Aerodynamic Streamlining Mini-Course

[ecomodded]

O.k I need to study this more..

I had a hard time understanding Yoshi at that point in his description, I lost his meaning at that point.

[Patrick]

Originally Posted by freebeard

The rabbit hole goes a long way down. Can anyone explain the scaling on the Re axis to me?

It's a logarithmic (Log) scale. Each major division increases by a power of 10. Where the 1 is would be Re = 1; where the 10 is would be Re = 10; the next one is 10 squared (see the 2 to the right just above it?), or 100; the next is 10 to the third power (cubed) or 1000; next is 10 to the fourth, 10,000; next is 10 to the fifth, 100,000; then 10 to the sixth, 1,000,000; and last is 10 to the seventh, or 10,000,000.

The other numbers in between the big divisions show how far that vertical line is to the next big division. Take that number and multiply it by the nearest big division before it to get the Re for that line. For instance, the vertical line to the left of the word Sphere is close to the line marked 4 at the bottom, but that line is to the right of the 10 to the fifth major line. 10 to the fifth is 100,000, so we multiply 100,000 by 4 to get 400,000 for the Re when the drag of the sphere drops way down.

Log scales are used to squish the graph down to a reasonable size.

[ecomodded]

Originally Posted by Patrick

FYI, a quick approximation for Reynolds number for objects in air at sea level is
Re = Length in feet X Speed in mph X 10,000.

So a 15 ft long car at 50 mph would have a Reynolds number of 15 X 50 X 10,000 = 7,500,000.

A 1/32 scale model in a 50 mph wind tunnel would have a Reynolds number of 15 X 1/32 X 50 X 10,000 = 234,375, or 1/32 of the full-size car. If you double the wind speed in the tunnel to 100 mph, the Re would be 1/16 of the full-size car. See the relationship?

Thanks for the insight.

I do see the relationship , thanks for that and the formula for scaling.

How the Re interacts with vortexes is still unclear , If you could simplify that , It would be much appreciated.

[freebeard]

Once you've got a handle on that, all you need is an ample air source, and an effective diffuser to straighten the flow. It's job is easier if the fan is behind the model.

How the Re interacts with vortexes is still unclear , If you could simplify that , It would be much appreciated.

I'll just point back to here:
http://ecomodder.com/forum/showthrea...tml#post433339

[ecomodded]

That mans research is what I need to study alright.

I came across this student thesis on a scale wind tunnel.

http://www.davidoprevatt.com/wp-cont...ors-thesis.pdf

[freebeard]

It's interesting that his focus was on buildings in hurricane force winds. It was a consideration in the dome I designed in 1980 for the Oregon coast.

I haven't read the student paper yet, but I will. Just glancing at the Contents it looks like what I was calling a de-fusser he calls a settling chamber.

Patrick -- Thanks. I knew it was logrithmic; looking at it today I see the superscripted exponents are interleaved with the subdivisions. It wasn't making no sense yesterday.

[ERTW]

You forgot "induced drag", which is caused by lift on a body. Trying to direct all the air on the trailing edge near the ground creates lift...especially if you don't have a 20 foot tail.

struts are at least 20x wider than their chords. End effects don't matter. Car bodies are relatively narrow, and the above rules of thumb do not apply.

wind drag *begins* to have a greater effect than friction at 50 mph. more streamlined the vehicle, the higher that speed. there is a point diminishing returns, where fuel economy doesn't increase, and the manufacturability, and style, go out the window.

GM seems to design its cars to a CdA of 7.0. Germans are a little better, at ~6.0. Engines are designed to operate with a load, and 2500-3500 rpm. A super low drag car, cruising at 1500 rpm, is not optimum. imo the aero civic owes more to its lean burn engine (wouldn't pass emissions in certain areas) than to its hideous boat tail.

Can OEM lower CdA to 5.0-6.0 without increasing cost? Yes. will it improve hwy mileage at 60 mph? not much, if at all. If the government, like aerohead wants, raises speed limits to 100 mph, then yes.

[ecomodded]

Well that's no fun is it.

I think your forgetting that work or force is not done without energy , adding a wind load will not benefit fuel economy , removing wind load will. Driving at 1500rpm will save fuel over driving at 2500-3500 because it takes more fuel to drive at higher rpm's.

My understanding is very different then yours.
The load theory does not trump all other conditions , not by far , the load theory is more about acceleration then cruising purposely with a wind load to strain your motor , or make it do more work while cruising then necessary..

[ecomodded]

quoted myself .. its catchy

Originally Posted by ecomodded

I am going to buy a 1:18 New beetle from ebay if I do not find something today.

eBay has a few Bburago 1/18 New Beetle's , made in Italy with opening doors, hood , hatch back and the steering wheel turns the front wheels. Its a very accurate cast model, even the mirrors look correct.

So I ended up buying one of the above mentioned models for $20 with free delivery. I had a few Beetle choices but choose a 1/18 New Beetle Cup car.

So I can test it in its race configuration then add a Kammback for further testing.

I will wind test with what I can , I remember that my grand dad made a shop ventilation fan by attaching metal blades to a 2hp electric motor - it was powerful.
I also want to make a water test system up , a fish tank to test with a inlet and outlet, yet to be designed or thoroughly looked into ..

I do not like the graphics or color much.. I bought it with no intent on reselling it as I will be damaging the paint in short order in the name of science ... ouch to the collectors with there neatly displayed boxes.

A photo of the "Cup" car and the body kit , basically it allows bigger tires and is lower to the ground.

[Xist]

Okay, so:

speed of sound at sea level =
761.207051 miles per hour

[Google]

So, to simulate a car going 55 MPH, you need a model that is 761.207051 / 55 = 13.8401282:1

Otherwise, you would be creating a sonic boom indoors.