Fluids in motion: Bernoulli, friction losses and network hydraulics
Fluid calculations rest on two conservation statements: mass (continuity) and energy (Bernoulli). Continuity, ρAv = constant through a closed conduit, immediately explains why a pipe narrowing to half the diameter must flow four times faster for the same volumetric rate. Bernoulli then tracks energy along a streamline:
P/ρg + v²/2g + z = constant (head form: pressure head + velocity head + elevation head)
Each term has units of length (metres of fluid), which is why engineers speak in "metres of head" — a 40 m head of water is 392 kPa at the bottom. In real systems the constant decays: the Darcy–Weisbach equation books the decay as friction,
h_f = f · (L/D) · (v²/2g)
with Darcy friction factor f obtained from the Moody chart as a function of Reynolds number and relative roughness. Turbulent flow in rough pipes makes f nearly constant — the regime behind most "f = 0.02" hand estimates — while laminar flow gives the exact f = 64/Re. Minor losses (valves, bends, tees, expansions) add as K·v²/2g per fitting; in compact piping their sum frequently exceeds the straight-run friction, which is why counting fittings is not optional. Networks close the loop: the Hardy Cross method balances loops by correcting trial flows with ΔQ = −Σhf/(2Σhf/Q) iteratively until every junction and loop satisfies continuity and head balance simultaneously.
Open channels replace pressure with gravity. The Manning equation — v = (1/n)·R^(2/3)·S^(1/2) (SI form) — relates velocity to hydraulic radius R and slope S, with roughness n tabulated per lining material (concrete ~0.013, natural channel ~0.03). Weirs and notches measure flow by measuring depth: a rectangular weir passes Q = C·b·h^(3/2), so discharge grows with depth to the three-halves power — small depth errors are amplified. Sediment transport adds the solid phase: once shear stress exceeds a critical threshold, particles entrain, and the channel that was stable last year starts silting or scouring. And water hammer is the unsteady extreme: a valve closing fast sends a pressure surge ΔP = ρ·c·Δv up the pipe at the speed of sound in the liquid (roughly 1000–1500 m/s), easily adding tens of bar — the reason slow-close valves and surge tanks exist.