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

Tuesday, December 15, 2020

Cabin Doors Made and Hung

 This was the last of the windows. The center window is fixed and the two side ones open to the inside of the cabin. These windows face the West but will be well shaded by the 3 foot roof overhang.

The glass for the doors needed to be tempered and took a bit longer to get. The challenge of making the frames for the doors was a little more complicated as there was no center post and I wanted the doors to interlock. With three layers of material this required some serious planning. First I made a "story stick" for the exact width and height of the 6 x 6 door frame.

In the sketch you can see the stack-up of the three layers. The yellow side door would be the door least often used and would be pinned in place at the bottom and top. The pink side door would be the primary door and the interlock needed to be constructed accordingly. The 11/16 inch glass would allow for a foam seal on each face. Stock 3/4" thick would be used to surround the glass. It would be cut to be flush with the frame on three sides. The mating side of the doors would need to be staggered as in the sketch. The 1 x 6 cedar material would make up the frame and hold the glass in place. All this was drawn full size on the "story stick" and made the job possible.

The doors were quite heavy, weighing in at about 75 pounds each so I transported them to the cabin on a sled since I could not drive up to the cabin with all the snow.

Doors hung. Mission accomplished. Very nice light and good view.

Looking at the cabin from the S.W. 

Looking at the cabin from the N.E.

Well this just about wraps up the cabin work for this year. May experiment with sizing up the BTU's needed for the propane fireplace that will be used for heat. Will have to think about that for awhile.

Wednesday, December 9, 2020

Gable Ends and Windows


A window was added to the South end to let in more light. Then the gable end was insulated and covered with salvaged barn board.

The North end of the loft was insulated and then covered with 1 x 8 tongue and groove pine to match the ceiling. In both ends the boards were set back to expose the 6 x 6 post and mortis and tenon brace work.

With the interior pretty much in hand it was time to move on to the windows. Thermo pane glass panels with interior dividers were ordered. The planning for this required a considerable amount of measuring and thinking.

The glass panels would be sandwiched between two layers of cedar. These layers had to be the correct size so as to fit into the cabin window framing. The inside of the layer had to be large enough to keep the glass in the frame. Lastly there needed to be a spacer the thickness of the glass between the two layers. This spacer had to be just the right size to fit the glass on the inside and be flush to the edge of the top and bottom layers to make the sandwich. Also the corner Lapp joints had to be alternated to provide the needed strength. Pocket screws were used to assemble the first layer. Then the spacers were nailed and glued in place. The glass was inserted and then the top layer was screwed on to finish the sandwich.

After a couple of hours the first two windows were framed and turned out well. Now it was out to the cabin to hang them.

Everything in the cabin is built heavy and oversized and the windows are no different. Locating and  inletting the hinges of these heavy windows would be a challenge. Getting them to hang and not bind would the the goal.

Mission accomplished ! Two down, with another three panel window and the doors to go. Should be an interesting next couple of days in the workshop and out at the cabin.