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Car aerodynamics and dimples

  1. Aug 1, 2010 #1
    Last semester, I learned that the dimples on golf balls and the fuzz on tennis balls make the balls go faster by inducing turbulent flow at a lower speed than it otherwise would. Turbulent flow reduces the drag on the object.

    When automobile engineers design cars, they seem to make them as smooth as possible. In many car commercials, they show the car in a wind tunnel with perfectly laminar flow passing by.

    If turbulent flow decreases drag, why don't car manufacturers design their cars to promote turbulent flow at lower speeds? Maybe a rough patch at the front.

    I'm sure my sophomore level of understanding is the problem, not the engineers who do this for a living. I assume a more laminar flow is better for cars, since that is what the engineers seem to strive for.

    But what makes a car different from a tennis ball aerodynamically? Why is a turbulent flow better for a ball and a laminar flow better for a car?
  2. jcsd
  3. Aug 1, 2010 #2
    A car with dimples would be more expensive and ugly.

    Aerodynamically, perhaps the fact that a ball spins, unlike a car, is relevant. But someone would have to verify this.
  4. Aug 1, 2010 #3
  5. Aug 1, 2010 #4
    Cars with massive hailstorm damage are dimpled all over, and are often totaled by insurance companies, and are available at very large discounts.

    Bob S
    Last edited by a moderator: Apr 25, 2017
  6. Aug 2, 2010 #5
    In general turbulence actually increases drag because of the increase in skin friction. Turbulence is useful on a golf ball because the golf ball is a bluff (not streamlined) body. When a sphere travels through a fluid the boundary layer is unable to remain attached to the surface because of the adverse pressure gradient. This means that the flow separates and a large region of low pressure forms behind the sphere. Low pressure in back creates a net force that opposes motion. Laminar flow separates more easily than turbulent flow. Turbulent flow promotes mixing between the boundary layer and the free stream so that the boundary layer has more energy and can travel further along the surface of the sphere before it separates. That means the wake is not as large so the net force due to the low pressure region is smaller.

    On a streamlined body such as an airplane at moderate angles of attack separation is not likely to occur because the adverse pressure gradient is not a large. So laminar flow is more desirable because it reduces skin friction. I suppose the same could be said for a car although it is not as streamlined as an airplane. And as was already mentioned a dimpled car would be expensive and ugly and that would certainly play a role in the design.
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