This set of Computational Fluid Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Turbulence Modelling – Turbulent Flows Characteristics”.

1. What is Reynolds stress?

a) Stress due to velocity fluctuations

b) Tangential component of pressure

c) Stress due to pressure fluctuations

d) Normal component of viscosity

View Answer

Explanation: Turbulent flows are highly chaotic and unstable. The high fluctuation in the turbulent flows creates highly varying velocities. These velocities create additional stresses called Reynolds stress.

2. The mathematical technique used to represent the random nature of the turbulent flow.

a) RANS

b) Reynolds decomposition

c) Parallel decomposition

d) DNS

View Answer

Explanation: The random nature of a turbulent flow needs a proper description of the motion of all the flow particles. Reynolds decomposition is used to represent this random nature. This decomposes the flow variables into two components.

3. Represent the velocity of turbulent flow using Reynolds decomposition.

a) Steady velocity + Mean velocity

b) Steady velocity + Fluctuating component of velocity

c) Variation in velocity + Fluctuating component of velocity

d) Mean variation + Fluctuating component of velocity

View Answer

Explanation: Reynolds decomposition separates the steady mean component and some statistical properties of their fluctuations. Total velocity = Steady velocity + Fluctuating component of velocity.

4. Eddies in turbulent flows result in _________

a) high diffusion coefficients

b) less diffusion coefficients

c) high value of the source term

d) low value of the source term

View Answer

Explanation: Because of the eddying motion of the turbulent flows, heat, mass and momentum are effectively exchanged. So, the values of diffusion coefficients of heat, mass and momentum are high.

5. Large turbulent eddies extract energy from the mean flow by this process.

a) Energy decomposition

b) Eddy extracting

c) Vortex stretching

d) Substantial variation

View Answer

Explanation: Vortex stretching is the lengthening of vortices with a corresponding increase in the component of vorticity in the stretching direction. This vortex stretching is responsible for the largest turbulent eddies to interact with the mean flow and extract energy from them.

6. Which of these is correct for large eddies?

a) High viscosity and linear momentum are conserved

b) Low viscosity and linear momentum are conserved

c) High viscosity and angular momentum are conserved

d) Low viscosity and angular momentum are conserved

View Answer

Explanation: Vortex stretching leads to conservation of angular momentum. As the large eddies are associated with vortex stretching, here angular momentum is conserved. They have a high Reynolds number which directly tells that they are relatively inviscid.

7. Which of these is highly energetic?

a) Kolmogorov micro-scale eddies

b) Small eddies

c) Medium eddies

d) Large eddies

View Answer

Explanation: Energy content peaks at low wavenumber. At low wave numbers, eddies are large. The larger eddies are the most energetic as they acquire energy from the mean flow through direct interaction.

8. Large eddies are _________

a) two-dimensional and isotropic

b) two-dimensional and isotropic

c) three-dimensional and anisotropic

d) two-dimensional and anisotropic

View Answer

Explanation: Turbulent flows are characterized by their three-dimensional fluctuation. So, the large eddies are also three-dimensional. The flow variables highly vary in all three directions. This leads to an anisotropic nature. Especially, large eddies are highly anisotropic.

9. Transfer of kinetic energy from large eddies to smaller eddies is called as _________

a) Energy cascade

b) Momentum cascade

c) Energy decomposition

d) Momentum decomposition

View Answer

Explanation: Smaller eddies are stretched strongly by large eddies. Thereby, kinetic energy is transferred from larger eddies to smaller and smaller eddies. This is called energy cascade.

10. Reynolds number gives the relative importance of __________

a) viscous force and tangential force

b) inertia force and viscous force

c) inertia force and pressure force

d) pressure force and viscous force

View Answer

Explanation: Reynolds number decides whether the flow is laminar or turbulent. Reynolds number of flow gives the relative importance of inertia force (convective forces) and viscous forces.

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