Kinematics of local entrainment and detrainment in a turbulent jet
Kinematics of local entrainment and detrainment in a turbulent jet
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DOI：
10.1017/jfm.2019.327
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发表时间：
2019-05
影响因子：
3.7
通讯作者：
Dhiren Mistry;J. Philip;J. Dawson
中科院分区：
文献类型：
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作者：
Dhiren Mistry;J. Philip;J. Dawson
本文研究了高雷诺数轴对称射流中流体元素在湍流/非湍流界面（TNTI）上携带和分离时的连续局部交换。为了阐明局部夹带和去除过程的特征运动学特征，同时进行了高速粒子图像测速和平面激光诱导荧光测量。利用界面跟踪技术，我们根据界面几何形状和局部流场，评估和分析了在随TNTI移动的参照系中局部夹带速度的条件依赖性。我们发现局部夹带速度是间歇性的，具有泰勒微尺度数量级的特征长度尺度，并且对净夹带速率的贡献来自局部夹带速率和夹带速率之间的不平衡，这种不平衡发生在两部分夹带与一部分夹带的比例上。平均而言，局部夹带的增加与TNTI向射流中心线的偏移有关，这些区域具有较高的流向动量，面向射流湍流侧和沿界面前缘的凸表面曲率。相反，沉积与TNTI从急流中心线向低流向动量、凹表面曲率和沿尾缘的区域漂移有关。我们发现，强夹带的特征是在与TNTI一起运动的参照系中存在局部逆流速度场，这增强了旋转和非旋转流体单元的输运。另一方面，沉淀的特征是局部均匀的流场，TNTI两侧的局部流体速度在同一方向上平流。这些局部流动模式和夹带或夹带率的强度也被观察到强烈地受到沿射流边界分布的湍流区域的泰勒微尺度的涡结构的存在和相对强度的影响。
In this paper we investigate the continuous, local exchange of fluid elements as they are entrained and detrained across the turbulent/non-turbulent interface (TNTI) in a high Reynolds number axisymmetric jet. To elucidate characteristic kinematic features of local entrainment and detrainment processes, simultaneous high-speed particle image velocimetry and planar laser-induced fluorescence measurements were undertaken. Using an interface-tracking technique, we evaluate and analyse the conditional dependence of local entrainment velocity in a frame of reference moving with the TNTI in terms of the interface geometry and the local flow field. We find that the local entrainment velocity is intermittent with a characteristic length scale of the order of the Taylor micro-scale and that the contribution to the net entrainment rate arises from the imbalance between local entrainment and detrainment rates that occurs with a ratio of two parts of entrainment to one part detrainment. On average, an increase in local entrainment is correlated with excursions of the TNTI towards jet centreline into regions of higher streamwise momentum, convex surface curvature facing the turbulent side of the jet and along the leading edges of the interface. In contrast, detrainment is correlated with excursions of the TNTI away from the jet centreline into regions of lower streamwise momentum, concave surface curvature and along the trailing edge. We find that strong entrainment is characterised by a local counterflow velocity field in the frame of reference moving with the TNTI which enhances the transport of rotational and irrotational fluid elements. On the other hand, detrainment is characterised by locally uniform flow fields with the local fluid velocity on either side of the TNTI advecting in the same direction. These local flow patterns and the strength of entrainment or detrainment rates are also observed to be strongly influenced by the presence and relative strength of vortical structures which are of the order of the Taylor micro-scale that populate the turbulent region along the jet boundary.