Fluid Dynamics
Density, viscosity, continuity, static/dynamic pressure, the venturi effect and Bernoulli's principle.
Fluid dynamics — summary notes
- Density and specific gravity: SG compares a substance's density to water's (1,000 kg/m³) — used constantly for fuel and hydraulic fluid specs.
- Viscosity is a fluid's resistance to flow — higher viscosity means more pumping energy, relevant to engine oil grade selection.
- Continuity: for an incompressible fluid, what flows in must flow out, so velocity increases where the cross-section narrows.
- Bernoulli's theorem: total pressure (static + dynamic) stays constant along a streamline — where velocity rises, static pressure falls. This is the physics behind airspeed indicators, carburettors and wing lift.
- Continuity
- A₁V₁ = A₂V₂
- Bernoulli
- P + ½ρv² = constant (along a streamline, constant height)
- Dynamic pressure
- q = ½ρv²
Air enters a venturi at 10 m/s through a section of area 0.02 m², and the throat area is 0.01 m². Air density is 1.225 kg/m³. Find the velocity at the throat and the pressure drop.
Continuity: V₂ = A₁V₁/A₂ = (0.02 × 10)/0.01 = 20 m/s. Pressure drop: ΔP = ½ρ(V₂² − V₁²) = 0.5 × 1.225 × (400 − 100) = 183.75 Pa — the same principle that generates lift over a curved wing surface.
Bernoulli & the venturi
Pressure & depth
Fluid-flow concept map
Fluid Flow
Fluid dynamics quiz
Fluid Dynamics
1. Density equals mass divided by volume; its unit is:
2. Specific gravity compares a fluid's density to that of:
3. Viscosity is a fluid's resistance to:
4. A thick oil has ___ viscosity than water.
5. As temperature rises, the viscosity of a liquid usually:
6. The continuity equation states that A × V is:
7. Where a pipe narrows, the fluid speed:
8. Bernoulli: where fluid speed increases, its static pressure:
9. Total pressure equals static pressure plus:
10. Dynamic pressure is given by:
11. A venturi is a:
12. In a venturi, pressure is lowest where the tube is:
13. Static pressure acts:
14. Streamlining a body reduces its:
15. Liquids are generally treated as: