Abstract

In this paper, the structure of the turbulent wake behind the inner tube of a suddenly expanding annular jet flow is studied. The flow field is measured using tomographic particle image velocimetry and analyzed using proper orthogonal decomposition (POD). It was found that both the instantaneous and time-averaged central wakes behind the inner pipe are highly asymmetric despite the axisymmetric structure of the geometry. This asymmetry is the result of a bifurcation at low Reynolds numbers which persists up to the turbulent regime. The asymmetry induces a pair of counter-rotating vortices in the jet which are aligned with the main flow direction. Moreover, the asymmetry also induces a highly dynamical flow field. Analyzing the flow structures using POD shows that the wake oscillates around the asymmetric equilibrium position at a very low Strouhal number in the order of 0.01. On top of this motion, the inner shear layer oscillates with Strouhal numbers in the range of 0.1-0.3. This oscillation causes an asymmetric shedding of vortices of the hairpin type in the inner shear layer. As such, a local asymmetric region of very intensive mixing is induced near the stagnation point.

Graphical Abstract

中文翻译：

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在突然膨胀的环形射流之后不对称涡旋脱落

摘要

在本文中，研究了环形喷射流突然膨胀的内管后面的湍流尾流的结构。使用断层摄影颗粒图像测速仪测量流场，并使用适当的正交分解（POD）分析流场。已经发现，尽管几何形状是轴对称的，但在内管后面的瞬时和时间平均的中央尾流都是高度不对称的。这种不对称性是在低雷诺数下分叉的结果，该分叉一直持续到湍流状态。不对称性在射流中引起一对与主流动方向对齐的反向旋转涡流。此外，不对称性还引起高度动态的流场。使用POD分析流动结构表明，尾流以非常低的Strouhal数（约0.01）围绕非对称平衡位置振荡。在此运动之上，内部剪切层以斯特劳哈尔数在0.1-0.3范围内振荡。这种振荡在内部剪切层中引起发夹型涡旋的不对称脱落。这样，在停滞点附近会引起非常强烈的混合的局部不对称区域。

图形概要