Submission + - A Fundamental Principle of Aeronautical Engineering Has Been Overturned (wired.com)
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It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case.
For more than 80 years, the principle of "the surface of an object must be smooth" has been the basic premise of aeronautical engineering worldwide 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" and turbulent transition, arguing that surface roughness prevented laminar flow from being realized.
At Tohoku University, a research team recently announced a discovery that significantly advances this trend. Aiko Yakino, an associate professor at Tohoku University, 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 the naked eye cannot distinguish it.
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.
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. Distributed micro-roughness delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.
The strength of DMR's aerodynamic drag reduction lies in its extremely high passivity and omnidirectional 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.
It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case.
For more than 80 years, the principle of "the surface of an object must be smooth" has been the basic premise of aeronautical engineering worldwide 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" and turbulent transition, arguing that surface roughness prevented laminar flow from being realized.
At Tohoku University, a research team recently announced a discovery that significantly advances this trend. Aiko Yakino, an associate professor at Tohoku University, 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 the naked eye cannot distinguish it.
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.
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. Distributed micro-roughness delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.
The strength of DMR's aerodynamic drag reduction lies in its extremely high passivity and omnidirectional 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.
