The structure of the absolutely unstable regions in the near field of low-density jets
The structure of the absolutely unstable regions in the near field of low-density jets
复制标题
低密度喷流近场绝对不稳定区域的结构
DOI：
10.1017/jfm.2012.441
复制
发表时间：
2012
影响因子：
3.7
通讯作者：
A. Sevilla
中科院分区：
文献类型：
--
作者：
W. Coenen;A. Sevilla
摘要：对从圆形喷嘴或管中产生的低密度层流圆形射流的粘性时空稳定性特性进行了数值研究，首次对实验中典型研究的两种特殊情况进行了单独处理：热气体射流排放到同一物种的静止冷环境中，以及由两种不同分子量气体的混合物组成的等温射流排放到较重物种的静止环境中。为此，使用基于边界层理论的基本速度和密度分布的实际表示，并考虑可变传输特性的影响。我们的结果显示出与之前的参数研究相比存在显着的定量差异，并表明热射流通常比轻射流更不稳定，因为它们对于实验中通常使用的雷诺数和动量厚度值具有更大的相关临界密度比。此外，对于射流与环境密度比 $S$ 的几个值，射流局部稳定性特性的下游演化计算为控制射流的两个主要控制参数的函数，即雷诺数 $\mathit{Re}$ 和初始速度剖面的动量厚度 ${\theta }_{0} / D$ 。结果表明，对于给定的 $S$ 值，$(\mathit{Re}, {\theta }_{0} / D)$ 参数平面可以分为三个区域。在由低值 $\mathit{Re}$ 或非常厚的剪切层定义的第一个区域中，流动在各处都是局部对流不稳定的。在第二个区域中，$\mathit{Re}$ 值适中且剪切层薄，射流表现出远离边界的绝对不稳定的局部区域。最后，在第三个区域中，在大多数 $(\mathit{Re}, {\theta }_{0} / D)$ 参数平面中普遍存在，绝对不稳定域以射流出口为界。文献中所有可用的实验都表明位于后一个区域，并且实验中观察到的全局转变被证明在绝对不稳定域变得足够大时发生。由此产生的全局自激振荡的边缘频率可以通过在射流出口处评估的绝对频率很好地描述，这与 Lesshafft 等人获得的数值结果一致。 (J. Fluid Mech., vol. 554, 2006, pp. 393–409) 用于合成射流。
Abstract The viscous spatiotemporal stability properties of low-density laminar round jets emerging from circular nozzles or tubes are investigated numerically providing, for the first time, a separate treatment of the two particular cases typically studied in experiments: a hot gas jet discharging into a quiescent cold ambient of the same species, and an isothermal jet consisting of a mixture of two gases with different molecular weight, discharging into a stagnant ambient of the heavier species. To that end, use is made of a realistic representation for the base velocity and density profiles based on boundary-layer theory, with account taken of the effect of variable transport properties. Our results show significant quantitative differences with respect to previous parametric studies, and reveal that hot jets are generically more unstable than light jets, in the sense that they have larger associated critical density ratios for values of the Reynolds number and momentum thickness typically used in experiments. In addition, for several values of the jet-to-ambient density ratio, $S$ , the downstream evolution of the local stability properties of the jet is computed as a function of the two main control parameters governing the jet, namely the Reynolds number, $\mathit{Re}$ , and the momentum thickness of the initial velocity profile, ${\theta }_{0} / D$ . It is shown that, for a given value of $S$ , the $(\mathit{Re}, {\theta }_{0} / D)$ parameter plane can be divided in three regions. In the first region, defined by low values of $\mathit{Re}$ or very thick shear layers, the flow is locally convectively unstable everywhere. In the second region, with moderately large values of $\mathit{Re}$ and thin shear layers, the jet exhibits a localized pocket of absolute instability, away from boundaries. Finally, in the third region, that prevails in most of the $(\mathit{Re}, {\theta }_{0} / D)$ parameter plane, the absolutely unstable domain is bounded by the jet outlet. All the experiments available in the literature are shown to lie in the latter region, and the global transition observed in experiments is demonstrated to take place when the absolutely unstable domain becomes sufficiently large. The marginal frequency of the resulting global self-excited oscillations is shown to be fairly well described by the absolute frequency evaluated at the jet outlet, in agreement with the numerical results obtained by Lesshafft et al. (J. Fluid Mech., vol. 554, 2006, pp. 393–409) for synthetic jets.