Experimental investigation on the mode-2 Internal solitary wave generated by bidirectional fluid gravity collapse in the continuous density pycnocline

Abstract

The mode-2 internal solitary wave (ISW) propagating independently in the stratified ocean is a high-mode internal wave (IW), and its complex wave-flow field will cause the harm to the safe operation of marine engineering structures. In this paper, a fluid environment with a continuous density pycnocline was prepared in a large experimental stratified fluid flume. Based on the principle of ISWs generated by the fluid gravity collapse, a physical experimental method for exciting the “oval” mode-2 ISW solitons by bidirectional fluid gravity collapse in a continuous density pycnocline was proposed. The experimental results showed that the evolution of the upper and lower double vortices near the center of pycnocline is a special generation mechanism of the generation process of mode-2 ISW generated by the gravity collapse of the stratified fluid. The number of generated mode-2 ISW internal solitons depends on the geometric size of fluid gravity collapse region and the geometric thickness ratio of the upper and lower layers of the fluid. The wave evolution process, wave-flow structure, wave-flow characteristics of the “oval” mode-2 ISW in the continuous density pycnocline and its stratified environment effect were obtained. The results showed that the wave speed of the mode-2 ISW increases with the increasing wave amplitude, and the wavelength increases first and then decreases with the increasing wave amplitude. Under the experimental conditions with same wave amplitude, the increase of the thickness ratio of the upper and lower layers of the fluid, the density difference and the thickness of the middle fluid layer would lead to the increase of the wavelength and wave speed of the mode-2 ISW. The velocity range of the mode-2 ISW is positively correlated with the wave amplitude. The increase of the thickness ratio of the upper and lower fluid layers, the density difference and the thickness of the middle fluid layer will lead to the enhancement of the horizontal and vertical flow field of the mode-2 ISW.