Experimental study on flow structure of transition boundary layer of the underwater vehicles

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 Re_L = 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.