This paper shows some numerical results obtained at DINMA on the fully nonlinear interaction of regular waves with four bottom-mounted circular cylinders [D’Este et al. 2001]. The phenomenon of interest is the so-called near trapping, i.e. the large magnification of the amplitude of the free surface elevation and induced pressures in the zone delimited by the cylinders surface. Linear theory [Linton & Evans, 1990] and experiments [Contento et al. 2000] show that this amplification depends strongly on the ratios of the column radius to the incident wavelength (ka parameter) and of the column radius to the side of the square (a/d parameter). In this specific case we analyse the ka condition that leads to extremely large oscillations of the second Fourier components of the free surface elevation in the inner zone and of the related pressure [Malenica et al., 1999] even in small amplitude incident waves. In particular the interest is focused on the behaviour of the higher order terms of the forces and run-up for two different configurations of the wave tank while keeping the same hydrodynamic parameters (ka, a/d).
From the preliminary results shown in the present work, it seems that first order in-line terms are only weakly affected by the dimensions of the basin whereas the first order side force on the lateral cylinder shows evident interference effects. At the same time, the amplitude of the second order terms of the forces show a smoother variation with time for the larger basin.
