Abstract:
The vortices merging in a turbulent boundary layer of a rectangular channel were experimental studied with a moving single-frame and long-exposure (MSFLE) imaging method which is a Lagrangian-type measurement technique. The process of vortices merging was studied in the streamwise-normal plane with
Reθ=97~194. During the experiments, the measurement system moved at a constant speed that is similar to the moving velocity of measured vortex to record continually the moving of vortices with long exposure time. An image processing algorithm based on skeleton extraction was employed to process the images of the flow field for obtaining the velocity of vortices and the flow field. The Liutex theory was adopted to process the flow field for characterizing the intensity of vortex structure. The study shows that the MSFLE imaging method is a cost effective method. It can intuitively show the spatiotemporal evolution of vortex and surrounding flow field in a turbulent boundary layer with a Lagrangian-perspective. The MSFLE image method combined with Liutex vortex recognition algorithm can be applied to the visualization and quantification of vortex structure in the channel turbulent boundary layer. The vortices merging most possibly occur in a pair of adjacent co-rotating vortices with the basically equal intensity and size. During the process of merging, the intensity of these two vortices changes in the opposite direction. Moreover, a new generated vortex has the same rotation direction as the two merging vortices and its size and intensity are about the sum of the two merging vortices at the initial merging.