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

1. The rotational flow structure of turbulent flows is termed as ___________

a) Turbulent eddies

b) Haida eddies

c) Whirlpool

d) Wake turbulence

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Explanation: The turbulent fluctuation has a three-dimensional spatial character. Visualizations of turbulent flows show that they have a highly rotational flow structure. These rotational flow structures are called Turbulent eddies.

2. According to Kolmogorov, the structure of the smallest eddies depends on ___________

a) rate of dissipation of kinetic energy

b) rate of dissipation of turbulent energy

c) rate of convection of kinetic energy

d) rate of convection of turbulent energy

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Explanation: Kolmogorov stated that the structure spectral energy of the smallest eddies depends only on the rate of dissipation of turbulent energy. But later studies revealed that only the spectral energy of the smallest eddies depends only on the rate of dissipation of turbulent energy and not their structure.

3. The details about the structure of the fluctuations are contained in ___________

a) sum of different variables

b) sum of different pairs of variables

c) moments of different pairs of variables

d) moments of different variables

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Explanation: Turbulent flows have complex rotating three-dimensional fluctuations. Details about the structure of these fluctuations are contained in moments constructed from pairs of different variables.

4. Consider the turbulent structure of thin shear layers. Which of these statements is correct?

a) The rates of change of the flow variables are very high in every direction

b) Flow variables do not vary much in any direction

c) The rates of change of the flow variables in the cross-sectional direction are smaller than that of the flow direction

d) The rates of change of the flow variables in the flow direction are smaller than that of the cross-sectional direction

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Explanation: Considering turbulence in thin shear layers, large variations are concentrated in thin regions. The variables do not change much in the flow direction. But, the rates of change in the cross-sectional direction are more.

5. The structure of free turbulent flow is controlled by ___________

a) Only the local environment

b) The source

c) The sink

d) The source along with the local environment

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Explanation: For a free turbulent flow, as the flow progresses, the effect of source degrades. Only the local environment tends to control the structure of the flow at any time. This is inferred from experiments.

6. For a turbulent flow, which of these is correct?

a) Fluctuating pressure is equal everywhere

b) Fluctuating velocity and pressure are not equal everywhere

c) Fluctuating velocity and pressure are equal everywhere

d) Fluctuating velocity is equal everywhere

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Explanation: Turbulent flow structures are highly anisotropic. The flow variables are not the same in a particular direction. Therefore, the fluctuating velocities are also not the same everywhere. Instead, they show continuous variation.

7. Methods of time-averaging statistics are applicable only to ___________

a) Free turbulent structures

b) Boundary layer turbulent structures

c) Coherent turbulent structures

d) Incoherent turbulent structures

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Explanation: For a time-averaged statistic to be applicable, the turbulent structure should have temporal coherence. Though turbulent flows are chaotic, they should be resolved into coherent structures to apply these methods.

8. The various fluid parcels come into contact in turbulent flow by ___________

a) Turbulent energy transfer

b) Turbulent vorticity

c) Turbulent dissipation

d) Turbulent diffusion

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Explanation: Turbulence increases the rate at which conserved quantities are mixed. Here, different fluid parcels are brought into contact. As this mixing is accomplished by diffusion, the process is called turbulent diffusion.

9. Dissipation in turbulent flows converts ____________ energy to ____________ energy.

a) kinetic, internal

b) internal, kinetic

c) kinetic, viscous

d) viscous, kinetic

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Explanation: Viscous effects reduce the velocity gradients and hence the kinetic energy of the flow. Thus, mixing is dissipative. This lost energy is converted into thermal internal energy of the flow. Thus, kinetic energy is converted into internal energy.

10. Direct Numerical Simulation of turbulent flows become difficult because of ____________

a) Viscosity scales

b) Time and length scales

c) Energy scales

d) Velocity scales

View Answer

Explanation: High fluctuation in turbulent flows result in a broad range of length and time scales. This makes the Direct Numerical Simulation (DNS) of turbulent flows difficult. It imposes a limitation to the DNS method.

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