USP wave basin: active wave absorption and generation algorithms

Abstract

This paper presents the analysis of wave absorption and generation algorithms that were latter applied in the new wave basin constructed at the University of São Paulo (USP), as part of the Numerical Offshore Tank (TPN) Laboratory. These algorithms calculate the motions of the wave makers both to generate and absorb the required wave field by taking into account the layout of the flaps and the limits of wave generation. In order to study different aspects of the implementation, the performance of a prototype device composed of 4 flaps was evaluated in a 2D wave flume, prior to the assembly of the complete system at the TPN wave basin. The generation algorithms are based on the summation of wave components (frequencies and directions) obtained from the required directional wave spectrum. The transfer function that relates the flap motion to the generated wave is considered. Absorption tests were conducted using two different algorithms: a frequency domain method based on Maeda et al. (2004), in which the controlled variable is the motor velocity, and the time domain algorithm proposed by Schäffer (2001). The latter is based on a digital filter and the position of the flap is the variable to be controlled. Both algorithms require hydrodynamic feedback based on the measurement of the surface elevation at each flap. The first algorithm needs an extensive test procedure to calibrate its control parameters, while the second one, after optimizing the digital filter, should be ready to use. Both algorithms presented similar results with reflection coefficients smaller than 10.7% for regular waves with frequencies ranging from 0.5 to 1.5 Hz.