Insights on the effects of aspect ratio and presence of bluff body on the dynamics and characteristics of flexible flags

This study explores the flapping behavior of flexible flags in low Reynolds number scenarios, particularly focusing on the influence of the flag's aspect ratio, flow velocity, and the presence of a bluff body placed at front of the flexible flag. Numerous studies have investigated the flapping dynamics of small aspect ratio flexible flags, however there is an absence of studies investigating change in flapping behavior of flexible flags with an increasing aspect ratio, especially for underwater applications. For this purpose, we experimentally and computationally investigate the impact of flow disturbances on the flag's dynamics, aiming to understand the fluid-structure interactions between the flag's motion and the surrounding fluid with and without the presence of vortices induced by a cylindrical bluff body. The experimental and numerical studies are conducted at two Reynolds numbers, approximating to values of 3600 and 6200, with aspect ratios ranging from 0.5 to 4.0, using a cylindrical bluff body. Results indicate a pronounced appearance of torsional-conventional flapping i.e. a coupling between torsional deformation and conventional flapping, for flags with aspect ratios greater than 2.0, where flag deformations are primarily attributed to an imbalance in vorticity distribution along the flag's height, as verified through three-dimensional computational fluid dynamics simulations. This imbalance in the vorticity distribution leads to an increase in the torsional response and a decrease in the amplitude of oscillation of the flexible flag. Furthermore, the inclusion of a bluff body changes the overall dynamics of the system due to higher vorticity and results in greater deflections for high aspect ratio flags compared to scenarios without the bluff body at low Reynolds numbers.