Computational Fluid Dynamics MCQs on “Direct Numerical Solution for Turbulent Models”.
1. Which of these equations is the starting point of the DNS method?
a) Continuity and momentum equations of homogeneous turbulent flow
b) Continuity and momentum equations of incompressible turbulent flow
c) Momentum and energy equations of incompressible turbulent flow
d) Momentum and energy equations of homogeneous turbulent flow
Answer: b
Clarification: The instantaneous continuity and Navier-Stokes equations (momentum equations) for an incompressible turbulent flow form the initial point of the Direct Numerical Solution method. These equations form a closed set of four equations with the four unknowns (pressure and three components of velocity).
2. DNS can solve _____________
a) transient 3-D equations
b) steady-state 3-D equations
c) transient 2-D equations
d) steady-state 2-D equations
Answer: a
Clarification: The starting set of equations is taken by the DNS system and the transient 3-D solution is done for a sufficiently fine spatial mesh and sufficiently small time-step sizes to resolve even the smallest turbulent eddies.
3. The grid size and time-step size of the DNS method depends upon the _____________
a) Schmidt number
b) Peclet number
c) Nusselt number
d) Reynolds number
Answer: d
Clarification: The grid size and time-step size of the DNS method is based on the largest and the smallest length and time scales of eddies in a turbulent flow. This, in turn, depends on the Reynolds number of the flow. So, Reynolds number decides the grid size and time-step size here.
4. When is the DNS method apt?
a) For complex flows
b) For design purposes
c) For precise details
d) For economic simulation
Answer: c
Clarification: The DNS method is used when we want the precise simulation with all the details in it. They are used for the development and validation of other turbulent models. The transport of any flow variable at any point can be precisely obtained using this model.
5. For which of these purposes, the DNS method is not suitable?
a) Designing
b) Simulation of the production of aerodynamic noise
c) Effects of compressibility on turbulence
d) To understand the mechanism of turbulence
Answer: a
Clarification: DNS method cannot be used to make the aerodynamic design of a model. They are computationally very expensive. To employ a model in designing, the same simulation should be done many times to improve the design. As DNS models are expensive, they cannot be used repeatedly.
6. DNS method is applicable for __________
a) Complex geometry and low Reynolds number
b) Simple geometry and low Reynolds number
c) Simple geometry and high Reynolds number
d) Complex geometry and high Reynolds number
Answer: b
Clarification: The DNS method needs highly refined grids to capture the detailed flow. So, it is not suitable for complex geometries as it will make the system more complicated. As the Reynolds number increases, the number of grids and time-steps also will increase. This will lead to higher complexity again.
7. Which of these is the simplest type of turbulent flows?
a) Homogeneous anisotropic turbulence
b) Incompressible turbulent flows
c) Homogeneous isotropic turbulence
d) Compressible turbulent flows