Engineering Questions with Answers - Multiple Choice Questions

# Computational Fluid Dynamics – Turbulence Modelling – Averaging Methods

1 - Question

The methods of averaging are collectively called as ______________
a) Reynolds averaging
b) Boussinesq averaging
c) Kolmogorov averaging
d) Schmidt averaging
Explanation: The averaging techniques include time averaging, Spatial averaging and ensemble averaging. These are collectively called the methods of averaging. They are used to simplify the algebra without actually disturbing the accuracy much.

2 - Question

What are the methods of averaging used to?
a) To decompose the flow variable
b) To get the mean component of the flow variable
c) To get the remove the fluctuating component
d) To solve the flow variables
Explanation: By Reynolds decomposition, the flow variables are decomposed into mean and fluctuating components. These methods of averaging are used to get the mean component during this decomposition.

3 - Question

Which of these averaging methods is useful for any kind of turbulent flows?
a) Ensemble averaging
b) Time averaging
c) Volume averaging
d) Spatial averaging
Explanation: Ensemble averaging is a method used in statistical mechanics. Here, it is used as one of the methods of averaging. This is suitable for any type of turbulent flows including unsteady turbulent flows.

4 - Question

Which of these represent time averaging?
a) 1VVϕdV
b) limV1VVϕdV
c) 1Tt+Ttϕdt
d) limT1Tt+Ttϕdt

Explanation: Time averaging represents the average of the flow variable based on a time interval ‘T’. It uses integration to sum up the flow variables at different times and then divides by the time interval limT1Tt+Ttϕdt.

5 - Question

The governing equations which are averaged using these methods of averaging are used in _____________
a) DNS model
b) SST model
c) RANS model
d) k-ε model
Explanation: The governing Navier-Stokes equation is averaged using the Reynolds averaging techniques and these averaged equations are used in the RANS method. This is the reason why the technique is named the Reynolds-Averaged Navier-Stokes equations method.

6 - Question

Time averaging method is useful for ____________
c) turbulent boundary layer flows
d) mixing flows
Explanation: Time averaging method is useful when we have to decompose the turbulent flow variables into mean and fluctuating components based on time. They are particularly applicable for steady turbulent flows.

7 - Question

Which of these represent spatial averaging?
a) 1Vϕ
b) limV1VVϕdV
c) 1TTϕdT
d) 1NNϕdN

Explanation: Spatial averaging represents the mean based on a particular space interval or volume. So, equation limV1VVϕdV represents spatial averaging.

8 - Question

Ensemble averaging represents the average of ____________
c) identical quantities
d) mean quantities
Explanation: This is useful for identical quantities. Identical in the sense, that they have similar properties in some concern. A number of quantities which have the simultaneous variations can be averaged using this method.

9 - Question

Which of these represent ensemble averaging if ‘N’ represents the number of identical quantities?
a) 1N∫NϕdN
b) limN→∞1N∫NϕdN
c) 1N∑Ni=1ϕi
d) limN→∞1N∑Ni=1ϕi

Explanation: Ensemble averaging is based on identical flow variables. It sums up the identical variables and then takes the average. The equation is limN1NNi=1ϕi.

10 - Question

Spatial averaging is suitable for ____________
a) homogeneous turbulent flows