playing with first gen areodynamics...

[Sumo]

I think they have incense at dollar stores too and if all else fails do what the guys at Mythbusters did. They used water to prove if a pickup is more efficient with the gate up or down. Although to see the flow they used oatmeal and the water became murky really quick. Either way i'm curious to see results in your experiment.

[OlderThanMe]

yeah I saw that and considered that path. Too messy and not portable enough. Plus my tube is cardboard. My younger brother is watching Predator so I am trying to ignore it.

[blueovalz]

This is a great project, and hopefully should provide some small amount of useful information, but...

 

yes exactly. Except I am using a 1/25 scale model. My $12 fan gives about 4.2 mph which is 105.9 mph at scale speed on the low fan speed. On high I get like 139 scale mph.

 

I believe this ratio is inversely propotional rather than proportional. In other words a 1/25 scale model would need to have an airflow velocity of 25 times the full scale speed, not 1/25th of the full scale speed. Or, you could pressurize the air to 25 times the atmospheric pressure to reduce the flow velocity to a full scale speed. The purpose of this is to maintain the Reynolds number for the air flowing around the object being tested so that it realistically matches full scale characteristics. This is why a compromise must be made in wind tunnels between size and speed (or density). Otherwise, the smaller the model, the easier it would be to test, which is not the case. I realize this is splitting hairs perhaps, but this becomes very important in really small scale testing, which would apply in this case.

 

 

__________________

[olie05]

wow!

 

so even if you did a 1:12 scale model, the air speed would have to be 1200mph to simulate 100mph?

 

thats faster than the speed of sound!

[nope]

yea inversely proportional. you want the same reynolds number.

 

so if char. length is smaller then you need to up the speed.

time for a new fan huh?

[thehelix112]

For those interested:

 

Reynolds Number (Re) = (pVL)/u

 

p is density, u is viscosity, V is velocity, L is length or a rough size of the model.

 

For air:

 

Re = 67778 VL (units: V m/s, L m)

Re = 6300 VL (units: V ft/s, L ft)

 

As others have said to get the same behaviour you have to maintain the reynolds number. So as you can see, if you are decreasing L by a factor of 25, you have to increase V by the same factor to maintain Re.

 

Sorry if that was boring, I just learnt this last night and it helps me to regurgitate it.

 

Dave

[mrparks]

OTM, you should have used the orange tube cement. Don't worry about the cancer, it only counts if you live in CA. The blue tube stuff (smells of oranges?) doesn't hold up that great.

 

I also recall one of the Japanese model companies saying that Revell-Monogram made some aspects of that Z smaller to retain the visual appearence of the body while in scale. Whether it's true or not is a matter of debate.

[OlderThanMe]

rg...time to pull out the leaf blower. I don't know what would provide enough smoke for that though. I have some of the orange stuff but house rules say none of my toxic fumes in the house...and this is happening on the kitchen table...

The fender lips seem to be about 3 times larger than stock but that seems about it for exagerating stuff.

[OlderThanMe]

I have been thinking about it...it can't be inversely proportional...the scale weight of a 2500lb Z at 1/25 scale is the cube root of the weight. That is .16 pounds...now how is that supposed to stay in place in 2500mph winds??? it only makes sense that if you want to make a scale 100lb of lift on a stock bodied 240Z that would be the cube root of 100 pounds...that would be .0016 pounds of lift on a 240Z...

[olie05]

from the nasa website:

In 1954 NACA began the difficult task of converting the Langley 19-Foot Pressure Tunnel for dynamic [80] testing of aircraft structures. The old circular test section was reduced to 16 x 16 feet, and slotted walls were added for transonic operation. A new 20 000-hp electric drive motor was installed; the tunnel designers knew that it would not come close to the desired speed of Mach 1.2 at required pressure levels. But they had an ace in the hole. They simply substituted freon for air. Freon, a fluorocarbon widely used in refrigerators, transmits sound at only half the velocity as air. A given Mach number and dynamic pressure could be attained with about one-half the power needed for an air-filled tunnel. Reynolds numbers also increased with freon-and that was advantageous. So, too, was the duplication of a key flutter parameter, Froudes number, which is used when gravity terms are involved in the equations.

[olie05]

this should make you a believer...

 

also from this page:

By 1921 more than a score of wind tunnels had been constructed the world over. But all those of substantial size were operating at normal atmospheric pressures. This meant that the experimental results obtained using scale models in the tunnels were open to question because a special parameter called the Reynolds number did not match those encountered in the actual flights of full-scale aircraft. In other words, the Reynolds number of 1/20-scale models being tested at operational flight velocities would be too low by a factor of 20. Reynolds' classic experiments had shown that airflow conditions could be radically different for model and full-scale aircraft. Since the Reynolds number is also proportional to air density, an obvious solution to the problem of scale effects would be to test 1/20-scale models at a pressure of 20 atmospheres. The Reynolds number would then be the same in the wind tunnel tests and actual full-scale flights.

[OlderThanMe]

hmmm...well it is sort of hard to make 20 atmospheres...that would be...pulling out calculator...294 psi...It is still hard to believe.

[thehelix112]

20 atmospheres is harder to believe than a 20000hp electric motor?

 

This is NASA, not a high school science project.

 

Dave

[OlderThanMe]

indeed...but I am not NASA...I guess I'll just get the fastest fan that I can find...

[olie05]

well if you build the tunnel in a pressurized box and fill it up with a standard compressor, an accurate 1:12 scale setup may still be within your grasp. You would need to run around 175psi, but you would still need a fan that can blow at 100mph.

 

another bennifit to using 1:12 is that the 1:12 scale models are really realistic... all the way down to using actual coil springs

[OlderThanMe]

so that means a closed circular wind system...I am on the grassroots kind of tunnel...I'll test it with what I already have...I don't have any air compressor anyway...

[rudypoochris]

Can you make a lexan cut out too btw (incase you havent already planned it) that way you can see!

[DavyZ]

Couldn't you use some plexiglass from TAP plastics or something like that to make a tunnel (bend plexiglass sheet over a piece of wood, affix at both ends of the wood and voila, a 'tunnel' that you can actually see into...) for this experiement? Man, if you are going through the trouble (cool project BTW) of doing this, you may as well see what you're wanting to look at anyway. Hopefully you haven't lost too many sheckles in your project already to make a modification if you desire to.

 

I like your ideas and it will be a great learning experience. Practical experience beats books anyday LOL!

 

Davy

[OlderThanMe]

yeah I am going to be cutting out several sections and replacing it with plexiglass. I have been thinking about getting a box fan and building a very small box that the car would sit in. Definitely windows are the plan. I haven't even worked on it today.

[Chewievette]

To create more wind speed you can use the venturi effect to your advantage. Use a large box fan and build a duct to route the air into your existing tube. You will have some air trying to escape back through the fan since its not a closed circuit, but it will increase your velocity a bit.

 

Even if you cant get wind speed up as high as you want, you can still track the airflow characteristics around different parts and pieces. By studying the airflow around the model the slickest design can be found, or at least you can get pretty close. You can also find something that will prevent the exhaust from building up at the tail panel.