Reflection Of Irregular Waves Research Articles

Experimental Study of Irregular Wave Reflection by a Perforated Caisson Breakwater Under Wave Overtopping Conditions

Experimental Study of Irregular Wave Reflection by a Perforated Caisson Breakwater Under Wave Overtopping Conditions

Numerical study of irregular wave propagation over sinusoidal bars on the reef flat

Irregular wave propagation over sinusoidal bars on the reef flat is studied based on a fully nonlinear shock-capturing Boussinesq model FUNWAVE-TVD. The model was first verified with existing experimental, theoretical, and numerical data to confirm its robustness in predicting irregular propagation over the reef bathymetry and Bragg reflection of irregular waves induced by sinusoidal bars on the horizontal bottom. A series of numerical experiments were then performed based on the verified model to investigate the Bragg reflection effect induced by sinusoidal bars set on the reef flat for irregular waves. Model results show when the natural frequencies of sinusoidal bars fall in the sea-swell wave band, sinusoidal bars on the reef flat can induce a Bragg reflection of sea-swell waves. However, no certain primary reflection peak is observed for sea-swell waves, and a substantial part of sea-well wave energy can still transfer to infragravity wave energy before reflection so that the reduction effect of infragravity waves is barely satisfactory. In this situation, increasing the amplitude of sinusoidal bars can still enhance the Bragg reflection of irregular waves while increasing the number of sinusoidal bars has no regular effect. When the natural frequencies of sinusoidal bars fall in the infragravity wave band, the reduction effects of both infragravity waves and sea-swell waves are better, but still no primary reflection peak is observed. In this situation, increasing the amplitude of sinusoidal bars can also enhance the Bragg reflection of irregular waves. Overall, the results presented in this paper show that the efficiency of idealized sinusoidal bars on the reef flat to reduce infragravity waves is not as good as expected.

Reflection of Irregular Waves from Perforated-wall Structures when the Chamber is Filled with Rocks

Kim, Y.-T. and Lee, J.-I., 2021. Reflection of irregular waves from perforated-wall structures when the chamber is filled with rocks. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 151–155. Coconut Creek (Florida), ISSN 0749-0208. The perforated-wall is normally applied to the quay walls and breakwaters such as vertical structures to improve the hydraulic characteristics. The perforated-wall could reduce the wave reflection, wave pressure, and wave overtopping comparing with the plane wall. The chamber of perforated-wall was generally designed empty, that is, the chambers were filled with only seawater. In this study, the hydraulic characteristics of perforated-wall were investigated with hydraulic model experiments when the chambers were filled with the armor rocks. The perforated-wall with vertical slits was applied in the tests. The numbers of the chamber and the sizes of the armor rock were changed. The reflection from the perforated wall was measured and compared for the various test conditions.

해수교환방파제가 설치된 주문진항에서의 파고분포 해석

항내 정온도를 산정 할 때에는 불규칙파의 회절과 반사가 정확하게 계산되어지는 것이 중요하다. 본 연구에서 수치모형의 기본방정식은 완경사 방정식을 사용하였으며, 이는 파랑 거동에 큰 영향을 미치는 비선선형성을 고려할 수 있는 장점을 가지고 있다. 그리고, 삼각격자망은 유한요소법을 이용하여 생성되었다. 비선형성의 효과를 검증하기 위하여 본 연구에서 개발된 수치모형의 계산결과들은 다른 연구자들의 실험결과 및 수치결과와 비교되어졌다. 그 결과, 비선형성의 파랑을 고려한 경우가 고려하지 않은 경우보다 파랑해석에 더 정확하다는 것을 보여주었다. 본 모형을 적용하기 위하여 해수교환방파제가 설치되어 있는 주문진항에 파고분포가 계산되어졌다. 수치해석결과, 이상 파랑이 해수교환방파제를 통한 유입 시에 항내 파고분포의 결과들이 높게 나타났다. 그러므로, 항내 파고를 낮게 유지하기 위해서는 이상 파랑의 유입을 차단할 수 있는 시설이 필요되어질 것으로 사료된다. When estimating the calmness in a harbor, it is important that diffraction and reflection of irregular waves should be exactly calculated. The basic equation of the numerical model in this study was used Mild-slope equation, which has the advantage of which non-linearity with great influence for the wave behavior can be considered, and a triangular mesh was generated by using finite element method. So as to verify the nonlinear effects, the results of the numerical model developed in this study are compared with the experimental and numerical results by other researchers. As a result, it is shown that the results in case of considering nonlinear wave are more exact for wave analysis than in case of not considering nonlinear wave. In order to apply this model, wave height distributions in Jumunjin fishery port installed a seawater-exchange breakwater are computed. From the results of this numerical analysis, when abnormal waves are intruded through the seawater-exchange breakwater, the results of the wave height distributions in the harbor are highly presented. Therefore, in order to get wave height low in the harbor, it is considered that the facility with the ability to protect the inflow of abnormal waves is needed.

Diffraction and reflection of irregular waves in a harbor employing a spectral model

The SWAN wave model is widely used in coastal waters and the main focus of this work is on its application in a harbor. Its last released version - SWAN 40.51 - includes an approximation to compute diffraction, however, so far there are few published works that discuss this matter. The performance of the model is therefore investigated in a harbor where reflection and diffraction play a relevant role. To assess its estimates, a phase-resolving Boussinesq wave model is employed as well, together with measurements carried out at a small-scale model of the area behind the breakwater. For irregular, short-crested waves with broad directional spreading, the importance of diffraction is relatively small. On the other hand, reflection of the incident waves is significant, increasing the energy inside the harbor. Nevertheless, the SWAN model does not achieve convergence when it is set to compute diffraction and reflection simultaneously. It is concluded that, for situations typically encountered in harbors, with irregular waves near reflective obstacles, the model should be set without the diffraction option.

An application of Boussinesq equations to Bragg reflection of irregular waves

A numerical model based on the second-order fully nonlinear Boussinesq equations of Wei et al. [1995. Journal of Waterway, Port, Coastal and Ocean Engineering 121 (5), 251-263] is developed to simulate the Bragg reflection of both regular and irregular surface waves scattered by submerged bars. Particularly for incident regular waves, the computed results are observed to agree very well with the existing experimental data as presented by Davies and Heathershaw [1984. Journal of Fluid Mechanics 144, 419-446] and Kirby and Anton [1990. Proceedings of the 22nd International Conference on Coastal Engineering, ASCE, New York, pp. 757–768). In the case of incident irregular waves, the simulated results reveal that the distribution of Bragg reflection from irregular waves becomes more flat than that of regular waves. Due to lack of experimental data, the numerical results for incident irregular waves are compared with those of the evolution equation of the mild-slope equation [Hsu et al., 2002 Proceedings of the 24th Ocean Engineering Conference in Taiwan, pp. 70–77 (in Chinese)]. In addition, several parameters such as the number of bars, the relative height of bars and the spacing of bars affecting Bragg reflection are also discussed.

사각형형상 불투과성 수증방파제에 의한 불규칙파의 변형

본 연구에서는 사각형형상 수중방파제에 의한 불규칙파의 반사에 대하여 수리모형실험과 수치모형실험을 수행한 후 실험결과를 비교하였다. 수치해석 모형에서는 Reynolds 방정식을 지배방정식으로 사용하고 난류해석을 위해 k-<TEX>$\varepsilon$</TEX>모델을 적용하였으며, 자유수면변위를 추적하기 위해 VOF기법을 사용하였다. 수리모형실험과 수치모형실험의 결과는 서로 잘 일치하였으며, 수중방파제의 배열이 증가함에 따라 반사율은 증가하였다. A combined experimental and numerical effort is presented for investigation of reflection of irregular waves due to rectangular submerged breakwaters. In the numerical model, the Reynolds equations are solved by a finite difference method and k-<TEX>$\varepsilon$</TEX> model is employed for the turbulence analysis. To track the free surface displacement, the volume of fluid method is employed. Numerical predictions of transmission and reflection coefficients are verified by comparing to laboratory measurements. Very reasonable agreements are observed. The reflection coefficients become stronger in proportion to numbers of submerged breakwaters.

Reflection of irregular waves from perforated-wall caisson breakwaters

An analytical model has been developed that predicts the reflection of irregular waves normally incident upon a perforated-wall caisson breakwater. To examine the predictability of the developed model, laboratory experiments have been conducted for the reflection of irregular waves of various significant wave heights and periods impinging upon breakwaters having various wave chamber widths. For frequency-averaged reflection coefficients, though the overall agreement is fairly good between measurement and calculation, the model somewhat over-predicts the reflection coefficients at larger values, and under-predicts at smaller values. The model also underestimates the energy loss coefficients as wave reflection becomes larger. These differences occur because the model neglects the evanescent waves near the breakwater, which increase the energy loss at the perforated wall. The frequency-averaged reflection coefficient shows a minimum when the wave chamber width is approximately 0.2 times the significant wavelength, and it decreases with increasing wave steepness. Finally, it is shown that the reflection of irregular waves from a perforated-wall caisson breakwater depends on the wave frequency, so that the reflected wave spectrum shows a frequency dependent oscillatory behavior.

Characteristics of Wave Reflection Spectra

Experimental results of irregular wave reflection from impermeable walls and rubble mound breakwaters suggest that there is a characteristic form of wave reflection spectrum. The results indicate that the reflection coefficient at a particular frequency within a random wave spectrum is determined by an Iribarren number based on that frequency. An empirical relationship between reflection coefficient and local, frequency-dependent Iribarren number is provided for both structure types, although the range of parameters used in the rubble mound tests is limited. Therefore, the reflection coefficient spectrum can be determined from the structure slope, frequency, incident significant wave height, and two fitted coefficients that vary with structure type. Specification of the reflection coefficient spectrum becomes more critical as the width of the incident wave spectrum increases and as the Iribarren number decreases. Otherwise, random wave reflection can be adequately determined using the bulk reflection coefficient across the frequency range of the incident sea.

Laboratory wave reflection analysis using co-located gages

Most coastal and ocean engineering laboratories employ techniques that use two or three spatially separated wave gages to estimate reflection of irregular waves in two-dimensional wave flumes. This paper presents a frequency domain method for separating incident and reflected wave spectra from co-located gages (gages located on the same vertical line). The technique is based on linear wave theory, and it can be applied to time series of sea surface elevation and horizontal water velocity collected in a vertical array, or it can be used with horizontal and vertical water velocity time series collected at the same point in the water column. Application of the method is limited to those frequencies showing good coherence between time series signals. Outside the range of good coherence, gross inaccuracies occur. The utility of the co-located gage method is illustrated using water velocity data collected in a wave flume with a laser Doppler velocimeter, and the method is validated for the case of complete reflection by a vertical wall. Side-by-side comparison to the spatially-separated wave gage method of Goda and Suzuki (1976) exhibited close agreement for a variety of irregular wave trains being weakly reflected by a mild sloping beach. The co-located gage method is useful in situations where there are spatial variations in the wavelength, such as on a mildly sloping bottom, or in the region close to highly reflective structures where errors arising from spatial variations in characteristic wave parameters would corrupt estimates made using the spatially-separated wave gage method.

Irregular Wave Reflection and Run‐Up on Rough Impermeable Slopes

Three irregular wave test runs were conducted to obtain detailed data on irregular wave reflection and run‐up on a 1:3 rough impermeable slope. The test results were also used to evaluate the capabilities and limitations of an extended numerical model for predicting the time series and spectral characteristics of the reflected waves and waterline oscillations on the slope. The numerical model is shown to predict the measured time series and spectra reasonably well, including the selective nature of wave reflection and dissipation as well as the appearance of low‐frequency wave components in the waterline oscillations on the 1:3 slope. The numerical model extended herein may not be as accurate as hydraulic model tests but can be used to predict hydrodynamic quantities that are difficult to measure directly. Moreover, any incident irregular wave train including that measured in the field or generated numerically for given incident wave spectrum can be specified as input to the numerical model. This will avoid the difficulties associated with the reproduction of specified incident irregular waves in experiments.

Wave Grouping Effect in Irregular Wave Agitation in Harbors

Hydraulic model tests were conducted on two different natural harbors to study the influence of diffraction and reflection of irregular waves compared to regular waves. Two types of reflective walls within one harbor were also tested. Irregular wave trains with different peak frequencies and groupiness factors were generated. These tests have shown that within the limits of the frequency band of the spectral energy density, irregular wave diffraction‐reflection coefficients show less variability and appear to give better information than do the ones obtained from regular wave tests. Outside the frequency range containing most of the energy of the incident wave train, high values of the transfer function were observed in the lower and upper frequency range. However, their energy content was relatively small, and the long wave energy attached to wave grouping could not be attributed to harbor resonance. In the high frequency range, the presence of noise is attributed to the high transfer function estimates.

Performance of a Wedge‐Type Absorbing Wave Maker

Design criteria for absorbing wave makers are presented and their implementation illustrated by reference to the conversion of a conventional wave maker with a wedge-shaped paddle. Measurements obtained with both regular and irregular waves demonstrate that the required feedback loop in the wave maker's control circuit can be successfully based on a conventional wave gauge. Furthermore, it is shown that good absorption characteristics can be achieved with a less-than-perfect circuit design. Comparison of results obtained both with and without absorption highlight some of the dangers of investigating the behaviour of reflective coastal structures in a laboratory channel equipped with a conventional wave maker. It is shown that the multiple reflection of irregular waves between a test structure and a nonabsorbing wave maker can produce systematic changes in the incident wave climate that it would be difficult to rectify. Generally the full consequences of such changes will be unknown but they could be very serious. Thus it is strongly recommended that tests on reflective structures should be carried out only with absorbing wave makers.