内孤立波与平顶海山作用的能量耗散实验研究

LABORATORY STUDY ON ENERGY DISSIPATION OF INTERNAL SOLITARY WAVES IN GUYOTS

内波破碎引起的能量耗散和混合是海洋内部的重要物理过程。通过在二维内波水槽进行实验室实验,分析内波与地形的作用,探究内孤立波与平顶海山地形作用时波要素、能量以及湍耗散率的时空变化。本实验利用重力塌陷法在两层流体中制造第一模态内孤立波,通过粒子图像测速技术(particle image velocimetry, PIV)获得内孤立波与地形作用时的流场结构,定量分析整个作用过程。结果表明,地形会改变波形甚至引起破碎,内波与地形作用时,振幅和能量密度会在内孤立波爬坡时迅速增大,在地形前缘产生强烈能量耗散。入射波的能量与塌陷高度呈二次函数关系,透射波能量随地形升高减小,反射波能量随地形升高增大。地形前缘局地湍耗散率极值时间序列在部分实验中呈双峰结构,对应内孤立波界面处剪切加强引起湍流耗散和波后缘翻转破碎。破碎引起的地形前缘区域平均湍耗散率量级在10^(-5)m^(2)/s^(3),局地湍耗散率极值与入射波振幅呈指数关系,所有实验中局地湍耗散率的最大值接近10^(-3) m^(2)/s^(3)量级。

Energy dissipation and mixing induced by internal wave breaking are important dynamic physical processes in the ocean.Upon 2-dimensional laboratory experiments in an internal wave tank,the evolution of an interfacial internal solitary wave(ISW)passing by an underwater trapezoidal topography mimicking a guyot was examined.The temporal and spatial variation of wave parameters,energy and turbulent dissipation rate during ISW interaction with guyots were analyzed.The first mode(Mode-1)ISW was generated in the two-layered stratified fluid system through standard lock-release method.The flow field structure of the internal solitary waves and the topography were obtained by particle image velocimetry(PIV),and the whole process was quantitatively analyzed.The velocity field of the Mode-1 ISW propagation was visualized by PIV.Results show that if the topography was moderately large,the wave was observed to shoal and break.The amplitude and energy could increase rapidly when the ISW climbed to the slope,during which intense energy dissipation appeared in the front edge of the topography.The energy of incident wave was in quadratic function to the collapse height,and the energy of transmitted wave and reflected wave decreased with the rise of topography.The time series of the local dissipation extremum shows bimodal peaks in several experiments,which corresponds to the shear dissipation boosting and plunging breaker.The average dissipation rate in the front edge of the topography was 10–5 m^(2)/s^(3) in magnitude and the maximum of local turbulence dissipation rate was close to the magnitude of 10–3 m^(2)/s^(3).The extreme value of local turbulent dissipation rate was exponential to the incident wave amplitude.