Abstract

This work investigates the flow structures of a precessing jet in an axisymmetric chamber with expansion ratio \(D/d=5\) and length-to-diameter ratio \(L/D=2.75\) at Reynolds number \({\mathrm{Re}}_{d}=2.4\times 10\) ⁴ by use of planar particle image velocimetry. Time-averaged and statistical results indicate that the precessing jet flow is symmetric, on average, in the streamwise plane. Large velocity fluctuations and vorticity exist in the inner and outer shear layers. In addition, vorticity is found in the boundary layer of the chamber wall, due to backflow and confinement. Subsequent correlative analysis indicates that coherent structures are mainly distributed in the shear layer and decay with its development. The results also indicate that precession occurs in the region \(x/d>5\) and may lead to changes in the structure and position of the shear layer. Further spectral analysis shows that a low-frequency structure with \(\mathrm{St}\approx 7.4\times 10\)⁻³, which can be interpreted as precession, exists in the flow field and that the corresponding dominant frequency decreases as the fluid flows downstream. Finally, proper orthogonal decomposition (POD) analysis reveals the most energetic mode representing the flow structure of precession, which has the highest proportion of the total downstream energy. The precession induces an alternating flow, switching between outflow from one side of the chamber and inflow from another side. In addition, four typical instances of flow structures of precession with different intensities or phases are presented through low-order reconstruction of the specified POD modes. These cases show that due to the instability of the reattachment point, the mainstream oscillates up and down in the region near the nozzle exit and twists back and forth in the region near the chamber exit, with changes of scale and position of flow structures on the measurement plane, exposing complex behavior of the instantaneous flow field of precession.

Graphical abstract

中文翻译：

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进动射流在轴对称腔室中的流动结构

摘要

这项工作研究了雷诺数为\ {{mathrm}时，轴对称腔内进动射流的流动结构，膨胀比为\（D / d = 5 \），长径比为\（L / D = 2.75 \）。{Re}} _ {d} = 2.4 \ times10 \） ⁴通过使用平面粒子图像测速仪。时间平均和统计结果表明，进动的射流在流平面内平均是对称的。内部和外部剪切层中都存在较大的速度波动和涡度。另外，由于回流和限制，在室壁的边界层中发现涡旋。随后的相关分析表明，相干结构主要分布在剪切层中，并随着剪切的发展而衰减。结果还表明进动发生在区域\（x / d> 5 \）中，并可能导致剪切层的结构和位置发生变化。进一步谱分析表明，用低频率结构\（\ mathrm {圣} \约7.4 \倍10 \）^-3可以解释为旋进，存在于流场中，并且随着流体向下游流动，相应的主频降低。最后，适当的正交分解（POD）分析揭示了代表岁差流结构的最活跃的模式，该模式在总下游能量中所占比例最高。进动引起交替流动，在从腔室的一侧流出和从另一侧流入之间切换。此外，通过指定POD模式的低阶重构，展示了具有不同强度或相位的岁差流结构的四个典型实例。这些情况表明，由于重新连接点的不稳定，

图形概要