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1.
Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect this region to deeper waters. Two proposed ways in which across-slope BBL fluxes can occur, in regions that have a slope current aligned to the bathymetry, are the frictional veering of bottom currents termed the ‘Ekman drain’ and through local wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom flow). We investigate the variability, magnitude and spatial scale of BBL fluxes on the Shetland shelf, which has a prominent slope current, using a high-resolution (~2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at the shelf break near the 200 m isobath and are found to have a seasonal variability with high/low volume transport in winter/summer respectively. By using a multivariate regression approach, we find that the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus mean-flow flux. We estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow, which has a much shorter decorrelation length compared to the Ekman flux. We conclude that both the speed and direction of the interior current determines the daily BBL flux. The wind does not explicitly contribute through local downwelling, but may influence the interior current and therefore implicitly the BBL fluxes on longer timescales. 相似文献
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《Continental Shelf Research》1999,19(15-16):1851-1867
To investigate the instabilities of steady and oscillating Ekman layers, an 8 m×2 m horizontal plate was moved at controlled speed in homogeneous water at rest in solid body rotation in the “Coriolis” 13 m diameter rotating tank. For a steady Ekman layer two distinct wave types were found, in agreement with previous experimental or numerical studies. Type I was stationary, was oriented positively with respect to the flow direction and had a wavelength of about 10 times the Ekman layer thickness. Type II was oriented negatively with respect to the flow direction and had a wavelength which was more than 20 times the Ekman layer thickness and a phase-speed between 0.3 and 0.5 the forcing interior velocity. The growth rates of both type I and type II waves for various Reynolds numbers Re (computed with the Ekman layer thickness) were estimated and their Re-variations qualitatively agree with previous numerical results. For an oscillating Ekman layer, experimental results depended strongly on Rot, the temporal Rossby number: only when Rot<1 was it possible to observe either type I or type II instabilities. Moreover, for all Rot and average to high Re, there was a noticeable upward turbulent transport occurring during each cycle between the flow maximum and the flow reversal. Such an upward turbulent transport is consistent with observations in the English Channel where maximum upward benthic movements and maximum turbidity were recorded at the flow reversal, hence Ekman layer instabilities and transition to turbulence are likely to occur in shallow tidal seas where they may be relevant for sediment resuspension and transport as well as for some biological processes. 相似文献
4.
Jun Iyama 《地震工程与结构动力学》2005,34(15):1799-1815
The response of an elasto‐plastic single degree of freedom (SDOF) system to ground motion is estimated based on wavelet coefficients calculated by discrete wavelet transform. Wavelet coefficients represent both the time and frequency characteristics of input ground motion, and thus can be considered to be directly related to the dynamic response of a non‐linear system. This relationship between the energy input into an elastic SDOF system and wavelet coefficients is derived based on the assumption that wavelets deliver energy to the structure instantaneously and the quantity of energy is constant regardless of yielding. These assumptions are shown to be valid when the natural period of the system is in the predominant period range of the wavelet, the most common scenario for real structures, through dynamic response analysis of a single wavelet. The wavelet‐based estimation of elastic and plastic energy transferred by earthquake ground motion is thus shown to be in good agreement with the dynamic response analysis when the natural period is in the predominant range of the input. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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The object of this paper is to introduce a procedure for the determination of elastic design earthquake input energy spectra taking into account the influence of magnitude, soil type and distance from the surface projection of the fault. Firstly, an accurate selection of a large set of representative records has been realized. Secondly, the construction of the design input energy spectra has required determining the spectral shapes and a normalization factor which measures seismic hazard in terms of energy. This factor, denoted as the seismic hazard energy factor, has been defined as the area under the earthquake input energy spectrum in the period interval between 0·05 and 4·0 s. Finally, due to the importance of the source-to-site distance in the evaluation of the input energy, an investigation into the attenuation of the seismic hazard energy factor has been carried out. © 1998 John Wiley & Sons, Ltd. 相似文献
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A computational method of energy evaluation is derived to study the elastic responses and energy distribution of actively controlled single‐degree‐of‐freedom (SDOF) structures during earthquakes. Contrary to the common perception that applying active control force pumps energy into the structure, the applied control force can actually reduce the energy in the structure by reducing the input energy from earthquakes to the structure. In addition, applying control force can dissipate a large amount of energy in the structure when this control force is applied in the direction opposite to the displacement and velocity responses. To demonstrate this energy mechanism in active controlled structures, the two most popular control algorithms, optimal linear control (OLC) and instantaneous optimal control (IOC) algorithms, are used to calculate the control response and energy spectra. One‐step time delay is incorporated into the algorithms to take into consideration the practical aspect of active control. The effects of different earthquakes and damping ratios on control energy and response spectra are studied. These studies show that both OLC and IOC are very effective in reducing the structural displacement and velocity responses by reducing the input earthquake energy as well as dissipating a large amount of energy in the structure. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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Adaptive neuro-fuzzy evaluation of wind farm power production as function of wind speed and direction 总被引:1,自引:1,他引:0
Dalibor Petković Shahaboddin Shamshirband Nor Badrul Anuar Sareh Naji Miss Laiha Mat Kiah Abdullah Gani 《Stochastic Environmental Research and Risk Assessment (SERRA)》2015,29(3):793-802
Wind velocity assumes a critical part for measuring the power created by the wind turbines. Nonetheless, power production from wind has a few weaknesses. One significant issue is that wind is a discontinuous energy source which implies that there exists substantial variability in the generation of vigor because of different variables, for example, wind speed. Wind direction is a significant variable for proficient turbine control for getting the most energy with a given wind speed. Taking into account the conjectures on wind heading, it might be conceivable to adjust the turbine to the wind bearing to get the most energy yield. Since both forecasts of wind speed and direction are basic for effective wind energy collecting it is crucial to develop a methodology for estimation of wind speed and direction and afterwards to estimate wind farm power production as function of wind pace and heading distribution. Despite the fact that various numerical functions have been proposed for demonstrating the wind speed and direction frequency distribution, there are still disadvantages of the models like very demanding in terms of calculation time. In this investigation adaptive neuro-fuzzy inference system (ANFIS), which is a particular sort of the artificial neural networks (ANN) family, was used to anticipate the wind speed and direction frequency dispersion. Thereafter, the ANFIS system was utilized to gauge wind homestead power creation as function of wind velocity and bearing. Neural system in ANFIS modifies parameters of enrollment capacity in the fuzzy logic of the fuzzy inference system. The reenactment outcomes exhibited in this paper demonstrate the adequacy of the created technique. 相似文献
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将理想化的南中国海海盆在垂直方向上划分为Ekman层、惯性层和摩擦层. Ekman层中的运动由大气风应力驱动,其底部的扰动压力将作为其下惯性层中运动的上边界条件. 惯性层中的运动是由f 平面三维非线性方程在准地转近似下位势涡度守恒控制,由此得到控制惯性层中运动关于扰动压力的三维椭圆型方程. 在惯性层以下考虑到深层的海盆水平尺度很小,由此引进带有底部摩擦的线性控制方程,方程的边界条件为惯性层和摩擦层交界面上的扰动压力连续,沿海盆边界假定海水与相邻的固壁间无热量交换,由此设在海盆边界上扰动温度为零. 在此基础上分别利用惯性层和摩擦层中的椭圆型控制方程计算了相应层次上冬、夏季的扰动压力和准地转流. 结果表明冬季各层上以气旋式环流为主,且随深度的增加流速减小;夏季各层上以反气旋式环流为主,流速也随深度增加而减小. 这在一定程度上和观测事实相符. 相似文献
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This paper presents qualitative investigations on the energy behaviour of structures into which hysteretic dampers are incorporated. Emphasis was given to the ratio of the structural stiffness after the yielding of hysteretic dampers to the initial elastic stiffness, with a premise that this ratio, termed α in this study, tends to be large for structures with hysteretic dampers. Structures concerned were represented by discrete spring–mass systems having bilinear restoring force behaviour, in which the second stiffness relative to the initial stiffness is α. It was found that with the increase of α the total input energy tends to increase, but the increase is confined to a narrow range of natural periods. Both the total input energy and hysteretic energy were found to become less sensitive to the yield strength with the increase of α. A simple formula was also proposed to estimate the maximum deformation given the knowledge of the hysteretic energy. Analysis of MDOF systems revealed that, even when α is large, the total input energy and hysteretic energy for MDOF systems are approximately the same as those of the equivalent SDOF system, and the hysteretic energy can be distributed uniformly over the stories if α is large. 相似文献
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《Journal of Atmospheric and Solar》2008,70(5):742-755
Since gravity waves significantly influence the atmosphere by transporting energy and momentum, it is important to study their wave spectrum and their energy dissipation rates. Besides that, knowledge about gravity wave sources and the propagation of the generated waves is essential. Originating in the lower atmosphere, gravity waves can move upwards; when the background wind field is equal to their phase speed a so-called critical layer is reached. Their breakdown and deposition of energy and momentum is possible. Another mechanism which can take place at critical layers is gravity wave reflection.In this paper, gravity waves which were observed by foil chaff measurements during the DYANA (DYnamics Adapted Network for the Atmosphere) campaign in 1990 in Biscarrosse (44°N, 1°W)—as reported by Wüst and Bittner [2006. Non-linear wave–wave interaction: case studies based on rocket data and first application to satellite data. Journal of Atmospheric and Solar-Terrestrial Physics 68, 959–976]—are investigated to look for gravity wave reflection processes. Following nonlinear theory, energy dissipation rates according to Weinstock [1980. Energy dissipation rates of turbulence in the stable free atmosphere. Journal of the Atmospheric Sciences 38, 880–883] are calculated from foil chaff cloud and falling sphere data and compared with the critical layer heights. Enhanced energy dissipation rates are found at those altitudes where the waves’ phase speed matches the zonal background wind speeds. Indication of gravity wave trapping is found between two altitudes of around 95 and 86 km. 相似文献
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Gaetano Manfredi 《地震工程与结构动力学》2001,30(4):485-499
In this paper, a method is proposed in order to obtain a simplified representation of hysteretic and input energy spectra. The method is based on the evaluation of the equivalent number of cycles correlated to the earthquake characteristics by the proposed seismic index ID. This procedure allows us to obtain peak values of the hysteretic and input energy that depend on the demanded ductility, on the seismic index ID and on the peak pseudo‐velocity. The assessment of the input energy represents a first step towards the definition of a damage potential index capable of taking into account the effect of the duration of the ground motions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
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太湖波浪数值模拟 总被引:8,自引:7,他引:1
在太湖实际波浪观测的基础上,采用率定验证后的第三代动谱平衡方程,考虑实际水底地形、波浪折射、浅化、反射、破碎、湖流等条件下,对太湖波浪进行数值模拟,研究太湖波浪的主要影响因素.结果表明:SWAN模型可以较好地模拟风作用下太湖风浪的生成和传播过程,模型在太湖应用是合适的;波高、波长、波周期等波浪参数在太湖的分布与风速、风向、水深等因素密切相关;在相同风向、不同风速情况下,太湖波浪发展至稳定状态的时间不一样;在不同风向,相同风速持续作用下,有效波高达到稳定的时间差不多,变化趋势也比较相同.说明波浪的发展不光取决于风速的大小,还同风的持续吹的时间和风区长度有关. 相似文献
14.
Wei Shen Klaus-Werner Gurgel George Voulgaris Thomas Schlick Detlef Stammer 《Ocean Dynamics》2012,62(1):105-121
Land-based high-frequency (HF) radars have the unique capability of continuously monitoring ocean surface environments at
ranges up to 200 km off the coast. They provide reliable data on ocean surface currents and under slightly stricter conditions
can also give information on ocean waves. Although extraction of wind direction is possible, estimation of wind speed poses
a challenge. Existing methods estimate wind speed indirectly from the radar derived ocean wave spectrum, which is estimated
from the second-order sidebands of the radar Doppler spectrum. The latter is extracted at shorter ranges compared with the
first-order signal, thus limiting the method to short distances. Given this limitation, we explore the possibility of deriving
wind speed from radar first-order backscatter signal. Two new methods are developed and presented that explore the relationship
between wind speed and wave generation at the Bragg frequency matching that of the radar. One of the methods utilizes the
absolute energy level of the radar first-order peaks while the second method uses the directional spreading of the wind generated
waves at the Bragg frequency. For both methods, artificial neural network analysis is performed to derive the interdependence
of the relevant parameters with wind speed. The first method is suitable for application only at single locations where in
situ data are available and the network has been trained for while the second method can also be used outside of the training
location on any point within the radar coverage area. Both methods require two or more radar sites and information on the
radio beam direction. The methods are verified with data collected in Fedje, Norway, and the Ligurian Sea, Italy using beam
forming HF WEllen RAdar (WERA) systems operated at 27.68 and 12.5 MHz, respectively. The results show that application of
either method requires wind speeds above a minimum value (lower limit). This limit is radar frequency dependent and is 2.5
and 4.0 m/s for 27.68 and 12.5 MHz, respectively. In addition, an upper limit is identified which is caused by wave energy
saturation at the Bragg wave frequency. Estimation of this limit took place through an evaluation of a year long database
of ocean spectra generated by a numerical model (third generation WAM). It was found to be at 9.0 and 11.0 m/s for 27.68 and
12.5 MHz, respectively. Above this saturation limit, conventional second-order methods have to be applied, which at this range
of wind speed no longer suffer from low signal-to-noise ratios. For use in operational systems, a hybrid of first- and second-order
methods is recommended. 相似文献
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对风速与风向边缘分布采用统一的极值概型描述,提出了一种可适用于多峰极值以及总体样本的风速风向联合概率分布函数的经验解析表达式。模型包括7个参数,可由实测数据利用非线性最小二乘方法拟合得到。对模型参数拟合时的初值选取方法提出了建议,并对典型的风向双峰值情况,给出了峰向区间的划分方法;利用双峰总体、双峰极值以及单峰极值3种不同类型的实测数据,检验了模型的适用性。结果表明,该模型可以较好地描述不同类型总体样本或极值样本的风速风向联合概率密度特性,可供风向设计风速的确定、风速评估及场地风能评估等工程问题参考。 相似文献
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通过对剪切型多自由度体系的地震能量反应分析,提出了描述多自由度体系整体脉冲能量反应的参数,瞬时输入能量等效平均速度,并在此基础上建立了瞬时输入能量分布系数的概念,提出了估计瞬时输入能量分布的方法。 相似文献
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This paper proposes energy input spectra applicable to seismic design of structures located in low‐to‐moderate‐seismicity regions. These spectra represent the load effect, in terms of input energy, of the most severe earthquake that the construction might encounter during its lifetime. The spectra have been derived through dynamic response analyses of over 100 ground motion records obtained from 48 earthquakes that have occurred in Spain. An empirical equation for estimating the energy input contributable to damage from the total input energy is also suggested. This equation takes into account both the damping and the degree of plastification of the structure. Finally, the proposed design energy input spectra are compared with the provisions of the current Spanish Seismic Code and with the response spectra of recent earthquakes that have occurred in Turkey and Taiwan. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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本文利用中国科学院空间科学与应用研究中心的瑞利激光雷达首次观测到了平流层地形重力波活动的现象,并结合美国国家环境预报中心(NCEP)的全球预报系统(GFS)的风场数据分析了该地形重力波的基本参数.与惯性重力波相比较,地形重力波的密度扰动没有下传的相位,在同一高度上,其扰动相位保持不变.北京空间科学与应用研究中心瑞利激光雷达自2012年开始观测实验以来,已经观测到多起地形重力波活动事件.本文以2013年11月11日的观测数据为例,研究北京上空的地形重力波活动,并结合GFS风场数据分析了北京上平流层地形重力波的波长、传播方向、传播速度等参量.通过分析得到在2013年11月11日北京上空存在一列传播方向为北偏西52.4°,水平波长为5.5km,平均垂直波长约为6.0km的地形重力波. 相似文献
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Interaction of waves,currents and tides,and wave-energy impact on the beach area of Sylt Island 总被引:3,自引:1,他引:2
Erosion due to waves is an important and actual problem for most coastal areas of the North Sea. The objective of this study
was to estimate the impact of wave action on the coastline of Sylt Island. From a 2-year time series (November 1999 to October
2001) of hydrological and wave parameters generated with a coupled wave–current modelling system, a period comprising storm
‘Anatol’ (3–4 December 1999) is used to investigate the effects of waves on currents and water levels and the input of wave
energy into the coastline. The wave-induced stress causes an increase of the current velocity of 1 m/s over sand and an additional
drift along the coast of about 20 cm/s. This produces a water level increase of more than 20 cm in parts of the tidal basin.
The model system also calculates the wave energy input into the coastline. Scenario runs for December 1999 with a water level
increase of 50 cm and wind velocity increased by 10% show that the input of the wave energy into the west coast of Sylt Island
increases by 30% compared to present conditions. With regard to the forecasted near-future (Woth et al., Ocean Dyn 56:3–15,
2006) increase of strong storm surges, the scenario results indicate an increased risk of coastal erosion in the surf zone of
Sylt Island. 相似文献