Dimples on a golf ball
How deliberately roughening a surface reduces drag rather than adding it.
A golf ball is covered in dimples because a rough surface, oddly enough, lets air flow round it more cleanly than a smooth one would, delaying the point where the airflow breaks away from the ball and leaving a smaller, less draggy wake behind it than a perfectly smooth ball of the same size produces.
Van Dyke's An Album of Fluid Motion is almost entirely photographs of moving fluid with barely a page of text, and one image in particular, a smooth sphere and a rough one photographed moving through the same tank of water at the same speed, made the whole dimpled-ball idea obvious in a way no written explanation had managed before.
How roughness keeps the air clinging on
Air flowing around a ball forms a thin boundary layer clinging to the surface. Past the widest point, where the surface curves away, that layer has to push against rising pressure. On a smooth ball the layer is moving smoothly, and smooth air carries little energy close to the surface, so it runs out of momentum early and peels away well before reaching the back. What it leaves behind is a wide, low-pressure, churning wake, and that wake accounts for most of the ball's drag.
Dimples stir the same thin layer from smooth into turbulent motion near the front of the ball. Turbulent flow mixes fast air from further out into the slow layer against the surface, giving it the extra energy to keep going against the rising pressure, so it clings on further round the back before letting go. The wake narrows, the pressure difference between front and back shrinks, and the rougher ball feels less drag.
Two boulders in a stream
A smooth, rounded boulder in a fast stream leaves a wide pool of slack, swirling water behind it, where the current peeled off its curve early. A craggy boulder of about the same size in the same current often leaves a narrower disturbed patch, because its rough face churns the passing water into small, energetic eddies that hug the rock's contours for longer before breaking away. It looks backwards that the rougher, more chaotic-looking rock should leave the calmer wake, yet it is the same pattern at work in a golf ball's dimples.
Speed, size, and how far the ball flies
A dimpled ball struck hard can travel roughly twice as far as an otherwise identical smooth one, almost entirely because of the smaller wake it drags along. The earliest golf balls were smooth and picked up their texture by accident, from nicks and scars left by play, and players found the battered balls flew further before anyone could say why.
The benefit depends on the ball moving fast enough for the dimples to trip the boundary layer, so a gently struck ball travelling slowly gains little over a smooth one. Too much roughness, or dimples the wrong size for the ball's speed, fails in the other direction, adding more drag from the surface itself than the narrower wake recovers. That is why dimple patterns are sized for the speeds a struck ball actually reaches.
The same trick, deliberately tripping a boundary layer so it stays attached longer, appears elsewhere once it is recognised: small vortex-generating bumps and ridges on aircraft wings and some vehicle bodies, placed at exactly the point where the airflow would otherwise peel away too soon.