Summary: This is an instructable for a blowdown, open circuit, climatic, wind tunnel driven by a leaf blower and provided by a fog stream produced by a garden humidifier.

The World Health Organization (WHO) estimates there are 844 million people who lack basic drink water services. The exploration of unconventional water resources that have not yet been utilized, such as the water in the air, is necessary to meet (future) water demands.

Fog harvesting is a low cost, low tech, drinking water solution for arid regions where fog occurs on a regular basis. Most fog water collectors consist of a large mesh-like screen held by poles or a frame, perpendicular to the wind-driven fog.

For my MSc. thesis project at the technical university of Delft I researched the aerodynamics of fog harvesters. I built a climatic wind tunnel with the purpose to validate CFD results. This instructable outlines the construction of the set up I used. This wind tunnel can be used to test the designs of future fog harvesters, or any other project where you need to test the interaction of a stream of water droplets and an object.

Your wind tunnel might not turn out exactly the same because you are working with a different purpose, circumstances or resources. This instructable can still serve as useful inspiration for anyone building their own wind tunnel.

Many thanks go to to the people over at Sciencebuddies and the Instructable user beltenebros of rLoop, for your designs have helped me a lot with constructing this tunnel.

Sciencebuddies wind tunnel: https://www.sciencebuddies.org/science-fair-projec...

rLoop: https://www.instructables.com/id/DIY-Subsonic-Wind...

If you would like have more in-depth knowledge on low speed wind tunnels, the book "Low speed wind tunnel testing" by Jewel B. Barlow, William H. Rae and Alan Pope, is a great resource.

Step 1: Overview Design

The two most basic things needed to build a wind tunnel are something that makes wind and some sort of tunnel. In the case of this setup, also a fog stream has to be introduced.

The purpose of your tunnel and your budget will largely determine your design. I wanted to keep the costs as low as possible, so I didn’t want to build a very large tunnel, yet not too small that scaling would become a problem. To keep the Reynolds and Stokes numbers in the right order of magnitude, wind speeds of 3 to 5 m/s and prototype dimension of 10cm x 10cm are necessary. The frontal area of the scale model should not cover more than 10% of the area of the test section, this means the test section should be at least be 30cm x 30cm. I decided on a length of 60cm to leave enough space for the wake to develop behind the prototype.

For small scale, low speed, wind tunnels, such as the one made here, fans are often used to make the wind. Although the climate room I could work in was a godsend, it also meant that I could not work with anything that can’t stand that kind of levels of wetness, such as an electrically powered fan. Luckily there was a hole in the wall of the climate room that exactly fits the outlet of a leaf blower.

The wind tunnel consists of five parts: the inlet, a diffuser, a straightener, a contraction cone, and the test section.

From the sketch on the wind tunnel, you can get a rough idea on the measurements. The ratio between the different parts is most important, the measurements can depend on your purpose and available budget.

Step 2: Materials

Materials:

* leafblower (Check in the specifications if the provided power will be sufficient, most manufacturers provide you with the wind speed and outlet area, you can use this information to estimate the discharge and wind speed in you test section, take into account that some losses will occur)

* tropical plywood (I specifically used tropical wood because it is better resistant to water)

* triangular beams for connecting the corners (Triangular because this causes less disruption of the air stream than rectangular beams)

* wood dye/staining to make water resistant

* 40cm x 20cm plexiglass

* 1 garden hose sprinkler mist system --> see picture

It is estimated that these low-pressure mist systems produce droplets with a size of about 100 micrometres, pressurized nozzles will produce smaller droplets.

* 2x egg crates

* screws

* wood glue

* 16 springs

* air sealing strip

* 1 role of duct tape

* 4 hook screws

* tie wraps

* double-sided tape

maybe (depending on your purpose and location):

* flexible air ventilator tube

* rain gauge

tools

* jig saw

* drilling machine

Step 3: Diffuser, Straightener and Contraction Cone

These three sections are the most straightforward to build, so a good place to start. All three follow a similar process, the only difference is that you need to add the egg crates to the straightener section.

The diffuser is there to make the air stream coming out of the leafblower wider and mix with the water droplets. In the straightener, turbulence is removed from the incoming air at a lower speed with the egg crates. In the contraction cone, the air is accelerated again and pressure differences are removed.

1) Saw the plywood and beams to size --> see sketch

2) Connect each of the four boards that belong to one section to create a tunnel form. I did this by first glueing one beam to two boards and let it dry. And then do the other two boards with another board. Then place the two L shapes together with the last two beams. It is nice to do this together with another person but not necessary.

After it is all glued together reinforce the connection with screws. Make sure to pre-drill.

4) Make the section water resistant with some sort of wood seal or dye and let it dry overnight. Add an extra layer for security if you have the time.

5) To make sure the wind tunnel has a modular set up that is easily taken apart and moved around the sections are connected with springs. Therefore add screws on the outside of all the corner edges so the springs can hook behind them. Think about the alignment between the different section. (In one of the pictures you can see what it is supposed to look like in the end).

6) Because the sections won't fit together perfectly you can add airstrips to all the edges to compensate for the slight irregularities. If your airstrip is a bit wider or thinner than the plywood plate it is not a big problem, just make sure the inside edges are aligned to avoid disruption of the air stream.

At last, you need to add the egg crates to the straightener. Cut or saw the egg crates to the right size, cut the corners off to fit with the beams. I placed each egg crate at 1/3, equally spacing them, in the straightener. If you cut them to be a good fit they already jam a bit, I added high quality (i.e. able to withstand a lot of water) double-sided tapes to keep them in place.

Step 4: Test Section

Depending on the use of your wind tunnel your test section will differ. The basics are the same, but, because I used this tunnel to test fog harvester prototypes I had to make an opening in the bottom for the drainage of the captured water and I added hooks to suspend the prototypes with.

I chose to make the test section out of plywood and make a window with plexiglass so I could observe what was going on. I have also seen designs where the whole test section was made out of plexiglass.

Think about what would suit your application best. The following instructions are based on the test section that I built.

1.1) Saw your plywood boards to size. Four boards of 60cm x 30cm in total. In two of them make openings, one for the drainage of water, the other one for the plexiglass window. Make sure to make the opening before putting them together, it is easier. Saw your beams to size. Also already add the hook screws on the place where you want to suspend your prototypes. It is a bit of hassle to put them in when your test section is already put together.

I made sure the prototypes could be suspended at 1/3 of the test section because then there is 2/3 of the test section left for the wake to develop.

1.2) Later I made the drainage area underneath the prototypes that I was testing larger. I did this by adding a slanted plated on the bottom of the test section. The 3D print for that drain can be found here: https://www.thingiverse.com/thing:3004841 If you don't have a 3D printer I am sure you can make something out of leftover plexiglass or a plate of plywood.

1.3) Because I wanted to measure the wind speed in my wind tunnel, I added a hole at the beginning of the test section. The beam of a 3 cup anemometer would fit through the hole and I was able to determine the wind speed inside the tunnel. (See picture)

2) Assembly the four boards with the beams in the same way as you did for the other sections.

3) Make the section water resistant with some sort of wood seal or dye and let it dry overnight. Add an extra layer for security if you have the time.

4) Add the plexiglass to create a window in your test section. I added the plexiglass on the inside of the test section and secured it with screws. I made sure the plexiglass would cover exactly one whole side of the test section so the side would still be one smooth side and the airstream wouldn't be interrupted.

5) At the edge that will be connected to the contraction cone, add the screws for hooking the springs and the airstrip.

Step 5: Inlet

Just as with the test section, your inlet might differ from mine depending on what you have available and the purpose of your setup. I was working in a climate room and had to bring in the wind through a hole in the wall with a diameter of 12cm, the leaf blower was perfect for that. I added the mist at the inlet too.

Think about what would suit your application best. The following instructions are based on the inlet that worked best for my application.

1) Saw plywood and beams to size, add the hole where the tubing of the airstream will go through. The tube that I used had a diameter of 12cm, by making the diameter of the hole in the inlet a bit smaller the tube will jam.

2) Add holes in the side boards for the nozzles of the garden mist system to go through. The diameter of the holes will depend on the nozzles that you are using.

3) Assemble the boards with the beams in a similar way as with the previous sections.

4) Make the section water resistant with some sort of wood seal or dye and let it dry overnight. Add an extra layer for security if you have the time.

5) At the edge that will be connected to the diffuser, add the screws for hooking the springs and the airstrip.

5) Add the nozzles to the inlet. I was able to suspend the mist-making nozzles in the inlet by taking the nozzle part of the tubing, placing the tubing through the holes you made earlier and placing the nozzle back on the tube. (See picture)

6) Add the tubing for the airstream. As you can see in the picture, I added 4 small wooden blocks on the outside of the inlet around the tubing. I did this to make sure the airstream would enter the inlet parallel to the floor and not slightly up or down disturbing the air stream.

Step 6: Assembly and Operation

With all the section built, made water resistant, screws for the springs, and airstrips in place it is time for to assemble them all.

1) Place all components end-to-end in the place where you are going to use the device. Be sure you can attach the hose for the mist nozzles to a tap close by. You will notice that all parts are not the same height and you will need to lift some parts. You can construct nice stands. Or make it easy for yourself and place some scrap pieces of wood underneath. For me, that worked fine.

2) One by one, connect the sections by their connecting screws, using the springs and make sure the inside of the tunnel is as smooth as possible.

3) You might notice that the transitions between the boards and beams aren't that smooth and the connections aren't too good either. Therefore, each time I added a new part I smoothed the corners and connections with duct tape. It is an easy and quick solution to ensure a smooth airflow inside your tunnel. Beware! Don't go cheap on your tape, choose a high-quality brand that can resist the water.

4) Connect the nozzle system to the tap, plug in the leafblower and start testing!

5) Enjoy!