There is aerodynamic drag the main “barrier” to high-speed transport planes, cars and armored trains. This is because a design with less aerodynamic drag allows the aircraft to travel at higher speeds while using less energy.
When the body of an airplane or car moves at high speed, a slender layer of air called the “boundary layer” forms on its surface. This boundary layer exists in two states: laminar flow, in which air flows in an neat manner, and turbulent flow, in which turbulence occurs.
The longer the air remains in a state of laminar flow with low friction, the lower the air resistance becomes, but as the air speed increases it changes to turbulent flow. The key to reducing aerodynamic drag is to delay the transition to turbulence.
For over 80 years, the principle “the surface of an object must be smooth” has been a fundamental tenet of aerospace engineering around the world in order to stop the transition to turbulence and reduce aerodynamic drag. This assumption was based on the results of research conducted in 1940 by Ichiro Tani, a Japanese aerodynamicist, who quantitatively demonstrated the relationship between “surface roughness” (an indicator of the condition of a machined surface) and turbulent transition, arguing that surface roughness, which was inevitable with the production technology of the time, made it impossible to realize laminar flow.
However, in 1989, Tani reinterpreted experimental data on rough-surfaced pipes obtained by fluid engineer Johann Nikulase in the 1930s, bringing a recent perspective that “roughness does not necessarily merely promote turbulent transformations and increase fluid drag.” Inheriting this idea, a research group led by Yasuaki Kohama of Tohoku University demonstrated experimentally in the 1990s that fibrous, ragged surfaces that have petite fibrous irregularities on their surface retard the transition under certain conditions.
The same research team at Tohoku University recently announced a discovery that significantly accelerates this trend. Aiko Yakino, associate professor at the Institute of Fluid Science at Tohoku University, and his research group are the first in the world demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by using diffuse micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be recognized with the naked eye.
This technology is fundamentally different from the “stream (shark skin)” process known as typical aerodynamic drag reduction technology. The jetting process mimics the fine longitudinal grooves in a shark’s skin and, by carving approximately 0.1 mm wide grooves along the direction of airflow, evens out vortices that occur near the wall surfaces of areas of turbulent airflow. On the other hand, DMR delays the transition from laminar to turbulent flow by random and diminutive irregularities. The flow zones it affects and the mechanisms it uses are based on completely different concepts.
Precise measurement in a wind tunnel without support bars
The key factor in this achievement was the employ of a different experimental method in the wind tunnel than before. Conventional wind tunnel experiments had structural limitations: the support bars and wires necessary to support the model disrupted the airflow, canceling out petite changes in air resistance caused by microscale roughness.
The world’s largest 1-meter magnetic support balancing system (1m-MSBS), owned by Tohoku University’s Institute of Fluid Science, has fundamentally solved this problem. This device can levitate a streamlined model approximately 1.07 m long in a wind tunnel without contact using electromagnetic force. Because it doesn’t employ any support bars or other means, it completely eliminates disruptions to airflow around the model.
Yakino and his team precisely measured the total drag coefficient on sleek and DMR-coated surfaces over a wide range of Reynolds numbers (the ratio of inertial to viscous forces acting on a fluid) (Re = 0.35 x 10⁶ to 3.6 x 10⁶).