Unsteady aerodynamic characteristics of flexible flapping plate at low Reynolds numbers

Flexible deformation impacts the aerodynamics of flapping wings and is accompanied by severe fluid-structure interaction effects in the case of flexible flapping wings. This study examines the fluid-structure interaction aerodynamic properties of a flexible flapping plate at low Reynolds numbers in order to clarify the process by which flexible deformation influences unsteady aerodynamic forces. By applying the motion patterns of dragonflies to both flexible and rigid flapping wings and conducting fluid-structure interaction numerical simulations, it was found that, under the same motion patterns, the average lift of flexible flapping wings over an entire cycle increased by 60.5% compared to rigid flapping wings. During the initial downstroke, the lift decreased by 31.4%, but increased by 76.7% during the mid to late downstroke. The thrust generated by both flexible and rigid flapping wings throughout the cycle was nearly zero. The study revealed the spatiotemporal influence mechanism of flexible deformation on aerodynamic forces. Lift is increased by the flexible flapping wings' spanwise bending deformation during the downstroke, which keeps the leading-edge vortex's spanwise distribution on the wing surface. At the beginning of the downstroke, flexible flapping wings experience downstroke lag, which is detrimental to the formation of the leading-edge vortex, thus reducing lift. As the downstroke progresses, spanwise bending deformation increases the downstroke speed, affecting the structure of the leading-edge vortex and subsequently increasing lift.