Fluid properties

A fluid is a substance that deforms continuously under any shear stress, however small. Liquids and gases are both fluids. Civil engineers deal mostly with water, sometimes with air (wind loads, ventilation of tunnels) and with slurries, sewage and fresh concrete, which behave in more complex ways.

Density ρ is mass per unit volume, about 1 000 kg/m³ for water; specific weight γ = ρ g, about 9.81 kN/m³; and specific gravity compares a substance with water. Viscosity measures resistance to shear: Newton's law of viscosity says shear stress equals dynamic viscosity μ times the velocity gradient. Kinematic viscosity ν = μ / ρ appears in the Reynolds number. Water's viscosity falls markedly as it warms, which affects settling tanks and pipe friction. Fluids that follow Newton's law are Newtonian; others, such as fresh concrete, drilling muds and sludge, are non-Newtonian and need a yield stress or variable viscosity to describe them.

Surface tension makes water rise in thin tubes and in fine soils, causing capillary rise above the water table. Compressibility, described by the bulk modulus (about 2.1 GPa for water), is negligible in most hydraulics but important in water hammer. Vapour pressure matters at pump inlets and siphons: if pressure falls to vapour pressure, water boils at ambient temperature, cavities form and collapse, and cavitation damages pumps, spillways and valves.