Hydraulics Engineering: Why does Velocity stay the same when Roughness

[cdux]

I used the E.P.A.'s EPAnet program to run a network analysis. I noticed that if I change the roughness of the main pipe connecting the reservoir with the first junction, the pressures do change on the network but all the velocities are same!

Also if I do change the roughness on an internal different pipe, pressures but also velocities change.

Why does it have that behavior? I heard that it's related to the supply remaining the same and I will look into it but I was also wondering for more sources of information.

I'm particularly surprised of the fact the main pipe's roughness change doesn't change any velocity on the network (even though it does change pressures) but an internal pipe's roughness affects both.

Formula for head losses used: Darcy-W. I was instructed it's the most suitable.

 

[Valkarie]

The reason for the behavior you see is related to the principle of conservation of energy. The supply of energy remains constant in a closed system, so when a change is made to one part of the system, it affects the other parts of the system in order to maintain equilibrium. In the case of the roughness of the main pipe, the change only affected the pressure in the network because the velocity remained unchanged. This is because the energy supplied by the reservoir is the same regardless of the roughness of the pipe. The supply of energy must remain constant, so the pressure changes in order to account for the additional head losses due to friction caused by the increased roughness.On the other hand, when the roughness of an internal pipe changes, both the pressure and velocity change. This is because the energy supplied by the reservoir is not enough to account for the additional head losses caused by the increased roughness. Therefore, the velocity decreases in order to maintain equilibrium and conserve energy.