Study on Wave Energy Absorption System with the Plural Oscillating Water Columns : Part1 Wave Energy Absorption Characteristics

Abstract

Many wave energy absorption system currently proposed can absorb wave energy with fairly high efficiency, regarding wave energy at a specified frequency band ; but they are said to lower their absorption efficiency in irregular waves, This is due to the fact that while waves themselves are composed of wide-range frequencies, the natural frequency of the wave energy absorption system itself is basically a single one, and is so arranged that the wave energy absorption efficiency will become the greatest in the vicinity of its natural frequency. To overcome this discrepancy, ingenuity has been used to estimate the incident waves and functionally change the natural frequency, but this method has not yet been able to enhance the efficiency. The present study has conceived that the wave energy absorption system is made to correspond to wide frequency bands, which the irregular wave has, by providing a plural number of natural frequencies in a single system, not by changing the natural frequency of the system itself. An attempt has been made to grasp experimentally the wave energy absorption characteristics. In the present system, it was impossible to provide a plural number of natural frequencies in a single unit, owing to the restr ction in scale. The authors solved this problrem by paying attention to the fact that the water column in the U-shaped tube had a natural frequency, and the length of this water column, not its volume, can determine the natural frequency. This paper verifies the validity of determing the natural frequency in the oscillating water-column-type wave energy absorption system, using the same concept as that of the water column in the U-shaped tube, and reports the absorption characteristics when a plural number of water columns, which have different natural frequencies, is used in a single system to absorb wave energy.