Mean velocity, spreading and entrainment characteristics of weak bubble plumes in unstratified and stationary water
Mean velocity, spreading and entrainment characteristics of weak bubble plumes in unstratified and stationary water
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非分层静止水中弱气泡羽流的平均速度、扩散和夹带特性
DOI：
10.1017/jfm.2019.461
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发表时间：
2019
影响因子：
3.7
通讯作者：
S. Socolofsky
中科院分区：
文献类型：
--
作者：
Binbin Wang;Chris C. K. Lai;S. Socolofsky
本文对非分层和静止水中的弱气泡羽流进行了实验和理论研究。我们将弱气泡羽流定义为传播速度比经典羽流的线性速度慢的气泡羽流。这项工作的重点是羽流的平均流量特征，包括中心线速度、羽流扩散和周围水的夹带。本文采用了一种新的理论来描述气泡及其相关羽流的横向扩散，而不是夹带假说。新理论得到了实验数据的支持。根据液体体积通量的实测数据和新理论，我们得出弱气泡羽流是一个递减的夹带过程，弱气泡羽流的夹带系数$\unicode[STIX]{x1D6FC}$随高度的增大而减小$z$，其次是$\unicode[STIX]{x1D6FC}\sim z^{-1/2}$，其值远小于经典气泡羽流。还需要一个额外的无量纲扩散系数$\hat{E_{t}}\sim E_{t}U_{s}^{2}/B_{0}$来描述弱泡羽流中平均流动的体积和运动动量通量的演变。其中$E_{t}$为有效湍流扩散系数，$U_{s}$为气泡末端上升速度，$B_{0}$为源的运动浮力通量。最后，我们给出了经典和弱气泡羽流的平均流动特征的统一框架，包括体积通量、动量通量和羽流扩散，这也为从经典到弱气泡羽流行为的转变提供了见解。
In this paper we present an experimental and theoretical study of weak bubble plumes in unstratified and stationary water. We define a weak bubble plume as one that spreads slower than the linear rate of a classic plume. This work focuses on the characteristics of the mean flow in the plume, including centreline velocity, plume spreading and entrainment of ambient water. A new theory based on diffusive spreading instead of an entrainment hypothesis is used to describe the lateral spreading of the bubbles and the associated plume. The new theory is supported by the experimental data. With the measured data of liquid volume fluxes and the new theory, we conclude that the weak bubble plume is a decreasing entrainment process, with the entrainment coefficient $\unicode[STIX]{x1D6FC}$ in the weak bubble plume decreasing with height $z$ , following $\unicode[STIX]{x1D6FC}\sim z^{-1/2}$ , and taking on values much smaller than those in a classic bubble plume. An additional non-dimensional diffusion coefficient, $\hat{E_{t}}\sim E_{t}U_{s}^{2}/B_{0}$ , is also needed to describe the evolution of the volume and kinematic momentum fluxes for the mean flow in the weak bubble plume. Here, $E_{t}$ is the effective turbulent diffusion coefficient, $U_{s}$ is the terminal rise velocity of the bubbles, and $B_{0}$ is the kinematic buoyancy flux of the source. Finally, we provide a unified framework for the mean flow characteristics, including volume flux, momentum flux and plume spreading for the classic and weak bubble plumes, which also provides insight on the transition from classic to weak bubble plume behaviour.