One of the fundamental principles of aerospace engineering has been overturned

Aerodynamic drag is A major “barrier” at high speed planes, Cars and fast trains. This is because a design with less aerodynamic drag allows the aircraft to move at higher speeds with less energy.

When the body of an airplane or car moves at high speed, a thin layer of air called the “boundary layer” forms on its surface. This boundary layer has two states: laminar flow, in which air flows in an orderly manner, and turbulent flow, which involves turbulence.

The longer the air remains in a state of laminar flow with low friction, the smaller the air resistance becomes, but as the air speed increases, it switches to turbulent flow. The key to reducing aerodynamic drag is how to delay this transition to turbulence.

For more than 80 years, the principle that “the surface of an object must be smooth” has been the basic premise of aeronautical engineering worldwide in order to suppress the transition to turbulence and reduce aerodynamic drag. This hypothesis was based on the results of a 1940 study by Ichiro Tani, a Japanese aerodynamicist who demonstrated quantitatively the relationship between “surface roughness” (an indicator of the condition of a machined surface) and turbulent transmission, arguing that surface roughness, which was unavoidable with the manufacturing technology of the time, prevented the achievement of laminar flow.

However, in 1989, Tani reinterpreted experimental data on tubes with rough surfaces obtained by fluid engineer Johann Nicolasi in the 1930s, offering a new perspective that “roughness may not necessarily lead to enhanced turbulent transformation and only increased fluid resistance.” This idea was inherited by a research group led by Yasuaki Kohama of Tohoku University, which showed experimentally in the 1990s that rough fibrous surfaces, which have fine fibrous anomalies on their surface, have the effect of delaying transformation under certain conditions.

The same research team at Tohoku University recently announced a discovery that greatly reinforces this trend. Aiko Yakino, an associate professor at the Institute of Fluid Science at Tohoku University, and her research group were the first in the world to do this. It is clear 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 discerned with the naked eye.

This technology is fundamentally different from the “river (shark skin) process”, which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving approximately 0.1 mm wide grooves along the direction of airflow, it aligns the vortices that occur near the wall surface of areas of turbulent airflow. On the other hand, DMR delays the transition from laminar to turbulent flow by random and fine irregularities. The flow zones they affect and the mechanisms they use are based on completely different concepts.

Accurate measurement in the wind tunnel without support bars

The main factor in this achievement was the use of a different method of wind tunnel experiment than before. Traditional wind tunnel experiments had structural limitations: the support rods and core wires to support the model disrupted airflow, canceling out subtle changes in air resistance caused by small-scale roughness.

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

Yakino and his team precisely measured the total drag coefficient on smooth, DMR-coated surfaces over a wide range of Reynolds numbers (the ratio of inertial to viscous forces acting on the fluid) (Re = 0.35 x 10⁶ to 3.6 x 10⁶).