• 한국방재학회:학술대회논문집
• /
• 2008.02a
• /
• pp.439-442
• /
• 2008

Diffraction of multi-directional random waves passing semi-infinite breakwater is investigated by using analytic solution derived by Penny and Prices(1952). An irregylarity of period and incident angle of waves and regular periods for regular waves are considered in addition by expanding from the past study which used only monochromatic wave in general. The Bretschneider-Mitsuyasu frequency spectrum and Mitsuyasu directional spectrum are used for incident waves. And diffraction of multi-directional random waves is reappeared by decomposing numerical results of several monochromatic waves which have variable period and incident angle. Analytic solution on the diffraction of regular waves and multi-directional random waves calculated in this study.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.2 no.1
• /
• pp.51-57
• /
• 1990

Based on second-order Stokes wave and parabolic approximation, a refraction-diffraction model for linear and nonlinear waves is developed. With the assumption that the water depth is slowly varying, the model equation describes the forward scattered wavefield. The parabolic approximation equations account for the combined effects of refraction and diffraction, while the influences of bottom friction, current and wind have been neglected. The model is tested against laboratory experiments for the case of submerged circular shoal, when both refraction and diffraction are equally significant. Based on Boussinesq equations, the parabolic approximation eq. is applied to the propagation of shallow water waves. In the case without currents, the forward diffraction of Cnoidal waves by a straight breakwater is studied numerically. The formation of stem waves along the breakwater and the relation between the stem waves and the incident wave characteristics are discussed. Numerical experiments are carried out using different bottom slopes and different angles of incidence.

• Water for future
• /
• v.9 no.1
• /
• pp.70-80
• /
• 1976

The efficient breakwater design requires a knowledge of the behaviour of the waves passing the breakwater. Wave Diffraction is an important factor and phenomeon in this behaviour. The diffraction ocean waves entering a gap in a breakwater normal to the incident wave direction in water of uniform depth has been investigated, applying a solution previously given in the author's paper, based on the theory of light diffraction by Sommerfeld. The wave profiles and heights on both the leeward side of the breakwater and the gap side have been studied and summarized in the form of diagrams with diffraction coefficients in range of x/L, y/L 0∼100, b/L=0.5∼12, with some extension of the diagrams made previously. The results of the theoretical approaches have not been experimentally verified. The theory ad computation methods with computer program in Fortran IV developed in this study make an efficient use for estimating the diffraction about a breakwater gap.

• Water for future
• /
• v.7 no.1
• /
• pp.55-64
• /
• 1974

The knowledge of the waves passing through the breakwater makes an important role in the efficient breakwater design. Wave diffraction is an important factor in this role, but some usable development about it have not been made in our country as yet. The diffraction of sea-water waves round the end of a semi-infinite impermeable breakwater has been investigated, applying a solution of the water wave diffraction problems given by Penney & Price. The wave pattern and heights on both the leewardside and the windward-side of the breakwater have been calculated and summarized in the form of diagrams with diffraction factors between $r/{\lambda}=0~50$. This involves some extension of the diffraction diagrams made previously. The theory and computation methods with computer program in fortran IV developed in this study make an efficient use for estimating the diffraction effects at a semi-infinite breakwater.

• Water Engineering Research
• /
• v.3 no.3
• /
• pp.203-213
• /
• 2002

A wide-angle parabolic approximation equation model considering the interaction between wave and current is employed to simulate the deformation of irregular waves over a submerged shoal. It is found that the model gives qualitative agreements with experimental data for the cases of breaking waves around the shoal. Thus, the effect of breaking-induced current on the refraction-diffraction of waves is well understood.

• Journal of Ocean Engineering and Technology
• /
• v.15 no.1
• /
• pp.12-18
• /
• 2001

This study concentrates on the second-order diffraction problem around circular cylinders in multi-frequency waves. The method of solution is a time-domain Rankine panel method which adopts a higher-order approximation for the velocity potential and wave elevation. In the present study, the multiple second-order quadratic transfer functions are extracted from the second-order time signal generated in random waves, and the comparison with other bench-mark test results shows a good agreement. This approach is directly applicable to prediction of nonlinear forces on offshore structures in random ocean.

• ETRI Journal
• /
• v.42 no.6
• /
• pp.827-836
• /
• 2020

Measuring the diffraction loss for high frequencies, long distances, and large diffraction angles is difficult because of the high path loss. Securing a well-controlled environment to avoid reflected waves also makes long-range diffraction measurements challenging. Thus, the prediction of diffraction loss at millimeter-wave frequency bands relies on theoretical models, such as the knife-edge diffraction (KED) and geometrical theory of diffraction (GTD) models; however, these models produce different diffraction losses even under the same environment. Our observations revealed that the KED model underestimated the diffraction loss in a large Fresnel-Kirchhoff diffraction parameter environment. We collected power-delay profiles when millimeter waves propagated over a building rooftop at millimeter-wave frequency bands and calculated the diffraction losses from the measurements while eliminating the multipath effects. Comparisons between the measurements and the KED and GTD diffraction-loss models are shown. Based on the measurements, an approximation model is also proposed that provides a simple method for calculating the diffraction loss using geometrical parameters.

• 한국연소학회:학술대회논문집
• /
• 2006.10a
• /
• pp.9-11
• /
• 2006

Visualization has played an essential role in the development of our understanding of the complex unsteady flows associated with the initiation, propagation, and extinction of detonation waves. These methods and application to various aspects of detonation are illustrated by results obtained in my laboratory, particularly using combinations of the PLIF technique with other methods. Examples shown will include detonation initiation by projectiles, diffraction over ramps and steps, diffraction out of tubes, detonation implosion, and the cellular structure of detonation waves.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.5 no.1
• /
• pp.45-50
• /
• 1993

The phenomenon of wave diffraction due to structure is an important factor in the wave climate at the site As an approximation, the propagation characteristics of a regular wave train are usually used. instead of those of irregular waves. However, there are great differences between the diffraction coefficients of the irregular waves and monochromatic waves, as shown by Goda (1985). The spectral calculation method. one of the methods to deal with the transformation of random sea waves essentially consists of decomposing a spectrum of the irregular sea state Into various monochromatic components, and assembling the component results by linear superposition. Monoch romatic wave transformation model developed by Chen(1987) is used to make spectral calculation. These calculations agree closely with Goda et al. (1978)'s diffraction diagram for a thin semi-infinite breakwater.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.33 no.6
• /
• pp.275-286
• /
• 2021

In this study, the analytical solution for diffraction near a vertical detached breakwater was suggested by superposing the solutions of diffraction near a semi-infinite breakwater suggested previously using linear wave theory. The solutions of wave forces acting on front, lee and composed wave forces on both side were also derived. Relative wave amplitude changed periodically in space owing to the interactions between diffracting waves and standing waves on front side and the interactions between diffracting waves from both tips of a detached breakwater on lee side. The wave forces on a vertical detached breakwater were investigated with monochromatic, uni-directional random and multi-directional random waves. The maximum composed wave force considering the forces on front and lee side reached maximum 1.6 times of wave forces which doesn't consider diffraction. This value is larger than the maximum composed wave force of semi-infinite breakwater considering diffraction, 1.34 times, which was suggested by Jung et al. (2021). The maximum composed wave forces were calculated in the order of monochromatic, uni-directional random and multi-directional random waves in terms of intensity. It was also found that the maximum wave force of obliquely incident waves was sometimes larger than that of normally incident waves. It can be known that the considerations of diffraction, the composed wave force on both front and lee side and incident wave angle are important from this study.