A row of fifty identical, truncated vertical cylinders is submitted to regular head
waves, with wave periods in a narrow range around the period of the so-called
Neumann trapped mode. The free-surface elevation is measured at 14 locations along
the array. Response amplitude operators of the free-surface motion are compared
with numerical predictions from a potential flow model. Resonance effects, at wave
periods equal to or larger than the critical one, are found to be much less than given
by the numerical model. It is advocated that these discrepancies are due to dissipative
effects taking place in the boundary layers at the cylinder walls. An artificial means is
devised to incorporate dissipation in the potential flow model, whereby the cylinder
walls are made slightly porous; the inward normal velocity of the flow is related to the
dynamic pressure. The coefficient of proportionality is based on existing knowledge
for circular cylinders in oscillatory flows. With this modification in the numerical
code, excellent agreement is obtained with the experiments. The numerical model is
further used for the case of a very long array composed of 1000 cylinders; it is found
that with dissipation at the cylinder walls, the wave action steadily decreases along the
array, even for wave periods substantially larger than the critical one. On the other
hand, at wave periods less than the critical one, dissipation plays a negligible role;
the observed decay is solely due to diffraction effects. Implications of these results
for very large structures such as column-supported floating airports are discussed.
In particular, it is concluded that scale effects may be an important issue in the
experimental analysis of such multi-column structures.