# Specifying a wave generator

The type of research to be conducted in the tank determines the choice of tank size and wavemaker. First determine the sea state that is to be modelled: how deep is the water and what is the amplitude and frequency range of the waves in the open ocean? The open full-scale sea wave spectra should be split into component wave fronts so that the amplitudes at different frequencies can be determined. The next step is to set the scale factor for the tank and models. There are many arguments that big is better however model scale is ultimately determined by the available budget. For most tanks this is in the range between 1/10 to 1/100 scale. The wavemaker type will depend on the relationship between the waves and the water depth. If the water depth is less than half the wavelength, or will be varied, a piston should be chosen.

The angular motion of a flap paddle is determined by the quality of the control system. With position feedback it is reasonable to run up to +/-12 degrees.  With force feedback or other 2nd order correction they will run well up to a displacement of +/-18 degrees. Piston paddles can move larger distances and are typically designed with a stroke of 50-100% the water depth. A paddle for generating solitons will require a total travel of at least twice the water depth.

The first analysis of wave generation was published by Biesel and Suquet[1] in 1951 and provides solutions for relationship between wave height, stroke and force for hinged and piston wave generators. This was refined by Gilbert, Thompson and Brewer[2] who produced design charts that give engineering solutions for wavemaker design. The analysis is based on linear theory takes no account of breaking waves.  Higher frequency waves are limited by breaking; for regular waves the limiting steepness is 1:7 so the linear wave height curve is combined with the breaking wave limit. This tends to overestimate the size of the maximum breaking wave so a practical solution is to truncate the top 15% of the curve. The paddle will create waves above this height but they will be unsuitable for research but useful for demonstrating the tank to visitors.  Lower frequency waves are limited by the displacement of the paddle. As an approximate guide a flap paddle should extend about 35% of the hinge depth above the waterline.