Fluid Mechanics - 20 Key Questions and Answers
1. What is the primary distinction between a fluid and a solid?▼
A solid resists deformation and has a definite shape, while a fluid continuously deforms under any applied shear stress.
2. Define density.▼
Density is the mass per unit volume of a substance, typically expressed in kg/m³.
3. What is specific weight?▼
Specific weight is the weight per unit volume of a fluid, calculated as γ = ρ × g.
4. Define specific gravity.▼
Specific gravity is the ratio of the density of a fluid to the density of a reference substance (usually water).
5. What is dynamic viscosity?▼
Dynamic viscosity is a fluid's resistance to flow under an applied force, measured in Pa·s or Ns/m².
6. Define kinematic viscosity.▼
Kinematic viscosity is the ratio of dynamic viscosity to density, expressed as ν = μ / ρ in m²/s.
7. How does viscosity vary with temperature in liquids?▼
Viscosity decreases with increasing temperature in liquids.
8. How does viscosity vary with temperature in gases?▼
Viscosity increases with increasing temperature in gases.
9. State Newton's law of viscosity.▼
Shear stress τ is proportional to the velocity gradient: τ = μ × (du/dy).
10. What is a Newtonian fluid?▼
A Newtonian fluid is one whose viscosity remains constant regardless of the applied shear rate and obeys Newton’s law of viscosity.
11. What is vapor pressure?▼
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature.
12. Define boiling point in terms of vapor pressure.▼
The boiling point is the temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure.
13. What is surface tension?▼
Surface tension is the force per unit length acting along the surface of a liquid, causing it to behave like a stretched elastic sheet.
14. What causes surface tension in fluids?▼
Surface tension is caused by cohesive forces between liquid molecules at the surface being unbalanced, pulling them inward.
15. Define capillarity (capillary action).▼
Capillarity is the ability of a liquid to flow in narrow spaces without external forces, due to the interplay between cohesive and adhesive forces.
16. How does surface tension affect capillarity?▼
Higher surface tension increases capillary rise in narrow tubes, provided adhesive forces with the walls are also strong.
17. What is the bulk modulus of elasticity?▼
It is a measure of a fluid's resistance to uniform compression, defined as the ratio of pressure change to relative volume change.
18. What does a high bulk modulus indicate about a fluid?▼
A high bulk modulus means the fluid is nearly incompressible.
19. Define compressibility of a fluid.▼
Compressibility is the measure of how much a fluid's volume changes under pressure, the inverse of the bulk modulus.
20. Why are liquids often considered incompressible in fluid mechanics?▼
Because their compressibility is very low (bulk modulus is high), leading to negligible volume changes under normal pressure.
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