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191.
The equations of motion applying to the wind field in a forest canopy are simplified to a balance between the shearing stress gradient and either the form-drag of the leaves in the upper dense canopy, or the overall horizontal pressure gradient in the more open space beneath. The equations imply that, in descending through the forest, the stress and wind vectors turn through an angle which depends on the forest characteristics and on the stability and the speed of the airflow above the forest. The turning is roughly confirmed by an overall average measured on a very flat site near Thetford, Norfolk, covered by an extensive uniform pine forest. 相似文献
192.
Francois E. Heuzé Richard E. Goodman Ann Bornstein 《Rock Mechanics and Rock Engineering》1971,3(1):13-24
SummaryNumerical Analyses of Deformability Tests in Jointed Rock — Joint Perturbation and No Tension Finite Element Solutions Sound design of structures in or upon rock requires a thorough knowledge of the rock mass deformability. Field test data interpretation has generally relied upon analytical or Finite Element linear elastic solutions. However, when testing in jointed rock, these can no longer be readily used since the medium cannot resist the tension induced upon loading. Accordingly, other solutions are required. If the discontinuities of the rock mass can all be mapped and their surface properties determined, the Finite Element model will use a Joint Perturbation solution where the joint elements have variable stiffnesses to account for movements along the fractures. If the discontinuities cannot be satisfactorily mapped, a global approach is provided by a No Tension analysis of the stress transfer type. The two techniques are compared in the case of bore hole jack deformability tests and found to agree remarkably well, indicating an appreciable difference from results of linear elasticity.
Research supported by the National Aeronautics and Space Administration — Contract NAS 8-21432.
With 8 Figures 相似文献
ZusammenfassungNumerische Analyse von Verformungsversuchen in geklüfteten Fels — Lösungen mittels Kluft-Perturbation und No Tension Analysis auf Grund der Methode der Finiten Elemente Der Entwurf von Bauwerken in oder auf Fels erfordert eine genaue Kenntnis des Formänderungsverhaltens der Felsmasse. Im allgemeinen basiert die Interpretation der Ergebnisse von Feldversuchen auf den Lösungen des zugehörigen Randwertproblems der linearen Elastostatik mittels klassischer analytischer Methoden oder der Methode der Finiten Elemente. Wenn die Versuche aber in geklüftetem Fels durchgeführt werden, müssen die theoretischen Methoden modifiziert werden, da das Medium keine Zugspannungen aufnehmen kann. Zwei Fälle können unterschieden werden. Erstens: die Kluftflächen der Felsmasse können geometrisch erfaßt und ihre Oberflächeneigenschaften können bestimmt werden; in diesem Falle kann die Finite-Elemente-Lösung eine Kluft-Perturbations-Methode verwenden, in welcher spezielle Kluft-Elemente verschiedener Steifigkeiten normal und tangential zu den Kluftflächen die Bewegungen entlang derselben bestimmen. Zweitens: die Diskontinuitäten der Felsmasse können nicht hinreichend beschrieben werden; für diesen Fall ist vorgekehrt durch die Finite-Element-Analyse vom Typ der Spannungsumlagerung unter Ausschaltung der Zugspannungen (No Tension Analysis), um das globale Verhalten der geklüfteten Felsmasse zu erfassen. Die Lösungen dieser beiden Methoden wurden für den Fall der Bohrloch-Verformungs-Tests verglichen; der Vergleich zeigt ausgezeichnete Übereinstimmung, im Gegensatz zu den Lösungen als lineares Elastizitätsproblem.
RésuméLa méthode des Eléments Finis appliquée aux résultats d'essais de vérins cylindriques dans les roches fissurées — solutions dites Joint Pertubé et Sans Traction La construction de structures sur ou dans des roches nécessite une connaissance approfendie de la déformabilité du massif rocheux. L'analyse des résultats d'essais en place se fait généralement à l'aide de solutions analytiques ou par Eléments Finis, qui supposent une élasticité linéaire du milieu. De fait, dans les roches fissurées, ces solutions sont inadéquates puisque le milieu ne peut résister aux sollicitations de traction. Il s'agit donc d'utiliser d'autres méthodes d'analyse. Lors qu'il est possible de déterminer la géométrie et les propriétés de toutes les discontinuités, l'on pourra utiliser une solution par Eléments Finis, dite Joint Perturbé dans laquelle la rigidité de surface du joint (ou fissure) peut varier lorsque des movements prennent place au niveau du joint. Si cela est impossible, il faudra utiliser une approche globale dite Sans Traction qui redistribue les contraintes de traction et les dissipe. On compare ces deux types de solutions dans le cas des essais de vérins cylindriques unidirectionnels. Leurs résultats sont identiques et ont nettement différents de ceux obtenus avec une solution élastique linéaire.
Research supported by the National Aeronautics and Space Administration — Contract NAS 8-21432.
With 8 Figures 相似文献
193.
Wayne V. Burt David B. Enfield Robert L. Smith Henry Crew 《Boundary-Layer Meteorology》1973,3(3):385-391
Harmonic analysis of longshore and onshore components of surface winds over an upwelling area off the coast of Peru shows regular, nearly sinusoidal, diurnal variations in both velocity components. Spectral analysis of wind speed also indicates strong diurnal variations in kinetic energy. The data are compatible with a recent model of coastal winds proposed by Lettau and Lettau. 相似文献
194.
Two experiments were conducted in the Carboniferous Limestone aquifer of the Mendip Hills, Somerset, to compare the relative merits of differing groundwater tracers. The tracers employed were lithium acetate, polyethylene powder, Lycopodium spores and the fluorescent dye Pyranine Conc.; all were introduced into the aquifer at stream sinks. The first two tracers were totally unsuccessful. In the two experiments the rates of travel of the fluorescent dye and Lycopodium spores were very similar. Only relative concentrations can be measured from the recovery of spores as opposed to absoute concentration for dye which were determined using a field continuous flow fluorometer. The quantitative dye concentration data can be used with simultaneous measurements of discharge at the spring to deduce the flow geometry of the system; the input, tributary, distributary and maximum discharges in the system and the volume of the underground conduits. Data are presented which indicate that exchange takes place between the flooded conduit, and the surrounding fissured bedrock in a manner analogous to bank storage in rivers flowing on alluvium.In conclusion fluoresent tracers are valuable for the detailed investigation of a single sink to rising system; Lycopodium spores, which can be used to trace several sinks simultaneously, are a valuable technique in exploratory tracing. 相似文献
195.
196.
A. D. N. Smith 《Journal of Geodesy》1967,41(4):469-475
Summary A method of adjusting intersecting chains of triangulation by direct elimination, using variation of coordinates, is given
which is suitable for a computer of modest storage capacity (about 4000–8000 words). The network is considered to be made
up of “nodes” (where two or more chains intersect) and “links” (which connect two nodes). The normal equations for the points
in each link are computed, the coefficients of the unknown coordinates of the associated nodes being treated as extra right
hand sides. These equations are then solved to express the coordinates of any link station in terms of the associated nodes.
The process is repeated for all links. The normal equations for the nodes are then set up and substitution made for all “link”
terms.
The method is particularly powerful for the adjustment of a few, long, interconnected chains since the reduced normal equation
coefficients are then banded about the diagonal, the semi band width being a little greater than the largest number of unknowns
in the nodes along a single tie chain. For a computer of 7000 words capacity it is possible to solve for a band width of over
100 unknowns i.e. more than 50 stations. 相似文献
197.
The response of the azimuthal component of the ionospheric electric field to auroral arc brightening
V. Safargaleev W. Lyatsky N. G. Gazey P. N. Smith V. Kriviliov 《Annales Geophysicae》1999,18(1):65-73
We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the short circuit of an external electric field by the conductor. The possible consequence of this phenomenon is discussed. 相似文献
198.
It is pointed out that simple models adopted so far have tended to neglect the obliquity of the magnetic field lines entering the Earths surface. A simple alternative model is presented, in which the ambient field lines are straight, but enter wedge shaped boundaries at half a right-angle. The model is illustrated by assuming an axially symmetric, compressional, impulse type disturbance at the outer boundary, all other boundaries being assumed to be perfectly conducting. The numerical method used is checked from the instant the excitation ceases, by an analytical method. The first harmonic along field lines is found to be of noticeable size, but appears to be mainly due to coupling with the fundamental, and with the first harmonic across field lines. 相似文献
199.
Boyle P Cooke T Halfacree K Smith D 《International journal of population geography : IJPG》1999,5(3):157-178
This paper investigates the problems in integrating different data sets for social science research. A cross-national analysis of the effects of family migration on labor market participation rates for female partners was used as an example to concretize these problems. The data used in the cross national analysis came from the 1991 British Census Sample of Anonymised Records and the 1990 United States Public Use Microdata Sample. The problems addressed were the following: data collection; manipulation and reliability; question type and definition; and measurement of individual and family variables. As part of the analysis, some empirical findings were also presented. Significant overall differences between the US and Great Britain (GB) samples were observed, particularly concerning graduate qualifications, marital and employment status, and ethnic group. There were higher numbers of married partners and full-time workers in the US sample, while female partners undertaking part-time employment were greater in GB. Furthermore, the US sample had more couples with a female graduate as compared with the GB sample who had a higher proportion of nongraduate couples. Finally, this paper recognized the importance of cross- national research in the evaluation of national ideological and institutional structures. 相似文献
200.