Abstract Unsteady turbulence structures in and downstream of a short elbow (radius to pipe diameter ratio 1.0) were studied experimentally for sub-to-post-critical Reynolds numbers (Re = 1 × 105 to 3 × 105). Measurement of the velocity field was carried out using particle image velocimetry (PIV) combined with proper orthogonal decomposition (POD) analysis. The mean velocity, turbulence intensity, power spectrum, spatial variation of POD modes, and time variation of instantaneous velocity and vorticity fields were evaluated. The experimental results indicate that a separation region forms around the elbow exit, which is almost independent of the Reynolds number for the post-critical Reynolds numbers. The power spectrum analysis indicates that the periodic oscillation is observed at Strouhal number 0.5, and that it originates from interaction of the secondary flow in the elbow and the unsteady separating flow behavior near the exit of the inner elbow wall. Furthermore, POD analysis was conducted to clarify the variation of POD modes in the elbow flow. It was found that the periodic velocity fluctuation is generated by the oscillation of the flow in spanwise direction downstream of the elbow, which consists of a secondary flow structure directed to the inner elbow wall, and a streamwise longitudinal structure along the centerline of the elbow. The POD reconstructed velocity and vorticity fields indicate periodic oscillation of the entire secondary flow arising from unsteady oscillation of a pair of vortices in the cross section of the elbow; this condition prevails in and downstream of the elbow.

中文翻译：

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后临界雷诺数下短弯管内和下游的不稳定湍流结构

摘要 针对亚临界到后临界雷诺数（Re = 1 × 105 至 3 × 105），对短弯头（半径与管径比 1.0）内和下游的不稳定湍流结构进行了实验研究。使用粒子图像测速 (PIV) 结合适当的正交分解 (POD) 分析进行速度场的测量。评估了平均速度、湍流强度、功率谱、POD 模式的空间变化以及瞬时速度和涡度场的时间变化。实验结果表明，在肘部出口周围形成了一个分离区域，对于后临界雷诺数，该区域几乎与雷诺数无关。功率谱分析表明，在 Strouhal 数 0.5 处观察到周期性振荡，它起源于弯管中的二次流和弯管内壁出口附近的非定常分离流动行为的相互作用。此外，还进行了 POD 分析以阐明弯管流中 POD 模式的变化。发现周期性速度波动是由弯管下游沿展向方向的流动振荡产生的，它由指向弯管内壁的二次流结构和沿弯管中心线的纵向流动结构组成。POD 重建的速度场和涡度场表明整个二次流的周期性振荡是由弯管横截面中一对涡流的不稳定振荡​​引起的；这种情况在肘部及其下游普遍存在。