Experimental study on flow structure of transition boundary layer of the underwater vehicles
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摘要: 采用激光诱导荧光（Laser Induced Fluorescence，LIF）和粒子图像测速（Particle Image Velocimetry，PIV）技术，对自由湍流条件下SUBOFF模型转捩边界层中的流动结构进行精细测量。实验在北京航空航天大学大型低速回流水洞中进行，SUBOFF模型长度为1.436 m，基于模型长度和来流速度的实验雷诺数为3.35 × 105。采用流动显示灰度场和有限时间李雅普诺夫指数（Finite-Time Lyapunov Exponents，FTLEs）对转捩边界层中的涡结构进行识别，并对转捩过程中发卡涡、二次涡等典型拟序结构的生成演化过程进行分析。采用两点相关方法提取转捩区拟序结构，同时采用椭圆拟合方法计算相干结构倾角，计算结果表明，结构倾角沿法向先增大后减小，在边界层附近达到最大值。为深入研究转捩流动结构特性，发展了基于流动显示的湍流/非湍流界面（T/NT）识别方法，并对界面几何特性进行了研究。研究结果表明，在转捩过程中，界面的法向高度和分形维数沿程增长。
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关键词:
- 边界层转捩 /
- 流动显示 /
- 有限时间李雅普诺夫指数 /
- 湍流/非湍流界面
Abstract: Laser induced fluorescence (LIF) and particle image velocimetry (PIV) are used to measure the flow field of the transition boundary layer of the SUBOFF model subject to the free-stream turbulence (FST). The experiment is conducted in the water tunnel at Beihang University. The length of the SUBOFF model is L = 1.436 m, and the Reynolds number is ReL = 3.35 × 105 based on the length of the model and the free-stream velocity. The vortex structures during the transition process are identified using the grayscale field of laser-induced fluorescence and Finite-Time Lyapunov Exponents methods, which illustrate the generation and evolution process of typical structures, such as hairpin vortices and the induced secondary vortices. The coherent structures of the transition boundary layer are extracted using two-point correlation. Simultaneously, the elliptical fitting method is used to calculate the inclination angle of coherent structures. The inclination angle increases initially, and then decreases along the normal direction, reaching its maximum near the boundary layer. The visualization-based method is proposed for identifying turbulent/non-turbulent interfaces, with a focus on studying the geometric characteris-tics of the interface during the transition process. It is shown that the height and the fractal dimension of the interface increase along the transition. -
图 3 LIF流动显示x–y平面内的瞬时流动结构
Figure 3. LIF-visualized vortical structures in x–y plane at four snapshots
图 4 FTLE在x–y平面内的涡结构演化过程。（a1～c1）孤立发卡涡结构及诱导的二次涡，（a2～c2）发卡涡包
Figure 4. Illustration of the evolution of vortex structures by Finite-Time Lyapunov Exponents method in x–y plane. （a1 – c1） Hairpin vortex and induced secondary vortex in transition, （a2 – c2） hairpin vortex packets
图 5 不同法向高度的两点相关系数分布，从上至下对应法向高度为yn/δ = 0.10、0.25、0.50、1.00、1.25，（a1～e1）基于LIF流动显示的两点相关系数分布，（a1～e1）基于FTLEs的两点相关系数分布
Figure 5. The contour of two points correlation coefficient at different normal positions. Five rows from the top to the bottom are the results at yn/δ = 0.10, 0.25, 0.50, 1.00 and 1.25, respectively. （a1 – e1） two-point correlation distribution based on LIF flow visualization, （a2 – e2） two-point correlation distribution based on FTLEs
图 6 x/L = 0.50处拟序结构结构倾角沿法向高度的变化
Figure 6. Inclination angle of coherent structure along normal direction at x/L = 0.50
图 8 不同流向位置界面高度概率分布
Figure 8. The probability density function of the interface position
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