The turbulent flow in a three-dimensional asymmetric diffuser was experimentally investigated using time-resolved, three-dimensional particle tracking velocimetry (3D-PTV). The diffuser geometry chosen for this study was a benchmark geometry devised and studied experimentally by Cherry et al. [Intl. J. Heat Fluid Flow 29, 3 (2008)], with a few subsequent numerical simulations at a Reynolds number of 10 000. In the present paper, we applied a state-of-the-art 3D-PTV to measure the 3D structure of the flow field in the entire diffuser for five Reynolds numbers (Re), ranging from 9200 up to 29 400. We found that the mean velocity fields were qualitatively similar across all Re studied. The volumetric fraction of the backflow region, when quantified using an intermittency factor of $\gamma <$ 0.8, was in the range of 11–14%, with a marginal decrease with increasing Re. The maximum values of normal and shear Reynolds stresses were located in the regions close to the edge of the backflow region, and the peak values for the streamwise normal Reynolds stress increased with Re. The corner vortices, which formed in the channel preceding the diffuser, showed the existence of secondary flow well within the diffuser region. The strength of these vortices reduced with increasing Re. A modal decomposition of the turbulent fluctuations using spectral proper orthogonal decomposition (SPOD) showed large-scale structures in the flow. The SPOD analysis revealed that these large-scale structures were associated with low frequency oscillations in the band of St = [0.003 0.03], with two frequency peaks at St = 0.012 and 0.028. The three-dimensional separation along the two diverging walls of the diffuser was characterized by detecting critical points in the near-wall streamlines. These critical points were investigated in relation to three large-scale vortical structures identified within the diffuser. The flow topology in the diffuser showed that two of these vortical structures originated from the wall with the largest diverging angle and were connected by a separation surface. The third vortical structure originated from the neighboring diverging wall and was bound by another large separation surface.

中文翻译：

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非对称扩散器中湍流和流动拓扑的体积测量

使用时间分辨的三维粒子跟踪测速仪（3D-PTV），对三维非对称扩散器中的湍流进行了实验研究。本研究选择的扩散器几何形状是Cherry等人设计和实验研究的基准几何形状。[国际 J. Heat Fluid Flow 29，3（2008）]，随后进行了一些雷诺数为10000的数值模拟。在本文中，我们应用了最先进的3D-PTV来测量3D结构5个雷诺数（Re）在整个扩散器中的流场的变化，范围从9200到29400。我们发现，在所有Re研究中，平均速度场在质量上都相似。回流区域的体积分数，使用$\gamma <$0.8的范围是11–14％，随着Re的增加而略有下降。正向和剪切雷诺应力的最大值位于靠近回流区域边缘的区域，并且沿流方向的雷诺应力的峰值随Re增大。在扩散器前面的通道中形成的角涡流表明，在扩散器区域内很好地存在二次流动。这些涡旋的强度随着Re的增加而降低。使用频谱固有正交分解（SPOD）对湍流涨落进行模态分解后，流动中出现了大规模结构。SPOD分析表明，这些大型结构与St = [0.003 0.03]频带中的低频振荡有关，在St = 0.012和0.028处有两个频率峰值。通过检测近壁流线中的临界点来表征沿扩散器两个分叉壁的三维分离。这些临界点是与扩散器中确定的三个大型涡旋结构有关的。扩散器中的流动拓扑结构表明，其中两个旋涡结构起源于具有最大发散角的壁，并通过分离表面相连。第三涡旋结构起源于相邻的发散壁，并被另一个较大的分离面所束缚。扩散器中的流动拓扑结构表明，其中两个旋涡结构起源于具有最大发散角的壁，并通过分离表面相连。第三涡旋结构起源于相邻的发散壁，并被另一个较大的分离面所束缚。扩散器中的流动拓扑结构表明，其中两个旋涡结构起源于具有最大发散角的壁，并通过分离表面相连。第三涡旋结构起源于相邻的发散壁，并被另一个较大的分离面所束缚。