Abstract
The flapping characteristics of perforated flags with various porosities in different flow environments are studied using XFlow-Abaqus co-simulation. It is proved that flapping characteristics significantly correlate with flag porosity in different flow environments. Specifically, as the flag porosity increases, the flapping speed and vortex strength decrease monotonically. Further study of the influence of perforated flags on the flow structures surrounding a square cylinder revealed that different flag porosities lead to different levels of flow control. As the flag porosity increases, the flag's modulation weakens, the average drag on the square cylinder rises, and the influence of the Kármán vortex street becomes more significant. The flag-induced vortices can generate a high-pressure zone on the rear side of the square cylinder's top surface, leading to a decrease in the average lift on the square cylinder. It is notable that flags with different porosities generate vortices at different flapping states, which results in phase differences between the lift on the square cylinder and the flag. Finally, the study discusses the peaks of the Reynolds stress in the x- and y-directions, which delineate the boundary of the recirculation zone and reflect force fluctuations on the flag and the square cylinder, respectively.