Aerodynamics - Boundary Layers, Laminar & Turbulent Boundary Layers Quiz(MCQ)

1 .

Which of these is not a property of boundary layer?

A)

No – slip condition at the surface

B)

Flow velocity increases along y – direction

C)

Thermal boundary layer is equal to velocity boundary layer

D)

Temperature of fluid at the surface is equal to wall temperature

Correct Answer : Thermal boundary layer is equal to velocity boundary layer

Explanation : When there’s a viscous flow over a flat plate, there’s a boundary layer formation which has certain properties. On the surface, there’s a no slip condition. Apart from this the temperature of the fluid which is immediately at the surface has the same temperature as the surface which is known as the wall temperature. The velocity profile inside a boundary layer increases along the y – direction until it becomes equal to the freestream velocity. The only property that is incorrect is that the thermal boundary layer is equal to the velocity boundary layer.

The boundary of thermal layer is defined as the layer where the outer edge temperature becomes equal to the freestream temperature. Similarly, at the velocity boundary layer, the outer edge velocity is equal to the freestream velocity.

2 .

How is the boundary layer thickness defined? (u_e is the outer edge velocity)

A)

u = 0.99u_e

B)

u = 0.89u_e

C)

u = 0.90u_e

D)

u = 0.50u_e

Correct Answer : u = 0.99u_e

Explaination : Inside the boundary layer, the velocity increases along the y – direction until it becomes equal to the freestream velocity. The thickness of boundary layer δ is defined as the point from the surface where the velocity is 0.99 times the free stream velocity.

3 .

What is the thermal boundary layer?

A)

T = 0.90T_e

B)

T = 0.99T_e

C)

T = 0.89T_e

D)

T = 0.97T_e

Correct Answer : T = 0.99T_e

Explaination : The flow temperature just like velocity varies within the boundary layer. It is a function of y – direction. The temperature ranges from T_w which is the temperature at the wall (y = 0) to T = 0.99T_e at y = δ_t, where δ_t is the thermal boundary layer thickness. This variation of temperature is known as temperature profile.

4 .

In which case are the velocity and thermal boundary layer equal?

A)

Pr = 0

B)

Pr = 1

C)

Pr > 1

D)

Pr < 1

Correct Answer : Pr = 1

Explaination : In most of the cases the two thermal and velocity boundary layers are not same except in one exceptional case when the Prandtl number = 1, in which case δ_t = δ. When Prandtl number is greater than 1, δ_t < δ and when Prandtl number is less than 1, δ_t > δ. In real life scenario, the Prandtl number is equal to 0.71 thus the thermal boundary layer thickness is greater than the velocity boundary layer thickness.

5 .

Which of these relations is applicable for turbulent and laminar boundary layer?

A)

δ_turbulent < δ_laminar

B)

δ_{T_turbulent} = δ_{T_laminar}

C)

δ_turbulent > δ_laminar

D)

δ_{T_turbulent} < δ_{T_laminar}

Correct Answer : δ_turbulent > δ_laminar

Explaination : In case of turbulent flow, there is a high energy and momentum exchange compared to the laminar flow due to the presence of eddies. This leads to the thermal and velocity boundary layer thickness of the turbulent flow to be higher than that of the laminar flow. Thus, δ_turbulent > δ_laminar and δ_{T_turbulent} > δ_{T_laminar}.

6 .

What leads to shear stress between the adjacent layers of fluid inside the boundary layer adjacent to the surface?

A)

Viscosity

B)

Wave drag

C)

Pressure drag

D)

Aerodynamics drag

Correct Answer : Viscosity

Explanation : There is shear stress between the adjacent layers of fluid in both laminar and turbulent flow inside the boundary layer. This is due to the viscosity and it is given by the relation:

7 .

When the Reynolds number approaches infinity, what happens to the boundary layer thickness?

A)

Remains same

B)

Approaches zero

C)

Approaches once

D)

Approaches infinity

Correct Answer : Approaches zero

Explanation : As the Reynolds number increases, the boundary layer thickness decreases when compared to the length of the body. Usually for very large aircrafts, the value of δ/L is around 0.01 which is a very small value. So hypothetically, as Re ? ∞, δ ? 0.

8 .

When there’s flow over a flat plate, there’s laminar boundary layer at the leading edge followed by turbulent boundary layer.

A)

True

B)

False

C)

Can Not Say

D)

None of the above

Correct Answer : True

Explanation : When a flat plate is kept in the freestream flow, there’s a formation of a laminar boundary layer at the leading edge. This boundary layer thickness grows to a point where a transition point is reached. Beyond that point, there’s turbulence due to the presence of eddies and there’s turbulent boundary layer formation whose thickness keeps on increasing.

9 .

According to the y – momentum equation, how does the pressure vary inside the boundary layer normal in the direction normal to the surface?

A)

Increases

B)

Decreases

C)

Remains constant

D)

First increases then decreases

Correct Answer : Remains constant

Explaination : The y – momentum equation for a boundary layer is given by :

According to the formula, at any x point in the boundary layer, pressure remains constant in the direction normal to the surface.

10 .

Turbulent boundary layer is considered to be a combination of inner and outer layer.

A)

True

B)

False

C)

Can Not Say

D)

None of the above

Correct Answer : True

Explanation : Turbulent boundary layer consists of inner and outer layers. There is a viscous – dependent part of the profile very close to the surface and different length scaling parameters are needed for the remaining Reynolds – stress – dependent part of the profile.

11 .

Which of these models is a hybrid of LES and RANS?

A)

WALE model

B)

DNS model

C)

RNG – LES model

D)

DES model

Correct Answer : DES model

Explanation : DES model is a hybrid of LES and RANS model and makes use of advantages of both. For treating near – wall regions, DES works like the RANS model and for rest of the region, it works as the LES model.

12 .

Which of these models solves turbulent kinetic energy and specific dissipation?

A)

RANS model

B)

RNG – LES model

C)

K – omega model

D)

K – epsilon model

Correct Answer : K – omega model

Explanation : K – omega is one of the popular turbulence model. It is a two-equation model which solves two transport equations – turbulent kinetic energy and specific dissipation. Turbulent kinetic energy determines the energy whereas the specific dissipation determines the turbulence scale.

13 .

Which of these is a two – equation model?

A)

K – Epsilon model

B)

Baldwin – Lomax model

C)

Spalart – Allmaras model

D)

Prandtl’s equation model

Correct Answer : K – Epsilon model

Explanation : K – epsilon model is a two – equation model which includes two transport equations representing the turbulent flow properties. The first transport equation is solved is the turbulent kinetic energy, and the second one is the turbulent dissipation rate.

14 .

Which of these models compute large vortexes directly while neglecting small scale eddies?

A)

DNS

B)

LES

C)

FANS

D)

RANS

Correct Answer : LES

Explanation : In Large Eddy simulation, large eddies are computed by resolving large time and length scales. In LES, the smaller length scales are ignored making it an economical and less time consuming than DNS.

15 .

What are the shortcomings of DNS?

A)

Time consuming

B)

Low scale resolution

C)

Possible for high Reynolds number

D)

Possible for low Reynolds number

Correct Answer : Possible for low Reynolds number

Explanation : The DNS solves the time – dependent Navier’s Stokes equation by resolving eddies of all scales for a sufficient time before reaching statistical equilibrium. The only shortcoming is that DNS is only applicable for low Reynolds number flow which has simple geometry.

16 .

Why are turbulence models used?

A)

Model hypersonic flows

B)

Define reynolds stress for closure problems

C)

Closure of continuity equation

D)

Lack of system to accurately find turbulent flow characteristic

Correct Answer : Define reynolds stress for closure problems

Explanation : While solving Navier’s Stokes equation for turbulent flow which governs the velocity, pressure of the fluid, the quantity is decomposed into mean and fluctuating components. While solving these using RANS equation, we get a reynolds stress term that needs to be closed in order to solve it. Hence we make use of turbulence modeling which defines these reynolds stresses in terms of the known averaged quantities.