Time-frequency analysis of internal waves generated by a towed and self-propelled submerged body model

The spectrogram, based on a short-time Fourier transform, can visualize the time-dependent frequency spectrum of waves and is easy to compute. This time-frequency analysis method provides crucial information about waves generated by moving vessels and has been utilized to analyze Kelvin ship waves and internal waves. To further study the internal waves induced by a submerged body, an experiment is conducted for the towed and self-propelled SUBOFF model in a stratified fluid. The internal wave elevation signals are captured using electronic conductivity probes. Comparing with the calculation results of theoretical model, the high-frequency component of internal waves is identified. The high-frequency component has the exact same characteristics in both the towed and self-propelled model experiment and is consistent with the theoretical results for all Froude numbers. Therefore, this component is composed mainly of lee waves. Through spectral characteristics identification, a low-frequency component is discovered in the spectrogram in addition to the lee wave component. The intensity of the low-frequency component is tightly related to the vortex structure behind the submerged body. The vortex structure depends on the net momentum imparted by the submerged body. Therefore, this component is composed mainly of wake waves induced by the vortex structure.