I am a mechatronics student. we do a lot of fluid dynamics too.. i don't know if you'll need this, but that what we learn!
flow through pipes differs from country to another, but it should be between 2 and 4 meters per second.. to calculate the volumic flow rate, you need to have the coeeficient of discharge (Cd) of the pipe (a constant between .5 and .8)
then the volumic discharge Q =
Q = Cd * Area_of_pipe * velocity
Now if you are able to meaure velocity, you can deduce the Cd of any pipe with this experiment:
- take a bassin, of known volume,
- start a stop-watch, and let the water flow from the pipe into this bassin, until its full, then stop the time.
- calculate the volume that should be in the bassin using:
Volume = Velocity*time*Area_of_pipe,
- the coeefefient of discharge is equal to: Volume of bassin / Calculated volume.
all this to tell you that, you cannot bill people only knwoing the velocity of water and the diameter of their pipes.. yould be cheating them!
the real volume getting out of the pipes is less than what you calculated. that's why we use the Cd constant.