The three-dimensionality of turbulent motions in a massively separated Mach 2.49 longitudinal cylinder wake is investigated using tomographic particle image velocimetry (TPIV). Measurements were acquired within six volumetric subregions of the flow, including the separated shear layer, the recirculation region, the shear layer reattachment region, and the trailing wake, with a large ensemble of measurement volumes (between 2100 and 3300) acquired and processed per region to allow for adequate statistical convergence in the various analyses used. Large streamwise-elongated turbulent structures with a quasiaxial orientation were commonly observed to exist in regions of the flow encountering the adverse pressure gradient associated with shear layer reattachment. The statistical geometry and orientation of these structures within this region of the flow are demonstrated using linear stochastic estimation. The snapshot proper orthogonal decomposition method was also used to decompose the TPIV data into mode shapes. Both an energy-based and an enstrophy-based decomposition were performed, as the three-dimensional TPIV data allow for calculation of all components of the vorticity vector. It is demonstrated that the highest energy-containing modes for many of the TPIV subregions are associated with coherent turbulent structures, such as hairpin vortices or the aforementioned quasiaxial structures. The highest energy-containing mode from the recirculation region data represented 30% of the turbulent kinetic energy within a single mode, and appears to indicate a large-scale global flow interaction between the recirculation region and the separated shear layer with a helical orientation. This large-scale behavior appears consistent with past computational simulations of this flow that also predicted that low-order azimuthal and helical instability modes within the recirculation region play a significant role in the generation of large-scale structures and the production of pressure drag.

中文翻译：

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轴对称分离/附着超音速流中的三维流动结构

使用断层成像颗粒图像测速仪（TPIV）研究了在质量上巨大的分离性马赫2.49纵向圆柱尾流中的湍流运动的三维性。在流的六个体积子区域中获取测量值，包括分离的剪切层，再循环区域，剪切层重新附着区域和尾随尾流，每个区域都需要采集和处理大量的测量体积（在2100和3300之间）以便在使用的各种分析中进行足够的统计收敛。通常观察到具有准轴向取向的大的沿流向伸长的湍流结构存在于流区，该流域遇到与剪切层重新附着有关的不利压力梯度。使用线性随机估计来证明这些结构在流动区域内的统计几何形状和方向。快照适当的正交分解方法还用于将TPIV数据分解为模式形状。由于三维TPIV数据允许计算涡度矢量的所有分量，因此进行了基于能量和基于熵的分解。已经证明，许多TPIV子区域的最高能量包含模式与相干湍流结构（例如发夹涡旋或上述准轴结构）相关。再循环区域数据中包含最高能量的模式代表了单个模式内湍流动能的30％，似乎表明再循环区域和分离的剪切层之间呈螺旋状的大规模整体流动相互作用。这种大规模的行为似乎与该流动的过去计算模拟相一致，该模拟还预测再循环区域内的低阶方位角和螺旋不稳定性模式在大规模结构的产生和压力阻力的产生中起重要作用。