Shape Selection Research Articles

Three-dimensional viscous fingering: A numerical study

Multidimensional viscous fingering is accurately simulated by an extension of the Hartley spectral methods to three dimensions. Two types of initial conditions were used in the rectilinear displacements−white noise and polygonal cells of the wave number of the mode of maximum instability identified by the linear theory. White noise initial conditions demonstrated that the mechanisms of nonlinear interaction of viscous fingers found in two-dimensional (2-D) simulations persist to three dimensions. Further, the long-time rate of advance of viscous fingers remains unchanged from two dimensions, suggesting that 2-D simulations are sufficient to capture essential features of nonlinear viscous fingering. Simulations with polygonal cellular symmetries, specifically square cells and hexagonal cells, illustrated that weak nonlinear theory cannot predict the shape selection of viscous fingers, as the mechanisms of finger interactions that dominate shape selection lack transverse symmetry and symmetry with respect to the displacement front.

Design sensitivity analysis of nonlinear dynamic response of structural and mechanical systems

This paper describes a unified variational theory for design sensitivity analysis of nonlinear dynamic response of structural and mechanical systems for shape, nonshape, material and mechanical properties selection, as well as control problems. The concept of an adjoint system, the principle of virtual work and a Lagrangian-Eulerian formulation to describe the deformations and the design variations are used to develop a unified view point. A general formula for design sensitivity analysis is derived and interpreted for usual performance functionals. Analytical examples are utilized to demonstrate the use of the theory and give insights for application to more complex problems that must be treated numerically.

Structural polymorphism in the major groove of a 5S RNA gene complements the zinc finger domains of transcription factor IIIA.

Metal complexes that bind to DNA on the basis of shape-selection have been used to map the conformational features of the DNA binding site for transcription factor IIIA. Conformationally distinct segments are detected on the 5S rRNA gene that correspond closely to the binding sites identified for the individual zinc finger domains of the protein. The local conformations are characterized by a major groove opened because of a change in base pair inclination and/or displacement at a central 5'-pyrimidine-purine-3' step, flanked by a widened minor groove, as would arise at the junctions between alternating B- and A-like DNA segments. Docking experiments with a consensus structure of a zinc finger reveal that the mixed A-B binding site accommodates the peptide domain better than either canonical B- or A-DNA helices. The close structural matching of the conformational variations in the 5S rDNA both to the proposed sites of zinc finger binding and to the shape of an individual zinc finger domain points to DNA structural polymorphism as providing an important determinant in recognition. In particular, shape selection in the 5' half of the internal control region may orient the multiple finger domains.

Shape selection in the association of diaminoguanidinium cation with counterions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTShape selection in the association of diaminoguanidinium cation with counterionsJames P. Ritchie, K. Y. Lee, D. T. Cromer, E. M. Kober, and D. D. LeeCite this: J. Org. Chem. 1990, 55, 7, 1994–2000Publication Date (Print):March 1, 1990Publication History Published online1 May 2002Published inissue 1 March 1990https://doi.org/10.1021/jo00294a008RIGHTS & PERMISSIONSArticle Views68Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Steady-state cellular growth during directional solidification.

We study cellular structures moving at constant velocity, in a symmetric model of directional solidification. This is done by employing Newton's method to solve a discretized version of the integro-differential equation for the solid-liquid boundary. Our results indicate that there is a continuous band of allowed wavelengths and that there is a generic fold in the solution diagram. This fold provides a maximum allowed wavelength for any given velocity. Also, we argue that the mechanism of microscopic solvability serves to fix the tip shape at fixed wavelength. The implications of our results for wavelength and shape selection are briefly discussed.

Variational method for design sensitivity analysis in nonlinear structural mechanics

This paper describes a unified variational theory of design sensitivity analysis of linear and nonlinear structures for shape, nonshape, and material selection problems. The concepts of reference volume and adjoint structure are used to develop the unified viewpoint. A general formula for design sensitivity analysis is derived. Several analytical linear and nonlinear examples are used to interpret various terms of the formula and demonstrate its use. Such analytical examples give insight into the application of the general formula to more complex problems that must be treated numerically. Adjoint state fields are also interpreted as sensitivities for the functional related to the primary structure.

L'attention sélective : l'effet du critère de sélection

Summary : Selective attention : effect of selection criterion. We investigated how subjects ability to ignore the global and local aspects of a nonattended objet was affected by the criterion used to select the attended object. Two large letters (global aspect) constructed front small letters (local aspect) were presented ; one large letter was surrounded by a square while the other was enclosed in a circle. Subjects were required to attend either to the global or the local aspect of one of the compound stimuli (attended object) and to ignore a distractor located at the global or local aspect of the nonattended object. Three selection criteria were used to define the attended object : a) shape, b) brightness, or c) spatial location. The results show that when shape and brightness selection were required, the ease of ignoring the aspects of the nonattended object was dependent upon the direction of attention. When attention was global-directed, subjects could ignore the local, but not the global aspect of the nonattended object. On the other hand, under local-directed attention, subjects were unable to ignore the local aspect, while they could easily ignore the global aspect of the nonattended object. However, selectivity of attention was greatly improved when the relevant object was located on the basis of its spatial location and, regardless of the direction of attention, subjects were able to ignore both aspects of the nonattended object. The results are discussed in terms of the attentional spotlight metaphor. Key words : selective attention, selection criterion, global/local aspects.

Narrow fingers in a Hele–Shaw cell

Narrow fingers that occupy much less than one-half the cell width have been observed in a carefully prepared Hele–Shaw cell. These fingers sometimes show dendritic instability. They are in most respects similar to those produced artificially by Couder et al. [Phys. Rev. A 34, 5175 (1986); Europhys. Lett. 2, 437 (1986)] and are believed to represent a new class of solutions to the shape selection problem.

Testing shape selection in directional solidification.

We report results from an experiment on the directional solidification of pivalic acid. In addition to the usual cellular interface patterns, we observe a new, metastable dendritic form that can be interpreted in the light of recent ideas about pattern formation.

Shape selection of Saffman-Taylor fingers.

Among all problems of pattern selection the one posed by the Saffman-Taylor finger is yet unsolved, although numerics and experiments indicate the very simple property that the relative width of the finger tends to 1 2 in the low–surface-tension limit. We explain this by performing an expansion beyond all orders leading to the formulation of a nonlinear eigenvalue problem with a discrete set of solutions. In particular we predict that the relative width of the finger tends to 1 2 as the surface tension to the power 2 3 . PACS numbers: 47.10. + g, 03.40 Gc, 68.10−m

Steady-state dendritic crystal growth.

We study the problem of velocity and shape selection in the growth of dendritic crystals. We demonstrate that the presence of surface tension destroys the Ivantsov family of solutions at all but a single velocity. This occurs at all undercoolings and for both two-dimensional and axisymmetric three-dimensional dendrites. The lack of a continuous family of solutions is due to a mismatch of terms exponentially small in velocity, in agreement with the mechanism of ``microscopic solvability'' previously derived for geometrical growth models.

The Effects of Tailing Dam Profiles on Relative Wind Erosion Rates

AbstractErosion from mine treatment and associated residue areas can pose a significant environmental problem for surrounding locations from dust and other transported materials. The shape of such residue areas can influence windfield behaviour by causing significant wind speed increases. Residue areas are often unprotected so that increasing the speed of wind passing over these areas will cause extra erosion. Values of wind speed‐up predicted by an empirical model for wind flow over hills of low slope were compared with measured values over approach slopes to tailings dam. Hunt's model used in this study relates wind speed from a point on the hill to that observed if there was no hill. Measured values are in agreement with those predicted by the model. Shear stress values calculated from the wind flow model are then used to determine the friction velocity which, in turn, predict the relative rates of erosion. This prediction is based on the cubic relation between the friction velocity and erosion rate observed by Bagnold. These calculations are repeated for the various possible hill shapes allowed by the plant layout and the need to integrate long term spoil heaps with existing topography. A strategy for minimizing erosion of mine tailings through shape selection can then form part of the environmental considerations associated with tailings dams.

Behaviour of underwater enclosures of optimum design

Experiments are described which support the application of the membrane theory of shell analysis in determining optimum shapes for underwater structures of constant strength. A relationship between marine life and such structures is considered and design procedures for shape selection are given. Proposals for the investigation of the behaviour of such structures under loadings other than the initial design forces are outlined both from an analytical! numerical and an experimental approach.

Study of Beam Parameters of 240-CM Isochronous Cyclotron

For two years beam parameter investigations have been carried out at the Kiev cyclotron for the extraction process and the beam transport system. The data obtained show that high quality beam is obtained with 0.1% accelerating voltage stabilization and shape selection of the isochronous magnetic field which limits phase shift to ±5°. The system for radial beam phase selection provides control of bunch phase duration within 5°-25°. In this case the radial oscillation amplitude does not exceed 2-3 mm and extraction efficiency from the cyclotron is about 70%. The measured beam emittance at the accelerator output is 20 mm-mrad. for vertical motion and 28 mm-mrad. for horizontal motion. Energy spread in the extracted beam is less than the energy gain per turn and is about 0.3%. The complete beam transport system is in use and provides beam without losses to any of six experimental rooms.

A pulse shape selection method to improve the timing properties of a large Ge(Li)-detector

The timing properties of a large Ge(Li)-detector of 46 cm 3 volume were studied. Theoretical calculations of the pulse shapes were undertaken and compared with experiments. By using a pulse shape selection method with ARC-discriminators we were able to improve the time resolution considerably. In coincidence with a plastic scintillation detector we achieved for gamma energies above 1 MeV for instance slopes with T 1 2 < 1 ns . The prompt curves were nearly symmetric for more than three decades and of exponential character.

Human factor in till-fabric analysis

Experiment shows that geologists accept a wide range of pebble shapes for till-fabric analysis. Most of this variation can be accounted for by a single variable—the B:A axial ratio of the clast. The individuals were classified into three groups, “conservatives,” “moderates,” and “liberals,” depending on the pebble elongation they would accept. Data on till pebble fabric were collected sequentially from 19 basal till sites in east-central New Hampshire. Fabric diagrams were produced for 11 different combinations of pebble elongation and quantity from each site, evaluated, and compared to independent indicators of ice-flow direction. The results show that the use of 100 elongate pebbles by conservative standards tends to produce uniform, intense, precise, accurate, and imbricated fabrics; whereas reversed and transverse fabrics are scarce. As shape selection becomes more liberal and as fewer pebbles are used, the fabrics tend to become irregular, weaker, less precise, less accurate, and less imbricated; whereas reversed and transverse fabrics are more frequently generated. At the extreme, where few pebbles are selected by liberal standards, the resulting fabrics approach those generated from random numbers. About 100 pebbles, selected by rather conservative shape standards ( B:A ≤0.59) are necessary to obtain dependable diagrams.

Vergence eye movements to pairs of disparity stimuli with shape selection cues

Vergence eye movements to pairs of disparity stimuli with shape selection cues

Subnanosecond timing with a planar Ge(Li) detector

The shape distribution of charge pulses obtained by uniform γ-irradiation of a 0.25 cm 3 planar Ge(Li) detector with a surface barrier window was studied. Leading-edge time resolution curves obtained with a plastic scintillation counter as time reference showed a strong Ge(Li) late tailing effect which was only partially reduced by the rise time compensation due to Chase. For a combination of the latter method and pulse shape selection time resolution curves were studied for different fractional numbers of all full energy events. By using about one half of all events the Ge(Li) late slope was improved one order of magnitude. For the 511 keV annihilation γ-ray from 22Na a fwhm time resolution of 1.1 ns and a Ge(Li) late T 1 2 slope of 0.13 ns over more than three decades were obtained. The time resolution was studied for different enrgies. The interaction number percebtage by total absorption for some energies are calculated and the influence on the pulse shape is discussed.

Wing shape selection for hypersonic speeds

Wing shape selection for hypersonic speeds

Wing shape selection for hypersonic speeds

Wing shape selection for hypersonic speeds