Abstract: Image technology is an important approach to fluid mechanics research widely used in flow visualization and flow measurement, and its application in such research is continuously deepening with the development of information technology. By taking the flow past a circular cylinder as an example, this paper analyzes the image characteristics of wake vortices in the Reynolds number (Re) range of 50 to 1725. We examine variations in the image feature parameters of grayscale histogram and texture with Re, and give an interpretation related to flow characteristics. The results show three factors, the turbulent diffusion rate of the wake flow, separation point locations, and the spatial distribution of wake vortices, are responsible for different characteristics of the grayscale histogram and texture. At Re < 600, the mean value of gray level decreases linearly with Re; at Re = 148 ~ 642, the vortex shedding period based on texture analysis agrees well with its empirical calculation; at Re > 170, the skewness of grayscale histogram increases with Re and follows a linear correlation.

Key words: wake flow behind a circular cylinder, image processing, grayscale histogram, skewness, texture features