On Frequency-Focusing Unidirectional Waves

Phase convergence to within lOin mechanically generated waves was achieved at a specified point in a wave flume by frequency focusing. Owing to the presence of nonlinear waves, the crest elevation reaches a maximum somewhat further from the waveboard, where also breaking occurs if the waves are of sufficient size. The critical amplitude for breaking was not in close agreement with previous measurements when normalized with respect to the central wave number of mechanically generated waves. It is suggested however that the limiting conditions are related to the phase speed near the focus point, where the wave group propagates with a considerable degree of coherence not present in a linear model. Predictions of a time-dependent nonlinear numerical model of Baldock and Swan (1994) are found to be in good agreement with the behavior of the crest in this region.