Important Questions for CFD Engineers

Topics to be Covered

CFD users have to understand the follwoing subject in details :

• Fluid Mechanics
• Basic of Computational Fluid Dynamics
• Turbulent flow physics and its Modeling
• Heat Transfer 
• Multiphase Flow MOdeling
• Combustion  Basics and  Turbulent Combustion Modeling 
• Basic pollutants NOx, SOx, Soot, and its modeling

Theroy, experiment and CFD simulations are importnat.  Each method has both sides. CFD is a tool of virtiual flow analysis. Without experimental validation, wwe can not relie only on CFD results.  Practical understandiong of flow or experiments is required to develop real products in industries.

CFD Questioner 

1. CFD modeling of physical phenomenon means

a) approximating the physical phenomenon based on certain assumptions

b) replacing the actual process with other physical process

c) neglecting the variable in a physical phenomenon

d) mathematical analysis of physical process

2.Computational fluid dynamics technique is used

a) To monitor the data of the actual flow system

b) To predict the boundary field data using the known flow field

c) To predict the moving flow fields in any system

d) To predict the flow field which can be either stationary or moving

3.When the numerical simulations are said to be converged

a) the relative error or residual of all variables is less than 10^-3

b) the relative error or residual of all variables is less than 10^-6

c) It depends on the number of iterations and selected convergence criterions

d) all variables show the constant residual values

4. To simulate any laminar flow with any geometry, which of the following equations are solved

a) Time averaged Navier-Stokes equations

b) filtered Navier-Stokes equations

c) instantaneous Navier-Stokes equations

d) equations for homogenous flow

5.To solve the energy equation of fluid flow for the conduction within the solid which term is neglected in the CFD solver

a) Unsteady

b) Advection

c) Diffusion

d) Heat generation

6. To simulate the two-dimensional laminar flow, for many how many variables the boundary conditions needs to be specified

a) two

b) three

c) four

d) five

7.The “Outflow” boundary condition is used when

a) all gradients normal to the boundary are zero except temperature

b) all gradients normal to the boundary are zero except pressure

c) all gradients normal to the boundary are zero

d) None of above

8.The “Symmetry” boundary condition is suitable when

a) all gradients are normal to the boundary

b) all gradients are normal to the boundary are zero except pressure

c) No flow occurs cross the boundary

d) None of above

9.The “Periodic” boundary condition is selected to solve the flow systems when

a) statically periodicity is observed in the flow fields

b) all the flow variables are same with the inlet boundary irrespective of location

c) flow pattern is not changed in the flow domain

d) No flow occurs cross the boundary

10. Which of the following term is neglected when we simulate the forced convection heat transfer only

a) Buoyancy term

b) Pressure gradient

c) Viscous diffusion

d) None of above

11. To simulate the forced convection of air over at high Reynolds number which of the following term is not considered

a) inertia term

b) viscous heating

c) gravity term

d) pressure gradient

12. To compute the view factor for radiative heat transfer, which radiation model is used

a) P1- Radiation model

b) Discrete transfer radiation model (DTRM)

c) Surface to surface model

d) Discrete ordinate Model (DOM)

13. For a cold air flow over a vertical heated surface, the which pressure scheme is used for numerical simulations

a) PRESTO!

b) SIMPLE

c) SIMPLEC

d) PISO

14.The viscous heating term is neglected in in-compressible flows because of

a) Difficult to model

b) it is important only for high velocity flows

c) This term is very small compared other mode of heat transfer for high velocity flows

d) This term is very small compared other mode of heat transfer

15. To simulate the turbulent flow with the k-Ɛ model, which of the following equations are solved

a) Time averaged Navier Stokes equations

b) filtered Navier-Stokes equations

c) Instantaneous Navier Stokes equations

d) Space averaged variables

16. Compared to the laminar flow equations which is an additional term in the Reynolds averaged momentum equations

a) Fluctuating velocity

b) Mean velocity

c) Reynolds stress tensor

d) Turbulent kinetic energy

17. Compared to the laminar flow energy equation which is an additional term in the Reynolds averaged energy equations

a) Fluctuating temperature

b) Mean temperature

c) Turbulent stress tensor

d) Turbulent heat diffusion

18.In the k-ε or k-ω model, two additional equations over the averaged mass, momentum and energy are solved

a) To compute the averaged velocities

b) To compute the turbulent viscosity

c) To model the fluctuating velocities

d) None of above

19. To simulate the separated flow with adverse pressure gradient which of the following turbulence model is well suited

a) Prandtl mixing length model

b) Standard k-Ɛ model

c) Standard k-ω model

d) SST k-ω model

20.Which of the following radiation heat transfer model is not suitable for combustion

a) P1- Radiation model

b) Discrete transfer radiation model (DTRM)

c) Surface to surface model

d) Discrete ordinate model (DOM)

21. For turbulent flow over a heated surface, which two variables are required to compute turbulent heat flux term in the averaged energy equations apart from the mean temperature gradients

a) Molecular Prandlts number and density

b) Turbulent Prandlts number and turbulent viscosity

c) Heat conduction in fluids

d) Radiative heat flux

22.To model species transport model in turbulent flow which additional term needs to be modeled

a) Averaged velocities of mass fraction of species

b) Fluctuating mass fraction of species

c) Turbulent convective transport

d) Turbulent diffusive transport

23.For modeling the particle laden flow, the discrete phase model is suitable when the concentration of dispersed phase is

a) Less than 20%

b) Greater than 10%

c) Less than 8%

d) Between 10-20%

24.The Eulerian continuum model is used for dispersed phase e when the concentration of dispersed phase is

a) Less than 20%

b) Greater than 20%

c) Less than 8%

d) Between 10-20%

25.The Discrete element model (DEM) is used mainly

a) to consider the collision between two particles

b) to model the heavy particle loaded flows

c) to model two -way coupling of particle laden flow

d) to mode the fluid flow

26. CFD modeling of physical phenomenon means

a) approximating the physical phenomenon based on certain assumptions

b) replacing the actual process with other physical process

c) neglecting the variable in a physical phenomenon

d) mathematical analysis of physical process

27. Which flow is considered to be in-compressible

a) pressure variation in the flow is negligible

b) the flow is isothermal

c) density variation in the flow is negligible irrespective of pressure and temperature

d) density variation in the flow is negligible with time

 

28.The flow is taken to be compressible

a) Pressure variation in the flow is negligible

b) The flow is isothermal

c) Density variation in the flow is significant irrespective of pressure and temperature

d) Density variation in the flow is negligible with time

29.The flow is considered to be steady because

a) time variation of physical quantities is neglected

b) time variation in velocity is neglected

c) transient changes in flows are significant

d) none of above

30.The flow is considered to be unsteady because

a) time variation of physical quantities is neglected

b) time variation in velocity is neglected

c) transient changes in flows are significant

d) none of above

31.To discretise the convective flow for high Peclet number which scheme is used

a) central scheme

b) SIMPLE

c) upwind scheme

d) PRESTO!

32.In the first order Updwind scheme the ……variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

33. In the second order Updwind scheme, which variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

34. In the QUICK scheme, which variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

35. For the adiabatic condition, which of the following condition is assumed at the wall

a) zero value of temperature

b) Fixed gradient of temperature

c) Zero gradient of temperature

d) None of above

36.To consider the effect of density variation, … term is added in the momentum equation

a) Buoyancy

b) Pressure gradient

c) Coefficient of expansion

d) Compressibility

37.To coefficient of expansion of fluid (β) for ideal gas is calculated as

a) ∆P/∆x

b) ∆ρ/∆Tρ

c) ∆P/∆Tρ

d) 1/T

38.Which dimensionless number affect the pressure, temperature in the compressible flow significantly

a) Reynolds number

b) Mach number

c) Damkohler number

d) Prandtl number

39.Which the flow the effect of compressibility on the flow cannot be neglected

a) Reynolds number , Re > 50000

b) Mach number, Ma > 0.3

c) Damkohler number, Da > 0.2

d) Prandtl number, Pr > 0.7

40.The flow inside the duct is considered to be turbulent when the Reynolds number defined based on the hydraulic diameter (D) and mean velocity at the inlet (U)

a) Re_D < 1000

b) Re_D < 1500

c) Re_D < 2000

d) Re_D > 2300

41.The flow over the open channel is considered to be turbulent when the Reynolds number defined based on the width of channel (L) and mean velocity at the inlet (U)

a) Re_L < 10000

b) Re_L < 15000

c) Re_L < 20000

d) Re_L > 500000

42.The turbulent intensity is defined if u’ is the fluctuating velocity and U is the mean velocity

a) u’/U

b)

c) 2u’/U

d) 1.5u’/U

 

43. The unit of the dissipation rate of turbulent kinetic energy (ε) is given as

a) m²/s²

b) m²/s

c) m³/s²

d) m²/s³

44.The unit of the specific dissipation rate of turbulent kinetic energy (ω) is given as

a) 1/s²

b) m²/s

c) 1/s

d) m²/s³

45. In turbulent flow, which is not the property of fluid

a) Mass density

b) Dynamic viscosity

c) Mass diffusivity

d) Eddy viscosity

46.In turbulent flow, the viscosity ratio is defined as ……..

a) Eddy viscosity / dissipation rate

b) Eddy viscosity / density of fluid

c) Eddy viscosity / dynamic viscosity of fluid

d) Eddy viscosity / dynamic viscosity of fluid

47.To resolve the viscous sub- layer adjacent to the wall in the CFD simulation of turbulent flow, the non-dimensional wall coordinate, y⁺ is taken as

a) y⁺ < 5

b) 5 < y⁺ < 30

c) 30 < y⁺ < 500

d) y⁺ > 500

48.For a given turbulent flow, k is the turbulent kinetic energy, the fluctuating velocity is calculated as

a) sqrt (2/3k)

b) sqrt (3/2k)

c) sqrt (3k)

d) sqrt (k/2)

49.For low Reynolds number flow, the very fine mesh normal the wall is required because of

a) High velocity of fluid

b) High density of fluid

c) High steep gradient near the wall

d) High level of turbulence near the wall

50.In the reactive flow,the Damkohler number is defined as

a) Turbulent mixing time / chemical time

b) Heat diffusion time / chemical time

c) Mass diffusion time / reaction time

c) Momentum diffusion time / reaction time

51.In the coal fired boiler, which statement is true for Damkohler number (Da)

a) Da >>1

b) Da = 1

c) Da << 1

c) Da < 1

51. The Damkohler number (Da) for plume combustion is

a) Da >>1

b) Da = 1

c) Da << 1

d) Da = 2

52. The Eddy dissipation combustion model is used when

a) Da >>1

b) Da = 1

c) Da << 1

d) Da < 1

53. The coal combustion, the single reaction devolatilization model is used

a) To find volatile yields at high heating rate

b) To find volatile yields at high reaction rate

c) To find devolatile products at high combustion rate

d) To find devolatile products

54.The coal combustion, the two reaction devolatilization model is used

a) To find volatile yields at high heating rate

b) To find volatile yields at high reaction rate

c) when the high volatile products in the bituminous coals

d) To find devolatile products

55. The Arrehunis reaction model is used

a) To model turbulent-chemistry reaction model

b) To model turbulent flow in combustion

c) To model the reactive flow in combustion

d) To model the species flow with turbulence

56. The model reactive flow with the k-ε turbulent model, the reaction rate is proportional to

a) Mixing time scale (k/ε)

b) Chemical time scale (ρ_a/R_slow)

c) Rate of thermal diffusivity (K/ρ_agC_pg)

d) Rate of mass diffusivity (D_g)

57.The Arrehenius reaction rate in turbulence chemistry coupling , , here

a) A is the activation energy and E is the internal energy

b) A is the activation factor and E is the storage energy

c) ) A is the frequency factor and E is the activation energy

d) A is the reaction constant and E is the molecular energy

58.In the single first order reaction (SFOR) devolatilization model , the rate of devolatilization is assumed to be dependent on

a) First order of the amount of volatiles remaining in the particle

b) First order of the amount of char formation

c) First order of the amount of reaction rate

d) Temperature of coal

59.In the single first order reaction (SFOR) devolatilization model , the kinetic rate

k_v = A_v exp (-T_v/T_p) is determined by

a) Numerical simulation

b) Experimental data

c) Analytical method

d) None of above

60.In the char combustion with heterogeneous reactions, the following product of combustions are formed

a) CO₂ and H₂O

b) CO₂ and CO

c) CO₂ and H₂

d) CO₂ and H

61. For major formation of NO emissions, … is not considered in pulverized coal combustion

a) Fuel NO mechanism

b) Thermal NO mechanism

c) Pressure NO mechanism

d) Prompt NO mechanism

62.Which of the following NO has the significant contribution in fossil fuel combustion

a) Fuel NO

b) Thermal NO

c) Prompt NO

d) None of above

63.Which of the following are the products of devolatilization

a) Volatiles (HC) and C (char)

b) Ash and moisture

c) Water vapor

d) Coke

64. Finite rate chemistry model is applicable …. flames

a) Turbulent flames

b) Laminar flames

c) Transition flames

d) Diffusion flames

65.In coal-fired flames, the eddy dissipation model (EDC) is used to model

a) Turbulent flow

b) Turbulence -chemistry interaction

c) Dissipation of turbulent kinetic energy

d) Diffusion of turbulent fluxes

66.The governing equation solved for thermal and prompt NOx mechanism apart from the equations for mass, momentum and species of reactant and product of combustion

a) one

b) Two

c) Three

d) Four

67.The number of governing equations solved for Fuel NOx mechanism apart from the equations for mass, momentum and species of reactant and product of combustion

a) one

b) Two

c) Three

d) Four

68.The species transport equations solved for Fuel NOx mechanism are ……

a) NO

b) HCN, NO

c) NH₃, N₂O, NO

d) NO, HCN, NH₃, N₂O

69.In coal combustion, the sulfur is decomposed into the following products in the gas phase during devolatilization which are

a) H₂S, COS and SO₂

b) H₂S and SO₂

c) H₂S, COS, SO₂ and CS₂

d) NH₃, N₂O and NO

69.In Sulfur Oxidation (SO_X) model, to track the sulfur -containing intermediate species,which species transport equations are solved in addition to the SO₂ species

a) H₂S, SO, SO₃ and SH

b) H₂S and SO₂

c) H₂S, COS and CS₂

d) NH₃, SO₂ and SO

 

70. In the first order Updwind scheme , what variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

71. In the second order Updwind scheme what variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

72. In the QUICK scheme the  what variation between two nodes is considered

a) Fixed value

b) Linear

c) Parabolic

d) Hyperbolic

73.For the adiabatic condition, which of the following condition is assumed at the wall

a) zero value of temperature

b) Fixed gradient of temperature

c) Zero gradient of temperature

d) None of above

74. To consider the effect of density variation, … term is added in the momentum equation

a) Buoyancy

b) Pressure gradient

c) Coefficient of expansion

d) Compressibility

 

75.The coal combustion, the two reaction devolatilization model is used

a) To find volatile yields at high heating rate

b) To find volatile yields at high reaction rate

c) when the high volatile products in the bituminous coals

d) To find devolatile products

76.The Arrehunis reaction model is used

a) To model turbulent-chemistry reaction model

b) To model turbulent flow in combustion

c) To model the reactive flow in combustion

d) To model the species flow with turbulence

78.The model reactive flow with the k-ε turbulent model, the reaction rate is proportional to

a) Mixing time scale (k/ε)

b) Chemical time scale (ρ_a/R_slow)

c) Rate of thermal diffusivity (K/ρ_agC_pg)

d) Rate of mass diffusivity (D_g)

79.The Arrehenius reaction rate in turbulence chemistry coupling , , here

a) A is the activation energy and E is the internal energy

b) A is the activation factor and E is the storage energy

c) ) A is the frequency factor and E is the activation energy

d) A is the reaction constant and E is the molecular energy

80. In the single first order reaction (SFOR) devolatilization model , the rate of devolatilization is assumed to be dependent on

a) First order of the amount of volatiles remaining in the particle

b) First order of the amount of char formation

c) First order of the amount of reaction rate

d) Temperature of coal

81. In the single first order reaction (SFOR) devolatilization model , the kinetic rate

k_v = A_v exp (-T_v/T_p) is determined by

a) Numerical simulation

b) Experimental data

c) Analytical method

d) None of above

82.In the char combustion with heterogeneous reactions, the following product of combustions are formed

a) CO₂ and H₂O

b) CO₂ and CO

c) CO₂ and H₂

d) CO₂ and H

83. For major formation of NO emissions, … is not considered in pulverized coal combustion

a) Fuel NO mechanism

b) Thermal NO mechanism

c) Pressure NO mechanism

d) Prompt NO mechanism

84. Which of the following NO has the significant contribution in fossil fuel combustion

a) Fuel NO

b) Thermal NO

c) Prompt NO

d) None of above

 

Answer of Above CFD Questions 

1. – (a),    2.-(d),   3.-(c),    4.- (c),   5 .-(b),    6. -(b),  7. -(b),  8 -(c),  9. (a),  10.- (c), 11. (c),  12- (c), 13. -(a),  14.-(c),  15-(a), 16.-(c)  17-(c), 18-(b), 19.-(c), 20.-(C)