• Structural Engineering and Mechanics
• /
• v.63 no.1
• /
• pp.37-46
• /
• 2017

This paper is concerned with the numerical study on the resonance response of a rigid spar-type floating platform in coupled heave and pitch motion. Spar-type floating platforms, widely used for supporting the offshore structures, offer an economic advantage but those exhibit the dynamically high sensitivity to external excitations due to their shape at the same time. Hence, the investigation of their dynamic responses, particularly at resonance, is prerequisite for the design of spar-type floating platforms which secure the dynamic stability. Spar-type floating platform in 2-D surface wave is assumed to be a rigid body having 2-DOFs, and its coupled dynamic equations are analytically derived using the geometric and kinematic relations. The motion-variance of the metacentric height and the moment of inertia of floating platform are taken into consideration, and the hydrodynamic interaction between the wave and platform motions is reflected into the hydrodynamic force and moment and the frequency-dependent added masses. The coupled nonlinear equations governing the heave and pitch motions are solved by the RK4 method, and the frequency responses are obtained by the digital Fourier transform. Through the numerical experiments to the wave frequency, the resonance responses and the coupling in resonance between heave and pitch motions are investigated in time and frequency domains.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.5 no.4
• /
• pp.614-624
• /
• 2013

The resonance power buoy is a convincing tool that can increase the extraction efficiency of wave energy. The buoy needs a corresponding draft, to move in resonance with waves within the peak frequency band where wave energy is concentrated. However, it must still be clarified if the buoy acts as an effective displacement amplifier, when there is insufficient water depth. In this study, the vertical displacement of a circular cylinder-type buoy was calculated, with the spectrum data observed in a real shallow sea as the external wave force, and with the corresponding draft, according to the mode frequency of normal waves. Such numerical investigation result, without considering Power Take-Off (PTO) damping, confirmed that the area of the heave responses spectrum can be amplified by up to about tenfold, compared with the wave energy spectrum, if the draft corresponds to the peak frequency, even with insufficient water depth. Moreover, the amplification factor of the buoy varied, according to the seasonal changes in the wave spectra.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.20 no.6
• /
• pp.583-588
• /
• 2008

Swell data observed in the East Sea in February, 2008 were analyzed using wavelet method. The wavelet analyzed results show detailed time series variation of wave group, peak frequency and spectrum. The comparison of time averaged wavelet spectrum with fourier spectrum turn out to be very similar in terms of spectrum shape and peak frequency evolution but the peak frequency wave energy and the significant wave height show discrepancies. Wavelet analysis can detect the change of spectrum in time as well as in frequency and very efficient to study transient and irregular phenomena such as freak waves and abnormal swells in the ocean. More analysis with more wave data are needed for future application.

• Journal of the Optical Society of Korea
• /
• v.19 no.3
• /
• pp.297-302
• /
• 2015

Terahertz (THz) wave generation from bulk and periodically poled $LiTaO_3$ (PPLT) with a Cherenkov phase matching scheme is numerically investigated. It is shown that by using the crystal birefringence of bulk $LiTaO_3$ and a grating vector of PPLT, THz waves can be efficiently generated by difference frequency generation (DFG) with a Cherenkov phase matching scheme. The frequency tuning characteristics of the THz wave via varying wavelength of difference frequency waves, phase matching angle, poling period of PPLT and working temperature are theoretically analyzed. The parametric gain coefficient in the low-loss limit and the absorption coefficient of the THz wave during the DFG process in the vicinity of polariton resonances are numerically analyzed. A THz wave can be efficiently generated by utilizing the giant second order nonlinearities of $LiTaO_3$ in the vicinity of polariton resonances.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.15 no.4
• /
• pp.207-213
• /
• 2003

This study is to numerically investigate the characteristics of wave energy variations propagating over impermeable submerged breakwaters with irregular waves. Two-dimensional numerical wave flume based on the VOF method was used. VOF method is the most efficient capable of simulating free surfaces including wave breaking. From the computed frequency spectrum results, wave breaking play important role in ability of the submerged breakwaters to dissipate incident wave energy. In case of occurring wave breaking, our analysis shows that wave energy moves to short wave period on one-row impermeable submerged breakwater's lee side and is widely distributed not having peak period on two- row impermeable submerged breakwater's lee side.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.13 no.7
• /
• pp.653-665
• /
• 2001

The characteristics of the pulsating flow in a hydraulic pipe have been investigated. It is necessary to study the power control of the power transmission system in the landing gear system of aircraft and the design of robots. In this system, the power transmission pipeline is composed of a hydraulic system, and the operating flow is unsteady flow. The wave equation varying with frequency is analyzed in order to investigate the characteristics of unsteady flow in such a pipe. This wave equation involves the propagation coefficient in terns of frequency and viscosity. The theoretical result of this wave equation are compared with experimental result. Each wave equation, varying with the propagation coefficient, is analyzed theoretically. then, a sinusoidal wave generator is built in order to make better sinusoidal waves, and a rectifier is built to eliminate the noise from the hydraulic pump. The theoretical results of the wave equation in the flow of viscous fluid agree well with experimental results.

• Journal of Ocean Engineering and Technology
• /
• v.35 no.5
• /
• pp.347-359
• /
• 2021

This study aims to evaluate the dynamic responses of the jacket-type offshore wind turbine using FAST software (Fatigue, Aerodynamics, Structures, and Turbulence). A systematic series of simulation cases of a 5 MW jacket-type offshore wind turbine, including wind-only, wave-only, wind & wave load cases are conducted. The dynamic responses of the wind turbine structure are obtained, including the structure displacement, rotor speed, thrust force, nacelle acceleration, bending moment at the tower bottom, and shear force on the jacket leg. The calculated time-domain results are transformed to frequency domain results using FFT and the environmental load with more impact on each dynamic response is identified. It is confirmed that the dynamic displacements of the wind turbine are dominant in the wave frequency under the incident wave alone condition, and the rotor thrust, nacelle acceleration, and bending moment at the bottom of the tower exhibit high responses in the natural frequency band of the wind turbine. In the wind only condition, all responses except the vertical displacement of the wind turbine are dominant at three times the rotor rotation frequency (considering the number of blades) generated by the wind. In a combined external force with wind and waves, it was observed that the horizontal displacement is dominant by the wind load. Additionally, the bending moment on the tower base is highly affected by the wind. The shear force of the jacket leg is basically influenced by the wave loads, but it can be affected by both the wind and wave loads especially under the turbulent wind and irregular wave conditions.

• Journal of Astronomy and Space Sciences
• /
• v.39 no.1
• /
• pp.11-22
• /
• 2022

It is suggested that magnetosonic waves (also known as equatorial noise) can scatter radiation belt electrons in the Earth's magnetosphere. Therefore, it is important to understand the global distribution of these waves between the proton cyclotron frequency and the lower hybrid resonance frequency. In this study, we developed an empirical model for estimating the global distribution of magnetosonic wave amplitudes and wave normal angles. The model is based on the entire mission period (approximately 2012-2019) of observations of Van Allen Probes A and B as a function of the distance from the Earth (denoted by L^*), magnetic local time (MLT), magnetic latitude (λ), and geomagnetic activity (denoted by the Kp index). In previous studies the wave distribution inside and outside the plasmasphere were separately investigated and modeled. Our model, on the other hand, identifies the wave distribution along with the ambient plasma environment-defined by the ratio of the plasma frequency (f_pe) to the electron cyclotron frequency (f_ce)-without separately determining the wave distribution according to the plasmapause location. The model results show that, as Kp increases, the dayside wave amplitude in the equatorial region intensifies. It thereby propagates the intense region towards the wider MLT and inward to L^* < 4. In contrast, the f_pe/f_ce ratio decreases with increasing Kp for all regions. Nevertheless, the decreasing aspect differs between regions above and below L^* = 4. This finding implies that the particle energy and pitch angle that magnetosonic waves can effectively scatter vary depending on the locations and geomagnetic activity. Our model agrees with the statistically observed wave distribution and ambient plasma environment with a coefficient of determination of > 0.9. The model is valid in all MLTs, 2 ≤ L^* < 6, |λ| < 20°, and Kp ≤ 6.

• 제어로봇시스템학회:학술대회논문집
• /
• 1996.10b
• /
• pp.173-176
• /
• 1996

The transient behavior of the complex-LMS adaptive filter is studied when the adaptive filter is operating on a fixed or sweeping complex frequency sine-wave signal. The first-order difference equation is derived for the mean weights and its closed form solution is obtained. The transient response is represented as a function of the eigenvectors and eigenvalues of input correlation matrix. The mean-square error of the algorithm is evaluated as well. An optimal convergence parameter and filter length can be determined for sweeping frequency sine-wave signals as a function of frequency change rate and signal and noise powers.

• Ocean Systems Engineering
• /
• v.7 no.1
• /
• pp.53-74
• /
• 2017

Ship shaped FPSO (Floating Production, Storage and Offloading) units are the most commonly used floating production units to extract hydrocarbons from reservoirs under the seabed. These structures are usually much larger than general cargo ships and have their natural frequency outside the wave frequency range. This results in the response to first order wave forces acting on the hull to be negligible. However, second order difference frequency forces start to significantly impact the motions of the structure. When the difference frequency between wave components matches the roll natural frequency, the structure experiences a significant roll motion which is also termed as second order roll. This paper describes the theory and numerical implementation behind the calculation of second order forces and motions of any general floating structure subjected to waves. The numerical implementation is validated in zero speed case against the commercial code OrcaFlex. The paper also describes in detail the popular approximations used to simplify the computation of second order forces and provides a discussion on the limitations of each approximation.