Wind Tunnel Design Question Re: Test Section and Diffuser/Fan Size

Ryan29

My original choice was actually to use PVC pipe with a couple of windows cut out and some lexan over top, but the largest diameter I could find readily available was 4", which is smaller than I'm willing to use because in a classroom full of kids, it just wouldn't provide enough visibility.

I'll talk to some plumbing supply stores and see if I can get my hands on some 8" or 10" PVC pipe. Shoving a duct fan booster in one end and a flow straightener in the other would sure be a hell of a lot easier than building out what I'm planning now.

etudiant

If PVC pipe is too hard to get in the right diameter, maybe just use ducts.
I don't think that the flow would be disrupted if you made some large cutouts in it and bent thin plastic sheet to cover it. Even the plastic from loose leaf separators would give an 8x11 inch window. You could then just turn the model 90 degrees to show the top of the flow as well as the side view.
Please keep us posted.

Aero51

Speaking of ducts..why not air conditioning ducts! They come in all shapes and sizes! Also, if you really wanted to you could heat the PVC then form it.

AlephZero

I would be careful about taking too many "short cuts" because "this is only demos for kids". First, because demos that don't work properly can be worse than useless, and second because (as the OP obviously already knows) if the Canadian air cadet organization is anything like the UK one, some of those "kids" will probably end up on the fast track to becoming air force pilots.

The NASA site you linked to has detailed plans for a few different designs. They maiy look more complcated that you hoped for, but all that stuff is there for a reason. And after a while you will probably want to progress from qualitative demos to actually measuring something...

There are good reasons for wanting to make something small and portable, but (within the limits of common sense) bigger, and higher air speeds are easier to use and more capable. If nothing else, if you can't use hand tools with both hands insde the working section of the tunnel, you are making it hard for yourself setting up experiments and demos!

Ryan29

I'm looking into some large diameter PVC pipe. I'll make some calls tomorrow. If I can get 10" or 12" diameter pipe, then the booster fans make a very nice fit and a very good CFM compared to the fan system I was planning to use. Unfortunately, the downside is that large diameter pipe only seems to be sold in 10' lengths. I don't need nearly that much and the expense is significant.

Ryan29

Certainly a good point. It's a bit of a catch 22 though because the Cadets meet weekly at a school, not at their own permanent location, meaning the tunnel will need to be brought to cadets and assembled when needed, thus it absolutely has to be portable and modular. Otherwise, I can build it as big as I want, but the cadets won't be able to use it.

That being said, the design I've done the most planning around is essentially the Baals wind tunnel as demo'd on that NASA site. The only difference being that mine will be smaller, and the fan system will be smaller. The modular design I'm going for allows me to test that, and then if the larger fan is absolutely necessary, I can remove the old diffuser/power module and build in a new one with a larger, more powerful fan.

As it stands though, for pure flow rate, the large diameter pvc pipe driven by the duct booster fan seems to be by far the best option. I'll have to continue my research and decide on one method or the other once I've got some more complete info on sourcing the large diameter pipe.

Thanks again to everyone for all the great help this far! I really appreciate what's been offered up, and please, post any other thoughts that come to mind!

etudiant

The construction and utility companies often have that size pipe on their jobs and after cutting to fit the unused ends are discarded. That might solve both the length and the cost problem.
Air conditioning ducts come in both rectangular as well as round cross section. They are cheap, light and easy to work with.
AlephZero is entirely right that you need good access to the test section, else the mounting of the models is a chore. Maybe just cut the tunnel in half at that point with a hinge, so it opens like a clamshell.
Presumably the NASA documents give some guidance on what instrumentation to consider and how to accomodate them, measurement is the soul of engineering and the kids should get that experience early.

RandomGuy88

Will you be trying some sort of surface flow visualization such as fluorescent oil flow? If so, these techniques may not work well at very low speeds because the shear stress is not large enough.

Keep in mind that if your model is too big relative to your test section then you can have some pretty serious wall effects that will alter behavior of your model. These effects can be severe enough to qualitatively change the flow so it is no longer representative of what the flow should be.

Along the lines of what Alephzero said about taking too many short cuts, you should use the symmetrical inlet to avoid significantly skewing your flow in the est section.

RandomGuy88

Another point worth making is that it will be important to have good quality flow in your test section if you want to do visualization. If your free stream turbulence is too high then smoke flow will be difficult because the smoke will quickly diffuse and you will not get nice clear lines.

Ryan29

I got some quotes on large diameter PVC and sure enough, it's just too damn expensive. I've considered the scavenging route, but I've decided I'm just going to go with my original basic design using a lexan box for the test section and 1/2" cabinet grade plywood for most of the rest of the structure. I've also decided to switch to a car radiator fan for the power, as they're cheap, come in a ton of sizes, are thinline, easy to install, and push a lot of CFM.

As far as access to the test section, it'll have a large lexan access hatch that attaches to the top via wing nuts much like the one on the NASA WMT tunnel. Instrumentation will be left out. Right now the curriculum the kids learn through this organization doesn't include anything that would require them to actually need to take quantifiable measurements (i.e. little to no math).

Ryan29

The plan so far is just to use simple smoke stream visualizations. I can get smoke pellets and run the exhaust through a tube into the tunnel, or I can just use some dry ice and water in a coke bottle and then use a tube to direct it.

Very good point about the wall effects. Right now the test section will have an inside cross section of 8" x 8". The wingspan on a 1:72 F/A-18 is about 6.5 inches. That might be enough space, but it might be a bit much. A 1:72 F-16 would only be about 6 inches, which I assume would be ok.

That being said, the larger plane models will mostly just be a fun thing. The actual important demonstrations will be with airfoil models, which I will construct, so I'll make sure to ensure they have ample space at the ends.

Ryan29

Yea this is definitely one of the reasons I'm building in modular sections. Hopefully the flow straightening grid I'm going to build will do the job well enough, but if not I can always re-design that section, and/or the contraction cone in order to get it better.

Aero51

Oh for the record, in a typical wind tunnel the test item is much smaller than the test section. For instance if you have a 12x12x24 section and you wish to test an airfoil, you airfoil should have a chord of about 4 inches(at most). The reason is that large objects will create pressure blockages in the flow. I do not know if it will affect flow viz but it will affect measurements.

Aero51

How is this project coming along?