Reynolds number and its significance MCQs With Answer

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Reynolds number and its significance MCQs With Answer is an essential topic for B. Pharm students studying fluid dynamics in drug formulation, mixing, and respiratory delivery. This introduction explains the Reynolds number as a dimensionless parameter that compares inertial and viscous forces to predict laminar, transitional, or turbulent flow regimes. Understanding Reynolds number helps in scale-up, reactor design, aerosol science, and dissolution processes. Key keywords: Reynolds number, flow regime, laminar flow, turbulent flow, mixing, scale-up, drug formulation, dimensionless number. Mastering these concepts sharpens your practical skills in formulation development and process optimization. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the mathematical definition of Reynolds number for a Newtonian fluid?

  • The ratio of viscous to inertial forces: μ / (ρ V L)
  • The product of density, velocity and characteristic length divided by dynamic viscosity: ρ V L / μ
  • The ratio of surface tension to viscous forces: σ / μ
  • The ratio of thermal diffusivity to momentum diffusivity: α / ν

Correct Answer: The product of density, velocity and characteristic length divided by dynamic viscosity: ρ V L / μ

Q2. Which physical forces does the Reynolds number compare?

  • Gravitational and buoyant forces
  • Inertial and viscous forces
  • Surface tension and gravitational forces
  • Electrostatic and magnetic forces

Correct Answer: Inertial and viscous forces

Q3. What is the approximate critical Reynolds number for transition from laminar to turbulent flow in a smooth circular pipe?

  • Re ≈ 500
  • Re ≈ 2300
  • Re ≈ 10,000
  • Re ≈ 100

Correct Answer: Re ≈ 2300

Q4. Below which Reynolds number is flow in a pipe generally considered laminar?

  • Re < 100
  • Re < 5000
  • Re < 2300
  • Re < 10,000

Correct Answer: Re < 2300

Q5. Above which Reynolds number is pipe flow typically considered fully turbulent?

  • Re > 4000
  • Re > 230
  • Re > 100
  • Re > 200

Correct Answer: Re > 4000

Q6. What is the transitional Reynolds number range for pipe flow?

  • Re 0–1
  • Re 50–200
  • Re 2300–4000
  • Re 10,000–20,000

Correct Answer: Re 2300–4000

Q7. Is Reynolds number a dimensional or dimensionless quantity?

  • Has dimensions of length
  • Has dimensions of time
  • Dimensionless
  • Has dimensions of mass

Correct Answer: Dimensionless

Q8. Which fluid properties directly influence the Reynolds number?

  • Density and viscosity (along with velocity and characteristic length)
  • Surface tension and vapor pressure
  • Electrical conductivity and permittivity
  • Specific heat and thermal conductivity

Correct Answer: Density and viscosity (along with velocity and characteristic length)

Q9. What is the characteristic length used for Reynolds number in internal pipe flow?

  • Pipe radius squared
  • Pipe diameter
  • Cross-sectional area
  • Pipe circumference

Correct Answer: Pipe diameter

Q10. For flow around a sphere, what is the characteristic length used in Reynolds number?

  • Sphere surface area
  • Sphere diameter
  • Spherical volume
  • Spherical radius squared

Correct Answer: Sphere diameter

Q11. How does increasing dynamic viscosity (μ) affect the Reynolds number if other variables are constant?

  • Reynolds number increases
  • Reynolds number decreases
  • Reynolds number remains unchanged
  • Reynolds number oscillates

Correct Answer: Reynolds number decreases

Q12. Which form of Reynolds number is often used in formulations and stirred tanks involving impellers?

  • Re = ρ V L / μ using impeller tip speed for V and impeller diameter for L
  • Re = σ / μ using surface tension
  • Re = ρ g L^3 / μ using gravitational acceleration
  • Re = μ / (ρ V L)

Correct Answer: Re = ρ V L / μ using impeller tip speed for V and impeller diameter for L

Q13. Which Reynolds number regime is most favorable for predictable, diffusion-controlled mixing in small-scale dissolution tests?

  • High turbulent Re (>10,000)
  • Transitional Re (2300–4000)
  • Laminar Re (<2300)
  • Ultra-high Re (>100,000)

Correct Answer: Laminar Re (<2300)

Q14. What happens to the boundary layer thickness on a flat plate as Reynolds number increases (for the same plate length)?

  • Boundary layer thickness increases
  • Boundary layer thickness decreases
  • Boundary layer thickness remains constant
  • Boundary layer disappears

Correct Answer: Boundary layer thickness decreases

Q15. For very low Reynolds numbers (Re << 1), which flow regime and governing approximation apply?

  • Turbulent flow; Reynolds-averaged equations
  • Creeping or Stokes flow; viscous forces dominate and inertial forces negligible
  • Transitional flow; both inertial and viscous forces equal
  • Compressible flow; density changes dominate

Correct Answer: Creeping or Stokes flow; viscous forces dominate and inertial forces negligible

Q16. Which equation describes laminar viscous flow in a long straight circular pipe?

  • Bernoulli’s equation
  • Navier–Stokes general turbulent form
  • Hagen–Poiseuille equation
  • Euler’s inviscid flow equation

Correct Answer: Hagen–Poiseuille equation

Q17. Which nondimensional form of Reynolds number uses kinematic viscosity (ν = μ/ρ)?

  • Re = V L / ν
  • Re = ν / (V L)
  • Re = μ / (ρ V L)
  • Re = V / (L ν)

Correct Answer: Re = V L / ν

Q18. In a stirred tank, which expression correctly represents the Reynolds number using impeller speed N and diameter D?

  • Re = ρ N D^2 / μ
  • Re = μ / (ρ N D^2)
  • Re = ρ N^2 D / μ
  • Re = ρ D / (μ N)

Correct Answer: Re = ρ N D^2 / μ

Q19. Why is matching Reynolds number important during scale-up of mixing processes?

  • It ensures identical chemical composition
  • It guarantees identical temperature profiles
  • It achieves dynamic similarity of flow patterns and mixing behavior
  • It ensures the same power consumption per unit volume

Correct Answer: It achieves dynamic similarity of flow patterns and mixing behavior

Q20. Which Reynolds number is most relevant for particle settling and drag calculations?

  • Bulk pipe Reynolds number using pipe diameter
  • Particle Reynolds number using particle diameter and settling velocity
  • Reynolds number based on thermal diffusivity
  • Surface tension Reynolds number using particle surface area

Correct Answer: Particle Reynolds number using particle diameter and settling velocity

Q21. Which empirical drag law applies to spherical particles in the Stokes regime (low Re)?

  • Cd = 24 / Re
  • Cd = constant ≈ 0.44
  • Cd = Re^2
  • Cd = 2π / Re

Correct Answer: Cd = 24 / Re

Q22. How does turbulent flow affect mass and heat transfer compared to laminar flow?

  • Turbulent flow reduces transfer rates due to stable layers
  • Turbulent flow enhances mass and heat transfer due to eddies and mixing
  • Turbulent flow has no effect on transfer rates
  • Turbulent flow decreases transfer coefficients permanently

Correct Answer: Turbulent flow enhances mass and heat transfer due to eddies and mixing

Q23. Which Reynolds number form is commonly used for flow in microchannels and microfluidics?

  • Very high Re > 10,000 with inertial dominance
  • Re calculated using hydraulic diameter; typically Re << 1
  • Re based on gravitational acceleration
  • Re based on surface tension only

Correct Answer: Re calculated using hydraulic diameter; typically Re << 1

Q24. What is the hydraulic diameter (Dh) for an annulus with outer diameter Do and inner diameter Di?

  • Dh = (Do + Di) / 2
  • Dh = Do × Di
  • Dh = Do − Di
  • Dh = (Do^2 − Di^2) / (Do + Di)

Correct Answer: Dh = Do − Di

Q25. Roughly what Reynolds number (based on plate length) marks transition for boundary layer on a flat plate?

  • Re_L ≈ 500
  • Re_L ≈ 5 × 10^5
  • Re_L ≈ 10^2
  • Re_L ≈ 10^8

Correct Answer: Re_L ≈ 5 × 10^5

Q26. Stokes’ law for drag on small spheres is valid when particle Reynolds number is approximately:

  • Re_p < 1
  • Re_p ≈ 1000
  • Re_p > 10,000
  • Re_p ≈ 100

Correct Answer: Re_p < 1

Q27. Which statement about Reynolds number and temperature is generally true for most liquids?

  • Increasing temperature typically increases viscosity and decreases Re
  • Increasing temperature typically decreases viscosity and increases Re
  • Temperature has no effect on viscosity or Re
  • Increasing temperature always increases density and decreases Re

Correct Answer: Increasing temperature typically decreases viscosity and increases Re

Q28. Which number is commonly used to relate momentum and heat transfer in turbulent flows when Prandtl number is about unity?

  • Nusselt number
  • Schmidt number
  • Reynolds analogy
  • Biot number

Correct Answer: Reynolds analogy

Q29. For non-Newtonian (power-law) fluids, how is Reynolds number modified?

  • No modification; use Newtonian Re formula directly
  • Use an apparent or effective viscosity or a generalized Reynolds number (e.g., Metzner–Reade)
  • Replace density with surface tension
  • Use Reynolds number based on temperature only

Correct Answer: Use an apparent or effective viscosity or a generalized Reynolds number (e.g., Metzner–Reade)

Q30. Which parameter is used as characteristic length when calculating Reynolds number for flow in a packed bed?

  • Bed height
  • Particle diameter
  • Pore volume
  • Column radius

Correct Answer: Particle diameter

Q31. How does Reynolds number influence deagglomeration in inhaler aerosols?

  • Higher Re promotes deagglomeration due to increased shear and turbulence
  • Higher Re prevents deagglomeration by stabilizing agglomerates
  • Re has no effect on particle interactions
  • Lower Re always gives better deagglomeration

Correct Answer: Higher Re promotes deagglomeration due to increased shear and turbulence

Q32. Which of the following is a direct effect of increasing flow velocity V while keeping geometry and fluid constant?

  • Decrease in Reynolds number
  • Increase in Reynolds number
  • No change in Reynolds number
  • Re becomes negative

Correct Answer: Increase in Reynolds number

Q33. What velocity profile shape characterizes steady laminar flow in a circular pipe?

  • Flat (plug) profile
  • Parabolic profile
  • Sinusoidal profile
  • Exponential profile

Correct Answer: Parabolic profile

Q34. Which term in the Navier–Stokes equations becomes negligible at very low Reynolds numbers?

  • Viscous term (μ ∇^2 u)
  • Pressure gradient term (∇p)
  • Inertial term (ρ u · ∇u)
  • Body force term (ρ g)

Correct Answer: Inertial term (ρ u · ∇u)

Q35. Which phenomenon commonly appears downstream of bluff bodies at moderate to high Reynolds numbers?

  • Laminarization of flow
  • Vortex shedding
  • Complete flow reversal upstream
  • Elimination of drag

Correct Answer: Vortex shedding

Q36. Which empirical correlation relates drag coefficient and Reynolds number for spheres at intermediate Re?

  • Cd ≈ 24 / Re + 6 / (1 + sqrt(Re)) + 0.4 (empirical Schiller–Naumann type)
  • Cd = Re / 24
  • Cd = Re^3
  • Cd = 0 for all Re

Correct Answer: Cd ≈ 24 / Re + 6 / (1 + sqrt(Re)) + 0.4 (empirical Schiller–Naumann type)

Q37. Which dimensionless group must be matched along with Reynolds number for complete dynamic similarity when buoyancy effects are significant?

  • Schmidt number
  • Froude number
  • Biot number
  • Péclet number

Correct Answer: Froude number

Q38. What is the kinematic viscosity (ν) relation to dynamic viscosity (μ) and density (ρ)?

  • ν = μ × ρ
  • ν = μ / ρ
  • ν = ρ / μ
  • ν = μ^2 / ρ

Correct Answer: ν = μ / ρ

Q39. Which modeling approach is commonly used in computational fluid dynamics for high Reynolds turbulent flows?

  • Direct integration of Stokes equations only
  • Use of turbulence models such as k–ε or k–ω
  • Neglecting inertia entirely
  • Using only potential flow theory

Correct Answer: Use of turbulence models such as k–ε or k–ω

Q40. How is the particle Reynolds number for a settling particle defined (Re_p)?

  • Re_p = ρ g d_p^3 / μ
  • Re_p = ρ u_s d_p / μ where u_s is settling velocity
  • Re_p = μ / (ρ u_s d_p)
  • Re_p = d_p / (ρ μ u_s)

Correct Answer: Re_p = ρ u_s d_p / μ where u_s is settling velocity

Q41. In a laminar boundary layer on a flat plate, how does shear stress near the wall compare to turbulent flow?

  • Shear stress in laminar flow is typically lower and smoothly varying
  • Shear stress in laminar flow is higher and chaotic
  • Shear stress is identical in both regimes
  • Laminar flow eliminates shear stress at the wall

Correct Answer: Shear stress in laminar flow is typically lower and smoothly varying

Q42. Typical Reynolds number range for blood flow in large arteries is on the order of:

  • 10^-3 (very small)
  • 10^3 (thousands)
  • 10^6 (millions)
  • 10^9 (billions)

Correct Answer: 10^3 (thousands)

Q43. Which flow characteristic is most likely at Re ≈ 10^6 over an external body?

  • Viscous-dominated creeping flow
  • Fully laminar attached flow
  • Turbulent boundary layer with possible flow separation
  • Zero drag condition

Correct Answer: Turbulent boundary layer with possible flow separation

Q44. For mass transfer correlations in pipes, which nondimensional number often appears together with Reynolds number?

  • Nusselt number
  • Péclet number only
  • Schmidt number or Prandtl number (depending on mass or heat transfer)
  • Boiling number

Correct Answer: Schmidt number or Prandtl number (depending on mass or heat transfer)

Q45. When scaling up a reactor where turbulence strongly affects reaction rates, which strategy is most appropriate?

  • Match Reynolds number and turbulent Reynolds stresses if possible
  • Ignore Re and scale by geometric similarity only
  • Keep mixing speed constant regardless of size
  • Match only thermal properties

Correct Answer: Match Reynolds number and turbulent Reynolds stresses if possible

Q46. Which of the following is true about Reynolds number in open channel flows compared to closed pipes?

  • Re in open channels uses hydraulic radius or depth as characteristic length
  • Re in open channels uses pipe diameter always
  • Re is irrelevant in open channel flows
  • Re uses surface tension as characteristic length in open channels

Correct Answer: Re in open channels uses hydraulic radius or depth as characteristic length

Q47. Which flow phenomenon is more likely to increase with increasing Reynolds number in stirred tanks?

  • Viscous laminar layering
  • Turbulent eddy formation and enhanced mixing
  • Reduction of shear rates to zero
  • Suppression of circulation patterns

Correct Answer: Turbulent eddy formation and enhanced mixing

Q48. Which equation becomes a good approximation in the limit Re → 0 for flow around small particles?

  • Euler’s inviscid flow equations
  • Stokes (creeping flow) equations
  • Boussinesq approximation for buoyancy-driven flow
  • Bernoulli’s equation including compressibility

Correct Answer: Stokes (creeping flow) equations

Q49. How does particle Reynolds number affect drag coefficient trends for spheres as Re increases from very low to high?

  • Cd increases monotonically with Re
  • Cd decreases from 24/Re region, then transitions to near-constant plateau at high Re
  • Cd remains exactly 24/Re for all Re
  • Cd becomes negative at moderate Re

Correct Answer: Cd decreases from 24/Re region, then transitions to near-constant plateau at high Re

Q50. In pharmaceutical process design, which practical observation is often linked to operating at higher Reynolds numbers in mixers?

  • Lower homogeneity and longer mixing times
  • Increased shear leading to improved dispersion and reduced mixing time
  • No change in mixing characteristics
  • Complete prevention of chemical reactions

Correct Answer: Increased shear leading to improved dispersion and reduced mixing time

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