Rather than going off topic in a thread about inviscid fluid in a pipe, I decided to start a new thread. For this thread, I'm wondering about the effect of a ducted propeller operating in the air. Using this NASA article as a guide: https://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html The article has an idealized graph of the flow, in reality, with the propeller operating in a free stream, the exiting flow will draw the surrounding air to the flow due to viscosity, but the main issue I'm interested in is that the average pressure of the air affected by the fan or propeller doesn't return back down to ambient until some point downstream of the fan or propeller (the exit velocity point). Assume that in a static situation (zero free stream flow) that a particular combination of propeller and power result in this point being about 2 meters downstream of the propeller. Now consider enclosing the propeller in a 1 meter cylindrical tube (no taper), with the propeller in the middle of the tube. Is the point where the average pressure returns back to ambient reduced from 2 meters to only 1/2 meter (the end of the tube)? I'm also wondering about the viscous interaction between the flow into and out of the tube and the affected air outside the tube.