BOOMLAB - Sharing the Science of Boomerangs - Boomarology
The Boomerang Wind Tunnel BWT-1
This is a work in progress and probably always be. This is where it is up to.
I came up with the plan when I was stuck in hospital recovering from a burst appendix. I had to optimise it to fit in the space of my attic, but I had lots of time to plan it. I went for an 'open return type' with a closed test section. I decided sub-sonic for my first go.
Using the attic of my home. Probably the reason most people don't do this is a lack of space. Fortunatley we weren't doing much with the attic, and my wife was glad when I said I was thinking of putting some of my 'boomerang stuff up there'. Thanks to fellow boomeranger Jim Millar who helped and gave me confidence when the some of the structural beams needed removing to make space. We made a Scalextric track first but soon it was time for the wind tunnel
I used a hot glue gun to attach hardboard to the contraction frame. It was tricky but with help from Jim it was stuck together. The next job was to use cereal packets to make lots of little boxes to 'comb' the air on the way in. I also had to pinch quite a few menus from my local Wetherspoons bar
Construction under way. As much as possible of the materials were recycled from skips but the fan had to be bought new. I went for the biggest I could afford that runs off 240v. It shifts 2.57 m³/s.
Designing the shape of the inlet contraction zone was a bit mathematical. I got a funky polynomial equation by asking for help from Mazhar Ali and scaled it to my dimensions then plotted it on ply-wood using a ruler and a pencil. The expansion / diffuser part has to be at a 7 degrees angle up to fan. Thanks Oz for helping mount the fan and Mazhar for the equations
The Motor. When the main body of the tunnel was built the next job was to attach a motor to a shaft that spins the boomerang. Thanks to my mate Dave who has a metal turning lathe. I had to buy the motor but the power supply was a Mecano train set transformer I got from a skip. I had to buy a little gizmo to control the speed of the motor too. I found these quite easily on line for about £25 for both
Wind Speed - is measured with a dismantled hand held anenometer like the one shown. They cost about £8. I checked it for accuracy by asking my wife to drive around on a still day while the sensor was stuck out of the window. The readout part is separated by about 2m of wire.
The Spin rate of the boomerang (on the shaft) is measured by using a repurposed bike odometer / speedo that Ren gave me. I bought a Hantek oscilliscope to callibrate the speedo - I thought I'd need a scope eventually so I splashed out about £35 on this
Forces - I bought a Phidget Bridge and strain gauges to measure the forces on the boomerang in xyz directions. The boomerang is attached to a beam that lets it swing and move from an equilibrium poisiton in the centre of the test section. Stiff elastic then pulls on the strain gauges. These are basically what is inside electronic scales which I also experimented with. My brother in law Paul wrote some software for me to take the data from the USB connection and average it out of periods of 1 - 20 seconds to smooth out the vibrations caused by the spinning.
Torques - I need to work out a way to measure how the boomerang wants to twist and turn. I'm working on it for BWT - 1.2
Here is one of those 'panorama' pictures that mobile phones take which makes the diffuser look bulged in the middle, even though it is straight. It is had to take a photograph of the whole thing because it fills the attic. Have a look at the video link below. The whole project cost less than £1000 which is cheap in Wind Tunnel terms. My wife was very suprised when she went in the attic to get down the Christmas decorations. "Thats a big wind tunnel!" she said but actually its quite small. The next one will be twice the size. Stay tuned for BWT -2.0 ( Some people call it the Big Waste of Time)
Controls. I have a little control section to alter the wind speed and boomerang spin rate.
The angle of incidence to the air stream has to be manually altered by re-attaching the motor to the support arm which is quite a faff. A lot of these photos were taken with the boomerang inclined at a high angle of 40 degrees, which I recon is about all the tunnel has room for.
Strobe. I also rigged up bright LED lights that were switched by the rotating shart to make a 'strobe' light to 'freeze' the boomerang's appearance at any point in its spin. Hopefully this will help with flow visualisation, when I get a powerful enough smoke machine. This one came from a car boot sale but doesn't put out enough smoke / mist. Theres a video of the strobe working at the bottom of this page
The Test Section is 450mm across, 350mm high, and 500mm long. Its short because of the space limitations and has a small cross section to get a decent speed. I might make it less high to increase speeds but that will require a rebuilt in the condenser and the diffuser.
Maximum air speed is 14 m/s or about 32mph through the section. Optional flow straighteners (blocks of drinking straws can be inserted just before the test section but the reduce the maximum wind speed to about 9.5 m/s. I have not measured the turbulence yet but I expect it is pretty high by wind tunnel standards.
I am getting results for only one boomerang at the moment. It takes a while as therer are so many things to measure and so many things to change. There are dozens of graphs and hundreds of results to analyse. I need the help of some one brainy - Maxwell for my Faraday ha ha