Development of computational method for gravity currents in the ocean

在船闸换流的实验和计算中，对重力流水头的结构进行了研究。当闸门打开时，有限体积的流体被瞬间释放到另一种密度稍有不同的流体中。从能量角度出发，引入了一个箱形模型来描述重力流发展的不同阶段，用阴影法和染料示踪法进行了可视化实验，验证了箱形模型的正确性。释放流体的初始深度和体积影响前缘速度。在第一阶段，释放的流体以恒定的速度扩散，然后它减慢，使得前缘速度随时间减小到自相似区域中的-1/3次幂。在最后阶段，开始受粘性支配的水流进一步减慢，在此期间，前缘速度随时间减小到-4/5次方。 关于我们 在相同的实验条件下，研究了不同流态下的头部结构。用有限体积法求解了不可压缩非均匀流体的Navier-Stokes方程和溶质的输运方程。该计算产生了发展区之间的过渡，从而使由计算的密度场得到的波前速度与实验结果符合得很好。计算结果表明，过渡到下一个政权是伴随着显着的变化的几何形状和头部的内部结构。即使在低雷诺数下，头部也保持了尖锐的密度界面，并且穿过上界面的环境流体的相对大规模的夹带导致头部后面的强烈混合。无粘性边界的模型未能捕捉到过渡到第二阶段，这表明过渡可能需要一些与粘性运动相关的刺激。短锁长情况下的结果表明，在头部的体积和密度的突然减少所带来的旋涡脱落沿着上界面，这可能是一个机制，以解释在第一和第二区的强烈混合。少

The structure of the head of gravity currents is investigated in the experiment and the computation of lock-exchange flow. A finite volume of fluid was released instantaneously in another fluid of slightly different density in an open rectangular channel when a lock gate was opened. A box model was introduced to describe different regimes of a developing gravity current on the basis of energy argument.Visualization experiments were made by the shadowgraph method and with dye to trace the head of gravity currents, the results of which validate the box model. The initial depth and volume of the released fluid affect the front speed. At the first stage the released fluid spreads at a constant speed, and then it slows down so that the front speed decreases with time to the power of -1/3 in self-similar regime. The current that has begun to be dominated by viscosity slows down further in the final stage during which the front speed decreases with time to the power of -4/5.The computation wa … More s also made in the same condition as the experiments to study the structure of the head of currents in different regimes. The incompressible Navier-Stokes equation for an inhomogeneous fluid together with the transport equation for solute was solved by the finite volume method developed in the previous paper. The computation produces the transition between the development regimes so that the front speed obtained from the computed density field is in good agreement with the experimental result. The computational results indicate that the transition to the next regime is accompanied by remarkable change in the geometry and the inner structure of the head. The head has maintained a sharp density interface even at low Reynolds number and the relatively large-scale entrainment of ambient fluid across the upper interface causes strong mixing behind the head. The models with the inviscid boundaries failed to catch the transition to the second stage, which suggests that the transition may require some stimulus associated with viscous motions. The result for the case of short lock-length shows the sudden decrease in the volume and in the density of the head brought about by the vortex shedding along the upper interface, which may be one of the mechanism to account for the strong mixing in the first and the second regimes. Less