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Old 04-08-2008, 10:59 AM   #14 (permalink)
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We have to be careful when we look at other vehicles. Remember, there are folks involved besides the aero engineers--those darn stylists and marketing departments have their say, too, and these days, what a lot of them seem to be saying is "Make it look more macho". One way of doing that is putting big airdams on the front (e..g., the SUV shown above). The problem is, you can't tell *by looking* what thinking led to the air dam being the way it is: was it the stylist who said "add two more inches to it" or the engineer who discovered it lowered the Cd by 20 counts? What we have to do is understand the aerodynamics and work from there (imho).

Ditto for what you see on NASCAR bodies: they have different constraints and different goals (airspeed = 200mph, super smooth tracks (no speed bumps to negotiate), and a need for negative lift).

It turns out that front air dams have to do a lot of different things, depending on what's going on with the rest of the car. For example, I'm learning that wheel well size and shape have a lot to do with the aerodynamics of the front wheels--and that how the air dam affects air flow to that area has a lot to do with overall Cd. Second, you might notice that most air dams have a "chin" whereas the air dams lotsa people fabricate don't. I haven't researched that aspect yet, but just from my general knowledge of airflow, I'd have to say that I expect the chin has a major effect on helping the air flow to the sides rather than to the underside of the car (which seems to be the goal).

Also, there is a concept of "mixing distance". The air flowing over the front of the car forms a boundary layer (both above and below the car as well as on the sides) which grows in thickness as it moves from the front of the car toward the rear. At some point (assuming a typical ground clearance), the boundary layer below the car becomes thick enough to touch the ground. The distance of this point from the front of the car is known as the "mixing distance". According to Road Vehicle Aerodynamics (see my Intro), increasing the mixing distance and decreasing boundary layer thickness by careful attention to the front of the car can reduce the total Cd by up to 10% (please note the "up to"). The author then goes on to describe the effects of underbody curvature (front to back and side to side) and shows that curvature helps lower Cd and discusses the interaction of this curvature with what's happening at the front of the car.

The takehome message, it seems to me, is that a person can't just slap on a front air dam of an arbitrary size and shape and expect it to work (let alone be optimal). There's got to be some trial and error involved and the results *do* depend on what else is going on with the car. Wish it weren't so, but I'm afraid it is.


Last edited by SteveP; 04-08-2008 at 11:14 AM.. Reason: Added "as well as on the sides" to the boundary layer defn
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