Abstract We have recently discovered a new type of self-excited flapping jets due to a flexible film whose leading edge is fixed at the nozzle exit [Exp Ther Fluid Sci, 106, 226-233]. This paper is to report the experimental investigation on mixing characteristics of the jet induced by a rectangular FEP film. Hot wire anemometry and flow visualization are used to examine the flapping jet flow versus the non-flapping counterpart. Experiments are conducted under the following conditions: i.e., L/D = 1.0 (fixed), W/D = 0.03 ~ 1.0 (varying) and Re = 10000 ~ 45000 (varying); where W and L are the film's width and length, D is the nozzle-exit diameter, and Re is the Reynolds number defined by Re UoD/ν with Uo and ν being the jet-exit velocity and fluid viscosity. It is found that the jet-flapping frequency fF varies with W in a complex fashion while it grows roughly linearly with increasing Uo for W/D ≥ 0.5. The flapping Strouhal number StF fFD/Uo ranges in 0.13 ≤ StF ≤ 0.23 for Re = 15,000 ~ 45,000. These Strouhal numbers are substantially lower than that (≈ 0.45 ~ 0.7) for the primary vortex generation in the free jet, but one to two orders of magnitude higher than those from the conventional self-exciting fluidic devices. In general, the flapping jet decays and spreads more rapidly than does the free jet. As W increases, the decaying and spreading rates both grow. Of significance, the centerline evolutions of Taylor and Kolmogorov scales versus the integral scale are examined to characterize the small scales of turbulence against the large-scale motion.

中文翻译：

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薄膜激振拍动射流的混合特性

摘要 我们最近发现了一种新型的自激扑翼射流，这是由于其前缘固定在喷嘴出口处的柔性薄膜 [Exp Ther Fluid Sci, 106, 226-233]。本文报告了矩形 FEP 薄膜引起的射流混合特性的实验研究。热线风速测量和流动可视化用于检查扑翼射流与非扑翼对应物。实验在以下条件下进行：即 L/D = 1.0（固定），W/D = 0.03 ~ 1.0（变化）和 Re = 10000 ~ 45000（变化）；其中 W 和 L 是薄膜的宽度和长度，D 是喷嘴出口直径，Re 是由 Re UoD/ν 定义的雷诺数，其中 Uo 和 ν 是射流出口速度和流体粘度。发现喷气扑动频率 fF 以复杂的方式随 W 变化，而当 W/D ≥ 0.5 时，它随着 Uo 的增加大致呈线性增长。对于 Re = 15,000 ~ 45,000，扑动 Strouhal 数 StF fFD/Uo 的范围为 0.13 ≤ StF ≤ 0.23。这些 Strouhal 数大大低于自由射流中主要涡流产生的 (≈ 0.45 ~ 0.7)，但比传统自激射流装置高出一到两个数量级。一般来说，扑翼射流比自由射流衰减和扩散得更快。随着 W 的增加，衰减率和扩散率都会增加。重要的是，泰勒和柯尔莫哥洛夫尺度与积分尺度的中心线演变被检查以表征湍流的小尺度对抗大尺度运动。