Believe it or not, there are only a handful of criteria you need to use when selecting a wind tunnel facility for a test.
There are a surprisingly small number of tunnels in the world, almost all of them unique and specialized. Your options substantially narrow with every criterion you use to choose between them.
Here’s the rough order of evaluation that I’d recommend as you go about this:
Location. The point on the globe where the tunnel facility sits is of high importance. If you’re making a commercial product and have the budget available to travel internationally, the world is your oyster.
On the other hand, if you’re working on something that’s defense or military related in any way, I can almost guarantee you’ll need to stay within your country.
Speed. How fast does the air need to blow? If your vehicle is meant for subsonic, transonic, or supersonic flight, that will restrict you to a certain subset of tunnels.
Something to consider is not just the documented speed range of a wind tunnel, but also the range in which it was designed to operate best. As an example, Calspan’s Transonic Wind Tunnel in Buffalo, NY advertises online that it can operate at Mach numbers of 0.2 to 1.3. In practice though, the tunnel is happier operating in the middle of that range.
I’ll also note that you should find a tunnel in the speed range you need for this specific planned test. Many aircraft collect data for cruise conditions in a transonic or supersonic tunnel, but then visit a subsonic tunnel to get takeoff and landing data.
Size. Test section size determines how large a model you can actually, well, test. Low-speed wind tunnels often have the largest test sections at around 12 ft wide by 8 ft tall, allowing for nice big models and full-sized small UAVs. You can even “cheat” a little and get a larger model by only making half of your vehicle and mounting it sticking up from the floor.
Transonic tunnels usually have slightly smaller test sections. Supersonic tunnels have the smallest test sections of all, sometimes only 3 ft by 3 ft. Makes sense when you think about just how much air they’re pushing through that tunnel at two or three times the speed of sound.
You’ve likely narrowed your list down to maybe three or four solid options after going through these criteria. At this point you can start taking other differentiators into account: do they have a continuous power source available for propulsion testing, or would you need to bring your own? What security protocols are in place? How wide of angle of attack and sideslip ranges can they accommodate?
It’s also smart to reach out to these final contenders to see what their schedules look like. Tunnels are often booked out at least three months in advance; some popular ones, like Calspan, are booked a year or more into the future. Your first choice might not even be available when you need it.
And never hesitate to get in contact with staff if you need clarification or more information on anything. Every tunnel operator I’ve ever met has been happy to help folks understand their tunnel’s capabilities.