 3D. Results show significant differences in receiving basin configurations between salt-based and shale-based continental margins in the Gulf of Mexico, Angola, Nigeria, and NW Borneo, especially in the type, amount, and distribution of accommodation. Salt-based systems have more ponded accommodation than shale-based systems; however, all the margins have a relatively small percentage of ponded to total accommodation. Two exceptions to this are the central portion of the Northwest Gulf of Mexico where ponded accommodation constitutes 55% of the total accommodation and the Kwanza Basin of Offshore Angola with 39% ponded accommodation. Shale-based systems may be more prone to bypass on the upper to mid slope than salt-based systems. Evidence for this is found in the linear grade trend analyses where large below-grade areas (sinks) are pervasive on the upper and mid slope of salt-based systems for deepwater sediment to accumulate, while shale-based systems show extensive above-grade highs across the entire slope. Ponded accommodation trends and drainage analyses also demonstrate that shale-based systems are more susceptible to extensive bypass than salt-based systems. The tectonically active margin of NW Borneo has steeper slope profiles than the passive margins, with areally small ponded accommodation largely restricted between active toe-thrusts on the lower slope and locally distributed on the shelf. These and other quantitative analyses suggest that the type of substrate and overall tectonic setting are important factors to consider for receiving basin configuration and its potential impact on deepwater sedimentation patterns.

Accommodation to large disparity stereograms

Purpose: The purpose of this study was to test whether observers would show accommodation changes to depth depicted in large disparity stereograms and whether the amount of accommodation was related to the amount of depth seen in the stereograms. Previous studies have shown small accommodation changes to depth depicted in stereograms. We tested a larger range of depths than previously investigated and also asked observers to judge the depth depicted in the stereogram, because a previous study had suggested that this might be an important factor. Methods: All our observers had emmetropia. Accommodation was measured with a Canon Autoref R-1autorefractor as an observer viewed a depthful 4-by-4 cm planar target set against a 20-by-20 cm planar background 57 cm from the observer. The target was presented at a range of depths, -8 to +8 cm, either in stereogram stimuli or real stimuli. Each observer also made depth judgments of stereogram targets, after training with real targets. We also recorded the time taken for each observer to discern the depicted depths in stereograms. Results: The accommodation shown by all observers (n = 9) to stereogram targets was significant but was on average about 60% of that predicted. There were individual differences in the accommodation an observer showed to stereogram targets, but this was not related to the amount of depth the observer saw in the stereogram. Conclusions: Accommodation changes did occur to stereogram targets, but these were generally less than predicted and also less than those shown for real targets. Possible reasons for these findings and the implications for the use of stereograms in clinical tests are discussed. (J AAPOS 2002;6:377-84)

Transition flow in an impact pressure probe: Relevance of gas–wall interaction

Impact pressure probes may be used to measure the stagnation pressure of a spacecraft vehicle during reentry into the atmosphere. We model the flow in an impact pressure probe in the collision-free and transition regime using a kinetic simulation scheme of the Boltzmann equation. It is shown that, depending on the gasdynamic properties density, velocity, temperature of the inflow into the impact pressure probe, the measured pressure may deviate by almost a factor 2 from the stagnation pressure. Furthermore, we study the dependence of the flow in the impact pressure probe on the gas–wall interaction. The models by Maxwell, Cercignani and Lampis, and by Lord are implemented. The influence of the gas–wall interaction is sensible for small accommodation coefficients; it is more pronounced, however, for temperature and density than for the pressure.

Pressure‐dependent gas heat transport in a spherical pore

AbstractA mean free path gas kinetic theory is used to model the conductive heat transport of a gas within a void volume enclosed in a Fourier solid. A variational upper bound principle is derived for a void of arbitrary shape and applied to obtain a rigorous upper bound equation for the void gas conductivity in a spherical void. The variational void gas conductivity equation is exact in both the large and small Knudsen number (Kn) limits and provides a means to determine the accuracy of the reciprocal additivity interpolation formula as applied to thermal conductivity rather than diffusive mass transfer (maximum error 6% at Kn = 0.5 and α = 1). Temperature jump will occur even at atmospheric pressures and higher for sufficiently small thermal accommodation coefficients (α < 0.1). Experimental void gas heat conductivities vs. pressure data for H2, He, Ne, N2, CO2, and F12 in a polyurethane foam are compared with theoretical mean free path void gas conductivity vs. inverse Knudsen number curves drawn for various α. Estimates of the thermal accommodation coefficients for the gas‐ polyurethane surface exhibit a maximum with increasing molecular mass of the gas molecules, which qualitatively agrees with the predictions of Baule's classical theory. Results also point to a rather sharp shift of the S curve to higher pressures with decreasing thermal accommodation.

THE COSTS OF COMMUNITY RESIDENTIAL FACILITIES FOR ADULTS WITH A MENTAL HANDICAP IN ENGLAND

AbstractEstimates of the economic costs of a random sample of 123 community residential facilities for people with a mental handicap in England are presented. Total resource costs, including capital, average £38 per resident day and range from £16 per day to over £90 per day. Costs vary across the four types of provider. Voluntary and private sector facilities are the least expensive, health authority facilities are the most expensive and local authority facilities lie somewhere in between.A more systematic evaluation of the reasons why costs vary in this manner is being carried out but preliminary analysis presented here suggests high costs are associated with higher dependency of residents particularly those accommodated in small domestic scale accommodation.

Accommodation on downward gaze.

A study was conducted with the purpose of examining the possible effects of vertical and horizontal eye movements upon accommodation. A special Badal stimulator was made which was vertically rotatable around the center of an eye. The stimulator was attached to the Three-Dimensional Optometer III (TDOIII), which could measure accommodation, eye movement, and pupil diameter simultaneously. The reliability of the TDOIII measurements of accommodation was checked by comparing with a stigmatoscope measurement. Three young males served as subjects. A small but definite near accommodation was measured dynamically in association with vertical eye movement. We believe this to be the first report of this finding. The amount of near accommodation with downgaze showed a tendency to decrease when the target was brought near.

Area‐based training: Tourism training initiative

Abstract Recent government policy has enacted significant changes in the approach to training both north and south of the Border. In April 1991 the establishment of Scottish Enterprise and Highlands and Islands Enterprise, along with a network of Local Enterprise Companies, marked a shift in the direction to employer‐led and locally delivered training programmes. The importance of tourism to Scotland has attracted government attention and, in anticipation of the new training network, a Tourism Training Initiative has been set up to expand the take‐up of training opportunities. This area‐based approach is welcomed but it is argued that tourist businesses, especially the small accommodation sector, have particular characteristics that have made the uptake of training traditionally a low priority. The absence of consideration of these characteristics in the design of the new employment strategies is argued to be a significant weakness.

Continued enhancements to the commercial atlas launch system

Since the October 1988 update on the Commercial Atlas Launch System presented to the IAF, additional enhancements have been incorporated into the launch vehicles offered. Previous papers have dealt with performance and configuration details of the Atlas I, II, IIA launch vehicles. This paper discusses the latest addition to the line of Atlas launch vehicles, namely the Atlas IIAS, and other enhancements that have been made across the board. Discussion of the Atlas IIAS configuration includes details about structural upgrades made to the booster stage, the basic configuration of the solid rocket motors (SRMs) used, upgrades made to the launch facility, effects of the configuration change on spacecraft environments, and overall performance enhancement. By selective incorporation of the enhancement available, the performance of Atlas launch vehicles can be tailored to the specified requirements of particular missions. These capability enhancements include minimum residual shutdown of the Centaur upper stage, transverse spin separation, and small payload accommodations (SPA). When matched with existing capabilities of in-flight retargeting and precision pointing and injection accuracies, these improvements lead to the availability of a launch vehicle that can be tailored to a spacecraft's unique requirements. Finally, a review of performance capabilities for the entire line of Atlas launch vehicles is presented, capping a comprehensive view of the current state of the Atlas launch vehicle.

Concepts for Lunar Outpost Development

This paper summarizes the results of a qualitative investigation to identify concepts for design and construction of near-term lunar facilities. Accomplishing such construction will require an adaptation or transfer of current terrestrial technology and methods. Discussions on modularization, geosynthetic materials, aluminum materials, static load analysis, and dynamic load analysis provide illustrative examples of how terrestrial technologies can be adapted to lunar applications. These discussions provide support for the development of a phased lunar construction strategy. The initial stage of construction is characterized by small self-supporting accommodation and laboratory modules. The assembly facility stage is characterized by the construction of a large pressurized module-assembly facility. The module production is characterized by the fitting together of terrestrial or low earth-orbit subassemblies into completed modules within the module assembly facility. The completed modules are also tested and moved to their final location in this stage. The lunar materials state is characterized by the construction of facilities with maximum use of lunar materials.

Gas-Source Molecular Beam Epitaxy

The replacement of the elemental sources of conventional MBE with simple compounds, first reported in 1980 [1], was initiated in order to bring the advantages of molecular beam epitaxy to the growth of GaxIn1−xAs1−yPy/InP heterostructures. These advantages center about precision in layer thickness and abruptness in doping and heterojunction interfaces. This replacement of elemental sources was necessary because III–V semiconductors containing P, and particularly As and P simultaneously, are very difficult to grow by conventional MBE. A well controlled and useful beam flux from an effusion cell containing elemental phosphorus is difficult to achieve because of the presence of allotropic forms of solid P, each having a different vapor pressure, and because condensed P vaporizes to yield P4 molecules. The morphological observations of Asahi et al [2] of InP grown with P4, and the studies of the relative incorporation of As and P during MBE of GaAs1−yPy and InAsyP1−y by Foxon et al [3], suggest that P4 has a small accommodation coefficient on the III–V surface. It is possible, of course to thermally crack P4 to P2, and P2 can readily be used for epitaxy of P containing III–V compounds. Its accommodation coefficient is approximately unity [4]. However, the generation of P2 by adding a thermal cracker to a conventional effusion oven does not eliminate the underlying stability problem and has added control problems.

Mass accommodation coefficients from penetration measurements in laminar tube flow

The theory for penetration of ‘sticky’ gaseous molecules through cylindrical tubes in laminar flow is presented and implications of the theory for measurements of mass accommodation coefficients are discussed. For the case where the inner tube walls are coated with a liquid, a criterion is developed which can be used to determine whether re-evaporation is likely to contribute significantly to measured penetration values. This criterion can be useful in designing experiments. It is concluded that laminar flow penetration measurements can be useful for measuring very small mass accommodation coefficients (<10 −4). The theory can also be used to design diffusion denuders when accommodation coefficients are known.

Flow field effects on nucleation in a reacting mixing layer

Chemical nucleation has been studied numerically in a stagnation point mixing layer in which reactants in two counter‐flowing streams form a condensable monomer. The response of the subsequent nucleation kinetics to the velocity gradient in the flow is described in terms of a Damkohler number. Two limiting cases have been established. First, if the Damkohler number for monomer production is small, i.e., the rate of monomer production is slow, then the nucleation of particles can be strongly affected by the flow field in a manner which is equivalent to the effect of supersaturation in a uniform vapor. Second, if the Damkohler numbers for cluster growth are small (because of a small accommodation factor for monomer–cluster interactions), the concentrations of clusters do not achieve equilibrium levels. This can result in the suppression of particle formation over a critical range of Damkohler numbers. In this case the behavior of the nucleation kinetics is analogous to the transient phase of nucleation in a uniform vapor.

Slightly Rarefied Gas Flow over a Body with Small Accommodation Coefficient

The asymptotic behavior (for small Knudsen number) of the steady flow of a rarefied gas over a body with a small accommodation coefficient (of the order of the Knudsen number) is investigated on the basis of the Boltzmann-Krook-Welander equation and the Maxwell-type boundary condition. It is shown that the present results provide a bridge between the cases of zero accommodation coefficient (specular reflection) and of finite one. As applications of the theory, a sphere with constant temperature in a uniform flow of gas and that in a gas at rest with a uniform temperature gradient are considered.

Condensation of water via rarefaction wave

Condensation of water vapor via the rarefaction wave in a shock tube was investigated by measuring pressure as a function of time. At high temperature (260°K), condensation could be adequately explained by the classical nucleation theory combined with a small accommodation coefficient for particle growth. However, at low temperature (230°K), condensation was considerably faster than permitted by classical nucleation, and the classical theory of nucleation is therefore considered to be invalid. Condensation shocks due to the coupling of gasdynamics with condensation kinetics (heat release rate) were also observed.

Laser heterodyne study of water droplet growth

Droplet growth by condensation under simulated atmospheric conditions has been studied quantitatively in a diffusion cloud chamber. Droplet size distributions were obtained from heterodyne spectra of scattered laser light. The distributions were typically quite narrow, and the time dependence of the mean radius agrees well with theoretical predictions for supersaturations of 1.02–1.05 in the size range 3–7 μm. Our data indicate no need to postulate very small condensation and thermal accommodation coefficients in contrast to previously published work at higher supersaturations. However, the growth rates are anomalously low at supersaturations less than about 1.015.

Classical Theory of Small Energy Accommodation Coefficients. Application to 3He and 4He on W

Some recent experimental data on the energy accommodation coefficients of the systems 3He–W and 4He–W are compared with the predictions of the classical theories of gas–solid interactions. For reasons which are explained, these data should provide a particularly good test of theory. The classical lattice theory of energy accommodation coefficients is generalized, using results from other theories, to take account of the three-dimensional nature of the gas; this generalization involves, for example, elimination of the assumption of normal, head-on encounters of gas atoms with surface atoms and consideration of the hopping of gas atoms at the surface. The gas–solid interaction potential is the recently proposed modified exponential repulsion, the use of which allows considerable simplification of the analysis. The assumption that accommodation coefficients are independent of solid temperature, allowing use of a lattice model at zero temperature, is retained and discussed at some length. The application to He–W allows the calculations to be simplified by means of a perturbation procedure which is valid only for small accommodation coefficients. Although there are some disappointing features, in view of the approximations still existing in the theory the comparisons made are satisfactory. For example, there are no grounds for concluding, on the basis of these comparisons, that a classical model is inadequate for dealing with the accommodation of He–W.

Classical perturbation theory of the thermal accommodation coefficient in n dimensions

The theory of the thermal accommodation coefficient is studied using a classical perturbation theory, in which the solid is represented by various n-dimensional models. A general formula is found for the effective thermal accommodation coefficient of a single gas atom in terms of a summation over the normal modes of vibration of the model; this formula applies to any classical harmonic model of the solid. From this formula are derived expansions in closed form for the effective thermal accommodation coefficient which are valid in the regimes of high and low incident gas atom speed; attempts are made to define the conditions under which these expansions are valid. The theory restricts all motion to the direction normal to the solid surface, and all gas atom-surface atom collisions are head-on. A modified exponential repulsive gas-solid interaction potential function which includes realistically a long-range gas-solid attraction is used; a consequence of the form of this potential function is that the perturbation theory is not restricted in general to small accommodation coefficients. In the low-speed regime the theory is not restricted in general to small gas atomic mass, but this restriction applies in the high-speed regime. The theory is applied in turn to the following models of the solid: the n-dimensional lattice with different central and non-central spring force constants; the one-dimensional lattice with a surface impurity; the n-dimensional lattice with various types of surface impurity; the n-dimensional continuum. Where appropriate, the physical significance of the results of these models is considered, and relations among the results are studied; in this way, for example, the role of the three-dimensional continuum theory of Landau is clarified. It is shown that great care must be taken in using such perturbation theory expansions, as they apply only to a very limited group of experimental systems. It is concluded that, in fact, no satisfactory comparison with experimental data is possible at present, and the need is stressed for a more general theory. That the much-used one-dimensional model is not useful in any regime for realistic studies of gas-surface interactions is conclusively established in various ways; similar remarks will apply also to one-dimensional quantum-mechanical models.

Gas-Surface Interactions and Field-Ion Microscopy of Nonrefractory Metals

The performance of the helium field-ion microscope depends critically upon accommodating He atoms of 0.15-eV kinetic energy to the specimen tip. The small accommodation coefficient requires the field-trapped He atom to make several hundred contacts with the cold tip surface. The hopping He atoms diffuse preferentially to tip regions where the high local field permits ionization before full accommodation is reached. Improved accommodation is achieved with the provision of an intermediate collision partner in the form of adsorbed neon or, preferably, hydrogen or deuterium. Now a high-resolution He ion image is obtained at 70% of the field used before. As the addition of hydrogen promotes field evaporation, its partial pressure must be carefully controlled to achieve image stability of the nonrefractory metals. Low-field evaporation by the hydrogen reaction permits easy conditioning of the tip surface of the nonrefractory transition metals so that artifacts caused by yielding to He evaporation field stress are no longer a problem. The field evaporation end form obtained with hydrogen added to He more closely approaches the desirable spherical shape of the emitter than does field evaporation in vacuum or in a single imaging gas. As examples, ion images of niobium, nickel, iron, and high carbon steel are shown.

On the theory of accommodation coefficients—III: Classical perturbation theory for the thermal accommodation of light gases

A new classical theory of thermal energy exchange between gases and solid surfaces is proposed. A perturbation method is used and, because of the inherent errors in this method, the theory is restricted to light gases and small thermal accommodation coefficients. Realistic interaction potentials of the Morse type between the gas atoms and the solid surfaces are used; detailed consideration is given to finding the best possible values of the relevant interaction parameters for the systems considered. The experimental data with which the theory is compared are for the interaction systems He/Ni, He/W, He/Pt, Ne/Fe and Ne/W; it is shown that in each of these cases adequate quantitative agreement of the theory with experiment is obtained.