Fluid Properties

Fluids deform continuously under shear. Learn density, specific weight and gravity, viscosity (dynamic and kinematic), Newtonian vs non-Newtonian fluids, compressibility, surface tension, capillarity and vapour pressure.

A solid resists a shear force by deforming a fixed amount; a fluid keeps flowing as long as the force acts. That single difference makes liquids and gases behave so differently from solids.

A handful of properties — density, viscosity, surface tension, vapour pressure, bulk modulus — describe almost everything a fluid-machinery engineer needs.

An analogy

Viscosity is a fluid's 'thickness': honey resists stirring far more than water because its layers drag on each other more strongly.

Density and specific gravity

Water ≈ 1000 kg/m³ (998 at 20 °C), seawater 1025, mercury 13 600, air ≈ 1.2 at sea level. Specific gravity compares with water: mercury SG = 13.6, petrol ≈ 0.75.

Viscosity

Viscosity is internal friction between fluid layers. Water at 20 °C: μ ≈ 1.0 × 10⁻³ Pa·s; SAE 30 oil ≈ 0.3 Pa·s; air ≈ 1.8 × 10⁻⁵ Pa·s. Liquids get thinner when hot; gases get more viscous.

Newtonian and non-Newtonian fluids

Newtonian fluids (water, air, oils) have constant μ. Non-Newtonian fluids don't: shear-thinning (paint, ketchup, blood), shear-thickening (cornstarch slurry), Bingham plastics (toothpaste, concrete) which need a yield stress to flow.

Surface tension, capillarity and vapour pressure

Surface tension pulls liquid surfaces tight (droplets, capillary rise in thin tubes). Vapour pressure is the pressure at which a liquid boils at a given temperature — if local pressure drops below it, vapour bubbles form and collapse: cavitation, which destroys pump impellers and propellers.

Compressibility and the continuum

Liquids are nearly incompressible (water K ≈ 2.2 GPa) but not quite — water hammer and hydraulic stiffness depend on K. Gases are compressible; below Mach ≈ 0.3 density changes are < 5 % and flows can be treated as incompressible. The continuum assumption fails for rarefied gases (Knudsen number) — relevant in vacuum systems and microfluidics.

Formulas

• Newton's law of viscosity: τ = μ · du/dy
• Specific gravity: SG = ρ / ρ_water(1000 kg/m³)
• Capillary rise: h = 4σ cos θ / (ρ g d)
• Bulk modulus: K = −V · dp/dV

Worked example: Shear in an oil film

Given: Plate sliding at 2 m/s over a 0.5 mm oil film; μ = 0.1 Pa·s

Find: Shear stress.

• Linear velocity profile: du/dy = 2/0.0005 = 4000 s⁻¹
• τ = 0.1 × 4000 = 400 Pa

Answer: 400 Pa

Worked example: Capillary rise

Given: Water in a 1 mm glass tube; σ = 0.0728 N/m, θ ≈ 0°

Find: Rise height.

• h = 4 × 0.0728/(998 × 9.81 × 0.001) = 0.0297 m

Answer: ≈ 30 mm

Common mistakes

• Wrong: Confusing dynamic (Pa·s) and kinematic (m²/s) viscosity. — Right: ν = μ/ρ; Reynolds number can use either consistently.
• Wrong: Assuming gases get thinner when hot like liquids. — Right: Gas viscosity rises with temperature.

Applications

• Lubrication: Oil viscosity grade is chosen to keep a full film in bearings at operating temperature.
• Hydraulics: Fluid bulk modulus sets actuator stiffness; entrained air makes systems spongy.
• Pumps & propellers: Vapour pressure sets cavitation limits (NPSH).

Key takeaways

• τ = μ du/dy; ν = μ/ρ.
• Liquids thin with heat; gases thicken.
• Surface tension causes capillarity; vapour pressure causes cavitation.
• Treat flow as incompressible below Mach 0.3.

Going further

• Viscosity index: Multigrade oils (5W-30) use polymer additives to reduce viscosity change with temperature.
• Rheometry: Rotational rheometers measure τ vs shear rate to characterise non-Newtonian fluids like polymers and slurries.
• Effective bulk modulus: Hose compliance and 1 % entrained air can cut hydraulic system stiffness by more than half.