An investigation on internal waves generated by towed models under a strong halocline

Abstract

Experiments are performed for the characteristics of internal waves generated by four horizontally towed models, including a sphere and three slender models with different aspect ratios in a density stratified fluid with a strong halocline. The Reynolds number ranges within 3500 < Re = UD/ν < 126 000, and the Froude number ranges within 0.4 < Fr = U/NmaxD < 18. Based on the time histories of the density perturbation and analysis of the correlation velocity of internal waves generated by the towed models, it is shown that the body-generated waves are stationary multiple-mode Lee waves, while the wake-generated waves are non-stationary wake waves mainly produced by the large-scale coherent structure in turbulent wakes acting as a moving excitation source. The transition between body-generated and wake-generated internal waves occurs at a critical Froude number Frc which is linearly dependent on the model aspect ratio. Before the transition, when the wave height H changes with the variation of Fr, a peak value of H can be observed and its corresponding Froude number Frp increases linearly with growing aspect ratio. After the transition, the Froude number Friw associated with the internal wave correlation velocity fluctuates within a certain range. The wave height linearly increases with increasing Fr regardless of the aspect ratios, and the wave pattern is dominated by a non-stationary wake-generated wave. According to the experimental configuration, a theoretical model is proposed to simulate the body-generated and wake-generated internal waves. The numerical results obtained by this theoretical model show general agreement with the experiments.