A Fundamental Principle of Aeronautical Engineering Has Been Overturned | WIREDSkip to main contentMenuSECURITYPOLITICSTHE BIG STORYBUSINESSSCIENCECULTUREREVIEWSMenuAccountAccountNewslettersSecurityPoliticsThe Big StoryBusinessScienceCultureReviewsChevronMoreExpandThe Big InterviewMagazineEventsWIRED InsiderWIRED ConsultingNewslettersPodcastsVideoLivestreamsMerchSearchSearchRitsuko KawaiScienceMay 24, 2026 4:30 AMA Fundamental Principle of Aeronautical Engineering Has Been OverturnedIt's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case.Illustration: ktsimage/Getty ImagesCommentLoaderSave StorySave this storyCommentLoaderSave StorySave this storyAerodynamic 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 BarsA 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 ItselfAir 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’ ProcessingThe 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.CommentsBack to topTriangleYou Might Also LikeHow to find us: Add WIRED.com to your preferred sources in GoogleThese women are trying to optimize their vaginasBig Story: AI gig work is the new waiting tables—and it's soul-crushingThis summer, the American water crisis becomes realEvent: How to adapt, compete, and win in the next era of businessTopicsAviationphysicsengineeringaerodynamicsfrictionRead MoreMexico City Is Sinking. 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A fundamental principle of aeronautical engineering, which held that a smoother surface invariably resulted in lower aerodynamic drag, has been challenged. This principle was based on the premise that a smooth surface suppresses the transition from laminar flow to turbulent flow, thereby minimizing air resistance. Traditionally, this belief was rooted in research by Ichiro Tani in 1940, who quantified the relationship between surface roughness and turbulent transition, arguing that manufacturing limitations prevented the realization of ideal laminar flow. However, Tani reinterpreted earlier experimental data in 1989, suggesting that surface roughness may not solely dictate turbulent transition and resistance. This idea was expanded upon by research led by Yasuaki Kohama, who experimentally demonstrated in the 1990s that fibrous rough surfaces could delay this transition under specific conditions.

A significant advancement was made by Aiko Yakino and her research group at Tohoku University, who discovered that applying distributed micro-roughness (DMR)—surface irregularities too fine to be seen by the naked eye—can reduce aerodynamic drag by up to forty-three point six percent. This method differs fundamentally from established techniques like the rivulet process, which simulates shark skin by carving grooves to align vortices, and DMR, which achieves drag reduction by introducing random and minute irregularities to delay the laminar-to-turbulent transition.

Achieving this discovery required overcoming structural limitations inherent in conventional wind tunnel testing, where support rods interfered with minute air resistance measurements caused by micro-scale roughness. This challenge was resolved by the development of the Institute of Fluid Science, Tohoku University's one meter magnetic support balance system, which allows streamlined models to levitate within the wind tunnel without any physical contact, thereby eliminating airflow interference.

Yakino and her team performed precise drag measurements on surfaces treated with different DMRs, including convex patterns of glass beads and concave patterns from sandblasting, across various Reynolds numbers. The results indicated that the critical Reynolds number at which turbulent transition began increased from approximately 1.9 times one million to 2.2 times one million for the DMR-coated models. Furthermore, the DMR-applied surfaces consistently exhibited a lower drag coefficient than smooth surfaces up to the highest measured Reynolds number.

To determine the exact mechanism behind the observed drag reduction, the researchers employed large eddy simulation, a computational fluid dynamics method, combined with fluorescent paint visualization. The analysis revealed that the major factor in the reduction of aerodynamic drag by DMR was not the suppression of flow separation (pressure resistance), but rather the reduction of frictional drag itself. This finding contrasts with the effect of dimples on golf balls, which primarily reduce pressure resistance by inducing turbulence. The DMR mechanism operates by delaying the transition, thus suppressing wall friction.

In terms of practical advantages, DMR offers high passivity and omni-directional properties, meaning the surface roughness is independent of the flow direction, unlike the rivulet process which requires grooves aligned with the airflow. Moreover, DMR can be applied economically without requiring moving parts or external electricity, offering the potential for substantial reductions in operating costs and carbon dioxide emissions in aircraft by improving fuel efficiency. The research team is currently focused on further optimizing the shape and density of the DMR to expand its applicability across a wider range of speeds.