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1.
In this short paper the exclusion circles and vertical phase locities for gravity waves launched from the ground into a time-varying wind are studied using a ray-tracing technique. It is shown that waves with initial observed phase speeds that should place them within the local temporally varying exclusion circle, are often Doppler shifted outside of the circle. This, and the finite lifetime of some critical levels, allow waves to survive the critical layer and reach higher altitudes. Also, for slower phase-speed waves, the temporally varying wind can shift the observed frequency to negative values, so that the observed phase motions will be opposite (i.e. horizontally reversed and vertically upward), even though the energy still propagates upward. This effect can also cause the phase velocity to move inside the local exclusion circle. Due to the directional filtering of wave sources by the stratospheric wind, the percentage of such reverse-propagating waves will change systematically with local time and height in our simplified but realistic model. These results are related to ground-based systems, optical and radar, which sample the wind field and gravity waves in the middle atmosphere. 相似文献
2.
J. W. Meek 《地震工程与结构动力学》1978,6(5):437-454
When subjected to major earthquakes, core-stiffened buildings may begin to tip. That is, the overturning moment on the core's footing becomes so large that the footing breaks contact with the ground and begins to rock. A method is described for including the effects of tipping in the analysis of multistorey core-braced structures. Curves are presented which summarize the maximum response to both pulse and earthquake excitations; these data are elucidated via a typical design example. By comparison to fixed-base behaviour, tipping greatly reduces the base shear and moment. This makes possible a more economical design. However, attention must be devoted to avoiding potential soil-mechanics problems associated with the wobbling behaviour of the tipping core. 相似文献
3.
A discussion of the effects of soil-structure interaction on the dynamic response of linear structures which respond as single-degree-of-freedom systems in their fixed-base condition is presented. The structures are presumed to be supported at the surface of a homogeneous, elastic halfspace and to be excited at the base. The free-field ground motions investigated include a harmonic motion, a relatively simple pulse-type excitation and an actual earthquake record. Comprehensive response spectra are presented for a range of the parameters defining the problem, and the results are used to assess the accuracy of a simple, approximate method of analysis in which the system is represented by a viscously damped, simple oscillator. Special attention is given to defining the conditions under which the interaction effect is of sufficient importance to warrant consideration in design. The method of analysis used to obtain the numerical data reported herein is reviewed only briefly, the emphasis of the paper being on the interpretation of the results. 相似文献
4.
David Meek 《The Professional geographer》2019,71(1):65-74
The spatial nature of learning is increasingly a focus of geographic inquiry. I argue that the spatiality of education, which is where formal learning occurs, has the potential to shape students’ spatial imaginaries. I analyze the role the spatiality of agronomic education plays in the historical construction of the social and physical landscape in southeastern Pará, Brazil. In opposing ways, the first Green Revolution, and agrarian social movements’ more recent agroecological Green Revolution are found to structure agronomic education and spatial imaginaries. The perspectives of agricultural extension agents trained in traditional agronomic programs are compared with teachers from an agroecological school located in an Amazonian agrarian reform settlement of Brazil's Landless Workers’ Movement. I collected these data over 17 months of ethnographic fieldwork. To analyze these data, I employ a political ecology of education perspective, which highlights how education and political economy interact to mediate relations with, access to, and contestations over natural resources. The geography of education and the education of geography exist in a complicated feedback cycle: Education is not neutral but ideologically charged and affects conceptions of productive landscapes, providing students intellectual and economic power to put their visions of landscape into effect. 相似文献
5.
Chanda L. Meek 《Global Environmental Change》2013,23(1):217-228
Rapid environmental change in the Arctic has led to calls for new forms of environmental governance that consciously fit policy solutions to both the policy problem as well as the underlying social–ecological system dynamics. While efforts to evaluate the ecological fit of institutions to place have become more sophisticated, efforts to measure the social fit of policy remains underdeveloped. In order to examine the effect of institutional form on policy processes and socially relevant outcomes, I employ a mixed methods approach including ethnographic data and social network analysis to compare the implementation of two international wildlife regimes in two indigenous Iñupiaq towns in Alaska. My results yield three findings: (1) that separate institutions create differently structured policy networks, (2) differences in network structures and levels of power-sharing correlate with perceptions of policy, and (3) networks that reflect local social patterns are more likely to be considered fair and inclusive. These findings support congruence theory, which posits that public policy reflective of local constructions of legitimacy will achieve greater success than policy that is not reflective. 相似文献
6.
Saskatoon (52° N, 107°W) medium frequency (MF) radar data from 1979 to 1993 have been analyzed to investigate the climatology of irregular wind components in the height region 60–100 km. This component is usually treated in terms of internal gravity waves (IGW). Three different band-pass filters have been used to separate the intensities of IGWs having periods 0.2-2.5; 1.5-6 and 2–10 h, respectively. Height, seasonal and inter-annual variations of IGW intensities, anisotropy and predominant directions of propagation are investigated. Mean over 14 years’ seasonal variation of the intensity of long-period IGWs shows a dominant annual component with winter maximum and summer minimum. Seasonal variations of the intensity of short-period waves have a strong semi-annual component as well, which forms a secondary maximum in summer. Predominant azimuths of long-period IGWs are generally zonal, though they vary with season. For short-period IGWs, the predominant azimuth is closer to the meridional direction. Anisotropy of IGW intensity is larger in summer, winter and at lower altitudes. The IGW intensity shows apparent correlation with both solar and geomagnetic activity. In most cases, this correlation appears to be negative. The variations versus solar activity is larger for longer-period IGW. Possible reasons and consequences of the observed climatological variations of IGW intensity are discussed. 相似文献
7.
Manganiferous rock varnish, silica glaze and iron skins have formed on 20- to 40-year-old slag piles near Fontana, southern California. Rapid rock-varnish formation is associated with an unidentified cocci bacterium that grows rapidly in culturing experiments, combined with the likelihood that Mn-rich solutions flow over slag surfaces. A new model is proposed for the formation of silica glaze, involving soluble Al-Si complexes and the nature of wetting films. Iron skins occur where runoff is concentrated, and where iron bacteria grow. Observations indicate: (i) substrate can be important in supplying Mn, as well as maintaining a stable surface and channelling runoff to microspots where varnish grows; (ii) ignorance of boundary layer pH/Eh conditions makes chemical-physical models of varnish formation speculative; (iii) the balance between cementation of recently nucleated varnishes and erosional shear stresses is poorly understood, but is probably an important limiting factor on varnish growth rates; and (iv) there must be at least two steps in biotic varnishing-accretion of Mn-Fe in casts, followed by cementation of clays by either in situ cast residue or Mn-Fe remobilized from casts. Varnish accretion may be limited by the rate at which both steps occur. Rapid development of readily distinguishable manganiferous rock varnishes at this post-1952 site highlights the danger of speculation on the ages of landforms based solely on the appearance of rock-varnish coatings. 相似文献
8.
In lieu of the rigorous elastodynamical approach, many problems in foundation dynamics may be solved quite accurately via simple cone models of the soil. Although these cone models are amenable to very simple analysis, they have not yet been widely accepted in engineering practice. The reservations against their use are possibly due to the fact that cones are based on non-rigorous strength-of-materials assumptions, they neglect large portions of the half-space, and cannot represent Rayleigh surface waves. These potential objections are investigated step by step and proven to be unfounded. It turns out that cone models indeed incorporate and provide valuable insight into all the salient features of rigorous solutions; the aspects omitted by cones are revealed to be of minor physical importance. 相似文献
9.
To simplify the analysis, three-dimensional soil–structure interaction problems are often modelled by considering a two-dimensional slice without changing the material properties of the soil. This procedure, although convenient, is of questionable validity because two-dimensional modelling inherently overestimates the radiation damping for translational and rocking motions. To make matters worse, two-dimensional modelling always entails an underestimation of the dynamic-spring coefficient for the translational motions. The damping ratio of the two-dimensional case, which is proportional to the ratio of the damping coefficient to the spring coefficient, will thus be even larger. Thus, reliance upon a two-dimensional analysis based on an equivalent slice of a strip foundation may result in a dangerously non-conservative design. Valuable insights into the essence of radiation damping and the difference between two-dimensional and three-dimensional models may be obtained via approximate strength-of-materials solutions based on cone–wedge models and travel-time considerations. By examining the decay of the waves along the axes of the cone–wedge models, the essence of radiation damping can be grasped. The heuristic concept of more spreading of waves in three dimensions than in two is misleading. Indeed, just the opposite is true: The less the amplitude spreads and diminishes with distance, the greater is the radiation damping. Because the damping ratio is grossly overestimated, two-dimensional modelling of a three-dimensional case cannot be recommended for actual engineering applications. It is more feasible to take the opposite approach and idealize slender soil–structure interaction problems with a radially symmetric model. As an alternative, when defining the equivalent slice of the two-dimensional strip foundation, the impedance of the soil can be changed to achieve a much better agreement of the high-frequency limits of the damping coefficients. In the low-frequency range this modified two-dimensional model also overestimates radiation damping, although to a lesser extent. As a by-product, the dimensions of the equivalent slice of a two-dimensional strip foundation are discussed; and equations for the aspect ratios determining the opening angles of the corresponding wedges are derived. Also addressed is the quite separate but related topic of the transition from square to slender rectangular foundations. 相似文献
10.
The unfolded cone model used for calculating the dynamic response of a disk on the surface of a soil layer resting on flexible rock for translational motion is extended to rotational motion. The method is analogous to that for a layer on rigid rock, the only modification being that the reflection coefficient – α replaces the coefficient of total reflection – 1. The modified value of – α, which, in general, is frequency-dependent, is determined by considering one-dimensional wave propagation along the cone for the first impingement at the layer–rock interface. The low- and high-frequency limits of – α for the rotational motion are the same as for translational motion. As these limits do not depend on frequency, the dynamic analysis using cones can be performed in the familiar time domain. The transfer function constructed by addressing the reflections–refractions at the soil–rock interface and the reflections at the free surface in the unfolded cone model is highly accurate, resulting in the same accuracy of the dynamic response of a disk on a layer resting on flexible rock as that on a homogeneous half-space modelled with a cone. 相似文献