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761.
This paper investigates the instability of Poiseuille flow in a fluid overlying a highly porous material. A two layer approach is adopted, where the Darcy–Brinkman equation is employed to describe the fluid flow in the porous medium, with a tangential stress jump boundary condition at the porous/fluid interface. The basic velocity profiles are continuous due to the interfacial boundary conditions. It is shown that for certain parameter ranges the neutral curves are no longer bimodal, such that the two modes of instability corresponding to the fluid and porous layers, respectively, are not distinct. 相似文献
762.
763.
Hill Kulu 《GeoJournal》2000,51(3):135-143
This article examines the Estonian diaspora-policy and ethnic (return) migration from the Commonwealth of Independent States (CIS) to Estonia in the 1990s. Ethnic migration to re-independent Estonia began in 1992, when the Estonian government evacuated 170 ethnic Estonians from areas caught up in the Georgian-Abkhazian war. Along with this was created the institutional framework for receiving ethnic migrants from crisis areas of the CIS. Later, support and facilities were also granted to ethnic Estonians migrating from elsewhere in the CIS. As a result, about 1,200 ethnic Estonians from the CIS settled in Estonia in 1992–1996. The research supports that ethnic migration is a phenomenon in which the state policy plays a decisive role both in inducing migration and in selecting those who are granted the right to relocate. 相似文献
764.
Adopting an integrated approach the potential genotoxic, cytotoxic and developmental effects of tributyltin oxide (TBTO), a known endocrine-disrupting agent for neogastropods, have been evaluated in two ecologically relevant invertebrates: Mytilus edulis (blue mussel) and Platynereis dumerilli (rag worm). Following determination of the maximum tolerated dose (MTD) in terms of developmental and survival effects, the embryo-larval stages of these organisms were exposed to a range of concentrations of TBTO, and analysed for cytotoxic (proliferation rate index) and genotoxic (sister chromatid exchanges and chromosomal aberrations) effects. The study suggested that: (1) TBTO is both toxic and genotoxic to embryo-larval stages of both species; (2) at comparable concentrations, for developmental and genotoxic effects, P. dumerilii (non-target species) is more sensitive compared to M. edulis (target species); and (3) genotoxic effects are more closely tied with the development and survival of the organisms. The study emphasises the need of the evaluation of genotoxic potential of other endocrine-disrupting agents in different taxonomic groups. 相似文献
765.
P. Elebert P. J. Callanan A. V. Filippenko P. M. Garnavich G. Mackie J. M. Hill V. Burwitz 《Monthly notices of the Royal Astronomical Society》2008,383(4):1581-1587
We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1−2455. Our R -band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Hα emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km s−1 , placing a lower limit of 0.05 M⊙ on the secondary mass. For a 1.4 M⊙ primary, this implies that the orbital inclination is low, ≲20°. Utilizing the observed relationship between the secondary mass and the orbital period in short-period cataclysmic variables, we estimate the secondary mass to be ∼0.085 M⊙ , which implies an upper limit of ∼2.4 M⊙ for the primary mass. 相似文献
766.
M. E. C. Swanson Max Tegmark rew J. S. Hamilton J. Colin Hill 《Monthly notices of the Royal Astronomical Society》2008,387(4):1391-1402
As galaxy surveys become larger and more complex, keeping track of the completeness, magnitude limit and other survey parameters as a function of direction on the sky becomes an increasingly challenging computational task. For example, typical angular masks of the Sloan Digital Sky Survey contain about N = 300 000 distinct spherical polygons. Managing masks with such large numbers of polygons becomes intractably slow, particularly for tasks that run in time with a naive algorithm, such as finding which polygons overlap each other. Here we present a 'divide-and-conquer' solution to this challenge: we first split the angular mask into pre-defined regions called 'pixels', such that each polygon is in only one pixel, and then perform further computations, such as checking for overlap, on the polygons within each pixel separately. This reduces tasks to , and also reduces the important task of determining in which polygon(s) a point on the sky lies from to , resulting in significant computational speedup. Additionally, we present a method to efficiently convert any angular mask to and from the popular healpix format. This method can be generically applied to convert to and from any desired spherical pixelization. We have implemented these techniques in a new version of the mangle software package, which is freely available at http://space.mit.edu/home/tegmark/mangle/ , along with complete documentation and example applications. These new methods should prove quite useful to the astronomical community, and since mangle is a generic tool for managing angular masks on a sphere, it has the potential to benefit terrestrial mapmaking applications as well. 相似文献
767.
Frank Hill 《Journal of Astrophysics and Astronomy》2008,29(1-2):75-84
A major goal of helioseismology is to understand the mechanism of the solar cycle. In this paper, some results of helioseismic observations relevant to the cycle are briefly reviewed, the current state-of-the-art is discussed, and near-term future directions are sketched out. Topics covered include the internal rotation rate; activity-related parameter variations; the tachocline; far-side imaging; the torsional oscillation; and meridional flows. 相似文献
768.
Rodney A. Sheets Mary C. Hill Henk M. Haitjema Alden M. Provost John P. Masterson 《Ground water》2015,53(1):151-157
Simulating groundwater flow in a water‐table (unconfined) aquifer can be difficult because the saturated thickness available for flow depends on model‐calculated hydraulic heads. It is often possible to realize substantial time savings and still obtain accurate head and flow solutions by specifying an approximate saturated thickness a priori, thus linearizing this aspect of the model. This specified‐thickness approximation often relies on the use of the “confined” option in numerical models, which has led to confusion and criticism of the method. This article reviews the theoretical basis for the specified‐thickness approximation, derives an error analysis for relatively ideal problems, and illustrates the utility of the approximation with a complex test problem. In the transient version of our complex test problem, the specified‐thickness approximation produced maximum errors in computed drawdown of about 4% of initial aquifer saturated thickness even when maximum drawdowns were nearly 20% of initial saturated thickness. In the final steady‐state version, the approximation produced maximum errors in computed drawdown of about 20% of initial aquifer saturated thickness (mean errors of about 5%) when maximum drawdowns were about 35% of initial saturated thickness. In early phases of model development, such as during initial model calibration efforts, the specified‐thickness approximation can be a very effective tool to facilitate convergence. The reduced execution time and increased stability obtained through the approximation can be especially useful when many model runs are required, such as during inverse model calibration, sensitivity and uncertainty analyses, multimodel analysis, and development of optimal resource management scenarios. 相似文献
769.
Jonathan D. Hill Martin A. Uman Michael Stapleton Douglas M. Jordan Alexander M. Chebaro Christopher J. Biagi 《Journal of Atmospheric and Solar》2010,72(13):913-925
We describe attempts to create ball lightning by directing lightning, triggered from natural thunderclouds using the rocket-and-wire technique, through a variety of materials. Some of the observed phenomena have features in common with natural ball lightning or with laboratory attempts to create it: flame-like luminosity for up to 0.5 s above salt water; constant-luminosity silicon fragments falling for about 1 s under the influence of gravity; a 0.7 m region of stationary luminosity whose bottom was 0.3 m above a stainless steel surface to which arcing had occurred; and a glow for about 0.5 s above pine tree sections. 相似文献
770.
Joachim Miksat Kuo-Liang Wen Vladimir Sokolov Chun-Te Chen Friedemann Wenzel 《Bulletin of Earthquake Engineering》2010,8(4):847-858
Taipei, the capital of Taiwan, suffered from destructive earthquakes four times during the 20th century (M
L = 7.3 on April 15, 1909; M
L = 6.8 on November 15, 1986; the Chi–Chi M
L = 7.3 earthquake on September 21, 1999; and M
L = 6.8 on March 31, 2002). Analysis of recorded data shows a strong dependence of spectral amplification in the Taipei Basin
on earthquake depth and azimuth. At low frequencies (f < 3 Hz) significant larger amplifications are observed for shallow earthquakes as compared to intermediate depth events.
The former ones also display strong azimuthal dependence. As structures with large response periods such as bridges and tall
buildings are sensitive to these low frequencies the understanding of the associated wave effects within the basin and their
role for site effect amplification is critical. The tool we employ is 3D finite-difference modeling of wave propagation of
incident wave fronts. The available detailed model of the basin allows studying the wave effects. Modeling clearly reveals
that basin edge effects as observed in data are related to surface wave generation at the basin edges with a high degree of
azimuthal dependency. The reproduced site amplification effects are in qualitative agreement with the observations from strong
motion data. 相似文献