I'm an average joe here, trying to figure something out. Lets say I have a tube and a disk that travels from top to bottom pushing air kinda like a pump. Both tube ends are open so as air is pushed out of the bottom end, more air flows in through the top. Here is my question: Does pushing air through a tube produce more opposing force on that disk, than it would if that disk was doing the same motion outside the tube? If so, what could be a good estimate percentage wise? Also, would a couple of millimeters gap between the cylinder and the tube wall, negate any gains in performance? Thanks!