2D and 3D upper bound solutions for tunnel excavation using ‘elastic’ flow fields     

Assaf Klar  Ashraf S. Osman  Malcolm Bolton 《国际地质力学数值与分析法杂志》2007,31(12):1367-1374

This paper presents 2D and 3D upper bound solutions for the problem of tunnel excavation in soft ground. The solution invokes the use of incompressible flow fields derived from the theory of elasticity and the concept of sinks and sources. Comparison is made with previously published results. For some geometries the current calculation results in lower (better) upper bound values; however, the results were generally close to previously published values. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

The dynamics of jovian white ovals from formation to merger     

Ashraf Youssef 《Icarus》2003,162(1):74-93

In early 1998 two of the three, long-lived anticyclonic, jovian white ovals merged. In 2000 the two remaining white ovals merged into one. Here we examine that behavior, as well as the dynamics of three earlier epochs: the Formation Epoch (1939-1941), during which a nearly axisymmetric band broke apart to form the vortices; the Kármán Vortex Street Epoch (1941-1994), during which the white ovals made up the southern half of two rows of vortices, and their locations oscillated in longitude such that the white ovals often closely approached each other but did not merge; and the Pre-merger Epoch (1994-1997), during which the three white ovals traveled together with intervening cyclones from the northern row of the Kármán vortex street in a closely spaced group with little longitudinal oscillation. We use a quasi-geostrophic model and large-scale numerical simulation to explain the dynamics. Our models and simulations are consistent with the observations, but none of the observed behavior is even qualitatively possible without assuming that there are long-lived, coherent cyclones longitudinally interspersed with the white ovals. Without them, the white ovals approach each other and merge on a fast, advective timescale (4 months). A necessary ingredient that allows the vortices to travel together in a small packet without spreading apart is that the strong, eastward-flowing jetstream south of the white ovals is coincident with a sharp gradient in background potential vorticity. The jet forms a Rossby wave and a trough of the wave traps the white ovals. In our simulations, the three white ovals were trapped before they merged. Without being trapped, the amount of energy needed to perturb two white ovals so that they merge exceeds the atmosphere’s turbulent energy (which corresponds to velocities of ∼1 m s⁻¹) by a factor of ∼100. The mergers of the white ovals BC and DE were not observed directly, so there is ambiguity in labeling the surviving vortices and identifying which vortices might have exchanged locations. The simulation and modeling make the identifications clear. They also predict the fate of the surviving white oval and of the other prominent jovian vortex chains.  相似文献   

Vegetation and Climate Reconstruction of the Planalto in Rio Grande do Sul, South Brazil (Late Glacial and Holocene)     

M.Herrmann  A.R.Ashraf  V.Mosbrugger  M.M.Toigo 《东北亚地学研究》2003,(2)

1IntroductionThe ecosystem of Araucaria-Forest is one of the most important plant-associations in South Brazil. Today,however,the spreading area is a fraction of its natural size,originally spread over 200 km. It spanned from 18°to 30°of southern lati-t…  相似文献   

¹³C/¹²C ratio double cyclicity in a Miocene browncoal: isotopic signals and orbital forcing     

Timothy P. Jones  Steven M. Fortier  Volker Mosbrugger  Jochen Roessler  Thorsen Utescher  & Abdul R. Ashraf 《地学学报》1997,9(1):19-23

A detailed stable carbon isotopic profile of a late Miocene browncoal seam from the Lower Rhine Embayment, Germany, reveals two clear separate cycles of different frequency: this is the first instance of this type of isotopic signature being recognized within coals. The ratio of the two frequencies suggests this isotopic signal possibly resulted from climatic or vegetational responses to orbital forcing. The cyclicity has been enhanced by the application of conventional filtering methods on the data set. An analysis of the seam's palynology indicates a correlation between heavier isotopic compositions and the presence of Sequoiapollenites polyformosus , whose parent plant is believed to have favoured moist climates and higher groundwater tables.  相似文献   

Mapping soil electric conductivity using Bayesian kriging: A case study from Qasur,Pakistan     

Muhammad Imran  Ather Ashraf  Ateeq Ur Rehman 《Journal of the Geological Society of India》2016,88(6):711-717

Quality of soil data is vital to formulate agricultural policies at different scales. Current agricultural applications in Pakistan depend however, on average values of soil estimates over larger areas. In this work, model-based ordinary kriging (OK) and Bayesian kriging (BK) to interpolate soil data is used. The aim is to compare the two different methods for the accuracy of soil data prediction. For this soils were sampled for Electrical Conductivity (EC, dS m ^–1) at 759 different locations in the rural agricultural areas of Qasur Tehsil, Pakistan. Cross validation was used to compare the performance of OK and BK. Our results show strong skewness and spatial dependency of soil EC values in heterogeneous regions. Box-Cox transformation successfully reduced the level of skewness in the soil EC data (from 14.1 to 0.11). Contrary to OK, under-estimation of soil EC values was not evident in the BK interpolation. Mean square prediction error for BK (1.45) was significantly reduced as compared to that for OK (6.1). Considering these findings, BK is a better model to explain the sub-regional soil EC variability and estimating strategies for sustainable agricultural planning in Pakistan.  相似文献   

Effect of exo-polysaccharides producing bacterial inoculation on growth of roots of wheat (Triticum aestivum L.) plants grown in a salt-affected soil     

M. Ashraf M. Sc.  M. S.  S. Hasnain Ph.D.  O. Berge Ph.D. 《International Journal of Environmental Science and Technology》2006,3(1):43-51

Effect of soil salinity on physico-chemical and biological properties renders the salt-affected soils unsuitable for soil microbial processes and growth of the crop plants. Soil aggregation around roots of the plants is a function of the bacterial exo-polysaccharides (EPS), however, such a role of the EPS-producing bacteria in the saline environments has rarely been investigated. Pot experiments were conducted to observe the effects of inoculating six strains of EPS-producing bacteria on growth of primary (seminal) roots and its relationship with saccharides, cations (Ca²⁺, Na⁺, K⁺) contents and mass of rhizosheath soils of roots of the wheat plants grown in a salt-affected soil. A strong positive relationship of RS with different root growth parameters indicated that an integrated influence of various biotic and abiotic RS factors would have controlled and promoted growth of roots of the inoculated wheat plants. The increase in root growth in turn could help inoculated wheat plants to withstand the negative effects of soil salinity through an enhanced soil water uptake, a restricted Na⁺ influx in the plants and the accelerated soil microbial process involved in cycling and availability of the soil nutrients to the plants. It was concluded that inoculation of the EPS-producing would be a valuable tool for amelioration and increasing crop productivity of the salt-affected soils.  相似文献   

Natural environmental radioactivity and estimation of radiation exposure from saline soils     

N. Akhtar  M. Tufail  M. Ashraf 《International Journal of Environmental Science and Technology》2005,1(4):279-285

The study was conducted for the investigation of amount of radioactivity in the barren and cultivated soil of Bio saline Research Station in Pakka Anna, established by Nuclear Institute for Agriculture and Biology (NIAB) in 1990, 34 km. away from the city of Faisalabad, in the Punjab province of Pakistan. The studies were done on an area of about 100 hectares of two types of virgin and fertilized saline soils. The technique of gamma ray spectrometry was applied using HPGe (high purity germanium) gamma ray detector and a PC based MCA. Activity concentration levels due to ⁴⁰K, ¹³⁷Cs, ²²⁶Ra and ²³²Th were measured in 250 saline soil samples collected at a spacing of about 4 hectares at the depth level of 0–25 cm. with a step of 5 cm. depth. Activity concentrations ranges of the concerned radionuclides for both of the soils were as follows: ⁴⁰K, for virgin and cultivated saline soil was 500–610.2 and Bq/kg 560.2–635.6 respectively; ¹³⁷Cs, 3.57–3.63 and 1.98–5.15 Bq/kg ²³⁸U, 26.3–31.6 and 30.3–38.7 Bq/kg, and ²³²Th, 50.6–55.3 and 50.6–64.0 Bq/kg respectively. The absorbed dose rate in air lies in the region 63–73 nGyh^-1 and 68–83 nGyh^-1 for virgin and fertilized soils respectively. This indicates that this region lies in the area of higher radiation background, while comparing with the worlds’ average. The slightly higher value of dose in the fertilized farm may be due to the use of fertilizers for cultivation. Before the radiometric measurements, chemical analysis for concentration of Na, Ca and Mg was also carried out along with the measurement of electrical conductivity and pH of the soil samples.  相似文献   

Effect of earthquake energy input characteristics on hysteretic damper efficiency     

Hiroki Yamaguchi  Ashraf El‐Abd 《地震工程与结构动力学》2003,32(6):827-843

Robust performance of hysteretic dampers, used in controlling mid‐rise buildings, against change of earthquake characteristics is investigated in this paper. A shear type ten‐storey building incorporating hysteretic dampers is studied as a model under the assumption of elastic perfectly plastic behavior for inelastic frame and damper deformations. An energy‐based damper performance index is used to evaluate damper overall efficiency. Thirty‐five earthquake records are applied and the damper strength is optimized for each earthquake record to obtain the maximum performance index or the damper efficiency. Based on the obtained numerical results it is found that, besides the effect of maximum energy input on damper efficiency, other time‐dependant properties such as energy‐based effective duration and earthquake dominant period have great influence on the damper efficiency. A factor (α), which represents the combined effect of maximum energy input, effective duration and dominant earthquake period, is also derived for the prediction of damper efficiency. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Dynamic instability of frames having thin-walled columns     

Srinivasan Sridharan  M. Ashraf Ali 《地震工程与结构动力学》1988,16(5):705-717

Dynamic instability of single storey frames having thin-walled columns has been investigated. The lateral loads sustained by the frame are dynamic in character, while the axial loads are deemed to be quasi-statically applied. The analytical model employed by the authors has the capability of modelling the combined action of the two ‘companion’ local modes whose amplitudes are variable along the length of the column and any type of end conditions of the members. For given levels of axial loads sustained by the columns, the magnitudes of lateral loads causing instability can be significantly smaller than those corresponding to static buckling, provided the dynamic load is of sufficient duration. There exists, however, a threshold value of axial force carried by the columns, below which there is no elastic instability—static or dynamic. For columns with overall critical loads several times greater than the local critical load, there is no danger of elastic instability, but the deflections under dynamic lateral loads of less than 1 per cent of the axial load may reach such huge values that there is a serious danger of localized plastic collapse. It is also shown that moment frames having thin-walled columns such as those fabricated out of cold formed steel are extremely vulnerable to moderate seismic excitations.  相似文献   

Obliquely propagating nonextensive dust-ion-acoustic solitary waves in a dusty magnetoplasma     

S. Ashraf  S. Yasmin  M. Asaduzzaman  A. A. Mamun 《Astrophysics and Space Science》2013,343(1):145-151

An impact model of gravity designed to emulate Newton’s law of gravitation is applied to particles with relative motions at slow and relativistic speeds. Based on this model, a gravitational interaction mode is then conceived between photons and massive particles. This implies a deflection perpendicular to the propagation direction of a photon twice as large as expected from the mass-energy relation of photons—in accordance with observations and the General Theory of Relativity. The longitudinal interaction is compatible with the energy and momentum conservation principles applied to massless entities, and the results are consistent with the observed Shapiro delay.  相似文献