Experimental study on dynamic mechanism of vortex evolution in a turbulent boundary layer of low Reynolds number
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Experimental study on dynamic mechanism of vortex evolution in a turbulent boundary layer of low Reynolds number * Yan-ang Guo1, 2, Xiang-rui Dong1, 2, Xiao-shu Cai1, 2 , Wu Zhou1, 2 1. School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 2. Key Laboratory of Multiphase Flow and Heat Transfer in Shanghai Power Engineering, Shanghai 200093, China (Received March 30, 2020, Revised May 5, 2020, Accepted May 19, 2020, Published online September 21, 2020) ©China Ship Scientific Research Center 2020 Abstract: The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Re = 97 -194 is experimentally investigated. In this study, a moving single-frame and long-exposure (MSFLE) imaging method and a moving particle image velocimetry/particle tracing velocimetry (M-PIV/PTV) are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways, respectively. On the other hand, the Liutex vector, which is a new mathematical definition and identification of the vortex core proposed by Liu’s group, is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation. The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence. Furthermore, a quantitative investigation in terms of the critical vortex core boundary (size) and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV. According to statistics of the relation between vortex core size and the rotation strength during the whole process, the physical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the “synchronous linear segment (SL)-absolute enhancement segment (AE)-absolute diffusion segment (AD)-skewing dissipation segment (SD)”. Key words: Turbulent boundary layer, high- and low-speed fluid, vortex generation, moving single-frame and long-exposure, moving particle image velocimetry/particle tracing velocimetry, Liutex vector
Introduction Wall-bounded turbulent flow is a fundamental scientific topic that has drawn increased attention and studies in scientific research as well as engineering applications such as transition control and drag reduction[1], and many others. There is no doubt that the turbulent boundary layer is an essential component for most fluid dynamic research on transportation modes such as aircrafts, ships, and underwater vehicles. Turbulence remains the most important unsolved problem, and many researchers have treated it as a complex and random phenomenon. Coherent * Project supported by the National Natural Science Foundation of China (Grants Nos. 51906154, 51576130), t
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