Experimental investigation on the characteristics of directional focusing waves

Abstract

The analysis on fixed or floating structures exposed to directional extreme waves has been a topic of keen interest in ocean engineering. In this context, an attempt is made to study the characteristics of laboratory generated directional rogue waves, termed also as focusing waves. The first-order wave paddle displacements were generated using the linear superimposition principle by applying a constant steepness spectrum associated with directional components. The phase angles are set to be constant at a predefined location to create an extreme event. The experimental test cases span from narrow to broadband spectra having two central frequencies, namely, 0.68 and 0.85 Hz. The bandwidth ratios of 0.5 and 0.75 were considered with five discrete directions ranging from −25° to 25°. The tests include spilling breakers and non-breaking waves. The investigation involves studying the variations in the Atiltness parameter (ATP), Crest height ratio, and the nonlinearity parameter provided by Hastings, an index to measure the effect of nonlinearity. In addition, directional studies using the maximum likelihood method (MLM) are also presented. The critical combination of frequency spectrum and bandwidth is identified from the focusing wave characteristics and discussed in this paper for the tested conditions. The shift in directional spectrum due to nonlinearity was noticed in the present study.