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I'm trying to understand why the Mythbuster's dimple car gets better fuel economy - if it actually would as I'm not

I understand that a golf ball functions by creating turbulent air flow and via the disruption of the boundary layer it reduces drag and increases lift.

The only thing I can think of is maybe there's an area of low pressure directly behind the car and by adding the dimples it is creating turbulent flow that maybe prevents this? That seems like a stretch though.

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    $\begingroup$ Perhaps to save other people's time you could summarise. So what were the mpgs for the two configs? Did they ballast the original car to the same axle weights? $\endgroup$
    – Greg Locock
    May 17 at 0:02
  • $\begingroup$ Better fuel economy compared to what? A Unimog or Jeep Cherokee? no surprise there... A Mercedes diesel sedan *60 mpg easily... $\endgroup$
    – Solar Mike
    May 17 at 5:34
  • $\begingroup$ @SolarMike: As I recall, the comparison was to an otherwise identical car, but without the dimples. $\endgroup$
    – Jerry Coffin
    May 17 at 15:12
  • $\begingroup$ @JerryCoffin then the dtail needs to be in the question - link-rot is a thing. $\endgroup$
    – Solar Mike
    May 17 at 15:30
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    $\begingroup$ @SolarMike: if that was not what they did, it would deserve mention. But for a comparison, it seems to me that the obvious expectation would be "all else being equal". $\endgroup$
    – Jerry Coffin
    May 17 at 15:36

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The only thing I can think of is maybe there's an area of low pressure directly behind the car and by adding the dimples it is creating turbulent flow that maybe prevents this? That seems like a stretch though.

I don't think that's a stretch at all. In fact, it's extremely common for an airfoil to include a "trip strip" and/or turbulator spars for exactly that reason. And yes, they might easily have gotten similar (possibly superior) results using a normal trip strip instead of dimples.

Oh, one other point: the turbulent air flow probably doesn't prevent flow separation and the area of low pressure behind the car--it's much more likely to merely delay flow separation, so you end up with a smaller low pressure area.

It's probably difficult to translate this to other cars though. Even on an airfoil, it's difficult to predict the precise size/location for a good trip strip. Most cars already have enough seams and such that you'd expect to have at least some thin turbulent layer over much of the surface--but preventing flow separation depends pretty heavily on the thickness of the turbulent layer.

It might be interesting to plug the "polars" for a car into something like XFoil though. Assuming you get flow separation at the rear, you could experiment with various trip strips to see if you could delay separation at least a little.

When you add a Selig-style bubble ramp to your car's trunk, we'll know you're really getting into the spirit of it... ;-)

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  • $\begingroup$ You can't make a 1-to-1 comparison to airfoils. There the trip strips force transition from laminar to turbulent flow. In cars, there is not much laminar flow happening anyhow due to the shape, all the edges and creases etc. Look at this image. This means that trip strips don't add anything. $\endgroup$
    – Roy
    Sep 12 at 9:17
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This is my layman explanation if how it is supposed to work. At least in wings.

So we'll start with the basic premise:

  1. The more airflow remains attached to the wing, less drag and the more lift there is.

However, there is also the fact

  1. Laminar airflow is more easily disrupted than turbulent flow such that it detaches from the wing.

That means that the two points above somewhat conflict with each other. The laminar airflow is more "brittle" in performance in the sense that when it works it works better, but when it stops working rather easily and when it stops working it really stops working. All or nothing.

Turbulent flow on the other hand remains attached to the wing more easily but while it remains attached, it produces more drag than laminar flow. In this sense it is less brittle in performance, but the peak performance is not as high as full laminar flow.

So you can choose to chase a unicorn that performs the best but requires perfect conditions, or you can sacrifice a bit of theoretical peak performance for something that works a lot more often. That's what dimples are: they introduce energy (aka turbulence) into the airflow which allows it remain attached to the body for longer.

In context of a golf ball, my understanding was that it doesn't increase lift (it's a golf ball, not a wing) but it does reduce drag. If the flow remains attached to the golf ball for longer with dimples than without then the low pressure bubble formed by the flow detaching behind the golf ball would be smaller.

I'm skeptical whether this actually applies to cars though. Cars are not very aerodynamic to begin with and one would think just the regular features of a car would introduce plenty of turbulence, probably excessive turbulence. But in that case dimples to introduce even more turbulence would seem pointless.

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TLDR: you cannot accurately determine differences with casual testing like this.

I have experience in vehicle fuel economy testing of aerodynamic devices, and I have high doubts about the testing method as shown in the video. I have reasons to believe the dimples will not work, will add more on that at a later time.

Especially for relatively subtle changes like this (compared to drastic vehicle shape changes), a precise method is needed. With a few short runs like this, the uncertainty in the result is just too big.

When we did fuel economy testing, we would drive in highly homogeneous conditions (i.e. both test and control vehicles at the same time on an empty oval track). Still, there are things you cannot control such as wind and weather. This means that we would have to drive for hours on end to collect enough data to even out the conditions we cannot control. Even then, we would routinely discard data due to non-constant conditions.

They do not mention the distance or the time traveled. However, we can estimate it. With a total fuel usage of 478 grams, and a fuel density of 748.9 kg/m³ (source), it would amount to 0.638 liters consumed. Assuming that is for the fuel-efficient vehicle (which consumes 7.93 l/100km), that would amount to 8.04 km driven, which at 65 mph, would take 4.59 minutes. They mention a sequence of 5 runs, meaning that they probably slowed down and turned around 5 times, which further reduces the stability of the measurement. This is such a short measurement, that a small change in conditions can seriously influence the results.

Also, ICE engines take time to stabilize, so we would run the vehicles for a significant time to warm them up to operating conditions, and let the fuel usage stabilize before we started the measurements. Doing all this allowed us to reach reasonable accuracies (of around 1-2%). If they did not do any of this, their uncertainties will be much much bigger.

Furthermore, if you use two different vehicles, even when they are the same type, there will be significant differences in fuel consumption due to manufacturing tolerances, different wear and tear (of for example tires) etc. So it could very well be that the dimpled car was more efficient to begin with.

For the interested reader, we've described our testing procedure (including correction for differences between the vehicles) in more detail here, specifically section 5. Track Testing

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