Abstract
Low- and high-speed streaks (ejection, Q2, and sweep, Q4, events in quadrant analysis) are significant features of coherent structures in turbulent flow. Streak formation is closely related to turbulent structures in several vortex models, such as attached eddy models, streamwise vortex analysis models, and hairpin vortex models, which are all standard models. Vortex models are complex, whereby the relationships among the different vortex models are unclear; thus, further studies are still needed to complete our understanding of vortices. In this study, 30 sets of direct numerical simulation (DNS) data were obtained to analyze the mechanics of the formation of coherent structures. Image processing techniques and statistical analysis were used to identify and quantify streak characteristics. We used a method of vortex recognition to extract spanwise vortices in the x–z plane. Analysis of the interactions among coherent structures showed that the three standard vortex models all gave reasonably close results. The attached eddy vortex model provides a good explanation of the linear dimensions of streaky structures with respect to the water depth and Q2 and Q4 events, whereby it can be augmented to form the quasi-streamwise vortex model. The legs of a hairpin vortex envelop low-speed streaky structures and so move in the streamwise direction; lower-velocity vortex legs also gradually accumulate into a streamwise vortex. Statistical analysis allowed us to combine our present results with some previous research results to propose a mechanism for the formation of streaky structures. This study provides a deeper understanding of the interrelationships among different vortex models.
Highlights
Turbulence is generally not altogether chaotic, whereas there are many regular coherent structures in a turbulent flow
The coherent structures include streaky structures formed by low- and high-speed streaks, the bursting phenomenon that includes ejection and sweep events, vortex structure models, as well as superscale structures
We identified the positions of low- and high-speed streaks using image processing and calculated the characteristic dimensions of streaky structures in both the inner and outer layers using a statistical method
Summary
Turbulence is generally not altogether chaotic, whereas there are many regular coherent structures in a turbulent flow. The coherent structures include streaky structures formed by low- and high-speed streaks, the bursting phenomenon that includes ejection and sweep events (in quadrants Q2 and Q4), vortex structure models (streamwise vortex model, attached eddy vortex model, hairpin vortex model and hairpin vortex groups), as well as superscale structures. Low- and high-speed streaks are important in turbulence dynamics because of their large scale [1]. Experimental research into low- and high-speed streaks using hydrogen bubbles was first conducted by. The characteristic scales of streaky structures were identified by many researchers as the average nondimensional width W = 20–40y* and spanwise distance D = 100y* in the boundary layer region [3,4,5]. Lin et al [6] used particle image velocimetry
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