New mechanism reveals the aerodynamics of flapping insect wings

When insects flap their wings, they can stay in the air thanks to the leading-edge vortex, which creates a tornado-like force that augments the lift experienced by the insect. A new report may help explain why the leading-edge vortex remains stably attached to the top of a flapping wing, rather than moving or growing into an instability.

Werner et al. propose that radial planetary vorticity tilting – unique to rotational fluid systems – may help stabilize the leading-edge vortex. Using computational fluid dynamics, the authors show that this vorticity tilting acts in the direction opposite to the leading-edge vortex, consistently reducing the magnitude of its vorticity for different Reynolds numbers and wing aspect ratios.

Previous reports attributed the vortex’s stability to either the vorticity transport from the wing’s spanwise flow, or the Coriolis acceleration, which results from the rotation of the wing as the insect flaps back and forth. Neither mechanism, however, could completely account for the observed vortex stability.

Werner et al. instead suggest that the spanwise flow and Coriolis force both play a role in keeping the vortex from overgrowing. Because the spanwise flow is not uniform, it induces a velocity gradient in the Coriolis force that causes air in the system to rotate. This torque – or radial planetary vorticity tilting – counteracts the vorticity of the leading-edge vortex.

Despite the paper’s promising results, more formal analysis should be done to explain the exact mechanism of the leading-edge vortex and the specific role of planetary vorticity tilting in that stabilization, co-author Bo Cheng said. Understanding these fundamentals could help scientists design better micro air vehicles such as robotic insects.

Source: “Radial planetary vorticity tilting in the leading-edge vortex of revolving wings,” by Nathaniel H. Werner, Hojae Chung, Junshi Wang, Geng Liu, John M. Cimbala, Haibo Dong, and Bo Cheng, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5084967 .