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How can turbulence in a boundary layer reduce drag on a sphere or cylinder?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In fluid mechanics, turbulence in a boundary layer can reduce the drag force on objects like spheres and cylinders by delaying the point of flow separation.1

When fluid flows past a bluff body, the boundary layer typically separates from the surface due to an adverse pressure gradient, creating a large wake of low-pressure eddies behind the object that contributes significantly to drag. However, when the boundary layer becomes turbulent, it gains more momentum from the surrounding flow. This increased energy allows the boundary layer to remain attached to the object's surface for a longer distance, shifting the separation point further toward the rear of the object.231

By delaying separation, the size of the wake is reduced, which leads to a dramatic drop in the drag coefficient. This phenomenon is the principle behind the design of golf balls, where surface dimples are used to intentionally induce turbulence in the boundary layer, allowing the ball to travel farther by minimizing drag.1