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41.
Sergio Zlotnik Manel Fernández Pedro Díez Jaume Vergés 《Pure and Applied Geophysics》2008,165(8):1491-1510
A non-standard new code to solve multiphase viscous thermo–mechanical problems applied to geophysics is presented. Two numerical
methodologies employed in the code are described: A level set technique to track the position of the materials and an enrichment
of the solution to allow the strain rate to be discontinuous across the interface. These techniques have low computational
cost and can be used in standard desktop PCs. Examples of phase tracking with level set are presented in two and three dimensions
to study slab detachment in subduction processes and Rayleigh–Taylor instabilities, respectively. The modelling of slab detachment
processes includes realistic rheology with viscosity depending on temperature, pressure and strain rate; shear and adiabatic
heating mechanisms; density including mineral phase changes and varying thermal conductivity. Detachment models show a first
prolonged period of thermal diffusion until a fast necking of the subducting slab results in the break–off. The influence
of several numerical and physical parameters on the detachment process is analyzed: The shear heating exerts a major influence
accelerating the detachment process, reducing the onset time to one half and lubricating the sinking of the detached slab.
The adiabatic heating term acts as a thermal stabilizer. If the mantle temperature follows an adiabatic gradient, neglecting
this heating term must be included, otherwise all temperature contrasts are overestimated. As expected, the phase change at
410 km depth (olivine–spinel transition) facilitates the detachment process due to the increase in negative buoyancy. Finally,
simple plume simulations are used to show how the presented numerical methodologies can be extended to three dimensions. 相似文献
42.
A specific combination of spectral fine structures in meter – decimeter dynamic spectra of solar radio burst emission is
reported in observations carried out at the Astrophysical Institute Potsdam. We describe and interpret the occurrence of zebra
patterns in fast drifting (type III burst-like) envelopes of absorbed continuum emission. A possible mechanism of the origin
of such an involved spectral pattern is put forward, leading to a necessarily multinonequlibrium component coronal plasma.
The suggested mechanism is based on the fact that during the passage of a fast electron beam through the corona the loss cone
instability (which is caused by electrons captured in a magnetic trap generating the continuum) is quenched. As result, a
fast drift burst appears in absorption, and the zebra pattern becomes visible on the low background emission. This zebra pattern
is generated by a group of electrons with a nonequilibrium distribution over transverse velocities. In the absence of the
beam the pattern is invisible against the background of the stronger continuum. It is shown that the mechanism is sensitive
to the distribution parameters of the different electron ensembles. Therefore the effect in dynamic radio spectra is comparatively
rare but its proper existence underlines that the simultaneous presence of different ensembles of electrons in the flaring
corona can be quite a frequent situation. This can explain some problems in deconvolving X-ray photon spectra to electron
energy spectra. 相似文献
43.
Design of managed aquifer recharge (MAR) for augmentation of groundwater resources often lacks detailed data, and simple diagnostic tools for evaluation of the water table in a broad range of parameters are needed. In many large‐scale MAR projects, the effect of a regional aquifer base dip cannot be ignored due to the scale of recharge sources (e.g., wadis, streams, reservoirs). However, Hantush's (1967) solution for a horizontal aquifer base is commonly used. To address sloping aquifers, a new closed‐form analytical solution for water table mound accounts for the geometry and orientation of recharge sources at the land surface with respect to the aquifer base dip. The solution, based on the Dupiuit‐Forchheimer approximation, Green's function method, and coordinate transformations is convenient for computing. This solution reveals important MAR traits in variance with Hantush's solution: mounding is limited in time and space; elevation of the mound is strongly affected by the dip angle; and the peak of the mound moves over time. These findings have important practical implications for assessment of various MAR scenarios, including waterlogging potential and determining proper rates of recharge. Computations are illustrated for several characteristic MAR settings. 相似文献
44.
Observations of the active region AR 7962 obtained at 2–32 cm on the RATAN-600 radio telescope on May 10–12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background of the smooth spectrum of the local source associated with sunspots. This narrow-band emission is identified with a bright, pointlike, high-frequency source at 1.7 cm recorded on maps made using the Nobeyama radio telescope. The characteristics of the observed line (lifetime 3 days, brightness temperature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the third harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be due to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data can be used to probe the magnetic field, kinetic temperature, and electron density in the radiation source in the corona. 相似文献
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Analytical models of groundwater flow with a spatially varying elevation of a top boundary are widely used. However, a vast majority of previous analytical studies truncated the irregularly shaped top section with little to no analyses of the shortcomings of the approximate solutions for the resulting rectangles or parallelepipeds. We present an analytical approach based on a perturbation technique that treats complete domains. It is especially accurate near the top boundary, where fluid circulation is most pronounced and higher accuracy is typically needed, such as in regional or hyporheic systems flow. The approach is illustrated by analyzing flow for a Tóthian unit basin. 相似文献
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Asymmetric dipole-flow test in a fractured carbonate aquifer 总被引：1，自引：0，他引：1
In this study, a new method-the asymmetric dipole-flow test-is proposed and tested for characterization of conductive properties and structure of fractured aquifers. Analytical solutions were developed and then used for interpretation of a modification of the dipole-flow test with a single packer at the Bissen Quarry test site (Wisconsin, USA). The asymmetric dipole-flow tests were conducted by packing a well at various elevations, and fluids were pumped from the upper section (chamber) of the well to the lower section (chamber). The head was then monitored at 11 observation points and in both sections of the well, and the conductivities of the well segments were determined. The tests at seven packer elevations in the well were rapid (less than one hour to reach steady state). The asymmetric dipole-flow test demonstrates the potential to quantify heterogeneities of a fractured aquifer and delineate the applicability of the continuum and discrete approaches for conceptualization of ground water flow. 相似文献
49.
We investigate the close analogy between the solar radio emission with a quasi-harmonic spectrum structure and one of the microwave emission components of the Crab pulsar in the form of the so-called zebra pattern. The radio emission mechanism of this component can be provided by instability at double plasma resonance and can be realized in extraordinary (for a radio pulsar) conditions, namely in a nonrelativistic plasma with a relatively weak magnetic field. We point out possible models of the emission source in the form of a magnetic trap or a neutral current sheet with a transverse magnetic field localized in the corotating region of the pulsar magnetosphere far from the neutron star surface. 相似文献