Is it possible to increase an air column's diameter over a short distance? Let me explain:
Imagine a small 10 cm electric computer cooling fan that's pushing air into a cone-shaped duct 20 cm long. If the exit side of the cone-shaped duct has a diameter of 5 cm, I would think the diameter of the air "column" at the exit of the duct will be halved, and the pressure to be double of what it is at the beginning of the duct.
Now, if the diameter of the cone-shaped duct is instead increased to 20 cm on the exit side, will the air column have double the diameter and half the air pressure than at the beginning of the cone-shaped duct?