Research On Hydrodynamic Performance Of Comb-type Breakwater

Comb-type breakwater initiated in China during "Ninth-Five Year Plan" is a novel coastal structure with independent intellectual property. A type of open comb-type breakwater has been successfully applied in Dayao Bay, Dalian, China. Compared with conventional vertical wall breakwaters, the comb-type breakwater has been found to be effective in reducing wave reflection, wave forces and dissipating incident wave energy. In the continuing project of Dayao Bay, the impermeable comb-type breakwater was proposed with the purpose of ship berthing, however, in the process of construction, the side-plates and the superstructure were all damaged by a small wind wave. Thus, it can be deduced that the hydrodynamic characteristics of comb-type breakwater has be changed due to the different design of side-plate, which also leads to the complicated mechanism of comb-type breakwater. However, the existing studies about comb-type breakwater were almost exclusively based on laboratory experiments with limited wave conditions and dimension sizes. The empirical formulae for calculating the wave forces and wave refection on comb-type breakwaters based on the experiments are limited in practical application. Meanwhile, the working mechanism of it has not been investigated. This work aims to conduct a systematic investigation on the hydrodynamic characteristics of the comb-type breakwater with both experimental and numerical methods. The thesis work can be described as the following four major parts.Firstly, the hydrodynamic performance of an impermeable comb-type breakwater under waves is studied experimentally. The preliminary analysis indicated that the wave force on the side-plate is influenced significantly by the water depth and found not to increase with the increase of the water depth. When the water level is a certain distance below the bottom of the crest wall, the wave pressure on side-plate exhibit a notable characteristic of impact pressure, which dramatically exceeds the structural bearing capacity. Then three improved different kinds of non-open comb-type breakwaters are proposed and investigated experimentally in order to seek the optimum way that could reduce the wave pressure on side-plate. Based on the comprehensive comparison of wave pressure distribution on the side-plate and wave reflection coefficient, the structure with "â… " type baffle is selected as the optimum structure, which provides valuable information for the practical engineering.Then, a 3-D numerical wave flume is developed based on 3-D Reynolds-Averaged Navier-Stokes Equation. The source wave-generation method is employed to generate incident waves. The free surface is captured with VOF method and spongy layer is set at the both ends of the flume to absorb the reflected waves. Less numerical dissipation and excellent uniformity is obtained. Various wave trains, such as linear regular waves, Stokes waves, and irregular waves were generated by using different source functions. Further, the numerical wave flume is configured to simulate the standing waves in front of a vertical breakwater and floating identical rectangular structures to validate the dependency on the numerical grids and the effectiveness of the calculation of wave forces.Based on the above numerical work, an original numerical model for simulating the interaction between comb-type breakwater and irregular waves is developed. Through comparisons between the numerical and experimental results of the wave pressure, the characteristic of the impact pressure on side-plates are reproduced accurately by the numerical model. Further, the flow field in the cavity formed between side-plate and crest wall are analyzed to explore the mechanism of the impact pressure. The analysis indicates that the main cause of the occurrence of the impact pressure on side-plate is due to the cavity formed between side-plate and crest wall which prevent the moving of the flow in it. Thus the local flow velocity ascends suddenly and exerts a tremendous wave force on the side-plate. The relative wave height (the ratio of incident wave height to the distance between water surface and the super-structure) influences the impact pressure most. Then, the optimal measures are proposed to decrease the impact pressure, which includes raising the bottom of the crest wall and partly opening of the bottom of the crest wall.A total of 284 simulations are conducted to investigate the hydrodynamic characteristics of comb-type breakwater with both water level over and below the bottom face of crest wall, subject to variety of wave conditions and dimension sizes. When the water level exceeds the bottom face of crest wall, it's found that the reflection coefficient KR is influenced by not only b/L, but also c/d. KR increases lineally with the increase of c/d. The transmission coefficient KT is also affected by both b/L and c/d, An empirical formula for KR and KT are proposed based on numerical results.When the water level is below the bottom face of crest wall, the main factors of the wave force reduction coefficient KF-R, transmission KT are discussed, empirical formula for KF-R and KT are also proposed based on numerical results. Furthermore, the relationship between maximum wave force and average wave force when wave crest acting on the structure, are also investigated. The empirical formulae proposed in this paper essentially extents the application range of the existing formulae deduced from experiments by other researchers.