Test: Turbulent Flow In Pipes

QUESTION: 1

The cause of turbulence in fluid flow may be

A.
critical Reynold’s number
B.
abrupt discontinuity in velocity distribution
C.
very high Reynolds number
D.
none of these

Solution:

QUESTION: 2

In a turbulent flow in a pipe, the shear stress is

A.
maximum at the centre
B.
maximum at the boundary and decreases linearly to zero value at the centre
C.
maximum at the wall and decreases logarithmically towards the centre
D.
maximum at a finite distance from the wall

Solution:

In turbulent flow shear stress
τ = τ₀ (1 - y/r₀)
where,
r₀ = Radius of pipe
y = Distance measured from pipe wall
_ζ0 = Shear stress at boundary

QUESTION: 3

In the case of turbulent flows in circular pipes, the maximum velocity u_m and the mean velocity V are related as (u_m - V)lu =

A.
3.75 for rough boundary flow only
B.
3.75 for smooth boundary flow only
C.
3.75 for both rough and smooth boundaries
D.
5.75 for both rough and smooth boundaries

Solution:

Turbulent flow
For smooth pipe,

Average velocity,

g₀ = radius of pipe
y = distance measured from pipe boundary
u = shear velocity

from equation (i)

(iii) - (ii) gives
(b) Rough pipe

It gives,

same for both pipes rough or smooth

QUESTION: 4

The distance from pipe boundary, at which the turbulent shear stress is one-third the wall shear stress, is

A.
1/3 R
B.
1/2 R
C.
2/3 R
D.
3/4 R

Solution:

In turbulent flow shear stress at distance y from pipe boundary,

QUESTION: 5

A flow in which the quantity of liquid flowing per second is not constant, is called

A.
Stream line flow
B.
Turbulent flow
C.
Steady flow
D.
None of these

Solution:

A fluid motion is said to be turbulent when the fluid particles move in an entirely haphazard or disorderly manner, that results in a rapid and continuous mixing of the fluid leading to momentum transfer as flow occurs. At any point in a turbulent flow the velocity and pressure are the functions of time thereby rendering such a flow as unsteady.

QUESTION: 6

Which of following conditions would entail a greater energy dissipation in turbulent flow?
1. Smaller eddy size
2. Lower viscosity
3. Large intensity ofturbulence

Select the correct answer using the codes given below:

A.
1, 2 and 3
B.
1 and 3
C.
2 and 3
D.
1 and 2

Solution:

QUESTION: 7

Match List-I (Nature of flow) with List-II (Friction factor equation) and select the correct answer using the codes given below the lists:
List- I
1.Turbulent flow with rough boundary
2. Turbulent flow with smooth boundary
3. Turbulent flow with R_e < 10⁵
4. Laminar flow

A.
a
B.
b
C.
c
D.
d

Solution:

QUESTION: 8

Turbulent flow generally occurs

A.
at very low velocities
B.
in flows of highly viscous fluids
C.
in flows through very narrow passages
D.
in flows at high velocities through large passages

Solution:

Reynolds number, R_e = Vd/v
When R_e < R_ec Laminar flow occurs at very small velocities or through narrow passage (small d) or for highly viscous fluids (high v) the Re will be small and flow will remain laminar. For high velocities through large passage Reynolds number will be sufficient to cause turbulence.

QUESTION: 9

A pipe line carrying water has surface-protrusions of average height 0.10 mm. If the shear stress developed is 7.85 N/m², then the pipe surface will act as