Does reducing ground clearance REALLY reduce drag?

[tasdrouille]

Originally Posted by SteveP

OK, just to confuse things further, here's something I ran across late today. It's from an SAE paper, 951906, "Vehicle Design Strategies to Meet and Exceed PNGV Goals" by Timothy C. Moore and Amory B. Lovins, found here

https://www.rmi.org/images/PDFs/Tran...nStategies.pdf

Thanks for the link! I'm an avid areo paper reader

The following comes from page 11:

"Rather than smooth the underbody and attempt to tuck chassis
components up out of the flow, the industry strategy has
tended towards air dams below the front bumper to force much
of the flow around the vehicle rather than under.

Well, that is totally understandable! Lets suppose you have a given vehicule that looks like an aerodynamic wreck underside. What are you going to do? Redesign everything so you get a more or less streamlined underside? Or stick a plastic dam up in front of all that mess and acheive decent results? Accountants usually have the most convincing arguments when comes the time for management to make decisions.

This needlessly
increases frontal area and leads to the erroneous notion
that achieving very low aerodynamic drag requires extremely
low ground clearance[5]."

Since it actually reduced drag, I don't think you can say it needlessly increased frontal area. The guy seems to forget the answer depends on the vehicule you start with.

Footnote [5] says:

"If chassis components are streamlined or otherwise covered
by a smooth floorpan to prevent interference drag, there is
little reason, beyond the limited exposure of more of the tires’
frontal area, to prevent the airflow from passing under the car
(P. MacCready, AeroVironment, personal communication,
April 1995). Allowing the airflow to pass under the car can
actually aid in eliminating lift-induced drag."

I completely agree with that. However, I am inclined to beleive almost all the cars on the road today do not have the level of smoothness of the underside required not to benefit from reduced underside airflow.

It is clear in my mind that the average daily driver will see a drag reduction if lowered.

It is also clear in my mind that a streamlined concept vehicule will see a drag reduction if it is rised.

The only gray area that's left is for ground vehicules akin the lotus elise. Cause it's not just the smoothness that counts, the shape is at least as important.

[SteveP]

Originally Posted by trebuchet03

I'm not looking for transferability - but there was a blanket statement made that applies to vehicles with streamlined undersides.

Yes, but the statements I quoted were originally in the context of cars and trucks. And this thread started out being about cars. Sorry if that wasn't clear. Not saying your comment about lowering the height wasn't interesting or useful--just wanting to make sure I really understand the import of it.

tasdrouille: If you only look at the extremes (avg daily driver vs. streamlined concept car) maybe things are as clear as you suggest. But what do you tell the person who has put a smooth belly pan on his car and wants to know if he should also raise or lower the height? Also, many cars coming out today have smoother undersides than cars of ten years ago--what do you tell their owners? (And while I admire the spirit of "just try it", some of us have limited time/resources and need/want to put our time/money where we'll get the most bang for our buck.)

CoyoteX: if you're really interested in interactions between the wheel and the wheel well, have a look at SAE paper 2002-0105209. Reducing drag in that area isn't straightforward--even the guys with the moving floor wind tunnels and CFD technology take a lot of trial and error to improve it. The message from that paper (to me) was: just putting the wheel higher in the well doesn't necessarily improve overall Cd.

--Steve

[aerohead]

Originally Posted by SteveP

Maybe you mean something different by "fineness ratio" than I understand it to mean, so perhaps you should provide a short definition. Mine is "width/length" (or, for a cylinder, diameter/length). I therefore don't understand how lowering a car can alter its fineness ratio.

Also, would you be so kind as to expand on your comment that "Since road vehicles all suffer from the "mirroring" effect of the ground,anything which can be done to increase fineness-ratio is a shoe-in for lower drag". What does fineness ratio have to do with the ground effect? I could see it if you were *narrowing* the car, but lowering it doesn't do that.

Thanks,
--Steve

Steve,good question and I'll try and do it justice.From experimental investigation,it was discovered,back in the 1920s,that a body of revolution (say a Zeppelin body/fuselage) which had a drag coefficient of X in free air,would have a drag coefficient of 2X near the ground.The drag behaved as if you had placed the object on a mirror,and the air was striking both images,hence "mirroring".In early windtunnel photographs you will actually see car models joined at the wheels,one right side up,the other upside down,hanging in the air stream.When viewed from the side,if you drew a line which enveloped the double-car combination,hitting all the high points and spanning the voids,the line might describe an egg,as in the VW Beetle.The length of the "egg" divided by the height of the egg would define it's "fineness ratio".Its kinda like the aspect ratio of a wing,where the double-car is the airfoil.Kinda weird! Anyway,since all vehicles are in ground-effect,they all suffer from this virtual "mirroring" effect.So if you divide the length of your car by its height and say its 3-to-1,which would otherwise be very efficient,as far as the air is concerned,its only 1.5-to-1,an extremely blunt body,and not a good candidate for low drag.Hucho's first English translation edition from 1987 shows this whole affair on page 200.By lowering a car,you effectively stretch out the "egg",improving it's fineness as you approach 6-to-1,the ideal for anything in ground-effect.If you go longer,profile drag improves at a lower rate than skin friction which begins to climb,canceling out the benefit of any additional length.A wing has extremely low profile drag but extremely high skin friction and is unfit as a model for car bodies.A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body.Not really practical because of the necessary length,however it is the "pumpkin-seed" form chosen for world-class energy sippers.Our challenge is to get as close as we can to the efficiency of the pumpkin-seed while addressing all the other parameters of automotive operation.

[LostCause]

This is a tricky issue because it is a balancing act. Without proper testing, I can't imagine any of us offering more than an intuitive suggestion.

My intuitive suggestion is to lower the ride height until ~6 inches of clearance exists between the road and frame, fair everything in sight as smoothly and thoughtfully as possible, and install stiffer springs that keep the car stable at an AOA (angle of attack) of 0 degrees.

The only thing I remember having read on the subject is this online chapter that deals with race car aerodynamics. Here is the paragraph in question with its respective graph:

Originally Posted by Race Car Aerodynamics - Joseph Katz

The effect of ground proximity was shown to have a strong influence on the aerodynamic coefficients of generic bodies. For enclosed-wheel vehicles, with reasonably clean underbody flow, drag and lift usually decreases with decreasing ground clearance, as shown in Fig. 6.18. The increase in the downforce can be attributed to the higher speeds under the car, with decreased ground clearance. The drag reduction in automobile shapes with major rear flow separations is partially a result of this faster airstream emerging from under the vehicle, which reduces the size of the rear flow separation.

Another effect that causes this trend is the reduction in the frontal exposed area of the wheels (as they slide into the body), which decreases drag with the reduced ground clearance. In regard to lift, the interesting observation is that the trend (that less ground clearance equals less drag and less lift) is reversed at a certain small distance. This is a result of the viscous effects (thick boundary layer) of the too-small ground clearance blocking the flow between the road and the vehicle underpanel. Thus, in this case of very small ground clearance the flow resembles the case of the semi-ellipsoid and not that of the ellipsoid. In Fig. 6.18 the critical distance for the lift reversal is close to 3.5 cm, but for full-scale race cars with smooth underbody, moving on the track, this distance can be as small as 0-5 cm.

Fig. 6.18 - Cd vs. Ground Clearance

Lift and drag coefficient increments as a function of ground clearance (based on 1/5-scale wind tunnel test and model with smooth underbody and fixed ground plane).

I do believe decreased ground clearance will eventually have a detrimental affect on overall drag, but I have no idea at what point. All modern aerodynamic cars, from Shell Ecomarathons to car prototypes, have gone either way. I think experimentation is the only true answer.

- LostCause

[trebuchet03]

Originally Posted by SteveP

Yes, but the statements I quoted were originally in the context of cars and trucks. And this thread started out being about cars. Sorry if that wasn't clear. Not saying your comment about lowering the height wasn't interesting or useful--just wanting to make sure I really understand the import of it.

Gotcha

I'm still working on a converting a pretty complete model of my car... Once done, I can do the same test

[diesel_john]

CarloSW2[/QUOTE]
I was thinking something similar, aka only lower the front end. Are you thinking that the "larger volume" open space at the rear would aid in air exiting faster at the rear? CarloSW2[/QUOTE]



good point. cfg83,
But, i didn't think of that until i started thinking about a smooth underbody.
I found lowering the back end increased the drag (with a stock underbody), so would raising the back end decrease drag (with a smooth underbody). So now that i have smooth under, I will try to raise the back.

Probably each car should be optimized for front to rear height.
We say the shape of the rear is more important, so does that mean I can raise the rear ?to enhance the tail and while blunting the windshield, and still reduce drag, inexpensively.
Angle of attack effects so many things.
the grill to hood flow
hood to windshield flow
windshield to roof flow
roof to rear deck flow
on the bottom lift or downforce
under flow exit

lowering the front would compromise my alignment.

aerohead, thanks for the explanation. It gave me the answer to my question. What is optimal?
""A symmetrical airfoil of 6-to-1 aspect ratio,cut in half lengthwise,would describe the lowest drag car body""

[SteveP]

aerohead,

Thanks for the explanation--well done! I did know about the mirroring concept, but in a totally different context (nearly all of my knowledge/experience is with objects well above the ground). The bit about considering the ratio of length to height of the car plus its image was the part I was missing. When I (finally) get my copy of Hucho, I'm sure a lot of things will be cleared up.

--Steve

[SteveP]

Originally Posted by LostCause

This is a tricky issue because it is a balancing act. Without proper testing, I can't imagine any of us offering more than an intuitive suggestion.

<snip>

I think experimentation is the only true answer.

- LostCause

Just so folks understand where I'm coming from on this, I draw a distinction between what I call "blind" testing and "educated testing". Theory (or what we can understand of it) transforms the first into the second and helps us understand what tests are likely to prove fruitful. My model for this is the Wright brothers vs the other clowns that were, in some cases, killing themselves by just throwing a bunch of stuff together and jumping off of things. While the Wrights did a ton of testing, they also did a lot of thinking about theory, even developing new theory about such things as how a propeller works--they spent a lot of time with pencil and paper before they even built models.

If you have limited time/budget (and we all do) then you have to pick the *right* tests and the only way I know to do that is to absorb as much theory as possible and let that guide you. Of course, one can take that to the opposite extreme and never get around to trying anything, so there has to be a balance.

And, of course, it may be that the theory we would most like to have is all proprietary and we won't have access to it for another twenty years. But speaking personally, I have to start somewhere and since I can't start building my conversion for a while, I may as well learn as much theory as I can.

Besides, I really enjoy learning this stuff! When someone like trebuchet03 reports an Re of 184 and that seems to me to be an odd number, I figure I'm about to learn something--after all he's got access to this cool software and testing apparatus, so he probably knows a thing or two that I don't.

--Steve

[LostCause]

SteveP, I do agree with what you said. I just think everything we develop here needs to be understood under the pretext of "it depends." I like these types of threads...they make you think.

I think we'll best understand variances in drag due to ride height by making some simplifications. If we assume a catamaran configuration with completely smooth sides, we have essentially created a wind tunnel, which have been studied immensely.

Smooth Bodied Catamaran


I can calculate transistion points and total drag forces on flat plates easily, but I've never solved for ducts. I will try to determine how, but others studying aerodynamics have probably had to solve such a problem in at least one class.

If we vary the height of the duct, we can see when the flow starts to break down. Once we understand the "ideal" state, we can better infer how normal vehicles might fare. This is the most logical approach in my mind.

- LostCause

[cbergeron]

I'm glad this thread was formed... I didn't want to ruffle any feathers, but in a world where 90% of the [leading] populations have been led to believe that 19 guys with box-cutters destroyed the USA; it's time to re-think things from the ground up - the way Thomas Edison did things.

He once asked his students to determine the volume of a lightbulb and they scribed formulas on their notepads for hours upon hours on end. Eventually a solution was reached on paper. To wit - Edison took a lightbulb, unscrewed it and poured water into it until it was full. He then, poured the bulb full of water into a flask and measured the volume of a lightbulb - not by measure, but in practice (experience); and in a matter of seconds.

"In science, inducing controversy is the greatest way to achieve progress; particularly when great minds are involved."
-Chris Bergeron (unless it's been said before, that is