In engineering and physics, fluid-structure interaction (FSI) is used to describe the coupling between the laws describing the structural mechanics and fluid dynamics of a system. This phenomenon is common in natural and engineering systems and is often characterized by oscillatory or stable interactions between moving or deformable structures and internal or surrounding fluid flow. Usually, the strains and stresses exerted on the solid object when the fluid flow interacts with the structure could result in deformations, whose magnitude depends on the material properties of the structure and the velocity and pressure of the flow.
Research on FSI has grown owing to its relevance in understanding fundamental natural principles and extensive engineering applications involving fluid mechanics. The two main categories of FSI problems are the fluid-rigid and fluid-flexible interactions. Lately, research in this field has focused on achieving enhanced hydrodynamic performance by manipulating the surrounding fluid flows. Interestingly, this research focus is increasingly being inspired by various natural phenomena involving flow control for optimum propulsion, such as biological swimming and flight by fish and insects. Importing these novel ideas would significantly contribute to the advancement of this field.
Although many flow control techniques have been developed, their applications are limited as it is difficult to implement them in some circumstances. Moreover, recent studies on rigid-flexible coupling systems have raised new FSI problems involving the coupling of closed flexible structures with internal and external fluids to influence the behavior of each other. While such extended FSI systems are common, there are limited studies on the dynamics of such coupling systems, especially the rigid-flexible coupling system, as well as the corresponding drag reduction flow control mechanism.
Herein, Dr. Yuehao Sun, Associate Professor Ze-Rui Peng, Professor Dan Yang, Professor Yongliang Xiong, Dr. Lei Wang and Professor Lin Wang from Huazhong University of Science and Technology investigated the dynamics of two-dimensional viscous flow past a rigid flat plate with a trailing closed flexible filament that served a deformable afterbody. These numerical investigations were conducted using immersed boundary-lattice Boltzmann and finite element methods for fluid flow and filament motion, respectively. Subsequently, the effects of the length ratio and Reynolds number on the dynamics and flow patterns of the system were studied. Their work is currently published in the peer reviewed Journal of Fluid Mechanics.
The research team identified five distinct system state modes according to the flow patterns, filament motion and corresponding dynamics: micro-vibration, static deformation, periodic flapping, multi-frequency flapping and chaotic flapping. The occurrence of these systems modes was significantly dependent on the length ratio and Raynolds number. For flows at low Reynolds number, the drag monotonically increased with an increase in the length ratio, while for flows at higher Reynolds number confirmed the existence of maximum drag reduction accompanied by the emergence of system state transition.
Compared with the bare plate, the coupling system achieved a significant drag reduction up to 22%, attributed to the passive flow control enabled by the flexible filament. The instantaneous flow fields and time-averaged analysis revealed that the drag reduction was mainly due to the effective control of the shedding vortices movement around the flexible afterbody. The form drag was predominant at small length ratios but not at large length ratios where the effect of the friction drag was considered. The friction drag decreased with an increase in the Reynolds number.
In summary, the numerical simulations of the dynamics of rigid-flexible coupling system in a uniform flow were reported. Unlike experimental studies, numerical simulations offer the advantage of quantitative analysis, which is critical in exploring the mechanisms underlying complex system behaviors. The authors proposed using the scaling laws for the friction and form drag emanating from the viscous and pressure effects to estimate the overall drag acting on the system. In a statement to Advances in Engineering, Professor Ze-Rui Peng, the corresponding author explained their findings would shed light on the dynamical behavior of rigid-flexible coupling systems and their relevance in practical applications.
Sun, Y., Peng, Z.-R., Yang, D., Xiong, Y., Wang, L., & Wang, L. (2022). Dynamics of a rigid-flexible coupling system in a uniform flow. Journal of Fluid Mechanics, 943, A44–25.