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1.
Armando L. Brinca 《Planetary and Space Science》1977,25(9):879-885
Analysis of the modifications introduced in a turbulent whistler noise spectrum with the injection of a coherent whistler leads to a nonlinear dispersion equation for the stochastic modes. These modes are submitted to real frequency shifts and corrections to their growth rates which are in qualitative agreement with observations made in the Siple Station VLF wave injection experiment showing the creation of noise-free bands when CW whistler modes are transmitted. 相似文献
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Residual Exchange Flows in Subtropical Estuaries 总被引:1,自引:0,他引:1
Arnoldo Valle-Levinson Guillermo Gutierrez de Velasco Armando Trasviña Alejandro J. Souza Reginaldo Durazo Ashish J. Mehta 《Estuaries and Coasts》2009,32(1):54-67
Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that
these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every
estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents
were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and
outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density
gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that
drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves.
Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in
such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides.
This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange
flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous
theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings
advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual
flows are mainly driven by tides. 相似文献
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This paper reports improvements to algorithms for the simulation of 3-D hydraulic fracturing with the Generalized Finite Element Method (GFEM). Three optimizations are presented and analyzed. First, an improved initial guess based on solving a 3-D elastic problem with the pressure from the previous step is shown to decrease the number of Newton iterations and increase robustness. Second, an improved methodology to find the time step that leads to fracture propagation is proposed and shown to decrease significantly the number of iterations. Third, reduced computational cost is observed by properly recycling the linear part of the coupled stiffness matrix. Two representative examples are used to analyze these improvements. Additionally, a methodology to include the leak-off term is presented and verified against asymptotic analytical solutions. Conservation of mass is shown to be well satisfied in all examples. 相似文献
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Stephen Foster Antonio Pulido-Bosch Ángela Vallejos Luis Molina Armando Llop Alan M. MacDonald 《Hydrogeology Journal》2018,26(8):2781-2791
Intensive irrigated agriculture substantially modifies the hydrological cycle and often has major environmental impacts. The article focuses upon a specific concern—the tendency for progressive long-term increases in the salinity of groundwater recharge derived from irrigated permeable soils and replenishment of unconfined aquifers in more arid regions. This process has received only scant attention in the water-resource literature and has not been considered by agricultural science. This work makes an original contribution by analysing, from scientific principles, how the salinisation of groundwater recharge arises and identifies the factors affecting its severity. If not proactively managed, the process eventually will impact irrigation waterwell salinity, the productivity of agriculture itself, and can even lead to land abandonment. The types of management measure required for mitigation are discussed through three detailed case histories of areas with high-value groundwater-irrigated agriculture (in Spain, Argentina and Pakistan), which provide a long-term perspective on the evolution of the problem over various decades. 相似文献
10.
Pietro Catania Luigi Badalucco Vito Armando Laudicina Mariangela Vallone 《Environmental Earth Sciences》2018,77(9):348
Tillage, especially in semiarid Mediterranean environment, enhances the mineralization process of soil organic matter (SOM) and, in turn, decreases aggregate stability. Furthermore, continuous tillage leads to the formation of plough pan beneath the tilled layer. In the present study, we investigated the effect of an innovative self-propelled machine (spading machine, SM) for shallow tillage on SOM, water stable aggregates (WSA) and soil penetration resistance (PR). Such effects were compared to those of chisel plough (CP), rotary tiller (RT) and no tillage (NT). Each tilling method was applied up to a depth of 15 cm, whereas in NT only a brush cutter was used for weed control. Soil analyses were performed at the start of the experiment (March 2009, T0), in April 2010 (T1), May 2012 (T3), and June 2014 (T5) at both 0–15 and 15–30 cm. Compared to T0, soil PR increased with time in all the treatments and generally followed the order SM?<?RT?<?CP?<?NT. In soil tilled with the SM, soil PR never exceeded 2.5 MPa that was demonstrated to be a critical value for root elongation, and no evidence of the formation of plough pan beneath the tilled layer was observed. SOC as well as water content and WSA were higher in SM compared with CP and RT. In conclusion, the spading machine was proved to be more efficient in lowering the soil PR and in avoiding the formation of the plough pan. Furthermore, SM increased SOC and WSA. 相似文献