Turbulence structure of neutral and negatively buoyant jets
Turbulence structure of neutral and negatively buoyant jets
复制标题
DOI：
10.1017/jfm.2020.921
复制
发表时间：
2020-12
影响因子：
3.7
通讯作者：
K. Talluru;S. Armfield;N. Williamson;M. Kirkpatrick;L. Milton-McGurk
中科院分区：
文献类型：
--
作者：
K. Talluru;S. Armfield;N. Williamson;M. Kirkpatrick;L. Milton-McGurk
在中性射流(NJ)和负浮力射流(NBJ)中，分别将一股淡水垂直向下注入周围的淡水和咸水中，实现了速度和浓度的高保真测量。在所有实验中，基于管道入口直径($d)和源速度($W_o)的雷诺数($Re)约为5900，而在NBJ实验中，基于密度差的源弗劳德数约为30。用粒子图像测速仪和平面激光诱导荧光技术分别在1 7leq z/dleq 4 0$区域($z为轴向坐标)获得了速度和浓度测量结果。与有关急流的文献一致，在新泽西州，中心线速度($W_c)随z^-1而衰减，而在北半球，由于负浮力的作用，中心速度($W_c)沿z$衰减得更快。然而，当按局部中心线参数($W_c，C_c$)和喷流半宽($r^\ast_{W}，r^\ast_{C}$)标度时，两种流动中的平均速度($W$)和浓度($C$)分布均呈自相似的高斯分布。另一方面，NBJ中的湍流统计和雷诺应力并不随$W_c的变化而变化。自相关函数、积分长度尺度和二维相关图的结果表明，当轴向和径向距离被局部射流半宽归一化时，NJ和NBJ内的湍流结构相似。此外，谱函数和概率密度函数在轴线上是相似的，只有在喷流界面附近有微小的差异。上述发现与我们最近的分析(Milton-McGurk等人，J.Fluid Mech，2020b)基本一致，我们观察到NBJ中的平均统计和湍流统计具有不同的发展特征。总体而言，我们发现NBJ的湍流结构(用局部速度和长度尺度来衡量时)与动量驱动的NJ非常相似，而不同之处(如扩散速率、湍流强度的标度等)。新泽西州和新泽西州之间的关系似乎次要。
Abstract High-fidelity measurements of velocity and concentration are carried out in a neutral jet (NJ) and a negatively buoyant jet (NBJ) by injecting a jet of fresh water vertically downwards into ambient fresh and saline water, respectively. The Reynolds number ($Re$) based on the pipe inlet diameter ($d$) and the source velocity ($W_o$) is approximately 5900 in all the experiments, while the source Froude number based on density difference is approximately 30 in the NBJ experiments. Velocity and concentration measurements are obtained in the region $17 \leq z/d \leq 40$ ($z$ being the axial coordinate) using particle image velocimetry and planar laser induced fluorescence techniques, respectively. Consistent with the literature on jets, the centreline velocity ($W_c$) decays as $z^{-1}$ in the NJ, but in the NBJ, $W_c$ decays faster along $z$ due to the action of negative buoyancy. Nonetheless, the mean velocity ($W$) and concentration ($C$) profiles in both the flows exhibit self-similar Gaussian form, when scaled by the local centreline parameters ($W_c,C_c$) and the jet half-widths ($r^\ast _{W},r^\ast _{C}$). On the other hand, the turbulence statistics and Reynolds stress in the NBJ do not scale with $W_c$. The results of autocorrelation functions, integral length scales and two-dimensional correlation maps show the similarity of turbulence structure in the NJ and the NBJ when the axial and radial distances are normalised by the local jet half-width. Further, the spectra and probability density functions are similar on the axis and only minor differences are seen near the jet interface. The above findings are fundamentally consistent with our recent analysis (Milton-McGurk et al., J. Fluid Mech., 2020b), where we observed that the mean and turbulence statistics in the NBJ have different development characteristics. Overall, we find that the turbulence structure of the NBJ (when scaled by local velocity and length scales) is very similar to the momentum-driven NJ, and the differences (e.g. spreading rate, scaling of turbulence intensities, etc.) between the NJ and the NBJ seem to be of secondary importance.