Run-up and wave forces on an array of vertical circular cylinders:: Experimental study on second-order near trapping

This paper summarizes the results of experimental tests conducted at the Danish Hydraulic Institute (DHI) in the framework of the EU-IHP-ARI project "Wave Forces on an Array of Circular Cylinders: Experimental Investigation on the Higher-Order Near Trapping" The aim of this research project was to study in detail the so-called near trapping, and specifically the 2nd-order near trapping. This phenomenon regards diffraction effects on TLP-like structures in waves or, in general, on arrays of vertical cylinders that encircle a portion of free surface (inner domain). The result of the wave-structure interaction is a rather large amplification of the oscillations of the free surface in the inner domain. These magnifications are particularly large at the fluid-body interface with associated, large induced pressures; they show their maximum effect for specific ratios of the column radius a to the incident wavelength A (diffraction parameter ka) and of the column radius to the distance between the column axis d (a/d parameter). First-order near trapping occurs at the incident wave frequency, while 2nd-order near trapping is a double-frequency phenomenon that is expected to occur when double-frequency component waves exhibit a wavelength comparable to that of linear waves giving 1st-order near trapping. The paper describes the set-up of the experiments and the tests conducted, and finally presents the most meaningful results obtained. The behavior Vs ka of in-line and crosswise forces on the cylinders, of the wave run-up and of the free-surface oscillations at selected locations in the inner domain shows clearly the occurrence of the 2nd-order near trapping phenomenon with notable magnification factors.