Isotropization in bianchi type-I vacuum cosmology     

Enrique Guzman 《Astrophysics and Space Science》1993,199(2):289-292

We have found the most general solution for the Bianchi type-I equations in the Brans-Dicke theory (BDT) for the vacuum case. It is shown that forw > 500 the Universe will become isotropic for any amount of initial anisotropy, secondly it is shown that in the extended inflation scenario, the BDT-scalar field can avoid the inflation in one direction,R ₃. In the other scale factorsR ₁ andR ₂ there is inflationary expansion.  相似文献   

Star formation and AGN     

Rosa Maria González-Delgado  Enrique Pérez 《Astrophysics and Space Science》1993,205(1):127-134

We analyse the data from narrow band H and [O III] CCD imaging of a sample of nearby spiral galaxies with active nuclei. The results show that the large scale star formation depends on the level of nuclear activity, with Seyfert 2 presenting the strongest disk and circum-nuclear star formation, while Seyfert 1 show very little. We present particular results for the galaxies NGC 3982, NGC 4579, NGC 5427 and NGC 7469.  相似文献   

P and S Mach Waves Generated by the Detonation of a Cylindrical Explosive Charge - Experiments and Simulations     

Fr  d  ric Vanbrabant  Enrique P. Chac  n  Luis A. Qui  ones 《Fragblast: International Journal for Blasting and Fragmentation》2002,6(1):21-35

During blasting operations in a competent rock mass without open joints or faults, the structure of the wave fields generated by the detonation of a cylindrical charge depends mainly on the relation between the velocity of detonation ( c d ) and the velocity of the P and S waves, c P and c S , respectively, in the medium. If c d is larger than c P , detonation is supersonic and Mach P and S waves appear. If c d is between c P and c S , detonation is transonic, and only an S Mach wave is generated. If c d is less than c S , subsonic detonation occurs. In this paper, we describe an experiment conducted at the Andina mine (Chile) in order to demonstrate the differences between the wave fields generated by supersonic and transonic detonation. We also present a simulation of the detonation of a cylindrical charge using the finite differences Itasca software FLAC3.4. In particular, we show how important it is to secure a good adjustment of the rheological behaviour of the medium, in our case, a strain softening elasto-plastic model.  相似文献   

Is a small sea turtles rookery doomed to local extinction? Decreasing nesting trends at the Paria Gulf,Venezuela     

Clemente Balladares  Enrique Quintero‐Torres 《Marine Ecology》2019,40(5)

Small rookeries are rarely evaluated for marine turtles worldwide. Two species of sea turtles (Hawksbill, Eretmochelys imbricata; Leatherback, Dermochelys coriacea) nest on five main beaches of the northeast coast of the Paria Gulf in Venezuela. Population trends using generalized linear models at this rookery were assessed and compared with other small rookeries. Both species showed significant negative nesting trends: Nesting by critically endangered hawksbills decreased over nine seasons 2009–2017 (64–142 nests per year, General Linear Model Slope Value = ?0.061; data pooled for five beaches); similarly, vulnerable leatherback nests decreased across the same period in the main beach Los Garzos (0–69 nests/year; GLMSV = ?0.34). Besides human and natural predation of the nests, no significant environmental impacts affect the beaches except probably on Obispo Isthmus where a gas pipeline installation interrupted the nesting activity in 2014. Observed changes to the nesting trends in these small rookeries have a collective impact on broader conservation concerns for sea turtles in the region.  相似文献   

A new inherent damping model for inelastic time‐history analyses          下载免费PDF全文

J. Enrique Luco  Armando Lanzi 《地震工程与结构动力学》2017,46(12):1919-1939

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Coastal boulder deposit as evidence of an ocean-wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile)     

Manuel Abad  Tatiana Izquierdo  Miguel Cáceres  Enrique Bernárdez  Joaquín Rodriguez-Vidal 《Sedimentology》2020,67(3):1505-1528

A Late Holocene cliff-top deposit of large boulders well above the limits of modern storm waves is described from the southern coast of the Atacama Desert (northern Chile). The largest moved boulder weighs >40 t and field data point to a flood height >18·5 m above high tide level and an inland penetration greater than 284 m from the cliff edge. The minimum flow velocity needed for particle entrainment was estimated as 10·1 ms⁻¹ and the most likely processes of sediment deposition for different boulders were deduced. The boulder distribution, sorting and orientation of imbricated debris, together with the significant wave height of extreme storms reported and the occurrence of interplate earthquakes in the study area indicate that the deposit records a single event, interpreted here as a tsunami wave train rather than exceptional storm waves. The boulder field was dated to between the 13th and the 16th Centuries ce and possibly correlates with the 1420 Oei orphan tsunami, that affected the eastern coast of Japan. A magnitude of 8·8 to 9·4 has been estimated for the earthquake, which may be one of the larger events of a super-cycle of earthquakes in the southern Atacama Desert. These cycle-ending earthquakes involve large rupture areas (lengths in excess of 600 km) and highly destructive ocean-wide tsunamigenic events.  相似文献   

Evolution of the star formation rate in galaxies with increasing densities     

John M. Scalo  Enrique C. Vázquez 《Astrophysics and Space Science》1989,156(1-2):275-281

Some bursts of star formation are thought to be associated with situations in which a galaxy's density is increasing. Examples include protogalaxy collapse, mergers, inflow of gas into a galactic nucleus, or accretion of intergalactic gas. We have examined the evolution of the star formation rate (SFR) and other properties of galaxies with increasing density using one-zone cloud fluid equations describing an extension of the Oort cycle, for which the equilibrium state would give an SFR which increases monotonically with density. However, the calculations show that the energy input associated with the density increase generally dominates the evolution, and forces the system far from its normal equilibrium to a state in which cloud collisions are disruptive rather than coalescent. The calculations predict that starbursts associated with collapse, accretion, or inflow events should be preceded by a long incubation period with a very small SFR. For example, the initial star formation burst in a protogalaxy may be delayed for several billion years until nearly all the infalling material has been accreted onto the growing central object.  相似文献   

3D soft-sediment deformation structures: evidence for Quaternary seismicity in the Madrid basin, Spain     

Pablo G. Silva  Juan C. Cañaveras  Sergio Sanchez-Moral  Javier Lario  Enrique Sanz 《地学学报》1997,9(5-6):208-212

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Neural network based daily precipitation generator (NNGEN-P)   总被引：1，自引：0，他引：1  

Jean-Philippe Boulanger  Fernando Martinez  Olga Penalba  Enrique Carlos Segura 《Climate Dynamics》2007,28(2-3):307-324

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation.  相似文献   

Effects of artificial and natural recharge on chemical equilibrium in the Cerro Prieto reservoirs, Baja California, México     

Enrique Portugal  Rosa María Barragn  Jesús de Len 《Journal of Geochemical Exploration》2006,89(1-3):339

Chemical equilibrium modeling studies of two aquifers in the western part of the Cerro Prieto (CP-I) geothermal field were performed to obtain the activities of the main ions and complexes by using the EQQYAC program. The results provided a pH of 6.44 for the deep aquifer, which is slightly higher than the value of 5.5 found for the shallower reservoir. In the northern part of the field, the reservoir pH has decreased up to 1.5 units compared to that modeled for 1990. Variations of pH with respect to time were also found to occur in the southern part of the field. In spite of the large changes in hydrodynamic conditions occurring in the reservoir due to exploitation, the equilibrium between both K- and Na-feldspars and between K-feldspar and K-mica seems to control the chemical composition of the fluids, in both reservoirs.  相似文献