Aerodynamics Research on Vehicle Modifications
 

I did some research for college credit and tested various vehicle modifications to figure out Cd changes.
I ended up testing wheel well covers, vortex generators, and belly pans on all of the vehicles. I also tested tonneau covers, camper shells, and angled bed covers on the two pickups.
The results I got were fairly consistent with what I've been seeing on the website here, but there were a few results that were kind of surprising.
I'll get my results posted here and if you have any questions, just ask me!

(the vehicles I used are also listed on ebay if you would like to try to prove or disprove my results)

Results
 

Took me a few minutes to be able to figure out how to get these posted, and I hope they work!

If I interpreted your results properly, wheel well covers lowered your Cd, but vortex generators raised the Cd (didn't see any tests on belly pans in your spreadsheet). This is to be expected since the covers would smooth out the airflow along the side of the car. Vortex generators are usually used as bandaids to try to fix or reduce an aerodynamic flaw in a car's design and, depending on their positioning, can either raise or lower a car's Cd. Since their positioning is very critical and takes a lot of research or trial and error testing to find, for most people they will end up raising the Cd.

I see you made some dramatic improvements in the F350. The belly pan was worth 12%, the bedcover 17-20%, and the VGs 2%. That adds up to a 34% reductiion in drag, to Cd=0.32, which is very impressive if I interpreted that correctly.

What are 1.ozs and 2.ozs? How did you arrive at the figures in the "All Modifications" box? If this project also generates a report, I'd like to read it.

I'd be interested in the report as well. Thanks for posting.

Very interesting stuff Stephan. Thanks for sharing. Any pictures of the modifications or description of the methods?

Very interesting. I'm looking forward to some elaboration on your testing methods and some more interperetation of the results.

I don't know why all of the results aren't showing up, unless it had something to do with converting the Office 2007 file to the standard .xls?

I hope my results paper actually shows up correctly here. If you have any other questions or comments after reading my paper, just let me know. I'm no aerodynamics major, so please let me know if I made any mistakes.

As far as pictures, I do have some pictures and videos of some of the tests, and I'll try to get at least some of the pics up here soon.

Here are pictures of the F-350, I can't find any of the others at the moment. I'll get them up as I can find them.

The first time I read this, I thought it was hilarious. Yeah, I might spend $30k on a Ford F350 on Ebay to test some aeromods on it...

But if we're dealing with models, that changes things. First, if you test a scale model in a small wind tunnel, you need to scale the viscosity of the fluid, too. That's why some scaled-down tests are done in heated water.

Second, how accurately does the scale model represent the full-sized car? These models seem pretty good, but certainly not perfect.

Professors like it when you acknowledge and understand the limitations of your work.

Btw, I couldn't read your .docx. My copy of Word 2000, with the .docx compatibility filters, says it's not a valid file.

Dude, the scale models are on Ebay. Look at the pictures above.
Well theyre the scale models or he has really big hands !
More info would be cool.
Hope you get an "A" from the project.
So when do we get to see this translated to your full scale rides?

Great Job!!
 

Stephan, great job on this paper, you have certainly put a lot of good energy and work into this. :thumbup:

I have a question for the folks in here who know better. Someone wrote that you need to scale the viscosity? If you're doing comparative analysis, meaning you are simply comparing how a shape does before and after a given change, why would that matter?

It isn't as if he's saying "This is how the big one performs."

He is saying, "I took a model vehicle, got a base line force reading, then changed this on the model vehicle, and got this reading." Now I understand there may be some variables that don't "Scale up" but for the most part, this seems to be a pretty fair, accurate and economical way to test things.

My only thought with his modifications are they are not as good on the small scale as they would be when carefully crafted on a real vehicle, (Nothing personal Stephan) but this would slant his results in the direction of being less beneficial, meaning real world applications would work better than indicated in his tests.

The reason I chime in, is I'm really considering building a small scale wind tunnel myself in my garage to test what I can do to a pick-up truck. My thought was, that as long as I have a base line before any modification test, and the modification is the only variable in the test, the results should be close to accurate, I'm testing the modification.....not the whole system. Aerohed has repeated numerous times that from 20MPH to 250MPH air acts pretty much the same. I think that as long as you are getting 25 mph wind speed or more, and not making changes to the air speed from test to test, how can these results be too far off?

I have been able to open everything you've posted Stephan, so I don't think the problem is on your end.

I like his idea. Testing on models first to see what kind of results are possible. However I agree that seeing the real life versions of this to see what real world results come from these designs.

I commend you on your experiment. Maybe it will spark the interest of an owner of said cars to test it out.

Chaz, because the Reynolds number must be close enough between two scenarios to say there is similitude in the effects observed.

I'd like to know how exactly you were calculating or measuring Cd from the scale models?

Similitude (model) - Wikipedia, the free encyclopedia

You can get a dime to float in a glass of water, but a manhole cover the shape of a dime can not be made to float. Surface forces do not scale the way you might think.

If you test a scale model in a wind tunnel without achieving similitude, you will get different results.

Indeed, similitude. You have to slow down the air flow on the model until the Reynolds number is the same as on the full size truck.

Where did you get that scale model? It is exactly the same as my truck.

Tasdrouille & Robert, please explain how this makes a difference when comparing a change on a model. I understand that you can't scale the results on a case by case basis when taken individually. But when you make a change to something, why would Reynolds numbers & such make any difference? If all other conditions remain the same but, say, the addition of a square truck cap, why would the results not be legitimate? It is comparing “Cap On”-“Cap Off”.

The question is not a quantitative one such as finding out the horsepower it takes to go 50mph on a 60°F day, at 70% humidity, 30.02” barometric pressure, at 500’ above sea level. For this question I could fully understand why you need “Similitude”.

If you are incapable of explaining in detail why a change to the scaled model can not be tested, so that it is easy to understand, then please, do not reply.

The effects of changes on a scale model are just that, they apply solely to the scale model and can't be translated to full scale models unless Reynolds numbers matches between the two models. How fluid dynamics behave at a given Reynolds number is not the same as an other Reynolds number. Basically, what you'll get putting a 1:18 scale model in a wind tunnel with 60 mph airspeed will not scale properly for a full size model at 60 mph airspeed.

Or even a 1:18 model at (60/18) mph.

You may enjoy reading the first sentence here: Reynolds number - Wikipedia, the free encyclopedia
and this one: "[Reynolds Numbers] are also used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce random eddies, vortices and other flow instabilities."

The full-sized car might have turbulent flow where the model has laminar flow, etc. The flows will probably have different shapes, despite the model having the same shape as the full-sized car.

But if you test your scale model in a medium with the same Re, the flows are guaranteed to be equivalent.

If the manhole cover's size and weight were both scaled accordingly to the dime, I'm pretty certain it would float.

The problem with the comparison is that while a manhole cover might be 1,000x the volume, it's probably 10,000x the mass, making it less buoyant.

I do agree about the wind tunnel comparison, though. The changes in cd won't be scalar as the size of the object (and the size of features related to the object) increases.

Nope! Suppose the manhole cover has a diameter 100 times that of the dime. Its circumference is 100 times as large, and so is the force of surface tension. But the manhole cover is 100 times as thick, too, so its volume is 10000 times as large. The density is the same in this example, so mass, weight, and even buoyant forces are 10000 times as large.

Surface forces scale with L^2, while body forces scale with L^3.

The cross-sectional area of muscle and bone scales with L^2 while mass scales with L^3, which is why an elephant can't jump five times its height like a cat, and why we don't see large animals with the proportional strength of an insect.

dime = approx .5" dia. x100 = approx 50" dia (manhole cover)

dime = .196 area x100 = 19.6" area (manhole cover) right? Wrong. 1,963.5" circ.

dime = 1.57" circ x100 = 157" circ (manhole cover).

Since area is a surface calculation, and it just seems to have scaled to 10,000 times, I'd say there's still something wrong with the analogy.

If the manhole cover had the same density as the dime, but 10,000 times the surface area, and only increased spatial dimensions by 100 times the mass, it would float.

The point is that the manhole cover wouldn't be able to float not because of scaling, but because of the difference in density between the two materials.

Buoyancy is described as the force opposing an objects weight in a fluid because of displacement of said fluid. If the object displaces a weight of fluid that is equal to it's own weight, it will float. On the whole, the object has less mass than the mass of the fluid it displaces. If a manhole cover really had 10,000 times more surface area and only 100 or even 1,000 times the weight, it would also float. Even if it had 10,000 times the surface area and 10,000 times the weight, it would still have the potential to float (provided that you could make a dime float, which I don't recall ever having seen), with the exception of breaking surface tension due strictly to sheer size.

Since surface tension relies on molecules' ability to stick together under strain, a much larger object would have more of a tendency to break surface tension because it would require a much larger group of molecules to cooperate.

The kind of "floating" I'm talking about here is not buoyancy, it's a result of surface tension. No metals float buoyantly in water, but it's easy to get a paper clip to sit on the surface of water, and it's possible to get a dime to do the same. However, make the dime or the paper clip 10 times as long, 10 times as wide, and 10 times as thick, and its behavior will change.

The point is scale models do not behave the same as full-sized objects, because area and surface forces scale with L^2, while body forces scale with L^3.

Correct me if I'm wrong, but doesn't scale model testing take Reynolds into consideration by increasing the fluid flow proportional to the model?

I thought I'd read that if you use for example a 1/2 scale model, you would double the fluid flow to achieve comparable full scale fluid properties.

Hmm, yes, I did overlook that. You could preserve the Reynolds number of a half-scale model by doubling the airspeed, but it would become problematic for a 1:18 model of a car. It would take air at 1080mph, or Mach 1.4, to mimic the full-sized car at 60mph, and I don't know whether approaching or exceeding the speed of sound invalidates the results. However, if you use water, which is 50 times as viscous, you can run the 1:18 model at 22mph. A 6"x6" water channel flowing at 22mph (or maybe 11mph is good enough) is relatively easy to build.

However, for an Honors project, I think ignoring but acknowledging the lack of dynamic similitude is sufficient. Unless you want to play with your ME department's water trough.

Gotcha. I wasn't clear on exactly what point you were making, and I obviously took the wrong part of the post.

Yes, you are right, but they often test 1:2 or 1:3 models.