Hi, this is my first post. I teach high school physics and am trying to build a simple wind tunnel. For my first attempt I used a Utilitech high capacity 20 inch fan (nominal 6800 CFM) but the resulting air speed was disappointing – only 4.8 m/s. What type of fan would be better? I was thinking of trying a high-capacity, whole-house fan. A second issue is that I put a funnel in front of the fan: 20 inch diameter on the large end and 12 inch on the small end. I measured the average air speed directly in front of the fan to be 4.9 m/s so that the equation of continuity shows that the speed at the small end should be about 14 m/s, but I measured it to be 5.6 m/s. I also tried to use a cylinder and found similar results: the speed dropped from 6.6 m/s in front of the fan to 4.8 m/s on the opposite end, even though nominally the speed should be the same. I thought that perhaps there is a build up of pressure to explain the lower speeds which would produce a higher density. With these low velocities I suspected the flow would be relatively incompressible, but perhaps not? But the numbers do not work out: the increase in pressure is not enough to produce a higher density unless the air temperature drops significantly – which is not reasonable. Regardless of the fan used, shouldn’t the equation of continuity still work and produce a higher air speed through a smaller opening? Below are photos of the wind tunnel under construction. Any help would be greatly appreciated!