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This is an important constant as laminar vs turbulent flow comes with implications for drag. Recall that drag is a combination of skin drag and pressure drag primarily. An illustrative generalization would be that a bulk body has higher pressure drag where a streamlined body has higher skin drag. Despite this, a streamlined body will have superior drag characteristics to a bulk body as the skin drag will not be as large as pressure drag. This means for designing a body with the most important requirement being to have low drag, the first priority would be to prevent seperation by streamlining.

A streamlined body that maintains laminar flow for as long as possible is desirable , because it minimizes the pressure drag by delaying flow seperationbecause laminar flow results in less skin friction than turbulent flow. This is due to the proportionality of the normal velocity gradient to the drag force caused by viscous fluid effects. On the surface turbulent flow has a higher gradient than laminar flow which means more skin drag.

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titleDrag on a golf ball

The titular example of a design decision that violates these principles is the golf ball. You may have heard that golf balls are dimpled to improve their drag performance in the air. For a solar car; surface imperfections are undesirable, so why dimple a golf ball?

It has to do with the body shape. For a bluff body like the golf ball the fluid will tend to seperate from the surface creating a huge pressure drag. We still want to reduce drag, but we can’t streamline a golfball for obvious reasons. The priority for reducing drag is still to eliminate or at least delay flow seperation. How a golf ball does this is with dimples. The dimples create turbulent flow around the golfball, this still increases skin friction, but it delays flow seperation. This flow seperation delay more than makes up for the skin friction losses, and therefore improves the aerodynamics of a golf ball.

We don’t dimple solar cars because if designed correctly there should barely be flow seperation due to the streamlined shape. This means that creating turbulence adds a significant amount of skin drag with no real benefit.