When multiple cylinders are placed in waves with equal separation distance, the near-trapped mode may occur between cylinders at some critical frequencies. To discuss validity of a linear potential theory and nonlinear effects around near-trapping frequencies, spatially dense measurements are carried out for the free-surface elevation around four truncated circular cylinders placed at regular intervals in a straight line. At each of three wavelengths selected after preliminary measurements, two different waves in amplitude are generated with wave steepness H/X set approximately to 1/50 and 1/20. Measured data are Fourier analyzed, separating into first- and second-order quantities. At places where the first-order wave amplitude takes local maximum or minimum, the second-order wave amplitude becomes large. This experimental fact, however, cannot be explained only with the second-order component computed from quadratic products of the first-order quantities, implying that the contribution of the second order velocity potential is important in the wave interactions.
Understanding hydrodynamic interactions among multiple floating bodies is important in a study on column-supported very large floating structures. Recent numerical computations (e.g. Maniar & Newman, 1997) predict very large free-surface elevation and wave forces due to hydrodynamic resonant phenomena at some critical frequencies. To confirm whether this is true, Kagemoto et al. (1998) conducted an experiment using arrays of 50 elements (each element is a vertical circular cylinder with footing) placed at regular intervals along a straight line, and they measured the free-surface elevation at midst points between adjacent cylinders in waves propagating along the array. On the whole, good agreement was found between computed and measured results. However, around a critical frequency where the wave resonant phenomenon was observed among cylinders (which will be referred to as the neartrapping frequency hereafter), measured wave elevations were obviously smaller than computed ones by a linear theory.