A Fundamental Principle of Aeronautical Engineering Solved

Aerodynamic drag is A big “barrier” at high speeds plane, cars, and trains. This is because the low-gravity design allows airplanes to fly at high speeds with little downforce.

When an airplane or car body is moving at high speed, a thin layer of air called the “boundary layer” is formed above it. This boundary layer has two components: laminar flow, in which air flows in an orderly manner, and turbulent flow, which involves turbulence.

When the air spends a long time in laminar flow with low friction, the air resistance is low, but when the air pressure increases, it changes to a turbulent flow. The key to reducing aerodynamic drag is how to slow the transition to turbulence.

For more than 80 years, the principle that “the surface of an object must be smooth” has been the foundation of aircraft engineering around the world in order to prevent turbulence and reduce drag. This idea came from the results of a 1940 study by Ichiro Tani, a Japanese physicist who showed a connection between the “surface hardness” (an indicator of the working environment) and the change in turbulence, arguing that the surface hardness, which was inevitable with the production technology of the time, prevented laminar flow.

However, in 1989 Tani reinterpreted experimental data on underground pipelines conducted by fluid engineer Johann Nikulase in the 1930s, bringing the new idea that “friction can not only promote turbulence and increase fluid resistance.” Based on this idea, a research group led by Yasuaki Kohama of Tohoku University showed experimentally in the 1990s that a transparent surface, which has a fibrous surface on it, has the effect of slowing down some changes.

The same research team at Tohoku University recently announced findings that advance this trend. Aiko Yakino, assistant professor at Tohoku University Institute of Fluid Science, and her research team were the first in the world to develop show that aerodynamic drag can be reduced by up to 43.6 percent simply by using distributed micro-roughness (DMR), a surface roughness so subtle and unchanging that it cannot be detected by the naked eye.

This technology is very different from the “rivulet (shark skin)” technique, which is known as aerodynamic reduction technology. The rivulet system mimics the fine longitudinal grooves in the shark’s skin, and by carving lines about 0.1 mm wide along the direction of air flow, it connects the vortices found near the wall of the air flow areas. DMR, on the other hand, slows the transition from laminar to turbulent flow using random and momentary disturbances. The processes they influence and the methods they use are based on different perspectives.

Accuracy Measurement in the Airway Without Support Bars

A key factor in this success was the use of a more wind-tested method than before. Standard wind tunnel experiments had design limitations: the rods and wires needed to support the structure interfered with air flow, ignoring minute changes in air resistance caused by partial stiffness.

The world’s largest 1 meter magnetic support system (1m-MSBS), at the Institute of Fluid Science, Tohoku University, has solved this problem. This device is capable of producing a vertical beam of approximately 1.07 m in length within the wind tunnel without contact with electrical power. Because it does not use support rods or other methods, it completely eliminates the disturbance of the air around the model.

Yakino and his team precisely measured the total gravity on a smooth surface covered with DMR at a range of Reynolds numbers (number of non-viscous forces acting on water) (Re = 0.35 x 10⁶ to 3.6 x 10⁶).