How vortices behave on insect wings

Though insects and birds inspire aircraft design, the aspect ratio of the wing and its angle of attack are significantly different between natural and man-made flyers. By individually varying these parameters, Chengyu Li, Haibo Dong and Bo Cheng studied the effects of the Reynolds number, angle of attack and wing geometry on the vortices formed by a low-aspect-ratio wing over long travel distances.

“Our results demonstrated that the tip vortices have a more complex structure than what has been reported in the literature,” said Li.

While it was previously believed that vortex structures forming on the tip of a wing consist of a single loop structure extended from the leading-edge vortex, the group’s findings indicate the single loop can evolve into a counter-rotating dual vortex loop. As the leading-edge vortex intensifies as a result of increasing the wing’s aspect ratio or angle of attack, its counter-rotating trailing-edge vortex forms a secondary tip vortex. This dual-tip vortex formation remains consistent regardless of wing geometry.

Insect wing motion consists of two translational phases and two reversal phases, and a rotating wing configuration can be used as a canonical model for the translational phase. The group used this relationship to perform simulations of rotating rectangular wings within a fluid and calculated the wings’ lift and drag. They validated their simulated aerodynamic forces with data from experimental measurements.

“More formal analysis should be done to explain the correlation between the dual tip vortex loops and aerodynamic force oscillations during a long travel distance,” Li said. “Understanding these fundamentals could help scientists design better micro-air vehicles, such as robotic insects.”

Source: “Tip vortices formation and evolution of rotating wings at low Reynolds numbers,” by Chengyu Li, Haibo Dong, and Bo Cheng, Physics of Fluids (2020). The article can be accessed at https://doi.org/10.1063/1.5134689 .