Home made wind tunnel

[aerohead]

Originally Posted by onefstek

Cd asked me to show off my home made wind tunnel.

After a good time reading and trying to find info on my car's aero, I decided to just make a wind tunnel so I can see what results I can get.

Here are some pics

Wind tunnel



Model car in wind tunnel with stock spoiler



Model car in wind tunnel with kamm spoiler



This confirms the ideas I had and could possibly help me win some races next year.

Stefan

Stefan,your results may work out better in full scale.At 1/24-scale,your fan would have to deliver 480-mph air to develop a turbulent boundary layer and establish the kind of 3-D flow you'll encounter 'bigger.'

[3-Wheeler]

Originally Posted by aerohead

Stefan,your results may work out better in full scale.At 1/24-scale,your fan would have to deliver 480-mph air to develop a turbulent boundary layer and establish the kind of 3-D flow you'll encounter 'bigger.'

Hi Aerohead,

Do you have an equation that shows the relation between generated Reynolds number for full size objects and those of 1/24th scale?

Just off the top, 480 mph seems too fast for some reason. At a Mach value of 0.3 (225 mph), the air is starting to reach the "non-compressible" zone.

I know I've read here that the turbulent zone is fully established at speeds of only 20 mph or so for full size objects.

Jim.

[aerohead]

Originally Posted by 3-Wheeler

Hi Aerohead,

Do you have an equation that shows the relation between generated Reynolds number for full size objects and those of 1/24th scale?

Just off the top, 480 mph seems too fast for some reason. At a Mach value of 0.3 (225 mph), the air is starting to reach the "non-compressible" zone.

I know I've read here that the turbulent zone is fully established at speeds of only 20 mph or so for full size objects.

Jim.

Jim,I think it's just a scale relationship,or 'verisimilitude' as I think Don Sherman referred to it.
If the typical full-scale auto length/velocity @ 20 mph establishes the proper Reynolds number then,
*1/2-scale requires 40-mph
*1/4-scale 80-mph
*1/8-scale 160-mph
*1/16-scale 320-mph
*1/24-scale 420-mph
-------------------------------------------------------------------------
Lower air velocities can be used and a 'fudge-factor' arrived at.
When GM uses the Guggenheim tunnel at Cal Tech the Cds come out as much as 30% lower than what will happen in full-scale.
As long as a relationship is determined between the model test and full-scale results,a 'confidence' can be arrived at when predicting full-scale numbers.
Paul Jaray wrote about this issue back in the 20s.He used 1/10-scale models.
--------------------------------------------------------------------------
Without the correction factor you could end up with disappointing Cds as did Ledwinka with the Tatra,Tremulis with the Tucker,Breer with the Airflow,others I'm sure.

[Sven7]

Originally Posted by aerohead

Jim,I think it's just a scale relationship,or 'verisimilitude' as I think Don Sherman referred to it.
If the typical full-scale auto length/velocity @ 20 mph establishes the proper Reynolds number then,
*1/2-scale requires 40-mph
*1/4-scale 80-mph
*1/8-scale 160-mph
*1/16-scale 320-mph
*1/24-scale 420-mph
-------------------------------------------------------------------------
Lower air velocities can be used and a 'fudge-factor' arrived at.
When GM uses the Guggenheim tunnel at Cal Tech the Cds come out as much as 30% lower than what will happen in full-scale.
As long as a relationship is determined between the model test and full-scale results,a 'confidence' can be arrived at when predicting full-scale numbers.
Paul Jaray wrote about this issue back in the 20s.He used 1/10-scale models.
--------------------------------------------------------------------------
Without the correction factor you could end up with disappointing Cds as did Ledwinka with the Tatra,Tremulis with the Tucker,Breer with the Airflow,others I'm sure.

Thank you very much, Phil. My friend and I were thinking about testing his 1/4 scale clay model, and this information is very useful. I have a couple quick questions, though.

First, what size should the wind tunnel be? I imagine building one for a quarter scale model may be challenging if we can't find a dedicated space for it. Scaling that cardboard one up by a factor of six could take up an entire room. (The school has the space, but perhaps the CCS administration wouldn't appreciate that ) We would probably have to make it out of MDF or some kind of particle board.

Second, and this one is for onefstek too, how does one make the smoke and get it in that neat stream? I've seen the needle like tools they use in the full scale tunnels but don't really know where they get them or what the smoke is from.

Edit: does anyone know how fast box fans run? Edit edit: 2-8mph. I assume to speed the air up to 40mph you'd need at least a 3x3 array of fans on the back end of the tunnel, with quite the funnel on the other end. Perhaps there's a better solution?

Blah, questions keep popping into my head. I suppose it's pointless unless one can't record drag, but how does one figure out the "fudge factor"? All the stuff I'd like to try is either over my head or beyond my resources!

Thanks! Tyler

[aerohead]

Originally Posted by Sven7

Thank you very much, Phil. My friend and I were thinking about testing his 1/4 scale clay model, and this information is very useful. I have a couple quick questions, though.

First, what size should the wind tunnel be? I imagine building one for a quarter scale model may be challenging if we can't find a dedicated space for it. Scaling that cardboard one up by a factor of six could take up an entire room. (The school has the space, but perhaps the CCS administration wouldn't appreciate that ) We would probably have to make it out of MDF or some kind of particle board.

Second, and this one is for onefstek too, how does one make the smoke and get it in that neat stream? I've seen the needle like tools they use in the full scale tunnels but don't really know where they get them or what the smoke is from.

Edit: does anyone know how fast box fans run? Edit edit: 2-8mph. I assume to speed the air up to 40mph you'd need at least a 3x3 array of fans on the back end of the tunnel, with quite the funnel on the other end. Perhaps there's a better solution?

Blah, questions keep popping into my head. I suppose it's pointless unless one can't record drag, but how does one figure out the "fudge factor"? All the stuff I'd like to try is either over my head or beyond my resources!

Thanks! Tyler

*If you do a closed test section,the models projected frontal area should be no more than 5% of the tunnels test section cross sectional area.This prevents the walls from effecting streamlines.
* I'm not current on smoke generator state of the art but in past times,an oil like SHELL Ondina was electrically heated within a metal wool,flashing off as a cloud of smoke into a vessel which was in series with the tunnel.Air from the tunnel enters the vessel at test section speed,picks up the smoke and enters the smoke wand or smoke rake,discharging into the section airstream at an equivalent velocity so as not to create its own turbulence.
*As of 1991, commercial smoke generators started at around $1,100 (US).
*Your fan(s) will need to develop a volume of air equal to your test section velocity (80-mph) multiplied by the section area ( 100 sq ft ),or,704,000 CFM.
*A local HVAC company could help you with static pressure requirements,friction losses and necessary horsepower to drive it all.
*And if you do measurements you'll need to construct a 3-component balance.Texas Tech used the 'sting' from a military aircraft.It has strain load cells of 3-axis.$68,000.
* Alan Pope has a fine book on low speed wind tunnel design.

[Sven7]

Wow! That sounds like a ton of work and money. Maybe we won't be able to do it after all.

As much as I'd love to build a wind tunnel, openfoam sounds a lot easier, cheaper and of course more space friendly.

Thanks for all the info!

[KamperBob]

Originally Posted by aerohead

*If you do a closed test section,the models projected frontal area should be no more than 5% of the tunnels test section cross sectional area.This prevents the walls from effecting streamlines.
* I'm not current on smoke generator state of the art but in past times,an oil like SHELL Ondina was electrically heated within a metal wool,flashing off as a cloud of smoke into a vessel which was in series with the tunnel.Air from the tunnel enters the vessel at test section speed,picks up the smoke and enters the smoke wand or smoke rake,discharging into the section airstream at an equivalent velocity so as not to create its own turbulence.
*As of 1991, commercial smoke generators started at around $1,100 (US).
*Your fan(s) will need to develop a volume of air equal to your test section velocity (80-mph) multiplied by the section area ( 100 sq ft ),or,704,000 CFM.
*A local HVAC company could help you with static pressure requirements,friction losses and necessary horsepower to drive it all.
*And if you do measurements you'll need to construct a 3-component balance.Texas Tech used the 'sting' from a military aircraft.It has strain load cells of 3-axis.$68,000.
* Alan Pope has a fine book on low speed wind tunnel design.

Phil is right no doubt. (Since I recently had the privilege of meeting him, which was so worth the effort, I'm more convinced than ever.) All the experts fixate on Re and it makes perfect sense. That said, the Wright Brothers' wind tunnel produced extremely useful data. Also, considering orders of magnitude between jumbo jets and tiny insects, Re scaling does not seem to prevent flight. That leaves me to hope that, while scale wind tunnel testing without full Re compensation might not be perfect, it may be good enough to compare some shape effects. That could make me the most naive person on the planet for all I know (but I can also think of worse problems - wink).

[aerohead]

Originally Posted by Sven7

Wow! That sounds like a ton of work and money. Maybe we won't be able to do it after all.

As much as I'd love to build a wind tunnel, openfoam sounds a lot easier, cheaper and of course more space friendly.

Thanks for all the info!

Build your models,attach them to the hood of a car on a ground board,tuft them,drive and video record what happens.The Rutan Brothers do this out at the Mojave Airport where their Scaled Composites is located.
With limited resources we've got to skin cats creatively.

[Sven7]

I was actually planning on doing something very similar to that... great minds think alike?

Might be difficult with a 100lb clay model

[jime57]

And don't forget the weight of the smoke machine, which has to project out front by a ways