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Dynamics of double-diffusive gravity current and its modelling

双扩散重力流动力学及其建模

基本信息

批准号：
05455018
负责人：
UEDA Hiromasa
金额：
$ 2.43万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1993
资助国家：
日本
起止时间：
1993 至 1994
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-05455018/
关键词：
Gravity current Kelvin-Helmholtz instability Frontal line Pyroclastic flow Heavy gas Stratified wind tunnel Stratified turbulence Cutoff eddy

项目摘要

Dynamics and structure of double-diffusive gravity current in which both hear and mass transfers drive buoyanct effects were investigated experimentally and numerically. This type of gravity current includes dense gas outflow such as LNG,pyroclastic flow and frontal activity associated with heavy storm. Wind tunnnel experiment was madeon the outflow of methane which was cooled to heavy gas originally but, as it mixed with the surrounding air, became a rising plume. Numerical model of such a gravity current was also developed.Kelvin-Helmholtz (KH) instability was found to occur in the foremost part of thehead. Consequently KH billows were generated there and grew in amplitnde while traveling backward relative to the advancing front along the "zero-velocity boundary". The KH billows entrained the upper air into the gravity current. The resultant turbulent mixing and wave perturbations induced a friction force acting on the top boundary of the head.The KH instability (KHI) did not occur at all stages and this resulted in a significant variation of the fine structure and dynamics of the head. Rather, the KHI was produced when the gradient Richardson number attained the critical value of 0.25 in the middle stage of the life of gravity current. The induced top friction decelarated the gravity current penetration in the atmosphere.As the entrainment of the surrounding air at the gravity current head decresed the negative bouyancy, finally the foremost part of the head became less dense than the atmosphere and rose up like a plume. The entrainment rate was strongly dependent on the velocity and stability of the surrounding air. It was also found to depend on the KHI.

对热质传递驱动双扩散重力流的动力学和结构进行了实验研究和数值模拟。这类重力流包括液化天然气等稠密气体外流、火山碎屑流和与强风暴有关的锋面活动。对甲烷外流进行了风洞实验，甲烷最初被冷却成重气体，但随着与周围空气混合，成为上升的羽流。本文还建立了这种重力流的数值模型，发现在头部的最前部存在Kelvin-Helmholtz（KH）不稳定性。因此，KH巨浪在那里产生，并在振幅增长，而相对于前进的前锋沿着“零速度边界”向后移动。KH巨浪将高空空气带入重力流。湍流混合和波动扰动在封头上边界产生摩擦力，但在所有阶段均未发生KH不稳定性，这导致封头的精细结构和动力学发生显著变化。相反，当梯度Richardson数在重力流生命的中期达到临界值0.25时，KHI产生。由于重力流头部周围空气的卷吸作用，重力流头部的负浮力逐渐减小，最后头部的密度逐渐减小，呈羽状上升。卷吸率强烈地依赖于周围空气的速度和稳定性。它也被发现依赖于KHI。