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1
Elgar, Steve, e Britt Raubenheimer. "Field Evidence of Inverse Energy Cascades in the Surfzone". Journal of Physical Oceanography 50, n. 8 (1 agosto 2020): 2315–21. http://dx.doi.org/10.1175/jpo-d-19-0327.1.
Testo completoAbstract (sommario):
AbstractLow-frequency currents and eddies transport sediment, pathogens, larvae, and heat along the coast and between the shoreline and deeper water. Here, low-frequency currents (between 0.1 and 4.0 mHz) observed in shallow surfzone waters for 120 days during a wide range of wave conditions are compared with theories for generation by instabilities of alongshore currents, by ocean-wave-induced sea surface modulations, and by a nonlinear transfer of energy from breaking waves to low-frequency motions via a two-dimensional inverse energy cascade. For these data, the low-frequency currents are not strongly correlated with shear of the alongshore current, with the strength of the alongshore current, or with wave-group statistics. In contrast, on many occasions, the low-frequency currents are consistent with an inverse energy cascade from breaking waves. The energy of the low-frequency surfzone currents increases with the directional spread of the wave field, consistent with vorticity injection by short-crested breaking waves, and structure functions increase with spatial lags, consistent with a cascade of energy from few-meter-scale vortices to larger-scale motions. These results include the first field evidence for the inverse energy cascade in the surfzone and suggest that breaking waves and nonlinear energy transfers should be considered when estimating nearshore transport processes across and along the coast.
2
Fioranelli, Massimo, Maria Grazia Roccia, Aroonkumar Beesham, Dana Flavin, M. Ghaeni e Faissal AZIZ. "A model for considering effects of extremely low frequency electromagnetic fields on quail embryonic cells". AIMS Biophysics 9, n. 3 (2022): 198–207. http://dx.doi.org/10.3934/biophy.2022017.
Testo completoAbstract (sommario):
<abstract> <p>Previous experiments have shown that extremely low frequency electromagnetic fields could cause serious effects on the evolution of cells. We propose a mathematical model which confirms those results. In our model, electromagnetic waves could cause the motions of ions and charges and the emergence of some currents around and in the interior of cells. These currents produce some waves which interact with the DNAs and remove or attach some repressors. Consequently, some genes could be turned on or off, and cells could obtain some properties or lose them. The frequency of the external waves should be close to the frequency of the exchanged waves between the repressors and DNAs or even bigger than them. We test this idea and did some experiments on quail embryonic cells. We connected a sample of these cells to a battery and considered their evolution. We observed that after connecting the battery and the production of electrical current, some rings around the quail embryonic cells emerged. Maybe, these rings are the response of the cells to changes in electromagnetic waves and electrical currents.</p> </abstract>
3
Johnson, E. R. "Boundary Currents, Free Currents and Dissipation in the Low-Frequency Scattering of Shelf Waves". Journal of Physical Oceanography 19, n. 9 (settembre 1989): 1291–300. http://dx.doi.org/10.1175/1520-0485(1989)019<1291:bcfcad>2.0.co;2.
Testo completo
4
Torres, Hector S., Patrice Klein, Jinbo Wang, Alexander Wineteer, Bo Qiu, Andrew F. Thompson, Lionel Renault et al. "Wind work at the air-sea interface: a modeling study in anticipation of future space missions". Geoscientific Model Development 15, n. 21 (7 novembre 2022): 8041–58. http://dx.doi.org/10.5194/gmd-15-8041-2022.
Testo completoAbstract (sommario):
Abstract. Wind work at the air-sea interface is the transfer of kinetic energy between the ocean and the atmosphere and, as such, is an important part of the ocean-atmosphere coupled system. Wind work is defined as the scalar product of ocean wind stress and surface current, with each of these two variables spanning, in this study, a broad range of spatial and temporal scales, from 10 km to more than 3000 km and hours to months. These characteristics emphasize wind work's multiscale nature. In the absence of appropriate global observations, our study makes use of a new global, coupled ocean-atmosphere simulation, with horizontal grid spacing of 2–5 km for the ocean and 7 km for the atmosphere, analyzed for 12 months. We develop a methodology, both in physical and spectral spaces, to diagnose three different components of wind work that force distinct classes of ocean motions, including high-frequency internal gravity waves, such as near-inertial oscillations, low-frequency currents such as those associated with eddies, and seasonally averaged currents, such as zonal tropical and equatorial jets. The total wind work, integrated globally, has a magnitude close to 5 TW, a value that matches recent estimates. Each of the first two components that force high-frequency and low-frequency currents, accounts for ∼ 28 % of the total wind work and the third one that forces seasonally averaged currents, ∼ 44 %. These three components, when integrated globally, weakly vary with seasons but their spatial distribution over the oceans has strong seasonal and latitudinal variations. In addition, the high-frequency component that forces internal gravity waves, is highly sensitive to the collocation in space and time (at scales of a few hours) of wind stresses and ocean currents. Furthermore, the low-frequency wind work component acts to dampen currents with a size smaller than 250 km and strengthen currents with larger sizes. This emphasizes the need to perform a full kinetic budget involving the wind work and nonlinear advection terms as small and larger-scale low-frequency currents interact through these nonlinear terms. The complex interplay of surface wind stresses and currents revealed by the numerical simulation motivates the need for winds and currents satellite missions to directly observe wind work.
5
Berthelier, A., J. C. Cerisier, J. J. Berthelikr e L. Rnzeau. "Low-frequency magnetic turbulence in the high-latitude topside ionosphere: low-frequency waves or field-aligned currents". Journal of Atmospheric and Terrestrial Physics 53, n. 3-4 (marzo 1991): 333–41. http://dx.doi.org/10.1016/0021-9169(91)90117-p.
Testo completo
6
Matsumoto, Michio, Kazuo Sakai e Satoshi Takeuchi. "Low Frequency Waves with Azimuthal Currents in a Weakly Magnetized Positive Column". Journal of the Physical Society of Japan 61, n. 3 (15 marzo 1992): 844–48. http://dx.doi.org/10.1143/jpsj.61.844.
Testo completo
7
Yao, S. T., Q. Q. Shi, Q. G. Zong, A. W. Degeling, R. L. Guo, L. Li, J. X. Li et al. "Low-frequency Whistler Waves Modulate Electrons and Generate Higher-frequency Whistler Waves in the Solar Wind". Astrophysical Journal 923, n. 2 (1 dicembre 2021): 216. http://dx.doi.org/10.3847/1538-4357/ac2e97.
Testo completoAbstract (sommario):
Abstract The role of whistler-mode waves in the solar wind and the relationship between their electromagnetic fields and charged particles is a fundamental question in space physics. Using high-temporal-resolution electromagnetic field and plasma data from the Magnetospheric MultiScale spacecraft, we report observations of low-frequency whistler waves and associated electromagnetic fields and particle behavior in the Earth’s foreshock. The frequency of these whistler waves is close to half the lower-hybrid frequency (∼2 Hz), with their wavelength close to the ion gyroradius. The electron bulk flows are strongly modulated by these waves, with a modulation amplitude comparable to the solar wind velocity. At such a spatial scale, the electron flows are forcibly separated from the ion flows by the waves, resulting in strong electric currents and anisotropic ion distributions. Furthermore, we find that the low-frequency whistler wave propagates obliquely to the background magnetic field ( B 0), and results in spatially periodic magnetic gradients in the direction parallel to B 0. Under such conditions, large pitch-angle electrons are trapped in wave magnetic valleys by the magnetic mirror force, and may provide free perpendicular electron energy to excite higher-frequency whistler waves. This study offers important clues and new insights into wave–particle interactions, wave generation, and microscale energy conversion processes in the solar wind.
8
Deguchi, Ichiro, Mamoru Arita e Takumi Yoshii. "FLUCTUATION OF RIP CURRENT MEASURED IN SHALLOW WATER REGION WITH SMALL TIDAL RANGE". Coastal Engineering Proceedings 1, n. 32 (25 gennaio 2011): 44. http://dx.doi.org/10.9753/icce.v32.currents.44.
Testo completoAbstract (sommario):
Disappearance and formation processes of rip channel are discussed based on the field measurements of wave height, current velocity, bottom topography and flow pattern of near-shore current. Sudden increase in wave height together with the change in the wave direction took place during a half day caused these phenomena and rip current rose and fell according to the transition of the bottom topography. Furthermore, flow pattern of rip current was not steady but transformed itself with low frequency fluctuations of the period of few minutes. It is found that such low frequency fluctuations are caused by the intrinsic fluctuations of the incident waves (grouping waves) through the numerical simulations.
9
Stepanov, D. V., e V. V. Novotryasov. "Sub-inertial modulation of nonlinear Kelvin waves in the coastal zone". Nonlinear Processes in Geophysics 20, n. 3 (7 giugno 2013): 357–64. http://dx.doi.org/10.5194/npg-20-357-2013.
Testo completoAbstract (sommario):
Abstract. Observational evidence is presented for interaction between nonlinear internal Kelvin waves at the ωt,i (where the ωt is the semidiurnal frequency and the ωi is the inertial frequency) and random oscillations of the background coastal current at the sub-inertial Ω frequency in the Japan/East Sea. Enhanced coastal currents at the sum ω+ and difference ω-frequencies ω±=ωt,i ± Ω have properties of propagating Kelvin waves, which suggests permanent energy exchange from the sub-inertial band to the mesoscale ω± band. This interaction may be responsible for a greater-than-predicted intensification, steepening and breaking of boundary-trapped Kelvin waves. The problem of interaction between the nonlinear Kelvin wave at the frequency ω and the low-frequency narrowband noise with representative frequency Ω&ll;ω is investigated using the theory of nonlinear weak dispersion waves.
10
Van Dongeren, Ap, Ryan Lowe, Andrew Pomeroy, Trang Minh Duong, Dano Roelvink, Graham Symonds e Roshanka Ranasinghe. "MODELLING INFRAGRAVITY WAVES AND CURRENTS ACROSS A FRINGING CORAL REEF". Coastal Engineering Proceedings 1, n. 33 (15 dicembre 2012): 29. http://dx.doi.org/10.9753/icce.v33.currents.29.
Testo completoAbstract (sommario):
Low-frequency (infragravity) wave dynamics on a fringing coral reef were investigated using the numerical model XBeach (Roelvink et al, 2009). First, the skill of the one-dimensional model was evaluated based on its predictions of short waves (0.04-0.2 Hz), infragravity waves (0.004-0.04 Hz) and water level measurements (tidal and wave setup) obtained during a 2009 field study at Ningaloo Reef in Western Australia. The model calibration was sensitive to friction coefficients for short waves and current / infragravity bed friction, which were assumed independent in this model study. The infragravity waves were found to be generated primarily in the surf zone through the breakpoint generation mechanism rather than through offshore forcing. The infragravity waves were strongly also modulated over the reef by tidal depth variations, primarily due to the variability in frictional dissipation rates when the total water depth over the reef varied. The results reveal that short waves dominated bottom stresses on the fore reef and near the reef crest; however, inside the lagoon, infragravity waves become increasingly dominant, accounting up to 50% of the combined bottom stresses
11
Lamy, L., P. Zarka, B. Cecconi, R. Prangé, W. S. Kurth, G. Hospodarsky, A. Persoon, M. Morooka, J. E. Wahlund e G. J. Hunt. "The low-frequency source of Saturn’s kilometric radiation". Science 362, n. 6410 (4 ottobre 2018): eaat2027. http://dx.doi.org/10.1126/science.aat2027.
Testo completoAbstract (sommario):
Understanding how auroral radio emissions are produced by magnetized bodies requires in situ measurements within their source region. Saturn’s kilometric radiation (SKR) has been widely used as a remote proxy of Saturn’s magnetosphere. We present wave and plasma measurements from the Cassini spacecraft during its ring-grazing high-inclination orbits, which passed three times through the high-altitude SKR emission region. Northern dawn-side, narrow-banded radio sources were encountered at frequencies of 10 to 20 kilohertz, within regions of upward currents mapping to the ultraviolet auroral oval. The kilometric waves were produced on the extraordinary mode by the cyclotron maser instability from 6– to 12–kilo–electron volt electron beams and radiated quasi-perpendicularly to the auroral magnetic field lines. The SKR low-frequency sources appear to be strongly controlled by time-variable magnetospheric electron densities.
12
Richter, I., C. Koenders, H. U. Auster, D. Frühauff, C. Götz, P. Heinisch, C. Perschke et al. "Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko". Annales Geophysicae 33, n. 8 (19 agosto 2015): 1031–36. http://dx.doi.org/10.5194/angeo-33-1031-2015.
Testo completoAbstract (sommario):
Abstract. We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.
13
Sanchez-Arriaga, G., e J. R. Sanmartin. "Fast magnetosonic wave excitation by an array of wires with time-modulated currents". Annales Geophysicae 28, n. 2 (18 febbraio 2010): 577–86. http://dx.doi.org/10.5194/angeo-28-577-2010.
Testo completoAbstract (sommario):
Abstract. The excitation of Fast Magnetosonic (FMS) waves by a cylindrical array of parallel tethers carrying time-modulated current is discussed. The tethers would fly vertical in the equatorial plane, which is perpendicular to the geomagnetic field when its tilt is ignored, and would be stabilized by the gravity gradient. The tether array would radiate a single FMS wave. In the time-dependent background made of geomagnetic field plus radiated wave, plasma FMS perturbations are excited in the array vicinity through a parametric instability. The growth rate is estimated by truncating the evolution equation for FMS perturbations to the two azimuthal modes of lowest order. Design parameters such as tether length and number, required power and mass are discussed for Low Earth Orbit conditions. The array-attached wave structure would have the radiated wave controlled by the intensity and modulation frequency of the currents, making an active experiment on non-linear low frequency waves possible in real space plasma conditions.
14
Lepidi, S., L. Cafarella, P. Francia, A. Meloni, P. Palangio e J. J. Schott. "Low frequency geomagnetic field variations at Dome C (Antarctica)". Annales Geophysicae 21, n. 4 (30 aprile 2003): 923–32. http://dx.doi.org/10.5194/angeo-21-923-2003.
Testo completoAbstract (sommario):
Abstract. We conduct an analysis of the geomagnetic field variations recorded at the new Antarctic station Dome C, located very close to the geomagnetic pole, which has been operating for approximately one month during the 1999–2000 campaign. We also perform a comparison with simultaneous measurements at the Italian Antarctic station Terra Nova Bay, in order to investigate the spatial extension of the phenomena observed at very high latitude. Our results show that between the two stations the daily variation is similar and the fluctuations with f ~ 1 mHz are coherent, provided that in both cases the comparison is made between geographically oriented components, suggesting that ionospheric currents related to the geographic position, more than field-aligned currents, are responsible for the lowest frequency variations; conversely, higher frequency (Pc5) fluctuations are substantially decoupled between the two stations. We also found that at Dome C the fluctuation power in the 0.55–6.7 mHz frequency band is well related with the solar wind speed during the whole day and that at Terra Nova Bay the correlation is also high, except around local geomagnetic noon, when the station approaches the polar cusp. These results indicate that the solar wind speed control of the geomagnetic field fluctuation power is very strict in the polar cap and less important close to the polar cusp.Key words. Magnetospheric physics (MHD waves and instabilities; Polar cap phenomena; Solar wind-magnetosphere interactions)
15
Guerrero, Anlly Melissa, Luis Otero, Silvio Ospino e Jairo Cueto. "Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach". Journal of Marine Science and Engineering 12, n. 7 (6 luglio 2024): 1141. http://dx.doi.org/10.3390/jmse12071141.
Testo completoAbstract (sommario):
This study aims to investigate the hydrodynamic-morphological interactions on a microtidal intermediate-dissipative beach under low to moderate wave energy conditions using field measurements during two climatic seasons. The separate contributions of currents, sea-swell waves, and infragravity waves to high- and low-frequency sediment fluxes were analyzed. The infragravity wave energy was more relevant near the swash zone than in other areas. Although the currents are the primary suspended sediment transport mechanism, the results suggest that the waves are an important driver of sediment suspension from the seabed. The results indicate that Sea-Swell (SS) waves and cross-shore currents are the prevailing hydrodynamic factors in nearshore sediment transport, and the cross-shore suspended sediment transport rates are higher than those in alongshore transport. The submerged bar intensified during the wet season (1–4 November 2018) when the wave height intensities were lower, contrary to the dry season (24–25 March 2018). Significant accretion nearshore was identified (in the subaerial beach) during the wet season when the suspended sediments were greater, the SS-wave heights nearshore were lower, and sediment flux was directed onshore. A notorious erosion was distinguished during the dry season. The most representative volume changes occurred during the dry season (with high erosion), which is attributed to the high SS-wave energy.
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Sogut, Erdinc, e Ali Farhadzadeh. "ALONGSHORE VARIABILITY OF COASTAL MORHODYNAMICS IN EASTERN LAKE ERIE DUE TO LOW FREQUENCY OSCILLATIONS OF LAKE LEVEL". Coastal Engineering Proceedings, n. 36 (30 dicembre 2018): 23. http://dx.doi.org/10.9753/icce.v36.sediment.23.
Testo completoAbstract (sommario):
Lake Erie has the fourth largest surface area, shallowest water depth and smallest volume among the five Great Lakes in North America (NOAA). The dominant wind direction over Lake Erie’s is southwest-northeast, along the lake’s longitudinal axis. The atmospheric and water level data of the lake demonstrate that high wind and moving pressure systems can result in high storm surge of up to 3 m on the eastern end of the lake and significant drop in the water level at the western end of the lake Due to its shallow depth, such a water level gradient can trigger unique post-storm free water-level fluctuations or seiches in Lake Erie (Farhadzadeh, 2017). The morphodynamic implications of such low frequency oscillations are yet to be studied for the lake’s shorelines. Most of studies on the contributions of long waves to beach morphology changes focused on low frequency harmonics induced by short waves, e.g. infragravity waves, edge waves, etc., oscillations with periods of up to a few minutes. Wright and Short (1984) discussed the differences in hydrodynamic processes and relative contributions of various mechanisms to morphological changes of beaches of different states, i.e., reflective, dissipative or intermediate. They concluded that for reflective beaches, incident waves and subharmonic edge waves are dominant while for dissipative beaches currents associated with infragravity standing waves are dominant in nearshore areas. Russell (1993) stated that as low frequency wave energy increases toward a shoreline, the offshore-directed transport at low frequency can become more pronounced.
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Kolodziejczyk, Nicolas, José Ochoa, Julio Candela e Julio Sheinbaum. "Deep Currents in the Bay of Campeche". Journal of Physical Oceanography 41, n. 10 (1 ottobre 2011): 1902–20. http://dx.doi.org/10.1175/2011jpo4526.1.
Testo completoAbstract (sommario):
Abstract Data from five moorings deployed in the Bay of Campeche during November 2007–July 2008 are used to analyze subinertial motions of waters below 1000-m depth. To the authors’ knowledge, this is the first time such a comprehensive observational program of direct deep-current measurements has been carried out in the region. The mean currents are in agreement with a cyclonic circulation at 1000-m depth; however, this cyclonic pattern is not so clearly defined at deeper levels. Only at the deepest mooring, located at 3500-m depth, are the mean currents uniform all the way to the bottom. Over the Bay of Campeche’s smooth western slope, currents show features compatible with topographic Rossby waves having vertical trapping scales thicker than 700 m, periods between 5 and 60 days, and horizontal wavelengths of 90–140 km. In contrast, the eastern slopes are characterized by rough topography, and motions with periods longer than 28 days decrease toward the bottom, suggesting a substantial reduction in the low-frequency topographic Rossby wave signal. Velocities from one of the two neighboring moorings located over the eastern rough slope have a strong 3-day period signal, which increases toward the bottom and has a vertical trapping scale of about 350 m. These higher frequency motions are interpreted in terms of edge waves.
18
Koshevaya, S., N. Makarets, V. Grimalsky, A. Kotsarenko e R. Perez Enríquez. "Spectrum of the seismic-electromagnetic and acoustic waves caused by seismic and volcano activity". Natural Hazards and Earth System Sciences 5, n. 2 (2 febbraio 2005): 203–9. http://dx.doi.org/10.5194/nhess-5-203-2005.
Testo completoAbstract (sommario):
Abstract. Modeling of the spectrum of the seismo-electromagnetic and acoustic waves, caused by seismic and volcanic activity, has been done. This spectrum includes the Electromagnetic Emission (EME, due to fracturing piezoelectrics in rocks) and the Acoustic Emission (AE, caused by the excitation and the nonlinear passage of acoustic waves through the Earth's crust, the atmosphere, and the ionosphere). The investigated mechanism of the EME uses the model of fracturing and the crack motion. For its analysis, we consider a piezoelectric crystal under mechanical stresses, which cause the uniform crack motion, and, consequently, in the vicinity of the moving crack also cause non-stationary polarization currents. A possible spectrum of EME has been estimated. The underground fractures produce Very Low (VLF) and Extremely Low Frequency (ELF) acoustic waves, while the acoustic waves at higher frequencies present high losses and, on the Earth's surface, they are quite small and are not registered. The VLF acoustic wave is subject to nonlinearity under passage through the lithosphere that leads to the generation of higher harmonics and also frequency down-conversion, namely, increasing the ELF acoustic component on the Earth's surface. In turn, a nonlinear propagation of ELF acoustic wave in the atmosphere and the ionosphere leads to emerging the ultra low frequency (ULF) acousto-gravity waves in the ionosphere and possible local excitation of plasma waves.
19
Hernandez-Gonzalez, Nieves G., Juan Montiel-Caminos, Javier Sosa e Juan A. Montiel-Nelson. "An Edge Computing Application of Fundamental Frequency Extraction for Ocean Currents and Waves". Sensors 24, n. 5 (20 febbraio 2024): 1358. http://dx.doi.org/10.3390/s24051358.
Testo completoAbstract (sommario):
This paper describes the design and optimization of a smart algorithm based on artificial intelligence to increase the accuracy of an ocean water current meter. The main purpose of water current meters is to obtain the fundamental frequency of the ocean waves and currents. The limiting factor in those underwater applications is power consumption and that is the reason to use only ultra-low power microcontrollers. On the other hand, nowadays extraction algorithms assume that the processed signal is defined in a fixed bandwidth. In our approach, belonging to the edge computing research area, we use a deep neural network to determine the narrow bandwidth for filtering the fundamental frequency of the ocean waves and currents on board instruments. The proposed solution is implemented on an 8 MHz ARM Cortex-M0+ microcontroller without a floating point unit requiring only 9.54 ms in the worst case based on a deep neural network solution. Compared to a greedy algorithm in terms of computational effort, our worst-case approach is 1.81 times faster than a fast Fourier transform with a length of 32 samples. The proposed solution is 2.33 times better when an artificial neural network approach is adopted.
20
Kato, Y., S. Harisaki, W. Kubo, Y. Fujimura e K. Iwahara. "Initial Experimental Results on Ion Cyclotron Resonance Heating Selectively Mixed Low Z Ions to Enhance Production Efficiency of Multicharged Ions on Electron Cyclotron Resonance Ion Source". Journal of Physics: Conference Series 2743, n. 1 (1 maggio 2024): 012002. http://dx.doi.org/10.1088/1742-6596/2743/1/012002.
Testo completoAbstract (sommario):
Abstract According to the accessibility conditions of wave propagation in the magnetized plasma of an electron cyclotron resonance (ECR) ion source (ECRIS), it is speculated that the essential factor that determines the limitations in the multiply charged ion currents is the left-hand polarization wave cutoff (L-cutoff). It is necessary to overcome this limitation, we proposed the introduction of ion cyclotron resonance (ICR) or lower hybrid resonance (LHR) by lower frequency waves. We conduct heating low-mass ions selectively in enhanced producing multiply charged ion by mixing low-mass element gas, which has been conventionally performed in ECRIS, or relaxation of the potential well based on the existence of resonant electron particles by ECR. This paper will describe the initial experimental results of ICR application by introducing low-frequency RF electromagnetic waves in the ECRIS.
21
Teigen, P., e A. Naess. "Extreme Response of Floating Structures in Combined Wind and Waves". Journal of Offshore Mechanics and Arctic Engineering 125, n. 2 (16 aprile 2003): 87–93. http://dx.doi.org/10.1115/1.1554699.
Testo completoAbstract (sommario):
The paper highlights the problem of evaluating the extreme surge response of a floating, deep water structure subjected to stochastic loading from concurrent wind and waves. Additional load effects associated with ocean currents are also briefly discussed. Both long-crested and short-crested waves are considered, whereas the wind field is assumed to be unidirectional. The probability density function (PDF) of the combined wave frequency and low frequency response of the structure, due to waves, is calculated by an eigenvalue analysis and convoluted with the corresponding PDF from the wind loads, to obtain the PDF of the global response. The necessity of employing full, biquadratic transfer functions to evaluate the low frequency part of the wave loads is amply documented. The effect of short-crested versus unidirectional seas on the TLP motion response is discussed at some length, along with various numerical aspects related to the mathematical modelling and to the convergence and accuracy of the obtained solutions. Numerical solutions are presented for a wide range of harsh weather type, environmental parameters.
22
Li, Jiemeihui, Yang Shi, Yixin Yang e Xiaodong Huang. "Noise of Internal Solitary Waves Measured by Mooring-Mounted Hydrophone Array in the South China Sea". Journal of Marine Science and Engineering 10, n. 2 (8 febbraio 2022): 222. http://dx.doi.org/10.3390/jmse10020222.
Testo completoAbstract (sommario):
Internal solitary waves in the South China Sea have attracted attention because of their large amplitude and high rate of occurrence. Internal solitary waves have a substantial influence on underwater sound propagation and ambient noise. However, there have seldom been reports on the noise they cause. In this paper, we conducted an internal solitary waves cooperative observation experiment in the South China Sea in 2016. We analyzed the temperature, flow velocity and noise changes induced by internal solitary waves. The power spectra of noise generated by internal solitary waves at frequencies below 100 Hz was almost 20 dB higher than ambient noise. The observed low-frequency noise had uniform harmonics. Combined with the changes of flow velocity, we interpreted the low frequency noise as flow noise induced by vortex-induced vibration of internal solitary waves flowing past the cable mooring system. The noise spectra were related to the position of the cable where the hydrophone was mounted. The closer they were to the middle of the cable, the greater the vibration amplitude, and the stronger the noise. This study provided a passive acoustic monitoring and warning method for high marine currents.
23
Zhao, Xing, Bin Tu, Mengyao Li, Xiaojing Feng, Yuchun Zhang, Qiaojun Fang, Tiehu Li, Bartosz A. Grzybowski e Yong Yan. "Switchable counterion gradients around charged metallic nanoparticles enable reception of radio waves". Science Advances 4, n. 10 (ottobre 2018): eaau3546. http://dx.doi.org/10.1126/sciadv.aau3546.
Testo completoAbstract (sommario):
Mechanically flexible, easy-to-process, and environmentally benign materials capable of current rectification are interesting alternatives to “hard” silicon-based devices. Among these materials are metallic/charged-organic nanoparticles in which electronic currents though metal cores are modulated by the gradients of counterions surrounding the organic ligands. Although layers of oppositely charged particles can respond to both electronic and chemical signals and can function even under significant mechanical deformation, the rectification ratios of these “chemoelectronic” elements have been, so far, low. This work shows that significantly steeper counterion gradients and significantly higher rectification ratios can be achieved with nanoparticles of only one polarity but in contact with a porous electrode serving as a counterion “sink.” These composite structures act as rectifiers even at radio frequencies, providing a new means of interfacing counterions’ dynamics with high-frequency electronic currents.
24
Holden, C. J. "Observations of low-frequency currents and continental shelf waves along the west coast of South Africa". South African Journal of Marine Science 5, n. 1 (1 giugno 1987): 197–208. http://dx.doi.org/10.2989/025776187784522360.
Testo completo
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Cienfuegos, Rodrigo, L. Duarte, L. Suarez e P. A. Catalán. "NUMERICAL COMPUTATION OF INFRAGRAVITY WAVE DYNAMICS AND VELOCITY PROFILES USING A FULLY NONLINEAR BOUSSINESQ MODEL". Coastal Engineering Proceedings 1, n. 32 (29 gennaio 2011): 48. http://dx.doi.org/10.9753/icce.v32.currents.48.
Testo completoAbstract (sommario):
We present experimental and numerical analysis of nonlinear processes responsible for generating infragravity waves
in the nearshore. We provide new experimental data on random wave propagation and associated velocity profiles in
the shoaling and surf zones of a very mild slope beach. We analyze low frequency wave generation mechanisms and
dynamics along the beach and examine in detail the ability of the fully nonlinear Boussinesq- type model SERR1D
(Cienfuegos et al., 2010) to reproduce the complex dynamics of high frequency wave propagation and energy transfer
mechanisms that enhance infragravity wave generation in the laboratory.
26
Schlosser, Tamara L., Nicole L. Jones, Cynthia E. Bluteau, Matthew H. Alford, Gregory N. Ivey e Andrew J. Lucas. "Generation and Propagation of Near-Inertial Waves in a Baroclinic Current on the Tasmanian Shelf". Journal of Physical Oceanography 49, n. 10 (ottobre 2019): 2653–67. http://dx.doi.org/10.1175/jpo-d-18-0208.1.
Testo completoAbstract (sommario):
AbstractNear-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency feff, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves ωmin. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where feff was less than the Coriolis frequency f. If everywhere feff > f then NIWs were generated where ωmin < f and feff was smallest. The background current not only affected the location of generation, but also the NIWs’ propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because ωmin > f on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency ω within the anomalously low-frequency band (i.e., ωmin < ω < feff), which only exists if the baroclinicity is nonzero. We observed two periods when ωmin < f on the shelf, but the relative vorticity was positive (i.e., feff > f) for one of these periods. The process of NIW propagation remained consistent with the local ωmin, and not feff, emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.
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Yao, Y., e I. Parker. "Ca2+ influx modulation of temporal and spatial patterns of inositol trisphosphate‐mediated Ca2+ liberation in Xenopus oocytes." Journal of Physiology 476, n. 1 (aprile 1994): 17–28. http://dx.doi.org/10.1113/jphysiol.1994.sp020108.
Testo completoAbstract (sommario):
Inositol 1,4,5‐trisphosphate (InsP3) functions as a second messenger by liberating intracellular Ca2+ and by promoting influx of extracellular Ca2+. We examined the effects of Ca2+ influx on the temporal and spatial patterns of intracellular Ca2+ liberation in Xenopus oocytes by fluorescence imaging of cytosolic free Ca2+ together with voltage clamp recording of Ca(2+)‐activated Cl‐ currents. Oocytes were injected with a poorly metabolized InsP3 analogue (3‐F‐InsP3; see Introduction) to induce sustained activation of InsP3 signalling, and Ca2+ influx was controlled by applying voltage steps to change the driving force for Ca2+ entry. Positive‐ and negative‐going potential steps (corresponding, respectively, to decreases and increases in Ca2+ influx) evoked damped oscillatory Cl‐ currents, accompanied by cyclical changes in cytosolic free Ca2+. The source of this Ca2+ was intracellular, since oscillations persisted when Ca2+ entry was suppressed by removing extracellular Ca2+ or by polarization close to the Ca2+ equilibrium potential. Fluorescence recordings from localized (ca 5 microns) spots on the oocyte showed repetitive Ca2+ spikes. Their frequency increased at more negative potentials, but they became smaller and superimposed on a sustained ‘pedestal’ of Ca2+. Spike periods ranged from about 50 s at +20 mV to 4s at potentials between ‐60 and ‐120 mV. Ca2+ spike frequency decreased after removing extracellular Ca2+, but the spike amplitude was not reduced and low frequency spikes continued for at least 30 min in the absence of extracellular Ca2+. Membrane current oscillations decayed in amplitude following voltage steps, while locally recorded Ca2+ spikes did not. This probably arose because Ca2+ release was initially synchronous across the cell, leading to large Ca(2+)‐activated Cl‐ currents, but the currents then diminished as different areas of the cell began to release Ca2+ asynchronously. Fluorescence imaging revealed that Ca2+ liberation in 3‐F‐InsP3‐loaded oocytes occurred as transient localized puffs and as propagating waves. Polarization to more negative potentials increased the frequency of puffs and the number of sites at which they were seen, and enhanced their ability to initiate waves. The frequency and velocity of Ca2+ waves increased at more negative potentials. When the potential was returned to more positive levels, repetitive Ca2+ spikes at first occurred synchronously across the recording area, but this synchronization was gradually lost and Ca2+ waves began at several foci. We conclude that influx of extracellular Ca2+ regulates the temporal and spatial patterns of Ca2+ liberation from InsP3‐sensitive intracellular stores, probably as a result of dual excitatory and inhibitory actions of cytosolic Ca2+ on the InsP3 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)
28
Liu, Yonggang, Robert H. Weisberg, Clifford R. Merz, Sage Lichtenwalner e Gary J. Kirkpatrick. "HF Radar Performance in a Low-Energy Environment: CODAR SeaSonde Experience on the West Florida Shelf*". Journal of Atmospheric and Oceanic Technology 27, n. 10 (1 ottobre 2010): 1689–710. http://dx.doi.org/10.1175/2010jtecho720.1.
Testo completoAbstract (sommario):
Abstract Three long-range (5 MHz) Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde HF radars overlooking an array of as many as eight moored acoustic Doppler current profilers (ADCPs) have operated on the West Florida Shelf since September 2003 for the purpose of observing the coastal ocean currents. HF radar performance on this low-energy (currents and waves) continental shelf is evaluated with respect to data returns, the rms differences between the HF radar and the ADCP radial currents, bearing offsets, and radial velocity uncertainties. Possible environmental factors affecting the HF radar performance are discussed, with the findings that both the low-energy sea state and the unfavorable surface wave directions are the main limiting factors for these HF radar observations of currents on the WFS. Despite the challenge of achieving continuous backscatter from this low-energy environment, when acquired the data quality is good in comparison with the ADCP measurements. The rms differences range from 6 to 10 cm s−1 for hourly and from 3 to 6 cm s−1 for 36-h low-pass-filtered radial currents, respectively. Bearing offsets are in the range from −15° to +9°. Coherent variations of the HF radar and ADCP radial currents are seen across both tidal and subtidal frequency bands. By examining the HF radar radial velocities at low wave energy, it is found that the data returns decrease rapidly for significant wave heights smaller than 1 m, and that the rms differences between the HF radar and ADCP radials are degraded when the significant wave height is smaller than 0.3 m.
29
Pilipenko, V. A., D. Yu Klimushkin, P. N. Mager, M. J. Engebretson e O. V. Kozyreva. "Generation of resonant Alfvén waves in the auroral oval". Annales Geophysicae 34, n. 2 (16 febbraio 2016): 241–48. http://dx.doi.org/10.5194/angeo-34-241-2016.
Testo completoAbstract (sommario):
Abstract. Many independent observations have shown that resonant field line Pc5 pulsations (typical periods of about several minutes) are preferably excited inside the auroral oval. Consideration of the most evident interpretation schemes shows that there is no simple explanation of this effect. Here we consider theoretically the generation of toroidal Pc5 Alfvén waves by fluctuating magnetospheric field-aligned currents. It is shown that the Alfvén wave latitudinal structure has the same features as in the case of the classical field line resonance driven by external sources: a peak localized in latitude and a 180° phase shift across the resonant magnetic shell. This model suggests an additional mechanism of ultra-low-frequency (ULF) wave excitation which can operate at auroral latitudes.
30
Robles, Luis, e Mario A. Ruggero. "Mechanics of the Mammalian Cochlea". Physiological Reviews 81, n. 3 (1 luglio 2001): 1305–52. http://dx.doi.org/10.1152/physrev.2001.81.3.1305.
Testo completoAbstract (sommario):
In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the “base” of the cochlea (near the stapes) and low-frequency waves approaching the “apex” of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the “cochlear amplifier.” This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.
31
Cuperman, S., C. Bruma e K. Komoshvili. "Non-inductive current drive via helicity injection by Alfvén waves in low-aspect-ratio tokamaks". Journal of Plasma Physics 56, n. 1 (agosto 1996): 149–74. http://dx.doi.org/10.1017/s0022377800019152.
Testo completoAbstract (sommario):
A theoretical investigation of radio-frequency (RF) current drive via helicity injection in low aspect ratio tokamaks is carried out. A current-carrying cylindrical plasma surrounded by a helical sheet-current antenna and situated inside a perfectly conducting shell is considered. Toroidal features of low-aspect-ratio tokamaks are simulated by incorporating the following effects: (i) arbitrarily small aspect ratio, Ro/a ≡1/∈; (ii) strongly sheared equilibrium magnetic field; and (iii) relatively large poloidal component of the equilibrium magnetic field. This study concentrates on the Alfvén continuum, i.e. the case in which the wave frequency satisfies the condition , where is an eigenfrequency of the shear Alfven wave (SAW). Thus, using low-β magneto- hydrodynamics, the wave equation with correct boundary (matching) conditions is solved, the RF field components are found, and subsequently current drive, power deposition and efficiency are computed. The results of our investigation clearly demonstrate the possibility of generation of RF-driven currents via helicity injection by Alfvén waves in low-aspect-ratio tokamaks, in the SAW mode. A special algorithm is developed that enables one to select the antenna parameters providing optimal current drive efficiency.
32
Shevchenko, Georgy V., Alexander B. Rabinovich e Richard E. Thomson. "Sea-Ice Drift on the Northeastern Shelf of Sakhalin Island". Journal of Physical Oceanography 34, n. 11 (1 novembre 2004): 2470–91. http://dx.doi.org/10.1175/jpo2632.1.
Testo completoAbstract (sommario):
Abstract Ice-drift velocity records from coastal radar stations, combined with data from moored current meters and coastal wind stations, are used to examine sea-ice motion off the northeastern coast of Sakhalin Island in the Sea of Okhotsk. Ice motion is shown to be governed primarily by diurnal tidal currents and wind-induced drift, which explain 92%–95% of the total ice-drift variance. Diurnal tidal motions predominate off the northern Sakhalin coast, accounting for 65%–80% of the variance, while low-frequency wind-induced motions prevail off the south-central coast, accounting for over 91% of the ice-drift variance. Maximum diurnal tidal ice-drift velocities range from 90–110 cm s−1 on the north coast to 10–15 cm s−1 on the south coast, in good agreement with the barotropic model of Kowalik and Polyakov. The presence of diurnal shelf waves accounts for the strong diurnal currents on the steeply sloping northern Sakhalin shelf, while the absence of such waves explains the weak diurnal currents on the more gently sloping south-central shelf. Using a vector regression model, the authors show that wind-induced ice-drift “response ellipses” (the current velocity response to a unity wind-velocity forcing) are consistent with a predominantly alongshore response to the wind, with wind-induced currents most pronounced off the south-central coast where water depths are relatively shallow. Time–frequency analysis of wind and ice-drift series reveals that, in winter, when sea ice is most extensive and internally cohesive, the ice response is almost entirely aligned with the alongshore component of the wind; in spring, when sea ice is broken and patchy, the ice responds to both the cross- and alongshore components of the wind.
33
Uvarov, Vladimir. "Electromagnetic-Deformation Response of the Crust". E3S Web of Conferences 62 (2018): 03003. http://dx.doi.org/10.1051/e3sconf/20186203003.
Testo completoAbstract (sommario):
The analysis of mechanic-electromagnetic effects of the Earth’s crust made it possible to distinguish the relation of rock induced polarization with its deformation. On the basis of these relations, their generalized description is made. It is shown that, although the Earth’s crust is an energy-saturated structure in a critical state, but owing to electromagnetic properties, it is not a medium in which electromagnetic oscillations can be generated in the low-frequency region. At the same time, the Earth’s crust is favourable environment for the occurrences of seismo-acoustic oscillations. It is shown that the mechanisms of mechanic-electromagnetic transformations of the crust generate electromagnetic waves accompanying seismic-acoustic disturbances. Examples of these waves manifestations in electric currents and in electromagnetic radiation are given.
34
KUO, S. P. "Generation of extra and very low-frequency (ELF/VLF) radiation by ionospheric electrojet modulation using high-frequency (HF) heating waves". Journal of Plasma Physics 68, n. 4 (maggio 2002): 267–84. http://dx.doi.org/10.1017/s0022377802001964.
Testo completoAbstract (sommario):
Extra and very low-frequency (ELF/VLF) wave generation by modulated polar electrojet currents is studied numerically. Through Ohmic heating by the amplitude-modulated high-frequency heating wave, the conductivity and thus the current of the electrojet are modulated accordingly to set up the ionospheric antenna current. Stimulated thermal instability, which can further enhance the electrojet current modulation, is studied. It is first analysed analytically to determine the threshold heating power for its excitation. The nonlinear evolutions of the generated ELF/VLF waves enhanced by the instability are then studied numerically. Their spectra are also evaluated. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency in agreement with the Tromso observations. The efficiency enhancement by the stimulated thermal instability is hampered by inelastic collisions of electrons with neutral particles (mainly due to vibration excitation of N2), which cause this instability to saturate at low levels. However, the electron inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the heating power exceeds a threshold level, significant electron heating enhanced by the instability is shown, which indeed causes a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.
35
Sciffer, M. D., C. L. Waters e F. W. Menk. "Propagation of ULF waves through the ionosphere: Inductive effect for oblique magnetic fields". Annales Geophysicae 22, n. 4 (2 aprile 2004): 1155–69. http://dx.doi.org/10.5194/angeo-22-1155-2004.
Testo completoAbstract (sommario):
Abstract. Solutions for ultra-low frequency (ULF) wave fields in the frequency range 1–100mHz that interact with the Earth's ionosphere in the presence of oblique background magnetic fields are described. Analytic expressions for the electric and magnetic wave fields in the magnetosphere, ionosphere and atmosphere are derived within the context of an inductive ionosphere. The inductive shielding effect (ISE) arises from the generation of an "inductive" rotational current by the induced part of the divergent electric field in the ionosphere which reduces the wave amplitude detected on the ground. The inductive response of the ionosphere is described by Faraday's law and the ISE depends on the horizontal scale size of the ULF disturbance, its frequency and the ionosphere conductivities. The ISE for ULF waves in a vertical background magnetic field is limited in application to high latitudes. In this paper we examine the ISE within the context of oblique background magnetic fields, extending studies of an inductive ionosphere and the associated shielding of ULF waves to lower latitudes. It is found that the dip angle of the background magnetic field has a significant effect on signals detected at the ground. For incident shear Alfvén mode waves and oblique background magnetic fields, the horizontal component of the field-aligned current contributes to the signal detected at the ground. At low latitudes, the ISE is larger at smaller conductivity values compared with high latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; electric fields and currents; wave propagation)
36
Schlicht, Anja. "GRACE Accelerometers Sensitive to Ionosphere Plasma Waves: Similarities between Twangs and Whistlers". Geosciences 12, n. 6 (27 maggio 2022): 228. http://dx.doi.org/10.3390/geosciences12060228.
Testo completoAbstract (sommario):
Electrostatic space accelerometers are high-precision instruments used in gravity and magnetic fields and in fundamental physics missions. From their first use in space, these instruments show disturbances around the read-out frequency. In recent missions like Swarm and GRACE-FO, these instruments are seriously compromised. Their capability cannot be fully exploited or, in the worst case, not used for science data processing at all. We currently neither understand the mechanism nor the cause of the disturbance. For that reason, every correlation, direct or indirect, between signatures in the accelerometer data and other phenomena in the satellite environment is very important to study. In the GRACE mission, some of the disturbances relate to onboard current switching processes, such as switching of heater and torquer currents. In this paper, we examine the phenomena called twangs. The shape of these disturbances is responsible for their name, because they look like a decaying tone. Some of these twangs seem to relate to changing currents, too. They occur when sunlight hits surfaces of the satellite. This can cause the discharging of satellite parts. Here, we focus on special twangs, which we call cloud-related twangs, as recent investigations related them to cloud coverage. How can clouds in the troposphere influence a satellite, orbiting in the ionosphere? We investigated the troposphere–ionosphere coupling and found that the only coupling which can be responsible for disturbances on satellites and which may affect the accelerometers, are electromagnetic waves traveling in the so-called whistler mode at frequencies in the very-low-frequency (VLF) domain. This special propagation mode is a coupled electromagnetic and electrostatic mode. There, the electric field vector has a component in the direction of propagation due to the interaction with ions and the magnetic field in the ionosphere and magnetosphere. This led us to investigate the similarities between whistlers and twangs. The hypothesis that they are related is very important in our further understanding of disturbances on satellite instruments sensible to electromagnetic pulses in the range of very low frequencies and below.
37
Anh, Vu Tuan. "ASSESSMENT OF THE IMPACT OF BREAKWATER ON EROSION-DEPOSITION PROCESSES IN TAM QUAN RIVER MOUTH, BINH DINH PROVINCE". Tạp chí Khoa học và Công nghệ Biển 17, n. 4 (6 agosto 2018): 433–44. http://dx.doi.org/10.15625/1859-3097/17/4/11053.
Testo completoAbstract (sommario):
After the establishment of breakwater at Tam Quan river mouth in 2009, this area has been deposited quickly, especially in the rainy season (coinciding with northeast monsoon). The processes of litho-hydrodynamics and erosion-deposition change clearly over seasons. In the rainy season (September to December) prevailing wind directions are NE, NNE, N and NW. Most wind speeds range from level 2 to level 5. Waves generated by NE, N winds have the most impact on the Tam Quan river mouth. During propagation into the channel the waves reduce rapidly and tend to push the bed material from the sea to the channel that causes the alternate deposition and erosion in the river mouth and channel. Currents carry bed material from the river to the sea and cause the alternate deposition and erosion in the channel. Bed materials are accumulated in an arc-shaped area around the mouth. In the dry season (January to August) prevailing wind directions are SE, SSE and S, and wind speeds are rather small, from level 2 to level 4. The E wind has a low frequency but generates waves that cause direct impacts on the studied area. The wave height reduces rapidly during transmitting into the channel. The waves generated by the SE wind tend to push bed material from the sea and around Truong Xuan cape to the channel, then accumulating in the mouth. Wave-currents formed along the northern shore bring the materials to accumulate in the channel center. Currents just cause bed materials to move at the time of strong ebb-tide and then accumulate on a large area at the mouth, but with negligible intensity. Calculation results of the bed evolution from 15/12/2014 to 20/5/2015 fairly correspond with the measured results. This is the scientific basis of the solutions for long-term stability of channel.
38
Pinkel, Robert. "Advection, Phase Distortion, and the Frequency Spectrum of Finescale Fields in the Sea". Journal of Physical Oceanography 38, n. 2 (1 febbraio 2008): 291–313. http://dx.doi.org/10.1175/2007jpo3559.1.
Testo completoAbstract (sommario):
Abstract Continuous depth–time measurements of upper-ocean velocity are used to estimate the wavenumber–frequency spectrum of shear. A fundamental characteristic of these spectra is that the frequency bandwidth increases linearly with increasing wavenumber magnitude. This can be interpreted as the signature of Doppler shifting of the observations by time-changing “background” currents as well as by instrument motion. Here, the hypothesis is posed that the apparently continuous wavenumber–frequency spectrum of oceanic shear results from the advective “smearing” of discrete spectral lines. In the Arctic Ocean, lines at the inertial (ω = −f ) and vortical (ω = 0) frequencies (where f is the Coriolis frequency) account for most of the variance in the shear spectrum. In the tropical ocean, two classes of inertial waves are considered, accounting for 70% of the observed shear variance. A simple model is introduced to quantify the effects of lateral advection, random vertical advection (“fine-structure contamination”), and deterministic (tidal) vertical advection on these “otherwise monochromatic” records. Model frequency spectra are developed in terms of the probability density and/or spectrum of the advecting fields for general but idealized situations. The model successfully mimics the increasing frequency bandwidth of the shear spectrum with increasing vertical wavenumber. Excellent fits to the observed frequency spectrum of shear are obtained for the Arctic (weak advection and short-spatial-scale inertial waves) and low-latitude (strong advection and long and short inertial waves) observations. While successfully replicating the wavenumber–frequency spectrum of shear, the model does not even consider motion at scales greater than ∼250 m, the “energy containing” scales of the internal wave field. To a first approximation, the waves with the majority of the kinetic and potential energy constitute a population apart from those with the momentum, shear, and strain.
39
Li, Jianke, e Allan J. Clarke. "Coastline Direction, Interannual Flow, and the Strong El Niño Currents along Australia's Nearly Zonal Southern Coast". Journal of Physical Oceanography 34, n. 11 (1 novembre 2004): 2373–81. http://dx.doi.org/10.1175/jpo2645.1.
Testo completoAbstract (sommario):
Abstract The western equatorial Pacific Ocean El Niño signal leaks through the gappy western equatorial Pacific Ocean boundary to the western and southern coasts of Australia. Ocean Topography Experiment (TOPEX)/Poseidon sea level data and coastal tide gauge measurements show that off the northwest coast the low-frequency signal propagates westward as large-scale Rossby waves. However, along the nearly zonal southern coast, particle displacements are nearly zonal near the coast and experience no planetary vorticity change. As a consequence, the Rossby wave mechanism fails, and theory suggests that the signal should decay from the shelf edge with baroclinic Rossby radius-of-deformation scale. High-resolution along-track TOPEX/Poseidon sea level heights show that the interannual height signal does decay rapidly seaward of the shelf edge with this scale. The sharp fall in sea level and geostrophic balance imply strong (∼10 cm s−1) low-frequency currents seaward of the shelf edge. On the shelf, interannual flow is in the same direction as the shelf-edge flow but is much weaker. The anomalous flows tend to be eastward during La Niña, when the western equatorial Pacific and Australian coastal sea levels are unusually high, and westward during El Niño when coastal sea levels tend to be anomalously low. The anomalous low-frequency flows can transport larvae large distances, enhancing the recruitment of Australian salmon to nursery grounds in the eastern part of the southern coast when the coastal sea level is higher than normal and decreasing recruitment when it is lower than normal.
40
Thomas, Leif N. "On the effects of frontogenetic strain on symmetric instability and inertia–gravity waves". Journal of Fluid Mechanics 711 (20 settembre 2012): 620–40. http://dx.doi.org/10.1017/jfm.2012.416.
Testo completoAbstract (sommario):
AbstractThe dynamics of symmetric instability and two-dimensional inertia–gravity waves in a baroclinic geostrophic flow undergoing frontogenesis is analysed. A frontogenetic strain associated with a balanced deformation field drives an ageostrophic circulation and temporal variations in the basic state that significantly affect the properties of perturbations to the background flow. For stable stratification, perturbations to the basic state result in symmetric instability or inertia–gravity waves, depending on the sign of the Ertel potential vorticity and the magnitude of the Richardson number of the geostrophic flow. The kinetic energy (KE) of both types of motion is suppressed by frontogenetic strain due to the vertical shear in the ageostrophic circulation. This is because the perturbation streamlines tilt with the ageostrophic shear causing the disturbances to lose KE via shear production. The effect can completely dampen symmetric instability for sufficiently strong strain even though the source of KE for the instability (the vertical shear in the geostrophic flow) increases with time. Inertia–gravity waves in a baroclinic flow undergoing frontogenesis simultaneously lose KE and extract KE from the deformation field as they decay. This is because the horizontal velocity of the waves becomes rectilinear, resulting in a Reynolds stress that draws energy from the balanced flow. The process is most effective for waves of low frequency and for a geostrophic flow with low Richardson number. However, even in a background flow that is initially strongly stratified, frontogenesis leads to an exponentially fast reduction in the Richardson number, facilitating a rapid energy extraction by the waves. The KE transferred from the deformation field is ultimately lost to the unbalanced ageostrophic circulation through shear production, hence the inertia–gravity waves play a catalytic role in loss of balance. Given the large amount of KE in low-frequency inertia–gravity waves and the ubiquitous combination of strain and baroclinic geostrophic currents in the ocean, it is estimated that this mechanism could play a significant role in the removal of KE from both the internal wave and mesoscale eddy fields.
41
Brown, Jaclyn N., J. Stuart Godfrey e Susan E. Wijffels. "Nonlinear Effects of Tropical Instability Waves on the Equatorial Pacific Circulation". Journal of Physical Oceanography 40, n. 2 (1 febbraio 2010): 381–93. http://dx.doi.org/10.1175/2009jpo3963.1.
Testo completoAbstract (sommario):
Abstract In a numerical model of the equatorial Pacific Ocean, the ∼20-day period tropical instability waves, excited in the eastern half of the domain, are found to damp the strong zonal mean currents. The waves generate large, nonlinear, advection terms in the momentum balance, change the vorticity balance, and thus modulate the low-frequency state. The authors explore whether the effect of tropical instability waves on the background flow can instead be adequately parameterized by a constant-coefficient Laplacian friction scheme. On annual mean, a Laplacian friction coefficient that varies in space is required, for the coefficient is twice as large along the equator and a few degrees more to the north than elsewhere. In addition, wave activity varies in time. During active phases, such as the second half of the year and during La Niñas, the activity increases, which would require the Laplacian coefficient of friction to be at least twice as strong as during the inactive phases. Thus, a more sophisticated damping parameterization than simple Laplacian friction is required in ocean models that do not explicitly resolve tropical instability waves.
42
Archetti, Renata, e Maria Gabriella Gaeta. "WAVE RUN-UP OBSERVATION AND 2DV NUMERICAL INVESTIGATION ON BEACHES PROTECTED BY STRUCTURES". Coastal Engineering Proceedings 1, n. 33 (14 dicembre 2012): 20. http://dx.doi.org/10.9753/icce.v33.currents.20.
Testo completoAbstract (sommario):
The main parameter for the assessment of coastal vulnerability and sediment transport is the wave run-up on the beach, defining the limit of maximum flooding, but also hydrodynamic properties in the Swash Zone (SZ) are trivial for the comprehension of hydro-morphodynamic processes. Several studies have been carried out on the SZ but few literature is still available on the run-up and on SZ flows on beaches protected by Low Crested Structures (LCSs), where flow motion is driven by a combination of low frequency infra-gravity waves and incident waves. In presence of breakwaters, swash incident waves are transmitted through the structure. In the transmission area behind the structures, wave energy is shifted to higher frequencies with respect to the incident wave spectrum and in general its mean period considerably decreases with respect to the incident one. Collecting in situ run-up measurements during storms is essential to understand the SZ processes and properly calibrate their both empirical and numerical models but measuring extreme run-up is difficult, due to the severe sea conditions and due to unexpected nature of storms. The present paper present a numerical and experimental analysis of the wave run-up and of the flow properties on a beach: the study shows the different behavior of unprotected and protected beach, subjected to the same wave conditions. In particular the paper shows that submerged breakwaters reduce in general the run-up height, on the basis of the calibrated 2DV numerical simulations, under extreme wave conditions (TR >50 years), the effect of submerged breakwaters seems to be negligible on the run-up height. Moreover a preliminary empirical equation for run-up with protected beach is proposed
43
Alford, Matthew H., e Zhongxiang Zhao. "Global Patterns of Low-Mode Internal-Wave Propagation. Part I: Energy and Energy Flux". Journal of Physical Oceanography 37, n. 7 (1 luglio 2007): 1829–48. http://dx.doi.org/10.1175/jpo3085.1.
Testo completoAbstract (sommario):
Abstract Extending an earlier attempt to understand long-range propagation of the global internal-wave field, the energy E and horizontal energy flux F are computed for the two gravest baroclinic modes at 80 historical moorings around the globe. With bandpass filtering, the calculation is performed for the semidiurnal band (emphasizing M2 internal tides, generated by flow over sloping topography) and for the near-inertial band (emphasizing wind-generated waves near the Coriolis frequency). The time dependence of semidiurnal E and F is first examined at six locations north of the Hawaiian Ridge; E and F typically rise and fall together and can vary by over an order of magnitude at each site. This variability typically has a strong spring–neap component, in addition to longer time scales. The observed spring tides at sites northwest of the Hawaiian Ridge are coherent with barotropic forcing at the ridge, but lagged by times consistent with travel at the theoretical mode-1 group speed from the ridge. Phase computed from 14-day windows varies by approximately ±45° on monthly time scales, implying refraction by mesoscale currents and stratification. This refraction also causes the bulk of internal-tide energy flux to be undetectable by altimetry and other long-term harmonic-analysis techniques. As found previously, the mean flux in both frequency bands is O(1 kW m−1), sufficient to radiate a substantial fraction of energy far from each source. Tidal flux is generally away from regions of strong topography. Near-inertial flux is overwhelmingly equatorward, as required for waves generated at the inertial frequency on a β plane, and is winter-enhanced, consistent with storm generation. In a companion paper, the group velocity, ĉg ≡ FE−1, is examined for both frequency bands.
44
Kirincich, Anthony. "Improved Detection of the First-Order Region for Direction-Finding HF Radars Using Image Processing Techniques". Journal of Atmospheric and Oceanic Technology 34, n. 8 (agosto 2017): 1679–91. http://dx.doi.org/10.1175/jtech-d-16-0162.1.
Testo completoAbstract (sommario):
AbstractFor direction-finding high-frequency (HF) radar systems, the correct separation of backscattered spectral energy due to Bragg resonant waves from that due to more complex double-scattering represents a critical first step toward attaining accurate estimates of surface currents from the range-dependent radar backscatter. Existing methods to identify this “first order” region of the spectra, generally sufficient for lower-frequency radars and low-velocity or low-surface gravity wave conditions, are more likely to fail in higher-frequency systems or locations with more variable current, wave, or noise regimes, leading to elevated velocity errors. An alternative methodology is presented that uses a single and globally relevant smoothing length scale, careful pretreatment of the spectra, and marker-controlled watershed segmentation, an image processing technique, to separate areas of spectral energy due to surface currents from areas of spectral energy due to more complex scattering by the wave field or background noise present. Applied to a number of HF radar datasets with a range of operating frequencies and characteristic issues, the new methodology attains a higher percentage of successful first-order identification, particularly during complex current and wave conditions. As operational radar systems continue to expand to more systematically cover areas of high marine traffic, close approaches to ports and harbors, or offshore energy installations, use of this type of updated methodology will become increasingly important to attain accurate current estimates that serve both research and operational interests.
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Diaz-Hernandez, Gabriel, Fernando J. Mendez e Roberto Mínguez. "Numerical Analysis and Diagnosis of the Hydrodynamic Effects Produced by Hurricane Gordon along the Coast of Spain". Weather and Forecasting 29, n. 3 (1 giugno 2014): 666–83. http://dx.doi.org/10.1175/waf-d-13-00130.1.
Testo completoAbstract (sommario):
Abstract This paper presents a detailed hindcast for the generation and propagation of sea state variables—significant wave height Hs, peak period Tp, mean direction θ, and spectral shape γ –σ —associated with cyclonic events to numerically diagnose their possible hydrodynamic effects over the northeastern Atlantic. An example of such cyclonic events is Hurricane Gordon, which occurred during the second half of August 2012. Extreme hurricane-strength winds produced new and atypically low-frequency (about 14 s) packs of energy. The preexistent wave spectrum suddenly experienced an addition of low-frequency energy along the coast of Cádiz, Spain. This study presents the results of a comprehensive analysis developed to reconstruct the events produced by Hurricane Gordon (2012) along the coast of Cádiz. The analysis features the use of (i) parametric models for the characterization of hurricane winds and pressure fields, (ii) implementation of the Simulating Waves Nearshore (SWAN) model for the generation and propagation of waves in the northeast Atlantic Ocean, and (iii) its coupling with the MOPLA—taken from the Spanish acronym for wave propagation model, current, and morphodynamic evolution of beaches—model for the evaluation of longshore currents. The numerical wave characterization, generation, and propagation were validated with instrumental data from deep-water and coastal buoys.
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Yu, Chi, Sheng Zhang e Cheng Zhang. "Coupling Effects of a Top-Hinged Buoyancy Can on the Vortex-Induced Vibration of a Riser Model in Currents and Waves". Journal of Marine Science and Engineering 12, n. 5 (30 aprile 2024): 751. http://dx.doi.org/10.3390/jmse12050751.
Testo completoAbstract (sommario):
In order to investigate the effects of the top-end dynamic boundary of risers caused by floater motions on their vortex-induced vibration (VIV) characteristics, a combined model comprising a buoyancy can with a relatively simple structural form and a riser is taken as the research object in the present study. The aspect ratios of the buoyancy can and the riser model are 5.37 and 250, respectively. A set of experimental devices is designed to support the VIV test of the riser with a dynamic boundary stimulating the vortex-induced motion (VIM) of the buoyancy can under different uniform flow and regular wave conditions. Several data processing methods are applied in the model test, i.e., mode superposition, Euler angle conversion, band pass filter, fast Fourier transform, and wavelet transform. Based on the testing results, the effect of low-frequency VIM on the high-frequency VIV of the riser is discussed in relation to a single current, a single wave, and a combined wave and current. It is found that the coupling effect of VIM on the riser VIV presents certain orthogonal features at low current velocities. The effect of the cross-flow VIM component on VIV is far more prominent than that of its counterpart, the in-line VIM, with increasing flow velocity. The VIM in the combined wave–current condition significantly enhances the modulation of vibration amplitude and frequency, resulting in larger fluctuation peaks of vibration response and further increasing the risk of VIV fatigue.
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Andersson, L., J. E. Wahlund, J. Clemmons, B. Gustavsson e L. Eliasson. "Electromagnetic waves and bursty electron acceleration: implications from Freja". Annales Geophysicae 20, n. 2 (28 febbraio 2002): 139–50. http://dx.doi.org/10.5194/angeo-20-139-2002.
Testo completoAbstract (sommario):
Abstract. Dispersive Alfvén wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extremely low frequency (ELF) emissions below the lower hybrid cutoff frequency (a few kHz). Large-scale density depletions/cavities, as determined by the Langmuir probe measurements, and strong electrostatic emissions are often observed simultaneously. A correlation study has been carried out between the E and B field fluctuations below 64 Hz and 10 Hz, respectively, (the DC instruments upper threshold) and the characteristics of the precipitating electrons. This study revealed that the energisation of electrons is indeed related to the broadband ELF emissions and that the electrostatic component plays a predominant role during very active magnetospheric conditions. Furthermore, the effect of the ELF electromagnetic emissions on the larger scale field-aligned current systems has been investigated, and it is found that such an effect cannot be detected. Instead, the Alfvénic activity creates a local region of field-aligned currents. It is suggested that dispersive Alfvén waves set up these local field-aligned current regions and, in turn, trigger more electrostatic emissions during certain conditions. In these regions, ions are transversely heated, and large-scale density depletions/cavities may be created during especially active periods.Key words. Ionosphere (particle acceleraton; wave-particle interactions) Magnetospheric physics (auroral phenomena)
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Lee, Jiin-Jen, Ziyi Huang, Zhiqing Kou e Xiuying Xing. "THE EFFECT OF TIDE LEVEL ON THE TSUNAMI RESPONSE OF COASTAL HARBORS". Coastal Engineering Proceedings 1, n. 33 (15 dicembre 2012): 11. http://dx.doi.org/10.9753/icce.v33.currents.11.
Testo completoAbstract (sommario):
The tsunamis generated by the February 27, 2010 Chilean earthquake and the great Japan Tohoku earthquake on March 11, 2011 arrived at several Pacific Coast harbors when the tide levels were at low tides and persisted for several tidal cycles. Despite the significant difference in the recorded wave amplitude observed at Crescent City harbor between these two events, the energy spectrum as a function of frequency has been found to contain several spikes corresponding to the frequency range of 3×〖10〗^(-4)~10×〖10〗^(-4) Hz. This pattern in spectral density is different from several prior tsunamis observed and analyzed for Crescent City Harbor as presented by Lee, Xing and Magoon (2008). In the present study we present the reasons behind the differences in the response behavior associated with these two events. We prove that they are due to the effect of tide levels. We also show that in order to correctly decipher the resonant response characteristics to incident wave the response curves should be expressed as a function of the dimensionless wave number. The tsunami waves recorded at tide gauge station in San Diego Harbor (Southern California) are also analyzed and discussed.
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VITTORI, G., H. E. DE SWART e P. BLONDEAUX. "Crescentic bedforms in the nearshore region". Journal of Fluid Mechanics 381 (25 febbraio 1999): 271–303. http://dx.doi.org/10.1017/s0022112098003541.
Testo completoAbstract (sommario):
A wave of small amplitude is considered which approaches a straight beach normally and which is partially reflected at the coastline. By assuming that the local depth is much smaller than the length of the incoming wave, the shallow water equations are used to determine the water motion. The surf zone width is assumed to be small compared to the length of the incoming wave and hence the effect of wave breaking is included only parametrically. The time development of the cohesionless bottom is described by the Exner continuity equation and by an empirical sediment transport rate formula which relates the sediment flux to the steady currents and wave stirring. It is shown that the basic-state solution, which does not depend on the longshore coordinate, may be unstable with respect to longshore bedform perturbations, so that rhythmic topographies form. The instability process is due to a positive feedback mechanism involving the incoming wave, synchronous edge waves and the bedforms. The growth of the bottom perturbations is related to the presence of steady currents caused by the interaction of the incoming wave with synchronous edge waves which in turn are excited by the incoming wave moving over the wavy bed. For natural beaches the model predicts two maxima in the amplification rate: one is related to incoming waves of low frequency, the other to wind waves. Thus two bedforms of different wavelengths can coexist in the nearshore region with longshore spacings of a few hundred and a few tens of metres, respectively. To illustrate the potential validity of the model, its results are compared with field data. The overall agreement is fairly satisfactory.
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Xu, Jie, Yuchuan Wang, Baoying Mu, Huan Du, Yanlei Li, Zaijin You, Sheng Yan e Lixin Lu. "Modeling Rip Current Systems around Multiple Submerged Breakwaters". Journal of Marine Science and Engineering 12, n. 9 (12 settembre 2024): 1627. http://dx.doi.org/10.3390/jmse12091627.
Testo completoAbstract (sommario):
Multiple submerged breakwaters (MSBWs) are commonly used coastal protection structures due to their specific advantages over the emerged ones. Rip currents, as the inevitable natural hazard in the gaps of these constructions, are investigated numerically in the present study. A fully nonlinear mild-slope equation (NMSE) model possessing both fully nonlinear and fully dispersive properties is validated and adopted in the simulations. With four monochromatic wave conditions of different wave heights, periods and incidences representing low-energy, typical, storm and oblique waves tested, the flow patterns and the low-frequency oscillations of the rip currents are studied. For the convenience of risk assessment, the rip risk level is divided into three degrees according to the maximum rip flow speed. The effects of the configurations of the MSBWs on the rip current system as well as the rip risk level are examined, considering different breakwater widths, heights, forms, gap widths and gap numbers. Simulation results suggest that the cross-shore configurations of MSBWs influence the rip risk level by inducing different wave energy dissipations but the longshore configurations of MSBWs by changing flow field patterns.