层结背景下热液柱演化的实验模拟

摘要

为了研究洋中脊处海底烟囱喷出的热液柱的动力学特征以及其与周围海水之间的关系,在实验室内对热液柱的演化过程进行了实验模拟研究.在实验设计中,采用底部点源提供热通量的方法触发热液柱,使用粒子图像测速技术(Particle Image Velocimetry——PIV)获取速度信息,高灵敏度热敏探头采集温度信息.通过瞬时速度场分析热液柱的运动特征以及其与周围层结背景环境之间的卷挟过程.实验表明,层结的背景环境阻碍了热液柱的垂向上升以及水平延伸.层结的背景条件下,在热液柱发展演化的整个过程中,其呈现出明显的振荡特征,且振荡频率(0.10-0.35rad/s)与背景环境的浮力频率(0.20-0.30rad/s)数值相当.热液柱与周围纯水之间存在强混合,发现在热液柱的热源处和顶部的混合最为强烈,中性浮力层高度以下是热液柱与周围纯水混合的主要区域,混合强度以卷挟系数α表征,α值从-0.30到0.13之间变化,大致随着高度的增加而减小.通过瞬时速度场分析得到热液柱的湍流耗散率ε,发现在垂直方向上,其“颈部”的耗散最强,沿水平方向上,其中心处的耗散最大,向四周逐渐递减,呈现高斯分布.%In the mid-ocean ridge,there are usually a large number of submarine chimneys,riching in a variety of minerals hydrothermal plume,to provide heat and minerals for the ocean.In order to study the dynamic characteristics of these hydrothermal plumes and their interaction with the surrounding seawater,we report the results of an experimental study of the plume generated from a small heat source under a stable stratified environment in the laboratory.To carry out the experiment,a small heating resistance supply heat flux to the fluid continuously.Particle image velocimetry is used for the velocity field measurement,while thermal probes measure the temperatures at local positions.The instantaneous velocity fields support credible information to identify the characters of plume and the interaction with the stratified environment.The stratification prevents vertical rising and lateral extension of plume.In the existence of plume,the surrounding flow shows oscillating feature,which has a comparable frequency (0.10-0.35rad/s) with the background buoyancy frequency (0.20-0.30rad/s).Based on the results of entrainment coefficient a,it is found that the intension of mixing between plume and pure water,and recognize that the intense mixing is vigorous around the heat source and the plume's head.The mixing between the plume and the pure water mainly takes places below the neutral buoyancy layer.The mixing strength is characerized by the entrainment coefficient a.a varies from-0.30 to 0.13,and decreases with the increasing height.In order to study the budget of turbulent kinetic energy,the turbulent dissipation rate ε of the plume is calculated.It is found that the the plume stem has the largest dissipation rate in the vertical direction.It presents Gaussian distribution with peak at the central area in the horizontal direction.