Aerodynamics Research on Vehicle Modifications

[JasonG]

Originally Posted by RobertSmalls

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?

[ChazInMT]

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

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.

[spydyr]

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.

[tasdrouille]

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

[theycallmeebryan]

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

[RobertSmalls]

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.

[Big Dave]

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.

[ChazInMT]

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.

[tasdrouille]

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.

[RobertSmalls]

Originally Posted by tasdrouille

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.