A Fundamental Principle of Aeronautical Engineering Has Been Overturned
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. 空气动力阻力是高速飞机、汽车和高铁面临的主要“障碍”。这是因为阻力越小的设计，意味着飞行器能以更少的能量实现更高的速度。
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. 当飞机或车体高速移动时，其表面会形成一层薄薄的空气，称为“边界层”。该边界层有两种状态：空气有序流动的“层流”，以及伴随紊流的“湍流”。
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. 空气保持低摩擦的层流状态时间越长，空气阻力就越小；但随着空气速度增加，它会转变为湍流。减少空气动力阻力的关键在于如何延迟这种向湍流的转变。
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. 80多年来，“物体表面必须光滑”这一原则一直是全球航空工程的基本前提，旨在抑制湍流转变并降低空气动力阻力。这一前提基于日本空气动力学家谷一郎（Ichiro Tani）1940年的研究成果。他定量证明了“表面粗糙度”（加工表面状态的指标）与湍流转变之间的关系，并指出当时制造技术无法避免的表面粗糙度阻碍了层流的实现。
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. 然而，1989年，谷一郎重新解读了流体工程师约翰·尼库拉塞（Johann Nikuradse）在20世纪30年代获得的粗糙表面管道实验数据，提出了一个新观点：“粗糙度并不一定只会促进湍流转变并增加流体阻力。”继承这一思想，东北大学小滨泰昭（Yasuaki Kohama）领导的研究小组在20世纪90年代通过实验证明，表面具有细微纤维状不规则性的纤维粗糙表面，在特定条件下具有延迟转变的效果。
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. 最近，同一支东北大学研究团队宣布了一项重大发现，进一步推动了这一趋势。东北大学流体科学研究所副教授八木野爱子（Aiko Yakino）及其研究小组在世界上首次证明，仅通过应用“分布式微粗糙度”（DMR）——一种细微且不规则到肉眼无法分辨的表面粗糙度——就能将空气动力阻力降低高达43.6%。
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. 这项技术与作为典型减阻技术的“沟槽（鲨鱼皮）工艺”有着本质区别。沟槽工艺模仿鲨鱼皮细微的纵向沟槽，通过沿气流方向雕刻约0.1毫米宽的沟槽，使湍流区域壁面附近产生的涡流排列整齐。而DMR则是通过随机且细微的不规则结构来延迟从层流到湍流的转换。它所影响的流动区域和采用的机制基于完全不同的概念。
Precise Measurement in a Wind Tunnel Without Support Bars
无支撑杆的风洞精密测量
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. 这一成就的关键因素在于采用了与以往不同的风洞实验方法。传统的风洞实验存在结构限制：支撑模型所必需的支撑杆和钢丝会扰乱气流，从而抵消了微尺度粗糙度引起的微小空气阻力变化。
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. 东北大学流体科学研究所拥有的全球最大的1米磁悬浮平衡系统（1m-MSBS）从根本上解决了这个问题。该装置利用电磁力，可以在风洞内非接触地悬浮一个长度约1.07米的流线型模型。由于不使用任何支撑杆或其他手段，它完全消除了对模型周围气流的干扰。
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). 八木野及其团队在广泛的雷诺数（流体惯性力与粘性力之比，Re = 0.35 x 10⁶ 至 3.6 x 10⁶）范围内，精确测量了光滑表面和涂有DMR表面上的总阻力系数。
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. 本次实验使用了两种类型的DMR：一种是由直径在38至53微米（μm）之间的玻璃珠组成的凸起图案，另一种是通过喷砂工艺形成的凹陷图案。DMR涂层的高度仅为边界层厚度的1%，从流体动力学角度来看，它仍被归类为“光滑表面”。
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). 实验结果显示，对于涂有DMR的模型，湍流转变开始的临界雷诺数从约1.9 × 10⁶增加到了2.2 × 10⁶，且在转变区域内，阻力显著降低了高达43.6%。此外，在最高测量雷诺数（3.6 x 10⁶）下，DMR表面的阻力系数始终低于光滑表面。
A Mechanism to Suppress Friction Itself
抑制摩擦本身的机制
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. 空气阻力大致可分为两类：“压差阻力”和“摩擦阻力”。压差阻力是由“分离”引起的阻力，即气流在物体后方从表面分离；而摩擦阻力是由流经表面的空气粘度引起的，当流动保持层流状态时，摩擦阻力会降低。
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. 为了明确DMR效应是由哪种阻力引起的，研究团队使用了“大涡模拟”（LES），这是一种数值流体力学计算方法，直接计算大尺度湍流涡，并用模型近似小尺度涡。该实验的LES分辨率高达4538万个壁面网格，并使用荧光漆等材料在模型表面观察气流。综合分析结合了“油流可视化”技术，即在模型表面涂上荧光漆，以直观地检查气流情况。
According to the researchers, the LES analysis established a conservative upper limit of pressure r… 据研究人员称，LES分析确立了压差阻力的保守上限……