Sunday, December 27, 2020

High Speed Wind Tunnel for KidWind


This post is going to cover the development and evolution of the shroud design to increase wind velocity in the standard KidWind tunnel. After considerable testing and experimentation it was determined that the best way to increase the velocity was to reduce the inlet opening of the wind tunnel. Pictured here is the simplest of designs, a 32" diameter opening made out of flat cardboard! This brought the wind speed up from 4.5 m/s in the standard KW wind tunnel to a whopping 8.5 m/s! The early resulting out put of a turbine placed in this wind speed was considerable and approached 400 Joules of energy in 30 seconds with a 30 ohm load.

Could this "plain Jane flat cardboard" be the best design?  All the professional wind tunnel designs were more elegant and looked more scientific. So I began my improvements. Not so much for velocity but for the "looks".

I would go for a radius inlet opening. This was modeled after the successful larger tractor tire inner tube testing that was done earlier in the year. After deciding on using styrofoam for the material and solving the technical problems as to how I would cut and shape it with a hot wire construction began.

The optical result over the flat cardboard was impressive. The wind velocity still below the flat cardboard by about 1 m/s. To address this it was suggested that an extension tube (shown in pink) be added to the exit side of the radius. This was done and various lengths were tried. The longest length, about 12" was best. Test wind speeds were measured with a standard anemometer on the end of a stick. This got me  thinking about the testing method and I decided to shift to Joule output. I would keep the blades and generator the same for all testing. To my surprise the Joule output with the flat cardboard and radius opening shrouds were the same! My guess is that the turbulance caused by the flat cardboard design was the cause. This seemed to be demonstrated visually and graphically on the Vernier voltage display screen. So this radius would be the final design.


 Since the idea is to use the standard KW wind tunnel for regular KW Challenge testing and then add on the shroud for some "serious fun" testing at higher wind speeds after the main KW challenge I needed to keep the same dimensions inside the standard KW tunnel when the radius shroud was in place. A 3/4" plywood with a 32" diameter hole was hinged to the entrance of the standard KW wind tunnel. It was hinged on one side to allow it to be swung open so teams could  get inside and set their turbine for testing. 2x6 extensions were added to hold the radius portion of the design.

2X6 extensions and plywood hinged in place. You can see the turbine inside through the 32" diameter opening.

Important to note is the tight fit of the 3/4" plywood to the opening of the standard KW wind tunnel as shown here. No gaps mean fewer losses in wind speed.

The final step was to build the tube between the 3/4" plywood and the radius shroud opening. I choose to us light gauge metal coil stock.

The end results from the outside looks impressive and has the desired "cool factor".

What it looks like from the inside of the tunnel.

And with the high velocity shroud swung open allowing team access to the turbine for placement and set up.

The results
The Joule output numbers tell the story. 6 blades (110 mm wide, 520 mm long with 15 degree twist) NACA 2415 airfoil connected to a 30 watt generator with 1:30 gear ratio and a 45 ohm load.  Output: 35.957 volts, .753 amps, 27.075 watts and 807.51 Joules in 30 seconds! Sweet

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