Effect of Porous Breakwaters and the Harbor under Oblique Wave Impact
Date Issued
2007
Date
2007
Author(s)
Hsu, Hao-Jen
DOI
zh-TW
Abstract
Optimizing design to the harbor and coastal structures for the purpose of eliminating wave energy always depends on accurate analysis and simulation to the wave field, and the interaction between water waves and porous structures is the key factor. However, when the incoming water wave is parallel to the porous breakwater, a paradoxical phenomenon that the water wave permeates completely into the original quiescent wave field inevitably occurs in all the earlier researches. It is indeed questionable and obviously against our physical expectation.
For more reasonable analysis of harbor resonance and wave energy elimination, the wave field and physical mechanism of an oblique water wave impacting on a solid porous wall is investigated in the present study, and the result is further applied in numerical analysis of harbor wave field.
Firstly, the three-dimensional boundary value problem of an oblique small-amplitude water wave impacting on an infinite-depth porous wall is formulated in the present study. And according to the similar concept proposed by Lamb (1945) and Beavers and Joseph (1967), the partial-slipping boundary condition applicable for the thin porous wall impacted by oblique potential waves is thus obtained, followed by the analytical solution of the potential flow model. The obtained result eliminates the above mentioned paradox of parallel waves impacting on the thin porous wall. Moreover, in order to investigate the physical phenomenon in the neighborhood of porous wall surfaces, the boundary-layer approximation is adopted in this study to analyze the viscous boundary layer effect, which provides proper boundary conditions on the thin porous wall for laminar flow and gives more detailed flow information.
Secondly, in order to apply the partial-slipping boundary condition in analysis of wave field in the harbor with porous walls inside, the analytical solution of oblique waves impacting on a thin porous wall with a vertical impermeable bank behind is thus obtained in the present study. And a simplified reflection coefficient and the boundary condition applicable for porous breakwaters inside the harbor are thus found. Furthermore, the boundary element method (BEM) is adopted to analyze the wave field of the harbor in which porous breakwaters are arranged alongside the inner walls. By applying our partial-slipping boundary condition including undetermined incident angles, the iteration method is used in the BEM model to find out the most appropriate incident angle for each boundary element. And the wave field of the harbor with porous breakwaters inside is well investigated and proven in better agreement with physical expectation than the earlier investigations.
Due to the reasonable expression for oblique water waves impacting on the thin porous wall and the desirable analysis of the harbor wave field, it can be concluded that the present study is well expected to be the theoretical basis for optimization of harbor oscillation and design of coastal structures for more effective wave energy elimination.
Subjects
微小振幅水波
斜向
多孔介質薄牆
邊界層效應
港池共振
small amplitude water wave
oblique
porous wall
boundary layer effect
harbor resonance
Type
thesis