Experimental Investigation of Internal Waves Excited by a Sphere via Multiline Array Acquisition

The density stratification of seawater leads to the generation of internal wave wakes as a submerged body moves, and the characteristics of these wakes are closely related to the navigation speed of the submerged body. In this study, a wide-field density-stratified wave tank was used to simulate the stratified ocean environment using the top water injection method. The vertical displacement field data of internal waves generated by a submerged sphere, driven by a cyclic towing method, were indirectly measured using a high-precision multi-line array conductivity detection technique. Based on analysis methods such as the root mean square variance, symmetry, and Fourier transform using the Blackman-Harris window function, the amplitude characteristics and spectral distribution of the internal wave wakes were elucidated. The research demonstrates that the experimental system and method accurately capture key information, including the peak Froude number of Lee waves, transition Froude number, and other significant features. The experimental peak Froude number of Lee waves was Fr_p = 0.8, at which point the symmetric Lee wave has a dominant amplitude, and the overall antisymmetry of the wake wave system was minimized. The transition point is found to be within the towing Froude number range of 1.6 to 2.5. After the transition, higher-order mode Lee waves gradually weakened, and the wake-induced internal waves began to appear, with changes primarily reflected in the increase in wave amplitude. The dimensionless root mean square variance of the amplitude showed an approximately linear increase, with a fitted slope of approximately 0.0024, while the antisymmetry fluctuated within the range of 0.3 to 0.4. Similar characteristics were observed in the time-frequency domain, where the distribution of spectral intensity before the transition aligned well with the theoretical time-frequency curve. After the transition, the distribution of spectral intensity of the wake-induced internal waves in the time-frequency domain exhibited similar propagation characteristics to the theoretical time-frequency curve of Lee waves.