TPIV study for near-field tip vortex from an elliptical hydrofoil
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摘要: 梢涡空化作为一种常见的空化现象，广泛存在于水力机械及船舶推进领域。梢涡空化初生与桨叶梢部的旋涡流动密切相关，因此有必要深入研究梢涡流场，揭示其流动特征与空化的内在联系。基于高时间解析度的层析PIV技术，在高速空泡水洞中对椭圆水翼的近尾迹梢涡流场开展了实验研究。结果表明：梢涡在近尾迹区域内存在明显的摆动现象，未考虑旋涡摆动的时间平均会在时均流场中引入额外的误差，因此在梢涡特性的定量研究中有必要滤除旋涡摆动的影响；在水翼脱落剪切层的作用下，涡核中心两侧的切向速度分布明显不对称，且在剪切层与涡核之间存在高速轴向流动区域；梢涡流场中的湍流脉动能量主要集中在涡核内部，且由法向、展向速度脉动主导。结合前人研究，发现法向、展向速度脉动是涡核内部湍流压力脉动的主要来源。Abstract: Tip vortex cavitation （TVC） is a common type of cavitation in hydraulic machinery and marine propulsion. Since TVC inception is highly relevant to the vortical flow around the blade tip of turbines and propellers, it is essential to give more insights into the flow field of the tip vortex to reveal the inherent relationship between flow properties and TVC. Measurement for the tip vortex from an elliptical hydrofoil has been conducted in a high-speed cavitation tunnel utilizing tomographic particle image velocimetry （TPIV） with high time-resolution. The results show that the wandering motion of the tip vortex is noticeable in the near field. The time-averaging process without taking into account wandering motion can bring extra errors into the time-averaged flow field. Therefore, it is necessary to filter out the wandering motion for the quantitative analysis on vortex characteristics. The tip vortex is under roll-up process and can be greatly affected by the shear layer from the hydrofoil, which contributes to the asymmetric circumferential velocity distribution and a high-axial-velocity area between the shear layer sheet and the vortex core. The tip vortex contains the most of the turbulence energy within its core and the turbulence energy is dominated by the vertical and spanwise velocity fluctuations which are considered as the main source of the fluctuating pressure in the core center combining with previous researches.
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Key words:
- elliptical hydrofoil /
- tip vortex /
- TPIV /
- flow field measurement /
- vortex motion
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图 3 摆动滤除前后的时均涡流场
Figure 3. Time-averaged flow field before and after vortex wandering correction
图 4 近尾迹区域内涡核中心运动轨迹
Figure 4. Trajectory of the tip vortex core center in the near field
图 6 旋涡摆动滤除前后时均速度场，α = 10°
Figure 6. Time-averaged velocity field before and after vortex wandering correction, α = 10°
图 8 时均轴向速度三维分布
Figure 8. Three-dimensional distribution of time-averaged axial velocity
图 9 时均轴向涡量及轴向速度云图，α = 10°
Figure 9. Contour of time-averaged axial vorticity and velocity, α = 10°
图 10 梢涡时均切向速度分布，α = 10°
Figure 10. Distribution of time-averaged circumferential velocity, α = 10°
图 12 旋涡摆动滤除前后的法向湍动能分布，α = 10°
Figure 12. Distribution of turbulence kinetic energy along y axis before and after vortex wandering correction, α = 10°
图 13 不同流向位置涡核中心各速度分量脉动强度
Figure 13. Velocity fluctuation in the core center at different streamwise position
表 1 涡核中心各速度分量脉动对湍动能的贡献
Table 1. The contribution of each velocity component fluctuation to TKE in the core center
α/（° ） Rec 0.5u' 2U∞–2k–1 0.5v' 2U∞–2k–1 0.5w' 2U∞–2k–1 5 2.48×105 7.6% 40.4% 52.0% 4.70×105 5.7% 41.6% 52.7% 10 2.48×105 10.3% 40.6% 49.1% 
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