The addition of these ducts are useful when air needs to be drawn into an area which isn't exposed to the direct air flow the scoop has access to. Quite often you will see ducts along the sides of a car. These ducts take advantage of the Boundary layer, a layer of slow moving air that "clings" to the bodywork of the car, especially where the bodywork flattens, or does not accelerate or decelerate the air flow. Areas like the hood, roof, and side body panels are good examples. The longer the body panel, the thicker the layer becomes (a source of drag that grows as the layer thickens too).
These ducts scavenge this slower moving area by means of a specially shaped intake. The intake shape drops in toward the inside of the bodywork, and this draws the slow moving air into the opening at the end of the duct. Vorticies are also generated by the "walls" of the duct shape, aiding in the scavenging. The shape and depth change of the duct increase air flow to your intake manifold to cool it down, while drawing the warm air away from your air intake and other critical engine components that usually cause "heat soak" when warm air builds-up in those areas.
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