Methodology and experimental validation for generating periodic focused waves in a wave flume

A new methodology is proposed for generating multiple focused waves within a relatively short time. This can improve the efficiency of the hydrodynamic testing of coastal and offshore structures under extreme wave conditions with strong nonlinearity and randomness, such as in extreme wave impact testing of vessels and offshore platforms. In this study, two methods for generating periodic focused waves were proposed based on a first-order wavemaker theory and Schäffer's second-order wavemaker theory. Periodic focused waves were generated in a wave flume with an intermediate water depth with different peak frequencies, wave amplitudes, frequency bands, frequency resolutions, and nominal periods using the first-order and second-order wavemaker theories. The test results demonstrate that both first-order and second-order wavemaker methods can produce repetitive focused wave groups within a short time period. However, the second-order wavemaker method can reduce the influence of second-order free harmonics. The frequency band and resolution only slightly influence the envelope of the focused wave group for the NewWave-type focused waves in the present study. It is possible to generate periodic focused waves with any equivalent or different time intervals, but the interactions between different focused wave groups should be examined to determine an optimal nominal period.