Engineering Questions with Answers - Multiple Choice Questions

# Computational Fluid Dynamics – Turbulence Modelling – K-epsilon Model

1 - Question

What does the name k-ε model signify?
a) The seven extra transport equations used in the model
b) The variation of k and ε with the flow variables
c) The variation of k with ε
d) The two extra transport equations used in the mod
Explanation: k-ε is a turbulence model used to supplement the RANS equations in overcoming its non-linearity. This model uses two additional transport equations which govern the transport of k and ε.

2 - Question

What does k and ε stand for?
a) Turbulent kinetic energy and its dissipation rate per unit mass
b) Turbulent kinetic energy and turbulent diffusivity
c) Turbulent diffusivity and its dissipation rate per unit mass
d) Turbulent kinetic energy and mass transfer
Explanation: In the k-ε model, the two additional equations govern the transport of turbulent kinetic energy (k) and the rate of dissipation of the turbulent kinetic energy (ε). The behaviour of turbulent flow is given in terms of these two properties in this model.

3 - Question

The k-ε model focuses on the mechanism which affects ____________
a) the Reynolds stresses
b) the cross stresses
c) the transport of scalar fluxes
d) the turbulent kinetic energy
Explanation: The basic mixing length model cannot define a flow which involves flow separation or recirculation. So, a better turbulence model is developed in terms of k and ε. This model focuses on the dynamics of the flow and hence its turbulent kinetic energy.

4 - Question

___________ and _____________ are used in the k-ε model in addition to k and ε to formulate the transport equations.
a) Internal thermal energy and turbulent stresses
b) Internal thermal energy and kinetic energy
c) Rate of deformation and turbulent stresses
d) Rate of deformation and kinetic energy
Explanation: While forming the transport equations for k and ε, the rate of deformation term and the turbulent stresses are also used. These are used in their tensor form. Both of them can be expressed in terms of the mean velocity gradients.

5 - Question

If Sij represents the rate of deformation, μ represents the dynamic viscosity and V⃗ , the velocity of the flow, What does the terms div(2μV⃗ Sij) and 2μSij account for?
a) The effect of turbulent stresses
b) The effect of viscous stresses
c) The effect of Reynolds stresses
d) The effect of kinetic energy

Explanation: The term div(2μV⃗ Sij) represents the transport of kinetic energy due to viscous stresses. The term 2μSij represents the viscous dissipation of kinetic energy. Together, these two terms represent the effect of viscous stresses on kinetic energy.

6 - Question

The terms accounting for turbulence effects contain ____________
a) Reynolds stresses
b) Turbulent kinetic energy
c) Dissipation of turbulent kinetic energy
d) Length scale terms

Explanation: The terms accounting for turbulence stresses are div(ρV⃗ uiuj¯¯¯¯¯¯¯¯¯) and ρuiuj¯¯¯¯¯¯¯¯¯.div(ρV⃗ uiuj¯¯¯¯¯¯¯¯¯) represents the turbulent transport of kinetic energy by means of Reynolds stresses. ρuiuj¯¯¯¯¯¯¯¯¯ represents the net decrease of kinetic energy due to deformation work by Reynolds stresses. Both of these terms contain the Reynolds stress term ρuiuj¯¯¯¯¯¯¯¯¯.

7 - Question

In high Reynolds number turbulent flows _______________ terms dominate.
a) diffusion terms
b) convection terms
c) viscous stress terms
d) turbulent effect terms
Explanation: In high Reynolds number flows, the difference between the length scales will be very high. The large eddies are more energetic. So, the turbulent effect terms are much larger than the viscous stress terms in high Reynolds number flows.

8 - Question

Express the large scale velocity in terms of k and ε.
a) ε1/2
b) (ε/k)1/2
c) k1/2
d) (k/ε)1/2

Explanation: In the k-ε model, the properties of turbulence can be expressed in terms of the variables k and ε. The velocity scale of the large eddies are given by k1/2. k is the turbulent kinetic energy term.

9 - Question

Express the large scale length in terms of k and ε.
a) (k/ε)3/2
b) k3/2
c) ε/k3/2
d) (ε/k)3/2

Explanation: The length scale of the large eddies can be given as k3/2/ε. The small scale dissipation rate of the turbulent kinetic energy can be used to represent the large scale length as the rate at which the large eddies extract energy from the mean flow is matched with the rate of energy transfer to small eddies.

10 - Question

Let Cμ be a dimensionless constant and ρ be the density of the flow. Express the eddy dynamic viscosity in terms of k and ε.
a) ρCμk2
b) ρCμ k/ε
c) ρCμ ε/k
d) ρCμ ε2/k