Study On The Hydrodynamics Of Horizontal Twin-plate Breakwater

Horizontal twin-plate breakwater is a relatively new type breakwater and has environmental protection function. It has many advantages compared with traditional breakwaters:(1) No damage to the ocean environment because of its permitting water exchange from inside and out of the breakwater;(2) Easy to construct because of its simple shape;(3) The construction cost does not change significantly with water depth. The main obstacle to the practical application of this structure lies in the lack of quantitative evaluation method of wave dissipation and practical calculation method of load. So it is significant to deeply research the hydrodynamics of horizontal twin-plate breakwater. And it has practical significance for the promotion and application of this structure.This paper presents the experimental and numerical investigation of wave elimination and hydrodynamics of horizontal twin-plate breakwater.The wave dissipation mechanism is also discussed. The main achievements are as follows:(1) About calculation method of the transmission coefficient and total wave forceThe parameters such as relative plate width、relative wave height、wave steepness、 spacing ratio of twin plates and relative submergence are consided. The formula for calculating transmission coefficient and total vertical wave force are obtained using the least square method.(2) About velocity fields around the twin-plate breakwater and wave dissipation mechanismThe horizontal twin plate changes the kinestate of the water particle. Incorporate is:The water particle between the twin plates is in the state of horizontal reciprocating flow. And the water particle between the lower plate and the bottom has the same flow characteristics. For the submerged breakwater, the vorticity appears bwteen the upperside of the upper plate and the free surface. However, for the emerged breakwater, the vorticity appears bwteen the upperside of the lower plate and the free surface. The maximum vorticity moves with the wave propagation. The period of the vortex shedding coincides with the incident wave period. The pressure changes depend on the velocity field.The twin plates affect the velocity fields in front of the breakwater little, when the distance between the water particle and the breakwater is more than the width of the plate.However it has great influence on the velocity fields behind the breakwater. The wave energy flux in different positions has uniform difference. The wave energy flux between the plates of the submerged breakwater can transfer to the opposite direction. And the wave energy flux beneath the lower plate of the submerged and emerged breakwater has the same transfer mode. The wave dissipation mechanism is the energy loss due to the vortex and the reversion transmission of the wave energy flux.(3)About wave elimination of the horizontal plateWhen the wave interacts with the horizontal twin-plate breakwater, the breakwater not only can eliminate wave energy (reflected and dissipated part of the wave energy), but also can change wave frequency. Wave surface behind the submerged and emerged breakwater contains the second and high order harmonics. Especially when the relative plate width is less than0.5, the high order harmonics occupies a high proportion of wave energy, which is no less than the fundamental harmonics. Therefore, it is more reasonable to adopt the transmission coefficient defined by the harmonic energy superposition method (formula2.5). What’s more, the transmission coefficient of the horizontal twin-plate breakwater is influenced by many coupling parameters. For the monochromatic wave, the breakwater may not work better with wider twin-plate. The transmission coefficient shows a fluctuated type with relative plate width ranging from0to1. However, when the relative depth d/D is in the range (-0.25,0.17), the influence of the relative depth on the transmission coefficient is not significant.(4)About wave pressures and its distributionWave pressures of the submerged and emerged breakwater are dominated with primary frequency, and rarely with second-order frequency or three mode frequencies. High order frequencies can often be ignored. Surge pressure of Horizontal twin-plate breakwater may differ from different positions. When the upper plate of emerged breakwater rises above the water, wave pressures of the upside are small, and impact pressure occurs more easily on the front and rear of the downside. When the lower plate of emerged breakwater is submerged in the water, it exhibits the same time domain characteristics as those of submerged breakwater. But when the submerged breakwater is flooded deeply in the water, high frequency fluctuation pressure occurs more easily on the rear of the lower plate. Some influencing factors, such as the relative submerged depth d/D, the relative plate width B/L and the relative wave height H/D have effect on envelope distribution of wave pressures. The relative submerged depth d/D affects the wave pressures envelope distribution of the upside and downside of the upper plate greatly. That is to say, the upside of the upper plate of submerged breakwater faces bigger wave pressures, while the wave pressures of emerged breakwater is smaller. And the downside of upper plate of emerged breakwater faces impact pressures greatly. however it shows no signs of impact characteristic on the downside of the upper plate of submerged breakwater.