# Fluid Mechanics Questions and Answers – Loss of Head due to Friction in Viscous Flow

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This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Loss of Head due to Friction in Viscous Flow”.

1. Which among the following does not depend on the friction factor?
a) Pipe diameter
b) Fluid density
c) Viscosity
d) Weight

Explanation: The friction factor(f) depends on the velocity of flow, fluid density, pipe diameter and the viscosity of the pipe. Roughness of the pipe is also an important criteria to determine the friction factor.

2. Which among the following is the formula for friction factor(f)?
a) f = 0.079*Re0.25
b) f = 0.079/Re0.25
c) f = 0.079/Re0.5
d) f = 0.079*Re0.5

Explanation: To calculate the friction factor of a fluid, we use the Blasius equation. This equation is accurate for values within 5% having Reynolds number less than 105. (Blasius equation: f= 0.079/Re0.25)

3. How do we calculate losses for a larger range of Reynolds number?
a) Moody chart
b) Bar chart
c) Scatter chart
d) Column histogram

Explanation: Moody chart is a graph of frictional factor(f) vs Reynolds numbers. It gives various values corresponding to the ‘k/d’ ratios. Where ‘k’ is the measure of the wall roughness and ‘d’ is the pipe diameter.
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4. Darcy- Weisbach equation gives relation between__________
a) Pressure and temperature
b) Mass, volume and pressure
c) Head loss and pressure loss
d) Pressure loss only

Explanation: Darcy-Weisbach equation relates the head loss and pressure loss due to friction along a given pipe with a specified length. It contains a dimensionless friction factor called the Darcy friction factor. The equation was named after Henry Darcy and Julius Weisbach.

5. Which among the following is formula for friction factor of circular pipes?
a) 16/Re
b) 64/Re
c) Re/16
d) Re/64

Explanation: Circular pipes have a diameter treated in a round manner. For a fluid flow which is laminar head loss is directly proportional to the fluid velocity. Thus, friction factor is inversely proportional to its velocity. Therefore, the correct option is ‘64/Re’.

6. Loss of head due to friction is __________
a) Directly proportional to hydraulic radius
b) Inversely proportional to velocity
c) Inversely proportional to hydraulic radius
d) Directly proportional to gravitational constant

Explanation: Hydraulic radius is one of the properties of a fluid flow in a channel. It controls the water discharge. It also determines the amount of work the channel can do. (Rh=A/P). Thus, it is inversely proportional to loss of head due to friction.

7. The formula for hydraulic diameter is______
a) 4A/P
b) 4AP
c) 4AV
d) 4V

Explanation: Hydraulic diameter handles the flow in non-circular channels and tubes. The most suitable term to calculate the hydraulic diameter for a round tube is Dh= 4A/P. Where ‘A’ is the cross-sectional area and ‘P’ is the wetted perimeter.

8. What are the reasons for minor head loses in a pipe?
a) Friction
b) Heat
c) Valves and bends
d) Temperature

Explanation: Minor losses play an important role in calculating the flow, pressure and energy of the piping system. Fluid that moves through the pipe carries momentum and energy due to the forces acting on them. Thus, these minor loses are developed due to valves, pipe diameter and bending.

9. What happens to the head loss when the flow rate is doubled?
a) Doubles
b) Same
c) Triples
d) Four times

Explanation: If the flow rate is doubled, the head loss increases by a factor of four. Since, the head loss is directly proportional to the square of the flow rate. Option (d) is the correct option.

10. Relative roughness is_________
a) ϵ/D
b) ϵ*D
c) ϵ/Dm
d) ϵgD

Explanation: Relative roughness is defined as the quantity used to measure the roughness of the pipe’s surface. It is equal to the average height of the surface irregularities divided by the pipe diameter. Therefore, Relative roughness= ϵ/D.

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