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 involves turbulence.

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 how to delay this transition to turbulence.

For more than 80 years, the principle of “the surface of an object must be smooth” has been the basic premise of aeronautical engineering throughout the world in order to suppress the transition to turbulence and reduce aerodynamic drag. This premise was based on the results of a 1940 study by Ichiro Tani, a Japanese aerodynamicist who quantitatively 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-surface pipes obtained by fluid engineer Johann Nikulase in the 1930s, bringing a new perspective that “roughness may not necessarily only promote turbulent transition and increase fluid resistance.” Inheriting this idea, a research group led by Yasuaki Kohama of Tohoku University experimentally 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 trend. 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 known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm 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 different wind tunnel experiment method than before. 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 m 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 his team precisely measured the total drag coefficient on smooth and DMR-coated surfaces over a wide range of Reynolds numbers (ratio of inertial to viscous forces acting on the fluid) (Re = 0.35 x 10⁶ to 3.6 x 10⁶).

Two types of DMRs were used in this experiment: A convex pattern made of glass beads with diameters ranging from 38 to 53 micrometers (μm) and a concave pattern applied by sandblasting. The height of the DMR coating is only 1 percent of the thickness of the boundary layer and is classified as a “smooth surface” from a hydrodynamic point of view.

Experimental results showed that the critical Reynolds number at which the turbulent transition begins increased from approximately 1.9 × 10⁶ to 2.2 × 10⁶ for the DMR-coated model, and drag was dramatically reduced by up to 43.6 percent in the transition zone. Furthermore, the DMR-applied surface consistently showed a drag coefficient lower than that of the smooth surface up to the highest measured Reynolds number (3.6 x 10⁶).

A Mechanism to Suppress Friction Itself

Air resistance can be broadly divided into two types: "pressure resistance" and "frictional resistance." Pressure resistance is the resistance caused by "separation," where the airflow separates from the surface behind an object. On the other hand, frictional resistance is the resistance caused by the viscosity of the air flowing over the surface, and it decreases as the flow maintains a laminar state.

In order to clarify which of the two is responsible for the DMR effect, the research team used “large eddy simulation," a computational method for numerical fluid dynamics in which large scale turbulent eddies are calculated directly and small scale eddies are approximated by a model. This experiment had an LES with a resolution of up to 45.38 million wall cells, and also used fluorescent paint and other materials on the model's surface to see how air flows. The integrated analysis combined “oil flow visualization,” in which the surface of a model is painted with fluorescent paint to visually check the air flow.

According to the researchers, the LES analysis established a conservative upper limit of pressure resistance (Cp≈0.00021) that agrees with theoretical values within 1 percent from laminar flow calculations that do not intentionally introduce artificial disturbances. However, the amount of drag reduction observed in this study (ΔCD≈0.001) is approximately five times this upper limit.

Even if the separation at the rear of the object were completely eliminated, only about 20 percent of the observed reduction can be explained. In other words, the numerical analysis quantitatively confirmed that the main factor in the reduction of aerodynamic drag by DMR is not the suppression of delamination but the reduction of frictional drag itself.

This principle is fundamentally different from the effect of dimples on golf balls. Dimples reduce pressure resistance by intentionally turbulizing the airflow and suppressing backward separation. DMR, on the other hand, delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.

Advantages Over ‘Shark Skin’ Processing

The strength of DMR's aerodynamic drag reduction lies in its extremely high passivity and omni-directional nature. For the rivet process to be effective, grooves must be precisely cut along the direction of airflow. In contrast, DMR has a great advantage in that the surface roughness is random and does not depend on the direction of the flow.

In addition, since it requires neither moving parts nor electricity, a high drag reduction effect can be achieved at a low cost. If DMR is applied to aircraft, it is expected to significantly reduce operating costs and carbon dioxide emissions by improving fuel efficiency.

The research team plans to further optimize the shape and distribution density of the DMR and to expand the applicable speed range in the future.

This story originally appeared on WIRED Japan and has been translated from Japanese.