scholarly journals ANALYSIS OF LONG-CIRCUIT TYPE CAISSONS FOR ATTENUATION OF LONG-PERIOD WAVES

2018 ◽  
pp. 19
Author(s):  
Jose A. Gonzalez-Escriva ◽  
Jorge Molines ◽  
Josep R. Medina ◽  
M. Esther Gómez-Martín
Keyword(s):  
Low Frequency ◽  
Natural Oscillation ◽  
Maximum Efficiency ◽  
Destructive Interference ◽  
Mooring Lines ◽  
Cargo Handling ◽  
Short Period ◽  
Energy Amplification ◽  
Incident Waves ◽  
Sheltered Area

Breakwaters provide convenient shelter for short-period waves, thus limiting the amount of energy entering the harbor. This wave energy in the port basins may be amplified, due to the wave reflection, disturbing port operations. Anti-reflective Jarlan-type (ARJ) structures (see Jarlan, 1961) have been proposed in the literature, along with other structures, to alleviate wave disturbance within port basins. ARJ anti-reflective mechanism is based on the destructive interference of waves. An anti-reflective zone width, B, must be almost a quarter of the wavelength for maximum efficiency. Longer waves easily enter the port-sheltered area where resonance phenomena could magnify its energy under certain conditions. This low-frequency energy is much more difficult to dissipate. Port resonance is the phenomenon of energy amplification that takes place in a harbor or port basin if the incident waves have frequencies close to those of the natural oscillation of the mass of water in the port basin. These infragravity long waves are frequently reported as causing trouble in cargo handling areas of many ports (see Rabinovich, 2009), even causing breakages of mooring lines and other relevant damage (see Thotagamuwage and Pattiaratchi, 2014).

USING FIELD DATA FOR THE EVALUATION OF THE IMPACT OF ULTRA-LOW FREQUENCY MOTIONS ON THE FATIGUE OF MOORING LINES

2019 ◽  
Author(s):  
Guilherme Borzacchiello ◽  
Carl Albrecht ◽  
Fabricio N Correa ◽  
Breno Jacob ◽  
Guilherme da Silva Leal
Keyword(s):  
Field Data ◽  
Low Frequency ◽  
Mooring Lines ◽  
The Impact

scholarly journals Fluctuation of magnitude of wave loads for a long array of bottom-mounted cylinders

2019 ◽  
Vol 868 ◽  
pp. 244-285 ◽  
Author(s):  
Xiaohui Zeng ◽  
Fajun Yu ◽  
Min Shi ◽  
Qi Wang
Keyword(s):  
Incidence Angle ◽  
Local Maximum ◽  
Destructive Interference ◽  
Horizontal Distance ◽  
Wave Loads ◽  
Wave Load ◽  
Intrinsic Mechanism ◽  
Fluctuation Phenomenon ◽  
Analytical Formulae ◽  
Incident Waves

For wave loads on cylinders constituting a long but finite array in the presence of incident waves, variations in the magnitude of the load with the non-dimensional wavenumber exhibit interesting features. Towering spikes and nearby secondary peaks (troughs) associated with trapped modes have been studied extensively. Larger non-trapped regions other than these two are termed Region III in this study. Studies of Region III are rare. We find that fluctuations in Region III are regular; the horizontal distance between two adjacent local maximum/minimum points, termed fluctuation spacing, is constant and does not change with non-dimensional wavenumbers. Fluctuation spacing is related only to the total number of cylinders in the array, identification serial number of the cylinder concerned and wave incidence angle. Based on the interaction theory and constructive/destructive interference, we demonstrate that the fluctuation characteristics can be predicted using simple analytical formulae. The formulae for predicting fluctuation spacing and the abscissae of every peak and trough in Region III are proposed. We reveal the intrinsic mechanism of the fluctuation phenomenon. When the diffraction waves emitted from the cylinders at the ends of the array and the cylinder concerned interfere constructively/destructively, peaks/troughs are formed. The fluctuation phenomenon in Region III is related to solutions of inhomogeneous equations. By contrast, spikes and secondary peaks are associated with solutions of the eigenvalue problem. This study of Region III complements existing understanding of the characteristics of the magnitude of wave load. The engineering significances of the results are discussed as well.

Frequency Response Analysis of Piecewise Nonlinear Vibration Isolator

2005 ◽  
Author(s):  
Patrick Stahl ◽  
G. Nakhaie Jazar
Keyword(s):  
Low Frequency ◽  
High Amplitude ◽  
Relative Displacement ◽  
Response Analysis ◽  
Frequency Response Analysis ◽  
State Behavior ◽  
Vibration Isolators ◽  
Short Period ◽  
Secondary Suspension ◽  
Low Amplitude

Non-smooth piecewise functional isolators are smart passive vibration isolators that can provide effective isolation for high frequency/low amplitude excitation by introducing a soft primary suspension, and by preventing a high relative displacement in low frequency/high amplitude excitation by introducing a relatively damped secondary suspension. In this investigation a linear secondary suspension is attached to a nonlinear primary suspension. The primary is assumed to be nonlinear to model the inherent nonlinearities involved in real suspensions. However, the secondary suspension comes into action only during a short period of time, and in mall domain around resonance. Therefore, a linear assumption for the secondary suspension is reasonable. The dynamic behavior of the system subject to a harmonic base excitation has been analyzed utilizing the analytic results derived by applying the averaging method. The analytic results match very well in the transition between the two suspensions. A sensitivity analysis has shown the effect of varying dynamic parameters in the steady state behavior of the system.

A procedure for calibrating short-period telemetered seismograph systems

1988 ◽  
Vol 78 (6) ◽  
pp. 2077-2088
Author(s):  
M. C. Chapman ◽  
J. A. Snoke ◽  
G. A. Bollinger
Keyword(s):  
Dynamic Range ◽  
Low Frequency ◽  
Inertial Mass ◽  
System Response ◽  
Total System ◽  
Electronic Components ◽  
Amplitude Response ◽  
Motion System ◽  
Short Period ◽  
The Hilbert Transform

Abstract Efficient low-frequency calibration of the entire seismograph system can be accomplished by Fourier analysis of the system response to automatically generated transient test functions applied to the seismometer calibration coil. Typically, such calibrations are restricted to frequencies less than 10 Hz by the ambient ground motion, system noise, and limited dynamic range. To extend the calibration to a broader frequency range, we disconnect the seismometer and take advantage of the fact that the relative amplitude response of the electronic components in most systems can be measured with high accuracy at frequencies from as low as 0.02 Hz to the Nyquist frequency (e.g., 50 Hz) using standard electronics test equipment. The low-frequency amplitude response of the seismometer can then be isolated by dividing the total system response by that obtained for the electronic components. An iterative least-squares procedure is used to estimate the natural frequency and damping coefficient of the seismometer, along with a scaling parameter that specifies the absolute gain of the system. The phase response of the system is calculated directly from the amplitude response using the Hilbert transform. The procedure assumes that the seismometer is an ideal damped harmonic oscillator and that the system as a whole acts as a minimum phase filter. The only instrumental constants that must be known from independent measurement are the seismometer calibration coil force constant and the inertial mass.

scholarly journals A search for radio emission from exoplanets around evolved stars

2018 ◽  
Vol 612 ◽  
pp. A52 ◽  
Author(s):  
E. O’Gorman ◽  
C. P. Coughlan ◽  
W. Vlemmings ◽  
E. Varenius ◽  
S. Sirothia ◽  
...  
Keyword(s):  
Radio Emission ◽  
Mass Loss ◽  
Low Frequency ◽  
Large Mass ◽  
Low Frequencies ◽  
Surface Magnetic Field ◽  
Evolved Stars ◽  
Upper Limits ◽  
Short Period ◽  
Loss Rates

The majority of searches for radio emission from exoplanets have to date focused on short period planets, i.e., the so-called hot Jupiter type planets. However, these planets are likely to be tidally locked to their host stars and may not generate sufficiently strong magnetic fields to emit electron cyclotron maser emission at the low frequencies used in observations (typically ≥150 MHz). In comparison, the large mass-loss rates of evolved stars could enable exoplanets at larger orbital distances to emit detectable radio emission. Here, we first show that the large ionized mass-loss rates of certain evolved stars relative to the solar value could make them detectable with the LOw Frequency ARray (LOFAR) at 150 MHz (λ = 2 m), provided they have surface magnetic field strengths >50 G. We then report radio observations of three long period (>1 au) planets that orbit the evolved stars β Gem, ι Dra, and β UMi using LOFAR at 150 MHz. We do not detect radio emission from any system but place tight 3σ upper limits of 0.98, 0.87, and 0.57 mJy on the flux density at 150 MHz for β Gem, ι Dra, and β UMi, respectively. Despite our non-detections these stringent upper limits highlight the potential of LOFAR as a tool to search for exoplanetary radio emission at meter wavelengths.

scholarly journals A study of seasonal variations of gravity wave intensity in thelower thermosphere using LF D1 wind observations and a numerical model

2004 ◽  
Vol 22 (1) ◽  
pp. 35-45 ◽  
Author(s):  
N. M. Gavrilov ◽  
Ch. Jacobi
Keyword(s):  
Numerical Model ◽  
Drift Velocity ◽  
Seasonal Variations ◽  
Middle Atmosphere ◽  
Low Frequency ◽  
Internal Gravity Waves ◽  
Short Period ◽  
Summer Maximum ◽  
E Region ◽  
Standard Deviations

Abstract. The data of the regular low-frequency D1 E-region observations at Collm, Germany (52°N, 15°E) in 1983–1999 are used for estimations of the intensity of short-period perturbations of the horizontal drift velocity at 85–110 km altitude. A simple half-hourly-difference numerical filter is used to extract perturbations with time scales of 0.7–3 h. The average monthly standard deviations of short-period perturbations of the zonal velocity near altitude 83 km have a main maximum in summer, a smaller maximum in winter, and minimum values at the equinoxes. At higher altitudes the summer maximum is shifted towards the spring months, and a second maximum of perturbation amplitudes appears in autumn at altitudes near and above 100 km. The seasonal changes in the standard deviations of meridional velocity show the maxima in spring and summer. A numerical model describing the propagation of a set of harmonics modeling a spectrum of internal gravity waves in the atmosphere is used for the interpretation of observed seasonal variations of wind perturbation intensity. Numerical modeling reveals that the observed altitude changes in the seasonal variations of the drift velocity standard deviations may be explained by a superposition of IGWs generated at different levels in the troposphere and middle atmosphere. IGWs generated in the stratospheric and mesospheric jet stream may have substantial amplitudes at altitudes near and above 100 km, where they may modify the seasonal variations, which are typical for IGWs propagating from the troposphere. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) – Ionosphere (ionospheric irregularities)

Understanding the Dynamic Coupling Effects in Deep Water Floating Structures Using a Simplified Model

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Deep Water ◽  
Time Domain ◽  
Low Frequency ◽  
Wave Frequency ◽  
Computational Time ◽  
Coupled Analysis ◽  
Floating Platform ◽  
Coupling Effects ◽  
Mooring Lines

The global dynamic response of a deep water floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency, and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to the prohibitive computational time. This has spurred the development of more efficient methods, including frequency domain approaches. A good understanding of the intricate coupling mechanisms is paramount for making appropriate approximations in an efficient method. To this end, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is studied for physical insight. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are statistically linearized. The model allows key coupling effects to be understood; among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of some common approximations, and recommendations are made regarding the development of efficient modeling techniques.

Overview of observed seismic signals on Mars

2020 ◽  
Author(s):  
Savas Ceylan ◽  
John F. Clinton ◽  
Domenico Giardini ◽  
Maren Böse ◽  
Martin van Driel ◽  
...  
Keyword(s):  
Energy Content ◽  
Low Frequency ◽  
Careful Analysis ◽  
Atmospheric Effects ◽  
Seismic Signals ◽  
S Wave ◽  
Ground Segment ◽  
Short Period ◽  
Data Content ◽  
Set Up

<p>InSight landed on Mars in late November 2018, and the SEIS package, which consists of one short period and one very broadband sensor, was deployed on the surface shortly after. The data returned by the InSight is monitored in a timely manner by the Marsquake Service (MQS), a ground segment support group of InSight that has been set up to establish and maintain the seismicity catalogue. The MQS has at least one member on duty who routinely checks the data for any type of seismic signals. All suspicious signals are then communicated to the InSight team after evaluation.</p><p>To date, MQS has identified more than 365 events which are classified into two general families as high and low frequency, with each family having unique features in terms of their energy content. The most distinct quakes detected so far belong to the low frequency family that occurred on Sol 173 and 235, and have clear P and S-wave arrivals that reveal a distance around 30 degrees east of the lander, pointing the region in the vicinity of Cerberus Fossae. In addition to the signals of seismic origin, the SEIS data contain features that originate from other sources such as atmospheric effects or electronics. Part of these non-seismic observations may resemble quakes which may lead to wrong interpretations, and therefore require careful analysis.</p><p>Here, we show examples of signals of both seismic and non-seismic origins. We describe the characteristics of these observations in time and frequency domains in order to give an overview of martian data content.</p>

rTMS age-dependent response in treatment-resistant depressed subjects: a mini-review

CNS Spectrums ◽  
2012 ◽  
Vol 17 (1) ◽  
pp. 24-30 ◽  
Author(s):  
Stefano Pallanti ◽  
Andrea Cantisani ◽  
Giacomo Grassi ◽  
Sarah Antonini ◽  
Chiara Cecchelli ◽  
...  
Keyword(s):  
Elderly Patients ◽  
Rating Scale ◽  
Low Frequency ◽  
Treatment Resistant ◽  
The Third ◽  
Depressed Patients ◽  
Medical Disorders ◽  
Short Period ◽  
The Right ◽  
Low Frequency Rtms

BackgroundIn old age, depressive syndromes often affect people with chronic medical illnesses, cognitive impairment, and disability, which can worsen the outcomes of other medical disorders and promote disability. Repetitive magnetic transcranial stimulation (rTMS) is a simple and effective treatment in patients with treatment-resistant depression. Therefore the use of rTMS could be of particular potential benefit in treatment-resistant elderly patients, who often cannot tolerate the higher doses of drugs needed or show phenomena of intolerance and interaction. However, several studies assessing the efficacy of rTMS found smaller response rates in elderly patients when compared to younger samples. Nevertheless, the correlation between age and response is still a controversial issue, and there is no strong evidence to date. The aim of our study was to retest the effectiveness and safety of low-frequency rTMS in a 3 weeks active treatment in a group of resistant-depressed patients, and to investigate the role of age in the response to stimulation treatment.MethodsEnrolled in this study were 102 treatment-resistant depressed patients. The patients were treated with low-frequency rTMS over the right dorso-lateral prefrontal cortex (DLPFC) for 3 weeks with a simple protocol (420 pulses per session for 15 sessions). At baseline, at the end of the second week, and at the end of the third week of treatment, the Hamilton Depression Rating Scale (HAM-D) and the Hamilton Anxiety Rating Scale (HAM-A) were administered.ResultsLow-frequency rTMS on the prefrontal dorsolateral right area resulted in a statistically significant reduction of mean HAM-D scores in the entire group of patients at the end of treatment. The responder's rate in the whole group at the end of the third week was 56.86%. A significant inverse relationship between HAM-D reduction and age was found in the “older” (>60 years old) group, not in the “younger” (<60 years old) group.ConclusionResults from this study show that low-frequency rTMS over the right DLPFC, with a relatively low number of pulses (420 pulses per session) and a relatively short period of treatment, is effective in the treatment of resistant patients (in a sample also including elderly patients) in a 3-weeks treatment protocol with a low reduction with the progress of age. Furthermore, we found a greater response in younger patients and an inverse correlation between age and treatment response. Adaptations of the protocol according to age are reviewed.

scholarly journals Resonant behaviour of an oscillating wave energy converter in a channel

2012 ◽  
Vol 701 ◽  
pp. 482-510 ◽  
Author(s):  
Emiliano Renzi ◽  
F. Dias
Keyword(s):  
Wave Energy ◽  
Velocity Potential ◽  
Wave Energy Converter ◽  
Maximum Efficiency ◽  
Hypersingular Integral Equation ◽  
Energy Converter ◽  
Theory Application ◽  
Incident Waves ◽  
Resonant Mechanism ◽  
Recent Laboratory

AbstractA mathematical model is developed to study the behaviour of an oscillating wave energy converter in a channel. During recent laboratory tests in a wave tank, peaks in the hydrodynamic actions on the converter occurred at certain frequencies of the incident waves. This resonant mechanism is known to be generated by the transverse sloshing modes of the channel. Here the influence of the channel sloshing modes on the performance of the device is further investigated. Within the framework of a linear inviscid potential-flow theory, application of Green’s theorem yields a hypersingular integral equation for the velocity potential in the fluid domain. The solution is found in terms of a fast-converging series of Chebyshev polynomials of the second kind. The physical behaviour of the system is then analysed, showing sensitivity of the resonant sloshing modes to the geometry of the device, which concurs in increasing the maximum efficiency. Analytical results are validated with available numerical and experimental data.

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