Mechanical engineers have to deal with partial flow in pipes, launders, overflow pipes, and discharge through opeinings of various shapes and sizes at low head.
One example is the surge hopper at pump suction. The underflow nozzle into the suction pipe will not yield full calculated flow unless the head exceeds about 100 pipe diameters. That is one reason why suction lines are traditionally selected one size larger (Most pump suction calculations ignore this constraint, assuming instead that the exit Cd is applicable to all flows under all head conditions).
At the top end how much head is needed to ensure the overflow does not cause spillage due to flooding of the suction hopper? Most texts will not handle partial flow into a nozzle.
In slurry applications the launder slope is selected to allow scouring velocities to be achieved. But how much head is needed to discharge the desired flow into the launder? That is constrained by the shape and size of the exit opening. Often this constrain is overlooked - resulting in operational problems on commissioning.
By contrast, Seagull,your interest seems to be limited to precise instrumentation requirements. That is a different area of applied fluid mechanics.
