This set of Computational Fluid Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Turbulence Modelling”.
1. Which of these does not characterize a turbulent flow?
a) Time-independent
b) Rapid mixing
c) Three-dimensional fluctuation
d) Unstable
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Explanation: Turbulent flows are chaotic, diffusive causing rapid mixing, time-dependent as they are unsteady, and involve vorticity fluctuation in all three-dimensions. Turbulence develops an instability in flows.
2. Which of these methods is not used for turbulence modelling?
a) RANS
b) SIMPLE
c) DNS
d) LES
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Explanation: The SIMPLE algorithm is a widely used iterative solution technique for Navier-Stokes Equations for pressure linked systems. It is not used to solve turbulent models. The other techniques – RANS, DNS and LES – are all used to solve turbulent flows.
3. Which among these techniques is the first one invented for solving turbulent flows?
a) DNS
b) LES
c) RANS
d) FANS
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Explanation: DNS is the first method used to solve the turbulent flow models. DNS stands for Direct Numerical Simulation. It uses the energy cascade concept developed by Kolmogorov. Its time-steps are limited by Courant number.
4. Which of these methods is invented to overcome the disadvantage of DNS technique?
a) Temporal discretization
b) FANS
c) RANS
d) LES
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Explanation: The LES method is used to overcome the disadvantages of the DNS technique. LES stands for Large Eddy Simulation. This filters the Navier-Stokes equations to discard some scales and keep the rest.
5. What is the disadvantage of the DNS technique?
a) Time averaging
b) Spatial averaging
c) Computationally demanding
d) Large time-steps
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Explanation: The time-step sizes in DNS technique is limited by courant number. So, it involves many steps to reach the actual interval needed to be crossed. This makes the technique computationally demanding. This is the disadvantage of DNS technique.
6. Which of these methods is used to overcome the high resolution of turbulent flows?
a) Weighted average
b) Statistical analyses
c) Data analysis
d) Analytical method
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Explanation: Statistical analysis is used to simplify the resolution of turbulent flows. Statistical time averaging is used to reduce the random fluctuations in the time-dependent nature of turbulence flows.
7. Which of these correct about the LES method?
a) Navier-Stokes equation is not needed
b) Small turbulent models are directly simulated
c) Sub-grid scale models are used for small turbulent scales
d) Sub-grid scale models are used for large turbulent scales
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Explanation: Large scale turbulent structures are directly simulated and the small scales are simulated using sub-grid models. A spatial statistical filter is used to filter the Navier-Stokes equation to determine which scale to keep and which to discard.
8. Which of these methods use time-based averaging?
a) Statistical averaging with DNS
b) DNS
c) LES
d) RANS
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Explanation: The statistical averaging and LES methods use spatial statistical averaging. RANS stands for Reynolds Averaged Navier-Stokes equations where statistical averaging is based on time, unlike LES.
9. What is the expansion of FANS method?
a) Favre Averaged Navier-Stokes equations
b) Fully Averaged Navier-Stokes equations
c) Favre Averaged Numerical Simulation
d) Fully Averaged Numerical Simulation
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Explanation: The FANS model is another method like RANS. FANS stands for Favre Averaged Navier-Stokes equations. This employs Favre averaging technique used for turbulent compressible flows. Both RANS and FANS use modified Navier-Stokes equations.
10. The weight used to average time in FANS method is _________
a) volume
b) mass
c) area
d) density
View Answer
Explanation: The averaging in FANS method is a weighted average method which uses mass as the weight for it. Here, mass decides the importance of the terms and whether it can be filtered or not.
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