A Fundamental Principle of Aeronautical Engineering Has Been Overturned

Aerodynamic drag is a major “barrier” in high-speed airplanes, automobiles, and bullet trains. This is because a design with less aerodynamic drag allows the aircraft to move at higher speeds with less energy.

When an aircraft or car body moves at high speed, a thin layer of air called the boundary layer is formed on its surface. This boundary layer has two states: laminar flow, in which air flows in an orderly fashion, and turbulent flow, which is chaotic.

The longer the air stays in the laminar-flow state with low friction, the smaller the air resistance becomes, but as the air speed increases, it transitions to turbulent flow. The key to reducing aerodynamic drag is delaying this transition to turbulence.

For more than 80 years, a basic principle of aeronautical engineering has been that the surface of an object must be smooth in order to reduce aerodynamic drag. This premise was based on the results of a 1940 study by Ichiro Tani, a Japanese scientist who demonstrated the relationship between surface roughness (an indicator of the state of the machined surface) and turbulent transition, arguing that surface roughness, which was unavoidable with the manufacturing technology of the time, prevented laminar flow from being realized.

However, in 1989 Tani reinterpreted the experimental data on rough-surfaced pipes obtained by fluid engineer Johann Nikulase in the 1930s, suggesting that “roughness may not necessarily only promote turbulent transition and increase fluid resistance.” (In physics, air is considered a fluid.) Inheriting this idea, a research group led by Yasuaki Kohama of Tohoku University demonstrated in the 1990s that fibrous rough surfaces, which have fine fibrous irregularities on their surface, have the effect of delaying transition under certain conditions.

The same Tohoku University research team recently announced a discovery that significantly advances this idea. Aiko Yakino, associate professor at Tohoku University’s Institute of Fluid Science, and her research group were the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye.

This technology is fundamentally different from the rivulet (“shark skin”) process, which is a known air-drag-reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 millimeter wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.

Precise Measurement in a Wind Tunnel Without Support Bars

A key factor in this achievement was the use of a new wind tunnel method. Conventional wind tunnel experiments had structural limitations: The support rods and wires essential for supporting the model disrupted the airflow, negating the minute changes in air resistance caused by micro-scale roughness.

The world’s largest 1-meter magnetic support balance system (1m-MSBS), owned by the Institute of Fluid Science, Tohoku University, has fundamentally solved this problem. This device can levitate a streamlined model approximately 1.07 meter in length inside a wind tunnel without contact using electromagnetic force. Because it does not use any support rods or other means, it completely eliminates interference with the airflow around the model.

Yakino and her team precisely measured the total drag coefficient on smooth and DMR-coated surfaces over a wide range of Reynolds numbers, from 0.35 x 10⁶ to 3.6 x 10⁶. (A Reynolds numbers is the ratio of inertial to viscous forces within a fluid; it’s a key predictor of whether fluid flow will be laminar or turbulent.