A surface covered with small cavities reduced flow resistance by 13.2% in simulations.
These cavities, called dimples, alter the way a fluid moves in the immediate vicinity of a wall. The idea recalls the surface of a golf ball. Here, however, the researchers are primarily seeking to reduce friction on a surface. An unsuitable shape can, moreover, produce the opposite effect and increase resistance.
Pixabay illustration image
To find an effective geometry, the team varied four characteristics: the shape, depth, size and aspect ratio of the dimples. The parameter space contained 1,575 possible combinations. Testing each one with a detailed simulation would have required a great deal of computing power. The researchers therefore used an optimization method that progressively selects the most promising configurations to test.
Each selected configuration was then evaluated through a numerical simulation of turbulent flow in a channel. The flow rate remained identical from one test to the next. The researchers could therefore compare the force required to maintain this flow with that required by a perfectly flat wall. This comparison provides the drag-reduction rate.
The best configuration consists of diamond-shaped cavities, packed closely together and elongated in the direction of the flow. Under the simulated conditions, it reduced total drag by 13.2% compared with the flat reference surface. Earlier studies had achieved considerably more modest results with other geometries.
To understand this result, we need to look at what happens just above the wall. With the best patterns, the fluid remains attached to the surface and follows paths close to grooves. Some other shapes, by contrast, cause small local flow separations. These areas then add pressure-related resistance and can cancel out the reduction in friction.
Depth alone therefore cannot predict whether a dimple will be beneficial. Shape appears to be the parameter with the strongest influence in the researchers’ analysis. The proportion of covered surface and the aspect ratio also play a role, but their effect depends on their interactions with the other geometric characteristics.
The result does not mean that an aircraft wing covered with these dimples tomorrow would immediately consume 13.2% less fuel. The study concerns controlled flow in a numerical channel, chosen to isolate near-wall phenomena. A real wing encounters different speeds, curvatures, pressure variations and external conditions.
The authors plan to extend their work to flows at other speed and scale regimes. Real-world use will also have to take into account the manufacturing process, durability and fouling of these surface features. These constraints were not examined in this initial research.