In this paper the interaction in diffraction regime between regular waves and an array of 4 bottom-mounted circular cylinders with their axis at the comers of a square is studied on an experimental basis. Evidence of the existence of near-trapping phenomena is given in the case of incident waves travelling along the diagonal of the structure. Measurements include free surface elevation in the near field and pressure at the cylinder wall. The measured local quantities are compared with those obtained in the case of isolated cylinder showing a magnification factor between 3 and 4 for specific ratios of the cylinder diameter to the incident wave length.
The problem of the interaction between waves and an array of circular cylinders has been studied by many authors in the last ten years. The design of the deck height on the calm sea level and the estimate of high frequency loads from relatively short waves on multicolumn structures are the basic motivations of these works. In particular Linton and Evans [1990] have shown that large magnification factors of the global loads compared to the isolated cylinder case can be achieved for specific ratios of the wave length 2 to the cylinder radius a and of the wave length 2 to the distance between cylinder axis d. Their study is to be applied for cylinders with their axis at the comers of a regular polygon. In that work the phenomenon is explained with the occurrence of near-trapping free surface oscillation in the zone internal to the polygon. Most available results in the literature refer in any case to theoretical studies in the frame of inviscid flow and linearised free surface boundary conditions. Higher order solutions [Scolan & Malenica, 1998] are said to be still applicable in the frame of small amplitude waves. Malenica et al. [1999] have shown analytically the possibility of second-order near trapping when the incident wavelength is four times the wavelength causing first order near-trapping. The aim of this study is to give experimental evidence to the occurrence of first order near-trapping in the case of a 4 column structure in regular waves. In particular the columns are circularvertical- bottom mounted cylinders with their axis at the comers of a square. The direction of the incoming waves corresponds to the diagonal of the square.
