• Water Engineering Research
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• v.1 no.3
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• pp.187-197
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• 2000

This paper describes an adaptive quadtree-based 2DH wave-current interaction model which is able to predict wave breaking, shoaling, refraction, diffraction, wave-current interaction, set-up and set-down, mixing processes (turbulent diffusion), bottom frictional effects, and movement of the land-water interface at the shoreline. The wave period-and depth-averaged governing equations are discretised explictly by means of an Adams-Bashforth second-order finite difference technaique on adaptive hierarchical staggered quadtree grids. Grid adaptation is achieved through seeding points distributed according to flow criteria(e.g. local current gradients). Results are presented for nearshore circulation at a sinusoidal beach. Enrichment permits refined modelling of important localised flow features.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2004.11a
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• pp.220-224
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• 2004

The present paper deals with the numerical study to analyze the self-starting performance of impulse turbine in a reciprocating air flow generated by sinusoidal motion of wave inside oscillating water column. Result was compared to that of Wells turbine, well-known wave energy conversion device, and showed that the impulse turbine has a superior self-starting ability. More detailed parametric study was performed to demonstrate the effects of moment of inertia of rotor, loading torque, tip clearance and angle of guide vane.

• Journal of Korean Port Research
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• v.12 no.1
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• pp.75-86
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• 1998

To design a coastal structure in the nearshore region, engineers must have means to estimate wave climate. Waves, approaching the surf zone from offshore, experience changes caused by combined effects of bathymetric variations, interference of man-made structure, and nonlinear interactions among wave trains. This paper has attempted to find out the effects of two of the more subtle phenomena involving nonlinear shallow water waves, amplitude dispersion and secondary wave generation. Boussinesq-type equations can be used to model the nonlinear transformation of surface waves in shallow water due to effect of shoaling, refraction, diffraction, and reflection. In this paper, generalized Boussinesq equations under the complex bottom condition is derived using the depth averaged velocity with the series expansion of the velocity potential as a product of powers of the depth of flow. A time stepping finite difference method is used to solve the derived equation. Numerical results are compared to hydraulic model results. The result with the non-linear dispersive wave equation can describe an interesting transformation a sinusoidal wave to one with a cnoidal aspect of a rapid degradation into modulated high frequency waves and transient secondary waves in an intermediate region. The amplitude dispersion of the primary wave crest results in a convex wave front after passing through the shoal and the secondary waves generated by the shoal diffracted in a radial manner into surrounding waters.

• Wind and Structures
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• v.23 no.2
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• pp.127-142
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• 2016

Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number $Re_{Dm}$ = 16,500-22,000, based on the nominal width ($D_m$) of the prism and free-stream velocity ($U_{\infty}$). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of $6D_m$ and an amplitude of $0.15D_m$. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.

• Steel and Composite Structures
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• v.29 no.1
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• pp.53-66
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• 2018

A high-order nonlocal strain gradient model is developed for wave propagation analysis of porous FG nanoplates resting on a gradient hybrid foundation in thermal environment, for the first time. Material properties are assumed to be temperature-dependent and graded in the nanoplate thickness direction. To consider the thermal effects, uniform, linear, nonlinear, exponential, and sinusoidal temperature distributions are considered for temperature-dependent FG material properties. On the basis of the refined-higher order shear deformation plate theory (R-HSDT) in conjunction with the bi-Helmholtz nonlocal strain gradient theory (B-H NSGT), Hamilton's principle is used to derive the equations of wave motion. Then the dispersion relation between frequency and wave number is solved analytically. The influences of various parameters (such as temperature rise, volume fraction index, porosity volume fraction, lower and higher order nonlocal parameters, material characteristic parameter, foundations components, and wave number) on the wave propagation behaviors of porous FG nanoplates are investigated in detail.

• Proceedings of the Korea Water Resources Association Conference
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• 2004.05b
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• pp.479-483
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• 2004

A new calculation method of expected overtopping probability of rubble mound breakwater considering real tide occurrence has been proposed. A calculation method of expected overtopping probability of rubble mound breakwater was proposed by Kweon and Suh (2003). In their calculation, the fluctuation of tidal elevation was expressed by the sinusoidal change that yields the uniform distribution of occurrence frequency. However, the realistic distribution of tidal elevation should influence on the overtopping chance. In this study, the occurrence frequency of tidal elevation obtained from the real sea is included. The tidal elevation used in this study is collected from the east coastal part of Korean peninsular. Analyzing the annual data of the tidal fluctuation measured hourly during 355 days, the distribution of occurrence frequency is formulated utilizing by the normal distribution with one peak. Among the calculation procedures of annual maximum wave height, wave height-period joint distribution, wave run-up height and occurrence frequency of tide, only the annual maximum wave height is again chosen randomly from normal distribution to consider the uncertainty. The others are treated by utilizing the distribution function or relationship itself, It is found that the inclusion of the variability of tidal elevation has great influence on the computation of the expected overtopping probability of rubble mound breakwater. The bigger standard deviation of occurrence frequency is, the lower the overtopping probability of rubble mound breakwater is.

• Structural Engineering and Mechanics
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• v.52 no.3
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• pp.633-646
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• 2014

A semi-analytical numerical approach for the effective structural dynamic response analysis of spar floating substructure for offshore wind turbine subject to wave-induced excitation is introduced in this paper. The wave-induced rigid body motions at the center of mass are analytically solved using the dynamic equations of rigid ship motion. After that, the flexible structural dynamic responses of spar floating substructure for offshore wind turbine are numerically analyzed by letting the analytically derived rigid body motions be the external dynamic loading. Restricted to one-dimensional sinusoidal wave excitation at sea state 3, pitch and heave motions are considered. Through the numerical experiments, the time responses of heave and pitch motions are solved and the wave-induced dynamic displacement and effective stress of flexible floating substructure are investigated. The hydrodynamic interaction between wave and structure is modeled by means of added mass and wave damping, and its modeling accuracy is verified from the comparison of natural frequencies obtained by experiment with a 1/100 scale model.

• Journal of Korea Water Resources Association
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• v.36 no.5
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• pp.741-749
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• 2003

This study presents a combined experimental and numerical effort to investigate experimentally and numerically the Bragg reflection of sinusoidal waves due to trapezoidal submerged porous breakwaters. Numerical predictions of the study are verified by comparing to laboratory measurements. In the numerical model, the flow in porous structures is described by the spatially averaged Navier-Stokes equations and the volume of fluid method is employed to track the free surface displacements. Numerical solutions are agree well with laboratory measurements. The reflection coefficients of porous structures are smaller than those of non-porous structures and become stronger in proportion to the increase of number of submerged breakwaters.

• The Transactions of the Korean Institute of Power Electronics
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• v.8 no.1
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• pp.89-97
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• 2003

High frequency common mode voltage produced by PWM inverter fed Induction motor is a major cause of conducted EMI, creation motor ground currents, bearing currents and other harmful products. The zero switching states of inverter control invoke large in comparison with the non-zero switching state of Inverter control. We proposed a common mode voltage reduction method based on sinusoidal PWM technique. PWM signal are generated by comparing respective sinusoidal reference signal with three triangular carrier wave displaced of 120$^{\circ}$. Simulation and experimenta1 result show that common mode voltages in the proposed PWM technique are reduced by approximate 66% more than conventional FWM technique.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.36 no.5
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• pp.367-376
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• 1987

In this paper we have disigned and implemented a real time ALE (adaptive line enhancer) using a high speed digital processor,NEC 7720. For the ALE system, we have employed an adaptive LMS(least mean square) algorithm proposed by Widrow and Hoff and a 32-order FIR(finite impulse response) filter. Extensive computer simulations have been performed to investigate the peformance of the ALE and to determine necessary parameters for hardware design. The developed software for an NEC 7720 was tested in real time operation using an NEC7720 hardware emulator. The ALE has been tested by sinusoidal waves and real CW (continuous wave) signals. It was found that the experimental results were well agreed with the computer simulation results. Thus it can be concluded that the ALE is useful for detection and enhancement of a sinusoidal signal which is corrupted by an additive Gaussian noise.