Run-up and wave forces on an Array of Vertical Circular Cylinders: Experimental Study on the Second Order Near Trapping

This paper summarizes the results of experimental tests conducted at the Danish Hydraulic Institute (DHI) in the frame 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 thoroughly the so-called “near trapping”, and specifically the second order near trapping. This phenomenon regards diffraction effects of TLP like structures in waves, or in general arrays of vertical cylinders that enclose a portion of free surface (inner domain), and it shows itself with a rather large amplification of the oscillations of the mentioned portion of the free surface. 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 λ (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, whereas second order near trapping is a double frequency phenomenon that is expected to occur when double frequency component waves, according to the dispersion relation, exhibit a wavelength comparable to that of linear waves giving first order near trapping. The paper describes the set-up of the experiments, the tests conducted and finally presents the most meaningful results obtained. As far as the in-line and out-of-line forces acting on the cylinders, the wave run-up and the free surface oscillations at selected locations in the inner domain are concerned, their behavior Vs ka shows clearly the occurrence of the second order near trapping phenomenon with impressive quantitative aspects.