aerohead's baby wind tunnel

[aerohead]

Denis,in France asked about this,so I thought I'd do a separate thread in case any of you knuckleheads are toying with the idea of building one.
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I did the project over two weekends,with fabrication of the turning-vanes done by a local sheet metal shop during the week between.
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I used materials recycled off remodel jobs, scrounged at flea-markets, or found at yard sales.I've probably got $100(U.S.) in it for materials.
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Here are some particulars:
*Design type: modified Eiffel,with open test section,full recirculation
*Height 4-feet ( 1200mm )
*Width 16.75-inches ( 418.7mm )
*Length 8 feet ( 2400mm )
*Power 115-120 VAC,60 Hz,single phase,approx. 1-horsepower
*Fan: domestic HVAC centrifugal blower
*6-element, airfoil,constant-area turning-vanes @ three turns
*1/2-inch by 1/2-inch ( 13mmX13mm ) aluminum egg-crate flow straightener/turbulence-generator @ jet
*16-inch by 8-inch ( 400mm X 299mm ) jet
*test section: 8-inch by 16-inch by 35.5-inch (200mmX400mmX887.5mm)
*return air inlet: 16-inches by 12-inches (400mmX 300mm) with egg-crate debris guard
* flow rate - 1,800 cfm
* flow velocity 2,024 fpm (10.28 m/sec)( by Pitot-tube traverse/Magnehelic)
* application: flow visualization only,no force measurements
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Construction:
*side panels - 3/8" (9mm) plywood
*perimeter enclosure and test section - 5/8" ( 15mm ) plywood
*carpenter's glue/drywall screw butt-joints
*primer,2-coats top coat
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In his book,'Low-Speed Wind Tunnel Testing,' Alan Pope recommends a 'student' wind tunnel of 14"-30" ( 350mm-750mm) square jet,requiring 20-25 horsepower,and 100 mph airspeed ( 161 km/h )
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My tunnel gives me an otherwise 'impossible' look at flow.With tufts and a tuft-wand ( hobby shop model airplane aluminum streamline strut with yarn tuft ) I can observe for separation,realizing I have compromised Reynolds number in affect.
------------------------------------------------------------------------- I'll post photos as soon as Al can get away from his real work.
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Okay,photos are up.
As you can see,she's very simple and primitive.She does produce very good laminar flow across the models.Air is rock steady.
The second image is looking through the return inlet debris guard,showing one of the airfoil turning -vane sets.If you're considering building a tunnel I recommend these,as I don't think you can achieve laminar flow without them,and they give your electric meter its highest fuel economy!

[Bill in Houston]

too cool.

[Arragonis]

I posted something similar in a "what is your impossible project" thread a while ago, something based on a fish tank and a movable platform with a fan blowing through it. I didn't know about the other ideas though.

Do you have bigger pictures you could include in another post as this is very very cool.

[Frank Lee]

I use a leaf blower or a garden hose for my aero testing.*




*not

[ChrstphrR]

Phil,

I'm curious why you had the test section open, as opposed to having it closed in with say, acrylic or plexiglas?

The mention that it's for flow visualization only... only begs the question, do you have pictures of you smoke testing your models too?

Furthermore, what would have to be added to do force measurements, so that Cd figures could be deduced?

[aerohead]

Originally Posted by Arragonis

I posted something similar in a "what is your impossible project" thread a while ago, something based on a fish tank and a movable platform with a fan blowing through it. I didn't know about the other ideas though.

Do you have bigger pictures you could include in another post as this is very very cool.

Arragonis,the photos reflect the largest file size EcoModder.com can handle,so I'm stuck at this level of resolution.Sorry!
I don't do Photo-Bucket yet.Without a computer at home all this internet stuff is a challenge.

[aerohead]

Originally Posted by ChrstphrR

Phil,

I'm curious why you had the test section open, as opposed to having it closed in with say, acrylic or plexiglas?

The mention that it's for flow visualization only... only begs the question, do you have pictures of you smoke testing your models too?

Furthermore, what would have to be added to do force measurements, so that Cd figures could be deduced?

ChrstphrR,my first tunnel had a Plexiglas sidewall/access panel in to a closed test section.
It was a single-pass tunnel,exactly like the Wright Brothers used.
It had very little utility,as once the section is closed you are limited in how you can interact with the model inside.
With the open test area I'm free to move the tuft wand anywhere I choose,even change models without turning off the power.
Tunnel 'blockage' issues are not as prevalent as the flow is not confined.
A commercial smoke generator,in 1990 dollars are $1,100 ( US) and all the 'air' is wasted with the smoke and cannot be recirculated as it will soon foul the vision as food color will in a water tunnel.
Alan Pope recommends a 20-25 hp 17-foot length tunnel as a 'minimum' for a classroom,with a minimum 100 mph airspeed.I can't afford to operate anything like that.
Model scale verisimilitude for Reynolds number dictates a jet velocity @:
* full-scale 20-mph + air
* 1/2 scale needs 40 mph +
* 1/4 scale ------ 80 mph +
* 1/8 ------------ 160 mph +
* 1/16 ----------- 320 mph +
and so on
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The easiest way to do pure drag measurements is to suspend the model just above the tunnel floor on a wire-hung,pendulum ground board of streamlined construction which has been configured for proper boundary layer effects at the 'ground',and independently tested to determine its own aerodynamic 'tare' drag.
Another wire connects to the pendulum running over a low drag pulley,down to a scale or weight bucket.
When a model is tested on the pendulum,the force is 'balanced' and measured,then the 'tare' is subtracted out,leaving the drag force.
Knowing the frontal area of the model allows the Cd to be determined
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If you want to measure lift and yaw,a triple balance of precision knife edges must be constructed,or of strain gauges.Texas Tech has a "STING" out of the tail of a military jet which has three-axis strain gauges embedded within it.My memory remembers a figure like $66,000 for that piece of equipment.