Aerodynamic Streamlining Mini-Course

[freebeard]

1/32nd seems a little small-scale for wind tunnel testing. I've got a 1/18th New Beetle (in Cyber Green) somewhere, and the pros generally go to 1/4th or 1/5th.


have you looked at the work of Yoshi Suzuka? He uses 1/24th.

[ecomodded]

Its all I have , Its going ahead once I get my wind gauge I expect to gather some information and photos from it, despite it being just 5" inches long.

If I see a bigger one, I will get it, they come in large sizes for kids toys, The Barbie New Beetle is about 1/5th scale (28" long) A friends daughter had one at one point.. I will see what I can turn up in the months to follow at the second hand junk stores.


edit:

I just checked eBay , the biggest it I seen was a 1:18th scale for $30

I think I will check a couple of the 2nd hand stores in the next city over , when I'm there visiting tomorrow. It would be nice to find a 1/5 or 1/8th scale Beetle. I have a 1/8th scale RC truck ,it's about 2ft long , it would be a nice scale to work with for the Beetle..

[ecomodded]

Originally Posted by freebeard

1/32nd seems a little small-scale for wind tunnel testing. I've got a 1/18th New Beetle (in Cyber Green) somewhere, and the pros generally go to 1/4th or 1/5th.


have you looked at the work of Yoshi Suzuka? He uses 1/24th.

Freebeard You could (should) make a wind tunnel to test the 1:24 Beetle , You could test your designs and whatnot. Once the tunnel is built it could be used for some years testing scale models.

They have lots of really cool older cars in the 1:18 scale (11"x 5" ) Mercedes , fiats , Porsche's and so on, they would be interesting to wind test.
They also have Classic Beetles in many 1:18 forms / designs.

[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.

[freebeard]

Freebeard You could (should) make a wind tunnel to test the 1:24 Beetle...

Possibly so, but then I'd never get re-finishing my new kitchen cabinet done. I have made progress with 3D modeliing. Something I couldn't do before works now. ??? I can remove the bumpers and change tire sizes. As soon as I merge the Beetle and boattail geometries, I'll have something to show.

Found this, it seems on-topic for this thread.

To Predict Turbulence, Just Count the Puffs - Issue 15: Turbulence - Nautilus

To add to the difficulty of the problem, scientists have not even agreed on how to define turbulence to the present day. Depending on whom you ask, turbulence means rapid mixing, vortex stretching, an energy cascade from larger to smaller vortices, or sensitive dependence on initial conditions.

It then proceeds to a lucid description of Renynold's Number.

[NeilBlanchard]

Originally Posted by freebeard

1/32nd seems a little small-scale for wind tunnel testing. I've got a 1/18th New Beetle (in Cyber Green) somewhere, and the pros generally go to 1/4th or 1/5th.


have you looked at the work of Yoshi Suzuka? He uses 1/24th.

It must work at *very* high velocity, in order to compensate for the small scale. The sensors look to be pretty sensitive. Pretty darn cool!

[ecomodded]

A discussion on the Reynolds Number and windtunnels:
Wind tunnel tests and Reynolds Number

The Reynolds number is New to me , it is now making my head spin/ grind/stop when trying to apply it to a design.

[ecomodded]

I was watching this video (from the Yoshi link) when Yoshi mentioned the wind speed he would be using is 40 to 50 mph to simulate a full size car going 100 mph.
EDIT: I WAS WRONG , YOSHI ADDED THAT IT EQUALS 2MPH
It appears that you reduce speed to simulate a real car.
I had also thought I would need to increase the wind speed.. it just got easier to construct by a few fans at least and several mph.

http://www.youtube.com/watch?v=E1IIMFOFpjU

[freebeard]

The Reynolds number is New to me , it is now making my head spin/ grind/stop when trying to apply it to a design.

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

It appears that you reduce speed to simulate a real car.

The link you posted says no.

Originally Posted by Bob s

Re is a unitless number, so if D is reduced by a factor of 10, v would have to be increased by a factor of 10 to keep the Reynolds number the same.

[Patrick]

Originally Posted by ecomodded

I was watching this video (from the Yoshi link) when Yoshi mentioned the wind speed he would be using is 40 to 50 mph to simulate a full size car going 100 mph.

It appears that you reduce speed to simulate a real car.
I had also thought I would need to increase the wind speed.. it just got easier to construct by a few fans at least and several mph.

No, he said it would simulate a real car going about 2 mph.

Ideally, you want the Reynolds number of the model to be the same as the vehicle you are simulating. But since the car is 1/32 size, you would have to run the wind speed up to 32 times the real car speed that you want data for, which is not practical for most people.

But you can find out what the relative impact is for changes to the shape. Just try to make the model as big as possible and the wind speed as fast as possible.

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?