The airdam that didn't work

[MetroMPG]

Originally Posted by aerohead

Should you have had Hucho's book,you would have found that if the airdam projects below the elevation of the lowest suspension member,drag will begin to grow as a percentage of the percentage increase in frontal area.

Except I keep seeing examples where that rule doesn't appear to hold. The Car and Driver Pinto, for example. Even the new Tahoe hybrid's air dam appears to my eye to be much lower than any of the truck's dangly underbits:



Of course it's hard to tell from a photo, but I have also seen this on GM's full size panel vans. Maybe it's also a function of the relative "cleanliness" of the underside, apart from the height of the lowest parts.

[lovemysan]

The new silverado is equipped with a very stealthy airdam also. It looks to drop just below the suspension though.

[Otto]

Originally Posted by fabrio.

I am not surprised from your results.
with your airdam, you have added more frontal area, and because your car (with many aero mods) it is very efficient, your have gotten worse the things.
If I just remember, your car is equipped with full bellypan.
You can try to reduce central height of airdam, this, reduce the FA, and permit to pass the air under your car.
Look, the example opel calibra.
Calibra equiped with engine undercover, use an airdam tested in windtunnel.
After many tests, Opel are deliveryd an airdam shaped with raised central area, and high at the side.

look the image below:



It permit to deviate the air from wheels and the suspension arm.
I do not have any report, but in my car, I am insirated me to Opel for my front deviator
if you try it, tell us the change.

Does this mean that the gap in the middle of the spoiler should be ~900 mm wide? The spoiler should extend ~50 mm out from each side of the fender? The door sill should be ~50 mm lower and extend out ~50 mm horizontally from the side?

[SteveP]

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.

--Steve

[Otto]

Thanks, Steve, that's all good input and well reasoned.

I ask about the Calibra air dam because that car shape is quite similar to my Porsche 944 Turbo, which itself got significant wind tunnel attention in design stage. It has an undertray and chin fairing, although its nose shape has some unnecessary roughness in texture and contour.

That said, the nose of the Porsche could stand some improvement, and various spoilers and splitters have been designed for it, with anecdotal reports as to respective efficiency. Many on this website have made significant aerodynamic improvements to their vehicles by home-made experimentation and some pretty good intuition. From this, we can surmise that the factory original is not necessarily the final word on design.

Your post mentions some of your research and notes "mixing distance." Would you please tell us more about this, cite references we can Google, etc.?

Seems to me, "mixing distance" may be the reason for some of the geometry on the BMW air duct posted hereabouts a few days ago--that at given speed, the turbulent air behind the air dam moves such as to help extract radiator cooling air. Please help us with more info on mixing distance, etc..

[SteveP]

Otto:

That said, the nose of the Porsche could stand some improvement, and various spoilers and splitters have been designed for it, with anecdotal reports as to respective efficiency. Many on this website have made significant aerodynamic improvements to their vehicles by home-made experimentation and some pretty good intuition. From this, we can surmise that the factory original is not necessarily the final word on design.

I agree. However, just because *some* factory originals are not optimal doesn't mean *all* are not optimal. Again, be careful and be prepared to do some testing. However, my primary point was not that the front *can't* be improved--it was that rear improvement (if possible) would likely help more, though that's a generalization and may not apply to your particular car (I'm not familiar with the 944 and its aerodynamic history).

What I said about mixing distance came from Road Vehicle Aerodynamics, mentioned in my intro. To my knowledge it is not available for viewing on the net--you'd have to find it in a library, probably a local college library. It has quite an extensive section on how airlfow through the engine compartment is designed--apparently, until recently, this has been where the *majority* of wind tunnel time has been spent! The designers work really really hard to make sure the darned ICE is gonna stay cool (and warm in cold temps) and airflow through the grill, the engine compartment, and under the car is obviously critical (as well as very complex).

In general, the air under a car is at a lower pressure (because it's moving at a higher speed) than the air at the front--air moves from high pressure to low, so, yes, the air flow under the car has a lot to do with getting the air through the engine compartment.

The problem with the boundary layer (and I'm just learning this stuff) is this: it is the layer of air that is trying to "stick" to the car, and is trying to move with the car while the air farther from the car is happy to sail on by. The boundary layer forms because air is viscous. At Reynolds numbers for road vehicles, the boundary layer becomes turbulent very quickly; however, the boundary layer may or may not separate. Now from an energy point of view the vehicle will lose less energy to a laminar (non-turbulent) boundary layer than it will a turbulent one, but the real killer in terms of energy loss is when the boundary layer (turbulent or laminar) separates. It turns out that a turbulent boundary layer separates less easily than a laminar one, so sometimes turbulence (at the micro scale) is good. Vortices, which are "turbulence" at a macro scale are something else. Now, you probably knew all that, but I mention it for the following:

One problem (please notice I said *one*) with the boundary layer below the car is that it eventually thickens and meets the ground. The boundary layer (which is probably already turbulent and hopefully is still attached) is trying to move with the car but when it meets the ground, additional turbulence is created because the ground tries to retard the boundary layer. That additional turbulence (which may be "macro" turbulence--I'm not clear on this yet) represents a (small) loss of energy so the Cd has, in effect, just risen. If you can keep the boundary layer as thin as possible for as long as possible, you keep it from meeting the ground (i.e., you've increased the mixing distance) and you thereby lessen the gound-induced turbulence which means you've lessened the energy lost to turbulence.

There are other things going on as well, so this isn't the whole story--but it may help you understand the mixing distance and its effect on drag.

--Steve

[fabrio.]

Steve say, correctly, is it impossible to think that when are installed in other car is absolutely good for we cars.
we can to observe other poduction cars, look some aerodynamics elements and analysing they with in mind that in some cases, the marketing people change the shape for aesthetic motivation.
About the calibra airdam, my post it is only an example, and if we go to realize it for we cars, we must to inteprete little information that we have.
In particular Otto, the measure of gap in the middle of the spoiler, it is conseguence of some aerodynamic interference (actions).
Calibra, has full underbody pan, so permit to a good passage for air below car with minimal drag.
In this case, I think than, it is preferible to reduce the fronta area with gapped spolier.
Again, it is many important to deviate the air flow around the suspention ARM and any other element disturb of flow.

[jwxr7]

lovemysan, or anyone with data, do you still have any of your coastdown info? I'm looking for road load #s for saturns like your's because I'm thinking about building an EV saturn. All I would need would be the average time it took to coast from 55mph to 50 mph (use the data from before the airdam if you have it). thanks