From what I know, dimpling a car to reduce drag does not work and the Mythbusters did not have a well set up test. They are a TV show, not a science lab. Way too many variables.
Fun fact: A golf ball with dimples will fly about twice as far as one without.
We don't see dimples on cars is that the effect the dimples have only works well on a blunt body like a sphere or a cylinder. The biggest drag on a golf ball is due to pressure. In this case, the dimples are beneficial because they reduce the wake.
Shapes like wings and super efficient cars get more of their drag from skin friction drag.
Most streamlined bodies (bottom) have a teardrop shape that creates a much more gradual pressure gradient. This less severe gradient promotes attached laminar flow much further along the body that eliminates flow separation, or at least delays it as long as possible.
The ratio for length vs. height for laminar flow to stay connected on a downward slope is 6:1, meaning that for every 1 inch you want to drop, you need the shape to extend 6 inches rearward. Although this is nearly impossible on most cars, we get close on the super efficient models. The resulting wake is therefore very small and generates very little pressure drag.
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In the Mythbuster's clip they visited the NASA Ames Fluid Mechanics Lab (FML). I spent some time there during which we followed up the Mythbuster's test with our own. Rabindra Mehta, the head of the branch felt that their was inaccurate.
We were interested in the possibility of dimpling the surface of a passenger car. The (few) die cast models tested included a 350Z and (for fun) a 2002 VW Beetle (because it's shaped like a golf ball
). We covered the models with a thin layer of clay, tested in a 15x15 Wind Tunnel, and found the drag coefficients of each of them. We then applied a few versions of dimples in different sizes, depths and placements on the car's body and tested some more.
The results showed that on average, strategically placed dimples increased the drag coefficient on the model by about 8%. Dimpling the entire car (except windscreens and wheels) increased the drag coefficient by about 15%.
This was a casual test. I would not be blown away if it were to be refuted. We did test to fairly high speeds, about 250fps in the test section.
This test also has limited relevance to your car because we only tested Corvettes, 350Z's, etc.
In conclusion: I recommend keeping it as smooth as possible, but my tests were not the most accurate and I am not an aero engineer by trade.
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Vortex generators work differently than the dimples. On a car, they must be placed precisely and have an extremely small effect.
Vortex generators can be placed on the outside edge of a wing in order to promote a turbulent boundary layer that adds forward momentum to the flow. This helps the flow overcome the pressure gradient and remain attached to the surface longer than it would otherwise, just like they do on a golf ball.
The purpose of putting this on a plane wing is shown in the diagram, it allows the wing to reach a higher angle of attack before stalling.
(ex-Lada owner)
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