A numerical study of shear layer characteristics of low-speed transverse jets
A numerical study of shear layer characteristics of low-speed transverse jets
复制标题
低速横向射流剪切层特性数值研究
DOI：
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发表时间：
2016
影响因子：
3.7
通讯作者：
K. Mahesh
中科院分区：
文献类型：
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作者：
Prahladh S. Iyer;K. Mahesh
采用直接数值模拟（DNS）和动态模态分解（DMD）方法研究了横流中射流的剪切层特性。Megerian等人（J. Fluid Mech.，第593卷，2007年，pp. 93-129）在速度比（$R=上划线{v}_{j}/u_{infty }$为2和4以及雷诺数（$Re=上划线{v}_{j}D/{it
u}$）对上游剪切层从绝对不稳定向对流不稳定转变的影响。沿剪切层沿着不同点的点速度谱与实验结果吻合良好。当$R=2$时，相同的频率（$St=0.65$）沿剪切层长度沿着方向占主导地位，而当$R=4$时，主导频率沿剪切层沿着方向变化。全三维流场的DMD能够再现从DNS观察到的主频率，并表明剪切层模式在模拟的两种条件下都占主导地位。利用DMD获得的空间模态研究了剪切层不稳定性的本质。结果表明，在上游剪切层中形成了一个逆流混合层。相应的混合速度比获得，并看到描绘两个制度的绝对或对流不稳定。喷嘴的效果进行评估，通过执行模拟没有喷嘴，同时要求射流具有相同的入口速度分布，在喷嘴出口处获得的模拟，包括喷嘴。剪切层谱显示出良好的协议与模拟包括喷嘴。剪切层厚度的影响进行了研究，在速度比为2的基础上的峰值和平均射流速度。DNS/DMD的主导频率和空间剪切层模式显着改变射流出口速度分布。
Direct numerical simulation (DNS) and dynamic mode decomposition (DMD) are used to study the shear layer characteristics of a jet in a crossflow. Experimental observations by Megerian et al. (J. Fluid Mech., vol. 593, 2007, pp. 93–129) at velocity ratios ( $R=overline{v}_{j}/u_{infty }$ ) of 2 and 4 and Reynolds number ( $Re=overline{v}_{j}D/{it
u}$ ) of 2000 on the transition from absolute to convective instability of the upstream shear layer are reproduced. Point velocity spectra at different points along the shear layer show excellent agreement with experiments. The same frequency ( $St=0.65$ ) is dominant along the length of the shear layer for $R=2$ , whereas the dominant frequencies change along the shear layer for $R=4$ . DMD of the full three-dimensional flow field is able to reproduce the dominant frequencies observed from DNS and shows that the shear layer modes are dominant for both the conditions simulated. The spatial modes obtained from DMD are used to study the nature of the shear layer instability. It is found that a counter-current mixing layer is obtained in the upstream shear layer. The corresponding mixing velocity ratio is obtained, and seen to delineate the two regimes of absolute or convective instability. The effect of the nozzle is evaluated by performing simulations without the nozzle while requiring the jet to have the same inlet velocity profile as that obtained at the nozzle exit in the simulations including the nozzle. The shear layer spectra show good agreement with the simulations including the nozzle. The effect of shear layer thickness is studied at a velocity ratio of 2 based on peak and mean jet velocity. The dominant frequencies and spatial shear layer modes from DNS/DMD are significantly altered by the jet exit velocity profile.