### Mechanics of Fluids - MCQ from AMIE Exam (Summer 2021)

1. Shear stress in fluid flow is proportional to
(a) ∂u/∂x
(b) ∂u/∂y
(c) ∂v/∂x
(d) ∂v/∂y

2. If the contact angle at the surface of a glass tube immersed in a liquid is 100⁰, Then the liquid in the glass tube will
(a) Rise
(b) Fall
(c) Neither rise nor fall
(d) None of these

Hint: when contact angle θ is obtuse, i.e. θ > 90°
In case of non – wetting fluid, the level in the capillary tube will fall, and the phenomenon is capillary fall. For mercury glass tube, θ = 128°.

3. The atmospheric pressure at mean sea level is equivalent in terms of Hg is
(a) 70 cm
(b) 86 cm
(c) 76 cm
(d) 75 cm

(d) None of these

Hint:  The sum of pressure head and datum is known as piezometric head. It is given by
p/ρg + z

5. For uniform flow through pipe
(a) ∂u/∂x ≠ 0
(b) ∂u/∂x = 0
(c) ∂u/∂r = 0
(d) None of these

6. For non-Newtonian fluids, shear stress (τ) is equal to
(a) ∂u/∂x
(b) (∂u/∂x)n
(c) (∂u/∂y)n
(d) ∂u/∂y

7. For continuity equation for incompressible and steady flow, the discharge (Q) through pipe is
(a) 0
(b) Not constant
(c) Constant
(d) None of these

8. ln momentum equation, summation of forces on the control volume is equal to
(a) ρQV2
(b) ρQV
(c) ρV
(d) ρQ

9. Energy correction factor for laminar flow through pipe is equal to
(a) 1.0
(b) 1.5
(c) 2.0
(d) 1.7

Hint: Its value for a fully developed laminar pipe flow is around 2. It is 1 for a turbulent flow.

10. Head loss for laminar flow between parallel plates is equal to
(a) 12μVL/D2
(b) 12μVL/γD2
(e) 32μVL/D2
(d) 10μVL/γD2

11. Nominal boundary layer thickness (ઠ) is that distance (y) from the plate, at which
(a) u =1.0U₀
(b) u = 0.95
(c) u = 0.99U₀
(d) u = 1.01U₀
Where U₀ is the free stream velocity.

12. ln laminar boundary layer with appropriate velocity distribution, ratio of displacement thickness and displacement thickness (ઠ*/ઠ) is
(a) 1/2
(b) 3/10
(c) 2/15
(d) 3/15

13. Laminar sub-layer occurred at plate surface in
(a) Laminar boundary layer
(b) Turbulent boundary layer
(c) Transition boundary layer
(d) None of these

Hint:

14. Separation of the boundary layer developed over the curved surface takes place after that point, where the pressure gradient (∂p/∂x) is
(a) Greater than zero
(b) Equal to zero
(c) Less than zero
(d) None of these

Hint:  When the pressure goes on increasing in the direction of flow, the pressure force acts against the direction of flow in the boundary layer and hence thickening the boundary layer more rapidly. This and the boundary shear bring the fluid in the boundary layer to rest and causes backflow. Due to this the boundary layer no more sticks to the boundary but is shifted away from the boundary. This phenomenon is called as “Boundary layer separation”.

15. The dimensionless laminar boundary layer (δi/x) developed over the flat plate just from the leading edge is equal to
(a) 3.0/(Rex)0.5
(b) 5.0/(Rex)0.5
(c) 7.0/(Rex)0.5
(d) 1.0/(Rex)0.5

Hint: δ=5x/√Rex

16. The local skin friction coefficient (cf) for a turbulent boundary layer developed over a flat plate just from the leading edge, is proportional to
(a) Re1/5
(b) Re-1/5
(c) Re-2/5
(d) Re2/5
Where x is the distance from the leading edge.

17. The ratio of friction factor (f) and skin friction coefficient (f') is equal to
(a) 4.0
(b) 2.0
(c) 1.0
(d) 0.25

Hint: Friction factor = 4 x friction coefficient

18. In Blasius equation, the friction factor (f) is given by
(a) 0.416Re1/4
(b) 0.316Re1/4
(c)  0.216Re1/4
(d)  0.516Re1/4
Where R is the Reynolds number.

19. The exit loss coefficient for a pipe discharging freely into the atmosphere is
(a) 1.0
(b) 0.75
(c) 0.5
(d) 0.25

Hint:
At entry,  hL, entry = 0.5v2/2g
At exit =  hL, exit = (1.0)v2/2g

20. Adiabatic constant for diatomic gases is equal to
(a) 1.0
(b) 1.4
(c) 1.2
(d) 0.8

Hint:
γ for monatomic =  1.67
γ for diatomic =  1.40

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