We report upon a series of laboratory experiments and complementary (two-dimensional) direct numerical simulations that explore the lock release of a fixed volume of dense fluid into a two-layer density-stratified ambient. By initial condition, the lock release experiments/simulations fall into one of two categories: full-depth and partial-depth. Our particular focus is on the “tailwaters” limiting case where the lock fluid density matches that of the lower ambient layer. In either case the front speed of the advancing lock fluid is less than that of the excited interfacial disturbances. Consequently, the internal front propagates at constant speed for less time than, say, the downstream-propagating interfacial disturbance, which we term the dense gravity current (or GC1). Complementing GC1, there is an analogue flow of light ambient fluid into the lock, and this we refer to as the light gravity current (or GC2). Measured speeds for GC1, GC2 and the internal front are compared against analogue predictions from shallow water (SW) theory. From this comparison, positive agreement is noted in the case of GC1 and the internal front. Meanwhile, the speed of GC2 post reflection from the lock end wall is under-predicted by 10–20% depending on the initial depth of dense fluid within the lock. This under-prediction is believed to result from a mismatch between where the SW prediction is made (immediately following GC2 reflection from the back of the lock) and where the experimental GC2 speed is measured, usually 0.5–2.5 lock lengths downstream by which point the GC2 height has decreased due to dispersion. Although the GC1 height also undergoes a dispersive decrease in height, generally more positive agreement is noted when comparing measured and predicted gravity current heights. The distance travelled by the internal front prior to being arrested by the reflected GC2 agrees robustly with SW theory. Laboratory and DNS experiments exhibiting a thick ambient interface are also reported upon. We observe that the speed of the internal front and the downstream distance it travels at a constant speed increase with interface thickness. The insights gained from this investigation can be applied to realistic environmental flows such as nocturnal thunderstorm outflows.

中文翻译：

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尾水重力流及其与完美亚临界流的联系：实验室实验以及浅水和直接数值解

我们报告了一系列的实验室实验和互补的（二维）直接数值模拟，这些模拟探索了固定体积的致密流体向两层密度分层的环境中的锁定释放。按照初始条件，锁释放实验/模拟可分为两类之一：全深度和部分深度。我们特别关注“尾水”限制情况，其中锁定流体密度与下部环境层的流体密度匹配。在任何一种情况下，前进的锁液的前速度都小于激发的界面扰动的速度。因此，内部前沿以恒定速度传播的时间少于下游传播的界面扰动，我们称之为密集重力流（或GC1）。补充GC1，锁中有类似的轻质环境流体流，这称为轻重力电流（或GC2）。将GC1，GC2和内部前端的测量速度与浅水（SW）理论的模拟预测进行了比较。从该比较中可以看出，在GC1和内部方面的情况是一致的。同时，根据锁内部浓密流体的初始深度，GC2从锁端壁反射后的速度会被低估10–20％。据认为，这种预测不足是由于做出SW预测（紧接在GC2从锁的背面反射之后）与测量实验GC2速度之间的不匹配导致的，通常下游的锁定长度为0.5-2.5。由于分散，GC2高度降低了。尽管GC1高度也经历了高度分散性下降，但在比较实测和预测的重力流高度时，通常会发现更积极的一致性。内部前端在被反射的GC2捕获之前经过的距离与SW理论完全吻合。还报道了实验室和DNS实验显示较厚的环境界面。我们观察到内部前部的速度及其以恒定速度行进的下游距离随界面厚度的增加而增加。从这项调查中获得的见解可以应用于现实的环境流量，例如夜间雷暴流出。内部前端在被反射的GC2捕获之前经过的距离与SW理论完全吻合。还报道了实验室和DNS实验显示较厚的环境界面。我们观察到内部前部的速度及其以恒定速度行进的下游距离随界面厚度的增加而增加。从这项调查中获得的见解可以应用于现实的环境流量，例如夜间雷暴流出。内部前端在被反射的GC2捕获之前经过的距离与SW理论完全吻合。还报道了实验室和DNS实验显示较厚的环境界面。我们观察到内部前沿的速度及其以恒定速度行进的下游距离随界面厚度的增加而增加。从这项调查中获得的见解可以应用于现实的环境流量，例如夜间雷暴流出。