Density distribution in the flow past a sphere descending in a salt-stratified fluid
Density distribution in the flow past a sphere descending in a salt-stratified fluid
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DOI：
10.1017/jfm.2021.758
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发表时间：
2021-09
影响因子：
3.7
通讯作者：
S. Okino;Shinsaku Akiyama;Koki Takagi;H. Hanazaki
中科院分区：
文献类型：
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作者：
S. Okino;Shinsaku Akiyama;Koki Takagi;H. Hanazaki
用激光诱导荧光（LIF）法测量了盐层流体中下降球体周围的密度分布。采用粒子图像测速法（PIV）测量了相应的速度分布，并进行了数值模拟，作为LIF和PIV观测的补充。在稳定流动中，LIF观察到一个薄而垂直长的结构，与浮力射流相对应。在强分层和中等雷诺数（弗劳德数$Fr \lesssim 3$，雷诺数$50 \lesssim Re \lesssim 500$）条件下，还发现了钟形结构。盐度边界层和射流的密度分布与使用荧光染料施密特数的数值模拟结果一致（$Sc \sim 2000$）。研究了射流形成的初始非定常过程。在弱分层条件下，与均质流体条件下一样，LIF初始形成轴对称后涡。然而，随着时间的推移，分层效应变得明显，旋涡收缩消失，而急流垂直向上延伸。在强分层的情况下，薄射流的发展不会产生后涡，因为在涡产生之前的短时间内，分层的影响就会变得明显。
Abstract The density distribution around a sphere descending in a salt-stratified fluid is measured by the laser-induced fluorescence (LIF) method. The corresponding velocity distribution is measured by particle image velocimetry (PIV), and numerical simulation is also performed to supplement the observations by LIF and PIV. In steady flow, LIF observes a thin and vertically long structure which corresponds to a buoyant jet. The bell-shaped structure, which appears under strong stratification and moderate Reynolds number (Froude number $Fr \lesssim 3$, Reynolds number $50 \lesssim Re \lesssim 500$), is also identified. The measured density distributions in the salinity boundary layer and in the jet agree with the numerical simulations which use the Schmidt number of the fluorescent dye ($Sc \sim 2000$). The initially unsteady process of the jet formation is also investigated. Under weak stratification, the LIF shows an initial development of an axisymmetric rear vortex as observed in homogeneous fluids. However, as time proceeds and the effect of stratification becomes significant, the vortex shrinks and disappears, while the jet extends vertically upward. Under strong stratification, a thin jet develops without generating a rear vortex, since the effect of stratification becomes significant in a short time before the vortex is generated.