Dynamics of double-diffusive gravity current and its modelling

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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.