Puffing in planar buoyant plumes: BiGlobal instability analysis and experiments
Puffing in planar buoyant plumes: BiGlobal instability analysis and experiments
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DOI：
10.1017/jfm.2018.1022
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发表时间：
2019-01
影响因子：
3.7
通讯作者：
K. K. Bharadwaj-K.;D. Das
中科院分区：
文献类型：
--
作者：
K. K. Bharadwaj-K.;D. Das
本研究采用线性双全局稳定性分析和实验研究了平面浮力羽流的膨化行为。利用稳定性分析方法研究了二维羽流的双全局不稳定性特征，并与实验得到的矩形羽流和方形羽流的抽吸行为进行了比较。在研究的参数空间中，Richardson数的范围为0.03 <Ri<960$，不稳定性分析表明，平面羽流只在波动扰动下表现出双全局不稳定性，而在弯曲扰动下保持稳定。利用不稳定曲张模式的BiGlobal频率和增长率得到了Strouhal数的相关性和稳定性曲线。展向波数对BiGlobal不稳定性影响的研究表明，平面羽流对二维扰动比对三维扰动更不稳定。展向波数的增加倾向于稳定平面羽流而不影响它们的振荡频率。实验结果表明，矩形羽流中的抽吸频率严格遵循二维不稳定性分析得出的幂律，而对进气道展弦比的依赖性较弱。为了进一步探索长宽比对膨化行为的影响，在长宽比为1的羽流（即方形羽流）中进行了实验。方形羽流被发现是更稳定的，并表现出更高的吞吐频率比矩形羽流。矩形羽流和方形羽流在抽吸动力学方面的这些差异的原因已经从锁相流向和展向流动显示中进行了探索。除了抽吸，矩形和方形羽流的展向可视化显示在其拐角处存在二次流，类似于其等密度射流对应物。最后，通过实验，推导出了一个以水力直径为长度尺度的通用喷烟频率关联式，该关联式消除了长径比对喷烟频率的影响，适用于方形和低长径比矩形喷烟。
The present study investigates the puffing behaviour of planar buoyant plumes by employing linear BiGlobal stability analysis and experiments. The BiGlobal instability characteristics of two-dimensional plumes have been explored using stability analysis and compared with the puffing behaviour of both rectangular plumes and square plumes obtained from experiments. In the parameter space investigated, which spans a Richardson number range $0.03<Ri<960$ , instability analysis reveals that planar plumes exhibit BiGlobal instability only for varicose perturbations, while they remain stable for sinuous perturbations. The BiGlobal frequency and growth rates of the unstable varicose mode are used to obtain Strouhal number correlation and stability curves. An investigation into the effect of the spanwise wavenumber on BiGlobal instability indicates that planar plumes are more unstable to two-dimensional perturbations than to three-dimensional perturbations. An increase in the spanwise wavenumber tends to stabilize planar plumes without affecting their oscillation frequencies. Experiments suggest that the puffing frequencies in rectangular plumes closely follow the power law obtained from two-dimensional instability analysis while exhibiting a weaker dependence on inlet aspect ratio. To further explore the effect of aspect ratio on puffing behaviour, experiments have been carried out in plumes of aspect ratio 1, i.e. square plumes. Square plumes are found to be more stable and to exhibit higher puffing frequencies than rectangular plumes. The reasons for these differences in puffing dynamics between rectangular and square plumes have been explored from the phase-locked streamwise and spanwise flow visualizations. In addition to puffing, spanwise visualizations in both rectangular and square plumes show the presence of secondary flows at their corners, similar to their constant-density jet counterparts. Finally, from experiments, we deduced a new universal puffing frequency correlation with the hydraulic diameter as the length scale which eliminates the aspect ratio dependence, and is valid for both square and low-aspect-ratio rectangular plumes.