Dynamics of double-diffusive gravity current and its modelling

从实验和数值两方面研究了双扩散重力流的动力学和结构，其中HAR和质量传递共同驱动浮力效应。这类重力流包括液化天然气等致密气体外流、火山碎屑流和与大风暴相关的锋面活动。风洞实验是对甲烷的外流进行的，甲烷最初被冷却为重气体，但当它与周围的空气混合时，变成了上升的羽流。文中还建立了这种重力流的数值模型，发现开尔文-亥姆霍兹(KH)不稳定性发生在头部的最前面。因此，在那里产生了KH巨浪，并在相对于前进锋沿“零速度边界”向后移动时，波幅增大。KH巨浪将高空空气卷进重力流中。湍流混合和波动引起的摩擦力作用于头部顶部边界，KH失稳并不是在所有阶段都发生，这导致头部的精细结构和动力学发生了显着的变化。相反，KHI是在重力流生命中期梯度Richardson数达到临界值0.25时产生的。引出的顶部摩擦降低了重力流在大气中的渗透率，随着周围空气在重力流头的卷吸降低了负浮力，最终头部的密度变得低于大气密度，像羽流一样上升。夹带率强烈依赖于周围空气的速度和稳定性。它也被发现依赖于克什米尔革命制度。

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.