This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Drag on a Sphere”.

1. What is the dimension for drag coefficient?

a) Newton/s

b) m/s

c) kg/N

d) Dimensionless

View Answer

Explanation: In fluid dynamics, the drag coefficient has no dimensions. It is a dimensionless quantity. Drag coefficient is used to quantify the resistance of an object in a fluid environment. It is mainly used in air and water.

2. Skin friction acts on the component of _________

a) Profile drag

b) Surface blade

c) Vane angles

d) Parallel movement

View Answer

Explanation: Skin friction acts on the component of profile drag. Pressure drag is also called as form drag. It mainly arises because of the shape of the object. Thus, the correct answer is profile drag.

3. Bodies with a larger cross section will have________

a) Lower drag

b) Higher drag

c) Same drag

d) No drag

View Answer

Explanation: Bodies with a larger cross section will have higher drag. Pressure drag is also called as form drag. It mainly arises because of the shape of the object. Thus, the correct option ‘b’.

4. Drag coefficient is denotes as_______

a) C_{d}

b) B_{c}

c) D_{c}

d) T_{c}

View Answer

Explanation: In fluid dynamics, the drag coefficient has no dimensions. It is a dimensionless quantity. Drag coefficient is used to quantify the resistance of an object in a fluid environment. It is mainly used in air and water. It is denoted as C

_{d}.

5. The drag coefficient of a complete structure such as an aircraft includes________

a) Form drag

b) Pressure drag

c) Interference drag

d) Induced drag

View Answer

Explanation: The drag coefficient of a complete structure such as an aircraft includes interference drag. It results when an airflow around one part of an object. The two airflows must speed up in order to pass through the restricted area.

6. The drag coefficient is directly proportional to the ___________

a) Drag force

b) Mass density

c) Area

d) Flow speed

View Answer

Explanation: The drag coefficient is directly proportional to the drag force. In fluid dynamics, the drag coefficient has no dimensions. It is a dimensionless quantity. Drag coefficient is used to quantify the resistance of an object in a fluid environment.

7. If the friction is neglected, then_______

a) V_{r1} > V_{r2}

b) V_{r1} < V_{r2}

c) V_{r1} = V_{r2}

d) V_{r1} is a zero

View Answer

Explanation: The relative velocity of the jet is denoted as V

_{r1}. It is the relative velocity at the inlet to the vane. Relative velocity of inlet to the vane is obtained by subtracting vectorially the velocity of the vane with its absolute velocity. It happens in the same way for V

_{r2}. Thus, If the friction is neglected, then V

_{r1}= V

_{r2}.

8. Drag force is directly proportional to ________

a) Density of fluid

b) Mass density

c) Area

d) Flow speed

View Answer

Explanation: Drag force is directly proportional to density of the fluid. It is the force that acts opposite to the relative motion of any object moving with respect to its surroundings. Thus, the correct option is ‘a’.

9. Drag force can exist between two layers of liquid.

a) True

b) False

View Answer

Explanation: Drag force can exist between two layers of liquid. They can even exist in between two layers of solid surface. Unlike other resistive forces, they are dependent on velocity.

10. The efficiency of the vane is given by_________

a) 1-V_{2}^{2} / V_{1}^{2}

b) 1-(V_{2}^{2} / V_{1}^{2})

c) V_{2}^{2} / V_{1}^{2}

d) 1- V_{1}^{2}

View Answer

Explanation: In a velocity triangle at the inlet and the outlet, the control volume is moving with a uniform velocity. Therefore, the momentum theorem of the control volume is at a steady flow. Thus, the efficiency of the vane is given by 1-(V

_{2}

^{2}/ V

_{1}

^{2}).

11. Drag coefficient is a function of _________

a) Mach number

b) Froude’s number

c) Laminar flow

d) Reynolds number

View Answer

Explanation: Drag coefficient is a function of Mach number. In fluid dynamics, the drag coefficient has no dimensions. It is a dimensionless quantity. Drag coefficient is used to quantify the resistance of an object in a fluid environment.

12. For a streamlined body to achieve low drag coefficient, the boundary layer must_________

a) Flow over the body

b) Be attached to the body

c) Move away from the body

d) Move parallel to the body

View Answer

Explanation: For a streamlined body to achieve low drag coefficient, the boundary layer must be attached to the surface of the body for a long time as possible. This causes the wake to be narrow.

13. There will be a transition from laminar flow to turbulent flow when______

a) Reynolds number increases

b) Reynolds number decreases

c) Reynolds number is the same

d) Froude’s number increases

View Answer

Explanation: There will be a transition from laminar flow to turbulent flow with the increase in the Reynolds number. Reynolds number below 2000 is laminar flow and Reynolds number above 2000 is for turbulent flow.

14. With the increase in flow velocity, Reynolds number_________

a) Increases

b) Decreases

c) Same

d) Independent

View Answer

Explanation: With the increase in flow velocity, Reynolds number increases. Reynolds number below 2000 is laminar flow and Reynolds number above 2000 is for turbulent flow. Thus, the correct option is Increases.

15. With the decrease in the viscosity, Reynolds number ________

a) Increases

b) Decreases

c) Same

d) Independent

View Answer

Explanation: With the decrease in viscosity, Reynolds number increases. Reynolds number below 2000 is laminar flow and Reynolds number above 2000 is for turbulent flow. Thus, the correct option is Increases.

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