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Laboratory experiments on characteristics of perforated-type floating breakwaters

Floating breakwaters were treated as solid bodies without any perforation in previous studies. 

In this study, however, a floating breakwater is perforated to allow the partial absorption of the energy produced by incident waves and an air chamber is placed in the upper part to control the breakwater draft. A series of laboratory experiments for a floating breakwater installed with a mooring system is carried out. 

In general, a mooring system can be classified by the number of mooring points, the shape of the mooring lines, and the degree of line tension. In this study, a four-point mooring is employed since it is relatively easier to analyze the measured results. 

Furthermore, both the tension-leg and the catenary mooring systems have been adopted to compare the performance of the system. 

In laboratory experiments, the hydraulic characteristics of a floating breakwater were obtained and analyzed in detail. Also, a hydraulic model test was carried out on variable changes by changing the mooring angle and thickness of the perforated wall.

A hydraulic model was designed to produce wave energy by generating a vortex with the existing reflection method. Analysis of wave changes was conducted and the flow field around the floating  breakwater and draft area,  which have elastic behavior, was collected using the PIV system.  From the test results the strong vortex was identified in the draft area of the perforated both-sides-type floating breakwater.

Also, the wave control performance of the floating breakwater was improved due to the vortex produced as the tension in the mooring line decreased.

In conclusion, the flow field of the floating breakwater along with the incidental waves was collected by means of the PIV system and found the strong canal generation on the draft cross-section area of the perforated both-sides-type. 

Moreover,  wave pressure in the draft cross-sectional area of the perforated both-sides-type was measured high compared to the bottom wave pressure. This is an example showing that the wave energy was scattered and disappeared in the draft cross-sectional area from the vortex generation.

Therefore, the breakwater performance of the perforated both-sides• type was very high in the deep seawater wave area.

In the stability aspect, the perforated both sides-type significantly limited the rolling motion that induces excessive tensile stress during the two-dimensional free-motion compared to the solid and perforated-type.

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