Rain induced attenuation studies for V-band satellite communication in tropical region

Satellite communications operating at 10 GHz and above in the tropics suffer severe signal degradation due to rain. Attenuation due to rain at 38 GHz had been measured for a period of 20 months in Malaysia. Analyses carried out include seasonal variations, diurnal effects and the annual cumulative distributions. Obtained results were compared with several established prediction models including the ITU-R. The rain fade characteristics were also investigated in determining the levels of signal loss and fading. In addition, the studies highlight several potential fade mitigation techniques that can be embarked. These fundamental aprehensions are very critical for future earth space communication link design and can be exploited as preliminary groundwork plan for the researchers as well as engineers.     

Définition de la perméabilité équivalente des massifs fracturés par des méthodes d'homogénéisationDefinition of the permeability of fractured rock masses by homogenisation methods

In the methods used for determination of the hydraulic behaviour at large scale of fractured rock masses, based on the simulation of flow in a fracture network, the mean flux and the mean pressure gradient in the network are not rigorously determined. A method is given for deriving these quantities, in a heterogeneous permeable block, from pressure and flux values on the boundary of the block. A block conductivity tensor is then defined, based on the condition of linear variation of the pressure on the boundary of the block. It is shown that this conductivity tensor is symmetric and positive-definite. An example of application to a model of fractured medium is given. To cite this article: A. Pouya, A. Courtois, C. R. Geoscience 334 (2002) 975–979.     

Numerical Modelling Investigation of Wave Interaction on Composite Berm Breakwater

China Ocean Engineering - The breakwaters have experienced many changes during their construction history. These changes have been considered to improve their performance, depending on their...     

Effects of Drying on the Low-Frequency Electrical Properties of Tournemire Argillites

Nearly water-saturated argillite samples (initial water content near 3.4 wt%) were cored from an undisturbed area of an underground facility of the French Institute for Radioprotection and Nuclear Safety (IRSN), located at Tournemire (Aveyron, France). These samples were subjected to the following desiccation path: (a) A desaturation phase during which the samples were dried at ambient temperature conditions, relative humidity equal to 43% in average and (b) a heating phase during which the same samples were heated at four temperature levels from 70°C up to 105°C. During both phases, the low-frequency complex resistivity (0.18Hz–12 kHz) was recorded by a four-electrode device. The amplitude of the complex resistivity was extremely sensitive to water content change. At the end of the isotherm desaturation phase, it was multiplied by a factor of 3 to 5. During the heating phase, the resistivity increased by more than two orders of magnitude compared to the initial state. The percentage of Frequency Effect shows a low sensitivity to water content changes during the desaturation stage while it increased by two orders of magnitude during the heating phase. This result confirms that low-frequency spectral signature is extremely sensitive to textural changes (i.e., thermal-induced microcracking in this case) that occurred during heating. Moreover, the complex resistivity of the samples shows a strong anisotropy (a ratio of 10 between both amplitudes measured in the perpendicular directions). The classical Cole-Cole model cannot be used to fit the experimental data obtained in the heating phase. A generalized formulation of this model is required and was successfully applied to represent the complex resistivity data.     

Comparison of dynamic stiffnesses of rigid square and rectangular foundations by DBEM and IBEM

In this work, the Direct and Indirect Boundary Element Methods as applied to dynamic soil structure interaction problems, are compared. Both the methods are used to find the dynamic stiffnesses of rigid surface and embedded foundations. For surface foundations, the results obtained by the two methods were very close for the frequency range considered, but for embedded foundations at higher frequencies, the Indirect Boundary Element Method produced erroneous values for the real part of vertical compliance. The possible reasons for this discrepancy are also studied in this paper.     

Analysis of liquefaction susceptibility of nearly saturated sands

A new constitutive formulation for simulating the behaviour of nearly saturated sands under seismic loads is presented. The formulation is based on combining the Henry's law for dissolution of gas in water, the ideal or perfect gas law and the law of conservation of mass. The effects of transient air dissolution in water on the compressibility of partially saturated soils are also taken into account. The model was calibrated based on numerical simulations of isotropically consolidated cyclic triaxial tests conducted on partially saturated samples of Toyoura sand. A multi‐yield plasticity soil constitutive model implemented in the finite element code DYNAFLOW was used for these numerical simulations. It is shown that the formulation proposed here is able to reasonably predict the soil cyclic undrained behaviour at various degrees of saturation (95% and higher). Copyright © 2006 John Wiley & Sons, Ltd.     

Uranium and boron distributions related to metamorphic microstructure-evidence for metamorphic fluid activity

The distribution of uranium and boron in polymetamorphosed, granulite facies schists and gneisses has been studied using particle track methods. The concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into: (1) the behaviour of U and B in metamorphism, (2) the activity of a fluid phase in the metamorphic processes and (3) the nature of chemical processes during schistosity development. A low concentration of primary U occurs in micro-inclusions of apatite and zircon (many are metamict) in the granulite facies M₁ assemblage. This assemblage which lacks B, except for zoned sillimanite, has undergone a localized retrograde metamorphism RM₁ characterised by hydrous alteration products containing abundant U and B. The RM₁ metamorphism is attributed to fluids generated during granulite facies dehydration reactions. A schistosity S₂ defined by M₂ fibrolite aggregates overprints the M₁ events. It is associated with (1) intragranular U concentrated in M₂ apatite and titanium bearing minerals and (2) abundant intergranular U within the fibrolite aggregates. High B contents also occur with the fibrolite. The S₂ schistosity appears to develop in a metamorphic environment containing a fluid enriched in U and B.     

Microwave solar limb brightening

Previous models of microwave limb brightening have omitted the alignment of spicules along supergranule boundaries, have neglected the high temperature sheath around spicules, and have assumed an interspicular medium which was averaged over chromospheric network and non-network regions. We present a model which includes these factors. By constraining the model to conform to results from earlier UV and optical studies we are effectively left with two free parameters: the temperature at the core of the spicules, T _core, and (at solar minimum), the interspicular chromospheric network density model of the lower transition zone. The absence of limb brightening at the short millimeter wavelengths implies T _core 6000 k. Differences between the model and certain deconvolved observations near 9 mm are expected as a consequence of an extension of emission beyond the optical limb, predicted by the model, which affects the accuracy of the deconvolution technique. Unlike models which assume homogeneous spicules in a random distribution, ours does not require an abnormally high spicule area.     

Dynamic stress analysis of a class of geomechanics problems by the boundary element method

This paper presents the application of an advanced BEM for periodic and transient dynamic stress analyses of a class of geomechanics problems. For transient dynamic analysis, the problem is first solved in the Laplace transform space, which happens to be similar to the periodic dynamic analysis, and then the time domain solution is obtained by numerical inversion of transform domain solutions. The numerical implementation of the BEM used to present the results in this paper is complete and most general available to date. It is capable of treating very large, multi-layered problems by substructuring and satisfying the equilibrium and compatibilities at the interfaces. With the help of this substructuring, capability problems related to layered media and soil–structure interaction have been analysed. A number of examples are presented and through comparisons with available analytical and numerical results, the applicability and usefulness of the present analysis to real geomechanical problems are established.