LABORATORY STUDY ON THE MODULATION EVOLUTION OF INITIALLY UNIFORM WAVE TRAINS

New experiments on the modulation evolution of initially uniform wave trains are carried out. Waves with initial steepness varying from 0.18 to 0.32 are considered. The results show that, due to the instability of the wave front, the initially uniform trains are gradually transformed into a series of wave groups, and the wave groupiness increases with the propagation distance. In addition, the modulation evolution for shorter waves is more obvious than that of longer waves in the same effective distance, indicating that wave length plays an important role in wave evolution. The frequency modulation of waves is studied using the instantaneous frequencies, which is extracted through wavelet transform. The instantaneous frequencies reveal that the modulation of frequency occurs at local time firstly, and then gradually expands. Moreover, even without the effective frequency downshift, some local frequency downshift is observed, implying that frequency downshift begins at local time.