Scientists from Hiroshima University undertook a study of dragonfly wings with the intention to higher understand the connection between a corrugated wing structure and vortex motions. They found that corrugated wings exhibit larger lift than flat wings.
Their work was published within the journal Physical Review Fluids on December 7, 2023.
The researchers set out to find out if the corrugation of a dragonfly’s wing is a secret ingredient for enhancing lift. While past research has largely zoomed in on the regular flow across the wing during forward motion, the impact of vortices spawned by its corrugated structure on lift has remained a mystery.
The wing surfaces of insects like dragonflies, cicadas, and bees, are usually not flat just like the wings on a passenger plane. The insect wings are composed of nerves and membranes, and their cross-section shapes consist of vertices (nerves) and line segments (membranes). The geometry of the form appears as a connection of objects with a V-shape or other shapes.
Earlier studies have shown that corrugated wings, with their ridges and grooves, have a greater aerodynamic performance than smooth wings at low Reynolds numbers. In aerodynamics, the Reynolds number is a quantity that helps predict the flow pattern of fluids. The sooner aerodynamic studies on corrugated wings have contributed to applications in small flying robots, drones, and windmills. Because insects possess low muscular strength, indirectly their corrugated wings must give them aerodynamic benefits. Yet scientists haven’t fully understood the mechanism at work due to the complex wing structure and flow characteristics.
The researchers used direct numerical calculations to investigate the flow around a two-dimensional corrugated wing and compared the corrugated wing performance to that of a flat wing. They focused their study on the period between the initial generation of the leading-edge vortex and subsequent interactions before detachment. They found that the corrugated wing performance was higher when the angle of attack, that angle at which the wind meets the wing, was greater than 30°.
The corrugated wing’s uneven structure generates an unsteady lift due to complex flow structures and vortex motions. “We have discovered a boosting lift mechanism powered by a novel airflow dance set off by a definite corrugated structure. It may possibly be a game-changer from the straightforward plate wing scenario!” said Yusuke Fujita, a PhD student on the Graduate School of Integrated Sciences for Life, Hiroshima University.
The researchers constructed a two-dimensional model of a corrugated wing using a real-life dragonfly wing. The model consisted of deeper corrugated structures on the leading-edge side and fewer deep, or flatter, structures on the trailing-edge side. Using their two-dimensional model, they further simplified the wing motion and focused on unsteady lift generation by translating from rest. Translational motion, or sliding motion, is a principal component of wing motion, along with pitching and rotation. The researchers’ evaluation expands the understanding of the nonstationary mechanisms that dragonflies use during flight.
The research team considered two-dimensional models of their study. Nonetheless, their work focused on the aerodynamics of insect flight, where the flow is often three-dimensional. “If these results are expanded to a three-dimensional system, we expect to realize more practical knowledge for understanding insect flight and its application within the industry,” said Makoto Iima, a professor on the Graduate School of Integrated Sciences for Life, Hiroshima University.
Looking ahead, the researchers will focus their investigations on three-dimensional models. “We kicked things off with a two-dimensional corrugated wing model in a sudden burst of motion. Now, we embark on the search to explore the lift-boosting across a broader range of wing shapes and motions. Our ultimate goal is crafting a brand new bio-inspired wing with high performance by our lift-enhancing mechanism,” said Fujita.
The research team includes Yusuke Fujita, a PhD student, and Makoto Iima, a professor, each from the Graduate School of Integrated Sciences for Life, Hiroshima University. Their research is funded by the Japan Society for the Promotion of Science KAKENHI and the SECOM Science and Research Foundation.
