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. A force of 500 N acts on an area of 0.02 m². The pressure produced is:
25 kPa10 kPa2.5 kPa2. The pressure due to a 5 m depth of water, taking density as 1000 kg/m³ and g as 9.81 m/s², is approximately:
49 kPa5 kPa490 kPa3. A material has a density of 2700 kg/m³. Its relative density (specific gravity) is:
2.7270.374. Density equals mass divided by volume; its unit is:
kg/m³NPa5. Specific gravity compares a fluid's density to that of:
WaterAirMercury6. Viscosity is a fluid's resistance to:
FlowCompressionHeating7. A thick oil has ___ viscosity than water.
HigherLowerEqual8. As temperature rises, the viscosity of a liquid usually:
DecreasesIncreasesStays constant9. The continuity equation states that A × V is:
ConstantZeroIncreasing10. Where a pipe narrows, the fluid speed:
IncreasesDecreasesStays the same11. Bernoulli: where fluid speed increases, its static pressure:
DecreasesIncreasesStays the same12. Total pressure equals static pressure plus:
Dynamic pressureDensityViscosity13. Dynamic pressure is given by:
½ρV²ρghmgh14. A venturi is a:
Constriction in a tubeType of pumpHeat exchanger15. In a venturi, pressure is lowest where the tube is:
NarrowestWidestAt the inlet