‘Zero-thickness’ designs of airfoils to minimise drag or maximise lift. Evolutionary algorithms have shown that designs which have the least drag have a strongly corrugated shape (top), while those which maximise lift have a large, smooth curvature with a dip at the trailing edge (bottom). Credit: Tokyo Metropolitan University.
Evolutionary search using exclusively computational ‘zero-thickness’ airfoils reveals new design strategies
Researchers from Tokyo Metropolitan University have used an evolutionary algorithm to identify optimal shapes for ultra-thin, bio-inspired corrugated airfoils, the two-dimensional cut-throughs of wings.
Lightweight membrane-like wings are a strong contender for use in unmanned aerial vehicles (UAVs) in thin atmospheres, like on Mars, and in UAV miniaturisation.
By using a ‘zero-thickness’ model, they were able to focus on the effect of the airfoil corrugation pattern without the influence of a wing’s leading-edge thickness and shape.
They discovered designs which were optimised for minimal drag or maximal lift.
Other planets
Unmanned aerial vehicles (UAVs) (or drones) are seeing increasing use in our daily lives, from package deliveries to live media coverage.
They also promise scientists the ability to explore exotic environments, like other planets.
This comes with significant challenges. For example, if we consider Mars, the atmosphere is significantly thinner.
This means that wing shapes optimised for high-speed movement on Earth are no longer the best: a ground-up redesign is required which takes into account the ‘low-Reynolds number’ nature of the fluid in which they move, where the effect of energy dissipation due to air viscosity dwarfs the influence of inertia.
The same fluid properties apply to wing design for very small aircraft on Earth.
This has led scientists to seek inspiration from the wings of insects, whose size and speed put them squarely in the low-Reynolds number regime.
A notable spotlight is on ultra-thin, membrane-like wings, which not only resemble those seen in nature, but are also lightweight and portable, both crucial factors for a UAV being carried to another planet.
In recent work, a team led by Professor Masahiro Kanazaki from Tokyo Metropolitan University has used computer algorithms to explore designs for a new UAV wing in low-Reynolds number atmospheres.
They focused on how bio-inspired corrugations on the airfoil can help reduce drag or increase uplift.
They used evolutionary optimisation, where an algorithm runs aerodynamic simulations on different designs, judges them based on multiple objectives, and continues to tweak them to get better results.
‘Zero-thickness’
To focus on airfoil corrugation, they adopted a ‘zero-thickness’ simulation: while impossible in the real world and only realisable in a computer, these simulations can help teach us the sole effect of wing shape, separate from how the thickness of the leading-edge might affect performance.
The team discovered important trends in designs optimised for low drag or high lift.
For low drag, there were designs which had less drag than a flat sheet; corrugations near the leading-edge led to rolls of air which reduced the frictional drag.
In contrast, airfoils optimised for lift had very few corrugations, an overall convex shape, and concave features near the trailing edge.
While insects can deform their membrane-like wings to adopt both corrugated and cambered designs, incorporating elements of both into a 3D design can optimise performance in a wide range of environments.
The team believes that their findings will guide UAV miniaturisation, as well as wing designs for thin atmospheres.
The study is published in Computers & Fluids.
