Fluid Mechanics
What to master
Fluid properties (density, specific weight, viscosity, surface tension, vapor pressure), hydrostatic pressure distribution and manometry, forces on plane and curved submerged surfaces, buoyancy and stability, continuity and the Bernoulli/energy equation with head losses, the Reynolds number and laminar/turbulent classification, Darcy-Weisbach friction factors (Moody chart, laminar f = 64/Re), minor losses, pipe networks and parallel/series piping, pump and system curves (head, efficiency, NPSH, cavitation), open-channel flow (Manning's equation, Froude number), orifice and weir flow, and drag on immersed bodies.
Traps
- Gauge versus absolute pressure: Bernoulli works consistently in either, but vapor-pressure comparisons need absolute.
- Applying Bernoulli across a pump, turbine or friction loss without adding the head terms.
- Using volumetric flow rate where velocity is needed (divide by area) and mixing diameter with radius.
- Confusing the friction factor f with the loss coefficient K.
- Unit weight of water: 9.81 kN/m^3 or 62.4 lb/ft^3 -- watch the unit system.
Worked example
Water at 1000 kg/m^3 and dynamic viscosity 1.0 x 10^-3 Pas flows at 2 m/s through a 0.05 m pipe. Re = rhoV*D/mu = (1000)(2)(0.05)/0.001 = 1.0 x 10^5, which is turbulent (laminar below roughly 2100).
Rule of thumb: compute the Reynolds number first in every pipe-flow problem; it selects the friction-factor correlation and tells you whether minor losses matter.