卷波破碎致湍流混合的实验室实验研究

Surface waves play an important role in the mixing of the upper ocean. Part of the turbulence kinetic energy (TKE) in the upper ocean is transferred from wave breaking. Field observations and laboratory experiments offer the original data to investigate the wave-breaking-induced turbulence. Unfortunately, most of the field observations focused on the statistical results of mixing or dissipation rate of TKE rather than the whole and detailed evolution of turbulence induced by wave breaking, which had been measured in the laboratory experiment conducted by Rapp and Melville (1990). However, there are not travelling waves after the wave breaking in Rapp and Melville experiment, which is different from the actual phenomena observed in the ocean because the wave breaking always takes place in the travelling wave field in the real ocean. Our early experiment showed that both the travelling waves that follow the breaking wave and the repeated wave breaking evidently affected the penetration rate of turbulence induced by wave breaking. Considering that the plunging breaker penetrates deeper, this program will investigate the turbulence induced by plunging breaker by using laboratory experiment. Two series of experiments will be conducted, one is about the effect of the travelling waves on turbulence induced by single wave breaking and another one is about turbulence induced by repeated wave breaking. Laser-induced fluorescence will be used to measure the penetration depth of turbulence and PIV to obtain the velocity. The main purpose of this program is to clarify the mechanism of enhanced dissipation rate of TKE and deeper penetration depth due to the travelling waves and repeated breaking. If the purpose is achieved, it is believed to be helpful for understanding the wave-breaking-induced turbulence and wave-turbulence interaction, even to offer support for the parameterization of mixing in the upper ocean.

海浪在上层海洋混合中起重要作用，波浪破碎直接将波动能量传递到湍流混合。外海观测和室内实验是研究破碎致混合的重要手段，然而目前的外海观测多侧重于破碎致湍流结果的观测，少有跟踪破碎全过程的精细测量；经典的室内实验研究了单次破碎致湍流过程，但没有考虑表面波对湍流演变的影响，而实际海洋中的波浪破碎总是发生在表面波场中。申请人的初步工作表明后续表面波和多次破碎可明显改变破碎致湍流下传速度。鉴于卷波破碎影响更深，本项目拟采用室内实验研究卷波破碎致湍流混合过程，具体开展后续表面波对单次卷波破碎致湍流演变过程的影响以及同一地点多次卷波破碎致湍流过程的系列实验，利用激光-荧光成像技术测量破碎致湍流过程的影响范围，利用PIV测量速度场。通过实验结果的比较分析和机制研究，探讨后续表面波和多次破碎在湍流混合加强和加深过程中的作用，为深入认识波破碎致湍流混合、波-湍相互作用和海洋上层混合参数化提供一定实验数据支持。

海浪在上层海洋混合中起重要作用，波浪破碎直接将波动能量传递到湍流混合，但海洋中的波浪运动与湍流过程是否存在相互作用尚存争议。本项目开展了波-湍相互作用的实验室实验和外海观测研究，结果表明波浪对湍流具有明显的调制作用，且该调制作用具有波谷锁相特征。研究结果对于深入开展海洋上层混合过程研究具有一定的科学意义。.在实验室实验中，鉴于振动格栅产生湍流具有更好的可控性，本项目利用振动格栅代替波浪破碎在水槽中生成湍流。主要开展了三类系列实验，一是单纯机械造波实验，波动周期1.0s；二是单纯格栅振动产生湍流实验；三是首先格栅振动10分钟，湍流稳定后机械造波，波动周期1.0s。利用声学多普勒流速计（ADV）高频测量了水体流速。首先分析了水槽中振动格栅产生湍流的基本特征，随后利用傅里叶分析、高维全息谱分析的方法，通过三类实验结果的比较，研究了波浪-湍流的相互作用过程，高维全息谱结果表明波动对高频湍流具有明显的调节作用，能量谱分析结果也清晰显示了有波动情况下，湍流能量明显增强。利用水槽实验结果，还分析了机械波水质点水平速度与垂直速度不正交的现象。.基于广东茂名气象观测平台，组织了波湍相互作用外海观测，利用ADV高频测量了海洋上层三维流速，采用坐底式海床基（浅水型ADV和浪龙）进行了海浪、海流剖面和底边界层湍流过程的观测。实验室过程中台风威马逊影响南海，设备损毁严重，仅获取了台风影响前期的数据。利用经验模分解方法将波浪-湍流信号分离，进而利用全息谱（Holo-Hilbert spectra）分析方法研究了波浪-湍流相互作用，结果表明波浪对湍流过程存在明显调制作用，这种调制作用具有波谷锁相特征。