Experiments on internal waves generated by a towed revolution body with different depths in a stratified fluid

Abstract

Internal waves generated by submerged bodies such as towed spheres, towed cylinders or self-propelled models in stratified fluids are of great interests in oceanic hydrodynamics. Several classifications of internal waves were proposed based on experimental observations, including the Lee waves, the random waves and the gravitational collapsed waves. A more general classification is proposed in recent studies, consisting only two types. The first type is the body-generated internal wave which is stationary to the towed sphere and the second is the wake-generated internal wave which is non-stationary to the towed sphere. According to Robey’s work, experiments in this paper were performed for the time-space characteristics of internal waves generated by a horizontally towed revolution slender body with the aspect ratio λ =7.7, located at five different submerged depths below the strong pycnocline of a stratified fluid. The Reynolds number range is from 2500 Re = UD / v Fr = U / N max D Fr c=3.64 which is independent of submerged depths of the revolution slender body. For Fr Fr c, the internal waves are dominated by body-forced effect, and their correlation velocities are equal to the towing speeds of the revolution slender body, as well as the degree of anti-symmetry for such internal waves becomes larger with the increasing Fr . Moreover, the dimensionless maximum peak-peak amplitudes for such internal waves dominated by body-forced effect firstly increase with the growth of Fr until Fr reaches a certain value of approximately 1.33 regardless of submerged depths, and then begin to decrease. For Fr > Fr c, the internal waves are dominated by wake-forced effect, and their correlation velocities are noticeably lower than the towing speeds of the revolution slender body, as well as the Froude numbers with respect to the correlation velocities for such internal waves vary within the range from 0.7 to 1.4. Results further show that the degree of anti-symmetry for such wake-generated internal waves fluctuates around a constant smaller than 0.5, indicating that the symmetric component is slight stronger than the anti-symmetric component in the wake-generated waves, as well as the opening angles of the wave patterns in the symmetric and anti-symmetric components remain at relatively constant value of approximately 20.6 ° regardless of submerged depths and towed speeds of the revolution slender body. Moreover, the dimensionless maximum peak-peak amplitudes for such internal waves dominated by wake-forced effect linearly increase with Fr for different submerged depths, the slopes of such linear relations decrease with submerged depths and follow an exponential law. In particular, the present work demonstrates that it is unreasonable to assume that the wake- generated internal waves for an axis-symmetric revolution body are symmetric about the longitudinal section in center plane of the body.