DIY wind tunnels

[AndyL]

Ok so this is making me nutty... 2D simulators suck in real world applications

I'm hunting around, find lots of homebrew wind tunnels... Problem is, they're "smoke and mirror" based, yes you can see eddies, vortices, etc... But they don't put numbers to the tests... I'm a numbers guy...

So, request of you folks who understand the mechanics better... What do I need to do - to measure Cd from a model?

I'm going to run with this as a starting point,
https://wiki.engr.illinois.edu/displ...HKK)+by+Team51
But I'm thinking of using Breakout Board MPXV7002DP instead of the pitot tube setup used in the HAWT...

But thats only part of the equation, it still doesn't give us a Cd, so need to measure the drag, load cells are relatively simple and easy to get my hands on - then the question is, put a stationary model on a slideable platform?

Any thoughts/suggestions before I start hacking up some acrylic?

Thanks in advance,

[ChazInMT]

I heard of a small wind tunel where they had a sliding base attached by lever to a digital scale. Then you have a number to go by to track changes.

Crude illustration.

[AndyL]

Yup, have seen that one - I like the NASA school project method, string to shiv to 100g weight on a zero'd scale, as drag increases weight lifts...

[aerohead]

The bigger challenge is the 'verisimilitude',where the model has the proper Reynolds number,as boundary layer conditions which determine Cd must match the full-scale car.
At around 20 mph,the Cd will be constant for a 1:1 scale car.
*40 mph @ 1/2-scale
*80 mph @ 1/4-scale
and so on.
The model's frontal area cannot be more than 5% of the test section cross-sectional area.
It takes a big powerful blower to make all this work.

[AndyL]

Big blower wasn't too big of an issue, have a lovely fan sitting here that pulls 5900cfm, was figuring on a 24" cross section...

Now where does that 5% come from - doesn't make much sense to me, 6" square model shouldn't need a 5' cross section tunnel...

[nimblemotors]

I would suggest that you build a tether car track,
American Miniature Racing Car Association - Tethered Car Racing
and then you can build a model, keep the engine constant and change the body and measure how fast it goes. The top speed improves when the aerodynamics have improved. in fact, power it with electric motor to make it more constant than a gas engine.
Jack

Originally Posted by AndyL

Ok so this is making me nutty... 2D simulators suck in real world applications

I'm hunting around, find lots of homebrew wind tunnels... Problem is, they're "smoke and mirror" based, yes you can see eddies, vortices, etc... But they don't put numbers to the tests... I'm a numbers guy...

So, request of you folks who understand the mechanics better... What do I need to do - to measure Cd from a model?

I'm going to run with this as a starting point,
https://wiki.engr.illinois.edu/displ...HKK)+by+Team51
But I'm thinking of using Breakout Board MPXV7002DP instead of the pitot tube setup used in the HAWT...

But thats only part of the equation, it still doesn't give us a Cd, so need to measure the drag, load cells are relatively simple and easy to get my hands on - then the question is, put a stationary model on a slideable platform?

Any thoughts/suggestions before I start hacking up some acrylic?

Thanks in advance,

[AndyL]

Tether car is interesting idea I had one of those once upon a time, Cox 049 powered but, not sure how statistical data can be sampled from it...

Wind tunnel I understand, I can play with smoke - just ordered a Casio exilim for some high speed video, and I can sample pressure zones via probe, and get some force numbers to try to work back to a drag level...

The question becomes can one work out close enough in a 1:24 scale without hitting near Mach speeds in a huge tunnel, where improvements are needed and get a rough idea on how those modifications translate to full scale?

[aerohead]

Originally Posted by AndyL

Big blower wasn't too big of an issue, have a lovely fan sitting here that pulls 5900cfm, was figuring on a 24" cross section...

Now where does that 5% come from - doesn't make much sense to me, 6" square model shouldn't need a 5' cross section tunnel...

If you were going to do a 1/24-scale model of my CRX,you'd need a fan capable of delivering 650,918 cfm of air volume,while overcoming the skin friction of the tunnel.
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The 5% is a hard and fast rule of wind tunnel design if you're going to do force measurements.
At this scale:
*the model is 0.77 sq ft.
*the test section is 15.41 sq ft.
*The air velocity must be 42,240 feet per minute
*Which yields 650,918 cfm flow volume.
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If you want to model at this scale,you should do a water tunnel.An overhead gantry crane which straddles the tank can support the sting on which the model is secured.
Since water is 833 times more dense than air,you can get your proper Reynolds number at much lower velocity.

[AndyL]

Dunno about a gantry tow, but the water tunnel is feasible, but my Google foo is lacking - is there some papers around on design principles?

[aerohead]

Originally Posted by AndyL

Dunno about a gantry tow, but the water tunnel is feasible, but my Google foo is lacking - is there some papers around on design principles?

All the serious scientific work is done in a tank,with the gantry riding rails on either side moving the model through the water.
The U.S.NAVY's David Taylor Model Basin is probably the most famous.
Texas Tech constructed a smaller tank setup to test a 1/3-scale ( I think) Ford Taurus model with complete engine compartment and clear hood to photograph under hood cooling flow.That model cost $63,000.
Somewhere I have a SAE Paper of tests on 1/24-scale F-1 or Indycar models that were pulled underwater.I'll see if I can find it.
Texas Tech has a water tunnel in which dye (red food coloring) is released from ports drilled in the models body,which can be observed through the glass test section.No force measurements can be obtained.
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I did find GM's Paper.
It's from SAE Transactions,Volume 70,1962.The research was conducted at Cal Tech,in the Guggenheim Aeronautical Laboratory by Harold Flynn and Peter Kyropoulos,using 1/8-scale models.
The model was @ 4% test section blockage ratio,Rn=17,000,000,@ 160 mph wind velocity,with a 600-hp blower.
There is a pictorial representation of their data which I put together over at the trailer thread.
Their work seems to verify the 'Denominator-Rule' if you will,where in order to achieve proper boundary layer verisimilitude,you merely multiply 20-mph times the denominator of the fraction which defines the model scale.
i.e.GM's 1/8-scale = 8X20= 160 mph.