I am trying to find a solution for my small wind tunnel that will allow me to maintain a set wind velocity of 0.06 – 0.3 m/s within the tunnel. Currently, I am using a 5v computer fan w/a potentiometer, but I can’t get the fan slow enough. It just stops turning. I am also using a simple telescoping hot wire anemometer to verify wind velocity.
Most computer fans are brushless, which means that they are either "ON" or "OFF". If you want to control the speed, you'll need a PWM controller for the fan so that you can control the on-off at the rate which will give you different speeds.
To get the full torque of your motor, you will need a PWM control. What I said about the on or off is not correct - your fan will turn slower with a lower voltage. However, a pot control (unless it is a 2 ohm reostat) will likely starve your motor of the current it needs.
I would consider the possibility that the used brushless fans simply don't support the intended slow speed. PWM won't help much in this case, unless you accept a few Hz "stutter" operation as continuous pwm. Lookout for special brushless motors with speed control input.
0.06m/s ..0.3m/s is just a 1:5 ratio.
Therefore i recommend to run the fan at full speed and us some mechanical wind shiled to get the desired 0.3m/s (maybe add a bit of headroom).
Then reduce the voltage and see if you reach the 0.06m/s. If not then use another fan with more dynamics.