Drag is caused by two forces: pressure differential and skin friction. Most bluff bodies are dominated by pressure drag due to flow separation, e.g. regular cars. Flow separation is disadvantageous because it creates a low pressure region in the back which doesn’t counteract the high pressure region at the front. Turbulence delays flow separation, thereby reducing drag. Note, the transition from laminar to turbulent flow creates a sudden drop in drag known as the “drag crisis” which golf balls take advantage of via dimples to travel farther. However, turbulent flow also causes greater skin friction, so it can be a balancing act. Some surfaces, such as airplane wings, use “trippers” to force turbulence at a certain point downwind to get the best of both worlds, i.e. laminar flow at the front to minimize skin friction and turbulent flow at the tail to delay separation.

Rough surfaces can be more aerodynamic than smooth ones

“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.”

Didn’t they already discover something very similar when they started putting dimples on golf balls?