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FLUID MECHANICS

机译：流体力学

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The concurrence of stable and unstable stratification in stratified flows leads to dramatically different features of turbulent mixing. This unique flow is experimentally studied by introducing a horizontal jet of dense fluid into a pool of light fluid. The buoyancy flux from simultaneous velocity-density measurements is an indicator for competition between a stabilising mechanism and another destabilising mechanism. The difference of mixing efficiency, quantified by flux Richardson number Rif, between the (un)stable stratification is mainly contributed by the large-scale mixing. The behaviour of Rif can be modelled by the gradient Richardson number Rig linearly in the low-Ri case and nonlinearly in the high-Ri case (especially in a region where the counter-gradient flux emerges). The turbulent diapycnal diffusiv-ity, quantifying the combined effect of reversible and irreversible mixing processes, increases as the buoyancy Reynolds number Reb increases only when Reb is large. The irreversible mixing diffusivity, which quantifies the sole irreversible mixing process, increases linearly as the turbulent Peclet number with the data points from the (un)stable stratification overlapped. The turbulent Prandtl number approaches 0.75 as Rig approaches zero, but does not show clear dependence on Rig in the examined regime.
机译：分层流中稳定分层和不稳定分层的并发导致湍流混合的显着不同。通过将稠密流体的水平射流引入轻质流体池，对这种独特的流动进行了实验研究。来自同时进行的速度密度测量的浮力通量是稳定机制与另一种不稳定机制之间竞争的指标。 （不稳定）分层之间，混合通量Richardson数Rif量化的混合效率差异主要是由大规模混合引起的。 Rif的行为可以通过梯度Richardson数Rig在低Ri情况下线性建模，而在高Ri情况下（尤其是在出现反梯度通量的区域中）非线性建模。仅当Reb很大时，随着浮力雷诺数Reb的增加，湍流的对流扩散率（量化可逆和不可逆混合过程的组合效果）就会增加。量化唯一的不可逆混合过程的不可逆混合扩散率随着湍流Peclet数与（不稳定）分层数据点的重叠而线性增加。当Rig接近零时，湍流的Prandtl数接近0.75，但在所检查的状态下并未显示出对Rig的明显依赖性。

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    《Oceanographic Literature Review》 |2017年第1期|222-235|共14页
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