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ABSTRACT Computation of the wavefield due to reflection from an irregular surface is carried out for subsurfaces with large radii of curvature. The Kirchhoff approximation is proved to be sufficiently accurate provided that the acoustic wavelength is sufficiently small with respect to the asperities of the rough surface. For cases where the irregular surface does not fulfil this condition, a series solution is proposed. The first term of this series appears to be the result obtained by conventional Kirchhoff approximation. The series, initially developed in the space–wavenumber domain by Meecham, is transformed into the space–time domain, and the general expression for the series is obtained by calculation of the normal derivative of the field function. The series solution, restricted to the first two terms, is illustrated by application to three synthetic examples. Applications show that the series approximation obtained by the Kirchhoff method contributes significantly to the modelling of narrow, steep and deep structures and consequently it appears that the second term in the series cannot be ignored in the computation of the wavefields arising from a rough surface. 相似文献
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Li Xiao-Feng Blenkinsop Stephen Barbero Renaud Yu Jingjing Lewis Elizabeth Lenderink Geert Guerreiro Selma Chan Steven Li Yafei Ali Haider Villalobos Herrera Roberto Kendon Elizabeth Fowler Hayley J. 《Climate Dynamics》2020,54(11):4823-4839
Climate Dynamics - We investigate the global distribution of hourly precipitation and its connections with the El Niño–Southern Oscillation (ENSO) using both satellite precipitation... 相似文献
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Multiple intertidal bars are common features of wave‐dominated sandy beaches, yet their short‐term (<1 month) and small‐scale (<1 km) morphology and dynamics remain poorly understood. This study describes the morphodynamics of multiple intertidal bars in North Lincolnshire, England, during single and lunar tidal cycles under two contrasting conditions – first when significant wave height was <0·5 m and second when significant wave height frequently exceeded 1 m. The relative importance of swash, surf and shoaling processes in determining morphological change was examined using detailed field observations and a numerical model. The beach featured four intertidal bars and both cross‐shore and longshore bar morphology evolved during the field investigation, particularly under medium to high wave‐energy conditions. Numerical modelling suggests shoaling processes are most common on the seaward two bars under calm wave conditions (Hs < 0·5 m) and that surf zone processes become more common during neap tides and under more energetic (Hs < 0·5 m) conditions. Surf processes dominate the inner two bars, though swash influence increases in a landward direction. The numerical modelling results combined with low tide survey data and high‐resolution morphological measurements strongly suggest changes in the intertidal bar morphology are accomplished by surf zone processes rather than by shoaling wave or swash processes. This is because shoaling waves do not induce significant sediment transport to have any morphological effect, whereas swash action generally does not have enough scope to act as the swash zone is much narrower than the surf zone. It was found, however, that the absolute rate of morphological change under swash action and surfzone processes are of similar magnitudes and that swash action may induce a significant amount of local morphological change when the high tide mark is located on the upper bar, making this process important for bar morphodynamics. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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Crystallisation of sodium sulfate solutions by evaporation under controlled climatic conditions has revealed the existence
of crystalline hydrated sodium sulfate salts not previously reported. The sodium sulfate phase crystallising and the concentration
of the solution at the point of crystallisation depends on the climatic conditions (temperature and evaporation rate). During
the rehydration of the anhydrous sodium sulfate phase, thenardite, another previously unreported phase was formed prior to
the nucleation of the stable phase, mirabilite Na2SO4 · 10H2O. The addition of organic inhibitors changes both the crystallisation and the rehydration behavior in this system. 相似文献
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Atike Nazik Engin Meri? Niyazi Av?ar Selma ��nl�� Vildan Esenli Erkan G?ka?an 《Geo-Marine Letters》2011,31(2):75-86
In this study, the fossil microfaunal composition of 28 Recent sediment samples from Lake İznik and 19 samples from Lake Sapanca
was investigated. The presence of at least ten marine ostracod and 23 foraminifer species in the sediments of Lake İznik,
and at least one marine ostracod and two foraminifer species in the sediments of Lake Sapanca confirm that these lakes must
have been connected to the world ocean sometime during the late Quaternary. The most obvious former link between these lakes
and the Mediterranean Sea would have been via the Marmara Sea with an eastward extension of the Gemlik and İzmit bays. The
proximity of Lake Sapanca to the lower course of the Sakarya River, however, also supports earlier suggestions that there
may have been a temporary connection with the Black Sea via this river valley. Whatever the case, the findings of this study
clearly demonstrate that vertical tectonic movements need to be taken into account when reconstructing the late Quaternary
sea-level history of this region. 相似文献
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We have compared the observed distribution of the quantity log(V
2
2
rP) for a sample of 233 pairs of galaxies with Monte-Carlo simulations. From such an analysis we have derived an average mass-to-luminosity ratiosM/L
B
=18±11. Our result is consistent with a linear increase of the mass with radius at least until distances of about 30 kpc. 相似文献
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Cenk Yalt?rak Umut B. ülgen Cengiz Zabc? Sven Oliver Franz Sena Ak?er ?n Mehmet Sak?n? M. Nam?k ?a?atay Bedri Alpar Kurultay ?ztürk Cemal Tuno?lu Selma ünlü 《Geo-Marine Letters》2012,32(3):267-274
The identification of past connection routes between the Black Sea and the Sea of Marmara, other than the traditional one through to the Bosphorus Strait, would be of considerable interest to the international scientific community. Nazik et al. (Geo-Mar Lett 31:75?C86 (2011) doi:10.1007/s00367-010-0216-9) suggest the possibility of two alternative waterway connections via lakes Sapanca and ?znik. Their Black Sea to Sea of Marmara multi-connection hypothesis, which is based on undated marine fossils collected in both lakes from surficial grab samples, conflicts with many earlier studies. In this contribution, the hypothesis and the underlying data are discussed in the light of previous tectonic, sedimentological and limnological findings showing that it is impossible to have had marine connections through lakes Sapanca and ?znik during the last 11.5?ka. Global sea-level trends and tectonic uplift rates would accommodate a connection between the Sea of Marmara and Lake ?znik in the middle Pleistocene. Uplift rates for the northern block of the North Anatolian Fault, when compared with the global sea-level curve, clearly indicate that there cannot have been a connection through the ?zmit Gulf?CLake Sapanca?CSakarya Valley for at least the past 500?ka. Moreover, borehole sediments along the western shores of Lake Sapanca, which reach down to the bedrock, do not contain any marine fossils. 相似文献