# Fluid Mechanics Questions and Answers – Dimensionless Numbers

This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Dimensionless Numbers”.

1. The rate at which the particles of fluid can spread is called_______
a) Surface tension
b) Diffusivity
c) Viscosity
d) Kinetics

Explanation: Diffusivity is defined as the rate of diffusion. It is a measure of particles at which the fluids or heat can spread. They are measured in different mediums. It can be defined on the basis of its properties.

2. Which among the following is the standard symbol for Archimedes number?
a) A
b) AR
c) Ar
d) a

Explanation: The standard symbol for Archimedes number is Ar. Archimedes number in fluid mechanics deals with the motion of fluids. This takes place due to the differences in their densities. It was followed by the Archimedes principle.

3. The Prandtl Number approximates ___________
a) Momentum diffusivity to thermal diffusivity
b) Thermal diffusivity to momentum diffusivity
c) Shear stress to thermal diffusivity
d) Thermal diffusivity to kinematic viscosity

Explanation: The Prandtl number is a dimensionless number. It approximates the ratio of momentum diffusivity to thermal diffusivity. It can be expressed as Pr = v/ α(1). Where α= thermal diffusivity and v= momentum diffusivity.

4. Which among the following is the standard symbol for Atwood number?
a) A
b) AR
c) Ar
d) a

Explanation: The standard symbol for Atwood number is A. Atwood’s number in fluid mechanics deals with the onset of instabilities in mixtures of fluid. It is due to the density differences in fluid.

5. Which among the following is the standard symbol for Blake number?
a) Bi
b) ba
c) Bl
d) b

Explanation: The standard symbol for Blake number is B or Bl. Blake number in fluid mechanics deals with geology, fluid mechanics and porous media. It is due to the inertial over the viscous forces in fluid flow through porous media.
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6. Which among the following is the standard symbol for Darcy friction factor?
a) F
b) Fd
c) C
d) Cd

Explanation: The standard symbol for Darcy friction factor is Fd. Darcy friction factor in fluid mechanics deals with fractions of pressure losses. This is due to the development of friction in the pipe.

7. Fanning friction factor is _________
a) 0.25 times Darcy friction factor
b) Same as Darcy friction factor
c) 2 times Darcy friction factor
d) Independent

Explanation: Fanning friction factor is 0.25 times Darcy friction factor. Fanning friction factor in fluid mechanics deals fraction of pressure losses due to friction in the pipe.

8. Which among the following is the standard symbol for Froude number?
a) F
b) Fo
c) Fr
d) f

Explanation: The standard symbol for Froude number is Fr. Froude number in fluid mechanics deals with wave and surface behaviour of fluid particles. This is with the ratio of body’s inertia to gravitational forces.

9. Which among the following is the standard symbol for Peclet number?
a) P
b) p
c) Pe
d) pe

Explanation: The standard symbol for Peclet’s number is Pe. Peclet’s number in fluid mechanics deals with heat transfer. It is defined as the ratio of transport rate over molecular diffusive transport.

10. Which among the following is the formula for Knudsen number?
a) λL
b) λ2L
c) λ3L
d) λ4L

Explanation: The formula for Knudsen number is λL. Knudsen number in fluid mechanics deals with gas dynamics. It is defined as the ratio of the molecular mean free path length to the representative scale length.

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